Influence of manganese and magnesium on oxygen bleaching in carbonate media after a nitrogen dioxide pretreatment

Kraft pulps from softwood subjected to pretreatment with 2% NO2 which removed magnesium and catalytically active transition metal compounds were delignified by more than 80% after oxygen bleaching with low charges of sodium carbonate. With 12 gmol Na2CO3 per 100 kg pulp, addition of 0.11 gmol of MnSO4 per 100 kg pulp resulted in a catalyzed delignification and a retarded attack on the cellulose. The retardation decreased with increasing charge of carbonate and turned to a catalytic effect on the cellulose degradation at high carbonate concentration. Of the added manganese 50% or more was dissolved within 30 min. A large proportion was precipitated after 120 min when no magnesium compounds were added. Both the delignification and the depolymerization of the cellulose were retarded by addition of magnesium sulfate. In bleaches with large charges of Na2CO3 the magnesium salt gave rise to an increased selectivity (increased pulp viscosity at a given kappa number). With charges within the range attractive in practical applications the effect was small. No precipitation of magnesium compounds occurred.