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ISSN: 1402-1544 ISBN 978-91-7439-309-5
Luleå University of Technology 2011
Elisaveta Potapova Adsorption of Surfactants and Polymers on Iron Oxides: Implications For Flotation and Agglomeration of Iron Ore
Department of Civil, Environmental and Natural Resources Engineering
Division of Sustainable Process Engineering
Adsorption of Surfactants and Polymers on
Iron Oxides: Implications For Flotation
and Agglomeration of Iron Ore
Elisaveta Potapova
Adsorption of surfactants and polymers on iron
oxides: implications for flotation and
agglomeration of iron ore
Elisaveta Potapova
Luleå University of Technology
Department of Civil, Environmental and Natural Resources Engineering
Division of Sustainable Process Engineering
September 2011
Cover illustration: schematic illustrations of surfactan adsorption on a mineral particle (top),
mineral flotation (bottom left), and a wet agglomerate (bottom right).
Printed by Universitetstryckeriet, Luleå 2011
ISSN: 1402-1544
ISBN 978-91-7439-309-5
Luleå 2011
www.ltu.se
ABSTRACT
Iron ore pellets are an important refined product used as a raw material in the production of
steel. In order to meet the requirements of the processes for iron production, the iron ore is
upgraded in a number of steps including, among others, reverse flotation. Under certain
circumstances the flotation collector may inadvertently adsorb on the iron ore particles
increasing the hydrophobicity of the iron ore concentrate, which in turn has been shown to
have an adverse effect on pellet strength. To minimize the influence of the collector on pellet
properties, it is important to understand the mechanism of collector adsorption on iron oxides
and how different factors may affect the extent of adsorption.
In Papers I-III, the adsorption of a commercial anionic carboxylate collector Atrac 1563 and
a number of model compounds on synthetic iron oxides was studied in-situ using attenuated
total reflectance Fourier transforms infrared (ATR-FTIR) spectroscopy. The effect of
surfactant concentration, pH, ionic strength, calcium ions and sodium silicate on surfactant
adsorption was investigated. The adsorption mechanism of anionic surfactants on iron oxides at
pH 8.5 in the absence and presence of other ions was elucidated. Whereas silicate species were
shown to reduce surfactant adsorption, calcium ions were found to facilitate the adsorption and
precipitation of the surfactant on magnetite even in the presence of sodium silicate. This
implies that a high concentration of calcium in the process water could possibly enhance the
contamination of the iron ore with the flotation collector.
In Paper III, the effect of calcium, silicate and a carboxylate surfactant on the zeta-potential
and wetting properties of magnetite was investigated. It was concluded that a high content of
calcium ions in the process water could reduce the dispersing effect of silicate in flotation of
apatite from magnetite. Whereas treatment with calcium chloride and sodium silicate made
magnetite more hydrophilic, subsequent adsorption of the anionic surfactant increased the
water contact angle of magnetite. The hydrophobic areas on the magnetite surface could result
in incorporation of air bubbles inside the iron ore pellets produced by wet agglomeration,
lowering pellet strength.
Based on the adsorption studies, it was concluded that calcium ions could be detrimental for
both flotation and agglomeration. Since water softening could result in further dissolution of
calcium-containing minerals, an alternative method of handling surfactant coatings on
i
magnetite surfaces was proposed in Paper IV. It was shown that the wettability of the
magnetite surface after surfactant adsorption could be restored by modifying the surface with
polyacrylate or sodium silicate.
In Paper V, the results obtained using synthetic magnetite were verified for natural
magnetite. It was illustrated that the conclusions made for the model system regarding the
detrimental effect of calcium ions were applicable to the natural magnetite particles and
commercial flotation reagents. It was confirmed that polyacrylate and soluble silicate could be
successfully used to improve the wettability of the flotated magnetite concentrate. The fact that
polyacrylate improved the wettability of magnetite more efficiently at the increased
concentration of calcium ions indicates that this polymer is a good candidate for applications in
hard water.
Finally, it was concluded that in-situ ATR-FTIR spectroscopy in combination with zetapotential and contact angle measurements could be successfully applied for studying surface
phenomena related to mineral processing.
ii
ACKNOWLEDGEMENTS
First, I would like to acknowledge the financial support of this work provided by the
Hjalmar Lundbohm Research Centre (HLRC).
Secondly, I would like to express my gratitude to the people who made these four years an
exciting and fruitful journey: my supervisor, Prof. Jonas Hedlund, for his guidance and trust in
me; my assistant supervisor, Dr. Mattias Grahn, for all his help and encouragement, no matter
what; and Assoc. Prof. Allan Holmgren for being able to solve any problem and answer any
question.
Further, I am grateful to Dr. Seija Forsmo and Dr. Andreas Fredriksson for valuable advice
and feedback about my work from an industrial perspective.
The assistance of Dr. Johanne Mouzon, Lic. Eng. Ivan Carabante, and Lic. Eng. Iftekhar
Uddin Bhuiyan in working with the new SEM and of Dr. Annamaria Vilinska in zetapotential and contact angle measurements is highly appreciated.
I would like to thank my close colleagues Lic. Eng. Ivan Carabante, Dr. Xiaofang Yang, Lic.
Eng. Magnus Westerstrand, and Lic. Eng. Richard Jolsterå for their co-operation, sharing ideas
and experiences.
Ulf Mattila and Oniel Albino, thank you for saving me from the scariest PhD nightmare – a
broken computer during the writing of the thesis.
Dear administrators, thank you for being helpful and patient when settling all the
administrative issues and for not talking work and football during coffee breaks.
I would like to thank my colleagues at the former Department of Chemical Engineering and
Geosciences and especially my colleagues at the Division of Sustainable Process Engineering
for being such great people. You are the best colleagues I could ever have!
A big hug goes to all my friends outside the department, outside the university and outside
Sweden for being there when I needed you and for making my life a fantastic, unforgettable
adventure.
A final and very special thank you goes to my family, who have always supported me from a
distance, and especially to my mom for her guidance through the moments of confusion. I
love you!
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LIST OF PAPERS
This thesis is based on the following papers:
Paper I: Studies of collector adsorption on iron oxides by in-situ ATR-FTIR
spectroscopy
E. Potapova, I. Carabante, M. Grahn, A. Holmgren, and J. Hedlund
Industrial and Engineering Chemistry Research 49 (2010) 1493-1502
Paper II: The effect of calcium ions and sodium silicate on the adsorption of
anionic flotation collector on magnetite studied by ATR-FTIR spectroscopy
E. Potapova, M. Grahn, A. Holmgren, and J. Hedlund
Journal of Colloid and Interface Science 345 (2010) 96-102
Paper III: The effect of calcium ions, sodium silicate and surfactant on charge
and wettability of magnetite
E. Potapova, X. Yang, M. Grahn, A. Holmgren, S. P. E. Forsmo, A. Fredriksson, and J.
Hedlund
Colloids and Surfaces A: Physicochemical and Engineering Aspects 386 (2011) 79-86
Paper IV: The effect of polymer adsorption on the wetting properties of partially
hydrophobized magnetite
E. Potapova, M. Grahn, A. Holmgren, and J. Hedlund
Submitted to Journal of Colloid and Interface Science
vv
Paper V: Interfacial properties of natural magnetite particles compared with
their synthetic analogue
E. Potapova, X. Yang, M. Westerstrand, M. Grahn, A. Holmgren, and J. Hedlund
Full-length paper to be submitted to Minerals Engineering and accepted for presentation at the Flotation
2011 Conference in Cape Town, South Africa
Author’s contribution to the appended papers
Papers I, II, and IV: All experimental work and evaluation, and almost all writing.
Paper III: Approximately one-third of experimental work, two-thirds of evaluation, and
almost all writing.
Paper V: Approximately half of experimental work and evaluation and almost all writing.
vi
CONTENTS
INTRODUCTION............................................................................................................... 1
SCOPE OF THE PRESENT WORK .................................................................................. 3
BACKGROUND .................................................................................................................. 5
Upgrading of iron ore........................................................................................................ 5
Froth flotation.................................................................................................................... 6
Flotation of iron oxides...................................................................................................... 8
Flotation effect on wet agglomeration of iron ore.............................................................. 9
Surface wettability and contact angle measurements ........................................................ 10
Adsorption at the solid/liquid interface............................................................................ 12
Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) ........ 16
EXPERIMENTAL PART ................................................................................................... 19
Materials .......................................................................................................................... 19
Methods........................................................................................................................... 21
Film preparation............................................................................................................ 21
ATR-FTIR spectroscopy ............................................................................................... 21
Contact angle................................................................................................................ 22
Zeta-potential ............................................................................................................... 22
RESULTS AND DISCUSSION.......................................................................................... 23
Characterization of iron oxides (Papers I, II, V)............................................................... 23
Surfactant adsorption and factors affecting the adsorption (Papers I-III)........................... 26
Adsorption mechanism ................................................................................................... 26
Factors affecting surfactant adsorption on iron oxides........................................................... 29
The effect of surfactant adsorption on the properties of the magnetite surface (Paper III) 32
Zeta-potential ............................................................................................................... 32
Contact angle................................................................................................................ 34
Verification for natural magnetite (Paper V) .................................................................... 35
Summary and implications for flotation and agglomeration of iron ore ........................... 36
Restoring magnetite wettability after surfactant adsorption (Papers IV, V) ...................... 37
Modification with sodium silicate ..................................................................................... 37
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Modification with hydrophilic polymers............................................................................. 38
Verification for the flotated magnetite concentrate (Paper V) .......................................... 40
Summary and implications for agglomeration of iron ore ................................................ 41
CONCLUSIONS................................................................................................................. 43
FUTURE WORK ............................................................................................................... 45
BIBLIOGRAPHY................................................................................................................ 47
viii
INTRODUCTION
Adsorption of surfactants and polymers on mineral surfaces is important for many industrial
applications such as detergency, coatings, flocculation of fibres and fine mineral particles,
dispersion of pigments, stabilization of colloidal suspensions in cosmetics and pharmaceuticals,
flotation, and agglomeration.
Undesired adsorption of surfactants and polymers can be of interest, too, in cases where it has
an adverse effect on the performance of a certain process. For instance, interaction of fulvic and
humic acids with iron oxides is a subject of many research publications, since the adsorption of
these natural polymers has been shown to impair the remediation of arsenic-contaminated soils
using iron oxides [1]. Another example of surface contamination that has received much
attention in the literature is the surfactant coating on iron ore concentrates upon flotation.
Flotation of iron ore is performed in order to reduce the amount of certain elements, such as
phosphorous, in the concentrate to an acceptable level for iron production. Phosphorouscontaining minerals, like apatite, associated with the iron ore are separated by reverse flotation
using anionic carboxylate surfactants [2]. Any contamination of the iron ore concentrate with a
surfactant decreases wettability of the concentrate and thus has an adverse effect on the
subsequent pelletizing process and strength of the iron ore pellets produced [3-5].
In order to minimize any negative effects of surfactant adsorption on the surface properties of
the iron ore concentrate after flotation, it is important to elucidate the mechanism of
interactions between anionic carboxylate surfactants and iron oxides and to identify the factors
that may affect these interactions. This information can further provide an idea about the
possibilities of reducing surfactant adsorption on iron oxides or to restore the wettability of the
iron ore concentrate after flotation.
Several different natural polymers and their derivatives have been proposed as depressants of
iron oxides in reverse flotation of iron ore. The depression phenomenon is complex and not
fully understood but the major mechanisms are believed to be by blocking surface sites for
collector adsorption and by co-adsorption resulting in a hydrophilic surface. Additionally, a
number of organic polymeric binders for agglomeration of iron ore have been developed in
the last decades (see references [3-14] in [6]). The main advantage of organic binders compared
to traditional inorganic binders, like bentonite, is that the former are completely eliminated
1
during the heat treatment of iron ore pellets and thus do not introduce any contaminants to
the final product. Therefore, studies on interactions of different types of polymers with iron
oxides, especially in combination with anionic surfactants, could further extend the possibilities
of using polymers as depressants and binders in iron ore beneficiation and agglomeration.
Studies on the interactions between anionic carboxylate surfactants and iron oxides are rather
sparse. Even less common are investigations involving co-adsorption of anionic surfactants and
polymers. The existing studies primarily involve ex-situ methods, batch adsorption and
flotation experiments. Application of in-situ techniques like attenuated total reflectance Fourier
transform infrared spectroscopy (ATR-FTIR) could provide insight into the mechanism of
interaction between surfactants, polymers and iron oxides at the solid-liquid interface. Further,
with this technique, the adsorption and desorption kinetics may be monitored in-situ, as may
any possible changes in the adsorption mode at different experimental conditions.
ATR elements coated with thin films of synthetic analogues of natural mineral particles are
commonly applied in the adsorption studies by ATR-FTIR spectroscopy [7] to achieve a high
signal-to-noise ratio and to simplify interpretation of the spectroscopic results. However, a
possible difference in interfacial properties of synthetic and natural materials is an important
issue to consider and might require verification of the results, obtained using synthetic particles,
for their natural analogue.
2
SCOPE OF THE PRESENT WORK
The scope of the present work may be divided into two main parts:
x Studying the adsorption of surfactants on iron oxides and different factors that can
affect the adsorption;
x Investigating the possibilities of restoring surface wettability after surfactant adsorption.
To achieve the first goal, a method based on ATR-FTIR spectroscopy for in-situ studies of
the adsorption of organic and inorganic species from aqueous solutions on thin films of
synthetic iron oxides was developed. Adsorption and desorption of different surfactants on iron
oxides were investigated in order to elucidate the mechanism of interaction and to study the
stability of the surface complexes formed. The effect of pH, surfactant concentration, ionic
strength, presence of calcium ions and sodium silicate on the adsorption of surfactants on iron
oxides was also studied. In the next step, the change of the charge and wettability of the iron
oxide surface upon adsorption of calcium ions, sodium silicate, and an anionic carboxylate
surfactant was investigated.
Based on the information collected in the first part of the work, several means to reduce the
effect of surfactant adsorption on the wettability of the iron oxide surface were evaluated,
including treatment with sodium silicate and hydrophilic polymers. Finally, the results obtained
using synthetic iron oxide particles were verified for mineral magnetite concentrate.
3
4
BACKGROUND
Upgrading of iron ore
Being the most widely used metal in the world, iron is found in nature mainly in the form of
oxide and sulphide ores. Due to their wide occurrence in nature and their high iron content
[2], the most industrially important ores are: hematite (-Fe2O3), magnetite (Fe3O4) and
goethite (-FeOOH).
After extraction from the deposit, the iron ore is subjected to grinding and enrichment to
produce an iron ore concentrate with a required chemical composition and particle size
distribution. The main purpose of the ore enrichment is to separate the valuable ironcontaining mineral from the waste minerals (gangue) and to reduce the amount of certain
elements (like silicon, phosphorus, aluminium and sulphur) in the concentrate to an acceptable
level for the iron production. Concentration of the iron ore can be achieved by gravity
separation and/or magnetic separation, sometimes followed by froth flotation in order to
further reduce the silica and phosphorous content of the ore [2].
In order to make iron ore concentrates suitable for the blast furnace, fine iron ore particles
have to be agglomerated. Two commercial agglomeration processes exist today: sintering and
pelletizing. Pelletizing is a more energy-efficient process than sintering and requires less than
half the amount of fuel [8].
The pelletizing process starts with balling of wet, so-called green pellets from the iron ore
concentrate. This is done in balling drums using bentonite as a binder. Different additives can
be introduced to the pellet feed to produce pellets with required properties.
Balling of the green pellets is followed by screening where the desired size fraction is
separated from the under-size fraction, which is returned to the balling drum, and the over-size
fraction, which is first crushed and then returned to the balling drum.
Finally, the green iron ore pellets are dried, oxidized and sintered to give the final product.
Here, depending on the type of iron-containing mineral, fuel consumption can vary
significantly. When magnetite ore is used, a highly exothermic oxidation reaction takes place.
In this reaction, magnetite is converted to hematite, accompanied by a heat release that
5
accounts for more than two thirds of the total energy required for the subsequent sintering of
pellets [9].
The quality of the final pellets produced is highly dependent on the green pellets’ strength
and the pellet size distribution. Breakage of the green pellets results in creation of crumbs and
fines that increase the packing density of the pellet bed during drying, oxidation and sintering,
thus reducing the pellet bed permeability to air, which is undesirable since it negatively affects
both the production capacity and pellet quality [3].
Of all the process steps, froth flotation has probably the largest impact on the surface
properties of iron ore concentrate, which also affects pelletization.
Froth flotation
Froth flotation is based on the difference in surface properties of minerals, namely, their
affinity to air and water. Separation of two minerals by flotation can occur if the surface of one
of the minerals is hydrophobic and the surface of the other mineral is hydrophilic. Upon
introduction of air to the flotation cell, hydrophobic particles will be floated by the air bubbles
attached to the particle surface while hydrophilic particles will remain in the water. Fig. 1
illustrates the principle of froth flotation.
Figure 1. The principle of froth flotation.
6
Two types of flotation processes can be distinguished based on the floated fraction: direct
flotation refers to the process in which the valuable mineral is transferred to the floated fraction,
leaving the gangue in the slurry while in reverse flotation, the gangue is floated and the valuable
mineral remains in the slurry [10].
Most minerals are not hydrophobic by nature so their surface has to be modified by a flotation
collector, which selectively adsorbs on the surface of a mineral to be floated, rendering it
hydrophobic and thus easily attached to the hydrophobic air bubbles. Flotation collectors are
heteropolar organic molecules containing both a non-polar hydrophobic hydrocarbon group
and a polar head group. Depending on the properties of the head group, flotation collectors
can be classified as ionic or non-ionic. Ionic collectors become ionized upon dissolution in water
and are further subdivided into anionic (e.g. carboxylates, sulphonates, xanthates [11]), cationic
(e.g. amines, quaternary ammonium salts, pyridinium salts [12]) and amphoteric or zwitterionic
(e.g. amino acids, glycines, quaternary ammonium sulphonates [12]) based on the charge of the
head group after dissociation. Non-ionic collectors contain a head group that does not
dissociate in water, e.g. a polyoxyethylene glycol group [13]. Among all the collectors, anionic
collectors are most widely used in mineral flotation [10]. For instance, fatty acids and
petroleum sulphonates are applied in non-sulphide mineral flotation while xanthates are
commonly used for most sulphide ores [14].
In order to prevent the air bubbles holding mineral particles from bursting when they reach
the air-water interface, a frother is added to the flotation cell to facilitate the formation of a
stable froth, which is further transferred from the flotation cell surface to the collecting launder.
Additionally, different modifiers are typically used in order to increase flotation selectivity [10].
Modifiers can either enhance or reduce the effect of a collector on a certain mineral and are
therefore referred to as activators and depressants. Activators are usually soluble salts that become
ionized in solution and interact with the mineral surface altering its chemical nature and
making it more favourable for collector adsorption [10]. The action of depressants is more
complex and not always fully understood. However, one of the main mechanisms is blocking
of the surface sites by adsorbing the depressant to prevent collector adsorption [14]. Dispersants
may also be added to the flotation system to facilitate liberation of different small-size mineral
fractions (slime) from the surface of larger ore particles, thus facilitating increased floatability of
the larger particles, which in turn improves the recovery. Finally, pH regulators are added to
control the pH – one of the key variables in the flotation process that affects the surface
7
properties of the minerals and the speciation of both the flotation chemicals and naturally
occurring inorganic ions (e.g. carbonates, sulphates) in the process water.
Apart from modifiers added deliberately, different inorganic ions naturally present in the
process water may affect the flotation performance [15] by activating or depressing the flotation
of a certain mineral, changing collector solubility and zeta-potential of the mineral surface. For
instance, pyrite can be activated in the presence of copper ions [16] and activation of magnetite
for flotation with fatty acids can occur in the presence of calcium ions [17].
Flotation of iron oxides
The choice of flotation process in iron ore beneficiation depends on the nature of the gangue
associated with the iron-containing mineral, which can be siliceous or acidic (rich in silica) and
calcareous or basic (rich in calcium oxide) [10].
When iron oxide is to be separated from the siliceous gangue, either direct or reverse
flotation can be applied. Anionic collectors such as fatty acids and alkyl sulphates and
sulphonates [18] are most commonly used for flotation of iron oxides from siliceous gangue
minerals at pH values where the surface of iron oxide is positively charged. An example of a
process utilizing fatty acid flotation for the concentration of hematite is the Republic mine
process in the state of Michigan in the USA [18].
Quartz and silicate minerals can be floated from iron oxides with cationic collectors,
primarily amines, when their surface is negatively charged. In order to increase flotation
selectivity, iron oxides can be successfully depressed by starch or dextrin [18]. For instance, the
Empire and Tilden mines (Michigan, USA) and the Griffith Mine (Ontario, Canada) have
been using ether amines to float the siliceous gangue from the iron oxides [18]. This type of
flotation is also utilized in Brazil, Chile, India, Mexico, Russia, and South Africa [19].
Calcareous phosphate gangue minerals can be floated from iron oxides with modified fatty
acids. Selectivity is improved when sodium silicate or starch is used as a depressant [2]. The
Swedish mining company LKAB has been using reverse froth flotation with an anionic fatty
acid based collector for dephosphorization of magnetite concentrate. In order to improve
flotation
selectivity
and
phosphorous
recovery,
sodium
silicate
is
added
as
a
dispersant/depressant. A distinctive feature of the flotation of calcareous ores is the presence of
calcium ions in the process water [20]. Calcium is known to facilitate precipitation of fatty
acids [21] which may result in unnecessary increases in fatty acid collector consumption.
8
Moreover, high concentrations of calcium ions have been shown to activate magnetite for
flotation with a fatty acid collector by adsorbing on the magnetite surface and changing its
charge [17].
Flotation effect on wet agglomeration of iron ore
Wet agglomeration implies that fine particles in agglomerates are held together by a liquid,
which acts as a binder. The amount of liquid in the structure of agglomerates determines
agglomerate strength and is characterized by the liquid saturation (S) (Eq. 1):
S
100 ˜ F 1 H U P
˜
˜
,
100 F H
UL
(1)
where F – liquid content in the agglomerate; – fractional porosity; P – density of particles;
L – density of liquid.
Wet agglomerates can be in different liquid saturation states (see Fig. 2).
Figure 2. States of liquid saturation in wet agglomerates [22-24].
According to the capillary theory [25] developed for wet agglomerates with a freely movable
binder (like water), the tensile strength of agglomerates increases with the increase in liquid
saturation due to the development of the capillary forces. The tensile strength reaches
maximum in the capillary state (liquid saturation 80-90%) when all the pores inside the
agglomerate are filled with liquid and concave menisci are formed at the pore openings (see
Fig. 2). Complete wetting of the surface is required for full development of the capillary forces.
The relation between the tensile strength (c) of wet agglomerates in the capillary state and
surface wettability is described by the Rumpf equation [25]:
Vc
a˜
1 H
H
˜J ˜
1
˜ cos T LS ,
d
(2)
9
where a – constant; – fractional porosity; – liquid surface tension; d – average particle size;
LS – liquid-solid contact angle.
Strictly speaking, the Rumpf equation is only valid for agglomerates produced using a freely
movable binder and is not applicable to iron ore pellets balled with water and bentonite clay
[26, 27]. However, as illustrated below, similar trends as those described by the Rumpf
equation are observed for the wet strength of iron ore pellets.
Flotation of the iron ore prior to agglomeration may affect several parameters in the Rumpf
equation. The presence of a flotation collector in the water reduces the surface tension of the
water, which has been shown to decrease the wet strength of iron ore pellets [3, 22, 28].
Adsorption of flotation collector on the surface of the concentrate makes the surface more
hydrophobic and could be expected to further reduce pellet wet strength. Although no
experimental studies investigating the dependency of the agglomerate strength on the contact
angle of the feed have been found, an adverse effect of flotation collector adsorption on the
wet strength of iron ore pellets is commonly reported [3-5]. Additionally, collector adsorption
increases the affinity of the concentrate surface for air, which results in attachment of air
bubbles to the surface of the concentrate, followed by incorporation of bubbles inside green
pellets, increasing pellet porosity and decreasing the liquid saturation [3]. However, the authors
conclude that the decrease in pellet wet strength upon adding flotation collector was not due
to the decreased liquid saturation, but was due to the fact that air bubbles inside the green
pellets behaved like large plastic particles, increasing plastic deformations in pellets and
weakening the pellet structure.
To reduce the disturbances in balling circuits, variation in the properties of the pellet feed
should be minimized. Together with moisture content and fineness, wettability of the iron ore
concentrate should be monitored, so that necessary process adjustments could be made in both
flotation and pelletization.
Surface wettability and contact angle measurements
Wetting of a solid surface occurs due to adhesion forces between the surface and the wetting
liquid, which act against the cohesive forces within the liquid and make the liquid spread over
the surface at a certain contact angle (see Fig. 3).
10
Figure 3. Schematic illustration of the contact angle at the solid-liquid-gas contact line.
The solid-liquid contact angle (LS) of a liquid drop on a polished, flat, solid surface is defined
by the Young equation [29]:
J SG
J LS J LG ˜ cos T LS ,
(3)
where SG, LS, and LG are the solid-gas, solid-liquid and liquid-gas interface tension,
respectively.
For each pair of liquid and solid characterized by certain SG and LG, the solid-liquid contact
angle is determined by the liquid-solid interfacial free energy (LS). According to the van Oss
theory [30], the liquid-solid interfacial free energy can be divided further into the apolar
Lifshitz-van der Waals part (LW) and the polar part, with the latter comprising Lewis acid (+)
and Lewis base (-) components:
J LS
LW LW
J SG J LG 2 J SG
J LG J SG
J LG
J SG
J LG
.
(4)
Iron oxides have a large amount of acid and base sites [31], contributing to the polar
component of the surface free energy, and are consequently hydrophilic. For instance, a
contact angle of 25° ± 5° was reported [32] for water on the polished surface of natural
magnetite, measured using a static sessile drop method. However, it is not always possible to
obtain a completely smooth surface for contact angle measurements, which leads to the
problem of high variation in the results reported for the same iron oxide. Additionally,
chemical heterogeneity, introduced by impurities present in the natural mineral samples, for
example, may have a significant effect on the measured contact angle. Iveson et al. have shown
that the contact angle of the mixed hematite-goethite ores varied from 0° to 74° depending on
the relative content of these two minerals [33].
Depending on the particle size and morphology, different techniques are used for contact
angle measurements [34, 35]. Optical tensiometry methods (e.g. a static sessile drop method)
are based on capturing and analyzing images of a liquid drop placed on a surface and are
suitable mainly for measuring the contact angle on flat surfaces. However, the static sessile drop
11
method can also be used to assess the wettability of colloid particles, providing that a closely
packed layer of particles can be formed [36]. The contact angle of natural mineral powders is
commonly estimated by the Washburn method, which is an example of force tensiometry
methods and is based on measuring the sorption of a wetting liquid by a powder material upon
immersion. In this method, the packing of particles is also important since it may affect the
penetration rate of the wetting liquid and thus the measured value of the contact angle [37].
Although it might be a challenge to obtain a true value of contact angle for non-ideal
systems such as porous films and mineral powders, contact angle measurements can be
successfully used to characterize the changes in the wettability of these materials upon
adsorption of reagents related to flotation and pelletization.
Adsorption at the solid/liquid interface
Adsorption is a process of accumulation of adsorbate species from a bulk gas or liquid on the
surface of an adsorbent. In the case of interactions of surfactants and polymers with mineral
surfaces, it is the adsorption at the liquid/solid interface that is of interest. When a solid surface
is placed in contact with a polar liquid (like water), the surface may acquire a net surface charge
due to ionization of the surface groups, adsorption of ions from solution or dissolution of ions
comprising the surface [38]. Consequently, an electrical double layer may be formed, due to
the concentration of oppositely charged counter-ions at the charged surface to maintain
charge-neutrality (see Fig. 4).
Figure 4. Schematic figure of the electrical double layer at a liquid/solid interface.
12
In Fig. 4, represents the so-called Stern layer where the counter-ions have the highest
concentration and are held close to the surface. Beyond the Stern layer, the concentration of
counter-ions decreases until it reaches the bulk concentration. The charge of the surface in the
slip plane just outside the Stern layer is referred to as zeta-potential and can be estimated from
electrokinetic measurements.
Considering an iron oxide surface in contact with water, a fully hydroxylated surface should
be
expected.
The
net
charge
of
the
iron
oxide
surface
is
dependent
on
protonation/deprotonation of the hydroxyl groups when the pH of the solution changes (see
Eq. 5).
H
H
{ FeOH 2 m
 { FeOH 

o { FeO (5)
The pH at which the net charge of the surface is zero is referred to as the point of zero charge
(PZC). For iron oxides, the PZC is usually observed at pH 7-8 [39]. Above this pH, the
surface is charged negatively, whereas below the PZC the surface bears a positive charge. The
PZC of a surface can be determined by a potentiometric titration in an indifferent electrolyte.
When the charge of a surface is measured by electrophoresis, the pH at which the zetapotential is equal to zero is termed the isoelectric point (IEP). In the absence of specific adsorption
of non-potential-determining ions, the values of the PZC and the IEP should be the same.
The zeta-potential plays an important role in the adsorption of ionic species at mineral-water
interfaces. The change in the zeta-potential upon adsorption can be used as an indication of the
type of forces involved in adsorption [40]. If adsorption takes place only through electrostatic
interaction, the absolute value of the zeta-potential will decrease upon adsorption and will
eventually reach zero when the surface is fully saturated with the adsorbate. However, if apart
from electrostatic interaction, specific adsorption occurs due to affinity of certain species to the
surface, the zeta-potential of the surface can go through zero and then become reversed.
Another indication of specific adsorption is the shift of the PZC and the IEP of a surface in the
opposite directions upon adsorption.
Depending on the forces contributing to adsorption, specific adsorption can be either
physical or chemical. Chemical adsorption refers to when an adsorbate forms a covalent bond
with the surface of the adsorbent while physical adsorption implies contribution of weaker
forces such as hydrogen bonding and van der Waals interactions.
13
Quantitatively, adsorption of a certain compound on a solid surface is described by an
adsorption isotherm. It is obtained by plotting the measured amount of the adsorbate on the
surface against the equilibrium concentration of adsorbate in solution. Different adsorption
models have been developed to describe experimental adsorption data; the most common
models used for describing adsorption at the solid-liquid interface are the Langmuir and the
Freundlich models [38].
The Langmuir adsorption isotherm (Eq. 6) is based on the assumption of localised monolayer
adsorption and that the heat of adsorption is independent of surface coverage.
x
x max
aC
1 aC
In this equation,
(6)
x
is the fraction of the surface covered with the adsorbate; C is the
x max
equilibrium concentration of the adsorbate in solution and a is the adsorption constant.
The Freundlich adsorption isotherm (Eq. 7) can be derived from the Langmuir isotherm by
introducing an exponential change to the heat of adsorption with surface coverage. Thus, this
model implies adsorption on an energetically heterogeneous surface. The different adsorption
sites may be grouped patchwise, with sites having the same heat of adsorption grouped
together.
x
m
kC 1 / n
(7)
In this equation, x is the amount of the adsorbate adsorbed on a specific mass m of the
adsorbent; k and n are empirical constants.
Both the Langmuir and the Freundlich isotherms are applicable to the adsorption of
surfactants on mineral surfaces. However, due to specific properties of surfactant molecules
(e.g. their ability to form micelles or adsorbed multi layers) the adsorption of these molecules
can be characterized by other types of isotherms. For instance, adsorption of ionic surfactants
on oppositely charged surfaces is frequently described by an S-shaped isotherm when plotted
using a logarithmic scale and referred to as a “Somasundaran-Fuerstenau” isotherm [41]. This
isotherm has four characteristic regions as illustrated in Fig. 5 [42].
Region I represents adsorption at low surfactant concentrations due to electrostatic forces
between the surfactant species and oppositely charged surface sites. In Region II, surfactant
14
species on the surfaces begin to form two-dimensional surface aggregates due to hydrophobic
interactions between the hydrocarbon chains in surfactant molecules. Since the electrostatic
interactions are still active in this region, the adsorption density shows a sharp increase. In
Region III, the surface charge is fully neutralized by the adsorbed surfactant species and
electrostatic forces do not contribute to adsorption any longer. However, interaction between
hydrophobic chains in surfactant species still occurs, further increasing adsorption density,
though at a lower rate. In region IV, the surfactant concentration in solution reaches the
critical micelle concentration (CMC) and any increase in concentration contributes primarily
to formation of micelles in solution without changing the adsorption density on the surface
much.
Figure 5. Somasundaran-Fuerstenau isotherm [42].
Polymer adsorption on solid surfaces is commonly characterized by a so-called high-affinity
adsorption isotherm, exhibiting a sharp increase in surface loading at a very low polymer
concentration, which is followed by a plateau at higher concentrations [43]. This type of
adsorption isotherm has been reported for polyelectrolytes adsorbed on the surfaces of the same
[44] and opposite charge [45] as well as for the adsorption of non-charged polymers [46]. A
specific feature of polymer adsorption is that it can hardly be reversed by dilution [43] due to
the fact that a polymer molecule is bound to the surface through a number of segments, which
have to be detached from the surface in order to desorb the polymer molecule. The ability of
a polymer molecule to adopt a large number of configurations both in solution and at the
solid/liquid interface makes polymer adsorption rather complex as compared to adsorption of
small molecules and ions. Numerous theoretical models describing polymer adsorption have
been proposed and can be found elsewhere [43, 47].
15
Attenuated total reflectance Fourier transform infrared
spectroscopy (ATR-FTIR)
Fourier transform infrared (FTIR) spectroscopy is based on the ability of molecules to
undergo transitions from one vibrational energy state to another by absorbing infrared radiation
[48]. In order for absorption to occur, the transition must involve a change in the dipole
moment of a vibrational mode. Each molecule can only absorb radiation of certain frequencies,
that is, the natural vibrational frequencies of the molecule, resulting in a number of absorption
bands located at different frequencies in the spectrum. In infrared spectroscopy, the frequency
is traditionally expressed in wavenumbers (cm-1).
The most commonly used types of vibrational modes are stretching and bending. A stretching
vibration is characterized by a change in the length of a bond between atoms while a bending
vibration involves the change in the angle between bonds.
The amount of infrared radiation absorbed (A, Absorbance) by a certain species is described by
the Lambert-Beer law (Eq. 8).
A
log
I0
I
˜l ˜C
(8)
In this equation, I0 is the initial intensity of the radiation and I is the intensity of the radiation
after interaction with the sample; is the molar absorptivity of the species at a certain
wavelength; l is the path length of the radiation in the sample; C is the concentration of the
species of interest in the sample.
Thereby, the Lambert-Beer law illustrates that the intensity of a specific absorption band is
proportional to the amount of the corresponding group in the sample and can thus be used for
quantitative studies.
FTIR spectroscopy enables both ex-situ and in-situ studies. Attenuated total reflectance FTIR
spectroscopy (ATR-FTIR) is a technique well suited to in-situ studies, providing an
opportunity to study the interactions at the solid-liquid interface without changing the surface
characteristics of the sample [49].
The ATR technique is based on the phenomenon of attenuated total reflectance, which is
schematically illustrated in Fig. 6.
16
Figure 6. Schematic figure of an ATR waveguide illustrating the ATR phenomenon.
In this technique, the IR beam with the initial intensity I0 passes through a waveguide,
having a high refractive index n2 and being surrounded by a medium with lower refractive
index n1. The difference in the refractive indices results in attenuated total reflection of the
beam inside the waveguide, provided that the incident angle fulfils the relation shown in
Eq. 9.
sin T t
n1
n2
(9)
At each point of reflection, an evanescent wave of the IR radiation is formed perpendicular
to the waveguide. The wave can interact with the surrounding medium in the vicinity of the
waveguide resulting in attenuation in the intensity of the totally reflected beam. The amount
of radiation of a certain wavelength ( O ) absorbed by the surrounding medium depends on the
penetration depth dp (see Fig. 6), which is defined by Eq. 10.
dp
O
2
§
§n · ·
2Sn 2 ¨ sin 2 T ¨¨ 1 ¸¸ ¸
¨
© n2 ¹ ¸¹
©
(10)
1/ 2
The penetration depth is, by definition, the distance from the interface where the intensity of
the electric field (E) of the wave has declined to a value equal to:
E = E0·e-1
(11)
In this equation, E0 is the intensity of the electric field at the surface of the waveguide.
The values of the penetration depth typically vary in the range from some hundred
nanometres to a few micrometres, making the ATR-FTIR spectroscopy a surface-sensitive
17
technique. Furthermore, the short penetration depth significantly reduces the absorption of IR
radiation by water, facilitating studies of aqueous systems. In-situ spectroscopic measurements
open up possibilities for following the adsorption process in real time and to obtain
information about adsorption and desorption kinetics and equilibria, surface complexes formed
at the solid/liquid interface and the orientation of adsorbed species.
ATR-FTIR spectroscopy has been extensively used to study the adsorption of surfactants and
polymers on mineral surfaces [7, 49-51]. The fact that adsorption can be performed either
directly on a bare waveguide or on a waveguide coated by a thin layer of adsorbent makes the
technique applicable to a wide variety of systems [52].
18
EXPERIMENTAL PART
Materials
Three different iron oxide materials were used in the experimental work (see Table 1).
Table 1. Iron oxide materials used in the experimental work.
Iron oxide
Synthesis method/Origin
Paper(s)
Synthetic hematite
Matijevic [53]
I
Synthetic magnetite
Massart and Cabuil [54]
II-IV
*
LKAB, Kiruna, Sweden
V
Mineral magnetite
*
Cleaned by magnetic separation and flotation, stored at the ambient conditions for two years.
The iron oxides were characterized using X-ray diffraction (XRD), scanning electron
microscopy (SEM), electrophoresis, gas adsorption, and contact angle measurements.
Adsorption of a commercial flotation collector, Atrac 1563 from Akzo Nobel, and four
model compounds was investigated in this work (see Fig. 7).
Figure 7. Chemical structures of Atrac 1563 (a), ethyl oleate (b), maleic acid (c), poly
(ethylene glycol) monooleate (PEGMO) (d), and dodecyloxyethoxyethoxyethoxyethyl maleate
19
(e). R represents a linear alkyl chain in fatty acids or a C19H29 chain in resin acids, R’ –
CH3(CH2)7CH=CH(CH2)7, R’’ – CH3(CH2)11.
Commercial flotation collector Atrac 1563 has a complex chemical composition: 50-100 %
ethoxylated tall oil ester of maleic acid, and 1-5 % maleic anhydride (Akzo Nobel material
safety data sheet). Since the exact composition and chemical structure of Atrac 1563 are
unknown, four different reagents, as shown in Fig. 7b-e, were evaluated as model compounds
to be used in the experimental work instead of Atrac 1563.
Two types of soluble silicate were used in the experiments. Water glass, i.e. an aqueous
solution of sodium silicate, in this case with a SiO2:Na2O weight ratio of 3.25, is used as a
dispersant/depressant in the flotation of iron ore. Sodium metasilicate (Na2SiO3·9H2O) was
used as an analytical grade alternative of water glass.
In the experiments described in Papers IV and V, adsorption of four different polymers was
investigated (see Table 2).
Table 2. Polymers used for surface modification of magnetite.
Polymer name
Structural formula
Dispex A40 (ammonium
polyacrylate)
Dispex N40 (sodium
polyacrylate)
Average
molecular weight
4000
BASF
4000
BASF
ATC 4150
50000
(aliphatic quaternary
polyamine)
Soluble starch*
N/A
*
Supplier
Eka
chemicals
Merck
1 wt % aqueous starch solution containing 0.5 wt % NaOH was heated to 84°C for 10
minutes and then cooled to room temperature [55].
20
Methods
The main instrumental techniques used in the present work were ATR-FTIR spectroscopy,
contact angle and zeta-potential measurements.
Film preparation
For the spectroscopic and contact angle measurements, the appropriate substrate was coated
with a film of synthetic iron oxide. For the experiments described in Paper I, both sides of the
waveguide were coated with a hematite film by means of dip-coating. In the experiments
described in Papers II-V where synthetic magnetite was used, only one side of the waveguide
was coated with a film by spreading a certain amount of magnetite dispersion and air-drying it
at room temperature. The reason for this was to prevent the magnetite film from absorbing too
much IR radiation in the spectroscopic measurements.
ATR-FTIR spectroscopy
Spectral data were collected using a Bruker IFS 66v/S spectrometer equipped with a liquid
nitrogen cooled mercury-cadmium-telluride (MCT) detector and a deuterated triglycine
sulphate (DTGS) detector, a vertical ATR accessory and a stainless steel sample cell (see Fig. 8).
Trapezoidal ZnSe crystals (Crystran Ltd) with 45° cut edges and dimensions of 50x20x2 mm
were used as ATR waveguides.
Figure 8. Schematic illustration of the experimental setup. Thick solid lines represent liquid
flow whereas the dashed arrows indicate the IR beam.
Adsorption measurements were performed in-situ at room temperature with a continuous
flow of working solution pumped through the cell with recirculation, except for the
desorption experiments in which the solution was not recirculated. The pH during the
adsorption experiments was kept constant by a Mettler Toledo T70 titrator.
21
Contact angle
The static sessile drop method was used to determine the contact angle of the synthetic
magnetite nanoparticles (Papers III-V). Contact angle measurements were performed using a
Fibro 1121/1122 DAT-Dynamic Absorption and Contact Angle Tester equipped with a CCD
camera. The measurement was performed by placing a 4 L water droplet onto the magnetitecoated substrate using a microsyringe. A series of images were taken and analysed using the
DAT 3.6 software. To investigate the effect of different reagents on the wettability of the
synthetic magnetite particles, consecutive adsorption of the reagents was performed on the
magnetite film in the same way as in the spectroscopic measurements. Between the adsorption
steps, the contact angle of the magnetite film was measured.
The contact angle of the natural magnetite particles (Paper V) was determined by the
Washburn method using a Krüss K100 force tensiometer. Liquid sorption by the magnetite
powder was recorded as a function of immersion time, and Krüss LabDesk 3.1 software was
used to calculate the contact angle applying the Washburn equation. First, the capillary
constant of the Washburn equation was estimated for each sample using n-hexane. Thereafter,
the contact angle of the magnetite powder was measured using deionized water. The values of
the capillary constant and the contact angle were calculated as an average of three replicates.
To investigate the effect of different reagents on the wettability of the natural magnetite
particles (Paper V), batch adsorption was performed using suspensions containing 10 g
magnetite per ca 40 mL solution at pH 9 and room temperature. After adsorption, the solution
was decanted and magnetite was dried in an oven overnight at 50°C.
Zeta-potential
The zeta-potential of both synthetic and natural iron oxides as a function of pH was
determined by electrophoresis using a ZetaCompact instrument equipped with a chargecoupled device (CCD) tracking camera. The electrophoretic mobility data was further
processed by the Zeta4 software applying the Smoluchowski equation. For the case of
magnetite concentrate, the measurements were performed using the 0.22-8 m fraction of the
magnetite slurry collected at the LKAB concentrating plant in Kiruna, Sweden, after flotation.
The required size fraction of the particles was separated by vacuum filtration.
Further experimental details are available in the appended papers.
22
RESULTS AND DISCUSSION
Characterization of iron oxides (Papers I, II, V)
The X-ray diffraction data of the iron oxides used in the present study (Fig. 9) confirmed
pure crystalline phases of hematite (a) and magnetite (b, c), without any other phases present in
amounts detectable by XRD. The peak width decreases in the sequence synthetic magnetite >
synthetic hematite > natural magnetite, reflecting the increasing particle size of the iron oxide
materials (10 nm and 130 nm, as determined by SEM, see below, and < 45 m [56],
respectively).
Figure 9. XRD patterns of a synthetic hematite (a), a synthetic magnetite (b), and a natural
magnetite (c). The reflections originating from the corresponding iron oxides are indexed with
the appropriate Miller indices.
23
SEM images in Fig. 10 show examples of cross-sections of the films of synthetic hematite (a)
and magnetite (b) on a ZnSe substrate, which were used in the spectroscopic and contact angle
measurements. In both cases, porous films were formed with a thickness of ca 1 m and 250300 nm, respectively. Fig. 10 illustrates that the synthetic iron oxide crystals had a uniform
spherical habit and were slightly aggregated. The particles in the magnetite concentrate in
Fig. 10c exhibited high variation in both size and shape. The figure illustrates that the coarse
magnetite particles were covered by very fine particles (less than 1 m in size), some of which,
according to the EDS results, had a high content of silicon and aluminium and could be the
remains of aluminosilicate minerals, which are present in the iron ore before concentration.
Figure 10. Side view SEM images of a hematite film (a) and a magnetite film (b) on a ZnSe
crystal, a top view SEM image of the mineral magnetite particles on a carbon tape (c), and a
close-up SEM image of a magnetite particle shown in Fig. 10c (d).
24
Fig. 11 shows the zeta-potential of the iron oxides as a function of pH. The fraction of the
mineral magnetite concentrate used for the zeta-potential measurements was found to be
mainly comprised of other minerals that magnetite and will not be discussed here. The IEP for
the synthetic magnetite (empty triangles) was observed at pH 7, as per the literature [39],
whereas the IEP for the hematite particles (filled diamonds) was observed around pH 5, which
is lower than expected and could be caused by the adsorption of chloride [57] or carbonate
[58] ions on the surface.
Figure 11. Zeta-potential as a function of pH of the synthetic hematite in 10 mM KNO3 ()
and of the synthetic magnetite in 10 mM NaCl ().
Regarding the wetting properties of the iron oxides used in this work, the contact angle of
the synthetic magnetite was 15-25°, whereas the magnetite concentrate had a contact angle of
50-60°. The lower wettability of the magnetite concentrate was likely due to the hydrophobic
flotation collector species that have been reported to be present on the surface of the
concentrate after flotation [3]. However, the inconsistency in the obtained values could also be
due to the difference in particle size as well as the measuring techniques used.
Table 3 summarizes the morphological properties of the iron oxides used in this study.
Table 3. Morphological properties of the iron oxide materials.
Property
Synthetic hematite Synthetic magnetite
Mineral magnetite
Particle size
130 nm
5-15 nm
85% -45 m [56]
Particle shape
Spherical
Spherical
Irregular
BET (N2) surface area, m2 g-1
13
90
0.5 [56]
25
Surfactant adsorption and factors affecting the adsorption
(Papers I-III)
In the present work, the adsorption of one commercial flotation collector (Atrac 1563) and
four model compounds (PEGMO, maleic acid ester, ethyl oleate, and maleic acid) on synthetic
iron oxides was investigated. However, only three of the compounds were found to show
similar adsorption behaviour: Atrac 1563, PEGMO, and the maleic acid ester. The adsorption
of these three compounds will be discussed below.
Adsorption mechanism
Fig. 12 shows the spectra of Atrac 1563, PEGMO, and the maleic acid ester, as-received and
adsorbed on synthetic hematite and magnetite at pH 8.5.
Figure 12. ATR-FTIR spectra of Atrac 1563 (1), PEGMO (2) and the maleic acid ester (3)
as-received and spread over an uncoated ZnSe crystal (a); of Atrac 1563 (1) and PEGMO (2)
adsorbed on hematite from a 10 mg L-1 solution at pH 8.5, and maleic acid ester (3) adsorbed
on magnetite from a 25 mg L-1 solution containing 0.01 M NaCl at pH 8.5 (b).
As illustrated in Fig. 12, similar absorption bands are observed in the spectra of the
surfactants, confirming structural resemblance of the head groups in these molecules.
Assignment of the main absorption bands in the spectra of the surfactants is presented in
Table 4. More detailed discussions of the spectral features displayed by the surfactants used in
the present study are given in Papers I, II, and V.
26
Adsorption of the surfactants on synthetic iron oxides was performed from aqueous solutions
at pH 8.5, i.e. at an optimum pH for the flotation of apatite from iron oxide [2]. At this pH,
the free carboxylic groups in Atrac 1563 and maleic acid ester become deprotonated, forming a
negatively charged carboxylate ion as indicated by two new bands originating from the
symmetric and asymmetric stretching vibrations of the carboxylate ion (vs(COO-) and
vas(COO-), respectively) in the spectra of these compounds adsorbed on the iron oxides
(spectra (1) and (3) in Fig. 12b). No bands associated with the carboxylate ion were found in
the spectrum of PEGMO (spectrum (2) in Fig. 12b) suggesting that the ester bond in PEGMO
does not break upon adsorption on hematite at the conditions studied.
Table 4. Assignment of absorption bands originating from Atrac 1563, PEGMO, and maleic
acid ester adsorbed on synthetic hematite and magnetite in-situ at pH 8.5. The numbers in
parentheses represent the position of the corresponding absorption bands in the same
compounds as-received, spread over a ZnSe substrate.
Peak position, cm-1
Peak assignment
Atrac 1563
PEGMO
Maleic acid ester
1724 (1736)
1740 (1736)
1724 (1728)
(C=O) in ester [59]
(1709)
-
(1715)
(C=O) in acid [48]
(1645)
-
(1643)
(C=C) [60]
1564
-
1571
as(COO-) [60]
1424
-
1402
s(COO-) [60]
1171 (1159)
1175 (1173)
1178 (1161)
(C-O) in esters [61]
-
1095 (1115)
1104 (1105)
(C-O-C) [62]
-
1047 (1070)
-
(C-OH) [63]
At pH 8.5 the surface of the iron oxides was characterized by a negative zeta-potential (see
Fig. 11) and, consequently, no considerable adsorption of anionic carboxylate surfactants on
iron oxides would be expected at this pH due to electrostatic repulsion between the negatively
charged carboxylate ions and the surface bearing the same charge. In the present work, no
adsorption of maleic acid on hematite took place at pH 8.5, which agrees with the results
reported by Hwang and Lenhart [64]. However, both in the previous studies on oleatehematite systems [65, 66] and in this work (spectra (1) and (3) in Fig. 12), the anionic
carboxylate surfactants exhibited considerable adsorption on iron oxides even at pH values
27
above the IEP suggesting that the adsorption of anionic carboxylate surfactants on iron oxides
is not exclusively determined by the electrostatic forces.
Based on the results from adsorption of maleic acid, it may be concluded that the carboxylate
function is not likely to be responsible for the adsorption of the carboxylate surfactants onto
the iron oxides above their IEP. Similar to the ability of non-ionic surfactants (like PEGMO)
to adsorb on solid surfaces via the polar head group [42], the adsorption of Atrac 1563 and
maleic acid ester on iron oxides above their IEP could be determined by the presence of polar,
but not charged, groups such as ester carbonyl, hydroxyl, and ethoxy-groups. The suggested
mechanism of surfactant adsorption on iron oxides at pH values above the IEP is illustrated in
Fig. 13.
Figure 13. Proposed adsorption mechanism of Atrac 1563 (a), PEGMO (b), and maleic acid
ester (c) on iron oxides from aqueous solutions at pH 8.5. R represents a linear alkyl chain in
fatty acids or a C19H29 chain in resin acids, R’ – CH3(CH2)7CH=CH(CH2)7, R’’ –
CH3(CH2)11. Dashed ovals indicate the moieties interacting with the surface.
28
Additionally, hydrophobic interaction between the hydrocarbon chains of the surfactants
could possibly contribute to the adsorption, as indicated by the shift of the CH2 asymmetric
stretching vibration band in the spectra of Atrac 1563, PEGMO, and maleic acid ester with the
increase of surfactant loading on the surface (not shown) [67].
Thus, a conclusion can be made that both the hydrophobic tail and the polar head group
determine the ability of a surfactant to adsorb on iron oxides.
The desorption experiments (Fig. 13 in Paper I) showed that the adsorbed species of Atrac
1563 could be removed from the hematite surface only partially, even at increased pH,
implying rather strong interaction between the surfactant and the iron oxide.
It is important to mention here that carboxylate ions can be expected to facilitate the
adsorption of the surfactants on iron oxides below the IEP when the net charge of the surface
is positive. The contribution of electrostatic forces to the adsorption of surfactants containing
free carboxylic groups explains their strong adsorption dependency on the surface charge of the
iron oxide and consequently on pH and ionic strength [66], as will be discussed later.
Factors affecting surfactant adsorption on iron oxides
Surfactant adsorption on a solid surface can be affected by many factors, including surfactant
concentration, pH, temperature and presence of inorganic ions. In this chapter, the effect of
surfactant concentration, pH, and total concentration of ions (ionic strength) on surfactant
adsorption onto iron oxides is discussed. The results of adsorption of an anionic carboxylate
surfactant on magnetite in the presence of calcium ions and sodium silicate are also presented.
Surfactant concentration. Due to the fact that the absorbance of infrared radiation is proportional
to the concentration of the absorbing species according to the Lambert-Beer law (Equation 8),
the intensity of the bands in a spectrum of a surfactant adsorbed on iron oxide can be assumed
to be proportional to the amount of surfactant on the surface. This assumption is reasonable as
long as all the adsorbed species have transition dipole moments of similar value.
The absorbance of the C-H symmetric stretching vibration band in the spectra of PEGMO
and Atrac 1563 adsorbed on hematite at pH 8.5 plotted as a function of surfactant
concentration in solution (see Fig. 8 and 12 in Paper I, respectively) was in good agreement
with the Freundlich adsorption model (Equation 7). This type of adsorption implies that the
heat of adsorption changes depending on the surface coverage [68], as discussed above.
29
Ionic strength. For the adsorption of the maleic acid ester on magnetite at pH 8.5, a ten-fold
increase in ionic strength (from 10-2 to 10-1 M NaCl) resulted in a 20-25% increase in the
intensity of the bands originating from the surfactant adsorbed on magnetite (see Fig. 7 in
Paper II), indicating the contribution of electrostatic forces to adsorption. This can be regarded
as further evidence for the formation of outer-sphere surface complexes.
Calcium chloride and sodium silicate. Fig. 14 illustrates the effect of calcium chloride and
sodium silicate on the adsorption of maleic acid ester on magnetite.
Figure 14. Intensity of the ester C=O stretching vibrations band as a function of time during
in-situ adsorption of maleic acid ester on magnetite at pH 8.5 from a 25 mg L-1 aqueous
solution without Ca2+ and Na2SiO3 added (), with 4 mM Ca2+ (), 0.4 mM Na2SiO3 (),
and with 4 mM Ca2+ and 0.4 mM Na2SiO3 (). Background electrolyte: 10 mM NaCl.
The adsorption of maleic acid ester on magnetite in the presence of calcium ions (open
triangles in Fig. 14) increased dramatically compared to when no calcium ions were added
(open circles in Fig. 14). This result agrees with the findings reported by Rao et al. [17] that
activation of magnetite for flotation with anionic collector occurred in the presence of calcium
ions. Calcium ions are also known to facilitate precipitation of fatty acids by forming calcium
soaps [21], which may also adsorb on the surface of magnetite [4, 17].
Considering the effect of sodium silicate, competitive adsorption of silicate and surfactant
species on magnetite was observed resulting in a three-fold decrease in surfactant adsorption
(filled circles in Fig. 14) as compared to when no silicate was added (open circles in Fig. 14).
However, desorption experiments (see Fig. 10 in Paper II) revealed higher stability of the
30
surfactant-magnetite complex as compared to the silicate-magnetite complex, suggesting that
silicate species in solution are not likely to replace the surfactant molecules already adsorbed on
magnetite. Similar results were recently reported by Roonasi et al. for silicate-oleate adsorption
on magnetite [69].
The adsorption behaviour in the silicate-surfactant-magnetite system changed significantly
with the introduction of calcium ions. Despite the fact that silicate adsorption slightly increased
in the presence of calcium ions (see Fig. 5 in Paper II), almost no silicate adsorption was
observed when the surfactant was added to the system, resulting in nearly as high adsorption of
the surfactant (filled triangles in Fig. 14) as with calcium ions only (open triangles in Fig. 14).
Thus, the depressing activity of sodium silicate on surfactant adsorption was almost completely
suppressed in the presence of calcium ions. One explanation for such behaviour could be a
much higher affinity of the surfactant for the calcium ions as compared to that of sodium
silicate, which is not surprising since carboxylate surfactants are known to adsorb on calcium
sites on apatite and other calcareous minerals [70].
pH change. Fig. 15 illustrates the adsorption of maleic acid ester on magnetite as a function of
pH.
Figure 15. Intensity of the ester C=O stretching vibration band originating from the maleic
acid ester adsorbed on magnetite in-situ from a 25 mg L-1 solution at different pH. The pH
was gradually decreased from pH 10. The surfactant was allowed to adsorb for 5 hours at each
pH. The background electrolyte was 10 mM NaCl.
The spectral data indicates that the amount of surfactant on the magnetite surface decreased
with increasing pH, as typically observed for the adsorption of anionic surfactants on the
31
surfaces bearing the same charge. As the pH decreases, the surface charge first becomes less
negative and then turns positive (see Fig. 11) thus making the surface more electrostatically
favourable for adsorption of the negatively charged deprotonated surfactant species. An
increased precipitation of the surfactant on the magnetite surface at acidic pH could further
contribute to the surfactant loading on the surface.
When the magnetite surface was pretreated with calcium ions and sodium silicate prior to
surfactant adsorption, the adsorption of the surfactant in the pH range 7.5-9.5 went through a
maximum at pH 8.5, in concert with the results reported by Morgan [71] for oleate adsorption
on hematite and explained by the formation of an acid-soap complex [(RCOO)2H]- [66, 72].
Fig. 6b in Paper III further illustrates that surfactant adsorption was denser at pH 9.5 than at
pH 7.5, which opposes the trend in Fig. 15. Such behaviour could be explained by the affinity
of the surfactant towards calcium ions, which are expected to be present on the magnetite
surface in a larger amount at higher pH, as becomes evident from the zeta-potential results
presented in Fig. 16a. An increased calcium-surfactant precipitation at higher pH would
contribute to this behaviour.
The effect of surfactant adsorption on the properties of the
magnetite surface (Paper III)
In this chapter, the effect of adsorption of an anionic carboxylate surfactant (maleic acid ester)
onto synthetic magnetite in the presence of calcium ions and sodium silicate is discussed.
Zeta-potential
Fig. 16 shows the zeta-potential of synthetic magnetite as a function of pH in the presence of
calcium chloride, sodium silicate, and maleic acid ester. Whereas calcium ions were capable of
reversing the zeta-potential of magnetite at pH values above the IEP (empty squares in
Fig. 16a), sodium silicate exhibited the opposite effect, making the magnetite surface more
negatively charged and shifting the IEP to lower pH (empty triangles in Fig. 16a). Considering
the combined effect of calcium ions and silicate species, the resulting zeta-potential of the
magnetite particles was determined by the ratio of these compounds in solution (filled
diamonds and empty squares in Fig. 16b). As the calcium-to-silicate ratio increased, the IEP of
32
the magnetite particles shifted to higher pH values and the zeta-potential above the IEP
became less negative.
When maleic acid ester was added to the solution containing calcium chloride and sodium
silicate (filled triangles in Fig. 16b), the zeta-potential of the magnetite particles became slightly
more negative as compared to that with only calcium and silicate, probably due to the
adsorption of the surfactant on the magnetite surface via positively charged calcium ions. The
adsorption of the surfactant in a bi-layer structure due to hydrophobic chain-chain interactions
could also result in an additional negative charge introduced by the deprotonated surfactant
head groups oriented towards the solution in the second adsorbed layer [73, 74].
Figure 16. Zeta-potential as a function of pH: (a) of the magnetite crystals in 10 mM NaCl
(), 3.3 mM CaCl2 (), and 1 mM Na2SiO3 (); (b) of the magnetite crystals in 3.3 mM CaCl2
and 0.4 mM Na2SiO3 (), in 3.3 mM CaCl2 and 1 mM Na2SiO3 (), 3.3 mM CaCl2, 0.4 mM
Na2SiO3, and 25 mg L-1 maleic acid ester (), of the maleic acid ester (no magnetite crystals)
in a 15 mg L-1 aqueous solution containing 10 mM NaCl and 2.4 mM CaCl2 ().
The zeta-potential of the magnetite particles in the presence of surfactant, calcium, and
silicate was nearly constant in the entire pH range studied, with the IEP expected to be below
pH 5, suggesting specific interaction between the surfactant and magnetite. Similar results were
reported by Rao et al. [73] for oleate adsorption on fluorite and were explained by the
adsorption of calcium oleate precipitate, characterized by a strongly negative and nearly
constant zeta-potential at pH 5-10.
Regarding the maleic acid ester, it is difficult to say whether calcium-surfactant complexes
were formed at the surface or already in solution, followed by the adsorption of the calciumsurfactant complexes onto magnetite. The zeta-potential of the surfactant in solution
33
containing calcium chloride (without magnetite particles, empty triangles in Fig. 16b) showed
similar dependency on pH as the zeta-potential of magnetite in solution containing calcium
chloride, sodium silicate, and maleic acid ester (filled triangles in Fig. 16b). However, the
values of the zeta-potential in the latter case were significantly less negative, confirming the
proposed mechanism of surfactant adsorption on magnetite in the form of ternary complexes
with calcium ions.
Contact angle
Table 5 illustrates the effect of calcium chloride, sodium silicate, and surfactants on the
wettability of synthetic magnetite.
Table 5. Water contact angle of the as-synthesized synthetic magnetite and magnetite after
consecutive conditioning with calcium ions, sodium silicate and a surfactant. The background
electrolyte was 10 mM NaCl. The values reported were measured 1 second after a drop of
water was deposited on the surface and are presented as an average value ± one standard
deviation.
Treatment
As-synthesized
magnetite
4 mM CaCl2 0.4 mM Na2SiO3
20 ± 3
15 ± 4
10*
22 ± 3
19 ± 2
10*
Contact angle, °
25 mg L-1 surfactant
43 ± 8 (Atrac 1563)
44 ± 3 (maleic acid
ester)
*
The exact value of the contact angle could not be estimated since the contact angle after
silicate adsorption was below the detection limit of the instrument (10°).
Whereas treatment with sodium silicate improved magnetite wettability as discussed in detail
in Paper III, adsorption of the surfactants resulted in an increased hydrophobicity of synthetic
magnetite. Nearly the same contact angle was obtained after treatment with Atrac 1563 or
maleic acid ester, suggesting that these compounds had a similar effect on magnetite wettability
at the conditions studied. A higher variation of the contact angle for the case of Atrac 1563
could possibly be the result of a complex chemical composition of the surfactant. The results in
Table 5 further indicate that sodium silicate did not prevent the adsorption of the surfactants
on magnetite in the presence of calcium ions, in agreement with the spectroscopic result
discussed above.
34
Verification for natural magnetite (Paper V)
In order to test whether the conclusion regarding surfactant adsorption on synthetic
magnetite in the presence of calcium ions and sodium silicate were applicable to natural
magnetite particles, adsorption of Atrac 1563 and water glass in the presence and absence of
calcium ions was performed on the mineral magnetite (see Fig. 9, 10 and Table 3 for material
characterization). Fig. 17 shows the results of the contact angle measurements performed after
batch adsorption experiments.
Figure 17. Water contact angle of the natural magnetite particles after adsorption of 1 mg g-1
water glass at pH 9 for 1 h, followed by the adsorption of Atrac 1563 for 20 minutes at the
same pH. Adsorption of both compounds was performed in the presence of either 10 mM
NaCl () or 4 mM CaCl2 (). The points at 0 mg g-1 represent the contact angle of the
magnetite concentrate after adsorption of 1 mg g-1 water glass at pH 9 for 1 h.
In the absence of calcium ions, the contact angle of the natural magnetite did not change
upon the adsorption of Atrac 1563 indicating that no or very little adsorption took place on
magnetite pretreated with water glass. However, the adsorption of Atrac 1563 in the presence
of calcium ions resulted in an increased contact angle of the magnetite particles, despite the
pretreatment with water glass, due to the activation of the magnetite surface for surfactant
adsorption by calcium ions. These findings agree with the spectroscopic and contact angle
results obtained using synthetic magnetite.
35
Summary and implications for flotation and agglomeration of
iron ore
Based on the results discussed above, the following conclusions regarding surfactant
adsorption on iron oxides and its effect on the surface properties of iron oxides can be made:
1. Anionic carboxylate surfactants were capable of adsorbing on iron oxides at pH values
above the IEP of iron oxides. The adsorption increased in the presence of cations,
especially calcium, and could be reduced by preconditioning with sodium silicate, but
only in the absence of calcium ions. Desorption of the surfactants from the surface was
only partial, even at elevated pH (up to pH 10).
2. The zeta-potential and the IEP of magnetite particles in the presence of calcium
chloride and sodium silicate was determined by the relative content of these
compounds. Adsorption of an anionic carboxylate surfactant did not have any drastic
effect on the zeta-potential of magnetite.
3. Magnetite wettability improved after treatment with calcium chloride and sodium
silicate, whereas subsequent surfactant adsorption made the magnetite surface more
hydrophobic.
For the flotation of iron ore, these results imply that a certain amount of the flotation
collector would likely adsorb on magnetite increasing the required collector dosage, especially
at high concentrations of calcium ions in the process water. Calcium ions may also have an
adverse effect on the dispersing performance of water glass. The adsorbed flotation collector on
the magnetite surface could facilitate flotation of magnetite to a certain extent, resulting in
reduced flotation selectivity.
The fact that adsorbed collector species can hardly be removed from the magnetite surface by
rinsing with water suggests that a certain amount of flotation collector would remain on the
surface of the magnetite concentrate after flotation and would be carried over to the balling
drums. This would affect the pelletizing process negatively and would reduce the strength of
the pellets produced, as discussed above.
Contamination of magnetite with flotation collector may be expected to increase with
increased concentration of calcium ions in the process water. Consequently, the second part of
the present work was focused on finding the means to minimize the effect of adsorbed
flotation collector on magnetite wettability.
36
Restoring magnetite wettability after surfactant adsorption
(Papers IV, V)
Based on the discussion above, the best way to reduce the adsorption of flotation collector on
magnetite would probably be by decreasing the concentration of free calcium (and
magnesium) ions in the process water. Application of different chelating agents [17, 75-78] and
ion exchangers [79] has been proposed for that purpose. However, the removal of calcium ions
from the process water would result in further dissolution of sparingly soluble calcareous
minerals, again increasing the calcium concentration.
Alternatively, modification of the magnetite surface after flotation could be performed in
order to increase surface wettability. In the present work, two types of hydrophilizing agents
were investigated, namely, hydrophilic polymers and sodium silicate. Their effect on the
wettability of synthetic magnetite after surfactant adsorption is presented in this chapter.
Modification with sodium silicate
Since sodium silicate is known to have a depressing effect on iron oxides in flotation with
anionic carboxylate collectors, the ability of sodium silicate to improve magnetite wettability
after surfactant adsorption was investigated (see Table 6).
Table 6. Water contact angle of synthetic magnetite after consecutive adsorption of calcium
chloride, sodium silicate, and a surfactant, followed by treatment with sodium silicate in the
presence of calcium chloride for 24 hours. Concentrations of the reagents were the same as in
the experiments described in Table 5. Adsorption was performed at pH 8.5.
Treatment
Contact angle, °
CaCl2, Na2SiO3, and surfactant
CaCl2 and Na2SiO3
42 ± 2 (maleic acid ester)
21 ± 1
49 ± 3 (Atrac 1563)
16 ± 1
A considerable decrease in the magnetite contact angle was achieved after 24 hours of
conditioning with sodium silicate in the presence of calcium ions. The observed effect could
be caused by desorption of surfactant due to the difference in concentration at the surface and
in solution or by substitution of the surfactant species for silicate. However, the latter would
contradict the results reported by Roonasi et al. [69] for competitive adsorption of sodium
37
oleate and sodium silicate on magnetite, stating that silicate in solution could not easily replace
oleate adsorbed on the magnetite surface.
Modification with hydrophilic polymers
Three types of polymers, viz. cationic, anionic, and non-ionic (Table 2), were investigated
regarding their ability to adsorb on surfactant-coated magnetite and improve the wettability of
the magnetite surface. Although all the polymers tested adsorbed on magnetite (Fig. 2 in
Paper IV) independent of their charge and functionality, only anionic ammonium polyacrylate
could increase magnetite wettability after surfactant adsorption (see Table 7).
Table 7. Water contact angle of synthetic magnetite after consecutive adsorption of calcium
chloride, sodium silicate, and a surfactant, followed by treatment with a polymer, and storage
in air for 24 h. Concentrations of the reagents were the same as in the experiments described
in Table 5. Polymer concentration was 12.5 mg L-1. Adsorption was performed at pH 8.5.
Treatment
Contact angle, °
CaCl2, Na2SiO3,
Maleic acid ester
Atrac 1563
and surfactant
46 ± 5
43 ± 8
Polymer
24 h in air
Cationic aliphatic
polyamine
Starch
Anionic ammonium polyacrylate
68 ± 2
40 ± 4
24 ± 6
20 ± 6
49 ± 11
46 ± 3
20 ± 4
24 ± 8
Spectroscopic results (Fig. 2 in Paper IV) did not provide any evidence for the detachment of
the surfactant from the magnetite surface upon polyacrylate adsorption, since no negative
absorption bands originating from the surfactant were present in the spectra of polyacrylate
adsorbed on magnetite. Accordingly, the decrease in the contact angle of synthetic magnetite
upon polyacrylate adsorption was most likely due to shielding of the hydrophobic surfactant
moieties from the water phase by long, flexible polymer chains able to form loops on the
surface, especially in the presence of calcium ions [45], as illustrated in Fig. 18. The high
density of the carboxylic groups in the polyacrylate chain makes it highly hydrophilic, resulting
in an improved wettability of the magnetite surface.
A similar phenomenon was reported by Somasundaran and Cleverdon [80] for
amine/cationic PAM adsorption on quartz. The authors concluded that the polymer interacted
38
with the surface without affecting surfactant adsorption and that depression of quartz was
achieved due to masking of the surfactant by the polymer.
Figure 18. Schematic illustration of polyacrylate adsorption on magnetite pretreated with
surfactant. For the sake of simplicity, iron oxide surface sites and other adsorbed species are not
shown.
Considering the mechanism of interaction of polyacrylate with the magnetite surface,
polymer adsorption likely took place via calcium ions [81], similar to the adsorption of
carboxylate surfactants. Calcium ions have been shown to facilitate polyacrylate adsorption on
oxides [45] due to reduced electrostatic repulsion both between the carboxylate ions in the
polymer chain and the negatively charged oxide surface, and between the carboxylate ions
within the molecule. Such intramolecular bridging results in a more coiled conformation of
polyacrylate chains both in solution and on the surface, increasing packing efficiency of the
polymer species on magnetite.
The conclusion regarding the mode of adsorption of polyacrylate on magnetite was further
confirmed by the zeta-potential measurements (Fig. 5 in Paper IV), which showed that the
zeta-potential of calcium-polyacrylate complex in solution was nearly the same as the zetapotential of the magnetite particles treated with calcium chloride, sodium silicate, anionic
surfactant, and polyacrylate, suggesting that polyacrylate was adsorbed on magnetite as a ternary
complex with calcium ions.
39
Verification for the flotated magnetite concentrate (Paper V)
To investigate whether soluble silicate and polyacrylate were efficient in improving the
wettability of magnetite concentrate after flotation, the contact angle of the concentrate was
measured by the Washburn method before and after adsorption of water glass and sodium
polyacrylate.
Fig. 19 illustrates the effect of treatment with water glass on the wettability of the magnetite
concentrate.
Figure 19. Water contact angle of the magnetite concentrate upon modification of the
surface with water glass in 10 mM NaCl at pH 9 for 9 h. Prior to water glass adsorption, the
concentrate was preconditioned with 10 mM NaCl at pH 9 for 1 hour. The points at
0 mg g-1 represent the contact angle of the magnetite concentrate after conditioning with
10 mM NaCl at pH 9 for 1 hour.
In concert with the results obtained for the synthetic magnetite, wettability of magnetite
concentrate after flotation was significantly improved by water glass adsorption, with a contact
angle of 28° ± 3° obtained at the highest water glass dosage. As could be expected, the
hydrophilizing effect increased with increased water glass concentration.
The adsorption of sodium polyacrylate on magnetite concentrate was performed in the
presence of calcium ions, based on the conclusion about the adsorption mode of polyacrylate
on magnetite discussed above. The wettability of the concentrate slightly improved upon
polymer adsorption at the concentration of 0.04 mg g-1 (Fig. 9 in Paper V). However, the
increase in polymer concentration at constant concentration of calcium ions did not lead to a
further decrease in the contact angle of the magnetite concentrate. Since calcium ions are
40
known to facilitate interaction of polyacrylate with metal oxides [45], polyacrylate adsorption
in the presence of calcium ions could be determined not only by polymer concentration in
solution but also by the calcium-to-polymer ratio. Fig. 20 illustrates the effect of calcium
chloride concentration on the wettability of magnetite concentrate at constant concentration of
sodium polyacrylate.
Figure 20. Water contact angle of the magnetite concentrate upon modification of the surface
with 0.04 mg g-1 sodium polyacrylate at pH 9 for 1 hour measured with the Washburn
technique. The point at 0 mM represents the contact angle of the magnetite concentrate
treated with sodium polyacrylate in the presence of 10 mM NaCl and without calcium ions.
Without calcium ions, treatment with the polymer did not have any effect on the wettability
of the magnetite concentrate. On adding 4 mM CaCl2 at the same concentration of
polyacrylate, the contact angle of the magnetite concentrate decreased as discussed above. Even
better results were achieved when the CaCl2 concentration was increased to 6 mM, indicating
that the efficiency of polyacrylate in improving magnetite wettability was affected by the
concentration of calcium ions.
41
Summary and implications for agglomeration of iron ore
To summarize the results of surface modification of magnetite after surfactant adsorption, the
following conclusions can be drawn:
1. Soluble silicate and polyacrylate were shown to improve wettability of synthetic
magnetite after surfactant adsorption and of mineral magnetite concentrate cleaned by
flotation.
2. The effect of sodium polyacrylate on magnetite wettability improved in the presence
of calcium ions.
3. The adsorption of sodium polyacrylate did not have any major effect on the zetapotential of synthetic magnetite particles.
Consequently, both water glass and sodium polyacrylate may be used to improve the
wettability of magnetite concentrate after flotation and prior to agglomeration. Improved
wetting of magnetite concentrate could be expected to facilitate agglomeration and increase
the strength of the pellets produced. Compared to water glass, treatment with polyacrylate
would require less time and would not introduce any impurities to the final product after
sintering. The fact that calcium ions facilitate adsorption of polyacrylate on magnetite makes
the polymer suitable for the application in processes utilizing process water rich in calcium.
Since the zeta-potential of magnetite was not affected by polyacrylate adsorption to any
considerable extent, treatment with the polymer would not impair the electrostatic interaction
between the magnetite concentrate and bentonite binder in agglomeration.
Considering environmental issues, polyacrylate with a molecular weight similar to the one
used in this study (Mw 4500 and 4000, respectively) has not been found to have any adverse
effect on the environment [82]. Despite low biodegradability [83], polyacrylate and its products
of degradation are not toxic to aquatic, terrestrial, and mammalian species [82, 84]. In hard
water, polyacrylate can precipitate in the form of calcium polyacrylate when all the carboxylic
groups in the polymer become neutralized by calcium ions [82], resulting in polymer removal
from the aqueous phase.
Accordingly, polyacrylate seems to be a good candidate for use in improving wettability of
flotated magnetite concentrate prior to agglomeration.
42
CONCLUSIONS
A versatile method based on ATR-FTIR spectroscopy was developed and successfully used
for in-situ studies of the adsorption of surfactants, polymers, and inorganic compounds on thin
films of synthetic iron oxides.
Using the developed method, the adsorption mechanism of several surfactants on iron oxides
was elucidated.
The dramatic effect of calcium ions on the adsorption of carboxylate surfactants on magnetite
was for the first time confirmed in-situ. It was also illustrated that soluble silicate could reduce
surfactant adsorption on magnetite but only in the absence of calcium ions.
Among other factors that affected surfactant adsorption on iron oxides were surfactant
concentration, pH, and ionic strength. Variation of conditioning time with sodium silicate was
not found to have any considerable effect on surfactant adsorption on magnetite in the
presence of calcium ions.
Surfactant adsorption considerably decreased magnetite wettability. Once adsorbed, surfactant
species could not be completely removed from the surface by rinsing with water, even at
elevated pH.
Treatment with soluble silicate and polyacrylate proved to be a feasible means for restoring
wettability of magnetite after surfactant adsorption. The effectiveness of polyacrylate improved
in the presence of calcium ions, making this polymer a good candidate for applications in hard
water.
The results obtained using synthetic iron oxides were verified for natural magnetite particles,
suggesting that in-situ ATR-FTIR spectroscopy in combination with zeta-potential and
contact angle measurements on synthetic materials could be successfully applied to studying
surface phenomena related to mineral processing.
43
44
FUTURE WORK
Since it has previously been shown that storage of the magnetite concentrate prior to
agglomeration helps to improve green pellet quality, it would be interesting to investigate the
aging of flotation collector and other species (e.g. silicate) present on the magnetite surface.
To confirm or disprove the proposed adsorption mechanism of silicate, surfactant, and
polymer on magnetite in the presence of calcium ions, it would be useful to study the
distribution of these species on the magnetite surface using ATR-FTIR microscopy.
To test whether treatment of the flotated magnetite concentrate with water glass or
polyacrylate could improve green pellet quality, small-scale balling of the concentrate could be
performed, followed by porosity, strength, and plasticity studies of the green pellets.
Interactions between bentonite binder and magnetite modified with surfactant and/or
polyacrylate could also be studied in order to obtain information about possible effects of these
species adsorbed on magnetite concentrate on bentonite performance as a binder.
45
46
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53
54
PAPER I
Studies of collector adsorption on iron oxides by in-situ
ATR-FTIR spectroscopy
E. Potapova, I. Carabante, M. Grahn, A. Holmgren, and J. Hedlund
Industrial and Engineering Chemistry Research 49 (2010) 1493-1502
Ind. Eng. Chem. Res. 2010, 49, 1493–1502
1493
Studies of Collector Adsorption on Iron Oxides by in Situ ATR-FTIR
Spectroscopy
E. Potapova,* I. Carabante, M. Grahn, A. Holmgren, and J. Hedlund
DiVision of Chemical Engineering, Luleå UniVersity of Technology, SE-971 87 Luleå, Sweden
In this work, the adsorption of three model collectors, viz., poly(ethylene glycol) monooleate (PEGMO),
ethyl oleate, and maleic acid, as well as the commercial fatty-acid-type collector Atrac 1563, was studied in
situ on synthetic hematite using attenuated total reflectance Fourier transform infrared (ATR-FTIR)
spectroscopy. The adsorption behavior of the studied compounds on hematite was determined to a large
extent by the polar headgroup. Adsorption of Atrac and PEGMO as a function of concentration showed good
agreement with the Freundlich adsorption model, suggesting energetically heterogeneous adsorption. In situ
desorption experiments revealed that a large fraction of the Atrac was weakly attached to the hematite surface,
as it was partially removed by flushing with water at pH 8.5 and 10. These results suggest that a separate
washing unit after the flotation step could be beneficial in reducing the contamination of iron ore by flotation
chemicals.
Introduction
Iron ore pellets are an important refined product used as a
raw material in the manufacturing of steel. The production of
iron ore pellets comprises several stages: grinding and upgrading
of the iron ore; balling of wet, so-called, green pellets; and
drying, sintering, and oxidation of the green pellets to the final
product, to be transported to iron or steel plants.
LKAB is a Swedish mining company whose pelletizing plants
utilize magnetite iron ore from two deposits located in northern
Sweden: Kiruna and Malmberget. The Kiruna ore is a mixture
of magnetite and apatite having a phosphorus content of ca. 1
wt %. To reduce the phosphorus content to an acceptable level
for the blast furnace process1 (i.e., to less than 0.025%), the
ore is subjected to reverse flotation with an anionic fatty-acidtype collector reagent (Atrac 1563) with methyl isobutyl carbinol
(MIBC) used as a frother. To increase flotation selectivity and
phosphorus recovery, sodium silicate is added to the system.
The sodium silicate is used in fairly small amounts (300-500
g t-1) and thus acts primarily as a dispersant, and it has not
been found to prevent collector adsorption on the magnetite
surface to any great extent.2,3
Ideally, the collector should adsorb only on the apatite gangue,
rendering it hydrophobic and thus easily floated from the
magnetite. However, unwanted adsorption of the flotation
reagents on magnetite also occurs. It has been estimated that
the amount of Atrac adsorbed on the magnetite fed to balling
circuits is 10-30 g t-1.4 Once the collector is adsorbed on the
surface of magnetite, it is difficult to eliminate.5 Therefore, the
collector should be added in sufficiently small amounts so that
the apatite surface is rendered hydrophobic whereas the adsorption on magnetite is minimized. However, in the real process,
the Atrac dosage is adjusted based on the phosphorus content
in the pellet feed, and there is a slight degree of overdosage to
ensure that the desired phosphate levels are achieved. Typically,
the dosage varies in the range of 30-70 g per tonne of magnetite
concentrate.6
After the flotation step, the pulp passes through magnetic
separation and filtration steps where it is subjected to repeated
* To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +46 920 491 776.
dilutions and thickenings that can have a mild washing effect
on the ore, partly removing the flotation reagents from the
magnetite surface. However, no separate washing unit is used
for the purification of magnetite from the flotation chemicals at
LKAB.
It has been found that adsorption of the flotation collector
agent on the iron ore has a negative effect on the balling process,
as the collector adsorbed on the surface of magnetite makes
the particles more hydrophobic, which can lead to the attachment
of air bubbles onto the surface of the iron ore particles.6 The
air bubbles incorporated into green pellets decrease the green
pellet strength in both the wet and dry states. A low wet strength
tends to cause a wide size distribution of the green pellets
leaving the balling drums. The undersize fraction is recirculated
back to the balling drum, thus leading to increased energy
consumption and decreased capacity of the pelletizing plant.
Breakage of the pellets at the stage of drying and induration
causes dust formation and decreased pellet bed permeability,
resulting in lower production volumes and aggravated pellet
quality.7
To minimize the influence of the collector on the pelletizing
process, it is important to understand the mechanism by which
the collector interacts with the iron oxide. For instance, it has
been shown that the presence of Ca2+ ions in the process water
increases the adsorption of flotation collector reagent on
magnetite.8 In that work, the adsorption of the collector OS 130
on magnetite was studied in the batch experiments by measuring
the residual concentration of the collector in solution after
adsorption using the method of Gregory.9 According to the
suggested8 mechanism, positively charged calcium ions adsorb
at negatively charged surface sites on the magnetite surface,
which results in a more positively charged surface, rendering it
more favorable for the adsorption of negatively charged collector
species.
Spectroscopic techniques have been used extensively for
studying the adsorption of fatty-acid-based collectors onto
mineral surfaces.10-12 Fourier transform infrared (FTIR) spectroscopy is widely applied because it provides the possibility
of identifying complexes formed at the surface.13 Ex situ FTIR
techniques, such as diffuse reflectance infrared Fourier transform
(DRIFT) spectroscopy and reflection absorption infrared spec-
10.1021/ie901343f © 2010 American Chemical Society
Published on Web 01/07/2010
1494
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
Figure 1. Schematic image of an ATR waveguide illustrating the ATR
phenomenon. For the sake of clarity, the thickness of the sample (in this
case, iron oxide film) and the penetration depth dp are enlarged.
troscopy (RAIRS), imply drying of the sample after adsorption,
which can affect surface complexes and, hence, the validity of
the results. On the contrary, FTIR attenuated total reflectance
(FTIR-ATR) spectroscopy facilitates in situ studies of both
adsorption kinetics14 and complexes formed on the surface in
the presence of water.13,15 In the ATR technique (Figure 1), a
sample with a certain refractive index, n1, is placed in a close
contact with a waveguide characterized by a high refractive
index, n2. The IR beam is passed through the waveguide at a
certain incident angle θ. In the case when sin θ is larger than
the ratio n1/n2, the beam is totally reflected inside the waveguide,
forming a perpendicular evanescent wave of the IR radiation at
each point of reflection. This wave propagates through the
sample in the vicinity of the waveguide and interacts with the
sample, causing partial absorption of the radiation by the sample
material and thus a reduction of the intensity of the totally
reflected beam. The penetration depth, dp, is defined as the
distance from the interface at which the electric field of the wave
is equal to E0e-1 (see Figure 1), where E0 is the electric field of
the wave at the interface. Typical values of the penetration depth
are from some hundred nanometers to a few micrometers,
depending on the refractive indices of both the waveguide and
the sample, as well as on the wavelength and incident angle of
the radiation. Such a range of penetration depths makes FTIRATR spectroscopy a very surface-sensitive technique, providing
the possibility for in situ studies of surfaces in contact with
aqueous solutions.
During the past decade, the FTIR-ATR technique has been
developing, and new applications in surface chemistry have
evolved, for example, as a tool for studying adsorption13,16-19
and diffusion20-24 in thin films or at interfaces. This technique
has also been used in studies of catalytic reactions,25,26 as
well as in sensor applications.27,28 FTIR-ATR spectroscopy
has also been applied for studying the adsorption of surfactants at mineral surfaces, both qualitatively29,30 and quantitatively.14,31-33 Our group has developed the ATR technique for
studies of adsorption in zeolite films17,27,34-37 and on mineral
surfaces.14,18,38-41 The studies have been both qualitative35-38,40,41
and quantitative,14,17,18,27,34,39 and even the molecular orientations
of adsorbates have been determined.17,18,35,38,41
Although magnetite (Fe3O4) is the main iron-containing
mineral in the ore utilized by LKAB, it was found to become
partly oxidized during storage in air, forming first maghemite
(γ-Fe2O3), which has the same crystal structure but mostly Fe3+
on the surface, and then hematite (R-Fe2O3), which has a
different crystal structure than magnetite and maghemite.42 This
phenomenon was described earlier for submicrometer-sized
magnetite particles by both Haneda and Morrish43 and Gediko-
glu.44 In solution, the oxidation of magnetite was also observed
and explained by the leaching of Fe2+ ions from the surface.45
Thus, when magnetite ore is subjected to grinding and flotation,
apparent oxidation of the surface can occur both in water and
in air.
In this work, the adsorption of flotation collector reagents
on iron oxide was studied in situ using FTIR-ATR spectroscopy
for the purpose of obtaining essential information about the
interaction between the flotation collector reagents and the iron
oxide surface in the presence of water. This information is
crucial for the improvement of the pelletizing process, as it could
suggest possibilities to reduce the unwanted collector adsorption
on the iron ore surface and improve green-pellet strength.
Hematite was chosen as the adsorbent because it was reported
to be the final product of the surface oxidation of magnetite. In
addition, reports on the adsorption properties of collector-type
molecules on hematite were found to be very scant in the
literature.
Materials and Methods
Materials. Hematite crystals were synthesized from an FeCl3
solution according to the method described by Matijević.46 The
obtained hematite crystals were purified by repeated (five times)
centrifugation at 20000 rpm for 30 min and redispersion in a
0.06 M aqueous solution of acetic acid (glacial, >99.7%, Alfa
Aesar). The crystals were stored as a 2 wt % suspension in a
0.06 M aqueous solution of acetic acid at pH 3. For powder
X-ray diffraction analysis, the suspension was freeze-dried,
yielding a fine hematite powder.
The flotation collector reagent, Atrac 1563 (Akzo Nobel,
Sweden), was provided by LKAB. Atrac 1563 is a yellow
viscous liquid with a complex chemical composition: 50-100%
ethoxylated tall oil ester of maleic acid and 1-5% maleic
anhydride (Akzo Nobel material safety data sheet). Tall oil is a
byproduct of the Kraft pulp manufacturing process and is a
mixture of mainly fatty acids (e.g., oleic acid) and resin acids
(e.g., abietic acid). A similar collector for the froth flotation of
oxide and salt-type minerals that is a combination of a monoester
of a dicarboxylic acid and a monocarboxylic acid is described
in a patent.47 As described in the patent, the first component is
an aliphatic monocarboxylic acid containing 8-22 carbon atoms
bonded to a dicarboxylic acid containing 4-8 carbon atoms
through an alkylene oxide group with 2-4 carbon atoms, thus
resulting in a molecule with two ester carbonyls and one free
carboxylic group at the end of the molecule. Monocarboxylic
acid with 6-24 carbon atoms is added to increase the selectivity
and/or yield of the monoester.
Poly(ethylene glycol) monooleate (PEGMO) with a typical
number-average molecular weight (Mn) of 460 (Aldrich), maleic
acid (Fluka, g 99%), and ethyl oleate (Aldrich, 98%) were used
as model collector reagents. From the average molar mass, the
length of the poly(ethylene glycol) chain in the PEGMO model
collector reagent was estimated to be about four ethylene glycol
units. Oleic acid esters were chosen because oleic acid is one
of the main components of tall oil. Ethoxylated tall oil could
thus be modeled by using PEGMO, which has the same structure
as ethoxylated oleic acid. Maleic acid was chosen as one of the
model compounds because it is the tail group of the molecules
in Atrac. Ethyl oleate was used as the third model compound
to study the effect of the poly(ethylene glycol) chain on the
adsorption properties of oleate.
Working solutions of Atrac and the model collector reagents
were prepared in the following way: First, a 0.1 g L-1 stock
solution of the compound in distilled water was prepared. In
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010 1495
the next step, the required amount of the stock solution was
mixed in distilled water to give final solutions of the desired
concentration (1-25 mg L-1).
All aqueous solutions were prepared using distilled water.
The distilled water used for the spectroscopic measurements
was first boiled for 1 h, and then argon was bubbled through
the water to minimize the amount of dissolved carbon dioxide.
The pH was controlled during the experiments by a Mettler
Toledo T70 titrator using a 0.05 M aqueous solution of sodium
hydroxide (per analysis, Merck).
Dip-Coating. Hematite films were deposited on ZnSe substrates using a Nima DC-multi 8 dip-coater. Prior to deposition,
the substrates were washed in acetone (g99.5%, VWR), ethanol
(99.7%, Solveco Chemicals AB), and distilled water (10 min
in each). A 2 wt % hematite suspension was prepared by
dispersing the hematite crystals in a 6.27 M aqueous solution
of acetic acid. The substrates were immersed in the suspension
and withdrawn at a speed of 5 mm min-1, and the film thickness
was controlled by the number of dips. To prepare a film with a
thickness of ca. 1 μm, eight dips were needed.
Scanning Electron Microscopy (SEM). SEM images of the
hematite film on a ZnSe substrate were obtained using a Philips
XL 30 microscope with a LaB6 filament. The samples were
mounted on alumina stubs using carbon glue and subsequently
sputtered with a thin layer (ca. 10 nm) of gold to provide
conductivity.
X-ray Diffraction. X-ray diffraction patterns of both hematite
powder and film were collected with a Siemens D5000
diffractometer running in Bragg-Brentano geometry using Cu
KR radiation. To analyze the film, the hematite-covered ZnSe
substrate was mounted with carbon glue onto a custom-made
alumina holder.
Zeta-Potential Measurements. The point of zero charge
(PZC) of the hematite crystals used in this work was determined
by electrophoresis using a ZetaCompact instrument equipped
with a charge-coupled device (CCD) tracking camera. The
obtained electrophoretic mobility data were further processed
by the Zeta4 software applying the Smoluchowski equation. The
samples were prepared in the following way: One drop of the
hematite suspension was dispersed in 1 L of 0.01 M potassium
nitrate. The pH of the samples (10 samples, 100 mL each) was
adjusted using potassium hydroxide and nitric acid. The samples
spanned the pH range from 2 to 11. For each sample, the
measurement was repeated three times, and the final PZC was
calculated as an average of the obtained values.
FTIR-ATR Spectroscopy. Infrared spectra were recorded
using a Bruker IFS 66v/S spectrometer equipped with a liquidnitrogen-cooled mercury cadmium telluride (MCT) detector.
ZnSe ATR crystals (Crystran Ltd.) in the form of a trapeze with
45° cut edges and dimensions of 50 × 20 × 2 mm were used
in this study. Measurements of adsorption on the hematite-coated
ATR crystals were performed in situ in a cell with a flow
pumped through on both sides of the ATR crystal; see Figure
2. The incidence angle of the infrared beam was set to 45°.
All adsorption experiments on hematite were performed at
room temperature using water solutions of model collectors or
Atrac at pH 8.5 (the pH used in the flotation process at LKAB)
pumped continuously through the cell at a flow rate of 10 mL
min-1 with recirculation. Prior to the adsorption measurements,
the hematite film was flushed with a weakly alkaline solution
(pH 8.5) for 2 h to remove the residues of acetic acid and
carbonate species from the surface. A background spectrum was
recorded afterward with water at pH 8.5 in contact with a
hematite-coated ZnSe substrate. Spectra of the model collectors
Figure 2. Schematic image of the FTIR-ATR flow cell. For the sake of
clarity, the thickness of the iron oxide film is enlarged.
Figure 3. Images of (a) uncoated and (b) hematite-coated ZnSe ATR
crystals.
and Atrac in pure form were recorded in argon atmosphere using
a bare ZnSe substrate with a droplet of a collector spread over
its surface. ZnSe in argon atmosphere was recorded as a singlebeam background spectrum. All background and sample spectra
were obtained by averaging 500 scans at a resolution of 4 cm-1.
Data processing was performed using Bruker Opus 4.2 software.
Results and Discussion
Film and Powder Characterization. Figure 3 shows a
photograph of ZnSe crystals before and after being coated with
a hematite film. As is evident from the uniform red color of the
coated ATR crystal, the obtained film appeared to be quite
uniform, continuous, and even along the crystal surface and
stable under the conditions used for the in situ experiments,
because the visual appearance of the coated crystal did not
change after the experiments.
SEM images (Figure 4) showed that the hematite crystals
had a uniform spherical habit with a diameter of ca. 130 nm
and that the crystals were distributed evenly over the ATR
crystal surface, forming a porous film with an average thickness
of ca. 1 μm.
Figure 5 shows XRD patterns of freeze-dried synthetic
hematite powder and a hematite film on a ZnSe crystal. The
XRD pattern of the powder shows that the synthesized material
contained pure, randomly oriented hematite crystals without any
other iron oxide phase present in amounts detectable by XRD.
By comparing the relative reflection intensities in the powder
and in the film, it can be concluded that the crystals in the film
were also randomly oriented.
Figure 6 shows the ζ-potential of the synthetic hematite
crystals used in this work as a function of pH. The point of
1496
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
Figure 6. ζ-potential of the hematite crystals as a function of pH.
Figure 4. (a) Top- and (b) side-view SEM images of a hematite film on a
ZnSe crystal.
Figure 5. XRD patterns of hematite powder (top) and hematite film on a
ZnSe crystal (bottom). The peak labeled with an asterisk (*) emanates from
the ZnSe substrate.
zero charge (PZC) is at about pH 4.8. According to data
compiled by Fuerstenau48 and Cromieres,49 the PZC of hematite
has been reported to occur within a broad range of pH values,
viz., from 4.8 to 10.3. Factors that can influence the value of
the PZC are1 the technique used to determine the ζ-potential,
impurities (especially in the case of minerals), sample preparation procedure, and species adsorbed on the surface (e.g.,
carbonates).50,51
In the present study, the PZC of hematite was probably
affected by the carbonate species adsorbed on the surface, which
are known to lower the PZCs of iron oxides by ca. 1 pH unit.52
The presence of the carbonate species was confirmed by
spectra (not shown) recorded during flushing of the hematite
film with distilled water at pH 8.5 prior to the adsorption
experiments. Weak negative bands at ca. 1490 and 1340 cm-1
originating from the outer-sphere carbonate species52 were
observed in the spectra, indicating the desorption of carbonates
from the hematite surface. Desorption of carbonates had ceased
already after 30 min of flushing, suggesting that carbonate
species in solution and on the surface were in equilibrium.
Adsorption of Model Compounds. To better understand the
adsorption mechanism of Atrac on hematite, three model
compounds of Atrac were studied. As it is known that Atrac
contains maleic acid esterified with an ethoxylated tall oil (Akzo
Nobel material safety data sheet), poly(ethylene glycol) monooleate (PEGMO) and maleic acid were chosen as model
compounds. From the number-average molecular weight, Mn
) 460, it was estimated that an average poly(ethylene glycol)
chain in PEGMO contained four repeated ethoxy units
(-CH2-O-CH2-). Ethyl oleate was used as an additional
model compound to study the effect of the poly(ethylene glycol)
chain on the adsorption properties of oleate.
PEGMO is a nonionic surfactant53 characterized by a higher
solubility in water than the corresponding fatty acid because of
the poly(ethylene glycol) chain. PEGMO can be expected to
interact with the iron oxide surface in three different ways, viz.,
through the ester carbonyl, the ether oxygen linkages, or the
tail hydroxyl group. Figure 7 shows spectra of a droplet of pure
PEGMO on ZnSe and PEGMO adsorbed on hematite from
aqueous solution.
Strong absorption bands at 2922 and 2854 cm-1 in Figure
7a originate from asymmetric and symmetric stretching vibrations of the C-H bonds (νas and νs).54 Long-chain carboxylic
acids (e.g., oleic acid) contain significantly more methylene
groups than methyl groups and are thus characterized by much
stronger absorption bands corresponding to the CH2 groups
compared to the CH3 groups. The latter can be observed only
as shoulders. C-H deformation in methyl and methylene groups
is found around 1458 cm-1.55 The absorption band observed in
pure PEGMO at 1736 cm-1 (Figure 7a) is associated with
stretching vibrations of the CdO ester bond.56 It is only slightly
shifted upon adsorption on hematite (Figure 7b), indicating that
no significant interaction occurs between the ester carbonyl and
the surface. The absence of bands around 1570 and 1430 cm-1,
corresponding, respectively, to asymmetric and symmetric
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010 1497
Figure 8. Intensity of the band originating from the symmetric stretching
vibration (νs) of the C-H bond as a function of concentration of PEGMO
in solution (0). The solid line represents the fitted Freundlich adsorption
model.
Figure 7. Infrared spectra of (a) a droplet of pure PEGMO on ZnSe and
(b) PEGMO adsorbed on a hematite film in situ from a 10 mg L-1 aqueous
solution. Note that a.u. represents arbitrary units here and elsewhere.
stretching of a carboxylate group,12,57 suggests that the ester
bond in PEGMO does not break upon adsorption on hematite.
The intense band at 1115 cm-1 (Figure 7a) emanates from
the C-O-C stretching vibration in the poly(ethylene glycol)
chain.58 It has a shoulder on its low-frequency side at ca. 1070
cm-1 probably emanating from the stretching of the C-O bond
between the hydroxyl group and the CH2 group at the end of
the poly(ethylene glycol) chain.59 Both the peak frequency and
the shoulder frequency are shifted to lower wavenumbers when
PEGMO is adsorbed on hematite (see Figure 7b; 1115 f 1095
cm-1 and 1070 f 1047 cm-1, respectively), suggesting that the
poly(ethylene glycol) chain is involved in the bonding to the
surface.
Figure 8 shows the change in absorbance (measured as peak
height) of the 2854 cm-1 band during the adsorption of PEGMO
on hematite at different concentrations after 1 h of adsorption
at each concentration. The experiment was started at the lowest
concentration of 1 mg L-1; thereafter, the concentration in
solution was increased, and the measurement was continued.
Figure 8 shows that the intensity of the band originating from
the symmetric stretching vibration (νs) of the C-H bond
increased with increasing concentration in solution, suggesting
an increase of the adsorption of PEGMO on hematite. The
experimental data exhibited a poor fit with the Langmuir model
of adsorption, with a coefficient of determination (R2) of 0.87.
Plotting the data on a logarithmic scale yielded a straight line
with R2 ) 0.99, indicating that the obtained data were in good
agreement with the Freundlich model of adsorption, which
implies that the surface of adsorption is heterogeneous, for
instance, with different adsorption sites grouped patchwise based
on their adsorption energies, as suggested earlier.60 A desorption
experiment (not shown) showed that the intensity of the band
originating from the symmetric stretching vibration (νs) of the
C-H bond in PEGMO was reduced only by 5% upon flushing
the cell with water at pH 8.5 for 1 h, indicating that most of the
PEGMO was strongly adsorbed to the hematite surface.
Maleic acid was the second model compound studied. The
ability of maleic acid to form intramolecular hydrogen bonds
makes it easily soluble in water. In aqueous solution at pH 8.5,
it is expected to be fully deprotonated.61
From the spectroscopic measurements it was concluded that
maleic acid did not adsorb on hematite to any considerable
extent at pH 8.5 in the concentration range studied, viz., 1-25
mg L-1. This could be explained by the fact that, at pH 8.5, the
hematite surface is negatively charged and thus repels maleic
acid, which, at this pH, contains two negatively charged
carboxylate ions. Hwang and Lenhart60 studied the adsorption
of maleic acid on hematite at various pH values and concluded
that the adsorption was controlled by the surface charge of
hematite, suggesting that electrostatic interaction is the predominant force of adsorption of maleic acid on hematite, which
agrees well with our observations.
Ethyl oleate was the third model compound of Atrac used in
the experiments. It is almost insoluble in water due to the fact
that, instead of a hydrophilic poly(ethylene glycol) chain, it has
a hydrophobic ethyl group bonded to the carboxylate. This
change in chemical composition, of course, also affects the
adsorption properties of the molecule. The change in absorbance
of the 2852 cm-1 band during the adsorption of ethyl oleate on
hematite at different concentrations is shown in Figure 9. The
measurements were performed in the same way as for PEGMO.
Equilibrium was achieved at each concentration within 2 h.
Figure 9 shows that the absorbance of the band originating
from the symmetric stretching vibration (νs) of the C-H bond
is about a factor of 6 lower at all concentrations of ethyl oleate
than the absorbance of the same band at corresponding
concentrations of PEGMO. This indicates that about 6 times
less ethyl oleate is adsorbed compared to PEGMO at the
corresponding concentrations. The interaction between ethyl
oleate and the surface was probably very weak, as no significant
band shifts were observed in the spectrum of ethyl oleate on
hematite (not shown) compared to the spectrum of pure ethyl
oleate.
1498
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
Table 1. Assignment of Absorption Bands Originating from Pure
Atrac Spread over ZnSe and Atrac Adsorbed on Hematite in Situ
from a 10 mg L-1 Aqueous Solution
pure Atrac on ZnSe
Atrac adsorbed on hematite
peak assignment
2924
2855
1736
1709
2926
2855
1722
νas (CH2)54
νs (CH2)54
ν(CdO) in ester56
ν(CdO) in acid65
νas (COO-)66
νs (COO-)66
δ (CH2 and CH3)55
ν (C-O) in esters63
1456
1159
Figure 9. Intensity of the band originating from the symmetric stretching
vibration (νs) of the C-H bond as a function of the concentration of ethyl
oleate in solution.
Figure 10. Infrared spectra of (a) a droplet of pure Atrac on ZnSe and (b)
Atrac adsorbed on a hematite film in situ from a 10 mg L-1 aqueous solution.
Atrac Adsorption. Anionic fatty-acid surfactants are believed
to interact with iron oxides electrostatically, that is, to adsorb
on the positively charged iron oxide surface below its point of
zero charge (PZC).1 Nevertheless, oleate species are also known
to chemisorb on hematite at several pH units above the PZC,
forming iron oleate.12,62 Despite the fact that, at pH 8.5, the
hematite surface is charged negatively (see Figure 6), adsorption
of Atrac on hematite at this pH was still observed. Figure 10
shows the infrared spectra of a droplet of pure Atrac on ZnSe
and Atrac adsorbed on hematite from an aqueous solution.
1568
1427
1456
1175
Being a multicomponent system, Atrac presents a rather
complicated infrared spectrum with several absorption bands
in the 1000 and 3000 cm-1 regions (Figure 10). Several bands
in the spectrum of pure Atrac (see Figure 10a) are similar to
those observed in the spectrum of pure PEGMO (see Figure
7a), including CH2 stretching vibrations at 2924 and 2855 cm-1
and CH2 deformation at 1456 cm-1. The absorption bands
observed in pure Atrac at 1736 and 1159 cm-1 (see Figure 10a)
are associated with stretching vibrations of the CdO55 and
C-O63 bonds in esters, respectively. Upon complexation of the
ester group with a metal ion, ν(CdO) shifts to lower frequency,
and ν(C-O) shifts to higher frequency.64 These shifts are
observed in the case of adsorption of Atrac on hematite, with
ν(CdO) and ν(C-O) shifting by 14 and 16 cm-1, respectively
(see Figure 10b), suggesting rather strong interaction between
the ester carbonyls in Atrac and the hematite surface.
The stretching vibration of the CdO bond of free carboxylic
acids is observed in pure Atrac at 1709 cm-1 (see Figure 10a).65
Upon deprotonation of the carboxylic group in solution this band
disappears, and two new bands are observed at 1568 and 1427
cm-1 in the spectra of Atrac adsorbed on hematite (νas and νs,
respectively; see Figure 10b).66 As discussed above, adsorption
of maleic acid on hematite was not observed under the
experimental conditions used because of the electrostatic
repulsion of the carboxylate anion and negatively charged
hematite surface, suggesting that the adsorption of Atrac on
hematite through carboxylate ions is unlikely and that the most
probable interaction is through ester carbonyls connected by
an ethoxy group. However, the carboxylate ions in the adsorbed
molecules of Atrac are situated in the vicinity of the surface,
and the bands originating from them can thus be found in the
spectra of Atrac adsorbed on hematite. For band assignments,
see Table 1.
As one of the main components of Atrac is known to be
ethoxylated tall oil, stretching vibrations of the C-O-C group
in the ethoxy chain were expected to be observed around 1100
cm-1.58 However, rather weak absorption bands were found in
that wavenumber range in the spectra of both pure Atrac and
Atrac adsorbed on hematite (see Figure 10), indicating that the
degree of ethoxylation of the tall oil is quite low. This is further
supported by the information found in the patent47 indicating
that only one ethoxy group derived from an alkylene oxide with
two to four carbon atoms is found in the collector. A shorter
poly(ethylene glycol) chain and the possible presence of highly
nonpolar resin acids in Atrac result in a lower solubility of Atrac
as compared to that of PEGMO.
Prior to studying the adsorption of Atrac as a function of
concentration on hematite, the adsorption as a function of
concentration on an uncoated ZnSe crystal was investigated. In
Figure 11, the intensity of the 2855 cm-1 band is plotted as a
function of Atrac concentration in solution. The measurements
were carried out in a similar way as for PEGMO. At each
concentration, steady state was reached within 1 h.
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010 1499
Figure 11. Intensity of the band originating from the symmetric stretching
vibration (νs) of the C-H bond as a function of the concentration of Atrac
in solution.
Figure 12. Intensity of the band originating from the symmetric stretching
vibration (νs) of the C-H bond as a function of the concentration of Atrac
in solution (0). The solid line represents the Freundlich adsorption model
fitted to the experimental data.
Figure 11 illustrates that the absorbance increases with
increasing concentration, but not linearly, suggesting that the
recorded signal corresponds not to the bulk concentration but
to the amount of Atrac adsorbed on ZnSe. The measured
absorption intensities are quite low, as expected for a polished
ZnSe surface with a very low surface area. Atrac was detected
on the crystal even after the cell had been flushed with water
for 1 h and the crystal had been dried in air, indicating a rather
strong affinity for the ZnSe surface.
Figure 12 shows the change in the intensity of the band
originating from the symmetric stretching vibration (νs) of the
C-H bond during the adsorption of Atrac on hematite as a
function of concentration. The measurements were performed
in the same way as for PEGMO. Adsorption equilibrium was
achieved at each concentration within 3-5 h.
Figure 12 demonstrates that the adsorption of Atrac on the
hematite surface increased with increasing concentration in
solution. A poor fit of the experimental data was observed for
the Langmuir model of adsorption with a coefficient of
determination (R2) of 0.66. When plotted on a logarithmic scale,
the experimental data resulted in a straight line with R2 ) 0.99,
suggesting that the Freundlich adsorption model fitted the
experimental data quite well.
Desorption of Atrac from hematite with time was studied in
situ by flushing the cell with distilled water at two different pH
values; see Figure 13. Prior to desorption, adsorption of Atrac
Figure 13. Intensity of the band originating from the symmetric stretching
vibration (νs) of the C-H bond in Atrac as a function of time during the in
situ adsorption of Atrac onto hematite from a 25 mg L-1 solution at pH 8.5
(Δ) and desorption by flushing with water at pH 8.5 (() and pH 10 (0).
on hematite was performed in situ from a 25 mg L-1 solution
at room temperature and pH 8.5 until adsorption equilibrium
was reached. After the first desorption experiment at pH 8.5,
Atrac was adsorbed again on the same hematite film from the
same solution until adsorption equilibrium was reached. After
that, desorption at pH 10 was performed. As the two adsorption
curves leveled out at approximately the same absorbance values,
the second adsorption curve is not shown.
As illustrated by Figure 13, the absorbance was reduced rather
rapidly, even when the sample was flushed with distilled water
at pH 8.5 (i.e., the same pH as used during the adsorption),
indicating that Atrac desorbed rather rapidly. However, the data
indicate that, after 1 h of flushing, more than 50% of the Atrac
still remained on the surface, suggesting that some amount of
Atrac was strongly attached to the hematite surface. When the
sample was flushed with distilled water at pH 10, the data
indicate that Atrac desorbed more rapidly, probably because of
the electrostatic repulsion between the carboxylic groups and
the, at that pH, highly negatively charged hematite surface.
Nevertheless, after 1 h of flushing, the data indicate that still
about 40% of the originally adsorbed Atrac remained on the
hematite surface. These results suggest that mild washing of
the iron ore during magnetic separation and filtration in LKAB’s
process is not sufficient to completely remove the adsorbed
flotation collector from the iron ore, especially if the pH of the
water at these steps is below the flotation pH. At LKAB, the
yearly average pH of water in the clarifying pond, which
provides 80% of the process water, is reported to have varied
in the range from 7.8 to 8.1 during the period of time from
1992 to 2004.67 However, the seasonal variation of pH is much
higher and covers the pH range from 7.2 to 9.2, with lower
values during the period of snowmelt. A decrease of pH during
washing as compared to that during flotation (8.5) could lead
to further adsorption of the flotation reagent on the surface of
iron ore. A separate washing unit operating at a pH higher than
the flotation pH before the pelletization plant could be helpful
in reducing the amount of Atrac adsorbed on the magnetite
surface and possibly improving green-pellet strength.
Adsorption Mechanisms of Atrac and Model Compounds.
As shown above, the mode of adsorption of carboxylic acids
and their derivatives is determined by the tail group of the
molecule and its polarity. Maleic acid did not adsorb on hematite
at the pH of the experiments likely because of the electrostatic
repulsion between the carboxylate anion and the negatively
charged surface. Ethyl oleate showed a very weak interaction
1500
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
Figure 14. Proposed adsorption geometries of (a) PEGMO and (b) Atrac
on hematite from aqueous solutions at pH 8.5. R represents alkyl radical in
oleic acid [CH3(CH2)7CHdCH(CH2)7], and R′ represents alkyl radical in
fatty acids (including oleic acid). Dashed ovals indicate the groups involved
in adsorption. These groups are polar without negative ionic entities and
should interact with the polar and negatively charged Fe-O surface.
with hematite because of the nonpolar ester group. Both
PEGMO and Atrac were found to adsorb on hematite at the
experimental conditions. Based on the interpretation of the
spectroscopic data, PEGMO and Atrac are proposed to adsorb
as illustrated in Figure 14.
PEGMO showed rather strong absorption intensities in the
IR spectra when adsorbed on hematite, with significant shifts
of the bands originating from the ethoxy and hydroxyl groups,
which indicate that the adsorption of PEGMO probably occurs
through the long ethoxy chain and the tail hydroxyl group
(Figure 14a), where the latter is not likely to be deprotonated
at the pH used in this work.
Being a combination of ethoxylated fatty acids and maleic
acid, Atrac contains ester carbonyls, ethoxy groups, and a free
carboxylic group. The free carboxylic group is deprotonated in
solution at pH 8.5 and is not likely to adsorb on the negatively
charged hematite surface, whereas the ester carbonyls exhibited
rather strong interactions with the hematite surface, as indicated
by considerable shifts of the band originating from ester
carbonyls in the IR spectra. Thus, the suggested mode of
adsorption of Atrac on hematite is probably through ester
carbonyls and the short ethoxy chain, as illustrated in Figure
14b.
Conclusions
Continuous and evenly distributed hematite films of controllable thickness were deposited on ATR crystals by dip-coating.
It was shown that the adsorptions of both a flotation agent and
the selected model compounds on such films could be monitored
in situ by FTIR spectroscopy.
Maleic acid does not adsorb on hematite at pH 8.5 because
of the repulsion between the negatively charged hematite surface
and two carboxylate anions present in maleic acid at this pH,
suggesting that electrostatic interactions affect the adsorption
of maleic acid on hematite. No breakage of the ester bond was
observed in either the model compounds or Atrac at the
experimental conditions used in this work. Ethyl oleate showed
very low adsorption on hematite, suggesting that the ester
carbonyl does not form strong complexes with the hematite
surface. For PEGMO, the adsorption on hematite likely took
place through the tail hydroxyl group accompanied by the
interaction of the poly(ethylene glycol) chain with the surface.
Based on the adsorption behavior of maleic acid, it was
concluded that Atrac could not adsorb on hematite through
deprotonated carboxylic group at the chosen experimental
conditions. The most probable mode of adsorption of Atrac on
the hematite surface is through the ester carbonyls and the
ethoxy group. Adsorption isotherms for both Atrac and PEGMO
were in good agreement with the Freundlich model of adsorption
describing adsorption on energetically heterogeneous surfaces.
Based on the desorption experiments, it was concluded that
the strength of adsorption of Atrac on the surface of hematite
varied for different species. Some of them, probably those
attached directly to the surface, were strongly adsorbed and
remained on the surface even when the sample was flushed with
water at increased pH compared to the pH of adsorption. Other
Atrac species showed rather weak interaction and could be
removed from the surface by flushing with water at pH 8.5 (i.e.,
the same pH as during the adsorption). For PEGMO, the
intensity of the band originating from the symmetric stretching
vibration (νs) of the C-H bond in PEGMO was reduced by
only 5% upon flushing of the cell with water at pH 8.5 for 1 h,
suggesting a stronger interaction with hematite as compared with
Atrac, probably because of the longer ethoxy chain and the
presence of the tail hydroxyl group, which is not likely to be
deprotonated at pH 8.5 and is thus not causing electrostatic
repulsion of the molecule from the surface.
Further, the results of the desorption experiments suggested
that a separate washing unit after the flotation step could be
beneficial in reducing the contamination of iron ore by the
flotation collector and possibly improving green-pellet strength.
The method developed here will be used in future works in
which the effects of ionic strength, calcium ions, and silicates
on the adsorption of Atrac and selected model compounds will
be studied in detail.
Acknowledgment
The Hjalmar Lundbohm Research Center (HLRC) is gratefully acknowledged for financial support of this work.
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ReceiVed for reView August 27, 2009
ReVised manuscript receiVed December 3, 2009
Accepted December 19, 2009
IE901343F
PAPER II
The effect of calcium ions and sodium silicate on the
adsorption of anionic flotation collector on magnetite studied
by ATR-FTIR spectroscopy
E. Potapova, M. Grahn, A. Holmgren, and J. Hedlund
Journal of Colloid and Interface Science 345 (2010) 96-102
Author's personal copy
Journal of Colloid and Interface Science 345 (2010) 96–102
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science
www.elsevier.com/locate/jcis
The effect of calcium ions and sodium silicate on the adsorption of a model
anionic flotation collector on magnetite studied by ATR-FTIR spectroscopy
E. Potapova *, M. Grahn, A. Holmgren, J. Hedlund
Division of Chemical Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden
a r t i c l e
i n f o
Article history:
Received 25 November 2009
Accepted 14 January 2010
Available online 28 January 2010
Keywords:
Magnetite
Flotation collector
Calcium
Silicate
Adsorption
ATR-FTIR
a b s t r a c t
Previous studies have shown that agglomeration of the magnetite concentrate after reverse flotation of
apatite is negatively affected by the collector species adsorbed on the surface of magnetite. In this work,
the effect of ionic strength, calcium ions and sodium silicate on the unwanted adsorption of a model anionic flotation collector on synthetic magnetite was studied in situ using attenuated total reflectance
Fourier transform infrared spectroscopy (ATR-FTIR). The amount of collector adsorbed was found to
increase with increasing ionic strength at pH 8.5 providing evidence to the contribution of electrostatic
forces to the adsorption of the collector. Adding sodium silicate to the system resulted in a threefold
decrease in the amount of collector adsorbed compared to when no sodium silicate was added, confirming the depressing activity of sodium silicate on magnetite. Calcium ions were shown to increase the
adsorption of both the collector and sodium silicate on magnetite. The depressing effect of sodium silicate
on collector adsorption was completely suppressed in the presence of calcium ions under the conditions
studied. Furthermore, the amount of collector adsorbed on magnetite from the silicate-collector solution
increased 14 times upon addition of calcium ions suggesting that calcium ions in the process water may
increase undesired adsorption of the collector on the iron oxide.
Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction
Being the most commonly used metal in the world, iron is usually
found as oxide minerals in nature and is extracted from iron-containing ores, of which the most common are magnetite (Fe3O4),
hematite (a-Fe2O3) and goethite (a-FeO(OH)) ores. Due to their magnetic properties, magnetite and hematite can be separated from the
gangue minerals using magnetic separators [1]. However, additional
upgrading steps can be required in order to reduce the amount of
trace elements, such as phosphorous, to an acceptable level for the
blast furnace process. Dephosphorization of the iron ore is carried
out by reverse flotation with modified fatty acid based collectors
[2]. Ideally, the collector should selectively adsorb on the surface
of phosphorous-containing mineral rendering it hydrophobic and
thus easily floated and not affecting the surface of iron oxide.
In order to achieve a high recovery of phosphorous in the flotation of calcareous gangue containing minerals, such as apatite
(Ca5(PO4)3(F, Cl, OH)), fluorite (CaF2) and calcite (CaCO3), sodium
silicate is widely used as a dispersant. Depending on the dosage, sodium silicate can also have a depressing effect on iron oxides. Gong
et al. [3] showed that polymeric silicate species formed in concentrated solutions provided a depressing effect on iron oxide due to
* Corresponding author. Fax: +46 920 491199.
E-mail address: [email protected] (E. Potapova).
0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcis.2010.01.056
the ability of the polymeric silicate species to adsorb strongly on
the surface of the iron oxide, thus blocking it from collector adsorption. In another study, it was found that too high amounts of sodium
silicate resulted in decreased flotation selectivity as both iron oxide
and gangue minerals lost their floatability [4]. At moderate concentrations, silicate species adsorbed on the gangue mineral are replaced by the collector while the iron oxide surface is still
protected from the collector adsorption, thus a selective depressing
effect is achieved [5]. Low concentrations of sodium silicate
(<500 g t 1) have not been found to prevent collector adsorption
on the iron oxide surface to any greater extent [4,6,7].
In our previous study [8] it was shown that a commercial anionic fatty acid based collector readily adsorbs on hematite from
aqueous solutions at pH 8.5. Furthermore, from desorption experiments, it was proven to be difficult to completely eliminate the
collector adsorbed on the surface by flushing with water even at
pH 10, which was higher than the adsorption pH.
Besides the obvious fact that undesired adsorption of the flotation collector on the iron ore increases collector consumption in
the process, it may have a negative effect on the subsequent
agglomeration process where the iron ore is balled into pellets
[9]. Forsmo et al. [10] showed that even small amounts of the collector added to the balling feed caused a significant decrease in the
green (before thermal treatment) pellet strength. As suggested by
the authors, the collector adsorbed on magnetite decreases its
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E. Potapova et al. / Journal of Colloid and Interface Science 345 (2010) 96–102
wettability, resulting in the formation of stable air bubbles inside
the pellet, which in turn compromises green pellet strength. Low
green pellet strength, in turn, causes increased recirculation in
the pelletizing plant and breakage of the pellets during drying
and induration [9,11].
Apart from the flotation collector and dispersant, another parameter that may affect the flotation performance is the process water
chemistry. The presence of inorganic species may activate or depress
the flotation of a certain mineral, change collector solubility and the
zeta-potential of the mineral surface [12]. Monovalent ions with an
opposite charge compared to the charge of the surface have been
shown to reduce the zeta-potential of a mineral while polyvalent
ions are even capable of reversing the surface charge [13].
Previous studies indicate that high concentrations of Ca ions in
the process water may affect both the flotation step [14,15] and the
strength of the iron ore pellet [16,17]. During flotation, Ca ions in
the process water may form precipitates with the collector and/
or adsorb on the iron oxide surface reversing its charge and thus
making it more favourable for collector adsorption [14,15].
Infrared spectroscopy has been applied widely for studies of
interactions in flotation systems as revealed by several comprehensive reviews [18–20]. The mechanism of fatty acid adsorption
on calcareous and iron oxide minerals has been studied extensively
[21–26] since 1965, when Peck and Wadsworth published their results on the adsorption of oleate on fluorite and barite [27]. The
development of infrared external and internal reflection techniques provided the possibility to carry out in situ studies of collector/mineral systems, which are important since such studies
provide real-time information about the complexes formed at the
solid–liquid interface. Our research group has been utilizing
in situ ATR-FTIR spectroscopy for studying interactions of flotation
collectors [28–30], sodium silicate [31,32], bentonite [33] and inorganic ions [34,35] with iron oxides as well as adsorption of hydrocarbons in zeolite films [36–39].
In our previous work on the interactions between fatty acid
based collectors and iron oxides, the application of ATR-FTIR spectroscopy provided the possibility to elucidate the mechanism by
which different collectors and model compounds were adsorbed
on the hematite surface in the presence of water, to follow adsorption and desorption kinetics in situ and to make a conclusion about
the stability of the complexes formed [8].
The objective of the present work is to study the effect of ionic
strength, calcium ions and sodium silicate on the adsorption of a
model flotation collector reagent on magnetite. A better understanding on how sodium silicate and the process water chemistry
affect the unwanted adsorption of the collector on magnetite may
lead to insights on how to minimize the adsorption of the collector
on magnetite.
2. Materials and methods
2.1. Materials
Magnetite nanoparticles were synthesized by co-precipitation
of Fe(II) and Fe(III) according to the method described by Massart
and Cabuil [40]. In short, 50 mL of an aqueous solution containing
0.33 M FeCl24H2O (pro analysi, KEBO) and 0.66 M FeCl36H2O (pro
analysi, Riedel-de Haën) was added dropwise under continuous
stirring to 450 mL of a 1 M solution of NH3 (25%, Suprapur, Merck)
in degassed MilliQ water. The black precipitate formed was purified by repeated sedimentation and redispersion in degassed MilliQ water until the supernatant remained turbid. The suspension
was further dialyzed until the conductivity of the water outside
the dialysis tube reached 1.6 lS cm 1 and remained constant upon
replacing it with the fresh water. The obtained suspension of fine
magnetite crystals was then diluted with methanol resulting in a
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water-to-methanol ratio of 3:1 by volume. The dry solid content
of the suspension was estimated to ca 1.2 mg mL 1 by weighing
a known volume of the suspension after drying in an oven at
100 °C. The suspension was stored in a refrigerator in order to minimize oxidation of magnetite.
Dodecyloxyethoxyethoxyethoxyethyl maleate (Sigma–Aldrich),
for the sake of clarity and simplicity hereafter referred to as ‘collector’ was chosen as a model flotation collector reagent. This collector
was chosen based on the information given in a patent [41] describing a similar collector for froth flotation of oxide and salt type minerals, which consists of a long aliphatic hydrocarbon group
connected by an alkylene oxide group to a dicarboxylic acid. The
chemical structure of the collector used in this work is shown in
Fig. 1.
Stock solutions of the collector, sodium silicate (Na2SiO39H2O,
98%, Sigma) and calcium chloride (CaCl22H2O, 95%, Riedel-de
Haën) were prepared by dissolving appropriate amounts of the corresponding chemicals in 0.01 M aqueous solutions of NaCl (pro
analysi, Riedel-de Haën), which was used as a background electrolyte. Further, working solutions were prepared by diluting appropriate amounts of the stock solutions with the 0.01 M aqueous
solution of NaCl to give the required final concentrations.
All aqueous solutions were prepared using distilled water degassed by applying vacuum. The distilled water used for the spectroscopic measurements was, after degassing, saturated with argon
in order to minimize the amount of dissolved carbon dioxide.
2.2. Film deposition and general characterization of the film
Prior to film deposition, the trapezoidal ZnSe crystals (Crystran
Ltd.), with the dimensions 50 20 2 mm and 45 cut edges, were
first rinsed in ethanol (99.7%, Solveco chemicals AB) and distilled
water. Thereafter, 0.3 mL of the magnetite suspension described
above was spread evenly over one side of the ZnSe crystals and
dried producing a thin film. The other side of the crystal was left
uncoated in order to reduce the attenuation of the IR radiation
by magnetite and thus obtain a higher throughput to the detector
in the spectroscopic measurements.
An X-ray diffraction pattern of the magnetite film was recorded
with a Siemens D5000 powder diffractometer operating in Bragg–
Brentano geometry and utilizing Cu Ka radiation. In order to analyse the film, a magnetite-coated silicon wafer was mounted in a
custom-made aluminium holder. The pattern of the assembly
without magnetite film was also recorded in order to identify the
peaks belonging to the substrate and the holder.
The magnetite film on a ZnSe crystal was investigated with an
FEI Magellan 400 field emission high resolution scanning electron
microscope (HR-SEM) using an accelerating voltage of 1 kV. No
gold coating or similar was deposited on the film to render it electrically conductive. The magnetite coated ZnSe crystal was cut in
the middle and mounted vertically in the sample holder in order
to measure the film thickness and investigate the morphology of
the magnetite particles.
2.3. ATR-FTIR spectroscopy
Spectral data were collected using a Bruker IFS 66v/S spectrometer equipped with a liquid nitrogen cooled mercury–cadmium–
Fig. 1. Chemical structure of the collector. R represents the linear alkyl chain
CH3(CH2)11.
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telluride (MCT) detector. A ZnSe crystal coated with magnetite was
placed in a stainless steel flow cell which was further mounted on
the ATR accessory in the spectrometer (see Fig. 2). The IR beam,
guided by the mirrors, enters the ZnSe through the cut edge at
an angle of incidence of 45° and passes through the crystal via a
number (ca 25) of total reflections. At each point of reflection, an
evanescent wave of IR radiation is formed, which penetrates into
the sample perpendicular to the surface of reflection and interacts
with the sample material, simultaneously losing a part of its
intensity.
Adsorption measurements were performed in situ with a flow of
the solution pumped continuously through the cell on one side of
the ATR crystal at a rate of 10 mL min 1 with recirculation of the
solution. The other half of the cell was evacuated. Both single beam
background and sample spectra were obtained by averaging 500
scans at a resolution of 4 cm 1. Data evaluation was performed
using the Bruker Opus 4.2 software.
A spectrum of the pure collector was recorded in argon atmosphere by spreading a droplet of the collector over a bare ZnSe crystal; as a background, a spectrum of the bare crystal in argon was
used. All adsorption and desorption experiments were performed
at room temperature and pH 8.5. The pH was controlled by a Mettler
Toledo T70 titrator using a 0.05 M aqueous solution of sodium
hydroxide (pro analysi, Merck). The concentration of the background
electrolyte was 0.01 M NaCl in all the experiments if not stated
otherwise. Prior to adsorption, the magnetite film was equilibrated
with 75 mL of a 0.01 M aqueous solution of NaCl at pH 8.5 for
30 min and at this point a background spectrum of the solution in
contact with the magnetite coated ZnSe substrate was recorded.
In the experiments where calcium ions or sodium silicate were
pre-adsorbed on magnetite, a 4 mM solution of CaCl2 or a 0.4 mM
solution of Na2SiO3, both at pH 8.5, was pumped through the cell
for 1 h. Infrared spectra were recorded every 5 min and a new
background spectrum was recorded when the pre-adsorption
was completed. After that, an appropriate amount of the collector
was added to the solution to give a final concentration of the collector of 25 mg L 1 whereas the concentration of calcium or silicate
ions was kept constant. Adsorption of the collector on magnetite
was followed by recording infrared spectra every 5, 10 or 30 min.
In the experiment where the influence of calcium ions on the
depressing effect of sodium silicate was studied, calcium ions were
first pre-adsorbed, thereafter a new background spectrum was recorded and then sodium silicate was added to the solution and the
adsorption was monitored with time. After silicate adsorption, another background spectrum was recorded and finally the adsorption of the collector from a 25 mg L 1 solution was performed.
Desorption experiments were carried out by flushing the cell
with a 0.01 M NaCl solution at pH 8.5 without recirculation of
the solution.
Fig. 2. Schematic image of the ATR-FTIR setup.
3. Results and discussion
3.1. Characterization of the synthetic magnetite
Fig. 3 shows an HR-SEM image of a cross-section of a magnetite
film on a ZnSe crystal.
The image illustrates that the film thickness is about 250–
300 nm and the size of the individual particles vary between about
5–15 nm.
Fig. 4 shows an X-ray diffraction pattern of magnetite crystals
deposited on a silicon wafer.
Except for the narrow diffraction peaks emanating from the silicon wafer, the XRD pattern is characteristic for randomly oriented
magnetite crystals. The diffraction peaks from magnetite are quite
broad, which shows that the magnetite crystals are quite small in
accordance with the HR-SEM observations.
3.2. Effect of calcium ions on the adsorption of sodium silicate
Prior to studying the combined effect of calcium ions and sodium silicate on the collector adsorption on magnetite, the influence of calcium ions on the adsorption of sodium silicate on
magnetite was investigated in order to assess if calcium ions increased the polymerization of silicate species as proposed by Gong
et al. [5].
Fig. 3. Side view SEM image of a magnetite film on a ZnSe crystal.
Fig. 4. XRD pattern of synthetic magnetite crystals deposited on a silicon wafer. The
reflections originating from magnetite are indexed with the appropriate Miller
indices. The peaks labelled with () emanate from the silicon wafer.
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Silicate species adsorbed on magnetite at pH 8.5 (see spectrum
(a) in Fig. 5) are characterized by a broad combination of bands between 800 and 1300 cm 1. At this pH, the most pronounced band
is located at 1014 cm 1 with two shoulders at ca 1120 cm 1 and ca
950 cm 1.
The band at 950 cm 1 has been assigned to the monomeric surface bidentate complex while the band at 1014 cm 1 has been assigned to the oligomeric silicate species on the surface [31].
Further, the same authors attributed the band at 1120 cm 1 to
the 3-dimensional silica framework structure. With the increase
of surface polymerization of the silicate species the band at
1014 cm 1 is expected to shift to higher wavenumbers [31].
Spectra recorded when calcium ions were pre-adsorbed on
magnetite showed rather intense bands at 1490 and 1350 cm 1 assigned to the asymmetric and symmetric stretching vibrations,
respectively, of the carbonate species [42] (see spectrum (b) in
Fig. 5). This result indicates that some carbonate species were present in the water despite degassing, and further, these carbonate
species adsorbed on the surface of magnetite when calcium ions
were added to the solution.
As sodium silicate was added to the system after pre-adsorption
of calcium ions, negative bands associated with the carbonate species were observed in the spectra simultaneously as the positive
bands originating from adsorbed silicate species appeared, suggesting that silicate species were replacing the carbonates on the
magnetite surface (see spectrum (c) in Fig. 5).
The shape of the absorption bands emanating from the silicate
species was not particularly affected by the presence of calcium
ions; however, the band originating from the oligomeric silicate
species was slightly shifted (2 cm 1) to higher wavenumbers
indicating that the silicate species adsorbed on the magnetite surface in the presence of calcium ions were possibly polymerized to a
greater extent as compared with no calcium ions present [5].
Moreover, from the intensities of the bands it may be concluded
that the amount of sodium silicate adsorbed on magnetite increased by ca 35% when calcium was pre-adsorbed suggesting that
electrostatic forces contributed to the interaction between silicates
and magnetite.
Similar results were reported by Roonasi et al. for the system
calcium–sulfate–magnetite [34]. Calcium ions were found to
increase the adsorption of sulfate on magnetite at pH 8.5 while
Fig. 5. Infrared spectrum of silicate species adsorbed on magnetite without any
calcium ions present (a), spectrum of the magnetite film recorded after the preadsorption of calcium ions on magnetite (b), and spectrum of silicate species
adsorbed on magnetite after pre-adsorption of calcium ions (c).
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no effect on the adsorption was observed at pH 4 indicating the
importance of electrostatic interaction for the calcium/sulfate/
magnetite system and a similar behaviour could be expected for
the calcium/silicate/magnetite system.
3.3. Collector adsorption on magnetite
The collector studied in this work contains a carboxylic head
group (see Fig. 1), which, depending on pH, can be deprotonated
when dissolved in water. Fig. 6 shows a spectrum of the collector
in pure form (non-dissolved).
The strong absorption bands at 2922 and 2854 cm 1 in the
spectrum are characteristic for the molecules containing long alkyl
chain as these bands emanate from symmetric and asymmetric
stretching vibrations (ms and mas) of the CH2 group [22]. Stretching
vibrations of the CAOAC group in the polyethylene glycol chain
give rise to a characteristic intense band at 1105 cm 1 [43]. The
band at 1728 cm 1 is associated with the stretching vibrations of
the C@O bond [43]. As the collector molecule contains two carboxylic groups, one free and one esterified, two separate bands in the
carbonyl stretching region are expected to be observed. However,
when an ester carbonyl is conjugated with a C@C group (like in
maleate), the band from the ester carbonyl, typically observed
around 1740 cm 1, is shifted to lower wavenumbers viz. around
1725 cm 1 [44]. At the same time, intramolecular hydrogen bonding between the carboxylic groups in maleic acid may cause a shift
of the C@O stretching vibration to higher wavenumbers (1730–
1705 cm 1) [44] resulting in possible overlapping with the band
originating from the ester carbonyl.
In our previous work, it was shown that the adsorption of anionic flotation collectors on iron oxides is not fully governed by electrostatic forces. Anionic collectors can adsorb on iron oxides
despite repulsion between the negatively charged head group
and the surface bearing the same net charge. The influence of electrostatic forces on the adsorption can be revealed by studying the
effect of ionic strength on adsorption. Fig. 7 illustrates the effect of
increased ionic strength on the intensity of one of the bands originating from the collector adsorbed on magnetite.
Assuming that the band intensity is proportional to the amount
of the collector adsorbed on magnetite, the data presented in Fig. 7
suggests that higher ionic strength results in a higher adsorption of
the collector on magnetite due to the fact that the increased
amount of sodium ions reduces the negative effective surface
charge of the magnetite surface and thereby facilitating a greater
collector adsorption. These results indicate that electrostatic forces
contribute to the adsorption of the collector on magnetite.
Fig. 6. Infrared spectrum of a droplet of the pure collector on a ZnSe crystal in argon
atmosphere.
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Fig. 7. Intensity of the ester C@O stretching vibrations band as a function of time
during in situ adsorption of the collector on magnetite at pH 8.5 from a 25 mg L 1
solution containing 0.01 M NaCl (D) and 0.1 M NaCl (h). The ionic strength in the
working solution was increased after 13 h of adsorption by adding the appropriate
amount of NaCl.
Moreover, as the ionic strength was changed from 0.01 M NaCl
to 0.1 M NaCl it was also observed (not shown) that the band originating from the asymmetric stretching vibration of the CH2 group
in the spectra shifted from 2924 to 2922 cm 1 indicating that the
interaction of the alkyl chains in the collector increased, probably
due to a higher packing density of the molecules in the adsorbed
layer [45].
Fig. 8 shows spectra of the collector adsorbed on the magnetite
surface from aqueous solutions at pH 8.5. Compared to the spectrum of the pure collector, two new bands are observed in the
spectra of the collector (spectra (a–d)), viz. at ca 1570 and
1400 cm 1. These bands are assigned to the asymmetric and symmetric stretching vibrations of the carboxylate ion respectively
[46] indicating the deprotonation of the head carboxylic group.
However, the band originating from the stretching vibrations of
the C@O bond is still present in the spectra at 1724–1726 cm 1
suggesting that adsorption likely takes place without breaking
the ester bond.
When pre-adsorption of calcium ions was performed (see spectra (b) and (d) in Fig. 8), an additional band at 1425 cm 1 appeared
in the spectra of the collector adsorbed on magnetite suggesting
that calcium ions affected the adsorption mode of the collector
on magnetite.
When the effect of sodium silicate on collector adsorption was
studied, the magnetite film was pre-treated with sodium silicate
for 1 h before the collector was added to the system. The spectra
recorded during collector adsorption (spectrum (c) in Fig. 8), in
addition to the bands originating from the collector, contained an
intense band at 1030 cm 1 with a shoulder at 1120 cm 1 emanating from the silicate species adsorbed on the surface of magnetite.
The shoulder became evident when subtracting a spectrum of the
collector adsorbed on magnetite without sodium silicate (spectrum (a) in Fig. 8) from the spectrum (c) in Fig. 8. The position of
the band at 1030 cm 1 indicates higher degree of polymerization
of the silicate species adsorbed on magnetite as compared to those
after 1 h of adsorption prior to addition of the collector (see spectrum (a), the band at 1014 cm 1). These results suggest that the silicate species continued to adsorb on magnetite even in the
presence of the collector implying competitive adsorption between
the collector and sodium silicate for the magnetite surface sites.
In the case when the magnetite film was pre-treated with both
calcium ions and sodium silicate, which would be the most realistic conditions in a flotation process, no bands associated with silicate species were observed in the spectrum of the collector
Fig. 8. Infrared spectra of the collector adsorbed on magnetite at pH 8.5 from a
25 mg L 1 aqueous solution containing no added Ca2+ or Na2SiO3 (a), 4 mM Ca2+ (b),
0.4 mM Na2SiO3 (c), and 4 mM Ca2+ and 0.4 mM Na2SiO3 (d). All spectra were
recorded after 13 h of collector adsorption. For the sake of clarity, the absorbance of
the spectra (a) and (c) was multiplied with a factor 5.
adsorbed on magnetite suggesting that the adsorption of sodium
silicate was terminated by the collector when calcium ions were
also present. It should be noted that a new background spectrum
was recorded after the pre-adsorption of silicate and calcium ions.
However, silicate species already adsorbed on magnetite were not
substituted with the collector since no negative bands associated
with the silicate species were observed in the spectra. Thereby, it
may be concluded that adsorption and desorption of the silicate
species were equilibrated by the addition of the collector to the
system.
After 13 h of adsorption from a solution of 4 mM calcium chloride and 25 mg L 1 collector at pH 8.5, an in situ desorption experiment was performed. Fig. 9 shows spectra recorded during
flushing the cell with water containing 0.01 M NaCl at pH 8.5 for
2 h. As has been mentioned above, an additional band at
1425 cm 1 appeared in the spectra of the collector adsorbed on
magnetite when pre-adsorption of calcium ions was performed.
Upon desorption, the intensity of this band decreased much faster
than the band at 1402 cm 1 suggesting the presence of two kinds
of carboxylate complexes on the surface of magnetite: an innersphere complex characterized by the band at 1402 cm 1 [47] and
an outer-sphere complex characterized by the band at 1425 cm 1
[47] as was previously reported by Hwang and Lenhart for maleate
adsorption on hematite [47].
After 13 h of adsorption from a solution of 0.4 mM sodium silicate
and 25 mg L 1 collector at pH 8.5, an in situ desorption experiment
was performed. Fig. 10 shows spectra recorded during flushing the
cell with water containing 0.01 M NaCl at pH 8.5 for 2 h.
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E. Potapova et al. / Journal of Colloid and Interface Science 345 (2010) 96–102
Fig. 9. Spectra recorded during flushing the cell with water at pH 8.5 after collector
adsorption in the presence of calcium ions. The time interval between the spectra is
10 min.
The intensity of the bands emanating from the silicate species
was decreasing upon flushing with water at pH 8.5. After the first
10 min of flushing, the intensity of the band at 1030 cm 1 emanating from the oligomeric silicate species was reduced as much as
40%. The intensity of this band was decreasing more slowly as
the flushing continued. Furthermore, the peaks associated with
the collector adsorbed on magnetite were slightly increasing in
intensity at the beginning of desorption of silicate species likely
due to the increasing amount of vacant surface sites available for
adsorption of the collector. However, after 1.5 h of flushing with
water, the bands originating from the collector also started to decrease slowly, indicating that no more empty surface sites were
available for collector adsorption.
Thereby, the in situ desorption experiment revealed that the
collector was adsorbed on the magnetite surface much stronger
than the silicate species since the absorption bands originating
from the collector decreased very slowly or even increased upon
flushing with water.
Fig. 11 illustrates the effect of calcium ions and sodium silicate
on the intensity of the band originating from the collector adsorbed
on magnetite as a function of time.
The data presented in Fig. 11 shows that the intensity of the ester C@O stretching vibrations band increased slowly during in situ
adsorption of the collector on magnetite at pH 8.5 when no calcium
or silicate ions were present in the solution (open triangles).
Assuming that the band intensity is proportional to the amount
of the collector adsorbed on magnetite, the data presented in
Fig. 11 indicates an almost fivefold increase in the amount of the
collector adsorbed on magnetite in the presence of calcium ions
Fig. 10. Spectra recorded during flushing the cell with distilled water at pH 8.5 and
0.01 M NaCl after collector adsorption in the presence of silicates. The time interval
between the spectra is 10 min.
101
Fig. 11. Intensity of the ester C@O stretching vibrations band as a function of time
during in situ adsorption of the collector on magnetite at pH 8.5 from a 25 mg L 1
aqueous solution containing no Ca2+ and Na2SiO3 added (D), 4 mM Ca2+ (s), 0.4 mM
Na2SiO3 (N), 4 mM Ca2+ and 0.4 mM Na2SiO3 (d).
(open circles) as compared to when no calcium ions were added
(open triangles), supporting the results reported earlier [14,15]
and suggesting that calcium ions interact with the magnetite surface reducing the negative net charge of the surface thus making it
more favourable for collector adsorption. Similarly to collector
adsorption on semisoluble calcium-containing minerals like apatite, fluorite, and calcite [48], collector species can then interact
specifically with calcium ions adsorbed on magnetite forming
magnetite–calcium-collector complexes. Additionally, calcium
ions can facilitate the formation of calcium-collector precipitate,
which may subsequently adsorb on the magnetite surface [9,15].
A threefold decrease in the intensity of the band originating
from the collector adsorbed on magnetite was observed in the
presence of sodium silicate (filled triangles) as compared to when
no silicate was present (open triangles). The observed decrease is
an effect of competitive adsorption between the silicate species
and the collector on the magnetite surface sites as was also shown
in Fig. 8 by comparing spectra (a) and (c). The results confirm that
sodium silicate depresses the adsorption of the collector on magnetite in concert with previous findings [3,5].
In the experiment where both silicate and calcium ions were
pre-adsorbed, the intensity of the bands originating from the collector adsorbed on magnetite was reduced by less than 8% (filled
circles) as compared to the case when only calcium ions were present in the system (open circles) and was increased almost 12 times
as compared to when only sodium silicate was present (filled triangles). Thereby, the depressing activity of sodium silicate was less
pronounced in the presence of calcium ions, in contradiction to
the results previously reported by Gong et al. [5], who observed
much stronger depressing activity of the silicate–calcium ion mixtures on hematite in the flotation of apatite with tall oil fatty acid
as compared to that of pure sodium silicate. The reason for that
was probably much higher calcium/Si ratio used in the present
work (10) as compared to those in the study by Gong et al. (0.2–
0.6). In the present study, calcium ions were found to only slightly
increase the amount and polymerization degree of the silicate species adsorbed on magnetite (see Fig. 5), while they made much
more significant contribution to the increase of the collector
adsorption on magnetite (see Fig. 11).
The results presented in this work indicate that calcium ions
present in the water significantly increase the adsorption, and possibly precipitation, of the collector on synthetic magnetite. A similar effect may be expected in the industrial flotation process:
contamination of the iron ore with the flotation collector may be
enhanced upon high concentrations of calcium in the process
water increasing collector consumption and affecting the apatite
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flotation as has been previously reported by Rao et al. [15]. Further,
hydrophobic collector coating on the iron ore concentrate has been
shown to reduce the strength of iron ore green pellets [9,11] suggesting that high concentration of calcium ions in the process
water may have a negative impact on the green pellet strength
by increasing adsorption and/or precipitation of the collector on
the iron oxide surface.
4. Conclusions
The effect of ionic strength, calcium ions and sodium silicate on
the adsorption of a model flotation collector on magnetite was
investigated by in situ ATR-FTIR spectroscopy.
Monovalent cations were found to slightly increase the adsorption of the collector on magnetite at the studied pH by partly compensating the negative charge of the magnetite surface and thus
reducing the electrostatic repulsion between the surface and carboxylate ions suggesting that electrostatic forces contribute to
the adsorption of the anionic flotation collector on magnetite.
Divalent calcium ions were found to have a significant effect on
the adsorption of the flotation collector on magnetite. An almost
fivefold increase in the amount of the collector adsorbed on magnetite was observed when calcium ions were pre-adsorbed, in concert with previous findings for anionic fatty acid based collectors.
From the desorption experiments it became evident that in the
presence of calcium ions, collector adsorption took place via both
inner-sphere and outer-sphere complexes, the latter could be
rather easily removed by flushing with water.
When the magnetite film was pre-treated with sodium silicate,
a competitive adsorption of the collector and sodium silicate took
place on the surface of magnetite resulting in a threefold decrease
in the amount of the collector adsorbed on magnetite as compared
to the case when no pre-adsorption of silicates was performed confirming that sodium silicate can depress collector adsorption on
magnetite. However, the stability of the magnetite-collector complexes was greater as compared to the magnetite–silicate
complexes.
Furthermore, the depressing activity of sodium silicate on the
collector adsorption was almost completely suppressed in the
presence of calcium ions. Moreover, silicate adsorption on magnetite was terminated when the collector was added to the system
and even though the amount and the polymerization degree of
the silicate species adsorbed on magnetite in the presence of calcium ions were higher than without calcium, it was not sufficient
to prevent the collector adsorption to any greater extent.
The results presented in this work suggest that high concentrations of calcium in the process water may enhance collector
adsorption and precipitation on iron oxides, resulting in increased
collector consumption and a more hydrophobic surface. The latter
has been previously shown to decrease the green pellet strength.
Acknowledgments
The authors acknowledge the financial support from the Hjalmar Lundbohm Research Centre (HLRC) and the Knut and Alice
Wallenberg Foundation.
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PAPER III
The effect of calcium ions, sodium silicate and surfactant on
charge and wettability of magnetite
E. Potapova, X. Yang, M. Grahn, A. Holmgren, S. P. E. Forsmo, A. Fredriksson, and J.
Hedlund
Colloids and Surfaces A: Physicochemical and Engineering Aspects 386 (2011) 79-86
Author's personal copy
Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 79–86
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and
Engineering Aspects
journal homepage: www.elsevier.com/locate/colsurfa
The effect of calcium ions, sodium silicate and surfactant on charge and
wettability of magnetite
E. Potapova a,∗ , X. Yang a,b , M. Grahn a , A. Holmgren a , S.P.E. Forsmo c , A. Fredriksson d , J. Hedlund a
a
Division of Sustainable Process Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden
Research Centre for Eco-Environmental Sciences, Chinese Academy of Science, Beijing 100085, China
LKAB, SE-983 81 Malmberget, Sweden
d
LKAB, SE-981 86 Kiruna, Sweden
b
c
a r t i c l e
i n f o
Article history:
Received 31 March 2011
Received in revised form 23 June 2011
Accepted 30 June 2011
Available online 7 July 2011
Keywords:
Adsorption
ATR-FTIR
Contact angle
Silicate
Surfactant
Zeta-potential
a b s t r a c t
Anionic carboxylate surfactants and sodium silicate are used in the reverse flotation of iron ore to separate
magnetite from apatite. In this work, consecutive adsorption of sodium silicate and an anionic surfactant
on synthetic magnetite modified with calcium ions was studied in the pH range 7.5–9.5 using in situ ATRFTIR spectroscopy. The effect of these chemicals on the zeta-potential and wetting properties of magnetite
was also investigated. While adsorption of silicate increased with increasing pH, subsequent surfactant
adsorption went through a maximum at pH 8.5. Surfactant adsorption in the presence of calcium ions
was not affected by the amount of silicate adsorbed on magnetite. Calcium ions were found to render
the magnetite surface positive in the pH range 3–10 and could reduce the dispersing effect of silicate
in flotation of apatite from magnetite. While treatment with calcium chloride and sodium silicate made
magnetite more hydrophilic, subsequent adsorption of the anionic surfactant increased the water contact
angle on the magnetite surface from about 10◦ to 40–50◦ . Although the latter values are not high enough
to make magnetite float, the hydrophobic areas on the magnetite surface could result in the incorporation
of air bubbles inside the iron ore pellets produced by wet agglomeration, lowering the pellet strength.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Surfactant adsorption on mineral surfaces is important for
many industrial applications including detergency, dispersion of
pigments, stabilization of colloidal suspensions in cosmetics and
pharmaceuticals, flocculation of fine mineral particles, and ore
flotation.
Separation of minerals by flotation can occur if the minerals have
different affinities for air and water. A mineral can be flotated only
if the work of adhesion between a mineral particle and an air bubble is high enough to prevent the disruption of the particle–bubble
interface. The work of adhesion between a particle and an air bubble
increases with increasing contact angle at the surface–air interface
implying that the floatability of a mineral improves as the mineral surface becomes more hydrophobic. Most mineral surfaces are
highly polar and have a high Gibbs energy, which makes them
hydrophilic. To make the mineral float with the air bubbles, the
surface of such minerals has to be modified by a suitable surfactant in order to reduce the Gibbs energy. Providing that surfactant
∗ Corresponding author. Tel.: +46 920 491776; fax: +46 920 491199.
E-mail address: [email protected] (E. Potapova).
0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfa.2011.06.029
adsorption in a flotation system occurs selectively, good separation
of the minerals can be achieved.
Interactions between ionic surfactants and minerals are to a
large extent controlled by the charge density of the mineral surface [1]. Adsorption of different species as well as pH of the process
water can alter the charge density enhancing or reducing the interactions between the mineral and the surfactant and therefore
affect flotation performance. It is thus very important to know the
effects that various species in the process water may have on the
charge density of a mineral surface. The effect of adsorption on the
charge density of mineral particles can be determined from electrophoretic measurements and then expressed in terms of changes
in the zeta-potential.
Another phenomenon that is highly dependent on the charge
density of the mineral surface is the dispersion of the mineral particles. High dispersion is required to maximize mineral recovery and
flotation selectivity [2]. Dispersion is facilitated by increasing the
net surface charge density of the mineral particles, which results in
increased electrostatic repulsion. Increased surface charge can be
achieved by the adsorption of compounds forming charged complexes on the mineral surface.
One of the important parameters in flotation that affects both
the surface properties of the minerals and the speciation of the
flotation chemicals as well as species naturally occurring in the pro-
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cess water is pH. The net charge of a mineral surface and adsorption
of ionic collectors on the surface are dependent on pH, which has
also been shown to have a certain effect on natural wettability of
minerals [3,4]. In the recent paper by Puah et al. [5], the variation
of the contact angle of titania as a function of pH was explained by
the ionization of the surface hydroxyl groups above and below the
point of zero charge of the surface resulting in increased surface
charge and, consequently, surface wettability. Together with the
reagent dosing, pH determines to a large extent the selectivity in
separation of minerals by flotation [4].
In the reverse flotation of the iron ore rich in apatite, fatty acid
based surfactants are used as flotation collectors and sodium silicate as a dispersing agent [6]. Sufficiently high concentrations of
sodium silicate have been shown to have a depressing effect on the
iron oxides [7]. Flotation of apatite from the iron oxides is typically
conducted at pH > 7 whereas the optimum pH in any specific process depends on the type of apatite mineral which is to be flotated
[8].
If the iron ore concentrate is pelletized by wet agglomeration, it
is important that the surface of the iron oxide after flotation is sufficiently hydrophilic to ensure efficient balling and high strength of
the iron ore pellets in both wet and dry states [9]. Contamination
of magnetite with flotation collector renders magnetite hydrophobic and reduces pellet strength [10–12]. While sodium silicate can
to some extent protect magnetite surface from collector adsorption, calcium ions present in the process water enhance undesired
adsorption and precipitation of the collector on magnetite [13,14].
In our previous work [15] we have also shown that calcium ions
increase dramatically the adsorption of an anionic surfactant on
magnetite even in the presence of sodium silicate. The results further emphasize the importance of taking into account the influence
of additives and ions in the process water on the adsorption of flotation collectors on mineral surfaces in order to better understand the
phenomena in real flotation systems.
The scope of the present work is to show the effect of pH and
sodium silicate on the adsorption of an anionic surfactant onto magnetite in the presence of calcium ions and to investigate how the
adsorption of the different constituents in this system affects magnetite surface properties relevant to flotation and agglomeration,
namely, surface charge and wettability.
2. Experimental
2.1. Materials
Magnetite nanocrystals were synthesized and purified as
described earlier [15]. The magnetite crystals had a spherical habit,
the size of 5–15 nm and the surface area of 80–100 m2 g−1 . The
obtained suspension of magnetite in distilled water was further
diluted with methanol and degassed distilled water to give a working suspension containing 25 vol.% methanol and ca. 1.1 mg mL−1
magnetite. To minimize oxidation of magnetite, the suspension was
stored in a refrigerator at 6 ◦ C.
Stock solutions of dodecyloxyethoxyethoxyethoxyethyl
maleate (Sigma–Aldrich) used as a model flotation collector,
sodium silicate (Na2 SiO3 ·9H2 O, ≥98%, Sigma) and calcium chloride
(CaCl2 ·2H2 O, 95%, Riedel-de Haën) were prepared by dissolving
required amounts of the corresponding chemicals in a 0.01 M
aqueous solutions of NaCl (per analysis, Riedel-de Haën). Fig. 1
illustrates the structure of the model flotation collector used in
this work.
All solutions were prepared using distilled water degassed
under vacuum in order to minimize the amount of dissolved gases.
The distilled water used for the spectroscopic measurements was,
after degassing, bubbled with argon.
Fig. 1. Chemical structure of dodecyloxyethoxyethoxyethoxyethyl maleate. R represents the linear alkyl chain CH3 (CH2 )11 .
The pH of working solutions was adjusted using aqueous
solutions of sodium hydroxide (NaOH, per analysis, Merck) and
hydrochloric acid (HCl, 37%, per analysis, Merck).
2.2. Methods
2.2.1. Zeta-potential measurements
The zeta-potential measurements were performed using a ZetaCompact instrument equipped with a charge-coupled device (CCD)
tracking camera. The collected electrophoretic mobility data were
processed by the Zeta4 software applying the Smoluchowski equation. Six different aqueous dispersions of synthetic magnetite
crystals were prepared in: (a) 10 mM NaCl as ionic medium; (b)
3.3 mM CaCl2 ; (c) 10 mM NaCl and 1 mM Na2 SiO3 ; (d) 3.3 mM
CaCl2 and 1 mM Na2 SiO3 ; (e) 3.3 mM CaCl2 and 0.4 mM Na2 SiO3 ;
and (f) 3.3 mM CaCl2 , 0.4 mM Na2 SiO3 and 25 mg L−1 (0.06 mM)
maleic acid ester. The concentration of magnetite particles in the
dispersion was ca. 5 mg L−1 . The ionic strength of different samples was constant at 10 mM since the contribution of Na2 SiO3 and
the surfactant to the ionic strength is insignificant. Additionally,
the zeta-potential of 15 mg L−1 (0.04 mM) maleic acid ester was
measured in aqueous solutions containing (a) 10 mM NaCl; and (b)
10 mM NaCl and 2.4 mM CaCl2 . The pH of the samples was adjusted
using aqueous solutions of NaOH and HCl and the samples spanned
the pH range from 3 to 11. For each sample, the zeta-potential was
determined as an average of the values obtained in three replicate
measurements.
2.2.2. Contact angle measurements
Contact angle measurements were performed using a Fibro
1121/1122 DAT-Dynamic Absorption and Contact Angle Tester
equipped with a CCD camera. A magnetite film was prepared by
spreading 0.5 mL of the magnetite dispersion on a ZnSe crystal and
letting it dry in air. Thereafter, the film was rinsed with distilled
water and dried in a vacuum desiccator at ca. 1 kPa for 30 min.
The contact angle was measured by applying a drop of distilled
water, 4 ␮L in volume, onto the film using a microsyringe. A series
of images were taken at different time points and analyzed using
the DAT 3.6 software. Eight fresh drops placed at different sample locations were measured, and the average contact angle value
was calculated. After the measurement, the film was immersed for
60 min in an aqueous solution containing 4 mM of CaCl2 and 10 mM
of NaCl, then rinsed with distilled water and dried in a vacuum
desiccator for 30 min. Thereafter, the contact angle was measured
again. The measurement was repeated in the same manner twice
after subsequently adding 0.4 mM of sodium silicate and 25 mg L−1
of the maleic acid ester to the solution already containing CaCl2 and
NaCl. The pH of the solutions was kept constant at 8.5.
2.2.3. ATR-FTIR spectroscopy
Infrared spectra were recorded on a Bruker IFS 66v/S
spectrometer equipped with a liquid nitrogen cooled
mercury–cadmium–telluride (MCT) detector and a vertical
ATR accessory. A trapezoidal ZnSe crystal (Crystran Ltd.), with the
dimensions 50 mm × 20 mm × 2 mm and 45◦ cut edges, was coated
with a magnetite film as described earlier [15] and mounted in
a stainless steel flow cell. In situ adsorption measurements were
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E. Potapova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 79–86
performed by pumping the working solution continuously through
the cell at a rate of 10 mL min−1 with recirculation and recording
infrared spectra every 5 min.
All adsorption experiments were performed at room temperature. The pH was adjusted and controlled either by a Mettler Toledo
T70 titrator or manually using a conventional pH-meter. The concentration of the background electrolyte was 10 mM NaCl in all the
experiments. Prior to adsorption, the magnetite film was rinsed for
30 min by pumping a 10 mM aqueous solution of NaCl at the pH
of adsorption through the flow cell and thereafter a single beam
background spectrum of the solution in contact with the magnetite
coated ZnSe substrate was recorded. After that, calcium chloride
was added to give a 4 mM aqueous solution that was pumped
through the cell for 1 h. Thereafter, a new single beam background
spectrum was recorded and then sodium silicate was added to the
solution to give a 0.4 mM aqueous solution. The adsorption of silicate was monitored during 20 min by recording a spectrum every
5 min. After silicate adsorption, another single beam background
spectrum was recorded and finally the adsorption of the maleic
acid ester from a 25 mg L−1 solution was monitored for 1 h.
All spectra were acquired by averaging 500 scans at a resolution
of 4 cm−1 . Spectra evaluation was performed using the Bruker Opus
4.2 software.
3. Results and discussion
3.1. Zeta-potential measurements
The results of the zeta-potential measurements presented in
Fig. 2 illustrate that the isoelectric point (IEP) of the magnetite crystals in 10 mM aqueous NaCl (filled squares) was around pH 7, which
is similar to the values typically reported [16] for magnetite.
Addition of 3.3 mM calcium chloride made the magnetite surface positively charged in the whole pH range studied (open
squares). Furthermore, in the pH range between 4 and 7 the zetapotential of magnetite in 3.3 mM calcium chloride (open squares)
became more positive than the zeta-potential of magnetite in
10 mM NaCl (filled squares) indicating that the charge of the magnetite surface was affected by calcium ions even at pH below the
IEP for magnetite crystals in NaCl. At pH above the IEP, the acquired
positive net surface charge increased with increasing pH in the
range between pH 7.5 and 10 indicating that the affinity for calcium
Fig. 2. Zeta-potential of the magnetite crystals as a function of pH in 10 mM NaCl
(), 3.3 mM CaCl2 (), 10 mM NaCl and 1 mM Na2 SiO3 (), and 3.3 mM CaCl2 and
1 mM Na2 SiO3 ().
81
ions increased as the magnetite surface became more negatively
charged. Su [8] as well as Dixon [17] reported similar results for the
magnetite–calcium system and suggested that calcium ions reacted
with surface hydroxyls by the substitution of hydrogen ions for
Ca2+ , which increased the surface charge.
When 1 mM sodium metasilicate was added to the magnetite
dispersed in 10 mM NaCl (open triangles), the zeta potential shifted
to lower values in the whole investigated pH range, with the IEP
observed at pH 5.5. These observations are in agreement with
the recent study reported by Jolsterå et al. [18]. Based on results
obtained by potentiometric titrations, the authors suggested that
the negatively charged silicate surface complex FeOSiO(OH)2 −
starts to form by deprotonation already at pH below 5 and dominates at pH 7.0–9.8 giving the surface increased negative charge.
Pre-treatment of magnetite with both calcium ions and sodium
silicate (filled triangles) reduced the negative net surface charge in
the pH range between 6 and 10 as compared to silicate-modified
magnetite (open triangles in Fig. 2). This suggests that high concentrations of calcium in the process water (pH ∼ 8.5) can reduce the
efficiency of sodium silicate as dispersing agent due to decreased
electrostatic repulsion between the ore particles. However, in the
pH range between 3 and 6, calcium ions do not seem to have any
significant effect on the magnetite net surface charge since the values of the zeta-potential remained almost the same as in the case
when only sodium silicate was added to magnetite. Similar results
were reported by Dixon [17] for calcium adsorption on magnetite
coated with silica and were explained by the effect of solvation
energy. Adsorption from aqueous solutions is facilitated on surfaces
with dielectric constants close to that of water. Magnetite has much
higher dielectric constant than silica [19], consequently, less calcium is expected to adsorb on silica modified magnetite. However,
in the present work, magnetite particles were brought in contact
with calcium and silicate ions simultaneously and similar adsorption behaviour as described by Dixon was observed suggesting that
adsorption of silicate species on magnetite prevails over adsorption
of calcium at pH 3–6, most likely due to electrostatic interactions,
and that calcium ions do not adsorb on silicate species attached to
the magnetite surface.
Upon the decrease of silicate concentration from 1 mM to
0.4 mM while keeping the concentration of calcium ions (open diamonds in Fig. 3a) constant, the IEP of the magnetite surface shifted
to slightly higher pH and the net surface charge above the IEP
became less negative as compared to the case with 1 mM of silicate (filled diamonds in Fig. 3a) suggesting that the effect of silicate
on the surface charge was partly suppressed by calcium.
Another observation was that the zeta-potential of the magnetite surface in calcium–silicate solutions started to get less
negative above pH 9 (filled and open diamonds in Fig. 3a). The reason for that could be decreased silicate adsorption which is known
to go through a maximum around pH 9 [18,20,21] and at the same
time an increased positive contribution from calcium ions.
When the maleic acid ester was added to the calcium–silicate
mixture (open squares in Fig. 3a), the zeta-potential became more
negative, probably due to adsorption of the surfactant anions
onto calcium attached to the magnetite surface (ternary adsorption) partly compensating for its positive charge. Additionally,
the hydrophobic chain–chain interaction between the surfactant
molecules could result in formation of a bi-layer on the surface
with the head groups of the surfactant species in the second layer
oriented towards the solution and introducing additional negative
charge to the surface [22].
It is important to mention here that the magnetite dispersion
containing surfactant could not be analyzed at pH below 5.5 due
to the fact that the amount of particles in the sample was higher
than the tracking limit of the CCD camera. The observed particles
were smaller in size than the magnetite particles and most likely
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E. Potapova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 79–86
82
ions were added to the surfactant solution (filled triangles) the
charge became much less negative and independent of pH indicating the interaction between calcium and surfactant in solution.
Furthermore, the charge of surfactant–calcium species in solution
was rather similar to the charge of the magnetite particles treated
with calcium, silicate and surfactant (filled triangles in Fig. 3b and
empty squares in Fig. 3a, respectively) supporting the proposed
mechanism of surfactant adsorption on magnetite via calcium ions.
3.2. Contact angle measurements
Fig. 3. Zeta-potential of the magnetite crystals as a function of pH in 3.3 mM CaCl2
and 1 mM Na2 SiO3 (), 3.3 mM CaCl2 and 0.4 mM Na2 SiO3 (♦), and 3.3 mM CaCl2 ,
0.4 mM Na2 SiO3 and 25 mg L−1 maleic acid ester () (a); zeta-potential of the maleic
acid ester (no magnetite crystals) in a 15 mg L−1 aqueous solution containing 10 mM
NaCl (), and 10 mM NaCl and 2.4 mM CaCl2 () (b).
were the result of precipitation of the surfactant in solution as pH
decreased.
Fig. 3b shows the zeta-potential of the maleic acid ester in aqueous solution not containing magnetite particles. The fact that it was
possible to measure the zeta-potential of the surfactant dissolved
in water indicates that it was significantly aggregated. Surfactant
concentration used in this work was above the critical micelle concentration so the presence of micelles and possibly even larger
aggregates in solution could be expected. Surfactant aggregates
in a 10 mM aqueous solution of NaCl were found to be highly
negatively charged at pH 4–11 (empty triangles) indicating that
the head groups of surfactant species in aggregates were deprotonated and oriented towards the solution. Less negative charge
was observed below pH 6, probably due to protonation of surfactant anions since the dissociation constant (pKa) for this type of
molecules could be expected to be about 3.5 [23]. When calcium
Table 1 illustrates how consecutive conditioning with calcium
ions, sodium silicate and the maleic acid ester at pH 8.5 affected the
hydrophilic properties of the magnetite film.
The water contact angle of the as-prepared magnetite film was
determined to be slightly above 20◦ , which is rather close to the
value of 25 ± 5◦ previously reported by Wang and Ren [3] for magnetite in distilled water. Having a high concentration of acid and
base sites [24] contributing to the polar component of the Gibbs
energy according to the van Oss theory [25], iron oxides are commonly considered to be hydrophilic and thus not floatable without
collector.
In the present work, treatment of the magnetite film with calcium chloride lowered the water contact angle by ca. 3◦ . According
to a proposed mechanism [8,17], calcium ions can react specifically with surface hydroxyls releasing protons and adding positive
charge to the surface. In other words, adsorption of calcium ions
reduces the amount of surface hydroxyl groups and at the same
time increases the amount of unsaturated Lewis acid sites on the
surface. Gentleman and Ruud [26] recently showed that dehydration of the metal oxide surface decreases the water contact angle
of the surface due to the fact that metal–water interactions are
stronger than hydrogen bonding between the surface hydroxyls
and water. Similarly, when hydrogen in the hydroxyl groups is substituted with calcium, the polar contribution to the Gibbs energy
is expected to increase, interaction with water becomes stronger
thus increasing surface hydrophilicity and decreasing the contact
angle.
Subsequent adsorption of sodium silicate in the present work
lowered the water contact angle on the magnetite film even further,
to at least 10◦ . Decreased contact angle could possibly be a result
of the increased amount of hydroxyl groups [27] on the magnetite
surface introduced by the silicate species adsorbed. Deprotonation
of the silicate surface complexes as proposed by Jolsterå et al. [18]
would further increase surface polarity and consequently enhance
surface interaction with water reducing the water contact angle
and making the magnetite surface more hydrophilic.
Adsorption of the maleic acid ester increased the contact angle
between water and the magnetite surface to 40–50◦ , see Table 1.
For alkyl sulfonate with a hydrocarbon chain containing 12 carbon atoms (as the surfactant used in this work) the reported
contact angle of water on alumina varied from less than 20◦ to
ca. 80◦ depending on the concentration of surfactant in solution
[22]. According to the results presented by the authors, a contact
angle of 40–50◦ corresponds to the beginning of the formation of
hemimicelles on the surface. This value of the contact angle would
probably not be sufficient to facilitate flotation of magnetite since
Table 1
Contact angle of the as-synthesized magnetite film and magnetite film after consequent conditioning with calcium ions, sodium silicate and maleic acid ester. The background
electrolyte was 10 mM NaCl. The given values were measured 1 s after a drop of water was deposited on the surface and are presented as an average value ± one standard
deviation.
a
Treatment
As-synthesized magnetite
4 mM CaCl2
0.4 mM Na2 SiO3
25 mg L−1 maleic acid ester
Contact angle,◦
22 ± 3◦
19 ± 2◦
≤10◦ a
44 ± 3◦
The exact value of the contact angle could not be estimated since 5 out of 8 measurements after silicate adsorption were below detection limit of the instrument (10◦ ).
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E. Potapova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 79–86
in order to have natural floatability the mineral should have a contact angle above 60◦ [4]. However, the hydrophobic areas formed
by surfactant adsorbed on the magnetite surface may facilitate the
attachment of air bubbles and incorporation of these bubbles inside
the green pellets produced from the flotated magnetite concentrate
by wet agglomeration. Inclusions of air in the pellets have been
shown to lower pellet strength both in wet and dry state [10–12]
as discussed above.
It is important to mention that the variation of the contact angle
observed in the present work within the same sample was likely a
result of surface heterogeneity [28] including geometrical heterogeneity or surface roughness and chemical heterogeneity which is
enhanced by uneven distribution of adsorbate on the surface, e.g.,
by patchy adsorption of the surfactant [29].
3.3. ATR-FTIR spectroscopy
3.3.1. Adsorption of sodium silicate
The spectra of silicate species after 20 min of adsorption onto
magnetite at pH 7.5–9.5 in the presence of calcium ions are shown
in Fig. 4a. The broad absorption band observed in the spectra at
1200–800 cm−1 is associated with the Si–O stretching vibrations
and is commonly reported [30] for silicate species adsorbed on
Fig. 4. ATR-FTIR spectra of the silicate species on magnetite after 20 min of adsorption at pH 7.5–9.5 after pre-adsorption of calcium at the same pH (a), and integrated
absorbance of the silicate band between 1250 and 800 cm−1 as a function of adsorption time at pH 7.5 (♦), 8.5 () and 9.5 () (b). The background electrolyte was 10 mM
NaCl. The dotted lines in Fig. 5a indicate the shift of the absorption band upon the
increase of pH.
83
iron oxides. The band observed at ca. 950 cm−1 is assigned to the
monomeric surface silicate species whereas the bands observed
at 1200–1000 cm−1 are assigned to the oligomeric and polymeric
silicate species with increasing degree of polymerization as the
bands are shifted to higher wavenumbers [30]. The intensity of the
observed bands increases with increasing pH, see Fig. 4a, which
indicates that more silicate is adsorbed on the magnetite surface
at higher pH. The band assigned to the oligomeric silicate species
in the spectra recorded at pH 8.5 and 9.5 had the highest intensity and was shifted to higher wavenumbers as compared to the
same band observed in the spectrum recorded at pH 7.5 (the shift
is indicated by the dotted lines in Fig. 4a). This implies that more
oligomerized species were present on the magnetite surface at pH
8.5–9.5 than at pH 7.5 probably due to the higher surface loading
of silicate species at higher pH facilitating surface polymerization.
The adsorption of inorganic anions from aqueous solutions represents usually a two-step process with a fast and a slow stage [16];
the latter is often described by the Elovich equation [31]:
=
1
1
ln(˛ˇ) + ln(t)
ˇ
ˇ
(1)
where ˛ and ˇ are constants, t is time, and is the surface coverage
at the time t.
Upon the reasonable assumptions that the amount of silicate species adsorbed on the magnetite surface is proportional to
the integrated absorbance of the Si–O stretching vibration band
between 1250 and 800 cm−1 in the spectra (Fig. 4a), the surface coverage in the Elovich equation can be replaced by the integrated
absorbance and plotted as a function of ln(t) (Fig. 4b).
The obtained linear dependences of the integrated absorbance
vs. ln(t) in Fig. 4b indicate that the experimental data were in good
agreement with the Elovich equation (with a regression coefficient
R2 > 0.995 for all the pH studied).
Considering the amount of silicate adsorbed after 5 min (the
data points in the plot at ln(t) = 1.6), Fig. 4b shows that the adsorption increased with increasing pH, in concert with the previously
reported findings for silicate adsorption on maghemite [18] and
goethite [20,21] without pre-treatment of iron oxides with calcium chloride. The results obtained in the present work indicate
the same tendency of increased silicate adsorption onto magnetite
and that calcium ions did not change this tendency. As a result of
calcium ions adsorbing on the magnetite surface, the positive net
surface charge increased with increasing pH in the range 7.5–9.5
(see Fig. 2) resulting in an increased attraction between the magnetite surface and the negatively charged silicate species that start
to form in solution above pH 7 [32]. Additionally, surface precipitation of calcium silicate which increases with increasing pH [33],
may contribute to silicate loading on the magnetite surface.
The fact that the plots for pH 8.5 and 9.5 shown in Fig. 4b are
almost parallel (the slopes are 2.44 and 2.64, respectively) suggests
that pH did not have significant effect on the slow stage of silicate
adsorption onto magnetite in this pH range. However, the slope of
the plot at pH 7.5 is considerably lower (viz. 1.46) indicating that
adsorption at this pH proceeds with a lower rate. At this pH, the
surface charge density approaches zero, likely facilitating coagulation of the particles in the magnetite film. Coagulation would result
in decreased surface area available for adsorption, i.e. decreased
number of available surface sites, and consequently, would affect
the amount adsorbed.
3.3.2. Adsorption of the anionic surfactant
Fig. 5 shows the results from the adsorption of maleic acid ester
onto a magnetite film at different pH values followed by ATR-FTIR
spectroscopy. The pH was gradually decreased starting at pH 10.
The surfactant was allowed to adsorb for 5 h at each pH to allow the
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E. Potapova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 79–86
Fig. 5. ATR-FTIR spectra of the maleic acid ester on magnetite after 5 h of adsorption
from a 25 mg L−1 solution at pH 4, 6, 8, 8.5 and 10 (top to bottom) (a), and absorbance
of the carbonyl stretching vibration band originating from the maleic acid ester
adsorbed on magnetite at different pH (b). The background electrolyte was 10 mM
NaCl.
adsorption approach equilibrium. Assignment of the main absorption bands in the spectra in Fig. 5a is presented in Table 2 and
discussed in detail in our previous work [15].
The data in Fig. 5 suggest that the amount of surfactant on the
magnetite surface increased with decreasing pH, likely since the
surface charge first becomes less negative and then turns positive with decreasing pH (see Fig. 2) thus making the surface more
electrostatically favourable for the adsorption of the negatively
charged deprotonated surfactant species. Another possible explanation could be an increased precipitation of the surfactant on the
magnetite surface with decreased pH since the solubility of the
surfactant is reduced as pH gets more acidic.
The increase in intensity of the bands originating from the surfactant adsorbed on magnetite is accompanied by the increase of
the negative absorption bands at 1630 cm−1 and 1487 cm−1 originating from the bending vibrations of water [34] and asymmetric
stretching vibrations of the carbonate species [35], respectively.
Table 2
Assignments of the main absorption bands originating from the maleic acid ester
adsorbed on magnetite. The background electrolyte was 10 mM NaCl.
Peak position, cm−1
Assignment
1566, 1394
1721
1098
␯as (COO− ), ␯s (COO− ) in carboxylic acid [36]
␯(C O) in ester [37]
␯(C–O–C) in the polyethylene glycol chain [38]
Fig. 6. ATR-FTIR spectra of the maleic acid ester on magnetite after 1 h of adsorption
from a 25 mg L−1 solution at pH 7.5–9.5 after pre-adsorption of calcium and silicate
at the same pH (a), and absorbance of the carbonyl stretching vibration band originating from the maleic acid ester adsorbed at pH 7.5 (♦), 8.5 () and 9.5 () on
magnetite pre-treated with calcium ions and sodium silicate at the same pH (b).
The background electrolyte was 10 mM NaCl.
The presence of these bands in the spectra suggests that both water
and the carbonate species are partly removed from the surface
as surfactant loading increases. The displacement of water further
indicates that the surface gets more hydrophobic upon surfactant
adsorption, in concert with the contact angle results presented in
Table 1.
The point at about pH 8 in Fig. 5b diverges slightly from the trend
marked by the dotted line. Similar results illustrating an increased
surfactant adsorption in the neutral pH region were previously
reported by Morgan et al. [39] for oleate adsorption on hematite
and were explained by the formation of an acid soap complex
[(RCOO)2 H]− [40]. However, no evidence of the presence of this
type of aggregate has been found for the surfactant used in this work
suggesting that further experiments should be performed before a
plausible explanation for such behaviour can be given.
The spectra recorded after 1 h of surfactant adsorption at different pH values onto magnetite pre-treated with calcium ions and
sodium silicate are shown in Fig. 6a. The spectra in Fig. 6a are rather
similar to the ones shown in Fig. 5a, except for an additional band at
about 1030 cm−1 arising from the silicate species which continued
to adsorb on magnetite even after surfactant addition.
Fig. 6b shows the intensity of the carbonyl stretching vibration
band in the spectra of surfactant adsorbed on magnetite at different pH values as a function of time. The band intensity after 1 h
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E. Potapova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 79–86
of adsorption at pH 8.5 was higher than after adsorption at pH
7.5 or pH 9.5 (Fig. 6b) for several repeated measurements indicating that there would be a maximum in surfactant adsorption
on magnetite around pH 8 also when the magnetite surface had
been modified with calcium and silicate solutions. Fig. 6b suggests
that more surfactant was adsorbed at pH 9.5 than at pH 7.5, which
is opposite to the results obtained for surfactant adsorption on assynthesized magnetite and shown in Fig. 5b. Such behaviour cannot
be explained by the zeta-potential of the magnetite particles since
the charge of the magnetite surface with calcium and silicate is
rather constant and increases only slightly above pH 9 (diamonds in
Fig. 3a). It is more likely that adsorption increases due to high affinity of the surfactant towards calcium ions, which were expected
to be present on the magnetite surface in larger amount at pH
9.5 than at pH 7.5 in accordance with the zeta-potential results in
Fig. 2 (open squares) for calcium-treated magnetite. Another reason
could be the increased calcium-collector precipitation at higher pH.
Considering the fact that sodium silicate has previously been
shown to reduce flotation collector adsorption on iron oxides
[7,41,42], adsorption of the maleic acid ester could be anticipated to
decrease with increasing pH due to higher silicate adsorption. Furthermore, conditioning time with silicate could be expected to have
an impact on the amount of surfactant adsorbed afterwards. However, variation of the conditioning time with silicate in the range of
5–20 min at pH 7.5, 8.5 and 9.5 did not have any considerable effect
on the amount of the maleic acid ester adsorbed on magnetite at
any of the pH values (not shown), which accentuates the importance of the initial fast stage of the silicate adsorption. Together
with the observed increase in surfactant adsorption with increasing pH, these findings suggest that the maleic acid ester and silicate
probably adsorbed independently on different surface sites: silicate
species mainly on magnetite surface hydroxyls and the surfactant
mainly on the calcium ions adsorbed on magnetite. The presented
results also support the conclusion made in our previous work [15]
that in the presence of calcium ions adsorption of sodium silicate
does not affect the amount of the maleic acid ester adsorbed on
magnetite to any considerable extent at pH 8.5 used in the flotation
of apatite from magnetite.
4. Conclusions
ATR-FTIR spectroscopy in combination with zeta-potential and
contact angle measurements proved to be a powerful tool for studying simultaneous adsorption of several species from solution onto
mineral surfaces. In the present work, adsorption of sodium silicate
and maleic acid ester on magnetite in the presence of calcium ions
was investigated at different pH values.
Whereas calcium ions cannot be directly detected on the magnetite surface by ATR-FTIR spectroscopy, the zeta-potential results
provided evidence for specific adsorption of calcium on magnetite.
Adsorption of calcium ions reduced the negative charge of the
magnetite surface treated with silicate, suggesting that high concentrations of calcium in process water could have an adverse effect
on the dispersing performance of sodium silicate in flotation.
Adsorption of sodium silicate on magnetite pre-treated with calcium chloride increased with increasing pH in the range 7.5–9.5.
Subsequent surfactant adsorption in the same pH range was the
highest at pH 8.5 and was not very much affected by the amount of
silicate adsorbed on magnetite under the conditions studied suggesting that in the presence of calcium ions, silicate and collector
adsorbed independently on different surface sites. According to the
zeta-potential results, surfactant was likely adsorbed on magnetite
through calcium ions.
Whereas treatment with calcium chloride and sodium silicate
decreased the contact angle of the magnetite surface, subsequently
85
adsorbed surfactant species made the magnetite surface partly
hydrophobic. Although this degree of hydrophobicity is unlikely to
have a negative effect on the reverse flotation of iron ore, flotation
collector species adsorbed on the magnetite surface could cause
inclusion of air bubbles inside the green pellets produced by wet
agglomeration of iron ore thereby lowering the strength of the pellets. In forthcoming work, methods to improve the wettability of
magnetite surfaces contaminated with a flotation collector will be
investigated.
Acknowledgements
This is a contribution by Centre of Advanced Mining and Metallurgy. The financial support by the Hjalmar Lundbohm Research
Centre and Luossavaara-Kiirunavaara Aktiebolag (LKAB) is gratefully acknowledged. The authors would like to thank M.Sc. D.
Sammelin from Umeå University for performing the ATR-FTIR measurements as a part of her Master thesis project.
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[36] K.D. Dobson, A.J. McQuillan, In situ infrared spectroscopic analysis of the
adsorption of aliphatic carboxylic acids to TiO2 , ZrO2 , Al2 O3 , and Ta2 O5 from
aqueous solutions, Spectrochim. Acta. A: Mol. Biomol. Spectrosc. 55 (1999)
1395–1405.
[37] N.B. Colthup, L.H. Daly, S.E. Wiberly, Introduction to Infrared and Raman Spectroscopy, 3rd edn, Academic Press, London, 1990.
[38] P.C.J. Beentjes, J. Van Den Brand, J.H.W. De Wit, Interaction of ester and acid
groups containing organic compounds with iron oxide surfaces, J. Adhes. Sci.
Technol. 20 (2006) 1–18.
[39] L.J. Morgan, K.P. Ananthapadmanabhan, P. Somasundaran, Oleate adsorption on hematite: problems and methods, Int. J. Miner. Process. 18 (1986)
139–152.
[40] K.P. Ananthapadmanabhan, P. Somasundaran, Acid-soap formation in aqueous
oleate solutions, J. Colloid Interface Sci. 122 (1988) 104–109.
[41] W.Q. Gong, C. Klauber, L.J. Warren, Mechanism of action of sodium silicate
in the flotation of apatite from hematite, Int. J. Miner. Process. 39 (1993)
251–273.
[42] P. Roonasi, X. Yang, A. Holmgren, Competition between sodium oleate and
sodium silicate for a silicate/oleate modified magnetite surface studied by
in situ ATR-FTIR spectroscopy, J. Colloid Interface Sci. 343 (2010) 546–552.
PAPER IV
The effect of polymer adsorption on the wetting properties
of partially hydrophobized magnetite
E. Potapova, M. Grahn, A. Holmgren, and J. Hedlund
Submitted to Journal of Colloid and Interface Science
The effect of polymer adsorption on the wetting properties of partially hydrophobized
magnetite
*
E. Potapova, M. Grahn, A. Holmgren, and J. Hedlund
Division of Sustainable Process Engineering, Luleå University of Technology, SE-971 87 Luleå,
Sweden
* To whom correspondence should be addressed: [email protected]; tel.: +46 920 491776;
fax: +46 920 491199.
Abstract
Upon reverse flotation of iron ore, the surface of the iron ore concentrate may become partially
hydrophobized due to adsorption of flotation collector, which is facilitated by the calcium ions
present in the process water. Hydrophobic areas on the concentrate surface may introduce problems
in subsequent pelletization of the concentrate. A possible way to restore the wettability of the
surface could be by modifying the surface with a hydrophilic polymer. The effect of hydrophilic
polymers of different types, viz. cationic, anionic, and non-ionic, on the wettability of the magnetite
surface after adsorption of a surfactant was investigated. Although all the polymers could adsorb on
magnetite at pH 8.5, the contact angle measurements revealed that only anionic ammonium
polyacrylate could decrease the contact angle of synthetic magnetite after surfactant adsorption to a
level close to that of as-synthesized magnetite. Such effect was probably achieved due to shielding
of the hydrophobic surfactant chains from the aqueous phase by hydrophilic polyacrylate
molecules. The fact that polyacrylate adsorption on magnetite occurred via calcium ions makes
polyacrylate suitable for application in calcium-rich process water. The results presented in this
work illustrate that ammonium polyacrylate could be successfully used to improve the wettability of
magnetite after adsorption of surfactants.
Keywords
Adsorption, ATR-FTIR, contact angle, magnetite, polymer, wettability
1
1. Introduction
Hydrophilic properties of iron oxides are important for several industrial applications including
production of pigments, ferrofluids, and iron ore pellets. Pelletization of iron ore concentrates
cleaned by flotation has always been recognized as a potential problem [1-3] due to a combination
of decreased surface tension in the process water and increased hydrophobicity of the iron oxide
surface upon adsorption of flotation collector.
Numerous efforts have been made to minimize collector coating on iron oxides by either reducing
collector adsorption during reverse flotation or desorbing collector species from the iron oxide
surface after flotation. It has been suggested that collector adsorption could be reduced by removal
of calcium and magnesium ions [4-10] naturally present in the process water and known to increase
contamination of iron oxides with collector. However, water softening becomes a challenge in the
case of sparingly soluble calcareous gangue minerals that release calcium to the process water upon
partial dissolution.
Another way to reduce collector adsorption on iron oxide is to use depressants [5, 7, 11-16].
Whereas depressants make the mineral surface hydrophilic enough to prevent the mineral from
floating, the surface is not necessarily completely free from surfactant.
Once a flotation collector is adsorbed on the iron oxide surface, it has been proven difficult to
eliminate it by simple acidic or basic treatment [1]. To remove hydrophobic collector coatings from
the iron oxide surface various methods were proposed [1, 17].
In the present study, we propose an alternative way of improving wettability of the iron ore
concentrate after flotation by modifying the surface with a hydrophilic organic polymer.
Organic polymers have previously been tested and used as binders in agglomeration [18, 19].
However, rather high dosage of a polymer may be required to completely substitute the binder (e.g.
bentonite), which might not be economically beneficial because of the normally high cost of
synthetic polymers. Accordingly, the efficiency of using low dosages of polymers to improve
wettability of the iron oxide surface prior to binder addition was investigated in the present work.
A polymer, which could be used for increasing magnetite wettability prior to agglomeration, should
satisfy the following requirements:
x
The polymer should be hydrophilic by nature and should contain functional groups that can
facilitate adsorption of the polymer on magnetite.
x
Polymer performance should not be impaired by calcium and magnesium ions.
x
The polymer should not contain environmentally and metallurgically harmful elements (such
as sulphur and phosphorus) and should not introduce other impurities to the final product.
x
The polymer should not impair interaction between magnetite and binder.
x
The polymer should be easy to handle implying, first of all, good solubility in water.
2
x
The price of the polymer should be reasonable in relation to the required dosage.
Based on the above mentioned requirements, three different types of polymers were chosen for the
study, viz. anionic polyacrylate salt, cationic aliphatic quaternary amine, and non-ionic water
soluble starch. Adsorption of the polymers on magnetite was studied in-situ using ATR-FTIR
spectroscopy and the effect of polymer adsorption on the wettability and zeta-potential of magnetite
was determined using contact angle and electrophoretic mobility measurements, respectively.
2. Materials and Methods
2.1. Materials
Magnetite nanocrystals were synthesized and purified according to the procedure described
previously [20]. The crystals had a spherical habit and the size of 5-15 nm [20]. A dispersion of
magnetite in methanol and degassed distilled water containing 25 vol. % methanol and ca
1.1 mg mL-1 magnetite was prepared and stored in a refrigerator in order to minimize oxidation of
magnetite.
Magnetite films for the spectroscopic and contact angle measurements were prepared by spreading
0.3 and 0.5 mL of the dispersion, respectively, over a ZnSe substrate. The dispersion medium was
then allowed to evaporate in air at room temperature. We have previously reported [20] that films
prepared by spreading 0.3 mL dispersion on a ZnSe substrate are even, 250-300 nm thick layers of
particles. The layers are porous and the pore size is comparable with the crystal size.
Stock solutions of calcium chloride (CaCl2·2H2O, 95 %, Riedel-de Han), sodium silicate
(Na2SiO3·9H2O, 98 %, Sigma) and dodecyloxyethoxyethoxyethoxyethyl maleate (Sigma-Aldrich)
used as a model flotation collector were prepared by dissolving the required amounts of the
corresponding chemicals in 10 mM aqueous solutions of NaCl (pro analysi, Riedel-de Han). Fig. 1
shows the chemical structure of the surfactant used as a model flotation collector.
Figure 1. Chemical structure of dodecyloxyethoxyethoxyethoxyethyl maleate. R represents the
linear alkyl chain CH3(CH2)11.
The polymers used in this work are presented in Table 1. Stock solutions of the polymers were
prepared using distilled water.
Distilled water used in the experiments was degassed under vacuum to minimize the amount of
dissolved gases. The pH of the working solutions was adjusted using aqueous solutions of sodium
hydroxide (NaOH, pro analysi, Merck) and hydrochloric acid (HCl, 37 %, pro analysi, Merck).
3
Table 1. Polymers used for magnetite surface modification.
Polymer name
Structural formula
Average
Supplier
molecular weight
Dispex A40 (ammonium
4000
BASF
50000
Eka chemicals
N/A
Merck
polyacrylate)
ATC 4150
(aliphatic quaternary
polyamine)
Soluble starcha
a
1 wt % starch solution in distilled water containing 0.5 wt % NaOH was heated to 84°C during
10 min and then cooled to room temperature [21].
2.2. Methods
2.2.1. ATR-FTIR spectroscopy
A Bruker IFS 66v/S spectrometer equipped with a liquid nitrogen cooled mercury-cadmiumtelluride (MCT) detector and a vertical ATR accessory was used for collecting infrared data. Both
single beam background and sample spectra were acquired by averaging 500 scans at a resolution of
4 cm-1. Spectra evaluation was performed using the Bruker Opus 4.2 software. A trapezoidal ZnSe
crystal (Crystran Ltd.), with the dimensions of 50x20x2 mm and 45° cut edges, was coated with a
magnetite film as described above and mounted in a stainless steel flow cell. In-situ adsorption
measurements were performed by pumping the working solution continuously through the cell at a
rate of 10 mL min-1 with recirculation. All the spectroscopic experiments were performed at pH 8.5
and room temperature, with 10 mM NaCl as a background electrolyte. The working solution was
continuously bubbled with argon during the experiment to minimize the amount of dissolved carbon
dioxide. The pH of the solution was controlled by a Mettler Toledo T70 titrator. Prior to adsorption,
the magnetite film was rinsed with a 10 mM NaCl solution at pH 8.5 for 30 min. The chemicals
were added to the working solution in the following sequence:
4 mM CaCl2 0.4 mM sodium silicate 25 mg L-1 surfactant 12.5 mg L-1 polymer.
Prior to addition of each solute, a new background spectrum was recorded. Adsorption of each
component was monitored for 2 h by recording infrared spectra with 5 min interval. Thereafter,
4
desorption of the adsorbed species from the magnetite surface was attempted by flushing the cell
with a 10 mM aqueous solution of NaCl at pH 8.5 for 25 min.
2.2.2. Contact angle
The static sessile drop method was used to determine the contact angle of the synthetic magnetite
nanoparticles. Contact angle measurements were done using a Fibro 1121/1122 DAT-Dynamic
Absorption and Contact Angle Tester equipped with a CCD camera. Magnetite films for the contact
angle measurements were prepared as described above. Prior to the contact angle measurement, the
magnetite film was rinsed with distilled water and dried in a vacuum desiccator for 30 min. The
measurement was performed by placing a water droplet with a volume of 4 μL onto the magnetite
film using a microsyringe. A series of images were captured and analysed using the DAT 3.6
software. The value of the contact angle was determined as an average of the values measured for 8
fresh droplets placed on the same film. Thereafter, consecutive adsorption of CaCl2, Na2SiO3,
anionic surfactant, and a polymer was performed on the magnetite film at the same concentrations
and conditions as in the spectroscopic measurements. Between the adsorption steps, the film was
rinsed with distilled water and dried in a vacuum desiccator before the contact angle was measured.
After the last measurement (polymer solution), the film was left in air for 24 h and then a new
measurement was performed.
2.2.3. Electrophoretic mobility
The zeta-potential of the magnetite nanoparticles and a polymer in solution was determined by
electrophoresis using a ZetaCompact instrument equipped with a charge-coupled device (CCD)
tracking camera. The electrophoretic mobility data was further processed by the Zeta4 software
applying the Smoluchowski equation. The samples containing magnetite were prepared in the
following way: one drop of the magnetite suspension was dispersed in 1 L of distilled water
containing 10 mM NaCl, 4 mM CaCl2, 0.4 mM Na2SiO3, 25 mg L-1 maleic acid ester and 25 mg L-1
ammonium polyacrylate. The zeta-potential of the polymer in aqueous solution was determined
using 12.5 mg L-1 ammonium polyacrylate dissolved in 10 mM NaCl. In order to investigate the
effect of calcium ions on the charge of the polymer in solution, the measurement was performed on
a solution containing 10 mM NaCl, 25 mg L-1 ammonium polyacrylate, and 4 mM CaCl2. The
samples spanned the pH range from 4 to 10. For each sample, the measurement was repeated three
times and the final zeta-potential was calculated as an average of the obtained values.
5
3. Results and Discussion
3.1. ATR-FTIR spectroscopy
To determine whether the polymers adsorbed on magnetite, adsorption experiments were performed
on films of synthetic magnetite and monitored by in-situ ATR-FTIR spectroscopy. Infrared spectra
of the polymers adsorbed on magnetite at pH 8.5 after pre-adsorption of CaCl2, Na2SiO3, and the
anionic surfactant are shown in Fig. 2.
We have previously reported that in the presence of calcium ions adsorption of carboxylate
surfactant and sodium silicate on magnetite at pH 8.5 occurred independently on different surface
sites [22]. Silicate could be expected to interact mainly with the hydroxyl groups on the magnetite
surface whereas the surfactant likely adsorbed on magnetite in the form of a ternary complex with
calcium ions. The zeta-potential of the magnetite particles in the presence of calcium chloride,
sodium silicate, and anionic carboxylate surfactant was negative and nearly constant in the pH
range 5-10.
The increase in intensity of the absorption bands in Fig. 2 with time indicates that the three
polymers, independent of their charge and functionality, adsorbed on magnetite modified with the
anionic surfactant.
Assignment of the major absorption bands originating from the vibrations of different groups in the
polymers adsorbed on magnetite is presented in Table 2. Apart from the bands attributed to the
polymers, absorption bands originating from the anionic surfactant (at 1728 and 1582 cm-1 in
Fig. 2a and 2c) and silicate (at 1026 cm-1 in Fig. 2a) are present in the spectra of cationic polyamine
and starch, implying that both silicate and surfactant continued to adsorb on magnetite after addition
of the polymers to the working solutions.
No pronounced bands associated with the surfactant species are found in the spectra of polyacrylate
in Fig. 2b suggesting that adsorption of the polymer in that case prevailed over surfactant
adsorption. Furthermore, negative intensity between 1100 and 900 cm-1 in Fig. 2b indicates that
polyacrylate caused desorption of silicate from the magnetite surface.
Normally, polymers carrying multiple charged groups (polyelectrolytes) adsorb on oppositely
charged surfaces but can also adsorb on the surfaces bearing the same type of charge providing that
the charge is not too high to hinder adsorption [23].
6
Figure 2. ATR-FTIR spectra of the polymers adsorbed onto magnetite at pH 8.5 for 0, 15, 30,
45, and 60 min: cationic aliphatic polyamine (a), anionic ammonium polyacrylate (b), and
7
starch (c). The single beam spectrum of pre-adsorbed CaCl2, Na2SiO3, and the anionic
surfactant served as background. Ionic medium: 10 mM NaCl.
In the cationic polymer used in the present work, quaternary ammonium cations bear a
permanent positive charge which is independent of pH. However, primary amine
functionalities in the ethylene diamine monomer can become protonated forming –NH3+
groups. The protonation constants (log KH1 and log KH2) for ethylene diamine at 25°C and
zero ionic strength are 9.91 and 6.86 [24], respectively, so assuming that ethylene diamine
monomers in the polymer exhibit similar deprotonation behaviour as pure ethylene diamine,
one could expect the monoprotonated ethylene diamine [NH2(CH2)2NH3+] to be the
predominant specie at pH 8.5.
Apart from the fact that adsorption of the cationic polymer on the negatively charged
magnetite is favoured by electrostatic forces [25-27], specific adsorption through -OH and
-NH2 groups could also occur. Hydrophobic interaction between the polymer and the
hydrocarbon chains of the surfactant species adsorbed on magnetite may contribute to
adsorption as well.
Table 2. Assignments of the main absorption bands originating from the polymers adsorbed
on magnetite in-situ at pH 8.5 in the presence of CaCl2, Na2SiO3 and the anionic surfactant.
Ionic medium: 10 mM NaCl.
Peak position, cm-1
Assignment
Cationic aliphatic polyamine [25, 28]
1481, 1470
(CH2), (CH3)
1200-950
(CH-OH), (CH-NH2)
Anionic ammonium polyacrylate [29]
1555, 1410
as(COO-), s(COO-)
1731
(C=O) hydrogen bonded
1678
(C=O) in monodentate configuration
1457
(CH2)
Starch [30]
1153
(C-O-C) in glucosidic linkage
1081, 1026
(C-O) coupled with (C-C) and (O-H)
8
For ammonium polyacrylate, the pKa is strongly dependent of the overall dissociation degree
(
), especially at low ionic strength, since removal of a proton is hampered by the negative
potential of the polyanion [23]. The pKa of polyacrylic acid (MW = 2000 Da) at 25°C and
zero ionic strength when 1 was reported to be 6.95 ± 0.01 [31]. Accordingly, polyacrylate
at pH 8.5 could be expected to be nearly fully deprotonated.
In concert with previous studies [32, 33], the negatively charged polyacrylate was found to
adsorb on negatively charged magnetite surface indicating that forces other than Coulomb
interaction caused the adsorption. The intense bands at 1555 and 1410 cm-1 in the spectra of
polyacrylate adsorbed on magnetite (Fig. 2b) emanate from the asymmetric and symmetric
stretching vibration of the deprotonated carboxylic group, respectively, with = 145 cm-1
which is comparable to = 141 ± 4 cm-1 for calcium polyacrylate in solution [29]. It
indicates that polyacrylate was adsorbed on magnetite mainly via calcium ions as suggested
by Jones et al. [29]. According to Vermöhlen [34], the presence of calcium ions greatly
increases polyacrylate adsorption on oxide surfaces and makes the structure of the adsorbed
molecules more coiled due to the ability of calcium ions to stabilize coils through
intramolecular bridging. The presence of two weak bands at 1731 and 1678 cm-1 in the
spectra of polyacrylate adsorbed on magnetite (Fig. 2b), originating from the stretching
vibrations of the C=O bond, implies that some of the polyacrylate species were adsorbed via
hydrogen bonding and as a monodentate complex, respectively [29]. The band at 1731 cm-1
could also indicate that the carboxylic groups in the polymer were not fully deprotonated [35].
Considering adsorption of starch, no significant shifts of the absorption bands were observed
as compared to their position in the spectrum of non-adsorbed starch reported in the literature
[30]. Two mechanisms are commonly suggested for starch adsorption on metal oxides:
surface complexation with metal sites or hydrogen bonding with surface hydroxyls [30, 36].
In the present study, both mechanisms would be possible due to the presence of different
surface sites on the magnetite surface after the adsorption of calcium chloride, sodium silicate,
and the surfactant.
A negative absorption band at around 1640 cm-1 present in the spectra in Fig. 2 (more clearly
observed in the spectra in Fig. 2a and 2c) originates from the bending vibration of water [25].
The fact that the negative intensity of the band increased with time suggests that water was
removed from the surface upon adsorption of the polymers. Considering the intensity of this
band for different polymers, the intensity decreased in the sequence cationic polyamine >
starch > anionic polyacrylate, suggesting that the cationic polyamine caused the largest
9
displacement of water from the magnetite surface. Displacement of water implies an increased
surface hydrophobicity upon polymer adsorption.
Desorption of the polymers from the magnetite surface was performed using a 10 mM
aqueous solution of NaCl at pH 8.5, pumped through the cell without recirculation. No new
single beam background spectra were recorded prior to desorption. Fig. 3 shows the spectra
obtained by subtraction of the spectra recorded after 25 min of desorption from the spectra
recorded after 2 h of adsorption. The resulting spectra in all the three cases contained similar
absorption bands, independent of the polymer used, indicating that the desorbed species were
mainly those adsorbed prior to polymer addition, viz. surfactant (the bands at 1718, 1562,
1466, 1426, 1349, and 1102 cm-1) and silicate (the bands at 1022 and 953 cm-1). This
conclusion was supported by the fact that a similar spectrum was previously obtained by
spectral subtraction for the desorption of surfactant adsorbed in the presence of calcium
chloride and sodium silicate [20]. Thereby, the desorption results suggest that the adsorbed
polymers had high affinity to the magnetite surface.
Figure 3. ATR-FTIR spectra obtained by subtraction of the spectra recorded after 25 min of
desorption from the spectra recorded after 2 h of adsorption of polyacrylate (a), starch (b) and
cationic polymer (c). Desorption was performed by flushing with 10 mM NaCl at pH 8.5.
10
The shape and the position of the bands at 1800-1050 cm-1 in spectrum (a) in Fig. 3 are
slightly different from those in spectra (b) and (c) indicating that ammonium polyacrylate was
probably partially removed from the magnetite surface upon flushing with a 10 mM aqueous
NaCl solution. The desorption could be caused by depletion of calcium ions in solution upon
flushing with aqueous NaCl that resulted in repulsion of polymer molecules from each other.
However, the intensity of the bands in spectrum (a) in Fig. 3 is much lower than the intensity
of the bands in the last spectrum in Fig. 2b, suggesting that polyacrylate desorbed only to a
little extent.
The intensity of the bands originating from the silicate species in spectrum (a) is considerably
lower than the intensity of these bands in spectra (b) and (c), implying that the silicate species
were desorbed from the surface already during polymer adsorption and, accordingly, less
silicate left the surface when flushing with aqueous NaCl.
3.2. Contact angle of magnetite
Since all the three polymers were found to adsorb on magnetite, the effect of the polymers on
the wettability of the surfactant-coated synthetic magnetite particles was further investigated
using the static sessile drop method. The results are presented in Table 3 as an average of 8
replicates ± one standard deviation. The value of the contact angle for each water droplet was
collected one second after the droplet was placed on the surface. Since the flotated pellet feed
is normally stored for a certain period of time prior to agglomeration [37], the contact angle of
the magnetite film treated with a polymer was measured twice – directly after polymer
adsorption and after storage in air for 24 h.
Table 3 illustrates that the wettability of the magnetite particles was significantly reduced by
surfactant adsorption as compared to the wettability of as-synthesized magnetite and
magnetite after adsorption of calcium ions and sodium silicate, in agreement with the results
reported previously [22]. Whereas the resulting contact angle after surfactant adsorption
would probably not be high enough to facilitate flotation of magnetite, the adsorbed surfactant
species could facilitate attachment of air bubbles to the magnetite particles and incorporation
of air bubbles inside the iron ore pellets during agglomeration.
Adsorption of the polymers affected the contact angle very differently, most likely due to the
nature of the polymers. Treatment with cationic polyamine significantly increased
hydrophobicity of the magnetite surface. Such behaviour could possibly be explained by
orientation of the hydrophilic –OH and –NH3+ groups in the adsorbed polymer towards the
magnetite surface. In that case, the hydrophobic hydrocarbon chain of the polymer would be
11
exposed to water resulting in a high contact angle. The drop of the contact angle upon storage
in air could be caused by the alteration of the polymer conformation on the surface.
Table 3. Water contact angle of the as-synthesized magnetite and magnetite after sequential
adsorption of calcium chloride, sodium silicate, and surfactant, followed by treatment with a
polymer and storage in air for 24 h. The table shows three independent sets of measurements.
Treatment
As-synthesized magnetite
Contact angle (degrees)
14 ± 3
22 ± 3
2+
14 ± 2
a
Ca , Na2SiO3
10
Surfactant
44 ± 6
44 ± 3
49 ± 4
Polymer
Cationic aliphatic
Anionic ammonium
Starch
polyamine
polyacrylate
68 ± 2
24 ± 6
40 ± 4
49 ± 11
20 ± 4
46 ± 3
24 h in air
a
The contact angle could not be measured since it was below the detection limit of the
instrument (10°).
Treatment with starch decreased the contact angle of the surfactant-coated magnetite film to
ca 40°. Similar value was reported by dos Santos and Oliveira [15] for hematite after starch
adsorption. However, after 24 h in air the contact angle increased again, probably due to rearrangement of surfactant and starch in contact with the hydrophobic environment.
Adsorption of ammonium polyacrylate decreased the contact angle of the magnetite film
almost to the value of pure magnetite. Furthermore, the low contact angle was preserved even
after the film was kept in air for 24 h. Since no negative bands originating from surfactant
species were observed in the spectra of polyacrylate on magnetite (Fig. 2b), the decrease of
the contact angle was likely not due to surfactant desorption. Similar phenomenon was
observed by Somasundaran and Cleverdon [26] for amine/cationic PAM adsorption on quartz.
The authors reported that flotation of quartz was depressed by the adsorption of the polymer
whereas adsorption of the amine collector remained unchanged. The authors attributed such
behaviour to the masking of the adsorbed collector molecules by the massive polymer chains.
Similarly, the ability of polyacrylate to form loops, especially in the presence of calcium ions
[34], could facilitate adsorption of the polymer on the magnetite surface sites free from
surfactant and at the same time shield hydrophobic surfactant moieties from the water phase.
12
The high density of the carboxylic groups in the polyacrylate chain renders it highly
hydrophilic, resulting in an improved wettability of the magnetite surface.
It is important to point out that the effect of the polymers on the wettability of magnetite was
in agreement with the spectroscopic results illustrating water displacement from the magnetite
surface upon polymer adsorption. As discussed above, the negative absorption band of water
had the highest intensity in the spectra of the cationic polyamine. Accordingly, adsorption of
this polymer resulted in the highest contact angle of the magnetite surface. As the spectra of
ammonium polyacrylate on magnetite had the least intense negative water bands, treatment
with polyacrylate resulted in the most hydrophilic magnetite surface. These observations
suggest that the signal from water in the ATR-FTIR spectra recorded in-situ can be used to
predict the change of surface wettability.
Since ammonium polyacrylate was the only polymer that could restore magnetite wettability
after surfactant adsorption, this polymer was investigated further. In order to find out whether
the polymer could penetrate inside the magnetite particle film, contact angle, drop area, and
drop volume were analyzed as a function of time, see Fig. 4.
The contact angle of the magnetite surface prior to polymer adsorption decreased fast during
the first second and thereafter continued to decrease slowly throughout the measurement. On
the contrary, the drop area increased fast initially and then almost levelled off indicating that
the drop was slightly spreading on the surface at the beginning of the measurement. In
addition, the drop volume was almost constant suggesting that no penetration of water into
pores occurred and the decrease of the contact angle was solely due to water spreading on the
magnetite surface. According to Shang et al. [38], decreasing contact angle of polar liquids
like water could be a result of hydration and polar acid-base interactions between the wetting
liquid and different functional groups present on the surface. In the present work, apart from
the hydroxyl groups on the magnetite surface, water could interact with adsorbed species such
as calcium ions and silicate since these species enhance surface wettability [22]. Additionally,
surfactant head groups oriented towards the water phase could contribute to surface wetting.
After polyacrylate adsorption, the decrease of the contact angle during the first second
became much larger ( = 29° and 11° for polymer and surfactant, respectively), accompanied
by a significant increase in the drop area, which continuously expanded during the first 4 s
and then stabilized. The decrease in the contact angle during the first 4 s was probably due to
ionization of carboxylic groups in the polymer in the presence of water enhancing interaction
of the surface with water. No penetration of water into the pores took place since the drop
13
volume remained almost constant. However, after 4 s, the drop volume started to decrease
rather fast and dropped down even faster after 8 s.
Figure 4. Contact angle, drop area, and drop volume measured on the surface of the
magnetite film after consecutive adsorption of calcium ions, sodium silicate, and maleic acid
ester (¡), and ammonium polyacrylate ().
14
At the same time, the drop area decreased, implying that water at the droplet boundary
penetrated into the pores. Decreasing droplet volume indicates that water could penetrate into
the film, implying that polymer adsorption occurred even in the pores. In summary, the results
presented in Fig. 4 indicate that the wetting of the magnetite surface was significantly
improved by adsorption of the polymer.
The effect of pH on the contact angle of the magnetite surface was investigated using
DISPEX N40 – the sodium form of the polyacrylate used in the experiments discussed above.
The effect of sodium polyacrylate on the wettability of magnetite was similar to that of
ammonium polyacrylate, namely, the contact angle was decreased by ca 20° as compared to
the contact angle after surfactant adsorption.
The stepwise increase of pH from 8.5 to 9.5 and then to 10.5 during polymer adsorption did
not have any considerable effect on the contact angle measured after adsorption at each pH,
indicating that the hydrophilizing effect of the polymer was present also at alkaline
conditions.
Finally, as a control experiment, to illustrate the importance of using a polymer to render
magnetite hydrophilic after surfactant adsorption, the effect of a short dicarboxylic acid
(maleic acid, HOOC-CH=CH-COOH) on the contact angle of water on magnetite was
investigated. This molecule was chosen since it is a part of the surfactant head group and
could thus be expected to have a similar affinity to magnetite as the surfactant. Whereas we
have previously shown that maleic acid did not adsorb on hematite at pH 8.5 [39], the
presence of calcium ions could possibly facilitate the adsorption of maleic acid on magnetite.
Adsorption of maleic acid would illustrate if it is possible to render magnetite hydrophilic by
replacing surfactant molecules adsorbed on the surface by molecules without a hydrophobic
chain.
By means of the contact angle measurements it was concluded that treatment with maleic acid
at the same conditions as for the case of ammonium polyacrylate did not have any effect on
the wettability of magnetite. Furthermore, according to the spectroscopic adsorption data (not
shown), maleic acid did not adsorb on magnetite modified with calcium, silicate and anionic
surfactant. These findings indicate that both the length and the functionalities of a molecule
are important for its performance as a surface hydrophilizer.
3.3. Zeta-potential
One of the requirements set for the polymer that could be used to improve wettability of the
magnetite concentrate prior to agglomeration is that it should not impair the interaction
15
between the magnetite surface and the binder. Among the parameters that could affect the
interaction is surface charge. When bentonite is used as a binder, the magnetite surface should
not have too high negative charge not to cause electrostatic repulsion between the bentonite
platelets and the magnetite surface. In order to investigate the effect of polyacrylate
adsorption on the charge of the magnetite surface at different pH values, zeta-potential
measurements were performed, and the results are shown in Fig. 5.
The charge of the magnetite particles after polyacrylate adsorption (filled diamonds) was
negative and constant with pH, similarly to the charge of the magnetite particles after
adsorption of an anionic carboxylate surfactant [22]. The charge decreased by ca 5 mV, as
compared to the charge prior to polymer adsorption suggesting that polyacrylate adsorption
would not have any significant electrostatic effect on the interaction between the binder and
magnetite.
Similar results were reported by Pettersson et al. [33] for polyacrylate adsorption on alumina
and zirconia. The authors observed a shift in IEP to lower pH upon adsorption of the polymer;
however, in the present work, measurements below pH 5 could not be performed due to
precipitation of the surfactant at low pH.
Figure 5. Zeta-potential as a function of pH; of the magnetite particles after adsorption of
calcium ions, sodium silicate, maleic acid ester, and ammonium polyacrylate (); of
ammonium polyacrylate in aqueous solution (no magnetite particles) in the presence () and
absence () of calcium ions. Ionic medium: 10 mM NaCl.
The fact that polymer adsorption did not decrease the charge of the magnetite particles to any
considerable extent suggests that the charge of the ionized groups in the polymer was
16
compensated by counter-ions, both at the surface and in solution. Fig. 5 illustrates that the
charge of the polyacrylate species in aqueous solution in the presence of 10 mM NaCl, but
without any calcium ions present (empty triangles), decreased with increasing pH, probably
due to the gradual deprotonation of carboxylic groups in the polymer.
The charge of the polyacrylate species in solution in the presence of calcium ions (empty
squares) was nearly constant with pH, probably due to complexation of polyacrylate with
calcium in solution. The presence of divalent calcium ions in solution screens neighbouring
negative charges better than monovalent sodium ions. Additionally, as has been discussed
earlier, calcium ions can facilitate intramolecular interactions stabilizing polyacrylate.
Furthermore, the charge of calcium polyacrylate in solution was rather similar to the charge of
the magnetite particles after polymer adsorption, suggesting that polyacrylate was adsorbed
on magnetite by forming a ternary complex with calcium ions.
4. Conclusions
Three types of polymers (cationic, anionic, and non-ionic) were tested for their ability to
restore wetting of synthetic magnetite pre-treated with a surfactant. All the three polymers
could adsorb on magnetite although they were differently charged. However, only the anionic
polyacrylate could improve magnetite wettability to the level of pure magnetite under the
conditions studied. No desorption of the surfactant was observed upon polymer adsorption
suggesting that the improved surface hydrophilicity was achieved due to shielding of the
hydrophobic surfactant tails by hydrophilic polymer chains. From the results of the
ATR-FTIR and zeta-potential measurements, it was concluded that polyacrylate was adsorbed
on the surface of magnetite via calcium ions. Polyacrylate adsorption only slightly increased
the negative zeta-potential of the magnetite surface and is not likely to have any significant
electrostatic effect on the interaction between bentonite and magnetite in agglomeration.
Magnetite treated with polyacrylate remained hydrophilic at alkaline pH and during storage in
air, suggesting that treatment with polyacrylate could be a feasible means of improving
wettability of the flotated magnetite concentrate prior to agglomeration.
Acknowledgements
This is a contribution by the Centre of Advanced Mining and Metallurgy (CAMM). The
financial support by the Hjalmar Lundbohm Research Centre (HLRC) is gratefully
acknowledged.
17
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Hedlund, Colloids Surf. A 386 (2011) 79.
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19
20
PAPER V
Interfacial properties of natural magnetite particles compared
with their synthetic analogue
E. Potapova, X. Yang, M. Westerstrand, M. Grahn, A. Holmgren, and J. Hedlund
Full-length paper to be submitted to Minerals Engineering Abstract and accepted for presentation at the
Flotation 2011 Conference in Cape Town, South Africa
INTERFACIAL PROPERTIES OF NATURAL MAGNETITE PARTICLES
COMPARED WITH THEIR SYNTHETIC ANALOGUE
E. Potapova,*, a X. Yang, b M. Westerstrand,c M. Grahn,a A. Holmgren,a and J. Hedlunda
a
Chemical Technology, Luleå University of Technology, SE-971 87 Luleå, Sweden
b
Research Centre for Eco-Environmental Sciences, Chinese Academy of Science, Beijing 100085,
China
c
Applied Geology, Luleå University of Technology, SE-971 87 Luleå, Sweden
*
To whom correspondence should be addressed: [email protected]; tel.: +46 920 491776;
fax: +46 920 491199.
ABSTRACT
Understanding of the interactions between iron oxides and flotation reagents is important both for
flotation and agglomeration of iron ore. Model systems comprising synthetic iron oxides and pure
chemical reagents are commonly applied in experimental work in order to obtain high quality data
and to ease the interpretation of the empirical data. Whether the results obtained using model
systems are valid for iron ore minerals and commercial reagents is a question seldom addressed in
the literature. It is shown in this work that previously reported results obtained from a model
system, concerning adsorption of a carboxylate surfactant and sodium metasilicate onto synthetic
magnetite nanoparticles, as obtained by in-situ ATR-FTIR spectroscopy and contact angle
measurements, are applicable to adsorption of flotation reagents on magnetite concentrate.
Additionally, the problem of restoring magnetite wetting after flotation is addressed since good
wetting of a magnetite concentrate is required to produce iron ore pellets by wet agglomeration.
The results from the present work indicate that the wettability of both synthetic magnetite coated
with surfactant and magnetite concentrate after flotation can be improved by adsorbing a
hydrophilizing agent such as silicate or polyacrylate.
KEYWORDS
Flotation reagents, Iron ores, Particle size, Surface modification
1
1 INTRODUCTION
Interfacial phenomena such as adsorption at the solid/liquid interface and wetting are important for
several mineral processing operations including desliming, flocculation, flotation and
agglomeration. In-situ infrared spectroscopy and contact angle measurements have proven to be a
powerful combination of tools for studying adsorption on mineral surfaces and its effect on surface
wettability (Fuerstenau (2007)). ATR-FTIR spectroscopy allows monitoring interactions in-situ in
the presence of water and provides information about adsorption kinetics and equilibrium as well as
the surface complexes formed (Smart et al. (2007)).
Since most of the minerals are not transparent for infrared radiation, ATR elements coated with thin
films of mineral particles are commonly applied in the adsorption studies by ATR-FTIR
spectroscopy (Smart et al. (2007)). Natural mineral particles are not always appropriate for this type
of measurements since they may have relatively low surface area resulting in a low signal-to-noise
ratio and may contain impurities that would complicate the interpretation of the spectroscopic
results. For that reason, synthetic mineral particles with high surface area are commonly applied in
the adsorption studies by ATR-FTIR spectroscopy.
In our previous work (Potapova et al. (2010a), Potapova et al. (2010b)), a method based on in-situ
ATR-FTIR spectroscopy was developed and successfully used for studying adsorption of flotation
related chemicals from aqueous solutions on thin films of synthetic iron oxides. The spectroscopic
data were later complemented by contact angle and zeta-potential measurements (Potapova et al.
(2011b)).
The interactions between iron oxides and flotation reagents are important, not only for the
performance in flotation of apatite from magnetite, but also for the subsequent agglomeration of
magnetite concentrate to produce iron ore pellets. Reduced wetting of the magnetite concentrate due
to adsorption of a flotation collector results in lower pellet strength (Forsmo et al. (2008),
Gustafsson and Adolfsson (1997), Iwasaki et al. (1967)) and may hamper the production capacity of
a pelletizing plant.
The scope of the present work was to characterize the interfacial properties of natural magnetite
particles cleaned by magnetic separation and flotation and to compare with the properties of the
synthetic magnetite nanoparticles used in our previous experimental work, especially to test
whether the conclusions drawn regarding adsorption behaviour and wettability of synthetic
magnetite are valid for mineral particles. Whereas this aspect is seldom addressed in the literature,
the difference in interfacial properties of synthetic and natural materials is an important issue to
consider when substituting synthetic particles for their mineral analogue in experimental work.
2
2 MATERIALS AND METHODS
2.1 Materials
Magnetite nanocrystals were synthesized and purified according to the procedure described
previously (Potapova et al. (2010b)). A dispersion of magnetite in distilled water was further diluted
with methanol and degassed distilled water resulting in a working dispersion containing 25 vol. %
methanol and ca 1.1 mg mL-1 magnetite. The dispersion was stored in a refrigerator in order to
minimize oxidation of magnetite.
Magnetite pellet concentrate after flotation and magnetic separation from the pelletizing plant in
Kiruna, Sweden was provided by LKAB. The concentrate had been stored at ambient conditions for
more than two years. Moist when received, the concentrate was stored in a refrigerator in a sealed
plastic bag. Prior to usage, the concentrate was dried in an oven at 50°C.
Flotation collector Atrac 1563 (Akzo Nobel) and dispersant/depressant water glass were provided
by LKAB, Sweden.
Water glass is an aqueous solution of sodium silicate, in this case with a SiO2:Na2O weight ratio of
3.25. Sodium metasilicate (Na2SiO3·9H2O, 98 %, Sigma) was used in the experiments with
synthetic magnetite as an analytical grade alternative of water glass.
Flotation collector Atrac 1563 has a complex chemical composition: 50-100 % ethoxylated tall oil
ester of maleic acid, and 1-5 % maleic anhydride (Akzo Nobel material safety data sheet). Since
exact composition and chemical structure of Atrac 1563 were not specified by the supplier,
dodecyloxyethoxyethoxyethoxyethyl maleate (Sigma-Aldrich) was used as a model flotation
collector to ease the interpretation of the spectroscopic data. Fig. 1 shows the supposed chemical
structure of flotation collector Atrac 1563 and the structure of the surfactant used as a model
flotation collector.
Figure 1. Supposed chemical structure of Atrac 1563 (a) and the structure of
dodecyloxyethoxyethoxyethoxyethyl maleate (b). R represents the alkyl chain in fatty acids, R’
represents the linear alkyl chain CH3(CH2)11.
Calcium chloride (CaCl2·2H2O, 95 %, Riedel-de Han) was used to provide a solution with calcium
ions. Sodium chloride (NaCl, pro analysi, Riedel-de Han) at a concentration of 10 mM was used as
3
ionic medium in spectroscopic, zeta-potential and contact angle measurements, if not stated
otherwise.
Dispex A40 and Dispex N40 (ammonium and sodium polyacrylate, respectively, BASF) with the
average molecular weight of 4000 were used for surface modification of magnetite after surfactant
adsorption.
All aqueous solutions were prepared using distilled water. Distilled water for the spectroscopic and
contact angle experiments was degassed under vacuum to minimize the amount of dissolved gases.
The pH of the working solutions was adjusted using aqueous solutions of sodium hydroxide
(NaOH, pro analysi, Merck) and hydrochloric acid (HCl, 37 %, pro analysi, Merck).
2.2 Methods
2.2.1 Characterization of the synthetic and natural magnetite particles
X-ray diffraction (XRD). XRD patterns of both synthetic and natural magnetite particles were
collected with a Siemens D5000 diffractometer running in Bragg-Brentano geometry using Cu-K
radiation.
High-resolution scanning electron microscopy (HR-SEM). The morphology of synthetic and
natural magnetite particles without any coating was investigated with an FEI Magellan 400 field
emission extreme high resolution scanning electron microscope (XHR-SEM) using an accelerating
voltage of 1 kV.
Energy dispersive X-ray spectroscopy (EDS). Chemical analysis of the natural magnetite
particles in the size range from 0.22 to 8 μm was carried out by SEM-EDS using an Oxford
instruments X-Max50 SDD detector, combined with a Zeiss Merlin field emission SEM. In the
EDS measurements, the accelerating voltage of 20 kV was applied.
BET surface area. Specific surface area of synthetic magnetite particles was estimated using the
BET method from nitrogen adsorption data recorded at liquid nitrogen temperature using a
Micrometrics ASAP 2010 gas adsorption analyzer. Degassing was performed by evacuating the
sample at 130°C overnight.
Zeta-potential. The zeta-potential of both synthetic and natural magnetite at different pH was
determined by electrophoresis using a ZetaCompact instrument equipped with a charge-coupled
device (CCD) tracking camera. The electrophoretic mobility data was further processed by the
Zeta4 software applying the Smoluchowski equation. The zeta-potential was calculated as an
average of three replicates.
The natural magnetite sample for the zeta-potential measurements was collected in the form of a
slurry after flotation at the LKAB concentration plant in Kiruna, Sweden. Due to the particle size
limitations of the measuring technique, the zeta-potential was measured using the particles with a
4
size of 0.2-8 μm, obtained by vacuum filtration of the slurry using cellulose filters. Prior to
filtration, the filters were washed with a 5% aqueous solution of acetic acid (Odman et al. (1999)).
Process water used in the zeta-potential measurements on the synthetic magnetite was obtained
from the filtration step at the LKAB concentration plant in Kiruna, Sweden. The process water was
filtered through a cellulose filter (Millipore, 0.22 μm pore size), washed with a 5% aqueous solution
of acetic acid (Odman et al. (1999)). The concentration of dissolved species in the process water
was analyzed at ALS Scandinavia in Luleå, Sweden, accredited according to the international
standards ISO 17025, ISO 9001:2000, SS EN 1484 and ISO/IEC Guide 25. The samples were
analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) and
inductively coupled plasma sector field mass spectrometry (ICP-SFMS). Total organic carbon
(TOC) was analyzed using a Shimadzu TOC-5000 high-temperature combustion instrument.
Contact angle. The static sessile drop method was used to determine the contact angle of the
synthetic magnetite nanoparticles. A thin film of synthetic magnetite was produced on a substrate
by spreading 0.5 mL of the magnetite dispersion described in Section 2.1 and drying it in air at
room temperature. Contact angle measurements were performed using a Fibro 1121/1122
DAT-Dynamic Absorption and Contact Angle Tester equipped with a CCD camera. The
measurement was performed by placing a 4 μL water droplet onto the magnetite coated substrate
using a microsyringe. A series of images of the droplet were recorded and analysed using the DAT
3.6 software. For each film, the measurement was repeated 8-10 times by applying fresh droplets.
The value of the contact angle was determined as an average of the replicates. Consecutive
adsorption of CaCl2, Na2SiO3, collector, and a hydrophilizing agent (either ammonium polyacrylate
for 1 h or sodium metasilicate for 24 h) was performed on the magnetite film at the same
concentrations and pH as in the spectroscopic measurements. After each adsorption step, the contact
angle of the magnetite film was measured. Further experimental details are described elsewhere
(Potapova et al. (2011a)).
The contact angle of the natural magnetite particles was determined by the Washburn method using
a Krüss K100 force tensiometer. Liquid sorption by the magnetite powder was recorded as a
function of immersion time and Krüss LabDesk 3.1 software was used to calculate the contact angle
applying the Washburn equation. First, the capillary constant of the Washburn equation was
estimated for each sample using n-hexane. Thereafter, the contact angle of the magnetite powder
was measured using deionized water. The values of the capillary constant and the contact angle
were calculated as an average of three replicates. For a single measurement, ca 1 g of the magnetite
powder was used.
To investigate the effect of different reagents on the wettability of the natural magnetite particles,
batch adsorption was performed using suspensions containing 10 g magnetite per ca 40 mL solution
5
at pH 9 and room temperature. After adsorption, the solution was decanted and the magnetite was
dried in an oven overnight at 50°C.
To investigate the effect of calcium ions and water glass on collector adsorption onto the natural
magnetite particles, magnetite powder was preconditioned for 1 h in an aqueous solution containing
water glass at a concentration of 1.0 mg g-1 magnetite (or ca 1.1 mM [Si]), together with 10 mM
NaCl or 4 mM CaCl2. Thereafter, the flotation collector Atrac 1563 was added to the suspension
and was allowed to adsorb for 20 min.
To investigate the effect of sodium polyacrylate and water glass on the wettability of flotated
magnetite concentrate, as-received magnetite concentrate was treated with these reagents for 1 h
and 9 h, respectively. The effect of sodium polyacrylate was examined in the aqueous solutions of
either calcium chloride or sodium chloride.
2.2.2 In-situ ATR-FTIR spectroscopy
Infrared spectra were recorded using a Bruker IFS 66v/S spectrometer equipped with a deuterated
triglycine sulphate (DTGS) detector and a liquid nitrogen cooled mercury-cadmium-telluride
(MCT) detector. Magnetite coated ZnSe ATR crystals (Crystran Ltd) in the form of a trapeze with
45° cut edges and dimensions of 50x20x2 mm were used in this study.
The incidence angle of the infrared beam was set to 45°. Both single beam background and sample
spectra were obtained by averaging 500 scans at a resolution of 4 cm-1. Data processing was
performed using the Bruker Opus 4.2 software. All the spectroscopic experiments were performed
on solutions at pH 8.5 and at room temperature. The pH of the solution was controlled by a Mettler
Toledo T70 titrator. Further experimental details are described elsewhere (Potapova et al. (2010b),
Yang et al. (2008)).
3 RESULTS AND DISCUSSION
3.1 Characterization of the synthetic and natural magnetite particles
XRD patterns presented in Fig. 2 indicate pure crystalline magnetite without any other phases
present in amounts detectable by XRD. Broader reflections in Fig. 2a compared to the ones in
Fig. 2b indicate that the crystal size was much smaller for the case of the synthetic magnetite than
for the magnetite mineral, in concert with SEM observations, see below.
6
Figure 2. XRD patterns of the synthetic (a) and natural (b) magnetite crystals. The reflections
originating from magnetite are indexed by the appropriate Miller indices. For the sake of clarity, the
intensity of pattern (a) was multiplied by a factor 2 and shifted.
Fig. 3 shows HR-SEM images of the cross-section of a thin layer of synthetic magnetite
nanoparticles deposited on a ZnSe substrate and natural magnetite particles spread over a carbon
tape. Synthetic magnetite particles (Fig. 3a) showed a spherical habit with a diameter of 5-15 nm.
The particles were partially aggregated and formed a continuous porous film on the ZnSe substrate.
The particles in the magnetite concentrate (Fig. 3b) had irregular shape and varied in size. The
coarse magnetite particles in Fig 3b were covered by very fine particles (less than 1 μm in size),
some of which, according to the EDS results, had a high content of silicon and aluminium and could
be the remains of aluminosilicates present in the iron ore before concentration.
Figure 3. HR-SEM images of synthetic magnetite particles on a ZnSe substrate (a) and natural
magnetite particles on a carbon tape (b).
7
The specific surface area of synthetic magnetite determined from nitrogen adsorption data was
about 90 m2 g-1, which is much higher than the specific surface area of natural magnetite that is only
about 0.5 m2 g-1 (Forsmo (2005)). The synthetic magnetite thus had much more surface sites per
gram material available for adsorption, which motivates the use of synthetic magnetite in model
systems.
Regarding the contact angle of synthetic and natural magnetite particles, different measuring
techniques had to be used due to the difference in the particles’ size. The static sessile drop method
is suitable for measuring the contact angle of colloid particles (like synthetic magnetite
nanoparticles used in this work), providing that a closely packed layer of particles can be formed
(Shang et al. (2008)). Using this method, the contact angle of synthetic magnetite in the present
work was determined to be 15-25°. A value of 12 ± 1° was previously reported (Galindo-González
et al. (2005)) for spherical magnetite nanoparticles with a mean size of 11 ± 2 nm. Wang and Ren
(Wang and Ren (2005)) measured the contact angle of water on a polished surface of natural
magnetite using the static sessile drop method and reported a value of 25 ± 5°. The contact angle
determined in the present work is thus comparable to previously reported contact angles.
The contact angle of natural mineral powders is commonly estimated by the Washburn method. In
this method, packing of particles is also important since it may affect the penetration rate of the
wetting liquid and thus the measured value of the contact angle (Kirchberg et al. (2011)).
For the natural magnetite particles used in this work, the contact angle was estimated to be 50-60°,
which is similar to previously reported values. A contact angle of 72-75° (Kirchberg et al. (2011))
was reported for unsieved magnetite powder with a particle size of 146 μm and irregular particle
shape. For the magnetite with a size of 86 % -74 μm, the contact angle was determined to be 46°
(Qiu et al. (2004)). Thereby, the contact angle of natural magnetite measured by the Washburn
method is normally reported to be higher than the contact angle of synthetic magnetite
nanoparticles. This difference could be partly due to the measuring technique but also the particle
size and impurities present on the surface of natural magnetite particles. Apart from that, the contact
angle measured by the sessile drop method may vary depending on the surface roughness, resulting
in an underestimated contact angle for hydrophilic surfaces and an overestimated contact angle for
hydrophobic surfaces. For the case of natural magnetite used in this work, the high contact angle
could also be caused by adsorbed flotation collector since the iron ore was concentrated both by
magnetic separation and flotation. The amount of flotation collector adsorbed on the magnetite
surface after flotation was previously estimated to be 10-30 g t-1 (Forsmo et al. (2008)). Already a
partial coverage by a surfactant could be enough to obtain a hydrophobic surface (Holmberg et al.
(2003)).
8
Fig. 4 illustrates the zeta-potential of the synthetic and natural magnetite particles as a function of
pH. The zeta-potential curve for the synthetic magnetite in a 10 mM NaCl solution (empty
triangles) exhibits the shape typically observed for iron oxides, with the IEP at pH 7 (Potapova et al.
(2011b)).
Figure 4. Zeta-potential as a function of pH; of the synthetic magnetite particles in 10 mM NaCl
() and in the process water from the LKAB concentrating plant in Kiruna, Sweden (¡); of the
0.22-8 μm fraction of the iron ore concentrate in the process water ().
However, the zeta potential of the 0.22–8 μm particle fraction of the magnetite slurry in the process
water after flotation (filled squares) showed a completely different dependency on pH, with two IEP
observed at about pH 11 and at pH 2. Such behaviour could be due to the specific adsorption of
soluble species present in the process water onto the surface of the particles and due to the presence
of mineral impurities in the slurry.
To test the first hypothesis, the zeta-potential was measured on the synthetic magnetite particles
dispersed in the process water from the LKAB concentrating plant in Kiruna, Sweden (open
diamonds in Fig. 4).
The curves for the iron ore concentrate after flotation and the synthetic magnetite in the process
water are fairly similar and characterized by a weakly negative zeta-potential almost in the whole
pH range with an increase in the zeta-potential at highly alkaline pH. These results suggest that the
dissolved species present in the process water can have a significant effect on the zeta-potential of
magnetite, in concert with the results reported previously (Potapova et al. (2011b)). The measured
concentration of dissolved species present in the process water is shown in Table 1.
The decrease of the zeta-potential of the magnetite particles in the process water at pH < 8 and the
shift of the IEP to a lower pH as compared to the zeta-potential of the synthetic magnetite in 10 mM
NaCl could be explained by the specific adsorption of anions such as sulphate, bicarbonate, and
9
silicate on the magnetite surface. The specific adsorption of cations such as calcium and magnesium
on the magnetite surface could result in the increase of the zeta-potential of the magnetite particles
in the process water at pH > 8 and could cause a shift of the IEP to a higher pH as compared to the
zeta-potential of the synthetic magnetite in 10 mM NaCl.
Table 1. Concentration of solutes in the process water.
Specie
Ca
S
Na
Cl
K
NO3
Mg
HCO3
Si
TOC
Concentration, mM
8.8
10.6
9.5
9.0
2.4
2.0
1.6
1.1
0.5
4.3a
a
Concentration in mg L-1.
To determine whether the natural magnetite particles used in the zeta-potential measurements
contained any mineral impurities, the particles of the 0.22-8 μm fraction of the magnetite slurry
obtained after flotation were investigated by SEM-EDS, which showed that the measured particle
fraction contained mainly silicon, aluminium, and oxygen. Thereby, it was concluded that the
mineral particles used in the zeta-potential measurements were to a large extent aluminosilicate.
Whereas aluminosilicate minerals at the LKAB concentrating plant in Kiruna, Sweden, are removed
from the iron ore by magnetic separation, small aluminosilicate particles still remain in the iron ore
concentrate after magnetic separation.
3.2 Adsorption behaviour of model compounds and commercial flotation reagents
Careful analysis of the ATR-FTIR spectra of model compounds and commercial flotation reagents
can provide important information about their chemical structure and adsorption mechanisms.
Fig. 5 shows spectra of the flotation collector Atrac 1563 and the model collector adsorbed on
synthetic magnetite at pH 8.5. Atrac 1563 and the model collector display rather similar spectral
features, in agreement with the structural resemblance of the head groups in these compounds
(Fig. 2). A significant difference that should be pointed out is the absence of the band at 1104 cm-1
in the spectrum of Atrac 1563, associated with the stretching vibrations of the C-O-C groups
(v(C-O-C)) in the ethoxy-chains (Beentjes et al. (2006)). This implies a low degree of ethoxylation
of the molecules in Atrac 1563 as compared to the model collector, in concert with the chemical
structures shown in Fig. 2.
10
Figure 5. ATR-FTIR spectra of maleic acid ester (a) and Atrac 1563 (b) adsorbed on magnetite for
6 h from 25 mg L-1 aqueous solutions at pH 8.5 with 10 mM NaCl as background electrolyte.
In aqueous solutions at pH 8.5, the carboxylic acid groups in Atrac 1563 and the maleic acid ester
become deprotonated forming a negatively charged carboxylate ion as indicated by the presence of
the symmetric (vs(COO-)) and asymmetric (vas(COO-)) stretching vibrations bands (Dobson and
McQuillan (1999)) in the spectra of these compounds. Previously, it was concluded that carboxylate
ions were not likely to be responsible for collector adsorption on iron oxides at pH 8.5 in the
absence of a background electrolyte due to electrostatic repulsion (Potapova et al. (2010a)) and that
the adsorption took place via the non-charged polar ester carbonyl and ethoxy groups. However, the
presence of a background electrolyte in the present work could reduce the repulsion between the
ions and the magnetite surface further facilitating collector adsorption (Potapova et al. (2010b)).
For the adsorption of maleic acid on hematite, two co-existing surface complexes were reported
(Hwang and Lenhart (2008)) – an inner-sphere complex characterized by the vs(COO-) band at ca
1407 cm-1, and an outer-sphere complex characterized by the vs(COO-) band at ca 1430 cm-1.
Similarly, Atrac 1563 and the model collector both containing maleic acid in their head groups
could form an outer-sphere complex and an inner-sphere complex on magnetite.
Additionally, ester carbonyls present in the collectors could contribute to their adsorption on the
magnetite surface, as discussed elsewhere (Potapova et al. (2010a), Potapova et al. (2010b)).
Thereby, despite the difference in the structure of the head groups, similar adsorption mechanism
was observed for both collectors justifying the usage of the maleic acid ester as a model compound
for studies of the adsorption behaviour of commercial collectors such as Atrac 1563.
Fig. 6 shows infrared spectra of sodium metasilicate and water glass adsorbed on magnetite at pH
8.5. Despite the fact that these soluble silicates have different SiO2:Na2O weight ratio, the spectral
line shapes in Fig. 6 are rather similar.
11
Figure 6. ATR-FTIR spectra of silicate adsorbed on magnetite at pH 8.5 from a 1 mM aqueous
water glass solution for 110 min (a) and from a 1 mM aqueous sodium metasilicate solution for
150 min (b). For the sake of clarity, the absorbance of spectrum (b) was multiplied by a factor 2 and
spectrum (a) was shifted.
At this pH, the most pronounced band in the infrared spectrum is located at 1020 cm-1 with two
shoulders at ca 1120 cm-1 and ca 950 cm-1. The bands at 950 cm-1 and 1020 cm-1 originate from
adsorbed monomeric and oligomeric silicate species, respectively, whereas the band at 1120 cm-1 is
associated with the 3-dimentional silica framework structure (Yang et al. (2009)).
Although the time of adsorption was higher for sodium metasilicate, greater absorption intensity is
observed for water glass due to a larger amount of magnetite used in the adsorption experiment with
water glass.
According to previous results (Yang et al. (2008)), the speciation of the adsorbed silicate on the
magnetite surface is determined by pH and silicate concentration rather than by the SiO2:Na2O ratio
suggesting that silicate sources with different composition could be expected to show similar
adsorption behaviour.
3.3 Adsorption of flotation reagents and effect on magnetite surface properties
The effect of calcium ions and sodium metasilicate on the adsorption of the model collector on
synthetic magnetite was investigated in our previous work (Potapova et al. (2010b)). It was
concluded that sodium metasilicate could suppress the adsorption of the model collector on
magnetite but only in the absence of calcium ions. When calcium ions were present in the system,
the adsorption of the collector was dramatically increased both with and without sodium
metasilicate present in solution.
12
This conclusion was confirmed by contact angle measurements (see Table 2) showing that whereas
the contact angle decreased upon treatment with calcium and metasilicate, addition of the model
collector resulted in a considerable increase in the contact angle. These results imply that sodium
metasilicate could not prevent the adsorption of the model collector in the presence of calcium ions.
Approximately the same increase in contact angle was observed for the model collector and Atrac
1563, confirming that these compounds had similar effect on the wettability of magnetite.
Table 2. Contact angle of surface modified synthetic magnetite measured by the static sessile drop
method.
Contact angle (degrees)
Treatment
Test 1 (Potapova et al. (2011b))
Test 2
As-synthesized magnetite
22 ± 3
20 ± 3
4 mM calcium, 1 h
19 ± 2
15 ± 4
0.04 mM silicate, 1 h
10
10a
25 mg L-1 maleic acid ester, 1 h
44 ± 3
-
-
43 ± 8
-1
25 mg L Atrac, 1 h
a
a
The exact value of the contact angle could not be estimated since most of the measurements after
silicate adsorption were below detection limit of the instrument (10°).
Further, it was investigated whether the conclusions regarding the effect of calcium and silicate on
collector adsorption onto synthetic magnetite were applicable to natural magnetite particles.
Adsorption of Atrac 1563 and water glass on magnetite concentrate was performed in the presence
and absence of calcium ions and the contact angle of the concentrate was measured by the
Washburn method, see Fig. 7.
In the absence of calcium ions, the contact angle did not change upon collector adsorption
indicating that no or very little adsorption took place on magnetite concentrate pre-treated with
water glass. However, collector adsorption in the presence of calcium ions resulted in an increased
contact angle of magnetite concentrate, despite the pretreatment with water glass, due to the
activation of the magnetite surface for collector adsorption by calcium ions, in accordance with
conclusions drawn from spectroscopy data and reported previously (Potapova et al. (2010b)).
Adsorption of water glass is expected to result in a better wetting and protect magnetite from
collector adsorption, whereas collector adsorption (before bi-layer structure formation) has an
opposite effect on wettability. With a contact angle of 60°, the surface of the magnetite concentrate
still remains in the hydrophilic domain ( < 90º) implying that the particles would not necessarily
float with the air bubbles. However, hydrophobic areas on the magnetite surface impair wetting of
13
the concentrate and may result in air inclusions inside the green pellets produced by the
agglomeration of the magnetite concentrate, which has previously been shown to reduce pellet
strength in both wet and dry state (Forsmo et al. (2008)).
Figure 7. Water contact angle of magnetite concentrate upon modification of the surface with 1 mg
g-1 water glass and Atrac 1563 in 10 mM NaCl () and 4 mM CaCl2 () solution at pH 9 measured
with the Washburn technique.
3.4 Improvement of magnetite wettability after flotation
In order to improve the wettability of the flotated magnetite concentrate prior to agglomeration, the
effect of two hydrophilizing agents on the contact angle of synthetic magnetite coated with the
model collector or Atrac 1563 was investigated. Table 3 shows the contact angle of synthetic
magnetite nanoparticles measured by the static sessile drop method before and after adsorption of a
hydrophilizing agent.
When a magnetite film treated with a collector was subjected to conditioning with 4 mM CaCl2 and
0.4 mM sodium metasilicate at pH 8.5 for 1 day, the contact angle was lowered to 21 ± 1° for the
model collector and to 16 ± 1° for Atrac 1563. The increase in surface hydrophilicity was caused by
either desorption of the collector due to concentration gradient or replacement of the collector by
silicate species.
Conditioning with ammonium polyacrylate was performed by adding the polymer to a solution
already containing calcium chloride, sodium metasilicate and a collector (either Atrac 1563 or the
model collector) implying that the desorption of the collector due to the change of concentration
could not take place. As a result of the treatment, the contact angle of the magnetite film decreased
from 44 ± 3° to 24 ± 6° for the model collector and from 43 ± 8° to 20 ± 6° for Atrac 1563. The
observed increase in surface hydrophilicity was probably due to masking of the hydrophobic
collector species adsorbed on the magnetite surface by hydrophilic polymer molecules (Potapova et
al. (2011a), Somasundaran and Cleverdon (1985)).
14
Table 3. Change in the contact angle of synthetic magnetite pretreated with calcium chloride,
sodium metasilicate, and a collector upon adsorption of sodium metasilicate and ammonium
polyacrylate.
Contact angle of synthetic magnetite (degrees)
-1
25 mg L-1 Atrac 1563
25 mg L model collector
42 ± 2
44 ± 3
-1
49 ± 3
43 ± 8
0.4 mM sodium
12.5 mg L
0.4 mM sodium
12.5 mg L-1
metasilicatea
polyacrylateb
metasilicatea
polyacrylateb
21 ± 1
24 ± 6
16 ± 1
20 ± 6
a
In the presence of 4 mM CaCl2.
b
Polyacrylate was added to the solution already containing 4 mM CaCl2, 0.4 mM Na2SiO3,
25 mg L-1 collector, and 10 mM NaCl.
To verify that silicate and polyacrylate could be used for improving magnetite wettability after
flotation, adsorption of these compounds was performed on flotated magnetite concentrate at pH 9
and the contact angle of the magnetite concentrate was measured using the Washburn method.
Fig. 8 shows the effect of water glass adsorption on the wettability of the magnetite concentrate.
Figure 8. Water contact angle of the magnetite concentrate upon modification of the surface with
water glass in 10 mM NaCl at pH 9 for 9 h measured with the Washburn technique. Prior to water
glass adsorption, the concentrate was preconditioned with 10 mM NaCl at pH 9 for 1 h.
A contact angle of 57 ± 5º was obtained for the magnetite concentrate before water glass adsorption.
Such a high value could be due to residues of the collector adsorbed on the surface after flotation, as
has been discussed in Section 3.1.
15
Treatment with water glass clearly decreased the contact angle of the magnetite concentrate, and the
hydrophilizing effect improved with increased concentration of water glass. At the highest water
glass dosage (3 mg per g magnetite), the resulting contact angle was 28 ± 3º, which is rather close to
the values obtained for synthetic magnetite and sodium metasilicate presented in Table 3.
Sodium polyacrylate was adsorbed on the magnetite concentrate in the presence of calcium ions
(4 mM CaCl2). Fig. 9 shows the effect of polymer adsorption on the wettability of the magnetite
concentrate. The contact angle of the magnetite after flotation was 52 ± 1º, which, again, could
indicate that the magnetite surface was partly coated by the flotation collector. Upon polymer
adsorption from a 0.04 mg g-1 solution, the contact angle of magnetite concentrate decreased;
however, the variation of the contact angle within the replicates was in the range 34-54° indicating
an uneven distribution of the polymer on the magnetite surface. Increasing the polymer
concentration at a constant concentration of calcium ions did not result in further decrease of the
contact angle. A possible explanation could be the depletion of the magnetite surface sites available
for polymer adsorption already at lower concentration of the polymer, or the dependence of
polymer adsorption on the concentration of calcium ions, i.e. on the calcium-to-polymer ratio.
Figure 9. Water contact angle of the flotated magnetite concentrate upon modification of the
surface with sodium polyacrylate in 4 mM CaCl2 at pH 9 for 1 h measured with the Washburn
technique. The point at 0 mg g-1 represents the contact angle of as-received magnetite.
To test the latter hypothesis, polymer adsorption on magnetite was performed at different
concentrations of calcium ions keeping the concentration of the polymer constant at 0.04 mg g-1
magnetite, see Fig. 10. When polymer adsorption was performed without calcium ions, the contact
angle of the magnetite concentrate was virtually the same as that of the as-received concentrate
suggesting that in the absence of calcium, the polymer was not effective in improving the
wettability of the magnetite surface. The slight increase in the measured contact angle (to 55 ± 1°)
16
was possibly caused by desorption of hydrophilic silicate species, which are expected to be present
on the magnetite surface after flotation.
When 4 mM calcium chloride was added to the polymer solution, the contact angle of the magnetite
concentrate decreased to 41 ± 8° as discussed above. Further increase in calcium concentration to
6 mM resulted in an even lower contact angle (25 ± 11°) confirming the importance of the calciumto-polymer ratio for polymer adsorption onto magnetite. Since the concentration of calcium in the
process water at the LKAB concentrating plant in Kiruna, Sweden, is 8.8 mM (see Table 1), even
better effect of polyacrylate adsorption on the wettability of the magnetite concentrate could be
expected.
Figure 10. Water contact angle of the magnetite concentrate upon modification of the surface with
0.04 mg g-1 sodium polyacrylate at pH 9 for 1 h measured with the Washburn technique. The point
at 0 mM represents the contact angle of the magnetite concentrate treated with sodium polyacrylate
in the presence of 10 mM NaCl and no calcium ions.
An increased adsorption of polyacrylic acid on aluminium oxide in the presence of calcium ions
was reported (Vermöhlen et al. (2000)). The authors explained such behaviour by the ability of
calcium ions to screen the negative charge of the carboxylic groups in the polymer more efficiently
as compared to monovalent sodium ions, allowing the polymer to adopt a more coiled conformation
on the surface thus increasing surface loading. Hence, when the polymer concentration was
increased without increasing the concentration of calcium ions, the amount of calcium ions was not
sufficient to facilitate the adsorption of the highly negatively charged polyacrylate onto the
magnetite surface also being negatively charged at this pH.
It is important to mention here that the decrease in the contact angle in Fig. 10 is not likely to be due
to increased ionic strength upon increasing calcium concentration. According to Vermöhlen et al.,
polyacrylate adsorption on alumina in the presence of 3.3 mM calcium chloride was twice higher
17
than adsorption from solution containing 10 mM NaCl, having the same ionic strength (Vermöhlen
et al. (2000)).
The fact that polyacrylate improves the wettability of magnetite more efficiently in the presence of
calcium ions makes it suitable for application in processes having a process water rich in calcium
ions. Thereby, polyacrylate could be a good candidate for improving the wettability of magnetite
after reverse flotation from calcareous gangue minerals.
4 CONCLUSIONS
In spite of differences in surface properties and morphology of synthetic and natural magnetite
particles, similar tendencies were observed for adsorption of calcium ions, soluble silicates, anionic
carboxylate surfactants, and polyacrylate polymers on these materials, as illustrated by contact
angle measurements. It was confirmed that the wettability of magnetite was reduced by collector
adsorption when calcium ions were present in the system, despite pre-conditioning with water glass.
Wettability of the flotated magnetite concentrate could be significantly improved by prolonged
treatment with water glass or rather short conditioning with sodium polyacrylate, in agreement with
the results obtained for the synthetic magnetite nanoparticles. Better wetting of the concentrate
would facilitate wet agglomeration and could possibly increase the strength of iron ore pellets
produced.
ACKNOWLEDGEMENTS
This is a contribution by the Centre of Advanced Mining and Metallurgy (CAMM) at Luleå
University of Technology, Sweden. The authors acknowledge the financial support from the
Hjalmar Lundbohm Research Centre (HLRC). The Knut and Alice Wallenberg Foundation is
acknowledged for financial support of the Magellan SEM instrument.
18
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