Enhanced P Stages for ECF and TCF Bleaching, 1996 International Pulp Bleaching Conference Proceedings

Hydrogen peroxide has an established position in prebleaching and final bleaching of kraft pulps. The unproved methods for metals control have prepared the way to the use of high temperature and pressure to enhance the alkaline peroxide P stages. While the beneficial effect of oxygen pressure on the final brightening performance of alkaline hydrogen peroxide stages has only been identified quite recently, several (PO) systems are already in operation or under construction. The first part of the study deals with a systematic review of the effects of pressure, temperature, time and nature of the gas as a function of the kappa number entering the final brightening stage. These investigations seem to confirm that a short pressurized preretention followed by a conventional atmospheric retention can nearly equal the performance of a fully pressurized (PO) stage. Benefiting from the prevailing temperature and pressure conditions, hydrogen peroxide can be used to further enhance selective delignification with double oxygen stages. The second part of the study comments on a systematic review of place and configuration of hydrogen peroxide introduction in double oxygen stages. The results show how O-(OP) can be used to overcome the absence of interstage washing in standard double oxygen systems. Both O-(OP) and (PO) are examples of the newly emerging trend towards pressurized subsequences. They can be used alone or in combination to improve the environmental performance in ECF sequences and the brightness performance in TCF sequences.