Removal of hexenuronic acids by extended oxygen delignification of high-kappa eucalyptus kraft pulp

The presence of hexenuronic acids (HexAs) in kraft pulp interferes with bleaching by increasing the consumption of bleaching chemicals and compromising brightness stability. This study investigated the removal of HexAs during the extended oxygen delignification of eucalyptus kraft pulp with a high initial kappa number (64). Response surface methodology was used to evaluate the effects of temperature (80, 100, 120 and 140 °C) and alkali charge (15, 30, 45 and 60 kg t −1 ), both individually and interactively, under three conditions: without additives, with 0.05 % anthraquinone, and with 0.05 % ethanol (based on oven-dry pulp weight). The initial HexAs content (20.09 μmol g −1 ) was significantly reduced under severe conditions, especially in the presence of ethanol, which achieved a maximum reduction of 11.69 μmol g −1 (58.2 %). The removal of HexAs showed a strong dependence on the interaction between temperature and alkali charge, and the observed trends mirrored those of residual lignin, suggesting a similar response to the oxidative alkaline conditions. Among the treatments, ethanol was the most effective, whereas anthraquinone resulted in a limited reduction in HexAs content. These findings highlight the potential of optimized oxygen delignification for effectively reducing HexAs levels.