The Impact of Xylanase and Hot Acid Pretreatment on HexAs in Eucalyptus Kraft Pulp Bleaching

The influence of the presence of hexenuronic acid (HexA) in eucalyptus kraft pulp on the efficiency of D0EpD1 was investigated by separately using both commercial cellulase-free xylanase and hot acid pretreatments. In comparison to the xylanase pretreatment, the hot acid pretreatment was more than four-fold effective in removing the HexAs (61%) and concomitantly reduced the kappa number, but resulted in lower viscosity. After subsequent D0EpD1 bleaching, the final brightness and its stability were improved remarkably, especially under xylanase pretreatment. At the same brightness of 85% ISO, the xylanase pretreatment reduced the consumption of active chlorine (35.6%) more than the acid pretreatment did (27.5%). The total relative active chlorine savings for the xylanase pretreatment over the acid pretreatment was therefore nearly 23%. The HexA component can be removed considerably in both D0 and D1 stages during the D0EpD1 bleaching sequence, while only slightly in the Ep stage. The use of xylanase and ac...