Oxygen-Alkali Treatment of Kraft Pulp and Cellobiitol. Model Experiments and Applied Studies of Interest for Oxygen Bleaching

Abstract: During oxygen delignification of kraft pulp the alkalinity has a decisive influence upon the selectivity, defined as viscosity at a given kappa number. Using 0.2% pulp consistency in order to achieve constant pH-levels a minimum selectivity was reached at pH 13, corresponding to 0.1 M NaOH. At higher pH-levels the selectivity increased and the yield decreased. By lowering the pH the selectivity and the yield increased. With Na₂CO₃ as active alkali the selectivity increased with decreasing alkalinity.

Additions of large amounts of magnesium converted the alkali to Mg(OH). This led to a lowered pH and increased wet combustion of dissolved organic substances, generating alkali. Both the delignification and the depolymerization rates decreased, mainly due to the lowered pH. An improved selectivity was achieved.

At 0.1 M NaOH the addition of kraft lignin or spent bleach liquors resulted in a markedly decreased pH and improved selectivity. Higher alkalinities or injection of alkali in order to maintain a constant pH-level decreased the selectivity.

Additions of aniline or dimethyl aniline improved the selectivity significantly without negative effects on the delignification rate. This improvement was additional to the protective effect of magnesium.

In oxygen-alkali treatments of cellobiitol, a model substance for cellulose, addition of manganese(II) was found to exert protective as well as degrading effects, depending on the addition level of manganese, the purity, the temperature and the alkalinity of the system. The concentration at which the net manganese effect turned from retardation to promotion of the oxidation was lower the higher the purity of the system. The protective effect of manganese was different to that of magnesium. Magnesium compounds stabilized peroxide formed while manganese promoted its decomposition. Magnesium added to the system of the highest purity containing 10 mg/L manganese had a small but significant increasing effect on the degradation rate of cellobiitol.

Iron and copper additions exerted catalytic effects on cellobiitol. Copper added to a system containing iron suppressed the catalytic effect of iron. Manganese, 10 mg/L, substantially decreased the catalytic effects of iron or copper or both.