Understanding factors contributing to low oxygen delignification of hardwood kraft pulps

We observed that the O-2 delignification pattern of aspen kraft pulps was distinctly different from that of pine pulps based on a reduction in the apparent pulp kappa number. Much of this phenomenon can be attributed to the variation of the residual extractives and hexeneuronic acid (HexA) unit content in the unbleached and O-2-delignified pulps, as they interfered with the conventional lignin determination methods. However, after a correction for contributions from these non-lignin components, the hardwood kraft pulps still displayed a lower response to O-2 delignification than the pine pulps. It appears that the relative reactivity of residual lignin in the hardwood and softwood pulps differs appreciably with the extent of kraft delignification, and is a significant factor affecting the overall O-2 delignification reaction.