Parameter unit White liquor Orange liquor

Na2S g NaOH L–1 54.6 19.1

NaOH g NaOH L–1 73.0 102.2

Na2CO3 g NaOH L–1 19.4 23.1

Na2S2O3 g NaOH L–1 5.0 8.9

Polysulfide sulfur g s L–1 0.0 9.5

Active alkali g NaOH L–1 127.6 121.3

Effective alkali g NaOH L–1 100.3 111.7

Sulfidity % on AA 42.8 15.7

4.2 Kraft Pulping Processes 311

With a total active alkali charge of 25%, the polysulfide addition calculates to

1.9% on o.d. wood. The conventional kraft process showed a favorable response to

polysulfide pulping. The yield increase was in the order of 2.6% on wood, or about

5.5% on a pulp basis. Carbohydrate analysis revealed that the yield increase found

is a result of increased retention of glucans and mannans. The data suggest that

55% of the yield gain can be attributed to an increased cellulose retention, and the

residual 45% to an increased galactoglucomannan retention [184].

The addition of polysulfide to EMCC kraft cooking compensates for the yield

loss which occurs when pulping to kappa numbers below 20. A laboratory study

using Southern pine chips confirmed that extended modified cooking and polysulfide

pulping are compatible technologies [185]. The addition of 2% and 3% PS

to extended modified cooking was found to increase the pulp yield by about 2%

and 3% on wood, respectively, at a kappa number of 17 (Tab. 4.40).

Tab. 4.40 Laboratory study of the effect of polysulfide (PS)

addition to both conventional and extended modified kraft

pulping of Southem pine chips (according to [185]).