Pulp yield improvements using two-stage, extended oxygen delignification

Oxygen delignification is a process step commonly used to bridge the gap between cooking and bleaching. This process makes it possible to significantly reduce the kappa number of the pulp to be bleached and thereby the effluent load from the bleach plant. The yield loss during oxygen delignification is considerably less than that incurred in the final part of the cooking process. If conventional oxygen delignification is extended to produce very low kappa number pulps, it will also result in increasingly larger yield loss. For that reason, a unique, two-stage process has been developed to allow extended oxygen delignification with pulp yield and quality preservation. This paper describes laboratory cooking, oxygen delignification and bleaching results for softwood, covering a broad range of cooking kappa numbers. It quantifies the yield and quality improvements fi om this new method of extending oxygen delignification, which has recently been implemented at the commercial scale. Total pulp yield is increased by raising the cooking kappa number and extending the oxygen delignification step, while keeping the bleach plant kappa number constant.