Analysis of progress of oxidation reaction during oxygen-alkali treatment of lignin 2: Significance of oxidation reaction of lignin during oxygen delignification

Abstract

The progress of lignin oxidation during oxygen-alkali bleaching of pulp was evaluated by the method based on the difference between permanganate consumption of original pulp and that of the mixture of pulp and effluent after oxygen-alkali bleaching. By low-consistency oxygen bleaching of softwood kraft pulp, the permanganate consumption decreased very little. When the kappa number of the pulp was halved (from 25.4 to 13.4), the decrease in permanganate consumption of 1g pulp was only 1.22 ml of 0.02 mol/l potassium permanganate. This value was smaller than that obtained for the oxygen-alkali treatment of a corresponding amount of isolated residual lignin, 6.40 ml. This was not due to the slow oxidation of lignin in pulp but to the formation of permanganate-consuming substances from carbohydrates. Those newly formed substances cannot be oxidized by oxygen-alkali treatment. Taking these facts into account, lignin originally present in pulp was found to be oxidized well. Reduction of carbonyl groups in carbohydrates prevented the formation of such substances.