Hexenuronic acid (HexA) is formed during kraft pulping by the conversion of 4-O-methyl-glucuronic acid groups in strong alkali conditions. HexA can be removed from the pulp by including an acid hydrolysis (A) stage in the bleaching sequence. Despite a significant body of work in this area, very little research has been carried out to examine HexA in pulps produced from South African-grown tree species under the conditions of the local pulp and paper industry. The objective of this paper, which is part of a broader study, was to determine the optimum conditions in the A-stage that resulted in maximum removal of HexA, but with minimum impact on the physical and chemical properties of the pulp after bleaching. For this purpose, oxygen delignified pulps produced from a Eucalyptus hybrid clone, E. grandis x E. urophylla, were subjected to acid hydrolysis at varied temperature and reaction times. The pH was kept constant at 3.5 for all experiments. The results showed that the A-stage was efficient in the removal of HexA; acid hydrolysis carried out at 125°C-180min removed as much as 98% of the HexA, resulting in a kappa number reduction of approximately 6 units.

Hexenuronic acid (HexA) is formed during kraft pulping by the conversion of 4-O-methyl-glucuronic acid groups in the strong alkali conditions. The amount of HexA formed is strongly influenced by the pulping conditions used. It can be removed from the pulp by including an acid hydrolysis stage in the bleaching sequence. Very little research has been carried out to examine HexA in pulps produced from South African grown tree species under local pulp and paper industry conditions. We investigated the influence of genotype and site quality of two commercially important South African grown Eucalyptus hybrid clones on the formation of HexA. We also studied the efficiency of acid hydrolysis in removing HexA, and the subsequent effects on kappa number and bleaching chemical consumption. The results show that pulps produced from E. grandis x E. urophylla (GU) clones contain significantly higher amounts of HexA compared to pulps produced from E. grandis x E. camaldulensis (GC) clones, grown on sites of similar quality. Both clones grown on sites of poor quality also showed significantly higher amounts of HexA compared to the same clones grown on good sites.