Substrates, treatment Additives

[mmol L–1]

Viscosity

[ml g–1]

Carbonyl

Groups

[mmol kg–1]

Carboxyl

Groups

[mmol kg–1]

Cotton linters (CL) no 550 n.a. 3.6

CL, oxygen delignification (OD)a no 336 3.8 16.4

CL, OD 0.05 Fe(II) 262 6.6 18.7

CL, OD 0.05 Fe(II) + 3.0 Mg (II) 373 2.8 10.4

CL, OD 0.05 Co(II) 127 19.7 64.0

CL, OD 0.05 Co(II) + 3.0 Mg(II) 194 7.1 34.5

a. 98 °C, 60 min, 6 bar, 5% NaOH.

7.3 Oxygen Delignification 709

The addition of magnesium carbonate significantly retards cellulose degradation

and the formation of carbonyl and carboxyl groups in the presence of iron

salts. However, the catalytic effect of cobalt ions on cellulose degradation and oxidation

cannot be offset by the addition of magnesium carbonate.

The results of a detailed study on TCF-bleaching of a softwood kraft pulp indicated

that even a copper content below 1ppm on pulp promoted depolymerization

of cellulose during peroxide bleaching [68].

Hydrogen peroxide undergoes Fenton-type reactions with metal ions such as

iron and manganese, leading to hydroxy radicals that are then able to oxidatively

cleave and/or peel polysaccharide chains [69,70]. A recent study using methyl 4- O ethyl-

b-d-glucopyranoside as a model for cellulose indicates that Fe(II) ions are

the most harmful species to carbohydrates in the presence of hydrogen peroxide

[71]. The extent of glycosidic bond cleavage decreases with increasing initial pH

and in the presence of oxygen, presumably due to the preferred formation of lessreactive

hydroperoxyl radicals [71]. The Fenton reaction is drastically inhibited

under alkaline conditions at room temperature, this most likely being due to the

reduced solubility of Fe(II) and Fe(III) ions.

Radicals generated during the decomposition of hydrogen peroxide are also considered

to be involved in delignification reactions during oxygen delignification

[72]. The degradation rate of propylguaiacol (used as a lignin model) during oxygen

bleaching is however generally lowered in the presence of transition metal

ions [39]. Simultaneously, the maximum rate of degradation is shifted from pH 11

to pH 12. The highest rate of degradation is preserved when copper(II) ion is

added. In the presence of 0.1 lM copper (II) ion, the degradation rate of propylguaiacol

is reduced by about 20% while the degradation rate is decreased by more

than 40% in the presence of iron(II), cobalt(II) and manganese(II) ions at the

same concentration levels. One explanation of this behavior might be the formation

of a complex between the phenolate anion and the metal ion, which would

make the oxygen delignification process less efficient [39].

The addition of aluminum sulfate strongly retards delignification during oxygen-

alkali cooking of wood meal. It is suspected that redox active metal ions are

inactivated in the presence of aluminum salts by a type of coprecipitation together

with aluminum hydroxide, as is known similarly for magnesium salts [73]. The

addition of lanthanum nitrate prior to oxygen delignification exerts a protective

action similar to that of magnesium salts. Lanthanum salts precipitate as hydroxides

under the conditions prevailing during oxygen bleaching [67].

Since the discovery by Robert and associates of the protective effect of magnesium

carbonate on the cellulose fraction of the pulp, numerous investigations

have clearly confirmed that the addition of magnesium salts inhibits the degradation

of carbohydrates [74–76]. Consequently, several commercial oxygen delignification

plants add a magnesium salt to the caustic liquor in order to prevent the

metal ion-catalyzed degradation of polysaccharides. It has been proposed that

magnesium cations form precipitates with iron(II) and manganese(II) ions in the

presence of an anionic polymer (e.g., cellulose or polygalacturonic acid) and

change their physical characteristics into a negatively charged colloidal phase.

710 7Pulp Bleaching

Iron and manganese ions are redox-stabilized in their +II state by being incorporated

into a solid phase; in this way they cannot further participate in a Fentontype

reaction [69,77]. Magnesium must be precipitated as the hydroxide, carbonate

or silicate in order to act as stabilizer. The characterization of the precipitate

revealed the presence of a so-called solid solution at a pH > 10, with a variable

composition (Mg1 – xMnx)(OH)2(ss), that effectively binds a certain amount of transition

metal ion [78]. In a detailed study, it has been shown that the solubility of

Mn(II) decreases when the Mg(II):Mn(II) molar ratio increases. This implies that

the solid-state Mn(II) will be highly diluted with Mg(II), which means that the

Mn(II) will lose the opportunity to interact with dissolved hydrogen peroxide. By

considering the solubility products, it can be concluded that at pH < 10, this solid

solution can no longer keep the Mn(II) concentration below the catalytically active

level. The reason why the Mn(II) can still be found as a co-precipitate with Mg(II)

at the end of an alkaline oxygen stage with the pH below 10 has been studied in

detail [79]. As a first indication, it was found that the divalent metal ions in oxygen

delignification are present as carbonates, and not as hydroxides [80]. The analysis

of the precipitated particles indicated that the core consists of fairly insoluble

MnCO3(s) on which Mg(II), in the form of MgCO3(s), crystallizes in layers. The

latter originates from an initial precipitate, Mg5(OH)2(CO3)4(s). The crystallization

process is slow, which also coincides with the observation that the protection

against hydrogen peroxide degradation improves either with increasing Mg(II)

addition or prolonging the ageing time before hydrogen peroxide is applied. The

mechanism of redox stabilization of Mn(II) by Mg(II) in oxygen delignification

can be explained by assuming that manganese present in wood is transformed to

MnCO3(s) already during the kraft cook. After the addition of soluble Mg(II) salts

prior to oxygen delignification, MgCO3(s) crystallizes as a layer on top of the

MnCO3(s) particles, and thus assures the inertness of the Mn(II) ions against

hydrogen peroxide decomposition. In order to reach high redox stabilization of

the catalytically active transition metal ions, high temperature, and/or prolonged

ageing time is needed.

A study conducted by Lucia and Smereck indicates that the selectivity of oxygen

delignification, defined as the ratio of the extent of delignification divided by the

viscosity changes, Dkappa(initial-final)/Dviscosity(initial-final) [81], is significantly improved

by increasing the molar ratio Mg:Mn from 22 to 33. Interestingly, the lower lignin

pulp (kappa 25) seems to be more susceptible to carbohydrate preservation due to

the addition of magnesium salts as compared to the high-kappa number pulp

(kappa 40). One possible reason for this observation might be the greater amount

of oxidized structures in the low-kappa pulp (e.g., carboxylic groups), which

clearly are necessary to bind redox active metal ions efficiently [82]. The addition

of magnesium sulfate to oxygen delignification inhibits the formation of hydroxyl

radicals. The effect of increasing concentrations of magnesium ions on the rate of

hydroxyl radical formation in the treatment of a-d-glucose and creosol under oxygen

delignification conditions is shown in Fig. 7.45 [83]. The influence of magnesium

ions on the formation of hydroxyl radicals is apparently independent of the

substrate at concentrations <100 lM.

7.3 Oxygen Delignification 711

0.1 1 10 100 1000

0.000

0.005

0.010

0.015

creosol á-D-glucose

Ä[OH. ]/Ät [mM/min]

MgSO

[ìM]

Fig. 7.45 The effect of magnesium sulfate on the rate of of

hydroxyl radicals formation during treatment of creosol and

a-d-glucose under oxygen delignification conditions (according

to [83]). Substrate concentration 1.5 mM, pH 11.5, 5 bar

O2, 90 °C.

The difference in behavior between a-d-glucose and creosol with regard to hydroxyl

radical formation at magnesium sulfate concentrations >100 lM may be

ascribed to the formation of Mg2+carbohydrate complexes, and to thermal homolytic

cleavage of hydroperoxide intermediates derived from creosol to produce additional

hydroxyl radicals [83].

The treatment of an unbleached softwood kraft pulp with a solution of magnesium

and calcium acetate at pH 4.5 results in a decrease of the manganese content

from 40 ppm to 1.9 ppm [68]. The removal efficiency is comparable to a pretreatment

at pH 5.87 with 4 kg ethylen-diamine tetra-acetate EDTA t–1 pulp. Effective

manganese removal results in a comparable viscosity preservation as compared

to magnesium addition.

Although transition metal ions are removed prior to oxygen delignification, it is

confirmed that additional EDTA extraction is needed immediately prior to subsequent

hydrogen peroxide bleaching in order to avoid viscosity loss [84,85]. It has

been suggested that small amounts of previously embedded metals are released

during bleaching, and that the metal ions released give rise to the formation of

radical species during the subsequent hydrogen peroxide bleaching stage, causing

severe attack on cellulose [86].

In the presence of EDTA, the catalytic effect of iron salts during oxygen-alkali treatment

is enhanced. Chelation increases the oxidation potential of the redox reaction:

Fe 2_ _ Fe 3_ _ e _ _55_

712 7Pulp Bleaching

thereby stabilizing the +3 state relative to the +2 state while maintaining the possibility

of a redox reaction in the chelated form. However, the stability of the iron

chelates under the conditions prevailing during oxygen delignification is significantly

less than that of copper-EDTA. Thus, it can be assumed that decomposition

to simple ions takes place with accompanying catalysis. Therefore, iron activation

is probably not due directly to chelated iron but is simply the result of an

increased mobility of previously insoluble iron, such as oxides, by chelation, followed

by redeposition in the fiber [87]. Copper is known to produce stable complexes

with EDTA. However, the catalytic effect of copper is masked by EDTA only

at low hydroxide concentration. At high sodium hydroxide concentration no significant

protective effect of EDTA is obtained. This observation can be explained

by a gradual displacement of the EDTA by hydroxide ions in the copper-EDTA

complexes [67].

Дата добавления: 2015-10-21