Effects of alkali and oxygen extractions of kraft pulp on xylanase-aided bleaching

Alkali and oxygen extractions appear to have different effects on the respo nse of softwood kraft pulps to xylanase-aided bleaching. Alkaline extractio n (without oxygen present) of low- and high- xylan pulps increases their re sponse to xylanase-aided bleaching, as measured by percent savings in total active chlorine applied and total active chlorine multiple. After oxygen d elignification (with both alkali and oxygen present), the percent savings i n total active chlorine are similar to those of the corresponding unextract ed pulps. However there is no increase in pulp bleachability as measured by the pulp brightness achieved at a given total active chlorine multiple. Th e benefits of xylanase pretreatment of oxygen-delignified pulps are closely associated with the drop in kappa number before bleaching. The differences observed in pulp responses are not dependent on their hexenuronic acid con tent.