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 4-0-methylglucuronic acid groups of the native wood xylans are converted to 4- deoxyhex-4-enuronic acid groups (HexA). Because of its ene functionality, HexA reacts with several bleaching chemicals, such as chlorine, chlorine dioxide, ozone, and peracids, thus consuming these chemicals. Also permanganate attacks HexA which therefore contributes to the K-number of the pulps, We now report a method for selectively removing HexA through a mild acid hydrolysis of kraft pulps. The potential of the method as a part of ECF and TCF bleaching sequences was studied with several Finnish companies. During the selective hydrolysis HexA was converted to furan derivatives that could be quantified from the hydrolyzate by UV spectroscopy. The amount of the furan derivatives correlated linearly with the decrease in K-number of both unbleached and oxygen bleached softwood and hardwood pulps. Depending on the type of the pulp, the selective hydrolysis removed 20-60 meq HexA per kg of pulp and reduced the K-number by 2-7 units. Relatively speaking the change was biggest for oxygen bleached birch kraft pulps the K-number of which was reduced, as an average, by 50 %. The treatment reduced the chemical consumption in all bleaching sequences. In ECF bleaching of birch kraft pulp the reduction was 30-40 %. Because the selective acid hydrolysis removed a major part of the metal binding sites from the pulps, their transition metal contents were after the treatment as low as after an alternate treatment with complexing agents. Thus their use was unnecessary in TCF bleaching sequences. The selective hydrolysis had no profound effects on the paper technical properties of the fully bleached pulps but it significantly improved the brightness stability of oxygen and peroxide bleached pulps.