Combined PS and Anthraquinone (AQ) Effects

Modified pulping has made it possible to extend the cook to very low kappa numbers,

without impairing strength properties. However, the significant yield losses

which occur at low kappa numbers renders extended delignification economically

nonfeasible. The synergetic effect on yield of the combined use of PS and AQ

could compensate for the yield loss at low kappa numbers [188,191]. By applying

the concept of extended modified cooking of southern pine, the sole addition of

0.1% AQ increases brownstock yield by about 1% at kappa 25 [191]. The yield

increase becomes less than 0.5% by further extending delignification to kappa

number 16, and is not measurable at kappa number 10. Under these conditions,

only a small fraction of the AQ is available in the cooking liquor for carbohydrate

stabilization. Moreover, AQ can oxidize the C-2 and C-3 hydroxyl groups in the

anhydroglucose units, promoting chain cleavage and secondary peeling reactions.

The addition of 2% PS, however, results in an average yield increase of about 1.5%

within the kappa number range 8–12. The lower efficiency of PS in the lower

kappa number range can presumably be explained by the decreasing stability of

the retained hemicelluloses. The simultaneous addition of 2% PS and 0.1% AQ

results in a total yield increase of 3% at kappa number 10, which is 1.6% higher

314 4 Chemical Pulping Processes

as the additive effect from applying PS and AQ individually (Fig. 4.90). As one

possible mechanism which has been discussed in this regard is that PS participates

in the AHQ-lignin and AQ-carbohydrate redox system, where partial regeneration

of PS and/or stabilization of PS against disproportionation takes place.

According to the carbohydrate analysis, the yield increase originates from an

increased retention of glucomannan in the softwood pulp. The synergistic effect

of the combined addition of 0.1% AQ and 1.3% PS is also reported for conventional

batch cooking of southern pine in the kappa number range 20–35 [192]. At

kappa number 25, the total yield advantage amounts to 3.4% at kappa number 25,

which is approximately 1% more as compared to the additive yield effect from

applying PS and AQ individually. If PS and AQ are used in combination, the

H-factor can be reduced by 17% (from 1900 to 1580) compared to reference kraft

cooking to attain kappa number 25. The sole addition of PS shows no influence

on the delignification rate, whereas AQ cooking leads to a 10% reduction in H-factor

to reach kappa number 25. However, the reliability with respect to delignification

rate is somewhat doubtful, because in PS and PS/AQ-cooking the EA charge

was 23.7% as compared to 20.8% in the case of AQ and reference kraft cooks,

respectively.

5 10 15 20 25 30 35

Batch-Kraft Batch-Kraft-AQ Batch-Kraft-PS Batch-Kraft-AQ-PS

EMCC-Kraft EMCC-Kraft-AQ EMCC-Kraft-PS EMCC-Kraft-AQPS

Screened Yield [%]

Kappa number

Fig. 4.90 Effect of separate and combined

addition of polysulfide (PS) and AQ for both

extended modified cooking [191]and conventional

batch cooking [191]of southern pine.

EMCC cooking conditions: 21–25% NaOH

on wood, EA-split: 75% impregnation,

25% cooking, 170 °C; 0.1% AQ,

2% PS, WL sulfidity 30%; residual EA concentration

17–18 g L–1 as NaOH. Batch cooking

conditions: 20.8% NaOH charge on wood for

reference and AQ-cooks, 23.7% for PS and PS/

AQ-cooks; 30% sulfidity for reference and AQcooks,

16.5% for PS and PS/AQ-cooks; 166–

174 °C.

4.2 Kraft Pulping Processes 315

On the other hand, a recent kinetic study clearly states that the PS/AQ process

shows the highest delignification rate (equivalent to a H-factor reduction of

approximately 20%) as compared to kraft, kraft-AQ, and PS processes [193]. In

addition, compared with the kraft and kraft-AQ concepts, the PS and PS/AQ

cooks have lower cellulose degradation rates.

The effect of separate and combined addition of PS and AQ for both extended

modified cooking and conventional batch cooking of southern pine is illustrated

in Fig. 4.90.

Mill experience ofcombined PS/AQ pulping

The addition of PS sulfur amounts approximately to 0.5–1.5% on o.d. wood in

mill praxis. The yield increase observed is reported to be in the range of one- to

two-fold the amount of PS sulfur, dependent on the impregnation and cooking

techniques, kappa number and wood species [1]. However, if alkaline pulping is

preceded by impregnation of the wood chips with a PS-containing liquor, the yield

gain may be increased to 2.5- to 4-fold the amount of the charged PS sulfur [1].

Polysulfide pulping has been practiced at the Peterson kraft mill in Moss since

1973 [194,195]. The Moss mill is an integrated pulp and paper mill producing

linerboard specialties from pine and spruce. Cooking takes place in a two-vessel

steam/liquor continuous digester to a target kappa number of 65. The PS cooking

is prepared by catalytic air oxidation of sulfide in the white liquor using the

MOXY process [171,196]. Oxidation takes place in reactors with only a few minutes′

retention time in the presence of a special carbon catalyst, where about 70%

of the oxidized sulfide is converted to PS sulfur, and the remainder to thiosulfate

which behaves inertly under kraft cooking conditions. At the Moss mill, the PS

concentration of the cooking liquor is about 5–6 g L–1 sulfur, which occurs when

about 50% of the sulfide in the white liquor is oxidized. In addition, 0.35 kg AQ

per o.d. pulp is added to the cooking liquor. Polysulfide-AQ pulping at the Moss

mill results in an average reduction in wood consumption of about 4.3% per ton

of pulp, and an increased production capacity in the digester of about 4.5% –

which increases to 10% when chemical recovery evolves as bottleneck and a more

easily beaten kraft pulp is produced [194].

Anthraquinone (AQ) Pulping [197]

The beneficial effects of AQ on both the pulping rate and carbohydrate yield in

soda and kraft pulping were first discovered by Bach and Fiehn [198].

The stabilizing effect of AQ is explained by oxidation of the reducing endgroups

to form alkali-stable aldonic acid end-groups. Convincing evidence for this

type of stabilizing reaction has been provided by Sjostrom [199,200]and Samuelson

et al. [201]. Topochemical investigations using the method of selective bromination

of the lignin in nonaqueous system and subsequent determination of the

Br-L X-ray emission revealed that soda-AQ pulping was much more selective in

removing lignin from the middle lamella and cell corner regions as compared to

uncatalyzed alkaline processes [202]. The secondary wall, however, was delignified

faster by soda, followed by kraft, and finally soda-AQ pulping. It can be speculated

316 4 Chemical Pulping Processes

that lignin removal is retarded by the enhanced retention of carbohydrates being

linked to lignin structures [203].

Anthraquinone is clearly insoluble in water, whereas its reduction product [e.g.,

9,10-dihydroxyanthracene (AQ2–)]is soluble in alkaline aqueous solution. In addition

to the better solubility, use of the reduced form of AQ has been proposed as

being advantageous because of the considerably higher rate of penetration, resulting

in more homogeneous pulping [204,205]. The reduction of AQ is a reversible,

two-electron process with AQ2– as final product, as revealed by differential pulse

polarography of AQ in aqueous solution (containing 5% DMF to solubilize AQ)

[206]. The reduction of AQ in an aqueous solution can be described according to

the following equilibria [Eq. (141)]:

AQ 2 e _ 2 H _ AQH 2

AQ 2 e _ H _ AQH _

AQ 2 e _ _ AQ 2_

(141)

From the intersections of linear extrapolations of the E-pH plot, the dissociation

constants, pKaq1 = 9.0 and pKa2 = 12.05, can be determined. According to this

result (see Fig. 4.91), the reduction product of AQ is solely present as a dianion

under the conditions of kraft or soda pulping, with the standard redox potential,

E0

AQ/AQ

2– = –0.778 V(against saturated calomel electrode).

A thermodynamic study of the system Na2S/AQ under the conditions of kraft

pulping confirmed that AQ is reduced by the presence of hydrogen sulfide ions at

temperatures above 100 °C. Both increasing temperature and EA are favorable for

the reduction to the dianion. AQ oxidizes hydrogen sulfide ions in preference to

6 8 10 12 14

-0.9

-0.8

-0.7

-0.6

-0.5

pK

aq

pK = 12.05

aq

= 9.0

AQ

AQ2- AQH-

AQH

Potential, E [V]

pH value

Fig. 4.91 E-pH plot (Pourbaix diagram) for the equilibria of

AQ redox reactions: AQ//AQH2/AQH–/AQ2– measured at

25 °C in a 5% DMFaqueous solution (according to [206]).

4.2 Kraft Pulping Processes 317

sulfate ions and to thiosulfate ions, whereas oxidation to elemental sulfur is thermodynamically

not feasible at any of the temperatures studied (298–423 K). This

thermodynamic consideration suggests that AQ can be dissolved as AQ2– by mixing

it with white liquor at temperatures higher than 100 °C before its introduction

into the digester [206].

AQ is solubilized in the cooking liquor by sequential reduction, in the presence

of polysaccharides. Electrons are transferred from the reducing end groups of the

polysaccharide fraction in the wood. Simultaneously, the aldehyde groups are oxidized

to aldonic acid groups and thus stabilized against the alkaline peeling reactions.

This reaction is predominantly responsible for the increase in pulp yield,

and to some extent also for some alkali savings as a result of the reduction in the

formation of acids caused by suppression of stepwise depolymerization.

The AQ/AHQ redox system was extensively studied by Dence et al. [207]. During

pulping, AQ 1 is reduced beyond the hydroquinone 2 to anthrone 5 and further to

anthracene 7 and finally to dihydroanthracene 8. The complete AQ/AHQ system is

comprised of four individual redox systems, as shown in Scheme 4.26:

O

O

+ 2 e

-

O

H OH

O

H H

H

H H

OH

OH

OH

Ox. Red.

Ox.

Ox. Red.

Red.

H H

HO

-HOH

1 2

Scheme 4.26 The AQ/AHQ system in alkaline pulping

(according to [207]).

318 4 Chemical Pulping Processes

The most striking observation is the fact that the addition of extremely small

amounts results in both a significant improvement of yield due to carbohydrate

stabilization and in drastically enhanced delignification. An AQ charge of 0.05%

on wood corresponds to a molar ratio of AQ to a phenylpropane unit (C9-unit) of

about 1:500, and has a more pronounced effect on delignification as compared to

a conventional kraft process with the same EA charge at 25% sulfidity. This sulfidity

corresponds to a molar ratio of sulfur to C9-unit of about 1:2.5, this being two

orders of magnitude less efficient in removing lignin than AQ (on a stoichiometric

basis). The drastic improvements in delignification in the initial pulping stages and

the substantial yield preservation achieved by extremely small quantities of AQ have

been interpreted in terms of redox mechanisms. According to this highly simplified

concept, the quinone is initially reduced by the carbohydrates to the hydroquinone,

which in turn reduces lignin whereby the quinone is regenerated (Scheme 4.27).

-CH2OH > CHOH

O

O

OH

OH

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