GB468306A - Improvements in or relating to the refining of wood pulp - Google Patents

Abstract

Wood pulp is refined and undesired colouring matter removed therefrom by subjecting it, in high density, to the action of a free alkali and an agent substantially reducing the surface tension of the alkali solution, at an elevated temperature which is maintained throughout the treatment. The pulp may be impregnated at high density with the reagents and heated to the required temperature, e.g. by direct or indirect steam heating, or the pulp may be impregnated at low density, concentrated and then subjected to the heat treatment for \ba1/2\be--2 hours. Either method may be continuous or cyclic, the impregnating liquor being recovered, strengthened and re-used. It is advisable to add to the used liquor only so much caustic soda as is carried away by the pulp. Suitable surface tension reducing agents are oleic acid, linoleic acid, sulphonated oils, sulphonated alcohols and derivatives of these compounds. The substance should dissolve in dilute alkaline solution at ordinary or slightly elevated temperatures to yield a clear dispersion. After the heat treatment the pulp is washed with water having only a small content of lime and magnesium and containing if desired up to 0,5 per cent NaOH calculated on the fibre weight. With some emulsifying agents which form difficulty soluble calcium salts, the pulp must have a low content of lime. In examples: (1) unbleached sulphite pulp containing 1,4 per cent colouring matter was concentrated to a density of 20 per cent and treated in a mixer with steam and the refining agents. The pulp leaving the mixer contained 0,5 per cent turkey red oil, and 1 per cent NaOH and had a density of 16 per cent and a temperature of 55 DEG C. The temperature was maintained for 45 minutes. The pulp was then washed and subjected to a conventional two stage bleaching, the second stage at a pH higher than 8. After washing and slight acidification the pulp contained less than 0,25 per cent of extractable matter; (2) soft sulphite pulp partly bleached with hypochlorite and containing less than 0,4 per cent CaO and 0,9 per cent extractable matter was impregnated at a density of 1,5 per cent for 15 minutes with a liquor containing sodium lauryl sulphate and caustic soda. The pulp after conversion into laps of 35 per cent density contained 0,25 per cent sodium lauryl sulphate and 0,75 per cent NaOH. The laps were kept at 70 DEG C. for 30 minutes, washed, and bleached at pH greater than 8,5. After washing with slight acidification, the pulp contained less than 0,15 per cent extractable matter; (3) soft partially bleached wood pulp prepared from unbleached pulp by chlorination in presence of sodium chloride was impregnated at 1 per cent density for 15 minutes at normal temperature with liquor containing sodium linoleate and caustic soda. After concentration to 30 per cent the pulp contained 0,3 per cent sodium linoleate and 1,5 per cent NaOH, and was heated to 65 DEG C. for 90 minutes, washed, bleached at a pH above 8,5, washed, slightly acidified and rewashed. It contained 90 per cent a cellulose and less than 0,05 per cent extractable matter; (4) chlorinated pulp was concentrated to 27 per cent density and mixed with two solutions and steam until it contained 0,4 per cent sodium oleate and 7 per cent NaOH at a density of 22 per cent and a temperature of 98 DEG C. After 5 hours at this temperature it was washed and bleached at a pH above 9, washed and slightly acidified. Pulp produced by the process of the invention is suitable for cellulose derivatives, as a filler in the plastics industry or for paper of high colour stability. Specification 441,263, [Group XXIII], is referred to.