Mill Closure with Enhanced Peroxide Stages, 1997 Minimum Effluent Mills Symposium Proceedings

The development of ways to reduce bleach plant effluent towards closed loop operation has overtaken the intense debate on chlorine elimination (ECF) and chlorine dioxide substitution (TCF). Hydrogen peroxide is a well known bleaching agent in ECF and TCF pulp production, and for closed cycle operations there are some clear additional advantages. The use of alkaline hydrogen peroxide reduces the acid filtrate stream and facilitates the recycling of effluent in the kraft recovery loop. Enhanced hydrogen peroxide stages have been introduced in ECF and TCF pulp bleaching. The pressurized peroxide final brightening stage PO and the hydrogen peroxide reinforced double oxygen O-OP stage are both applications of enhanced peroxide technology based on engineering concepts used in medium consistency oxygen delignification. Hydrogen peroxide performance in delignification and bleaching depends on the transition metal content and dissolved organic matter concentration. With increased filtrate recovery, there is a build-up of transition metals and dissolved organic matter in the process water. Solutions are proposed to deal with increased metals and dissolved organic matter concentrations. In this paper, a newly developed, second generation method to remove metal ions from the pulp is briefly discussed in the context of closed loop bleaching. A second part of this study concentrates on ways to improve the resistance of enhanced hydrogen peroxide stages towards dissolved organic matter. Extending on experience in the design of double oxygen delignitication stages, double vessel configurations for PO final bleaching stages are examined. Laboratory results indicate that, under COD build-up conditions, an improvement in the bleaching performance can be noted in an O-PO type stage when compared with a PO type stage. An oxygen preretention...