Comparison of Various Hardwood Kraft Pulp Pre-Bleaching Techniques

Abstract

We examined five pre-bleaching methods with the same starting hardwood kraft brownstock pulp to determine their effects on reducing bleaching costs. Each pretreated pulp was subjected to a D(EP)D bleaching sequence. A D1 brightness curve as a function of the percent applied chlorine dioxide (ClO2) was obtained for each of these pre-bleaching methods and for a baseline pulp sample. For each of the pre-bleaching methods, we calculated the amount of ClO2 required to obtain D1 stage 79.1% ISO brightness and determined the net cost savings for each sequence. Pre-bleaching methods that produced a net savings in the D1 stage were also subjected to a four-stage bleaching sequence of D(EP)DD to 89%ISO brightness. We also determined net cost savings resulting from pretreat-ment of the fully bleached pulps. The methods examined were mild acid pretreatment, xylanase enzyme pre-bleach-ing, brownstock peroxide pretreatment, acidic brownstock peroxide treatment, and xylanase enzyme followed by acidic peroxide pretreatment. Enzyme pretreatment and small applications of acidic peroxide pretreatment resulted in net D1 stage cost savings at 79.1% ISO brightness. Only enzyme pretreatment resulted in a net savings at 89% ISO.