Brightness Development in the Final ClO2 Stages of an ECF Kraft Pulp Bleaching Sequence: Modeling and Effects of Pulping Conditions, 1996 Pulping Conference Proceedings

Thomas J. McDonough

Institute of Paper Science and Technology

To gain an understanding of the factors that determine bleachability in the final stages of an elemental chlorine-free bleaching sequence, conventional and modified Kraft pulps were prepared in the laboratory and bleached in a D0 (EO)D1 ED2 sequence with varied applications of C1O2 in the D1 and D2 stages. After delignification at a constant kappa factor, the extent of D1 brightening was determined by ClO2 consumption and pulp type, but not by unbleached kappa number. For a given brightness after the D1 stage, modified pulping produced pulps that consumed less C1O2 than conventional pulps. Brightening in the D2 stage was described by a simple equation relating final brightness to C1O2 consumed. The parameters in this equation may be interpreted in terms of a brightness ceiling and a rate of approach to the brightness ceiling, termed the D2 response factor. The brightness ceiling increased with increasing levels of brightness entering the D2 stage, and was higher for modified than for conventional pulps. The response factor decreased as the entering brightness was increased and was higher for pulps of lower unbleached kappa number.