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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-Oethyl-

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 α-D-glucose

Δ[OH.]/Δt [mM/min]

MgSO

4

[μM]

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:

Fe2_ _ Fe3_ _ 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].

7.3.6.2 Residual Lignin Structures

The principal pathway of oxidative degradation of phenolic units comprises the

abstraction of an electron from the phenolate anion by biradical oxygen, resulting

in the formation of the corresponding phenoxy and superoxide anion radicals.

The unstable cyclohexadienone hydroperoxide intermediates fragment to catechols

and various mono- and dicarboxylic acids, such as maleic and muconic

acids, by demethoxylation, ring-opening, and side-chain elimination reactions

[88]. The concentration of the catechols remain fairly constant throughout oxygen

delignification, implying that they are involved in the oxidation process as intermediates

[89]. Furthermore, the catechols also contribute to an improved solubility

of the oxidized lignin structures due to the increase in the hydrophilicity of the

lignin.

The overall outcome of these reactions is the degradation and elimination of

guaiacyl (softwood and hardwood) and syringyl phenolic (hardwood) units. The

degradation of the uncondensed phenolic units is more severe within the dissolved

lignins (51–67%) than in the residual lignins (3–46%) [90]. Condensed phenolic

units refer to those phenolic structures with C5 substituents other than

methoxyl. It is reported, that 56–58% of the total phenolic units present in the residual

softwood kraft lignin can be attributed to the condensed type [90,91]. The

proportion of condensed phenolic units in hardwood residual lignin is considerably

lower as compared to the softwood residual kraft lignin.

After applying oxygen delignification to a softwood kraft pulp, the content of

condensed phenolic units in the residual lignin is decreased only by 4–29%, while

the corresponding decrease within the dissolved lignin fraction is about 41–60%.

It is well established that 5,5′-biphenyl [92] and diphenylmethane structures [93]

are fairly resistant towards oxygen delignification. The latter structures were

found to accumulate during the whole oxygen delignification process. However,

some of the condensed phenolic structures seem to be susceptible to oxygen

delignification, as in general most reports demonstrate an overall decrease in the

total condensed phenolic units after oxygen delignification [90]. In a study by Lai

7.3 Oxygen Delignification 713

et al., it was indicated that b–5 linked phenylcoumaran-type structures are

degraded during the initial phase of oxygen delignification [94]. At very high temperatures

of about 140 °C the amount of condensed phenolic units decreases by

more than 50%, which confirms that the reactivity of softwood kraft lignin substantially

increases at temperatures above 110 °C.

The presence of carboxylic acid groups within the lignin structures is considered

to promote lignin solubilization during alkaline oxygen delignification. The

increase of the carboxylic content is particularly high in the residual softwood

kraft lignin, while the corresponding carboxylic content in the hardwood residual

lignin increases only moderately obviously due to the higher initial value [90].

Compared to the residual lignins, the dissolved lignins contain more carboxylic

acids. It is interesting to note that the rate of carboxylic group formation increases

drastically as the reaction temperature increases parallel to the degradation of the

uncondensed phenolic units. A significant increase in the carboxylic acid groups

is observed at temperatures greater than 120 °C [95]. After successive oxygen

delignification stages, the content of carboxylic acid groups in the residual lignin

decreases, which means that the lignin which is rich in carboxylic acid groups is

preferably removed in the subsequent oxygen delignification stages, leaving a residual

lignin containing fewer reactive groups [96]. The content of the residual lignin

(kappa number) also plays a decisive role in the overall reactivity of the lignin

structures. High-kappa number pulps are known to be easier to delignify than

low-kappa number pulps, clearly due to a lower proportion of condensed lignin

structures, particularly 5,5′-condensed lignin units and diphenylmethane structures

[93]. The extent of oxidation to carboxylic acid groups is found to be lower

than that in low-kappa lignins. It can be speculated that the lower amount of condensed

structures in these high-kappa number pulps requires less oxidation for

the removal. Nevertheless, it has been demonstrated in a study by Chirat and

Lachenal, and also later by Roost et al., that a significant fraction of the residual

lignin (>22–25% of the initial value) remains even after five oxygen treatments,

which suggests that the oxygen delignification levels off [97,98]. Elucidation of the

structures of residual lignin that is unsusceptible towards oxygen delignification

has certainly been the focus of recent research. Based on the experience that the

extent of lignin removal decreases with increasing hemicellulose content (particularly

xylan), it is assumed that the residual lignin is attached to carbohydrates by

lignin–carbohydrate complex (LCC) linkages [99–101]. In fact, xylan-linked lignin

is more resistant to oxidative reactions, while galactan-linked lignin is readily

degraded during oxygen delignification [96]. The removal of amorphous cellulose

during a first oxygen delignification stage causes an increase in cellulose crystallinity,

thus reducing the accessibility of residual lignin in the secondary wall.

Quite recently, the existence of p-hydroxyphenyl groups has been attributed to

the resistant lignin structures in residual kraft lignin [93,96,102]. Model experiments

indicated that these materials are less reactive than their guaiacol counterparts,

although they belong to the noncondensed phenolics. The content of p-hydroxyphenyl

groups in a residual softwood kraft lignin was shown to correlate

with delignification selectivity during oxygen delignification. The stability of p-

714 7Pulp Bleaching

hydroxybenzene during oxygen delignification is clearly confirmed by model compound

studies [93]. It can thus be concluded that the accumulation of 5,5′-biphenyl

structures occurring via phenoxy guaiacyl radical coupling reactions between

the liquor and the fiber and the p-hydroxyphenyl structures are amongst the major

factors impeding the efficiency of oxygen delignification.

The ratio between the light absorption coefficient and kappa number corrected

for the content of hexenuronic acids, k457 nm/kappa(**), is a measure of the specific

amount of chromophoric groups in the residual lignin (see Section 4.2.6, Influence

on bleachability). Quite surprisingly, the degree of oxygen delignification

was found to be somewhat higher for unbleached softwood kraft pulps with

increasing k457 nm/kappa(**) values, as shown in Fig. 7.46 [103].

58

60

62

64

66

68

0.6 0.9 1.2 1.5 1.8

k

457nm

/kappa, m2/kg

Degree of oxygen delignification, %

Fig. 7.46 Extent of oxygen delignification as a

function of the specific light absorption coefficient

[k457 nm/kappa(**)] (according to Gellerstedt

and Al-Dajani [103]). Unbleached Pinus sylvestris

kraft pulp, kappa numbers 15–19. Conditions

for oxygen delignification are given by Gustavvson

et al. [104]: 100 °C, 85 min, 12% consistency,

700 kPa pressure, 2.25% NaOH.

The slightly higher efficiency of oxygen delignification of the unbleached softwood

kraft pulps with the lower brightness was mainly attributed to the higher

content of reactive unsaturated aliphatic carbon atoms and phenolic hydroxyl

groups, as determined by a thorough analysis of the residual lignin structures

(two-dimensional NMR using the HSQC sequence) [103]. Prolonged cooking at a

low hydroxide ion concentration gives rise to the formation of these degraded lignin

structures due to preferred fragmentation reactions. However, the low hydroxide

ion concentration during residual delignification promotes the precipitation of

dissolved lignin, which then leads to a decrease in brightness and specific light

absorption coefficient. Despite their higher degree of delignification in the oxygen

stage, the pulps with a high unbleached k/kappa(**)-value require more bleaching

7.3 Oxygen Delignification 715

chemicals (expressed as OXE per kappa) for subsequent bleaching to full brightness

as compared to those pulps comprising a low unbleached k/kappa(**)-value

(see Fig. 7.47). The latter are produced with a high effective alkali charge and a

short cooking time.

0 300 600 900 1200 1500

0.6

0.8

1.0

1.2

1.4

1.6

1.8

k

457nm

/kappa, m2/kg

Consumed OXE / kappa

Fig. 7.47 k457 nm/kappa(**) versus quantity of oxidation equivalents

consumed (OXE) in an OQPQP-sequence to a brightness

of 87% ISO (according to Gellerstedt and Al-Dajani

[103]). The conditions for the QPQP-sequence are given by

Gustavvson et al. [104].

The better bleachability of pulps with low unbleached k/kappa(**)-values is associated

with a higher amount of b-O-4 structures [105]. It is generally agreed that

the bleachability of softwood kraft pulps in a TCF-sequence comprising oxygen

and peroxide as bleaching chemicals is positively correlated with the unbleached

pulp brightness, and thus with the amount of b-O-4-structures in the residual lignin

[106].

7.3.6.3 Carry-Over

The dissolved solids entering the oxygen stage originate from two different

sources: the black liquor from cooking; and the filtrate from the oxygen stage.

Experiments conducted by various groups have shown reproducibly that filtrates

from the oxygen delignification stage have no significant effect on the performance

of oxygen delignification. On the other hand, the spent liquor from the

cooking stage causes a clear reduction in delignification efficiency, as reported for

a hardwood kraft pulp [107]. At the same level of COD, the carry-over from the

cooking stage has a more detrimental effect on delignification as compared to the

filtrate of the oxygen stage (Fig. 7.48).

716 7Pulp Bleaching

0 10 20 30 40 50 60

30

35

40

45

50

Filtrate from O-stage Spent liquor from cook

Degree of Delignification [%]

carry-over [kg COD/odt]

Fig. 7.48 Influence of amount and type of carry-over on the

degree of delignification in a oxygen-alkali treatment of a

hardwood kraft pulp, kappa number 16.7 (according to [107]).

Conditions of oxygen delignification: 10% consistency, 15 kg

NaOH on pulp, 15 kg O2 on pulp, 30 min, 100 °C.

Therefore, efficient upstream washing is essential to ensure a good performance

of oxygen delignification. The brownstock washing losses are typically in

the range of 10–30 kg COD odt–1 of unbleached pulp, assuming a washing efficiency

of 98–99%. Black liquor solids entering the oxygen stage may also adversely

affect delignification selectivity. The effect of commercial lignin produced from

kraft black liquor (Indulin A from Westvaco) added to the bleach liquor on the

selectivity of oxygen delignification of a softwood kraft pulp was studied by using

different caustic concentrations, while maintaining time and temperature constant

[20]. The data in Fig. 7.49 show that the addition of dissolved lignin causes a

significant decrease in selectivity during bleaching in 0.5 M NaOH.

The results obtained are due to a markedly decreased delignification rate and a

disproportional increase in the rate of depolymerization of carbohydrates. The

drop in the delignification rate may be explained in part by the decreased hydroxide

ion concentration caused by the acid groups formed by the oxidation of the

dissolved lignin. Additionally, an insufficient supply of oxygen may contribute to

the limited delignification rate. At the same time, the depolymerization reactions

of the carbohydrates are accelerated in the presence of dissolved lignin. It is

assumed that the combined presence of dissolved lignin and a rather high hydroxide

ion concentration (0.5 M) promotes the formation of free radicals, which in

turn induces significant chain scissions. Analogous experiments with a low alkali

concentration (0.1M), however, reveal an improved selectivity in the presence of

7.3 Oxygen Delignification 717

8 12 16 20

700

800

900

1000

1100

0.1 M NaOH 0.1 M NaOH + 10 g/l Indulin

0.5 M NaOH 0.5 M NaOH + 10 g/l Indulin

Viscosity [ml/g]

Kappa number

Fig. 7.49 Effect of the addition of commercial

lignin compound, Indulin A from Westvaco,

on the selectivity of oxygen delignification of a

Scots pine kraft pulp, kappa 32, viscosity

1220 mL g–1. Indulin A is added at a concentration

of 10 g L–1 in the bleach liquor; experiments

were run at 97 °C, 0–14 h, 0.2% consistency,

0.7 MPa pressure [20].

dissolved Indulin A (see Fig. 7.49). At this low alkali charge, the dissolved lignin

consumes a great part of the hydroxide ions present. Hence, the pH falls from

12.5 to 8.8 within 1 h, and this explains the very low rate of delignification and

preservation of the carbohydrates due to a lack of free radical formation.

The effect of carry-over on the performance of oxygen delignification can be

understood as a competitive consumption of alkali and oxygen between the residual

lignin in the pulp and the dissolved material in the entrained liquor. Oxygen

delignification appears not to be impaired as long as sufficient caustic and oxygen

are available. The initial rapid phase of delignification is not affected by the presence

of carry-over from the cook, clearly because the hydroxide ion concentration

and oxygen supply are not limiting factors. However, in the continuation of oxygen

delignification, the extent of delignification is clearly impaired by the presence

of dissolved lignin. During this phase caustic and oxygen are consumed by

the dissolved organic matter rather than by the residual lignin. The sole contribution

of the dissolved organic matter on the performance of oxygen delignification

can be studied by neutralizing the carry-over to pH 7. Corresponding experiments

using a eucalyptus kraft pulp were performed by Iijima and Taneda [107]. The

results depicted in Fig. 7.50 show that alkali is preferably consumed by dissolved

black liquor, and this results in a rapid drop of pH.

It can be seen from Fig. 7.50 that delignification discontinues as soon as the pH

falls below 9.5, and at which most phenols are no longer ionized. Thus, it can be

concluded that the presence of carry-over from the cooking stage will increase the

718 7Pulp Bleaching

0 10 20 30 40 50 60

0

10

20

30

40

50

Degree of Delignification:

no carry-over

carry-over, pH 13

carry-over, pH 7

Degree of Delignification [%]

Reaction time [min]

0

8

10

12

14

pH

Course of pH:

no carry-over

carry-over, pH 13

carry-over, pH 7

Fig. 7.50 Effect of carry-over from cooking

stage on the performance of oxygen delignification

of a hardwood kraft pulp, kappa

number 16.7, and on the pH profile during the

reaction (according to [107]). Conditions of

oxygen delignification: 10% consistency, 20 kg

NaOH on pulp, 30 kg O2 on pulp, 60 min,

100 °C.

overall oxygen and alkali requirements due to their preferred consumption by

the dissolved organic and inorganic compounds. Moreover, under the conditions

of industrial oxygen delignification the black liquor solids adversely affect

selectivity.

7.3.6.4 Xylan Content

The controlled removal of hemicellulose by means of prehydrolysis prior to kraft

cooking gradually improves the efficiency of delignification in a subsequent oxygen

delignification stage [99]. The increasing degree of delignification, along with

the removal of xylan, has been attributed to the cleavage of LCCs [108]. Similar

results are obtained when the xylan content of a kraft pulp is further increased by

the addition of anthraquinone. Both the rate and the extent of oxygen delignification

are substantially reduced by increasing xylan content, as shown for a Northeastern

hardwood kraft pulp [109]. Parallel with a higher hemicellulose content of

the kraft pulp, the selectivity of oxygen delignification significantly improves.

Thus, the hemicellulose polymers (specifically xylan) act as viscosity protectors for

cellulose due to an enhanced accessibility of the xylan for the bleaching chemicals,

for example, caustic and active oxygen species (hydroxyl free radicals) [110]. The

loss in pulp yield during oxygen delignification results primarily from the xylan

fraction of the pulp. This supports the assumption that the alkali present in an

7.3 Oxygen Delignification 719

oxygen stage is consumed by peeling reactions in which xylan is preferably

involved. As a result, less alkali is available for both oxygen delignification and cellulose

degradation.

7.3.6.5 Selectivity of Oxygen Delignification

Delignification selectivity is commonly defined as the change in kappa number

over the change in viscosity (e.g., Dkappa, Dviscosity) or, more scientifically, as the

ratio between the reaction rates of lignin removal and chain scissions of carbohydrates.

Analogous to all delignification reactions, oxygen delignification is based

on competitive reactions of oxygen and oxygen-active species within pulp lignin

and carbohydrates. With progressive extent of delignification, the oxidation of carbohydrates

becomes a more favorable process. It can be concluded that process

selectivity is greatly influenced by the radical chemistry of active oxygen species as

they react with both lignin and carbohydrates.

Extending delignification by reinforcing the reaction conditions often results in

severe cellulose degradation. The selectivity of oxygen delignification can be estimated

by comparing the delignification and polysaccharide cleavage models. As

mentioned previously (see Section 7.3.3), Iribarne and Schroeder reported that

low temperature combined with high alkali and oxygen concentrations during the

initial delignification phase would improve the selectivity of oxygen delignification

[12]. The design and recommended conditions of commercial two-reactor oxygen

delignification processes are largely based on these results. Recently, it was shown

that the selectivity of oxygen delignification of North-Eastern softwood kraft pulp

decreases with increasing sodium hydroxide charge at given temperature and

reaction time [32] (Fig. 7.51). However, the chosen temperature during the first

stage was approximately 10 °C higher than that recommended by those promoting

commercial, two-reactor systems.

The selectivity is significantly improved when a given alkali charge (e.g., 4.5%

on pulp) in a single-stage oxygen delignification experiment with a total retention

time of 90 min is split into three stages of equal retention time (30 min for each

stage) in which 1.5% NaOH is added before each stage. The improved selectivity

can be attributed to the rather even alkali concentration profile throughout the

three stages and a lower average sodium hydroxide concentration as compared to

the single-stage control experiment [32]. The same authors claimed that the selectivity

of oxygen delignification is not significantly affected by raising the temperature

from 90 °C to 120 °C at a level of 4.0% NaOH charge.

As mentioned above, the addition of various magnesium ion compounds

(including magnesium sulfate and magnesium carbonate) provides favorable behavior

in maintaining pulp viscosity during oxygen delignification [111]. The

selectivity of oxygen delignification can be further improved in the presence of

both phenol and magnesium sulfate in a specific amount of 0.5% on dry pulp

[112]. However, this synergetic effect is limited to pulps with kappa numbers higher

than 30, presumably due to the greater proportion of lignin units to be oxidized.

Phenol as an additive mimics the phenolic lignin structure, and can take part in

720 7Pulp Bleaching

12 15 18 21 24 27

800

900

1000

1100

1200

1.5 % NaOH 2.5 % NaOH 4.0 % NaOH

Viscosity [ml/g]

Kappa number

Fig. 7.51 Selectivity of oxygen delignification of

a North-Eastern softwood kraft pulp with an

initial kappa number 26.7 (according to [32]).

Single-stage oxygen delignification at 90 °C,

different alkali charges (1.5%, 2.5%, and 4.0%)

in the range of 60 min retention time; 10% consistency,

780 kPa pressure, 0.2% MgSO4

charge.

the reaction with oxygen in an alkaline aqueous solution to produce active oxygen

species, as demonstrated by J.S. Gratzl [113]. The reaction of phenolic compounds

with oxygen produces active oxygen species, such as the hydroperoxy and hydroxyl

radicals that contribute to the efficiency of oxygen delignification. Furthermore,

the selectivity of oxygen delignification is improved in the presence of phenol due

to the preferred reaction of the hydroxyl radical with the former.

7.3.7

Process and Equipment

7.3.7.1 MC versus HC Technology

Following the implementation of the first commercial systems for oxygen delignification

during the 1970s, and the subsequent introduction of medium consistency

technology in the early 1980s, the MC and HC oxygen delignification technologies

have co-existed for some time. However, medium consistency has

emerged as the technology of choice due not only to simpler operation and maintenance

but also to fewer safety concerns.

7.3 Oxygen Delignification 721

7.3.7.2 Process Technology

Conventional oxygen delignification cannot remove more than 35–50% of the residual

lignin before sustaining detrimental oxidative carbohydrate degradation,

which is expressed as a loss in viscosity and fiber strength. The low selectivity of

oxygen delignification can be explained in part by the unfavorable conditions for

lignin leaching that seem necessary to achieve access to the more resistant lignin

structures (e.g., condensed phenolics). If lignin leaching does not take place, the

more resistant structures might not be attacked as easily as the carbohydrates

under typical conditions of a one-stage oxygen delignification process [93].

A clear overview of the physical and chemical behavior of leachable residual lignin

from a Scandinavian softwood kraft pulp during oxygen delignification has

been provided by Ala-Kaila and Reilama [114]. The residual lignin across an industrial

two-stage oxygen delignification process was divided into four fractions by

different leaching operations, representing wash loss lignin, easily leachable,

slowly leachable, and resistant fraction of lignin [114]. The investigated pulp samples

originate from the positions prior to the first reactor (brownstock), the transition

line from the first reactor blow tank to the second reactor (O1 blow), and

finally from the second reactor blow tank (O2 blow). The experiments revealed

that most of the leachable lignin is only weakly attached on the fiber–liquid interface,

and can be removed by a 5-min washing operation at 90 °C after being centrifuged

to a consistency of 38% (Tab. 7.24). The amount of wash loss lignin fraction

increases during the course of delignification. It can be assumed that part of the

resistant lignin fraction is converted into the wash loss lignin as a result of

delignification reactions in the first reactor. The unbleached softwood kraft pulp

contains approximately one kappa unit of each easily (30 min at 90 °C) and slowly

(24 h at 90 °C) leachable fractions, respectively. In the first reactor, the easily leachable

lignin fraction diminishes almost quantitatively, whereas the slowly leachable

lignin decreases significantly in the second reactor only. There, predominantly

resistant and slowly leachable lignin fractions are involved in the delignification

reactions. The results confirm the importance of mass-transfer processes between

fiber and liquor phases in the transition from cooking over washing to oxygen

delignification.

Tab. 7.24 Residual lignin contents measured as kappa number

of a softwood kraft pulp after different leaching operations in the

course of a two-stage oxygen delignification process (according

to [114]).

Parameter Brownstock O1 Blow O2 Blow

Centrifuged kappa number 22.9 16.8 13.1

Kappa after 5-min washing 19.7 11.5 7.6

Kappa after 30-min leaching 18.6 11.1 7.4

Kappa after 24-h leaching 17.5 10.0 6.8

722 7Pulp Bleaching

The different kappa number fractions of a birch kraft pulp were recently determined

through an industrial two-reactor oxygen delignification process [115]. The

leaching procedure was comparable to that described for the softwood pulp, with

the exception of a shorter leaching time (2 min instead of 5 min) for the removal

of the wash loss. The experimental procedure, the reaction conditions during oxygen

delignification, and the applied analytical methods are described elsewhere

[115]. The isolated lignin fractions were further characterized according to their

chemical natures, representing residual lignin, hexenuronic acids (HexA) [116],

extractives (acetone extractives) and other chemical structures contributing to the

kappa number of the pulps. The residual lignin is referred to as the “actual” lignin,

and is determined by means of the Oxymercuration-Demercuration (Ox-

Dem) kappa number, whereas the total kappa number is denoted as “apparent lignin”

[117,118]. The study results demonstrated that the amounts of wash loss

were rather comparable throughout oxygen delignification for softwood and birch

kraft pulps (Tabs. 7.24 and 7.25).

Tab. 7.25 Residual lignin contents measured as kappa number

of a birch kraft pulp after different leaching operations in the

course of a two-stage oxygen delignification process (according

to [115]).

Parameter Brownstock O1 Blow O2 Blow

Centrifuged kappa number 22.1 18.2 16.8

Kappa after 2-min washing 16.7 12.7 11.3

Kappa after 30-min leaching 15.9 12.5 11.0

Kappa after 24-h leaching 13.9 11.6 10.4

The easily removable (apparent) lignin was also of the same magnitude for both

softwood and hardwood kraft pulps, whereas the slowly removable fraction in the

birch brownstock pulp was somewhat higher for the brownstock birch kraft pulp.

The main difference in the performance of oxygen delignification between softwood

kraft and hardwood kraft pulp is reflected in the amount of the resistant

lignin. The responses for the birch kraft pulp were 2.3 units (13.9–11.6) in the first

reactor and 1.2 units (11.6–10.4) in the second reactor (see Tab. 7.25). The corresponding

values for the softwood kraft pulp were 7.5 units (17.5–10.0) in the first

reactor, and 3.2 units (10.0–6.8) in the second (see Tab. 7.24). It is well known that

a great part of the resistant lignin fraction in hardwood kraft pulp before and after

oxygen delignification consists of HexA (Tab. 7.26).

The high resistance of HexA towards oxygen delignification clearly limits the

removal efficiency of the resistant lignin fraction. The degree of delignification,

measured as a change in kappa number, was 43% from residual lignin and 20%

with regard to HexA and extractives. The leaching procedure was particularly effi-

7.3 Oxygen Delignification 723

Tab. 7.26 Partial kappa numbers of resistant lignin fractions

isolated from birch kraft pulp (received after 24 h of pulp

leaching) [115].

Parameter Brownstock O1 Blow O2 Blow

Pulp kappa no. 13.9 11.6 10.4

Residual lignin 6.5 4.7 3.7

Extractives 1.0 1.0 0.8

HexA 5.8 4.7 4.6

Others 0.6 1.2 1.3

cient with the unbleached pulp, with clear effects on the residual lignin, extractives

and HexA. The course of kappa numbers prior the leaching operations are

summarized in Tab. 7.27.

Tab. 7.27 Partial kappa numbers of lignin fractions after washing

isolated from birch kraft pulp (received after 2 min of pulp

washing) [115].

Parameter Brownstock O1 Blow O2 Blow

Pulp kappa no. 16.7 12.7 11.3

Residual lignin 8.2 5.3 4.1

Extractives 1.5 1.2 1.0

HexA 6.8 5.8 5.8

Others 0.2 0.4 0.4

The removal of residual lignin was more pronounced for brownstock pulp (1.7

units = 8.2 – 6.5) as compared to the oxygen-delignified pulps (0.6/0.4 units = 5.3 –

4.7 and 4.1– 3.7). The degree of oxygen delignification was 50% based on residual

lignin (from 8.2 to 4.1), but only 32% based on total lignin after washing (from

16.7 to 11.3). The removal of HexA during leaching was fairly constant for all

pulps investigated. It is assumed that the removal of HexA in the first reactor is

most probably caused by the dissolution of the HexA containing pulp xylan

located at the surface of the fibers.

This rather simple but very useful analytical procedure for the characterization

of different kappa number fractions provides valuable information on the performance

of delignification reactions in general, and on the reactivity of the apparent

residual lignin structures towards subsequent bleaching operations in particular.

Oxygen delignification in two stages exploits delignification reaction kinetics

and allows maximum delignification efficiency and selectivity when high levels of

724 7Pulp Bleaching

pressure and alkali concentration are applied in the first stage, followed by a second

stage at low pressure and low alkali concentration [11]. In addition, both

stages should be performed at the minimum temperature possible to achieve the

desired delignification. This proposal derived from theoretical considerations was

successfully put to practice [119]. The principle of two-stage operation was further

developed to an “extended” OO-process by further increasing the pressure up to

1.6 MPa in the first stage according to the conditions given in Tab. 7.28.

Tab. 7.28 Conditions proposed for the “standard” and the

“extended” OO-process (according to Roost et. al. [98]).

Standard”

O-process

Standard”

OO-process

Extended”

OO-process

Conditions Unit O O(1) O(2) O(1) O(2)

Pressure, pO2 MPa 0.6 0.9 0.6 1.6 0.6

Temperature °C 100 90 110 100 110

Time min 60 30 60 30 60

Pulp consistency % 10 10 10 12 12

MgSO4 % 0.5 0.5 0.5

NaOH % 3 3 3

Reinforcing the reaction conditions in the first short stage also improves the

efficiency of delignification. It was shown that differences in the chromophore

content of the incoming pulp originating from varying conditions in the cooking

plant can be leveled out when applying “extended” oxygen delignification. It is

well accepted that the bleachability of softwood kraft pulps is impaired by applying

high H-factor in combination with a low alkali concentration to reach a certain

kappa number [104]. The poor bleachability is characterized by a high light

absorption coefficient, k, and a low amount of b-O-4 structures as determined by

thioacidolysis [120]. If a conventional one-stage oxygen delignification process is

used, the content of b-O-4-structures determines the demand of OXE necessary to

reach full brightness (e.g., 89% ISO). If, however, the conventional O-stage is

replaced by an extended OO-process, the subsequent demand of bleaching chemicals

is no longer influenced by bleachability parameters, such as b-O-4-structures

or k-values [98]. The effect of an extended OO-process prior a Q-OP-Q-POsequence

for two different softwood kraft pulps of comparable kappa number but

significantly different amounts of chromophore groups, measured as brightness,

k-value and amount of b-O-4 structures, is indicated in Table 7.29.

7.3 Oxygen Delignification 725

Tab. 7.29 Bleachability of pulps with different amounts of chromophore

groups in an Q-OP-Q-PO sequence after oxygen

delignification using either standard O- or extended OO-process

technology (according to Roost et. al. [98]). Reaction conditions of

“standard” and “extended” OO-processes are listed in Tab. 7.28.

Parameters Units Low-alkali High-alkali

Cooking conditions

EA-charge % as NaOH 17.5 22.5

Time at cooking temp. min 300 105

Unbleached pulp characteristics

Screened yield % 43.8 43.8

Kappa number 18 17.2

Brightness %ISO 30.9 36.7

Viscosity mL g–1 973 913

Light absorption coefficient m2 kg–1 16.5 13.1

b-O-4 in residual lignin lmol g–1 71 115

Bleachability

After “Standard"-O

Total peroxide consumption in

Q-OP-Q-PO sequence

kg t–1 25.7 19.2

After “Extended"-OO

Total peroxide consumption in

Q-OP-Q-PO sequence

kg t–1 12.1 11.6

Thus, it can be concluded that extended oxygen delignification significantly

decreases the demand of bleaching chemicals and evens out variations from the

cooking stage. However, to date no information is available about the selectivity of

extended oxygen delignification.

There are a number of technical set-ups where the insights from delignification

reaction kinetics are realized. All of these seek to provide the best conditions in

order to maximize delignification efficiency and selectivity. The first commercial

two-stage oxygen delignification process was developed at Oji Seishi KK Tomakomai

mill in Japan in 1985, simply by adding a second reactor to an existing onestage

oxygen delignification system. A detailed description of the practical experiences

derived from this first commercial installation was provided [121]. In the

same year, the first patent for a two-stage reactor oxygen delignification process

was granted to Kamyr [122].

726 7Pulp Bleaching

The first commercial application of a two-stage oxygen delignification stage as a

pretreatment of a TCF-sequence for the manufacture of a high-purity eucalyptus

prehydrolysis kraft pulp (PHK) came on stream in 1996 at the Bacell S.A. mill

(since 2004, BahiaPulp) in Bahia near Salvador [123]. This concept of two-stage

oxygen delignification was developed in an extensive laboratory program with a

one-stage oxygen treatment as a reference [124]. In accordance with the final pulp

quality requirements, the task of oxygen delignification was to reduce the kappa

number from about 8–10 to below 3 in order to avoid too high ozone charges and

to be able to control cellulose degradation in the subsequent ozone stage. The preliminary

trials using the conventional one-stage oxygen delignification treatment

resulted in a significant drop in viscosity as soon as the lignin removal rate was

extended beyond 60%. Consequently, a two-stage delignification concept was

investigated to achieve a higher degree of delignification without impairing viscosity.

It was shown that if the given amount of caustic is split into the first and second

stage in a ratio of approximately 60/40 to 75/25, then delignification can be

extended in the final part of the second stage (Fig. 7.52).

The advantage of a higher delignification efficiency in a two-stage oxygen

delignification process at a given charge of sodium hydroxide can be attributed for

the most part to a higher pH-level (or residual effective alkali concentration) in

the final part of the treatment (Fig. 7.53). Similar to kraft pulping, the selectivity

of delignification improves due to the more even pH profile.

0 20 40 60 80 100

0

2

4

6

8

10

Kappa number

Reaction time [min]

one-stage oxygen two-stage-oxygen

Fig. 7.52 Course of kappa number of one- and

two-stage oxygen delignification of a eucalyptus-

PHK pulp, kappa 8.6, viscosity 1131 mL g–1

(according to [124]). One-stage: 25 kg

NaOH bdt–1, 110 °C, 10% consistency, 0.7 MPa

oxygen pressure; Two-stage: first stage 15 kg

NaOH bdt–1, 110 °C, 0.7 MPa oxygen pressure,

15 min; second stage 10 kg NaOH bdt–1,

115 °C, 0.4 MPa oxygen pressure; 10% consistency

in both stages.

7.3 Oxygen Delignification 727

0 20 40 60 80 100

11

12

13

pH-value

Reaction time [min]

one-stage oxygen two-stage oxygen

Fig. 7.53 pH-profile during one- and two-stage oxygen

delignification of a eucalyptus-PHK pulp, kappa 8.6, viscosity

1130 mL g–1 (according to [124]). Conditions as shown in

Fig. 7.52.

The optimum overall selectivity and efficiency of two-stage oxygen delignification

can be achieved by limiting the retention time in the first reactor to 10–

20 min, while about 60 min appears to be the optimum retention time in the subsequent

second reactor. Figure 7.54 illustrates the advantage in selectivity of a

two-stage concept at an extent of delignification higher than 55%. A rather moderate

and almost linear decline in viscosity can be observed as long as the kappa

number is above 3.5 in case of a one-stage, and 2.9 in case of a two-stage oxygenalkali

treatment. This corresponds to an improvement in delignification efficiency

from 61% to 68% (unbleached kappa number 9; see Ref. [124]).

In the meantime, the two-stage oxygen delignification process for the production

of high-purity dissolving pulp has been more than nine years in operation at

the Bahia pulp mill (Salvador, Brazil), and operational results have clearly

exceeded expectations based on laboratory experiments. The average performance

of this prebleaching stage is achieving values of about 76% delignification while

maintaining a moderate level of cellulose degradation of about 0.195 mmol AHG–1,

expressed as the number of chain scissions (corresponds to a kappa number of

2.2 and a viscosity of 785 mL g–1 when compared with the pulp used for laboratory

experiments in Fig. 7.54).

Extended oxygen delignification is certainly more important for paper-grade

kraft pulps than for dissolving pulps, mainly because of the difficulty in removing

the residual lignin present in paper-grade pulps (without prehydrolysis). Therefore,

much effort was undertaken to develop appropriate two-stage delignification

728 7Pulp Bleaching

1 2 3 4 5 6

400

600

800

1000

Viscosity [ml/g]

Kappa number

one-stage oxygen two-stage oxygen

Fig. 7.54 Comparative evaluation of one- and two-stage oxygen

delignification using a eucalyptus-PHK pulp, kappa 9 and

viscosity 1200 mL g–1. Conditions as in Fig. 7.52, and according

to Refs. [124,125].

concepts. In 1995, a two-reactor oxygen delignification process was applied for

patent by Sunds Defribrator (today, Metso) [126]. The delignification stage is manufactured

under the trademark OxyTrac. This technology operates as a two-stage

system with a high-shear mixer before each stage. It has been claimed that the

selected conditions in both stages of the OxyTrac system are based on the kinetics

for oxygen delignification [12,127,128]. The rather long retention time of 30–

40 min in the first reactor is not exactly in line with the current knowledge on

reaction kinetics (see Section 7.3.3). However, it cannot be concluded that the

longer residence time in the first reactor than was predicted from kinetic considerations

results in an inferior delignification performance. The first reactor is

operated at a rather low temperature of 80–85 °C, but with high oxygen pressure;

these conditions lead to a higher concentration of dissolved oxygen. Moreover, the

gas volume at a given charge of oxygen is smaller, and this facilitates mixing of

the three-phase system. All of the alkali and most of the oxygen are charged to the

first reactor (Tab. 7.30).

The second stage is designed as an extraction stage using a higher temperature,

a longer residence time, and a lower chemical concentration to extend delignification

without drastically reducing the pulp viscosity and pulp strength.

Figure 7.55 illustrates schematically a typical OxyTrac process flowsheet. Caustic

is added to the dilution screw of the press before the pulp drops down into the

standpipe of the medium-consistency pump. After oxygen is charged in the first

high-shear mixer, the pulp-liquor-gas mixture passes through the first oxygen

reactor. It then flows down to a static steam mixer, where medium-pressure steam

7.3 Oxygen Delignification 729

Tab. 7.30 Recommended operating conditions of the OxyTrac

system for two-stage delignification (according to

Refs. [127,129]).

Parameters Units First stage Second stage

Consistency % 12 >10.5

NaOH-charge kg adt–1 25 0

Oxygen charge kg adt–1 18–25 low charge

Retention time min 30 60

Temperature °C 80–85 90–100

Pressure (top) MPa 0.8–1.0 0.4

is injected to raise the temperature to the desired level for the second stage. A

booster pump ensures flow through the second high-shear mixer and the second

reactor to the blow tank, where gas and pulp slurry are separated. Finally, the pulp

is pumped to post-oxygen washing.

Fig. 7.55 Typical OxyTrac process flowsheet [130].

Implementation of the OxyTrac system in full scale confirms the superior

delignification performance as compared to single-stage oxygen delignification

systems [127,130]. It is reported that adoption of the OxyTrac system resulted in a

significant increase (from 39% to >60%) in the degree of pulp delignification for

the Arauco mill in Chile using radiata pine kraft pulp, while preserving strength

properties. The ability to level out variations of the incoming pulp is another

important feature of a two-reactor oxygen delignification system.

A second commercial solution to the two-reactor oxygen delignification system

is the Dualox™ system provided by Kvaerner. The major differences from the

OxyTrac system comprise the short retention time in the first reactor of only about

5 min, and the split addition of oxygen. The short retention time in the first reac-

730 7Pulp Bleaching

tor is chosen by following results from kinetic investigations [14]. The first reactor

was built as a long and thick tube, enabling a very space-saving design.

A typical Dualox process flowsheet is shown in Fig. 7.56. Caustic is added to the

standpipe before the first medium-consistency pump. The pulp slurry is then

pumped through the first high-shear mixer and the pipe-type prereactor and

further on, by a booster pump, through the second high-shear mixer and the

upflow reactor to the blow tank. Oxygen and steam are added both to the first and

the second high-shear mixers. In the situation when a greater temperature

increase is required, a dedicated static steam mixer is added to the system before

the second pump [131].

Fig. 7.56 Typical Dualox™ process flowsheet [131].

A characteristic feature of the Dualox™ system is that both reactors are run at

the highest possible pressures in order to extend delignification as much as possible.

In contrast to the OxyTrac system, the oxygen charge to the first reactor is

kept low, while the main part of oxygen is charged to the second reactor.

According to current experiences from mill-scale operations, the Dualox™ tworeactor

system provides delignification performances that are comparable to those

reported for the OxyTrac system [132]. In the case of delignifying softwood kraft

pulp, the kappa number reduction approaches values of about 65%, which typically

ensures kappa number values of 8–12 entering the bleaching plant. The conversion

of a conventional oxygen delignification system to the Dualox™ process in a

hardwood kraft pulp production line was reported to improve the viscosity, despite

extending delignification by almost 5 kappa number points [133].

7.3.7.3 Process Equipment

The process flowsheet of a typical single-stage oxygen delignification system is

shown in Fig. 7.57. Medium-consistency pulp coming from brownstock washing

drops into a standpipe and is mixed with caustic soda as it enters the mediumconsistency

pump. The pump forwards the pulp suspension to a high-shear

7.3 Oxygen Delignification 731

MC PUMP

HIGH-SHEAR

MIXER

OXYGEN

REACTOR

POST-OXYGEN

WASHING

BLOWTANK

O2 Steam

Pulp from Offgas

brownstock

washing

NaOH

Pulp to

next stage

Fig. 7.57 Process flowsheet of a typical single-stage oxygen delignification system.

mixer which is charged with oxygen and also with steam to control the reaction

temperature. The three-phase mixture then proceeds to a pressurized upflow reactor

where the delignification reaction takes place.

It is essential that the high-shear mixer creates stable micro-bubbles with a large

specific surface area. In addition, the consistency in the oxygen reactor should be

above 10%, so that the fiber network forces suppress the coagulation of microbubbles

and that sufficient mass transfer area remains available throughout the

reaction time. A higher consistency also minimizes the risk of channeling in the

reactor [134].

After its discharge from the reactor top, the pulp suspension is separated from

the gas phase in a blowtank. The offgas from the blowtank can normally be blown

to atmosphere. Depending on the feed requirements of the post-oxygen washing

equipment, the pulp slurry is discharged from the blowtank either at low or medium

consistency.

Two-stage oxygen delignification systems use two pressurized reactor in series.

Special features of such systems are described in Section 7.3.7.2.

Post-oxygen washing requires a better washing efficiency than any other washing

process in the bleach plant due to the relatively large amount of dissolved

organic material and its low degree of oxidation. Post-oxygen washing systems

need to have more than one washing stage. They may, for example, consist of a

multi-stage Drum Displacer™, a pressure diffuser followed by a press, two

presses, several vacuum drum washers, or a multi-stage belt washer.

The material from which the wetted parts in an oxygen delignification stage are

constructed is usually a lower grade of austenitic stainless steel.

Further information regarding oxygen delignification equipment, including

medium-consistency pumps and mixers, reactors and blowtanks is provided in

Section 7.2. Details of pulp washing are provided in Chapter 5.

732 7Pulp Bleaching

7.3.8

Pulp Quality

The kraft cooking process and subsequent oxygen delignification stage must be

regarded as combined processes from the point of view of yield and pulp strength.

The balance between cooking and oxygen delignification determines the quality

profile of the pulp. According to today’s knowledge, an unbleached kappa number

between 25 and 30 (preferably around 27) ensures the lowest productions costs,

taking into consideration both wood yield and chemical consumption combined

with the unchanged pulp quality of a softwood kraft pulp [37,135]. In agreement

with the results reported by Iribarne and Schroeder [12], strength properties are

not affected as long as a certain threshold viscosity is not exceeded [135]. If the

intrinsic viscosity of an oxygen-delignified softwood kraft pulp is kept above a level

of about 870 mL g–1, the relationship between tear index and tensile index seems

not to be affected. As an example, the data in Fig. 7.58 show that the zero span

tensile index of a loblolly pine kraft pulp increases proportionally to the intrinsic

viscosity, and reaches a plateau value above an intrinsic viscosity of about 750–

800 mL g–1. These values are somewhat lower than those reported before or elsewhere,

most likely because of the relatively low initial viscosity of the pulps used

in this study [136].

It is however important to point out that the unbleached kappa number must

not fall significantly below a level of 25.

0 200 400 600 800 1000

0

50

100

150

Unbleached Pinus taeda L. kraft pulps

Oxygen-delignified kraft pulps

Zero span tensile index [Nm/g]

Viscosity [ml/g]

Fig. 7.58 Relationship between intrinsic viscosity and zerospan

tensile index of unbleached Pinus taeda L. kraft pulps

and oxygen-bleached kraft pulps produced thereof according

to Iribarne and Schroeder (after recalculation) [12].

7.3 Oxygen Delignification 733

Unbleached kraft pulps – and also oxygen-delignified kraft pulps – show pronounced

swelling properties in an aqueous environment. The swelling properties

of the pulp fibers are affected by the acid groups, predominantly carboxyl groups.

These groups also determine the ion-exchange capacity of cellulose materials and

they contribute to the bounding of fibers. Furthermore, cations adsorbed by the

carboxyl groups play an important role in the discoloration mechanism of pulp

and products made thereof (e.g., paper, cellulose fibers, films). Carboxyl groups

originate from the hemicelluloses, the lignin fraction and, to a much smaller

extent, also from fatty and resin acids. The carboxyl groups in the hemicelluloses

are included in the 4-O-methyl-a-d-glucuronic acid side chains of the xylan fraction

which are, however, degraded to hexenuronic acids during alkaline cooking.

Hexenuronic acid and 4-O-methyl glucuronic acids are almost not degraded during

oxygen delignification. The observed decrease in carboxyl groups during oxygen

delignification of kraft pulp is attributed predominantly to the removal of lignin

[137]. The oxygen delignification of sulfite pulps, however, also contributes to

a reduction of carboxyl groups due to the dissolution of hemicelluloses.

7.4

Chlorine Dioxide Bleaching

7.4.1