GB601643A - Improvements in or relating to the bleaching of cellulose - Google Patents

Abstract

Cellulose fibres are bleached by subjecting them in aqueous suspension to a sequence of treatments comprising a chlorine treatment in acidic solution followed by a treatment with an alkaline solution in the absence of an oxidizing agent, the chlorine treatment being carried out in the presence of a dissolved nitrogen compound having at least one hydrogen atom attached directly to a nitrogen atom. The treatment with the alkaline solution may be followed by a second chlorine treatment in acidic solution and one or both of the chlorine treatments is or are carried out in the presence of the nitrogen compound. A suitable pH for the chlorine treatment is from 1.5 to 4.5. Nitrogen compounds specified are ammonia, aniline, toluidine, methylaniline, benzylamine, diphenylamine, dimethylamine, methylamine, ethylamine, ethanolamine, acetamide, formamide, butyric amide, urea, methyl urea, ethyl urea, diphenyl urea, diethyl urea, urethanes, thioureas, hydroxylamines, hydrazines, guanidines and chloramines. The amount used is ordinarily from 0.2 to 10 parts per million, bases on the weight of the solution, but the use of amounts equivalent to from 0.1 to 100 is referred to. The following examples are given: (1) Cotton linters which have been digested with dilute caustic soda are made into a 3 per cent slurry with water, and there are added 0.06 per cent of sulphuric acid and an amount of ammonium hydroxide equivalent to 4 parts per million calculated on the weight of the slurry. Calcium hypochlorite is then introduced at 86 DEG F. to bleach the linters, after which they are treated at 145 DEG F. with sodium hydroxide at a concentration of 0.015 per cent. The acid-hypochlorite-ammonia and alkaline steps are repeated, and finally a third acid-hypochlorite-ammonia step is given. (2) The first example is repeated, but the acid-hypochlorite-ammonia treatment is carried out at 56 DEG F. with various amounts of ammonia and the final treatment is with a solution containing 0.01 per cent of oxalic acid. (3) Caustic-digested unbleached cotton linters are made up into a 2.2 per cent slurry and there are added 0.05 per cent of sulphuric acid and 47 parts per million of ammonia, based on the weight of solution. Calcium hypochlorite is added to bleach the linters which are then washed and treated with 0.015 per cent of sodium hydroxide at 145 DEG F., and again treated with acid-hypochlorite-ammonia and washed. (4) Caustic-digested unbleached cotton linters are treated as in (3) except that the sequence of acid-hypochlorite-ammonia and alkali steps is repeated once. (5) Alkali-digested unbleached linters are made up into a 3.0 per cent slurry and treated at 43 DEG F. with 0.05 per cent of sulphuric acid, 4 parts per million of ammonia, and sufficient sodium hypochlorite to give an equivalent chlorine concentration of 0.02 per cent. They are then treated with 0.015 per cent of sodium hydroxide at 143 DEG F., washed, and finally treated with 0.02 per cent of oxalic acid, washed and dried.