GB1030416A - Improvements in the bleaching of cellulose pulp - Google Patents

Abstract

Cellulosic pulp, e.g. pulp obtained from wood, straw or bagasse, is bleached by a continuous process comprising treating it with an excess of a bleaching solution, subjecting the pulp-bleach mixture obtained to bleaching conditions, recovering the bleaching solution from the mixture when the pulp has been bleached to only 50-95% of the maximum brightness obtainable under the bleaching conditions or when the rate of consumption of the bleaching agent decreases or the rate of increase of the brightness in the pulp decreases, in an amount increasing the density of the remaining bleached pulp to at least three times the density of the pulp-bleach mixture and recycling 50-100% by weight of the recovered solution for the bleaching of fresh pulp, the ratio of recycled to fresh bleaching solution, the ratio of the densities prior to and after addition of the bleaching solution and the ratio of the density prior to addition of the bleaching solution to the increased density being maintained constant during the process. The bleaching agent may be an aqueous alkaline solution saturated with gaseous oxygen, liquid chlorine, aqueous solutions of chlorine, sodium hypochlorite, calcium hypochlorite, sodium peroxide, hydrogen peroxide, chlorine dioxide, calcium bisulphite, sodium bisulphite, sodium hydrosulphite or zinc hydrosulphite. Bleaching adjuvants, e.g. sodium hydroxide, sodium silicate polyphosphates and ethylene tetraacetic may be added to the bleaching solution. The density of the pulp-bleach mixture may be increased by using band pressure filters, suction drum filters, double screen presses, screw presses, roller presses or centrifuges. The ratio of the density prior to and after the addition of the bleaching agent may be at least 3:1.