Interactions between metal compounds during oxygen bleaching of kraft pulp in virtually closed recovery systems

Kraft pulps-freed from catalytically active transition metals and alkaline earth metals by soaking with SO 2 -water and alkaline EDTA-were oxygen bleached in the presence of small additions of manganese (II) sulfate to simulate oxygen bleaching in systems designed to approach effluent-free production of bleached kraft pulps. Small additions of magnesium resulted in severe depolymerization of cellulose, while greater magnesium additions markedly suppressed this attack, giving rise to pulps of high selectivity. Chemical analysis of raw, ultrafiltered, and ultracentrifuged spent liquors from oxygen bleaching showed that manganese and the lignin fragments produced during bleaching were linked by stable bonds to polydisperse magnesium oxides. Formation of the complex colloids relied on (a) covalent bonds, e.g., oxo bonds, (b) ionic bonds, e.g., ion exchange, and (c) coordinate (complexing) bonds of high stability. This permits complete removal of manganese from a closed system before final bleaching. Application : laboratory results suggest strategies for eliminating manganese from closed bleach-plant recovery systems.