Reduced brightness variations by extended oxygen delignification

The kappa number reduction for softwood kraft pulps in an alkaline environment was studied with and without oxygen present. It was shown that the major part of the kappa number reduction in oxygen delignification stages is due to the reaction chemistry but the importance of effective extraction washing was also proved in experiments with subsequent leaching in alkaline environments. During repetitive oxygen stages the kappa number reduction levelled off. Process parameters for extended oxygen delignification (OO) were chosen according to results from an investigation, using RSM (Response Surface Modelling) with a CCF (Central Composite Face-centred) design. The kappa number reduction and the brightness increase were mainly affected by the variations of pressure, pulp consistency and alkali charge. The extended oxygen delignification was shown to eliminate differences in the chromophore content of the pulp. Thus the conditions necessary for controlled charges of bleaching chemicals and stable bleaching conditions can be reached.