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

The progress of lignin oxidation during oxygenalkali 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 1 g pulp was only 1.22ml of 0.02mol/l potassium permanganate. This value was smaller than that obtained for the oxygen-alkali treatment of a corresponding amount of isolated residual lignin, 6.40ml. 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.Part of this paper was presented at the 10th ISWPC, Yokohama, June 1999; and at the 43rd Lignin Symposium, Fuchu, October 1998