Influence of sulfur compounds, manganese, and magnesium on oxygen bleaching of kraft pulp

Kraft pulp from softwood, freed almost completely from metals by soaking with SO2-water and EDTA, was oxygen bleached in the presence of different alkali sources, e.g., white liquor and oxidized white liquor. Sodium sulfide was found to affect the pulp quality adversely, while the influence of sodium thiosulfate was slightly favorable. The total amount of sulfur present during the bleaching appeared almost quantitatively in the spent bleach liquor, mainly as thiosulfate and sulfate. However, a large proportion of the sulfate that was formed during the bleaching could not be derived from thiosulfate. The addition of a substantial amount of manganese suppressed the depolymerization of the cellulose markedly, giving rise to increased selectivity, defined as the viscosity at a given kappa number. When magnesium protector was added in the molar ratio Mg:Mn = 10, a further gain in selectivity was obtained, but the rate of delignification was somewhat retarded.