GB1095279A - Bleaching pulps - Google Patents

Abstract

Low-cellulose wood pulp, i.e. wood pulp containing not more than 60% by weight of cellulose the remaining material being p non-cellulose wood constituents, is bleached by treating it at a temperature of 90 DEG - 300 DEG F for up to 4 hours in an aqueous pulp slurry having a pH of 5.5-8 and containing 1-20% by weight of pulp and 0.1-2% by weight of thiourea dioxide based on the weight of the dry pulp, the thiourea dioxide being used either as the acid or an alkali metal salt thereof, e.g. the sodium and potassium salts. The aqueous pulp slurry may also contain sodium bisulphite in a weight ratio to the thiourea dioxide of up to 20 to 1 and/or up to 1% on the dry weight of the pulp of a polyphosphate, e.g. sodium tripolyphosphate, tetrasodium pyrophosphate and sodium hexametaphosphate. The pH may be adjusted by the addition of sodium or potassium hydroxide or carbonate. The pulp may be a mechanical pulp, e.g. a groundwood pulp produced by grinding wood logs on grinding stones or a refiner pulp produced by passing wood chips through a disc or other refiner to mechanically subdivide the wood into fibres, or a chemi-mechanical pulp produced by softening the wood prior to mechanical subdivision of the fibres by treating it with aqueous sulphite, bisulphite or other softening agent. The pulp may be produced from spruce, fir, hemlock, pine, cottonwood, aspen or poplar. The aqueous pulp slurry may be heated by external heating means or by passing steam through the pulp slurry.