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

Alkali and oxygen extractions appear to have different effects on the response of softwood kraft pulps to xylanase-aided bleaching. Alkaline extraction (without oxygen present) of low- and high- xylan pulps increases their response to xylanase-aided bleaching, as measured by percent savings in total active chlorine applied and total active chlorine multiple. After oxygen delignification (with both alkali and oxygen present), the percent savings in total active chlorine are similar to those of the corresponding unextracted pulps. However there is no increase in pulp bleachability as measured by the pulp brightness achieved at a given total active chlorine multiple. The 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 content.