BEHAVIOR OF CALCIUM, MAGNESIUM AND MANGANESE COMPOUNDS DURING OXYGEN BLEACHING OF KRAFT PULPS

During oxygen bleaching of kraft pulps manganese compounds can, depend ing on their amounts, the charges of sodium hydroxide and of magnesium protector, and on the presence of trace amounts of catalytically acti ve transition metals retard or promote the depolymerization of the cel lulose. The present report deals with pulps pretreated with 2% nitroge n dioxide. As shown in forthcoming papers similar results were obtaine d with pulps without pretreatment with nitrogen compounds. Pretreatmen t with nitrogen dioxide at 80 degrees C followed by washing with water removed about 95% of the magnesium and manganese from kraft pulps. Su bsequent oxygen bleaching for 180 min removed 40-90% of the calcium re maining in the pretreated pulps. Produced hydroxycarboxylate ions gave rise to low molecular complexes of calcium and of magnesium, added as protector. Competition for complexing ligands contributed to a decrea sed dissolution of calcium with increasing addition of magnesium. When magnesium was applied large amounts of colloids of polynuclear magnes ium species were produced. These contained lignin fragments and less a bundant metal compounds such as manganese and calcium. Wet oxidation o f both low molecular complexing agents and of organic fragments of lar ger size contributed to a decreased or unchanged concentration of low molecular metal complexes and metal compounds linked to the colloids w ith hydrated magnesium oxide as the main constituent. The conditions w hich favored the wet oxidation were similar to those which led to an i ncreased attack on the lignin and carbohydrates in the fibers and thus promoted the dissolution of low molecular metal complexes and the for mation of metal ions containing colloids. Under certain conditions pre cipitation of sparingly soluble manganese compounds of oxidation state s higher than +2 had a predominant influence on the concentration of m anganese in the liquor.