GB758158A - Improvements in the purification of wood pulp - Google Patents

Abstract

Wood pulp is refined by first chlorinating, then treating with alkali solution below its boiling-point and finally treating the pulp with alkali solution at a temperature above that of the first alkali treatment and at least 90 DEG C. The pulp may be given a preliminary treatment with alkali and washed, before the chlorination, also it may be bleached between treatment in the two alkali baths or after the second alkali treatment, e.g. with hypochlorite, chlorite, chlorine dioxide or other bleaching agent. The two-stage alkali treatment gives a pulp which in sheet form is softer and more easily shredded than pulp which has been subjected only to the second stage alkali treatment. Suitable alkali baths contain 2-18 per cent caustic soda solution used at 0-100 DEG C. for the first bath and 1.5-6 per cent caustic soda solution used at 90 DEG C. or upwards for the second bath. Liquor which has been used in the first bath may be diluted and used in the second bath. In an example, sulphite pulp is first treated with 2 per cent of its weight of chlorine at 10 DEG C. for 10 minutes in an aqueous slurry, washed and drained, treated with a 6 per cent caustic soda solution at 20 DEG C. for 15 minutes, then diluted with water to reduce the caustic soda to 4 per cent, heated to 125 DEG C. in 20 minutes, kept at that temperature for a further 30 minutes and finally bleached and formed into a continuous sheet.