Lignin fragmentation

Reduced lignin

condensation

Lignin side chains

Lignin side chains

-CHO, > CO

base induced

fragmentation

Scheme 4.27 General scheme of redox mechanisms (according

to [203]).

It has been shown that in both carbohydrates and lignin, oxidative as well as

reductive processes are taking place. Consequently, each could drive the redox

cycle to some extent, and in particular lignin in which there appears to be a much

better balance between functional groups undergoing oxidation and reduction

than in the polysaccharides. The degradation of lignin is not only enhanced by

reductive processes such as the cleavage of phenolic b-aryl ether bonds by AHQ,

but also should be greatly facilitated by splitting the rather alkali-stable, nonphenolic

a-aryl ether substructures and the covalent carbon–carbon bonds between

C-a and C-b in side-chains [208]. Extensive experiments with appropriate model

compounds confirmed that oxidation reactions contribute to the degradation of

lignin [209]. By introducing carbonyl structures, lignin side-chains become prone

to base-induced fragmentations such as reverse aldol additions and b-eliminations

provided hydroxyl groups and ether functionalities are present in proper position

in the lignin.

The degradation paths of a-aryl ether structures by AQ are illustrated in Scheme

4.28.

4.2 Kraft Pulping Processes 319

O

O

MeO

O

MeO

CHO

O

OMe

O

MeO

O

MeO

O

O

MeO

O

MeO

O

O

H

O

O

OMe

H O 2 +

O

OMe

O

O

O

MeO

O

MeO

CH2OH

O

OMe

O

O

MeO

O

MeO

OH

O

OMe

â-Elimination

- -OAr

AQ AHQ

Retro-Aldol

+

Scheme 4.28 Degradation of a-aryl ether structures by

anthraquinone (AQ).

The interaction of AQ/AHQ with carbohydrates or lignin fragments can be visualized

by the interaction of their p-electron systems, as shown in Scheme 4.29.

The ionized AHQ acts as the p-base and forms a charge transfer complex (CTC)

with the lignin quinone methide (lignin QM) intermediate (p-acid), allowing an

electron transfer between both systems. The concomitant elimination of the b-aryl

ether brings about a fragmentation and the regeneration of AQ. The p-system of

AQ, now acting as p-acid, may accept an electron from a carbohydrate enolate (pbase),

present in alkaline solutions, also via formation of the corresponding CTC.

Here, the diketo sugar moiety is formed and AHQ regenerated.

Both, the adduct and the single electron transfer (SET) mechanisms have been

discussed in the literature. The adduct mechanism involves bond formation between

the lignin quinone methide intermediates and AHQ, which is followed by

fragmentation. The SET involves a single electron transfer between AHQ and a

lignin QM, again followed by fragmentation [210]. The observation of stable radical

formation within the AQ system and electron transfer to quinone methides

are in favor of the electron transfer processes [211].

The effect of AQ on the pulp yield–kappa number relationship in soda pulping

of water oak (Quercus nigra) is illustrated graphically in Fig. 4.92 [203].

320 4 Chemical Pulping Processes

O

OH

O O

O O

H

H R

- O O

H R

O

MeO

O R

H

O

OH

-

O

MeO

H

O O

O Râ

-

â

+

- +

+

Reduction of AQ by reducing end groups:

Oxidation of AHQ by lignin fragments:

Scheme 4.29 Proposed charge transfer complexes for the

reduction of lignin fragments and oxidation of carbohydrates.

20 40 60 80

cross-over

Soda-AQ (0.1 %) Soda-AQ (0.05%)

Kraft Soda

Total yield, %

Kappa number

Fig. 4.92 Soda-AQ pulping of water oak: time to = 90 min;

time at = 30–80 min; active alkali 10–19%; maximum temperature

170 °C; for kraft: sulfidity 25% (according to [212]).

4.2 Kraft Pulping Processes 321

The data in Fig. 4.92 show that the additive is most effective in cooking to high

kappa numbers. It is interesting to note that the regression lines for reference

kraft and soda-AQ pulping with 0.05% additive charge show a cross-over at a

kappa number of approximately 25. Only higher charges of AQ give a yield advantage

over kraft pulping at extended delignification. Carbohydrate analysis of pulps

indicated that soda-AQ pulping of water oak leads to a substantial stabilization of

cellulose rather than xylans [212]. This experimental evidence is in contrast to PS

pulping, where predominantly hemicelluloses are retained. The higher cellulose

stability has been explained by the fact that the quinone seems to be sufficiently

stable in the final cooking stage, and is thus able to stabilize the cellulose by oxidizing

the reducing end groups generated by chain cleavage [203]. Further evidence

for cellulose stabilization due to suppression of secondary peeling by AQ in

soda pulping was provided by pulping experiments with prehydrolyzed sweetgum,

where a significant increase in a-cellulose yield was obtained when AQ was

present [213].

The addition of AQ to conventional kraft cooking of loblolly pine revealed a significant

improvement in both delignification rate and selectivity of delignification

[14].

A kappa number of 30 can be achieved at 25% and 37% lower H-factors by

kraft-AQ pulping by applying AQ-charges of 0.05% and 0.1% on wood, respectively

(Fig. 4.93).

500 1000 1500 2000 2500 3000

Kraft Kraft-AQ (0.05%) Kraft-AQ (0.1%)

Kappa number

H-Factor

Fig. 4.93 Effect of anthraquinone (AQ) charge on the kappa

number–H-factor relationship (according to [14]). Constant

cooking conditions: 90-min rise to a maximum temperature

of 173 °C, liquor-to-wood ratio 4.0, sulfidity 35%, effective

alkali charge 18% as NaOH.

322 4 Chemical Pulping Processes

15 20 25 30 35 40 45 50

Kraft Kraft-AQ (0.05%) Kraft-AQ (0.1%)

Viscosity [ml/g]

Kappa number

Fig. 4.94 Effect of anthraquinone (AQ) charge on the viscosity–

kappa number relationship (according to [14]). Constant

cooking conditions: 90-min rise to a maximum temperature

of 173 °C, liquor-to-wood ratio 4.0, sulfidity 35%, effective

alkali charge 18% as NaOH.

It is apparent from the data in Fig. 4.94 that the presence of AQ accelerates

delignification at a given H-factor but has no effect on viscosity, and this results in

an overall improvement of selectivity. The advantage in viscosity as compared to

conventional kraft cooking increases with reducing kappa numbers (Fig. 4.94).

The effect of AQ addition on pulp yield being significant over the whole kappa

number range investigated is not only induced by lowering the cooking intensity,

but is primarily due to carbohydrate stabilization reactions. The major part of the

yield advantage is already given at low AQ charges. The addition of only 0.05%

AQ on wood results in a yield increase at a kappa number of 30 by 2.1%. A doubling

of the AQ charge leads to a further yield increase by 0.6% at the given kappa

number (Fig. 4.95).

The kraft-AQ process enables both the production of low-lignin pulps at

unchanged screened yield and high-yield pulps at a given kappa number. The former

are slightly more difficult to brighten in the final ECF bleaching stages of the

bleaching sequence, and deliver tear strengths which are approximately 10%

lower than those of conventional kraft controls. The latter, however, brings the

advantage of lower wood consumption and simultaneously helps to reduce bottlenecking

in the recovery area.

The addition of AQ to kraft pulping accelerates the production of southern pine

chips to produce linerboard-grade pulp. Mill trials demonstrated that a charge of

only 0.05% on wood allows a 25–35% reduction in H-factor combined with a 5%

4.2 Kraft Pulping Processes 323

15 20 25 30 35 40 45 50

Kraft Kraft-AQ (0.05%) Kraft-AQ (0.1%)

Screened Yield, %

Kappa number

Fig. 4.95 Effect of anthraquinone (AQ) charge on the

screened yield–kappa number relationship (according to

[14]). Constant cooking conditions: 90-min rise to a maximum

temperature of 173 °C, liquor-to-wood ratio 4.0, sulfidity 35%,

effective alkali charge 18% as NaOH.

reduction in EA charge, while keeping a comparable pulp quality as produced

without AQ addition [214]. At a given kappa number, yield gains of about 2–3%

on wood were obtained. In another linerboard production, the effect of AQ on

yield was confirmed [215], with a yield increase of 1.7% being achieved by adding

0.05% AQ to wood chips in kraft pulping of softwood to a kappa number level of

60–70. Simultaneously, the EA charge could be reduced from 14.2% to 12.6% on

wood without affecting either the kappa number of the pulp or the physico-chemical

properties of the linerboard.

On a chemical basis, AQ combines irreversibly with reactive parts of the lignin

structure [216]. Indeed, only 10–25% of the originally added AQ can be recovered

in the black liquor in an active form to be used in a subsequent cook [215]. A

minor portion remains on the pulp fiber, from where it is removed quantitatively

by oxidative bleaching treatment.

The effects of AQ on kraft pulping of Pinus radiata are less than are found for

other softwoods [217]. The yield gains are in the order of only 1% when adding

0.05% AQ on wood. In fact, AQ is effective in catalytic quantities, with only 0.01%

addition significantly accelerating delignification and raising the pulp yield. Comprehensive

studies have shown that the most cost-effective dose of AQ for pulps

in the kappa number range 25–30 is about 0.02%. At this level of addition, the

yield gain is about 0.5–0.7% on o.d. wood, while the reduction in H-factor calculates

to approximately 18% (which relates to a reduction in cooking time from 2.3

324 4 Chemical Pulping Processes

to 1.8 h at 170 °C). The use of AQ at this level should be economic in most applications,

and particularly so for a recovery or causticizing limited mill.

The effect of AQ was slightly improved by using Scots pine in kraft pulping. At

kappa number 30, the yield increase over the normal cook was about 1.7% unit

with 2 kg AQ t–1 wood. The reduction in H-factor was approximately 22%, while

the yield gain decreased slightly to 1.3% at kappa number 25 [188]

The influence of AQ addition was studied for soda and kraft pulping using

Eucalyptus grandis [218]. In soda pulping, a charge of 0.15% AQ on wood at given

H-factor (1115) and alkali charge (EA = 20.6% NaOH) increased the extent of

delignification by more than 30% (from kappa 33 to kappa 22.8), with a slight

improvement in pulp yield. In kraft pulping, the addition of 0.15% AQ also led to

a 30% reduction in kappa number (from 26.3 to 18.4) at 2% less alkali and 27%

less H-factor. The effect on pulp yield was more pronounced (+1.4%) as compared

to soda pulping (+0.4%).

4.2.7

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