Publication date: 2002-12-12 

Abstract

The invention relates to an improvement in bleaching of kraft pulp. In ECF bleaching chlorine dioxide is most often a prefered bleaching chemical in certain process positions. The disadvantage is that residual amounts of organically bound chlorine in form of AOX, i.e. organically bond chlorine in effluents, and/or OCl, i.e. chlorine organically bound in the produced pulp, is obtained. A normal approach has been to reduce charges of chlorine dioxide in the D-stage. According to the invention could substantial reduction in OCl levels be obtaiend if a substantial charge of sulphamic acid is added to the D-stage, preferably the last stage if several D-stages is used in sequence. A higher charge of chlorine dioxide could thus be used without increasing the residual content of OCl in pulp produced, which result in a mroe efficient D-stage as well as potential savings related to costs for bleaching chemicals in additional bleaching stages. A major reduction of OC1 up to 50 % could be obtained with the invention. The sulphamic acid added captures in situ formed elemental chlorine in the chlorine dioxide stage, which elemental chlorine would otherwise stick to the pulp.