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MICROWAVE ENHANCED RECOVERY OF
NICKEL-COPPER ORE:
COMMUNITION AND FLOATABILITY ASPECTS
R. Hendaa, *, A. Hermasa, R. Gedyeb and M.R. Islamc
a
Laurentian University
School of Engineering / b Department of Chemistry and Biochemistry, Sudbury, Ontario P3E 2C6, Canada
c
Faculty of Engineering/Dalhousie University, Halifax, Nova Scotia B3J 2X4, Canada
A study describing the effect of microwave radiation, at a frequency of 2,450 MHz, on the
processes of communition and flotation of a complex sulphide nickel-copper ore is presented. Ore
communition has been investigated under standard radiation-free conditions and after ore treatment in a radiated environment as a function of ore size, exposure time to radiation, and microwave
power. The findings show that communition is tremendously improved by microwave radiation with
values of the relative work index as low as 23% at a microwave power of 1.406 kW and after 10 s
of exposure time. Communition is affected by exposure time and microwave power in a nontrivial
manner. In terms of ore floatability, the experimental tests have been carried out on a sample of 75
μm in size under different exposure times. The results show that both ore concentrate recoveries
and grades of nickel and copper are significantly enhanced after microwave treatment of the ore
with relative increases in recovered concentrate, grade of nickel, and grade of copper of 26 wt%, 15
wt%, and 27%, respectively, at a microwave power of 1,330 kW and after 30 s of exposure time.
Submission Date: March 2005
Acceptance Date: January 2006
INTRODUCTION
Microwave radiation as a source of energy has been
developed primarily for communications and is finding
applications in many fields such as chemical synthesis,
environmental engineering, and materials processing. For
instance, in the area of chemical synthesis it is largely
documented that many chemical reactions experience an
increase in the kinetics when subjected to microwave radiation [Gedye et al, 1988; Whittaker and Mingos, 1994;
Gabriel et al., 1998]. Microwave radiation has also been
effectively used in sintering [Ramkumar et al., 2001],
leaching [Hua et al., 2002; Lovas et al., 2002], and catalyst selectivity [Ohgushi et al., 1999; Turner et al., 2000].
Several reviews on the applications of microwave energy
have been recently published [Whittaker and Mingos,
Keywords: microwave energy, mineral separation,
communition, flotation, process engineering.
International Microwave Power Institute
1994; Thostenson and Chou, 1999; Haque, 1999; Clark
et al., 2000; Jones et al., 2002].
Microwave radiation interacts with “lossy” materials
to achieve volumetric heating, in contrast to conventional
heating where energy is transferred due to thermal gradients. The ability of a material to dissipate and uniformly
distribute microwave energy is defined in terms of the
complex dielectric permittivity of the material [Von
Hippel, 1954]. The amount of energy dissipated per unit
volume of material increases with the imaginary part of
the permittivity known as the loss factor, ε”, and the power
penetration depth, Dp. The loss factor is a function of
wavelength, temperature, density, electric field orientation, etc., and Dp, is proportional to the wavelength of
radiation and inversely proportional to a complex function
of the loss factor [Metaxas and Meredith, 1983]. While
the physics of microwave radiation is well established,
devising better process parameters and operating conditions is crucial for microwave energy to be beneficial
to process engineering related operations. For instance,
7
the magnitudes of both microwave power and exposure
time to microwave radiation or, alternatively, their product must be kept to a minimum. A judicious choice of
process conditions close to those commonly used in the
mineral industry will also facilitate process scale-up
at a later time [Bradshaw, 1999]. Microwave heating
systems come in multi- or single-mode cavity designs.
The distinction between cavity types is determined more
so by the design characteristics than by the processes
that can be performed inside the cavity. A single-mode
cavity is a metallic box whose dimensions must be carefully controlled to correspond in a systematic way to the
characteristic wavelength of the microwaves. The wave
pattern is such that the intensity of the microwave field
is highest in a localised and small volume of the cavity.
While this feature allows the generation of a high electric
field in the material, it limits the amount of material to
be treated. Most importantly there is no practical way of
scaling up processes. A multi-mode cavity is the most
versatile type, and is typically a large box with dimensions
several space wavelengths long. The microwave field is
distributed throughout the cavity, but not as strong as in a
single-mode cavity, thus absorbing material can be heated
anywhere inside the cavity. Furthermore, the multi-mode
cavity design allows easy scale up of processes. It is clear
that a new type of cavity is necessary to increase cavity
efficiency, as recently suggested by Kingman et al. [2004],
and to allow for easy process scale up.
Communition is known to be one of the least energyefficient of all separation processes. While about 60% of
the total energy required in mineral separation is used
during the process of communition, only about 1% of the
energy delivered to the material is used to create a new
surface, according to the best estimates [McCabe et al.,
2001]. In composites and complex multi-phase materials
such as ores, microwave radiation can be used in selective
heating of the material. High loss phases (absorbing materials) will readily couple with microwave energy while
low loss phases (transparent materials) will not. Therefore,
the rapid heating of microwave radiation absorbing minerals in a non-absorbing gangue phase is likely to cause
some differential heat effects at the interface between the
different phases of the complex material, as shown in a
theoretical study by Salsman et al. [1996]. Early investigations into materials heating have focused on establishing
the heating rate characteristic [Ford and Pei, 1967; Chen et
al., 1984]. The phenomenon of selective heating can cause
thermal stresses within the material, and has been recently
utilized to enhance the potential for mineral separation
[Kingman and Rowson, 1998; Bradshaw, 1999; Vorster et
al., 2001]. Unfortunately, in most of these studies it was
concluded that energy required to improve communition
by microwave radiation was greater than that required
by conventional communition. The extent of structural
8
weakening, e.g., formation of micro-cracks at the phase
boundaries, is crucial in communition and depends on
the composition of the mineral, and the individual dielectric constants and thermal expansion coefficients of
the phases. The time response of structural weakening to
microwave radiation is equally important from the viewpoint of energy requirements.
The process of flotation is based on the differences in
chemical properties of mineral surfaces, and is designed
to separate hydrophobic and hydrophilic minerals from
each other [Willis, 1981]. Hydrophobic minerals attach to
the air bubbles and are lifted up to the surface of the pulp,
from where they are skimmed off to form the valuable
product (concentrate), while hydrophilic minerals remain
in the pulp, forming the waste product (tailing). Although
this concept appears to be simple and straightforward,
complications arise mainly due to the fact that minerals
in their natural state are all hydrophilic, with only a few
exceptions. Minerals are made hydrophobic by modifying
their surfaces through chemical treatment, i.e., use of collectors. However, such treatment needs to be selective, in
other words while some minerals in the ore pulp acquire
a hydrophobic property, others must remain hydrophilic.
The success of flotation relies largely on achieving such
selectivity, and although the flotation process has been
around for more than a century, this issue still poses a challenge in many applications. The use of benign (relatively
safe to use) sources of energy such as microwaves, with
their potential ability to affect the chemistry of flotation,
namely, increase in reaction rate, and thus to improve the
separation of useful minerals, can have a drastic impact
on the mineral industry.
This paper focuses on process engineering aspects
of the effect of microwave radiation on mineral separation of a nickel-copper ore obtained from the Sudbury
basin. The present work is aimed at investigating the
effect of microwave radiation, at a frequency of 2,450
MHz, on the communition and floatability of the ore. The
experimental runs have been carried out under different
operating conditions, such as ore size, exposure time, and
microwave power.
EXPERIMENTAL PROCEDURES
Material
The nickel-copper sulphide ore used in this work
has been obtained from the Sudbury basin. The ore “as
received” was previously crushed to about 18 mm in a
cone crusher. The chemical composition of representative
samples of the ore was analyzed by X-ray powder diffraction (XRD), and the corresponding quantification results
are shown in Figure 1. The major minerals present in the
Journal of Microwave Power & Electromagnetic Energy
Vol. 40, No.1, 2005
Pentlandite
1.7%
Biotite
3.8%
Chalcopyrite
6.1%
Quartz
20.1%
Pyrrotite
21.6%
Amphibole
7.9%
Plagioclase
38.7%
Figure 1: The chemical composition of the ore as analyzed by XRD.
ore are plagioclase (silicate), pyrrhotite (iron sulphide),
quartz (silicate), amphibole (silicate), chalcopyrite (iron
copper sulphide), biotite (silicate), and pentlandite (iron
nickel sulphide). While silicates are generally transparent
to microwave radiation, metals are excellent reflectors
thereof.
Grinding Tests
The ore used in the test has been divided into four
classes according to ore size: that is, ore as received with
a 80% passing size, d80, of 18 mm, and cone crushed to
d80 of 9.2 mm, 5.5 mm, and 1.2 mm. Standard samples
were prepared by using a riffling procedure to obtain
representative samples of 300 grams from each ore size.
The sampling was achieved by mixing the same amount
for each size fraction
as the
standard
so that each
R. Henda
et al.,
Figuresample,
1
sample will have the same d 80 and the same size
distribution as the standard sample. The method of Berry
and Bruce [1966], has been used to compare between
grindability power requirements of radiated samples and
non-treated samples. The relative work index, RWI, can
be expressed as
 10
10 

RWI = 
−
Fr 
 Pr
 10
10 


−
Ft  ,
 Pt
(1)
where P and F are the d80 of the product and feed in µm,
respectively, r is the reference ore sample (non-treated),
and t is the test ore (radiated).
International Microwave Power Institute
To determine the relative work index, a reference
sample was ground without microwave treatment, and
the values of the d80 of the feed, Fr, and of the ground
product, Pr, were determined. Then the same weight of
the reference sample was radiated at different exposure
times and constant power in a microwave oven with an
air atmosphere. The oven, BP-210 from Microwave
Research and Application, Inc., USA, is shown in Figure
2. It consists of a stainless steel cavity, a power supply
in the range 0 to 1,900 W (maximum power delivered
by the magnetron), a 2,450 MHz magnetron for generation of microwaves, and a control panel. The accuracy
of power measurement was assessed using a calorimetric method – using 3 litres of water subjected to microwave power for 62 s. The estimated error was found to
be less than 10%. The samples were placed in a highalumina tray and positioned at the center of the oven
cavity. The oven is furnished with a built-in thermocouple for in-situ measurements of sample temperature
(meter accuracy is±8 ºC at 1000 ºC). The thermocouple
and temperature measurement system were calibrated in
ice water and boiling water (a two-point calibration).
During the experiments, the tip of the thermocouple is
made to come in contact with the center of the sample.
In each experiment the d80 of the feed, Ft, and product,
Pt, for the test ore were measured. Each experiment was
repeated three times, and the corresponding standard
deviation was estimated, and reported in the figures. In
addition to sample size and exposure time, the relative
work index was also determined as a function of micro-
9
Figure 2: Snapshot of the microwave oven BP-210.
wave power.
The morphology of the radiated and non-treated
samples was analyzed by means of scanning electron
microscopy (JEOL 6400 SEM, BSE, 2nA, 20 kV, 38
mm).
dard deviation was calculated. Nickel and copper contents
of the concentrate and tailing were then determined by
atomic absorption spectrophotometry (λCu = 324.8 nm,
λNi = 232 nm).
Flotation Tests
RESULTS AND DISCUSSION
The ore was ground to a d80 of 75 µm in a ball mill
using the following grinding conditions: 70% pulp density
(pH = 11), ball mill charge of 45% of the mill volume, and
mill speed of 65 rpm at 70% of the critical speed. Eight
minutes are needed to recover the entire slurry from the
mill before the slurry is transferred to the 1.2-l flotation
cell (Denver D12, impeller speed 1,100 rpm) at a pulp
density of approximately 25%, followed by two minutes
of pH adjustment to 9.3. After the addition of the collector,
0.1% sodium isobutyl xanthate, the sample was transferred
to the microwave
oven for treatment under the following
conditions of exposure time and microwave power: 30
s, 60 s, 90 s, and 120 s at 70% (i.e., 0.7 x 1,900 kW =
1,330 W) of maximum microwave power. The sample
was placed in a Plexiglas container and positioned at
the center of the oven cavity. During the test the sample
was mixed by means of the built-in mechanical stirrer.
After sample conditioning for 10 minutes, nine ml of the
frother, 0.1% Dowfroth 250, was added to the flotation
cell and conditioned for one minute. Finally, the air valve
(13 scfh) was turned on and the sample was floated over
a period of five minutes (froth thickness = 15 mm). The
floated (concentrate) and gangue (tailing) materials were
filtered and dried for 24 hours at 100°C. Each experiment
has been repeated three times and the corresponding stan-
Heating Rate Characteristic
10
The heating rate, dT/dt, is related to the dissipation of
microwave energy that flows into the material. It can be
expressed through an energy balance on a control volume
of the material as follows
dT 2 π f ε oε E 2
=
,
dt
ρCp
(2)
assuming a uniform electric field intensity, E. The parameters ρ and Cp are the density and heat capacity of
the mineral, respectively, f is the frequency of microwave
radiation, and εo is the permittivity of vacuum (8.854 x
10-14 F/cm). Equation (2) is not necessarily linear as the
loss factor is known to depend on temperature in an intricate way.
In the tests three samples of 300 grams were heated
in the microwave oven at 70% of maximum power (1,900
kW) and their temperature, T, recorded in-situ as a function of exposure time, t. Experimental results, corresponding to the three samples, are depicted in Figure 3 (standard
deviation data are given in the inset). The samples differ in
size, with d80 of 18 mm, 9.2 mm, and 5.5 mm for samples
#1, #2, and #3, respectively. For a given sample size the
Journal of Microwave Power & Electromagnetic Energy
Vol. 40, No.1, 2005
900
800
Temperature (°C)
700
600
500
400
300
200
18.0 mm
9.2 mm
100
5.5 mm
0
0
20
40
60
80
100
Exposure Time (s)
Figure 3: Temperature versus microwave exposure time at 70% of maximum power (1,900 kW) for samples of different size as indicated in the inset. (The fitting lines are a guide to the eye).
temperature increases rapidly (rapid regime) with exposure time then reaches a quasi-steady state regime (slow
regime). The nontrivial relationship between temperature
and exposure time correlates well with the relationship
between the loss factor, ε”, and temperature of sulphidebearing minerals [AECL and Voss, 1990].
The other finding worth noticing in Figure 3 is that
the heating rate, which is defined as the average slope
of the rapid regime, seems to drastically decrease with
decreasing sample size. The calculated heating rates for
samples #1, #2, and #3 are ~11oC/s, 8oC/s, and 6oC/s,
respectively. The reason behind this behavior is not
R. Henda et al., Figure 2
quite clear, however, it may be attributed to an efficient
absorption/dissipation of microwave radiation, a function
of the penetration depth (Dp), by large ore particles compared to small ore particles. Furthermore, the smaller a
particle is, the larger the ratio (particle surface)/(particle
volume) becomes, and, consequently, more microwave
radiation is transferred to the gangue material from the
mineral particles by heat transfer. The other more plausible
cause for this behavior may be due to a change in the dielectric property, which appears in Equation (2), of the ore
with sample size, morphology, and phase change. This has
been noticed in some preliminary results obtained from
dielectric characterizations of the ore. This aspect of the
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heating rate is being investigated in a separate study.
Relative Work Index Determination
The relative work index is an indicator of how much
energy is needed to grind a sample after microwave treatment compared to grinding without microwave treatment.
It was determined from Equation (1) for four samples of
different sizes. Samples #1-3 are similar to those of the
previous section, and sample #4 has a d80 of 1.2 mm. The
typical variation of the relative work index of the samples
with exposure time to microwave radiation is shown in
Figure 4, corresponding to 70% of maximum microwave
power (1,900 kW). The grindability of the fine samples,
i.e., samples # 3 and #4, does not seem to be much affected by microwave radiation over time, but shows large
variations for the coarse samples, i.e., samples # 1 and
#2. For the latter samples the values of RWI vary in an
intricate way with exposure time: an initial decrease, then
an increase, and finally a decrease in RWI. The minimum
values of RWI are obtained after 10 s exposure to microwave radiation as shown in Figure 4. Similar trends have
been noticed in the variation of the relative work index
with microwave power at a given exposure time. A typical
example is depicted in Figure 5 for an exposure time of 10
s. In this case, the minimum values of RWI are obtained at
74% of maximum microwave power (1,900 kW). These
findings seem to suggest that ore grindability improves
with the increase in the magnitude of microwave power to
11
120
100
RWI (%)
80
60
40
18.0 mm
9.2 mm
20
5.5 mm
1.2 mm
0
0
10
20
30
40
50
60
Time (s)
Figure 4: Relative work index versus microwave exposure time at 70% of maximum microwave power (1,900 kW)
for samples of different size as indicated in the inset. (The fitting lines are a guide to the eye).
120
100
RWI (%)
80
60
R. Henda et al., Figure 3
40
18.0 mm
9.2 mm
20
5.5 mm
1.2 mm
0
70
75
80
85
90
Power (%)
Figure 5: Relative work index versus microwave power at 10 s for samples of different size as
indicated in the inset. (The fitting lines are a guide to the eye).
12
Journal of Microwave Power & Electromagnetic Energy
Vol. 40, No.1, 2005
(a)
(b)
(c)
(d)
Figure 6: SEM micrographs of the ore under different conditions: without microwave treatment (a), at 10s (b), at
40s (c), and at 50s of microwave radiation (d). Microwave power set to 70% of maximum power (1,900 kW).
a certain level only (i.e., 74% in this case) when exposure
time to microwave radiation is kept constant. This aspect
R. Hendalater
et al.,
5
is further discussed
inFigure
this section.
The non-monotonic variation of RWI with exposure time and microwave
energy is attributed to morphological considerations, viz.,
to the density of fractures as can be noticed in the SEM
monographs shown in Figure 6, at exposure time values
of 0 s, 10 s, 40 s, and 50 s, and at a microwave power
of 70% (1,900 kW).
From an energy-saving viewpoint, the amount of
energy input after 10 s exposure to 74% of microwave
power (1,900 kW) is roughly evaluated as follows
(.74 x 1.9) (kW) x
(10/3600) (h)/(0.3/1000) (t) = 13.0 kWh/t,
(3)
which is smaller than the work index, ~18.5 kWh/t, of the
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tested ore. The calculation is, of course, a rough estimate
on tests carried out on a laboratory scale, standard multimode microwave oven of low energy efficiency [Hermas,
2004]. As per Eq. (3), we expect that energy efficiency
of the process can be further improved, if exposure times
are reduced and microwave power is slightly increased,
keeping in mind that their product must be lowered. An
increase in the electric field strength is necessary to impart more damage to the ore (i.e., to reduce ore strength
and facilitate liberation of ore particles). For instance,
by reducing the exposure time by 100 and doubling the
magnitude of the electric field strength, the resulting
microwave energy input becomes 0.26 kWh/t. To reach
such energy reductions further developments (not within
the scope of the present study) in cavity designs with
higher powers and shorter exposure times are neces-
13
Table 1: Standard flotation test results for the sample with d80 = 75 µm.
Weight
Component Mass (g)
Component % Dist.
Product
Mass (g)
% Dist.
Cu
Ni
Cu
Ni
Concentrate
22.9
6.4
2.16
2.41
51.04
60.97
Tailing
335.6
93.6
2.07
1.54
48.96
39.03
Feed (calculated)
358.5
100.0
4.23
3.96
100.00
100.00
Feed (actual)
360
Table 2: Flotation results for the sample with d80 = 75 µm after 30 s of microwave treatment
at 70% of maximum power (1,900 kW).
Weight
Component Mass (g)
Product
Mass (g)
% Dist.
Cu
Ni
Cu
Ni
Concentrate
28.9
8.1
3.15
3.26
65.11
69.94
Tailing
328.9
91.9
1.69
1.39
34.89
30.06
Feed (calculated)
357.8
100.0
4.84
4.65
100.00
100.00
Feed (actual)
360
sary, as suggested in recent work [Kingman et al., 2004].
Floatability Study
The results of the flotation tests for the ore sample
with a d80 of 75 µm are shown in Tables 1 and 2. The data
in both tables correspond to flotation results without treatment (Table 1) and with microwave treatment (Table 2)
after 30 s exposure to 70% of microwave power (1,900
kW), as described in the previous section. The findings refer to the recovered fractions of floated material
(concentrate) and tailing, in addition to the composition
distribution of nickel and copper, measured using atomic
absorption spectrophotometry, in both fractions.
As it can be seen from the data the use of microwave
energy results in an increase (decrease) in the recovered
concentrate (tailing), and in the grades of the useful minerals, i.e., nickel and copper. After microwave treatment
of the ore, the amount of recovered concentrate increases
from ~23 g to 29 g, corresponding to a relative increase of
~26 wt%. The component distribution also improves with
microwave treatment and goes from ~51 wt% (standard
test) to 65 wt% for copper, and from ~61 wt% (standard
14
Component % Dist.
test) to ~70 wt% for nickel. It is to be noted that under
the present operating conditions, the process of flotation
does not seem to be much affected by exposure time to microwave radiation as depicted in Figure 7. Improvements
in the floatability of the ore may be attributed to kinetic
effects, viz., an increase in the rate of interaction between
the collector and the solid surface of mineral particles. Of
course, in order to draw conclusive observations a detailed
investigation of the effect of microwave radiation on the
reaction kinetics is required.
CONCLUSIONS
In this work the effect of microwave energy on the
mineral separation, viz., the processes of communition
and flotation, of a complex nickel-copper ore obtained
from the Sudbury basin has been investigated. In communition, the heating rate is found to be a function of
ore size and increases with sample size in a nontrivial
manner. The results have shown that ore communition,
expressed in terms of the relative work index, under microwave radiation is tremendously improved for coarse
Journal of Microwave Power & Electromagnetic Energy
Vol. 40, No.1, 2005
80
70
60
%; g
50
40
30
20
-.-.- Cu
----- Ni
10
-..-..- Conc.
0
0
20
40
60
80
100
Time (s)
Figure 7: Flotation results versus exposure time for the sample with d80 = 75 µm after 30 s of microwave
treatment at 70% of maximum power (1,900 kW). (#3) A comparison between the standard results
(dashed lines) and the results after microwave treatment is also shown. Units: % for Cu and
Ni (standard deviation shown in the inset), and gram for Concentrate (Conc.).
ore samples (d80 ≥ 9 mm), but experiences little change
for fine samples. The values of the relative work index
vary in a non-monotonic way with microwave exposure
time and power. The lowest values of the work index are
obtained at very short times (~10 s) making the use of miet al.,
Figure 6
crowave radiationR.toHenda
enhance
communition
economically
attractive. The preliminary results from the flotation tests
show substantial increase in the amount of recovered ore
(~26 wt%) and in the grades of nickel (relative increase
of ~15 wt%) and copper (relative increase of ~27 wt%)
in the concentrate, under the present process conditions.
In a forthcoming paper we will elaborate on dielectric,
chemical and morphological characterization of the ore
as it undergoes microwave treatment.
ACKNOWLEDGMENTS
Funding provided by LURF (to R.H.), and by Atlantic Innovation Fund, NSERC and Petroleum Research
Atlantic Canada (to M.R.I.) is acknowledged and appreciated.
International Microwave Power Institute
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