Process for dividing the sulphide content of the green liquor for the production of white liquors having high and low sulphidity respectively

摘要

PCT No. PCT/SE93/00782 Sec. 371 Date Mar. 23, 1995 Sec. 102(e) Date Mar. 23, 1995 PCT Filed Sep. 29, 1993 PCT Pub. No. WO94/09204 PCT Pub. Date Apr. 28, 1994The invention relates to a process for dividing up the sulphide content of green liquor into a sulphide-rich part and a sulphide-poor part by crystallizing out sodium carbonate (Na2CO3). The crystallization of the sodium carbonate is effected by evaporating green liquor with a ratio of hydroxide ions [OH-] and sulphide ions [HS-] such that the liquor which is obtained after evaporation and separation of the solid phase has high sulphidity and an acceptably low content of carbonate ions [CO32-]. Prior to evaporation, the hydroxide ion content in the liquor is increased by adding quicklime (CaO). The solid phase (sodium carbonate and calcium carbonate) which has been separated off is diluted with water so that the sodium carbonate crystals go into solution and at the same time the solution is given a cation content which favours the causticization which is subsequently undertaken. The causticization is carried out in a conventional causticization plant. After separating off the lime sludge (CaCo3) which is formed during the causticization processes, a white liquor of low sulphidity and low carbonate content is obtained for use in the cooking process and, after oxidation, also in the oxygen gas delignification prior to final bleaching of the pulp.