Download Scaling of manganese compounds in kraft mills

Scaling of manganese
compounds in kraft mills
-state of the art
Per Ulmgren
2005
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Scaling of manganese
compounds in kraft mills –
state of the art
Per Ulmgren
STFI-Packforsk report no.: 70 | November 2005
Cluster:Chemical Pulp Recovery
Restricted distribution to: AGA, AssiDomän Cartonboard, Billerud, Borregaard,
Eka Chemicals, Holmen Paper, Kemira, Korsnäs, M-real, Mondi Packaging,
Peterson & Son, Stora Enso, Södra Cell, Voith
a report from STFI-Packforsk
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Contents
Page
1
Summary
1
2
2.1
2.2
2.2.1
3
3
3
2.2.2
Introduction
Aim
Background
Survey of scale formation of manganese containing
compounds on the brown side of the fibre line
Manganese in kraft mills
3
3.1
3.2
3.3
3.4
3.5
3.5.1
3.5.2
3.5.3
3.5.4
Manganese process chemistry
Process conditions in kraft mills
Formation of solid solutions
Formation of metal carbonates
Oxygen delignification stage
Model calculations
Cooking conditions
Oxygen delignification
Hydrogen peroxide bleaching
Summary of Mn(II-IV) forms in the fibre line
5
5
9
11
12
13
14
15
16
17
4
Conclusions and future research work
19
5
References
20
Appendix
Theory
22
3
4
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1
Summary
The aim of this work was to clarify the state of the art regarding scaling of
manganese compounds in kraft mills, especially in the fibre line.
Three Swedish kraft pulp mills have or have had problems with hard black
scales of manganese containing compounds on their washing equipment in
the brown stock washing, especially in wash presses prior and after the
oxygen delignification stage. The scales are situated on the backside of the
channels of the drum and thereby clogging the wholes. The composition of
the scales, which has an amorphous feature, corresponds to manganese
oxide (probably MnO2(s)) and/or manganese oxide hydroxide (probably
MnO(OH)(s)).
Manganese is introduced into kraft mills with the wood. The amount of Mn
in wood is in the range of 50 to 200 g/ADt (100 % dryness) for Scandinavien
wood species. The main purging medium is the green liquor dregs.
Manganese is known to cause problems in the bleaching due to catalytic
decomposition of hydrogen peroxide.
Much knowledge has been gained during the last decade regarding the
chemistry of manganese under technical conditions. The possibility to redox
stabilise Mn(II) from oxidation by the formation of a solid solution,
(Mg,Mn)(OH)2(ss), or a mixed metal carbonate solid, MnCO3(s) coated with
MgCO3(s), by adding excess of Mg2+ has given new possibilities to solve the
problems with scaling of manganese compounds, besides the decomposition
of hydrogen peroxide under oxidative conditions.
The oxidising of Mn(II) to Mn(III,IV) can probably not be fully avoided
around an oxygen delignification stage, but the oxidising can be reduced by
adding excess of Mg2+ to the positions where there is a risk for Mn(II) to
oxidise. Manganese is redox stabilised in oxidation state +II by the
formation of a solid solution with Mg2+, and by precipitation of magnesium
carbonate on the surfaces of precipitated manganese carbonate.
The mechanisms behind the formation of scales of manganese compounds
in the fibre line is not known in detail in spite of all knowledge gained
during the last decade. There is still a lack of some vital knowledge for the
solution of this process problem. Future work should be devoted to the
following issues in order to gain an increased understanding of the
mechanisms:
•
Clarification of how the redox potential is changed from cooking to
post oxygen delignification positions, both in a kraft mill having
scaling problems and one without problems (reference case).
•
The changes of alkalinity (pH) and temperature should be recorded
at the same time.
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•
Other process conditions of importance to clarify are the change of
total concentrations of Na+, Mg2+ and Mn(II,IV) in pulp and
suspension, carbonate and hydroxide concentrations, and ionic
strength.
•
Testing the possibility to reduce the problems by addition of some
magnesium compound, e.g. MgSO4.
•
Simulation models that describe the scaling mechanisms should be
developed and mill balances calculated using e.g. WinGEMS.
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2
Introduction
2.1
Aim
The aim of this work was to clarify the state of the art regarding scaling of
manganese compounds in kraft mills, especially in the fibre line.
2.2
Background
Three kraft pulp mills in Sweden have reported problems with scaling of
manganese compounds in their wash presses prior and after the oxygen
delignification. The problems have started when the mills have
altered/modernised oxygen stage. The scales are black and very hard, and
difficult to remove from the presses. The scales clog the channels of the
press medium and impair the liquor flow through the press. The scales
consist mainly of some Mn(III,IV) containing compound.
2.2.1
Survey of scale formation of manganese containing
compounds on the brown side of the fibre line
A survey regarding scaling of manganese compounds in the brown stock
washing of kraft mills was done in 2003 to clarify the frequency of such
scaling problems in the fibre line of Finnish and Swedish kraft pulp mills.
According to mill experiences deposit formation of manganese
hydroxide/oxide can cause serious scaling problems on pre-oxygen and postoxygen washing equipment, especially presses, in kraft mills. The scales
have been very hard and difficult to remove.
11 (out of 13) Swedish and 9 (out of 15) Finnish mills responded to the
survey. Totally 3 of these mills (all Swedish) reported the occurrence of
scales of manganese hydroxide/oxide had been observed preferentially on
washing presses on the brown side.
Two mills reported to have continues problems due to scaling of manganese
compounds on the O2-stage wash presses, Table 1. In one case the scale
formation appeared when the old washing filtres were exchanged for new
wash presses prior and after the O2-stage, and in another case when
introducing O2-delignification into the fibre line. The third mill experienced
scales with manganese compounds when some changes were done in the
O2-stage in connection with a specific mill trial (no information regarding
type of trial was given).
No data of manganese concentrations in the different mill process streams
were gained in the survey.
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Tabel 1. Results from a mill survey on scaling of manganese hydroxide/oxide
compounds.
Mill A
Mill C
Mill B
Wood species
hard- and softwood
hard- and softwood
softwood
Cooking method
continuous
continuous
batch
prior and after O2-
prior and after O2-
stage
stage
Location of scale
Kappa number, in- and
out of O2-stage
after O2-stage
15 and 10
24 and 11
22 and 11
Scale content of Mn
(wt %)
70
70 (?)
32
Other NPEs present
Mg, Ca, Si, Zn
Mg, Ca
Mg (~ 67 wt %)
Occurrence of scales
during mill trials
in the O2-stage
after introducing
changes in the O2stage
when introducing
an O2-stage
2.2.2
Manganese in kraft mills
Manganese is introduced into kraft mills with the wood raw material. The
wood content of manganese is in the range of 50 to 200 g/ADt (100 %
dryness) for Scandinavien wood species (Ulmgren 1997). The main purging
medium is the green liquor dregs but some lesser amounts are also purged
together with the total effluents and the pulp (Ulmgren, Rådeström 2002).
Hydrogen peroxide and other oxygen containing bleaching chemicals is
catalytically decomposed by several transition metal ions, viz. Mn2+ and Fe2+,
which are easily oxidized in the presence of hydrogen peroxide forming radical
species. These radicals may then reduce the metal ions back to the divalent
stage while forming oxygen and water. It is, however, possible to stabilize
manganese and iron in their oxidation state II by co-precipitating Mn2+ and Fe2+
ions with Mg2+ in a solid solution with hydroxide or silicate anions, e.g. (Mg2+,
Mn2+)(OH)2(ss), or solid carbonates where the Mn(II)CO3(s) is coated with a
layer of MgCO3(s). In this way it is possible to enhance the stability of the
divalent oxidation state of Mn2+ and Fe2+ under strongly alkaline and oxidative
conditions (Lidén and Öhman 1997). The catalytic decomposition of hydrogen
peroxide by the metal ions can therefore be stopped. Polyelectrolytes, e.g.
polygalacturonic acid or kraft pulp, must also be present in order to stabilize
hydroxide precipitates.
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3
Manganese process chemistry
3.1
Process conditions in kraft mills
The process conditions vary to a large extent in a kraft pulp mill producing
bleached pulp, viz. in the fibre line from strongly reducing and alkaline
conditions in the cooking to strongly oxidising and alkaline conditions in
the oxygen delignification stage. In the bleach plant it is oxidising and
acidic, or oxidising and alkaline conditions depending on bleaching
sequence. The technical conditions affect the process chemistry of the metal
ions, and especially that of the transition metal ions, e.g. the oxidation
state of manganese.
pE
30
MnO4-
Z
20
P
MnO2(s)
10
A
O
Q
MnOOH(s)
0
Mn2+
-10
Mn(OH)2(s)
C
MnS(s)
-20
0
2
4
6
8
10
12
14
pH
Figure 1. Redox diagram for Mn at 25 ºC and in dilute solutions. Estimated pEvalues in the different process stages are indicated in the diagram as C =
cooking, O = oxygen delignification stage, P = hydrogen peroxide stage, Q =
complexing agent stage and Z = ozone stage. pE is a measure of redox conditions
(cf. Appendix).
The different dominating forms of manganese under different reducing and
oxidising conditions are shown in Figure 1. The wood manganese is to a
large part extracted in the cooking in oxidation state II. The change of
concentrations of Mn2+ and Mg2+ in the cooking liquor during cooking
deviates from that of Ca2+, cf. Figures 2 and 3. The concentrations of Mn2+
and Mg2+ are increased with cooking time while the Ca2+ concentration
passes a maximum and then drops off. The higher starting concentration of
carbonate ions the earlier the dissolved Ca2+ concentration starts to fall off.
The concentrations of Mn2+ and Mg2+ are not dependent on the carbonate
ion concentration since they are not re-precipitated as metal carbonates as
Ca2+ is. Mn2+ is probably a large part re-precipitated as MnS(s) and Mg2+ as
Mg(OH)2(s). MgCO3(s) is not formed although it has a rather low solubility.
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This is due to the need for a very high degree of supersaturation of the
precipitating ions to initiate a precipiation, higher than that for CaCO3(s).
2+
[Me ]tot (mmol/L)
0.3
Mg2+
2+
Mn
0.2
0.1
0.0
-50
0
50 100 150
time (minutes)
Figure 2. Total Mg2+ and Mn2+ concentrations, [Me2+]tot, in the cooking liquor
during kraft cooks. The cooks were performed in a laboratory digester sytem
with circulating cooking liquor. The Mg2+ and Mn2+ concentrations in the
cooking liquor should have been 4.05 and 0.84 mmol/L, respectively, when all
Mg2+ and Mn2+ in the wood had been dissolved (Hartler, Liebert 1973).
2+
[Ca ]tot (mmol/L)
1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150 200 250
time (minutes)
Figure 3. Total calcium ion concentration in the cooking liquor, [Ca2+]tot, during
pine kraft cook at 165 ºC. The cooks were performed in a laboratory batch
cooking sytem. The wood contained 700 mg/kg Ca2+. The aliquates were
immediately filtered (0.45 μm pore size). The dissolved calcium ion
concentration in the cooking liquor should have been 8.75 mmol/L, when all the
calcium ions in the wood had been dissolved (Lidén et al. 1996).
About 6 and 18 % of Mg2+ and Mn2+ were in the liquor aliquots at the end of
the cook while the Ca2+ part was less than 1 % when the carbonate ion
concentration was about 0.2 mol/L. Note that the aliquots for Mg2+ and
Mn2+ were not filtered directly after sampling, which the Ca2+ aliquots
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were. Part of Mg2+ and Mn2+ in cooking liquor can be present as colloidal
precipitates. Most of the Ca2+ is re-precipitated on the pulp as CaCO3(s)
since the dissolved part of Ca2+ is very small (about 0.1 mmol/L). The
dissolved part consists of Ca2+ and complxes between Ca2+ and inorganic
anions, and organic anions formed during the cooking.
wood
220
white liquor 5
Digester
Recovery
boiler
black
liquor
cooking 115
liquor
green liquor
dregs 170
165
O2-stage
reactor
oxygen
delignified
pulp 60
wash
filtrate
50
Figure 4. Manganese balance in a kraft mill. Based on data from Lidén 1994.
Unit: g Mn ptp.
Some colloidal MnS(s) enters the cooking with the white liquor (Lidén
1994). A large part, i.e. about 2/3, of the manganese is separated together
with the black liquor entering the black liquor evaporation train, Figure 4.
Mn(pulp)/Mn(wood) (%)
100
80
60
40
20
0
wood
pO
aO
aD0
aE
aD1
aD2
Figure 5. Remaining manganese in pulp (as % of the intake with wood) along
the fibre line including an elemental chlorine free (ECF) bleaching sequence in a
kraft mill (Ulmgren 1993). Prefixes p and a stand for prior to and after a stage.
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Manganese will be present mainly as solid particles of MnS(s) and probably
partly as Mn(OH)2(s) in the black liquor, but also in soluble species as
complexed to different organic anions such as orto phenols formed in the
cook. About 30 % of the wood manganese remained in the pulp after the
cook and part of it is extracted in the oxygen delignification stage and in
the bleaching, Figure 5. The data given in the Figure 5 were based on a
mill sampling campaign including an ECF blaching sequence (Ulmgren
1993).
Manganese is readily oxidised to Mn(III,IV) in the O2 stage under
formation of radicals due to the strongly oxidising and alkaline conditions,
Figure 6. This reaction can partly be hindered by addition of Mg2+. Many
theories have in the past been presented to explain the mechanisms of this
positive effect of Mg2+. The most plausible mechanism is at present that
given by Lidén and Öhman (1998).
HO• + HO–
Fe3+, Mn3+
½ O2 • – + HO–
O2 • –
O2 • – + H2O
H2O2
HO2–
HO– + HO2–
Fe2+, Mn2+
O2
Figure 6. Mn2+ and Fe2+ are easily oxidised in hydrogen peroxide solutions under
formation of radicals. The mechanisms are described by the so-called Fenton
cycle (Walling1975).
The remaining manganese in the pulp entering the bleaching after the
oxygen delignification is extracted as Mn2+ in the first acid stage, mostly a
chlorine dioxide (D) or a complexing (Q) stage. Manganese is oxidised to
Mn(III,IV) in the bleach plant under oxidizing and alkaline conditions such
as in hydrogen peroxide (P) and ozone (Z) stages unless redox stabilised by
addition of Mg2+.
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3.2
Formation of solid solutions
Oxidation of Mn(II) to Mn(III,IV) can to a large extent be hindered in
oxygen delignification and hydrogen bleaching stages by the addition of
magnesium sulfate. The metal ions Mg2+ and Mn2+ can form solid solutions
with hydroxide and silicate anions as counter-ions, i.e. (MgxMn1-x)(OH)2(ss)
and (MgxMn1-x)SiO3(ss), respectively, under alkaline and oxidative
conditions (Wiklund et al. 2001a), Figure 7.
= OH–, SiO32–
= Mg2+
= Mn2+ or Fe2+
Figure 7. Illustration of a solid solution. Part of the Mg2+ can be substituted for
Mn2+ and Fe2+ (Wiklund et al. 2002a).
The formation of solid solutions stabilizes the Mn(II) oxidation state of
manganese. However, the main part of the Mg2+ added will be present as
more or less colloidal precipitate of Mg(OH)2(s), since Mg2+ as a rule is
added in excess to Mn2+. Prerequisite for solid solution formation is that the
ionic radius of the metal ions is of the same magnitude. The unfamiliar
metal ion is incorporated into the lattice of the main constituent without
excessive distortion of the lattice, viz. Mn2+ is incorporated into the
crystalline lattice of Mg(OH)2. Mg2+ can also be substituted for Fe2+.
The apparent solubility product (L) of Mn(OH)2(s) is decreased as the molar
ratio of Mg/Mn is increased in solution, Figure 8. The apparent solubility
product is decreased by about two logarithmic units when the molar ratio is
increased to higher than 10. The apparent solubility product is in this
context defined as:
L = [Mn(II)]tot [OH-] 2
[1]
Thus, the total dissolved manganese concentration can be strongly
decreased by the addition of Mg2+ and consequently Mn2+ is to a large
extent stabilized by the incorporation inte the crystalline lattice of
Mg(OH)2(s). For example, at 90 °C, Mg/Mn = 31 mol/mol, OH/(Mg+Mn) =
1.5 mol/mol, and after a equilibrating time of 6 hours, the aqueous phase
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contained dissolved Mn2+ at a concentration 200 times lower than that
expected if pure Mn(OH)2(s) had been the solubility-controlling solid phase.
It should be noted that it is very difficult to form a solid soution between
Mg2+ and Mn2+ unless pulp is present.
log L
-12
-13
-14
0
10
20
30
Mg/Mn, mol/mol
Figure 8. The apparent solubility product of Mn(OH)2(s) (as log L) versus the
molar ratio of Mg/Mn (Wiklund et al. 2000). The molar ratio of OH/Me is 1.5,
temperature 90 ºC, equilibrating time 6 hours, and ionic strength 0.1 mol/L.
The apparent solubility products of Mn(OH)2(s) can be calculated as
(Wiklund et al. 2000):
log L = - 7.593 – (758/T) – 0.219 ln(t)
- 0.583 ln([Mn(II)]tot[Me(II)]tot) – 0.761([Mn(II)]tot[Me(II)]tot)2
- 0.427([OH–]tot[Me(II)]tot) ([Mg(II)]tot[Me(II)]tot)
[2a]
and that of Mg(OH)2(s) as:
log L = - 12.050 – (1055/T) – 0.248 ln(t)
- 0.181 ln([Mg(II)]tot[Me(II)]tot) –
- 0.427([OH–]tot[Me(II)]tot) ([Mn(II)]tot[Me(II)]tot)
[2b]
[Me(II)]tot = [Mg(II)]tot + [Mn(II)]tot
[2c]
where
Temperature, T, and equilibrating time, t, in these equations are expressed
in Kelvin and minutes, respectively, and the equations are valid for
323 ≤ T ≤ 363 K, 15 ≤ t ≤ 360 min and 0 ≤ [OH–]tot/[Me(II)]tot ≤ 2 mol/mol.
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3.3
Formation of metal carbonates
MgCO3(s) is readily precipitated on the surfaces of MnCO3(s) particles
(Wiklund et al. 2001b). Mn(II) is redoxstabilised in oxidation state +II by
the formation of the protective layer of MgCO3(s). Normally it is quite
difficult to precipitate MgCO3(s) from aqueous solutions due to the need for
a very high degree of supersaturation of the precipitating ions. However,
this requirement for supersturation is strongly reduced by the presence of
particles of precipitated MnCO3(s). Thus, Mn(II) is mainly in the core and
Mg(II) in the outer layer of the particles, Figure 9.
Mn
Mg
Figure 9. SEM-EDS photographs showing the elemental distribution of Mn(II)
and Mg(II) in a polished rhombohedral crystal originating from a suspension of
high [Mg(II)]tot/[Mn(II)]tot molar ratio (Wiklund et al. 2001).
Left side: Core dominated by Mn(II).
Right side: Outer layer dominated by Mg(II).
Some Mg2+ is also precipitated as the metastable compound
Mg5(OH)5CO3(s), Figure 10. MnCO3(s) precipitated with MgCO3(s) is for
simplicity hencefore denoted MgCO3*MnCO3(s) or just *MnCO3(s).
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MgCO3*MnCO3(s)
Mg5(OH)2(CO3)4(s)
Figure 10. A SEM photograph showing a precipitate resulting from a solution
with 18 mmol/L in Mg(II) and 2 mmol/L in Mn(II), and precipitated with HCO3(90 ºC, ageing time 24 hours). From x-ray powder diffraction measurements, the
flake structueres could be identified as Mg5(OH)2(CO3)4(s) and the rombic
structure as MgCO3*MnCO3(s) (Wiklund et al. 2001b).
3.4
Oxygen delignification stage
Mg2+ and Ca2+ are present as precipitated carbonates on or within the fibre
walls in oxygen delignified pulps (Norberg et al. 2002), Figure 11. The
calcium carbonate is to a large extent formed already during the cooking.
However, magnesium carbonate is probably partly formed in the oxygen
delignification stage since a rather large amount of Mg2+ is added in that
position. Mn(II) is present as manganese carbonate solid, where
Mn(II)CO3(s) is coated by a layer of MgCO3(s) (Wiklund et al. 2001b).
Mg(II) is also present as Mg5(OH)2(CO3)4(s). Na+ balances the ion exchange
capacity of the pulp.
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Na+
Na+
+
+
CaCO3(s)
Na+
Na+
Na+
+
Na+
Na+
Na+
+
+
+
+
+
+
MgCO3*MnCO3(s)
Na+
Fibre volume
Suspension
Figure 11. Model of a pulp suspension in an oxygen delignification stage based
on Donnan theory (Norberg et al. 2002).
The solid metal carbonates are dissolved in the first acid or near neutral
stage in the bleach plant. Mg2+ and Ca2+ are once more precipiated on the
subsequent alkalizing but now Mg2+ as Mg(OH)2(s) and MgxMn1-x(OH)2(ss)
are formed in the pulp suspension. The apparent solubility of Mn(OH)2(s) is
strongly decreased by the formation of the solid solution, i.e. Mn(II) is redox
stabilized.
3.5
Model calculations
The situations in cooking, oxygen delignification and hydrogen peroxide
bleaching can be illuminated by model equilibrium calculations using the
program WinSGW (Karlsson, Lindgren 2000) that was developed from
SOLGASWATER (Eriksson 1979). Formation constants developed from
laboratory experiments using clean solutions, i.e. no organic substance
present, were used in these calculations. The formation constant for
MnO2(s) is rather uncertain since there is no reliable data available in the
chemical literate. The conditions in the different stages are given in
Table 2. The program allows the usage of Donnan theory and charge
dependent acid-base equilibria for the active groups on the fibre surfaces.
Predominance diagrams were here used to illustrate the stability areas for
the main species of Mg2+ and Mn(II-IV) under the technical conditions. The
diagrams were plotted as the total carbonate ion concentration versus pH
(at 90 ºC). Along a bordering line both species bordering the line are
present in equal amounts.
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Table 2. Tentative summary of conditions in cooking, oxygen delignification and
hydrogen peroxide bleaching in kraft mills (cf. Figure 1).
Factor
Cooking
O2-stage
P-stage
pH (at 90 ºC)
11.5
11
8-9
pE
-15
7
15
temp., ºC
150
90+
90+
0.2 - 1
0.1 – 0.2
< 0.001
0.3
0.02 – 0.05
0.01
6
14
32
[OH–], mol/L
[CO32–], mol/L
Mg/Mn, mol/mol
3.5.1
Cooking conditions
2+
Mg and Mn2+ are to a large extent extracted during cooking (Hartler,
Liebert 1973), cf. Figure 2. Only a minor part remains in the cooking liquor
due to re-precipitation. The total molar ratio of Mg/Mn can be assumed to
be about 6 in cooking, Table 2. Both cooking and oxygen delignification
stages are alkaline. However, the cooking liquor is strongly reductive while
the oxygen delignification liquor is highly oxidative.
2-
log [CO3 ]tot
0
-1
2-
cooking and O2-stage
log [CO3 ]tot
0
MgCO3(s)
-1
-2
-2
MnS(s)
-3
-3
Mg2+
-4
cooking
Mg(OH)2(s)
-5
Mn(OH)2(s)
-4
-5
-6
4
6
8
10
pH
12
6
8
10
pH
12
Figure 12. Predominance diagrams for the system: Mg2+ - Mn2+ - CO32– - OH– HS-. Temperature is 90 ºC, ionic strength 0.1 mol/L, pE = -15 and equilibrating
time 6 hours.
Left side: Species of Mg2+. Mg2+ added is 5 mmol/L and Mn2+ is 0 mmol/L.
Right side: Species of Mn2+. Mg2+ added is 3 mmol/L and Mn2+ is 0.5 mmol/L.
Mg2+ is mainly present as Mg(OH)2(s) in both cooking and oxygen
delignification but some MgCO3(s) is most probably formed, especially in
oxygen delignification as coating of MnCO3(s), Figure 12 (left side). Mn2+ is
in cooking mainly precipitated as MnS(s). However, most probably some
Mn(OH)2(s) is also formed, Figure 12 (right side).
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3.5.2
Oxygen delignification
The conditions in an oxygen delignification stage are oxidative and
alkaline, and temperature and ionic strength are about 90 ºC and
0.5 mol/L, respectively, Table 2. The redox potential (as pe) was estimated
to be about 7, cf. Figure 1.
Mg2+ is under these conditions mainly present as Mg(OH)2(s), Figure 12
(left side). Here MgCO3(s) is precipitated as a coating on MnCO3(s).
2-
log [CO3 ]tot
0
-1
O2-stage
log [CO32-]tot
0
*MnCO3(s)
-1
*MnCO3(s)
O2-stage
-2
-2
-3
-3
MnO2(s)
Mn2+
-4
-4
Mn2+
(Mgx,Mn1-x)(OH)2(ss)
(Mg+Mn)/Mn = 32
-5
-6
-5
6
8
10
pH 12
6
8
10
pH
12
Figure 13. Predominance diagrams for the system: Mg2+ - Mn2+ - CO32– - OH–,
under oxygen delignification conditions. Temperature is 90 ºC, ionic strength 0.1
mol/L and pe = 7. Mn(II) as ([Mg(II)]tot + [Mn(II)]tot)/[Mn(II)]tot is 32 mol(mol and
equilibrating time 6 hours. *MnCO3(s) = MgCO3(s)*MnCO3(s). MnO(OH)(s) was
not formed according to equilibrium calculations.
Left side: All species included in the calculations.
Right side: The formation of MnO2(s) was suppressed assuming a slow
formation of crystalline MnO2(s).
The rate of oxidation of Mn(II) to Mn(IV) is dependent on both pe and pH.
Mn(II) should from an equilibrium point of view readily be oxidized to
Mn(IV) forming MnO2(s) under conditions found in an oxygen
delignificalion stage, Figure 13 (left side). However, the formation constant
of MnO2(s) is as pointed out very approximative since there is very few, and
rather uncertain literature values available. The formation of crystalline
MnO2(s) is most probably a very slow process. The composition of the
blackish brown precipiate of MnO2 is variable and should be written
~MnO2. MnO(OH)(s) was not formed under oxygen delignification
conditions according to the equilibrium calculations.
The formation of MnO2(s) can probably be neglected in the equilibrium
calculation under process conditions according to the discussion above,
Figuere 13 (right side). The solid solution (Mg,Mn)(OH)2(ss) is formed
under the conditions found in an O2-stage, and it is the most stable form of
Mn(II) above pH 7.5, at 90 ºC and ionic strength 0.1 mol/L.
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3.5.3
Hydrogen peroxide bleaching
The conditions in a hydrogen peroxide stage are alkaline and oxidative just
as in an oxygen delignification stage. The solid solution (Mg,Mn)(OH)2(ss)
is the most stable manganese form in this stage, Figure 14. The coprecipitation MgCO3*MnCO3(s) can also be formed when the hydroxide ion
concentration is low and the carbonate ion concentration is high.
2-
log [CO3 ]tot
0
*MnCO3(s)
-1
-2
P-stage
-3
2+
Mn
MgMn(OH)2(ss)
-4
-5
4
6
8
10
pH
12
Figure 14. Predominance diagrams for the system: Mg2+ - Mn2+ - CO32– - OH–,
under hydrogen peroxide bleaching conditions. Temperature is 90 ºC and ionic
strength 0.1 mol/L. pe was 15 and equilibrating time6 hours. ([Mg(II)]tot +
[Mn(II)]tot)/[Mn(II)]tot is equal to 32 mol/mol. *MnCO3(s) = MgCO3(s)*MnCO3(s).
The formation of MnO2(s) was supressed.
The situation in a hydrogen perxide stage is illustrated in Figure 15.
Na+
+
Na+
CaCO3(s))
+
(Mg,Mn)(OH)2(ss)
+
+
+
Ca2+
+
Mg2+
+
Na+
+
+
Na+
Fibre volume
Na+
MgCO3*MnCO3(s))
Suspension
Figure 15. Model of a pulp suspension in a hydrogen peroxide bleaching based
on Donnan theory (Norberg et al. 2001; 2002).
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3.5.4
Summary of Mn(II-IV) forms in the fibre line
Manganese (II-IV) exists in many different forms in kraft pulp mills ,
Figure 16. Mn(II) is mainly present as solids, i.e. MnS(s) and Mn(OH)2(s),
in the cooking. Mn(II) is also partly bound in soluble complex species
(MnL2-n) with some organic anios such as chatechole, formed in the lignin
decpomposition.
However, Mn(II) will be open for oxidation to Mn(III,IV) somewhere in
between cooking and oxygen delignification. The position for oxidation is
dependent on how pe and alkalinity is changed due to the counter-current
liquor flow from post oxygen stage to cooking in the brown stock washing.
Manganese can form both (MgxMn1-x(OH)2(ss) and MgCO3*MnCO3(s) in
oxygen delignification and in alkaline positions after the oxygen
delignification. Mn2+ is released in the first acidic stage in the bleaching, as
a rule D- or Q-stage, mainly from the dissolution of metal carbonates.
However, the metal ions within the pulp suspension passing on to an
alkaline stage are to a large part re-precipitate as hydroxides and/or
carbonates.
Mn(II) can be present as free Mn2+ in near neutral solutions towards the
acid side.
Positions prior to
O2-stage
pH
14
12
10
Positions after
O2-stage
pH
MnS(s), Mn(OH)2(s)
MnL2-n
14
12
10
8
8
6
6
4
4
(MgxMn1-x)(OH)(ss)
MgCO3*MnCO3(s)
Mn2+ - fibre
free Mn2+
Figure 16. Summary of the managanese (II-IV) forms in the fibre line. Based on
data from Lidén (1994). Ln- stands for organic anions such as chatechole.
Suggested forms of Mn(II) in different stages prior to and after oxygen
delignification stage. The stability ranges with respect to pH are approximative.
The dissolution of precipitated carbonates of Mg2+ and Ca2+ is favoured by
the strong attraction force of the fibre for divalent metal ions under process
conditions where the amount of divalent metal ions is low compared to the
ion exchange capacity of the pulp. This implies that the metal carbonates
but also metal hydroxides should dissolve at a somewhat higher pH than in
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a system without ion exchange capacity. This is a fact when the Na+
concentration is below 0.15 mol/L, i.e. when the partion constant is high.
The hydroxide phase is stable at pH above 12 (25 ∂C) and the carbonate
phase below pH 11. The separation of Mg(II) and Mn(II) in an oxygen
delignification stage can be described as a release in the next washing
stage of Mg-Mn(II) precipitates loosely held to the pulp.
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4
Conclusions and future research work
Much knowledge has been gained during the last decade regarding the
chemistry of manganese under technical conditions. The possibility of redox
stabilising Mn(II) from oxidation by the formation of solid solution and
mixed metal carbonate solids with Mg2+ has given new possibilities to solve
the problems with scaling of manganese compounds in the fibre line and
decomposition of hydrogen peroxide in the bleaching.
The oxidising of Mn(II) to Mn(III-IV) can probably not be fully avoided
around the oxygen delignification stage. However the oxidising can
probably be hindered to a large extent by adding excess of Mg2+ to the
positions where there is a risk for Mn(II) oxidation and formation of scales
of manganese containing compounds.
There is a lack of knowledge regarding the mechanisms behind the
formation of scales of manganese compounds in the fibre line. Future work
should cover the following issues:
•
Clarification of how the redox potential is changed from cooking to
post oxygen delignification positions, both in a mill having scaling
problems and one without problems.
•
The change of alkalinity and possibly temperature should also be
recorded.
•
Other process conditions of importance are the concentrations of
Mn(II-IV) and Mg2+ in pulp and liquor, carbonate and hydroxide
concentrations and liquor concentration of Na+ (ionic strength).
•
Testing the possibility to reduce the problems by addition of some
magnesium compound, e.g. MgSO4.
•
Simulation models describing the scale formation should be
developed and mill balanced calculated using e.g. WinGEMS.
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5
References
Donnan F G, Harris A B
The osmotic pressure and conductivity of aqueous solutions of Congo-red,
and reversible membrane equilibria
J Chem Soc 99:1554 (1911)
Eriksson G
An Algorithm for the Computation of Aqueous Multicomponent Multiphase
Equilibrium
Anal Chim Acta 112, 375 (1979)
Hartler N, Libert J
The behaviour of Certain Inorganic Ions in the Wood/White Liquor System
Svensk Papperst 76(12) 454 (1973)
Karlsson M, Lindgren J
WinSGW, Internet version of SOLGASWATER, 2000
Lidén J
The Chemistry of Manganese in a Kraft Mill
3rd European Workshop on Lignocellulosics and Pulps, Hasseludden,
August 28-31, 1994, Stockholm
Lidén J, Lindgren P, Lukkari I, Söderberg C
The relationship between wood species, CaCO3-scaling and calcium balance
in the kraft cook
5th Int. Conf. on New Available Techniques SPCI, June 4-7, 1996,
Stockholm, p. 498
Lidén J, Öhman L-O
Redox Stabilization of Iron and Manganese in the +II Oxidation state by
Magnesium Precipitates and Some Anionic Polymers. Implications for the
use of Oxygen-Based Bleaching Chemicals
J Pulp Paper Sci 23(5), 193 (1997)
Lidén J, Öhman L-O
On the Prevention of Fe- and Mn-Catalyzed H2O2 Decomposition Under
Bleaching Conditions
J Pulp Paper Sci 24(9), 269 (1998)
Lindgren J, Wiklund L, Öhman L-O
The contemporary distribution of cations between bleached softwood fibres
and the suspension liquid, as a function of -log[H+], ionic strength and
temperature
Nord. Pulp Pap. Res. J. 16(1), 24 (2001)
Norberg C, Lidén J, Öhman LO
Modelling the distribution of “Free”, Complexed and Precipitated Metal
Ions in a Pulp Suspension Using Donnan Equilibria
J. Pulp Paper Sci. 27(9): 289 (2001)
Norberg C, Lidén J, Lindgren J, Öhman LO
Some practical aspects of the metal ion chemistry in pulp processes
7th International conference on new available technologies, Stockholm,
Sweden, 4-6 June 2002, pp 36 (2002)
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Sillén L-G, Martell A E
Stability Constants of Metal-ion Complexes
The Chem Soc London, Chem Soc Spec Publ. No 17 and No 25 (1964, 1971)
Stumm W, Morgan J
Aquatic Chemistry
3rd Edition, John Wiley & Sons, New York (1996)
Towers M, Scallan A M
Predicting the Ion-Exchange of Kraft Pulps Using Donnan Theory
J Pulp Paper Sci 22(9):J332 (1996)
Ulmgren P
Unpublished data (1993)
Ulmgren P
Non-Process Elements in a Bleached Kraft Pulp Mill with a high Degree of
System Closure - State of the Art
Nord Pulp Pap Res J 12(1), 32 (1997)
Ulmgren P, Rådeström R
Process Chemistry on Trace Elements in a Kraft Pulp Mill
KAM-report A85 (“KAM” Ecocyclic Pulp Mill Program) (2002)
Walling C
Fenton's Reagent Revisited
Acc. Chem. Res. 8, 125 (1975)
Wiklund L
Mechanisms of Mn(II) Stabilisation by Mg(II) in Alkaline Aqeous Solution
Licentiate thesis, Umeå (2000)
Wiklund L, Lidén J, Öhman L-O
Solid Solution Formation Between Mn(II) and Mg(II) Hydroxides in
Alkaline Aqueous Solution
Nord. Pulp Pap. Res. J. vol. 16, no. 3, pp 240 (2001a)
Wiklund L, Lidén J, Öhman L-O
Surface Precipitation of MgCO3 on MnCO3 in Aqueous Solution at 90 °C
Nord. Pulp Pap. Res. J. vol. 16, no. 4, pp 339 (2001b)
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Appendix
Theory
The interaction between a metal ion and a fibre is mostly rather weak
because the sodium ion concentration as a rule is much larger than the
metal ion concentration and the total charge of the fibre. The interaction
between metal ions and the fibre active groups can, apart from a
traditional complex formation theory, be explained by the Donnan theory
(Donnan and Harris 1911), which has been presented by Towers and
Scallan (1996) and exemplified by Lindgren et al. (2001).
Classical complex formation
The formation of a species in the H+-Me2+-Xn--system, can be described by
the general formation reaction:
pH+ + qMe2+ + rXn- √ (H+)p(Me2+)q(Xn-)r
[3]
p, q and r are the numbers of protons, H+, divalent metal ions, Me2+, and
ligands, Xn-, respectively. The equilibrium reaction for the formation of
solid compound is thus:
Me2+ + X2- √ MeX(s)
[4]
The stoichiometric formation constant, ıpqr, for reaction [3] is defined as the
ratio of the concentration of the species formed to the product of the
concentrations of the individual components:
ıpqr = [(H+)p(Me2+)q(Xn-)r]/([H+]p[Me2+]q[Xn-]r)
[5]
Eq. [5] is only valid in so-called ideal solutions, where there are no
intermolecular forces between the solute molecules. In any real solution,
deviations from Eq. [5] and [6] occur. ıpqr is found to vary with both ionic
strength and temperature and is thus not a true equilibrium constant. The
concept of activity is introduced to deal with these non-ideal conditions, and
the thermodynamic formation constant Tıpqr, is defined as the ratio of the
activity of the species formed to the product of the activities of the
individual components:
Tıpqr
= {(H+)p(Me2+)q(Xn-)r}/({H+}p{Me2+}q{Xn-}r)
[6]
is valid at a given temperature and in standard state (Stumm, Morgan
1996). For aqueous solutions, the standard state is chosen as infinite
dilution, i.e. zero ionic strength. The activity, a, and the concentration, c,
and thus the thermodynamic and stoichiometric formation constants are
related through the activity coefficient, f:
Tıpqr
a = f*c
[7a]
ıpqr = Tıpqr · fH+p ·fMe2+q ·fXn-r/fpqr
[7b]
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The activity coefficient is a measure of deviation from the standard state.
The activity of a pure solid compound, and the activity of water are set to
unity. With increasing dilution, as the concentrations of all solutes
approach zero, the activity coefficients approach 1 and thus Tıpqr becomes
approximately equal to ıpqr. In a constant ionic medium, the activity
coefficients can be assumed to be constant, and thus concentrations can be
used instead of activities to evaluate the formation constants.
Donnan theory
The interaction between the metal ions and the carboxyl groups on the
fiber can apart from a traditional complex formation theory be explained by
the Donnan theory (Donnan and Harris 1911), which has been presented by
Towers and Scallan (1996).
The Donnan theory predicts that the concentration of mobile cations, Me
and anions, I, between the fibre volume, f, and the bulk suspension liquor,
s, are all related to the equation:
λ = [H+]f/[H+]s = [Me+]f/[Me+]s = ([Me2+]f/[Me2+]s)½
= [I¯]s/[I¯]f = ([I2¯]s/[I2¯]f)
[8]
Because electro-neutrality must prevail the following applies to the fiber
volume:
[≡COO¯]f + [≡O¯]f + Σ[I¯]f + 2Σ[I2¯]f = [H+]f + Σ[Me+]f + 2Σ[Me2+]f
[9]
Thus, the kraft fibre contains ionizable groups that are fixed to or in the
fibre wall. In order to fulfill the requirement of electro-neutrality, these
groups are balanced by an equivalent number of positive charges.
Especially in pulp suspensions at low ionic strengths this can give rise to a
marked uneven distribution of mobile ions between the fibre wall and the
bulk suspension liquor, the so-called Donnan effect. Carboxylic groups
contribute to the most significant part of these groups and, therefore, the
effective ion exchange capacity is most sensitive to pH changes in the range
of 2-6.
In order to utilise this equation, the effective anionic charge, the pulp
consistency, the amount of water in the fibre volume and the total amount
of cations are required. This implies that, with this model, only a limited
number of system dependent parameters are needed to mimic the metal ion
chemistry in pulp suspensions. As a satisfactory approximation for the
amount of water in the fibre volume, the water retention value (WRV) has
been found useful.
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Symbols used in the Donnan evaluation
[H+]ext
hydrogen ion concentration in external suspension solution
[H+]fiber
hydrogen ion concentration in fiber volume
[H+]tot
hydrogen ion concentration on total volume (external + fiber)
[Na+]ext
sodium ion concentration in external suspension solution
[Na+]fiber
sodium ion concentration in fiber volume
[Na+]tot
sodium ion concentration on total volume (external + fiber)
To utilize this equation, the effective anionic charge, the pulp consistency, the
amount of water in the fibre volume and the total amount of cations are
required. This implies that, with this model, only a limited number of systemdependent parameters are needed to mimic the metal ion chemistry in pulp
suspensions. As a satisfactory approximation for the amount of water in the
fibre volume, the water retention value (WRV) has been found useful.
Apart from the total metal ion content of the fibre suspension, the chemical
forms in which the various metal ions occur in the system, are also of great
importance for the numerical value of the Donnan distribution coefficient, λ.
For example, in the digester, a predominating part of the divalent metal ions
(typically 40-60 mmol/kg) are precipitated and will follow the pulp downstream
the fibre line. In an oxygen delignification stage, the added Mg2+ ions will also
precipitate and it has been shown that the resulting main chemical form is
MgCO3(s).
These metals, which exist in a non-ionic solid form, will not interact with the
ionized groups of the fiber, and should therefore not be included in the Donnan
calculations. If acid is added to such a pulp, the metal carbonates will dissolve
and the resulting divalent ions will to a considerable amount compete with the
sodium ions at the ion exchanger sites. If strong chelating agents, such as
EDTA or DTPA, are also added, some of these divalent ions may be converted
into di- or trivalent anions. In this form they will rather be expelled from the
fibre volume. When making the pulp alkaline again, as in a peroxide bleaching
stage, some of these divalent cations will be reprecipitated again.
The dominating cation in virtually all positions of the process is the sodium ion,
Na+. Its concentration varies from above 2000 mmol/l in the digester down to
below 5 mmol/l after the last chlorine dioxide bleaching stage.
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Redox potential
Aqueous solutions do not contain free protons and free electrons, but it is
nevertheless possible to define relative proton and electrone activities. The
pH measures the relative tendency of a solution to accept or transfer
protons, Eq. [10a].
pH = – log {H+}
[10a]
Similarly, it is equally convenient to define a parameter for the redox
intensity:
pE = – log {e–}
[10b]
pE gives the (hypothetical) electron activity at equilibrium and measures
the relative tendency of a solution to accept or transfer electrone.Thus, a
high pE indicates a relatively high tendency for oxidation, and a low pe for
reduction.
Nernst formula:
e = eo + (RT/nF) ln ({Ox}/{Red})
[11a]
Ox and Red stand for the oxidative and reductive forms of a redox couple,
respectively, according to reaction:
Ox + ne– √ Red
[11b]
The connection between the cell potential, e, and the electrode potential, is
given by:
e = e+ + e–
[11c]
The connection between redox potential, pE, and electrode potential, e, is
given by:
pE = Fe/RT ln10
[11d]
pE is as a rule plotted as a function of pH.
F is Faraday’s number (96487 C/mol), R the gas constant (8.314 J/(mol K)),
and T the temperature (K).
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STFI Database information
Title
Scaling of manganese compounds in kraft mills – state of the art
Author(s)
Per Ulmgren
Abstract
The aim of this work was to clarify the state of the art regarding scaling of
manganese compounds in kraft mills.
Much knowledge has been gained during the last decade regarding the
chemistry of manganese under technical conditions. The possibility of redox
stabilising Mn(II) from oxidation by the formation of solid solution and
mixed metal carbonate solids with Mg2+ has given new possibilities to solve
the problems with scaling of manganese compounds in the fibre line and
decomposition of hydrogen peroxide in the bleaching.
The oxidising of Mn(II) to Mn(III-IV) can probably not be fully avoided around
the oxygen delignification stage. However the oxidising of Mn(II) can probably
be hindered to some extent by adding excess of Mg2+ to the positions where
there is a risk for Mn(II) oxidation and formation of scales of Mn(III-IV)
containing compounds.
Keywords
Bleaching, Fiber line, Inorganic metal ions, Kraft pulping, Magnesium, Manganese,
Oxygen delignification, Scaling
Classification
100, 262, 560, 565
Type of publication
STFI-Packforsk report, Cluster program
Report number
Report No: xx
Publication year
Language
Project title
2005
English
Behaviour of NPEs under technical conditions
Project code
2381250
27