Kraft Cooking with Addition of Reducing Agent

Abstract

The changes of polysaccharides during ordinary and two per cent sodium borohydride added kraft cooking of Shirakaba birch were studied. The cooking cycle was divided in five stages, in each stage the changes of polysaccharide from wood residue (pulp) and cooking liquor were observed.The effects of borohydride addition to yield of pulp were considerably large at first stage, but only small toward the end of the cooking cycle.Addition of borohydride saves the peeling reaction of polysaccharides, and consequently alkali consumption will be lowered. According to avoid the lowering the yield of pulp by high hydrogen ion concentration, low ratio of effective-to-wood must be used.Stabilization of xylan by borohydride is not so noticeable as glucomannan. Even in ordinary kraft cooking liquor, much amount of xylan was found. Addition of borohydride increased the amount of xylan in cooking liquor and stabilization of xylan in cooking liquor was higher than that in wood residue. Xylan had also different behavior in this point from glucomannan.These facts indicate that xylan is more soluble in alkali and has higher resistance to peeling reaction than glucomannan. It seems to be due to branching structure of xylan. Recent report32) about chemistry of xylan indicates the presence of branching in xylan molecule.