Multistage Kraft Cooking as a way of Producing Bleachable Grade Alder Pulp

摘要

Modified kraft pulping as a way of obtaining bleachable grade pulp from black and grey alder wood was investigated. Extended kraft delignification as a three-stage cooking was performed. Chips were impregnated either with Na_2S solution or modelled green liquor. The lignin content in alder pulp was reduced to 1.8-1.9% without any reduction in pulp viscosity. Chips pretreatment with green liiquor had the same effect on delignifiication as the impregnation with Na_2S. Pretreatment of chips at 150 deg lowered the hemicelluloses content in the pulp. The increase of active alkali concentration in the liquor from 20 to 26 g/L in the final cooking stage had a positive effect on the degree of delignification. Both grey and black alder kraft pulps showed good brightness of 91-93% ISO after ECF and TCF bleaching sequence using xylanase. It has been shown that alkali-accessibility of lignin and hemicelluloses from unbleached pulp allows to forecat pulp bleachability.