Reduced brightness variations by extended oxygen delignification

The kappa number reduction for softwood kraft pulps in an alkaline environm ent was studied with and without oxygen present. It was shown that the majo r part of the kappa number reduction in oxygen delignification stages is du e to the reaction chemistry but the importance of effective extraction wash ing was also proved in experiments with subsequent leaching in alkaline env ironments. During repetitive oxygen stages the kappa number reduction level led off. Process parameters for extended oxygen delignification (OO) were c hosen according to results from an investigation, using RSM (Response Surfa ce Modelling) with a CCF (Central Composite Face-centred) design. The kappa number reduction and the brightness increase were mainly affected by the v ariations of pressure, pulp consistency and alkali charge. The extended oxy gen delignification was shown to eliminate differences in the chromophore c ontent of the pulp. Thus the conditions necessary for controlled charges of bleaching chemicals and stable bleaching conditions can be reached.