Voltammetric analysis of the bleachability of softwood kraft pulps

Abstract Kraft and oxygen delignified pulps with various kappa numbers were prepared from black spruce and western hemlock chips. The bleachability (ratio of kappa number decrease to chlorine dioxide applied) of the different pulps at the same kappa number varied with both wood furnish and delignification process. Thus, unbleached kappa number alone is not a reliable indicator of bleachability for these pulps. This may be due in part to the variable hexenuronic acid content of the pulps; those with higher ratio of hexenuronic acid content to kappa number (HexA/kappa) are harder to bleach. Voltammetric analysis of the same kraft pulps in the presence of redox mediators was found to measure both the content and reactivity of residual lignin. Peak current ratios of redox mediators correlated with pulp bleachability for all pulps. These ratios may therefore provide a more accurate prediction of bleachability than unbleached kappa number.