Ck1 ck2 ck3 emcc1 emcc2 emcc3

Polysulfide % S od w 0 0 2 0 2 3

Sulfidity % 30 30 30 31 30 30

Impregnation stage

Temperature °C 110 110 110 110 110 110

EA-charge % NaOH od w 19.71 20.61 21.9 14.1 14.1 16.8

First cooking stage

Temperature °C 170 170 170 165 165 165

EA-charge % NaOH od w 5.0 5.0 5.0

Countercurrent stage

Temperature °C 165 165 165

H-factor 1560 1720 1690 2960 3020 3050

Screened yield % od w 46.2 46.1 48.7 43.4 45.3 46.7

Kappa number 29.6 27.4 27.1 16.7 17.2 17.6

Intrinsic viscosity mL g–1 1115 1075 1150 1030 1080 1110

312 4 Chemical Pulping Processes

The addition of approximately 3% PS increases the pulp yield of an EMCC pulp

with kappa 17 to a level typical for conventional pulps with kappa number 30. The

results also show that charging with 2% PS increases the pulp yield by about 2.5%

when cooking to kappa numbers close to 30, suggesting that the yield increase

from PS addition is larger at a higher kappa number. This finding is in agreement

with results from previous studies where pulp yield was found to increase by

1.2–2% for every 1% of PS added. The specific yield gain further increases when

approaching higher kappa numbers [186]. In contrast to results from other studies,

the addition of PS does not appear to affect pulp tear, burst or tensile properties

[184]. The preservation of strength properties is also reflected in the high

intrinsic viscosity levels (see Tab. 4.40). Refining energy to a given freeness can be

reduced by approximately 10% and 20%, respectively, with 2% and 3% PS addition.

This is in agreement with previously reported data, and can be traced back to

a higher retention of hemicelluloses.

A PS kraft cook of a mixture of Pinus sylvestris and Picea abies with a 5% charge

of PS on wood gave an average yield increase of about 3% on wood within a kappa

number range 7 to 20 [187]. The yield increase related to PS addition is smaller

than has been reported at higher kappa numbers, probably because the reinforced

conditions when pulping to a lower lignin content cause a higher loss in hemicelluloses

[186]. Moreover, delignification selectivity, given as kappa number–viscosity

relationship, was improved as a result of the PS pretreatment. Interestingly, if

the hydroxide ion concentration is too low by using, for example a HCO3

–/CO3

2–

buffer system during the PS pretreatment, neither a yield increase nor a viscosity

improvement can be observed.

A decrease in kappa number from 35 to 20–25 in a normal kraft cook of Scots

pine reduces the pulp yield by 2–3% units. The yield loss is compensated for by

the use of PS which is produced by the MOXY process [171,188]. Using a highsulfidity

white liquor (sulfidity 52%), a PS concentration of 0.32 M S(0) is produced

applying the MOXY process. The conventionally bleached (CEHDED) polysulfide

pulps with low kappa number (21–23) revealed similar viscosity values (ca.

900 mL g–1 at 88% ISO brightness) and strength properties (ca. 14 mN m–2 g–1 tear

index at 70 Nm g–1 tensile index) as compared to the normal kraft pulps with

kappa number 35 after cooking [188]. Due to the prolonged cooking using PS-containing

white liquor to compensate for the yield loss, the consumption of chlorine

chemicals can be reduced by about 26% using a conventional CEHDED-sequence.

In the PS process, some alkali is consumed for the reaction between the PS and

the wood components according to Eqs. (137) and (138). Thus, approximately

16% more EA charge (20.4% instead of 17.6% on o.d. wood) is required in PS

cooking to attain the same degree of delignification at a given H-factor.

The addition of 1.6–1.7% PS sulfur during the impregnation stages of both the

MCC-type and ITC-type cooks using a mixture of Picea abies and Pinus sylvestris as

wood source gave an increase in carbohydrate yields of 1.2% at kappa number 24,

and of 1.5% at kappa number 19 as compared with the reference [189]. The lower

yield increase for the modified cooks can probably be explained by the higher

[OH– ]ion in the final cooking stages. The extraction of cooking liquor from the

4.2 Kraft Pulping Processes 313

digester in the Hi-Heat zone certainly counteracts the reprecipitation of dissolved

hemicelluloses. The carbohydrate yield increase associated with PS pulping can

be attributed predominantly to a rise in glucomannan retention. The pulps from

PS pulping showed a slightly lower tearing resistance, but comparable zero-span

tensile indices. Thus, it can be concluded that strength properties are not

impaired by PS pulping. The bleachability in an OD(E+P)DED bleaching

sequence was equal for both the PS ITC-type pulp and the reference ITC-type

pulp. The latter requires 135 OXE per ton of pulp and kappa number to reach a

brightness of 89% ISO, whereas the PS pulp required 133 OXE per ton of pulp

and kappa number.

Mill experience ofPS pulping

The Norwegian kraft mill, Lovenskiold-Vaekero, Hurum Fabriker, changed to the

PS process as early as 1967 [190]. Polysulfide is produced by the dissolution of elementary

sulfur in the white liquor. After one year’s experience, yield increases of

3.5–4.0% were obtained with a 2.2% sulfur addition on wood. The PS pulps are

characterized as easy-beating pulps, with the reduction in required beating energy

in the mill amounting to 25–30%. Due to the reduced fibers per unit area, a slight

reduction in the tear factor of the paper product was observed. Runnability on the

paper machine was, however, not significantly affected. Although economic calculations

are dominated by the currently available wood, sulfur and pulp prices, a

net gain of approximately 5 US$ adt–1 pulp can be expected. [190].

The additional costs in pulping due to PS addition and slightly higher EA

demand must be compared with the savings in bleaching chemicals and effluent

treatment costs, in order to estimate the economy of PS pulping in combination

with extended modified cooking.

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