FINAL CHLORINE DIOXIDE BLEACHING IN MULTI-STAGE BLEACHING AS AN OPTION FOR THE PRODUCTION OF VERY HIGH BRIGHTNESS AND OPTICALLY BALANCED PULPS - COMPARISON OF DD VERSUS DP BLEACHING SEQUENCES IN THE BLEACHING OF KRAFT PULPS

摘要

High Brightness and optically balanced pulp (as to their whiteness (L~*) and a~* and b~* values) is imperative to produce very high brightness ( > 92 %ISO) and high whiteness ( > 100%) paper products. The use of chlorine dioxide in the final bleach stage of a multi-stage bleaching has helped mills to produce high brightness and optically balanced pulps, enabling paper makers to produce very high brightness and whiteness paper products, necessitating the use of as little Optical Brightening Agents (OBA) as possible. Since bleaching is oxidative chemistry related, pulps have a tendency to revert when bleached at very high brightness. Therefore, the chemical used in final stages of bleaching to reach the brightness target is very critical. In this paper, the chemistry of chlorine dioxide and hydrogen peroxide, two predominant chemicals used in final bleaching stages, is reviewed and two distinct final bleaching stages, DD and DP are compared with respect to their ability to produce not only very bright and optically balanced pulps but also pulps with low brightness reversion.