Abstract
It was reported in the previous paper that hemicellulose becomes stabilized and the yield increases when kraft cooking is carried out by adding sodium borohydride as the reducing agent to the cooking liquor. However, as sodium borohydride is rather unstable at elevted temperatures, tests were carried out to find the most effective condition, i.e.
(1) carrying out two-stage cooking by reduction pretreatment, (2) changing the ratio of active alkali of the cooking liquor, (3) lowering the pH of cooking liquor on the halfway or at the end of cooking by addition of sulfuric acid.
As a result, it was found that (1) in case of softwood, two-stage cooking by reduction pretreatment for long time at room temperature is effective, and in this case, reuse of the pretreatment solution is possible and it is economical of the reducing agent, (2) in case of hardwood, while the effect of reduction pretreatment is small, cooking by addition of reducing agent under condition of low active alkali is effective, and especially lowering the pH at the end of cooking to make redeposition of hemicellulose is most effective.
