Pre-bleaching of kraft acacia pulp

Kraft pulps from acacia hybrid, Acacia mangium of 8 years old and Acacia auriculiformis of 6, 8 and 10 years old were pre-treated with oxygen, peroxyformic acid and acid treatment prior to bleaching. The kappa number reduction was 52-63% by oxygen delignification, 31-35% by peroxyformic acid (PFA) pre-treatment and 11-13% by acid pre-treatment. Oxygen delignified pulp required less chlorine dioxide charge to reach target brightness. At the consumption of 30 kg ClO2/ton of pulp, the pulp brightness reached to 65-71% for the untreated pulp, 81-85 % for the oxygen delignified pulp, 81-82% for the PFA treatment and 79-80% for acid pre-treated pulp. COD load in bleached effluent was much lower in oxygen delignified pulp. Cold alkali extraction of unbleached and oxygen delignified pulps was also carried out with varying alkali charge to remove hexenuronic acid (HexA) from the pulp. Xylan removal from the pulp was insignificant and resulted in no removal of HexA. Acid pretreatment removed 55.7% to 17.8% HexA from acacia hybrid, 57.5% to 16.3% from A. auriculiformis of 10 years and 58.6% to 20.1% from A. auriculiformis of 6 years old, resulting in improved final pulp brightness.