Delignification of chemical pulp - by peroxide oxidation in alkaline medium followed by mechanical pressing

摘要

Delignification of chemical pulp is effected by (a) oxidising the raw pulp with a peroxide, in alkaline medium, using a % ratio peroxide/pulp w.r.t. the pulp kappa index of 0.01-0.1, and at a pH of 9-12 (10-11), followed by (b) mechanically pressing the oxidised pulp, with recycling of the extraction effluents from the pressing stage, partly upstream and partly downstream of the oxidation step. The recycled effluents contain dissolved carboxylic acid salts (oxidised lignin) which have a buffering effect so that it is not necessary to use inorganic buffers and/or protectors of cellulose in the peroxide oxidation step. The mechanical pressing results in a better extraction and elimination of the oxidised lignins resulting in reduced pollution of the waste waters during the final bleaching step.