GB615604A - Improvements in and relating to the bleaching of cellulosic matter - Google Patents

Abstract

Cellulosic materials are bleached by means of aqueous hypobromite solutions which are kept at a pH value between 7 and 8.2. The hypobromite bleaching may be preceded or followed by a hypochlorite bleaching stage, and in the hypochlorite stage a bromide may be present as described in Specification 596,192 or 596,193. In examples: (1) sulphite pulp is chlorinated, washed with aqueous sodium hydroxide and then treated with a solution of potassium bromide and a solution of bleaching powder added slowly so that at no time does the mixture produce a blue colour on potassium iodide-starch paper, the pH being kept meanwhile at 7.5-the pulp is then successively treated with water, aqueous sodium hydroxide, water, hypochlorite, and sulphur dioxide; (2) kraft pulp is treated with bleaching powder, washed, treated with sodium hydroxide solution, washed, treated in two stages with bromide of lime or a mixture of potassium bromide and an equivalent amount of hypochlorite at a pH of 7.5, and finished by bleaching with bleaching powder, washing and after-treating with sulphur dioxide; (3) cotton piece goods are boiled with sodium hydroxide solution, washed, bleached with hypobromite at a pH of 7.5, washed, boiled with sodium hydroxide solution, washed and bleached in a second stage with hypochlorite; and (4) the process of (3) is repeated except that an amount of potassium bromide is added to the hypochlorite used in the second stage in an amount insufficient to produce a pure hypobromite liquor, and the liquor is kept at pH 7 to 7.5.