GB734216A - Process for the treatment of ligno-cellulosic fibres - Google Patents

Abstract

Fibrous ligno-cellulosic materials such as wood chips, bagasse or straw are treated, prior to alkali digestion, with steam at atmospheric pressure and at a temperature of substantially 100 DEG C., the steam containing sulphur dioxide and the treatment being carried out under such conditions that the quantity of dilute sulphurous acid condensed on the materials is so small, in relation to the amount of the materials, that substantially no flow of such dilute sulphurous acid occurs over the surfaces of the particles of the materials. The apparatus may consist of an autoclave for discontinuous treatment or of a tall column for continuous treatment. The proportion of sulphur dioxide may be 1-3 per cent, calculated on the dry material treated. In an example 100 kg. of eucalyptus wood chips (dry weight) containing 15 per cent of water are charged into a pressure cooker provided with a heat-insulating jacket, the air therein is partially exhausted and 2.5 kg. of gaseous sulphur dioxide is injected together with sufficient steam to increase the temperature to 100 DEG C. The mass is maintained at 100 DEG C. for 2-3 hours, during which time the water content of the chips increases to 40-50 per cent but no flow of condensate occurs over the chips. The pressure cooker is exhausted to remove the residual free sulphur dioxide, and the lignosulphonic acid formed during the treatment is neutralized and dissolved by the addition of black liquor from a previous cycle of operations. The mass is heated to 100 DEG C., drained and covered with an aqueous solution of caustic soda (110 g. per litre). Part of the caustic soda may be replaced by sodium sulphide. The mass is heated to 160-170 DEG C. for 1.2 hours, drained and washed. The black liquor thereby obtained is used for the neutralization of the next batch of hydrolized cellulosic material. The treated pulp may be bleached, e.g. by two or three treatments with chlorine in an acid medium followed by hypochlorite in an alkaline medium. Examples are given of the similar treatment of bagasse and chopped straw. The sulphur dioxide may be generated in situ in the cooker, e.g. by charging the cooker with aqueous solutions of sodium bisulphite and sulphuric acid.