Reduction of active chlorine charges in bleaching of xylanase-pretreated sulfite pulp

Biobleaching of acid bisulfite pulp with Aureobasidium pullulans xylanases and reduced amounts of chlorine dioxide (up to 50% as active chlorine) can produce dissolving pulp with improved properties such as alkali solubility (S-10 and S-18)and brightness over the conventionally ECF-bleached control. The bleachability of the xylanase-pretreated pulp was evaluated in a subsequent treatment with oxygen, hydrogen peroxide or three ECF-bleaching sequences. A brightness gain of 1.7, 1.1 and 1.6 points was achieved by the xylanase pretreatment respectively in the X-OD(1)E(0)D(2)P, X-OD(1)E(0)D(2)H and X-OD(1)E(0)D(2) bleaching combinations. The brightness of the X-OD(1)E(0)D(2)P-bleached pulp (94.2%) was approximately 2 and 3 units higher than that of the X-OD(1)E(0)D(2)H and X-OD(1)E(0)D(2) pulp, respectively. The S-10, S-18 and S-10-S-18 values of the X-OD(1)E(0)D(2)P dissolving pulp were reduced relative to the control by up to 22, 13 and 31%, respectively. In all instances the viscosities of the X-OD(1)E(0)D(2)H pulp were lower than those of the X-OD(1)E(0)D(2) and X-OD(1)E(0)D(2)P dissolving pulps.