Scaling of manganese compounds in kraft mills - … report 70.pdf · STFI-Packforsk report no ......

Scaling of manganese compounds in kraft mills

-state of the art

Per Ulmgren

2005

According to Innventia Confidentiality Policy this report is public since 2011-02-04

a report from STFI-Packforsk

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

According to STFI-Packforsk's Confidentiality Policy this report is assigned category 2 Scaling of manganese compounds in kraft mills – state of the art

STFI-Packforsk report 70

Contents Page

1 Summary 1

2 Introduction 3 2.1 Aim 3 2.2 Background 3 2.2.1 Survey of scale formation of manganese containing

compounds on the brown side of the fibre line 3 2.2.2 Manganese in kraft mills 4

3 Manganese process chemistry 5 3.1 Process conditions in kraft mills 5 3.2 Formation of solid solutions 9 3.3 Formation of metal carbonates 11 3.4 Oxygen delignification stage 12 3.5 Model calculations 13 3.5.1 Cooking conditions 14 3.5.2 Oxygen delignification 15 3.5.3 Hydrogen peroxide bleaching 16 3.5.4 Summary of Mn(II-IV) forms in the fibre line 17

4 Conclusions and future research work 19

5 References 20

Appendix Theory 22


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Scaling of manganese compounds in kraft mills – state of the art 1 STFI-Packforsk report 70

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.


According to STFI-Packforsk's Confidentiality Policy this report is assigned category 2 2 Scaling of manganese compounds in kraft mills – state of the art STFI-Packforsk report 70

• 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.




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 post-oxygen 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.



Tabel 1. Results from a mill survey on scaling of manganese hydroxide/oxide compounds.

Mill A Mill B Mill C

Wood species hard- and softwood hard- and softwood softwood

Cooking method continuous continuous batch

Location of scale after O2-stage prior and after O2-stage

prior and after O2-stage

Kappa number, in- and out of 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 O2-stage

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.




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.

-20

-10

0

10

20

30

0 2 4 6 8 10 12 14

pEZ

P

O

C

MnO4-

MnO2(s)

Mn2+ Mn(OH)2(s)MnS(s)

pH

MnOOH(s)A Q

Figure 1. Redox diagram for Mn at 25 ºC and in dilute solutions. Estimated pE-values 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.



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).

-50 0 50 100 1500.0

0.1

0.2

0.3Mg2+ Mn2+

time (minutes)

[Me2+]tot (mmol/L)

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).

0 50 100 150 200 2500.0

0.2

0.4

0.6

0.8

1.0

time (minutes)

[Ca2+]tot (mmol/L)

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




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.

wood220 white liquor 5

Digester Recoveryboiler

O2-stagereactor

green liquordregs 170

oxygendelignifiedpulp 60

wash filtrate

50

blackliquor

165

cooking 115liquor

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.

0

20

40

60

80

100

wood pO aO aD0 aE aD1 aD2

Mn(pulp)/Mn(wood) (%)

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.



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).

Fe3+, Mn3+HO• + HO–

½ O2 • – + HO–

H2O2

HO2– Fe2+, Mn2+

HO– + HO2–

O2 • –

O2 • – + H2O

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+.




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



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.

0 10 20 30

-14

-13

-12

log L

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]

where

[Me(II)]tot = [Mg(II)]tot + [Mn(II)]tot [2c]

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.




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).



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.




+

++

+

++

+

+Na+

+

Fibre volume Suspension

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

MgCO3*MnCO3(s)

CaCO3(s)

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.



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+

[OH–], mol/L 0.2 - 1 0.1 – 0.2 < 0.001

[CO32–], mol/L 0.3 0.02 – 0.05 0.01

Mg/Mn, mol/mol 6 14 32

3.5.1 Cooking conditions Mg2+ 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.

4 6 8 10 12-6

-5

-4

-3

-2

-1

0

pH

log [CO32-]tot

Mg(OH)2(s)

MgCO3(s)

Mg2+

cooking and O2-stage

6 8 10 12

-5

-4

-3

-2

-1

0

pH

log [CO32-]tot

MnS(s)

Mn(OH)2(s)

cooking

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).




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).

6 8 10 12-5

-4

-3

-2

-1

0

pH

log [CO32-]tot

MnO2(s)

*MnCO3(s)

Mn2+

O2-stage

6 8 10 12

-6

-5

-4

-3

-2

-1

0

pH

log [CO32-]tot

(Mgx,Mn1-x)(OH)2(ss)(Mg+Mn)/Mn = 32

*MnCO3(s)

Mn2+

O2-stage

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.



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 co-precipitation MgCO3*MnCO3(s) can also be formed when the hydroxide ion concentration is low and the carbonate ion concentration is high.

4 6 8 10 12-5

-4

-3

-2

-1

0

pH

log [CO32-]tot

MgMn(OH)2(ss)

*MnCO3(s)

Mn2+

P-stage

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+

+

Fibre volume Suspension

Na+

Ca2+

Na+Na+

Mg2+

Na+

(Mg,Mn)(OH)2(ss)

MgCO3*MnCO3(s))

CaCO3(s))

Figure 15. Model of a pulp suspension in a hydrogen peroxide bleaching based on Donnan theory (Norberg et al. 2001; 2002).




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 Positions after O2-stage O2-stage

141210864

pH141210864

pH

(MgxMn1-x)(OH)(ss)

MgCO3*MnCO3(s)

Mn2+ - fibre

free Mn2+

MnS(s), Mn(OH)2(s)

MnL2-n

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



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.




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.



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)




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)



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]

Tıpqr 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:

a = f*c [7a]

ıpqr = Tıpqr · fH+p ·fMe2+q ·fXn-r/fpqr [7b]




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.



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 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.

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.




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).



Scaling of manganese compounds – state of the art 27 STFI-Packforsk report 70

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 2005 Language English Project title Behaviour of NPEs under technical conditions

Project code 2381250


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