GB1095280A - Bleaching of wood pulps - Google Patents

Abstract

Wood pulps containing not more than 60% by weight of cellulose are bleached in an aqueous slurry containing 1-20% by weight based on the dry weight of the pulp, a bleaching amount of at least one reductive bleaching agent and 0.05 - 1% by weight of thiourea based on the dry weight of the pulp, under bleaching conditions at a pH of 2-8.5 and a temperature of 90 DEG - 300 DEG F for up to 4 hours. The pulp may be a groundwood pulp or pulp obtained by chemimechanical methods involving softening the wood with aqueous softening agents, e.g. sulphites and bisulphites, prior to mechanical subdivision of the wood into fibres and may be produced from spruce, fir, hemlock, pine, cottonwood, aspen or poplar. The reductive bleaching agent which may be present in an amount of 0.01 - 2% based on the dry pulp weight may be sodium or zinc hydrosulphite, thiourea dioxide or a mixture of an alkali metal borohydride and bisulphite ion. The thiourea may be added to the slurry prior to or simultaneously with the addition of the bleaching agents. The pH may be adjusted by the addition of sodium or potassium carbonate or hydroxide and the slurry may also contain sodium tripolyphospate, tetrasodium pyrophosphate or sodium hexametaphosphates or the corresponding potassium polyphosphates, diethylene triamine pentacetic acid or ethylene diamine tetraacetic acid.