Na2SO3 Na2CO3 NaOH

Wood 31.8 100.0 37.6 17.0 8.4

AS/AQ 31.0 0.0 0.0 95.9 12.1 68.7 37.5 10.6 6.6 3.9

AS/AQ 18.6 12.4 0.0 69.4 6.7 57.2 36.0 6.0 5.6 2.0

AS/AQ 18.6 0.0 12.4 28.9 2.4 48.0 35.6 4.8 5.4 1.5

Kraft 38.8 2.6 46.7 33.6 5.1 3.8 1.2

a) GGM: galactoglucomannan= (galactan + (4/3) mannan.

b) AX: arabinoxylan.

Cooking conditions: 0.1% AQ, I:s = 4.1, 90 min heating-up time to

175 °C, 3 h at 175 °C.

476 4 Chemical Pulping Processes

proved to be very effective in lignin removal, but also resulted in cellulose degradation

and further removal of glucomannan. Thus, liquor alkalinity throughout

AS/AQ cooking is the key parameter affecting delignification selectivity. It was

found that pulp viscosity passes a maximum when the pulping liquor contains

only a small concentration of free hydroxide ions. However, increasing the sodium

hydroxide concentration beyond a certain level results in a rapid drop in viscosity.

On the other hand, liquors containing no free sodium hydroxide but increasing

proportions of sodium bicarbonate also produce progressively lower viscosities

[44]. Another important parameter in AS/AQ cooking is the proportion of Na2SO3

to the total alkali charged. When the proportion of nonsulfite alkali is only about

20% of the total alkali charged (calculated as NaOH), the alkalinity prevailing during

most of the cook is rather insensitive to the initial ratio of NaOH to Na2CO3

because the dissolved lignin and carbohydrate degradation products neutralize

the charged alkali, forming an effective buffer system. In order actively to control

the alkalinity of the cooking liquor, the proportion of Na2SO3 must be reduced to

about 60% of the total alkali charged. As expected, the rate of pulping increases

continuously with liquor alkalinity but, unfortunately, the rate of carbohydrate

degradation and dissolution is similarly affected.

The final conclusion of all investigations into alkaline sulfite pulping conducted

during the 1980s was that the most promising applications of this process seem

to be packaging grades and news reinforcement. Ingruber et al. showed the yield

advantage of the AS/AQ over the kraft process to be about 7% on o.d. wood (62%

versus 55%) at a kappa number of 80 [45]. At the same time, the tensile strength

was 20% higher and the tear strength 20% lower, as compared to the corresponding

kraft pulp. The advantage of AS/AQ pulps in tensile strength can be used to

reduce refiner energy still further by operating at higher freeness (the beatability

of the AS/AQ pulp is superior in general) while simultaneously obtaining higher

tear resistance.

As mentioned earlier, the pronounced advantage of AS/AQ pulping diminishes

as the extent of delignification is increased to obtain bleachable grades (kappa

number range 20–30). Based on this conclusion, most activities on alkaline sulfite

pulping were halted, with the exception of a German research group headed by

Professor Patt at the University of Hamburg. This group found that the addition

of methanol to the alkaline sulfite liquor improved delignification considerably,

while preserving pulp viscosity. In a very systematic approach, the contribution of

each single component of the pulping liquor on delignification was evaluated [46].

The experiments were carried out with a cooking liquor containing a total alkali

charge of 25% on o.d. wood, calculated as sodium hydroxide, with a sodium sulfite:

sodium hydroxide ratio of 80:20. The reaction conditions were kept constant,

choosing a maximum temperature of 175 °C, a cooking time of 150 min, and

using spruce wood as the raw material. Under these basic conditions, only 20% of

the lignin could be dissolved, resulting in a kappa number of about 150. As mentioned

previously, the limited extent of delignification can be attributed to the low

liquor alkalinity. As soon as the charged sodium hydroxide is neutralized by the

degraded wood components, delignification slows down significantly. Replacing

4.3 Sulfite Chemical Pulping 477

part of the water in the cooking liquor with methanol promoted delignification,

and a final kappa number of about 100 was reached. The addition of AQ further

improved alkaline sulfite delignification very efficiently, and reduced the kappa

number to about 40. By combining both additives to the sulfite cooking liquor, the

delignification of spruce wood was extended to kappa number 25. Based on these

findings, the new pulping process – alkaline sulfite-anthraquinone-methanol

(ASAM) cooking – was developed [46,47]. Throughout the next 10 years, the

ASAM process was evaluated extensively on laboratory and pilot plant scale, using

all sources of raw materials (softwoods, hardwoods and annual plants) [48–55].

The ASAM process has proven to yield pulps of better strength properties, higher

yield and better bleachability as compared to the kraft process. The technical feasibility

of this process has been proven in extensive pilot plant trials, including TCF

bleaching, methanol recovery, liquor evaporation and combustion of concentrated

black liquor. Despite these clear advantages over kraft pulping, the ASAM process

has so far failed to become commercialized, for three main reasons:

1. The presence of methanol requires explosion-proof installations

and a methanol recovery system, both of which make

the investment more expensive.

2. The efficiency and reliability of the available sodium sulfite

recovery systems seem not to be sufficient, and measures

would have to be undertaken that, again, would lead to higher

costs.

3. The technology of kraft pulping has been improved extensively

during the past 15 years, and this has reduced the

advantages of the ASAM process (Modified kraft cooking).

Furthermore, with the general acceptance of ECF bleaching

procedures, the argument of a better TCF bleachability of the

ASAM pulp is no longer valid.

Nevertheless, the high selectivity and bleachability of AS pulps remains an attractive

basis for the development of superior pulping processes. To overcome the

problem of methanol recovery, an attempt was made to improve the efficiency of

AS pulping in the absence of an organic solvent. Based on the principles of modified

kraft cooking, part of the sodium hydroxide is added only after having

reached the maximum cooking temperature, the aim being to maintain an even

alkali profile throughout the whole cooking process [56]. In an initial approach,

the optimum alkali ratio with regard to delignification efficiency and selectivity

was evaluated to create a sound basis for further optimization. However, as known

from previous investigations [44], increasing the proportions of sodium hydroxide

promotes delignification at the expense of selectivity (Fig. 4.189).

The selection of an alkali ratio of 60:40 (Na2SO3: NaOH) ensures both sufficiently

low kappa number and reasonably good selectivity. Splitting of the NaOH

charge to different cooking stages alters the hydroxide ion concentration (pH) profile,

particularly during the heating-up period, as shown in Fig. 4.190.

478 4 Chemical Pulping Processes

15 20 25 30 35 40

Alkali ratio: Na

SO

:NaOH (as NaOH)

70:30 60:40 50:50

Viscosity [ml/g]

Kappa number

Fig. 4.189 Effect of alkali ratio on the viscosity–

kappa number relationship (according to

[56]). Conditions: spruce as raw material;

27.5% total chemical charge on o.d. wood

(calc. as NaOH); 0.1% AQ on o.d. wood;

90 min heating-up time; 175 °C cooking temperature;

60–270 min at cooking temperature.

0 20 40 60 80 100

Temperature [.C]

Initial NaOH charge [% of total charge]:

100 % 50% 0%

pH value

Heating time [min]

Temperature

Fig. 4.190 Course of the pH of the cooking

liquor (25 °C) in AS/AQ pulping with NaOH

splitting (according to [56]). Conditions: pine

as raw material; 27.5% total chemical charge

on o.d. wood (calc. as NaOH); 0.1% AQ on

o.d. wood; 90 min heating-up time; 175 °C

cooking temperature.

4.3 Sulfite Chemical Pulping 479

Splitting the alkali charge in a ratio 50:50 provides a rather even alkali profile

throughout the cook. At the start of the cooking phase, the pH increases almost to

starting level after addition of the residual amount of alkali. In the reference case

– without split addition – the hydroxide ion concentration continuously decreases,

leading to extensive carbohydrate degradation at the beginning of the cook and to

insufficient delignification rate during residual delignification. The split addition

of the NaOH charge is clearly reflected in a superior delignification efficiency and

selectivity (Fig. 4.191).

0 20 40 60 80 100

Selectivity (V/Ê)

Kappa number

Kappa number

Initial NaOH charge [% of total NaOH charge]

Selectivity = viscosity / kappa number

Fig. 4.191 Effect of alkali splitting in AS/AQ

pulping on the efficiency (kappa number) and

selectivity (viscosity/kappa number) of delignification

(according to [56]). Conditions: pine as

raw material; 27.5% total chemical

charge on o.d. wood (calc. as NaOH); alkali

ratio 60:40; 0.1% AQ on o.d. wood; 90 min

heating-up time; 175 °C cooking temperature;

150 min cooking time.

The selectivity plot shows that optimum selectivity is obtained when the initial

NaOH charge is limited to about 20–50% of the total charge. Compared to the reference

case, the kappa number can be decreased from 32 to about 22, while the

viscosity increases from 1130 mL g–1 to about 1200 mL g–1. These convincing

results clearly confirm the principles of modified cooking, where alkali profiling

leads to both better selectivity and enhanced delignification. Alkaline sulfite pulping

contributes to high carbohydrate yields, provided that the alkali charge

remains low and cooking intensity does not exceed a certain level.

Dissolution of the main wood components during AS/AQ cooking of spruce

with alkali splitting 37.5:62.5 was monitored. For comparison, the corresponding

results obtained from continuous batch kraft cooking of spruce are included in

Fig. 4.192 [57].

480 4 Chemical Pulping Processes

0 20 40 60 80 100

0 1

ASA: KRAFT (CBC):

Cellulose Cellulose

Glucomannan Glucomannan

Arabinoxylan Arabinoxylan

Wood Component Yield [rel%]

Lignin yield [rel%]

Fig. 4.192 Dissolution of the main wood components

cellulose, glucomannan, and arabinoxylan

as a function of lignin content during AS/

AQ and continuous batch kraft cooking of

spruce. AS/AQ cooking: 27.5% total chemical

charge on o.d. wood (calc. as NaOH); alkali

ratio 60:40; NaOH splitting ratio 37.5:62.5;

0.1% AQ on o.d. wood; 90 min heating-up

time; 175 °C cooking temperature [56,58]. For

CBC kraft cooking, see Fig. 4.72, Modified Kraft

Cooking [57].

The data in Fig. 4.192 confirm the better preservation of spruce carbohydrates

during AS/AQ cooking as compared to CBC kraft cooking, particularly in the early

and intermediate stages of the process. With progressive delignification, the yield

advantage of the AS/AQ cook, including the split addition of NaOH, diminishes

considerably. However, cellulose and xylan yields remain at a higher level as compared

to CBC kraft pulping, even when delignification is extended to kappa numbers

between 20 and 30. According to Patt et al., AS/AQ cooking of spruce with

split addition of NaOH results in a kappa number 23.7 and a viscosity of

1191 mL g–1 at a yield of 50.8% [56]. The corresponding results for CBC cooking

of spruce are kappa number 25.8 and a viscosity of 1188 mL g–1 at a total yield of

48.1% [57]. The comparison reveals a distinct yield advantage for the AS/AQ cooking

procedure, even at a low kappa number. The good response of this pulp to

oxygen delignification suggests that cooking should be interrupted at a higher

kappa number, and continued with two-stage oxygen delignification.

The AS/AQ process produces pulp with strength properties that are equal or

even slightly superior to those of kraft pulp [44]. This is illustrated in Fig. 4.193, in

which tear index is shown as a function of tensile index.

Unbleached AS/AQ pulps are slightly superior in tensile strength compared to

the corresponding CBC kraft pulps. As expected, the level of tear strength is below

that of CBC pulps. At a given tensile strength, the tear resistance reaches a

4.3 Sulfite Chemical Pulping 481

0 2 40 60 80 100 120

Tensile Index [Nm/g]

Tear Index [mNm2/g]

Unbleached Pulps Kappa number Viscosity [ml/g]

conventional Kraft: 30.5 1180

Kraft CBC: 26.0 1180

ASA: 21.4 1210

Fig. 4.193 Tear–tensile plots of unbleached spruce AS/AQ

and CBC kraft pulps. Strength properties of spruce AS/AQ

pulps are described in Ref. [58], and those of spruce CBC kraft

pulps in Ref. [57].

comparable level for both pulps, especially if the better beatability of the AS/AQ

pulp is considered.

Considering the manifold possibilities of modified cooking, it may be assumed

that the potential of the AS/AQ cooking concept has not yet been fully exploited.

For example, the rapid increase in yield loss at kappa numbers below 40 could

reflect deficiencies in sulfonation of the residual lignin in relation to the hydroxide

ion concentration. The introduction of displacement cooking technology may

provide a better basis for adjusting the reaction conditions within all single cooking

phases to further optimize the pulping performance.

One major disadvantage of AS pulping compared to kraft pulping is certainly

the low delignification rate, and this will not be easy to overcome. Currently, an

H-factor of about 3500 is necessary to attain a pulp of kappa number 25 in the

case of AS/AQ pulping, while for CBC cooking an H-factor of about 1200 is sufficient

to reach the same kappa number.

The only way finally to achieve industrial acceptance, however, is to develop a

reliable, cheap, efficient and flexible chemical recovery system. Low-temperature

gasification is likely to be the appropriate process that permits the highly energyefficient

pyrolysis of AS black liquor and, simultaneously, the separate recovery of

sodium and sulfur components. Together, this should render possible alkali splitting

in cooking (a prerequisite for modified cooking technology) and the generation

of sodium hydroxide for alkaline bleaching operations.

482 4 Chemical Pulping Processes

References 483

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