Selective hydrolysis of hexenuronic acid groups and its application in ECF and TCF bleaching of kraft pulps

During kraft pulping, a major part of the 4-O-methylglucuronic acid groups of the native wood xylans are converted into 4-deoxyhex-4-enuronic acid groups (HexA). We now report a method for selectively removing HexA through a mild acid hydrolysis of kraft pulps. The amount of HexA removed correlated linearly with a decrease in kappa number: Depending on the type of the pulp, the kappa number was reduced by 2-7 units. The removal of HexA significantly reduced chemical consumption in ECF and TCF blenching sequences. The transition metal contents of the the pulps were, after the treatment, as low as after an alternate treatment with complexing agents. The selective hydrolysis had no profound effects on the paper properties of the fully bleached pulps, but it significantly improved the brightness stability of oxygen- and peroxide-bleached pulps.