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 oxygen-alkali bleaching of pulp was evaluated by the method based on the difference between permanganate consu mption of original pulp and that of the mixture of pulp and effluent after oxygen-alkali bleaching. By low-consistency oxygen bleaching of softwood kr aft pulp, the permanganate consumption decreased very little. When the kapp a number of the pulp was halved (from 25.4 to 13.4), the decrease in perman ganate consumption of Ig pulp was only 1.22 ml of 0.02 mol/l potassium perm anganate. This value was smaller than that obtained for the oxygen-alkali t reatment of a corresponding amount of isolated residual lignin, 6.40 ml. Th is was not due to the slow oxidation of lignin in pulp but to the formation of permanganate-consuming substances from carbohydrates. Those newly forme d substances can not be oxidized by oxygen-alkali treatment. Taking these f acts into account, lignin originally present in pulp was found to be oxidiz ed well. Reduction of carbonyl groups in carbohydrates prevented the format ion of such substances.