GB671962A - Process of bleaching wood pulp - Google Patents

Abstract

Moist felted wood pulp containing substantial amounts of ligneous and other non-cellulosic materials and having a consistency of not less than 25 (preferably 35-50) per cent solids is bleached by impregnating it with an alkaline peroxide solution as the sole chemical addition to effect self-controlled bleaching of the pulp, said alkaline peroxide solution having pH of 9.5-11.5 and a total alkalinity of 0.35-1.1 per cent by weight sodium oxide equivalent based on bone dry pulp and comprising a peroxide (preferably hydrogen peroxide but including per-salts which yield hydrogen peroxide in aqueous solution, e.g. sodium perborate) and an alkaline salt including as an essential ingredient sodium silicate, trisodium phosphate, sodium pyrophosphate or sodium tetraphosphate, accumulating the impregnated pulp into a mass of substantial volume and relatively small exposed urea to inhibit drying out, allowing said mass to remain for from 5 hours to several weeks, during the initial period of which time the maximum brightness of the pulp is achieved and the pH automatically falls to at most 8, and omitting any further chemical additions to the bleached pulp, which retains substantially its maximum brightness for at least several weeks. The specified alkaline salts may be used alone or in combination with one another or with other alkaline materials (e.g. caustic soda, soda ash or borax). The process is applicable to the treatment of ground wood, pretreated ground wood or semi-chemical pulps or mixtures containing them, in the form of lap, web or crumbs. The initial pH of the alkaline peroxide solution is preferably 10.5-10.8 in the case of ground wood or pretreated ground wood pulp, and 10.8-11.2 in the case of semi-chemical pulp, and the solution is preferably applied at a temperature of 15-30 DEG C. Examples describe the bleaching of ground wood pulps from spruce and balsam and from poplar, a neutral sulphite pretreated birch ground wood pulp and an aspen semi-chemical pulp. Bactericides and fungicides may be added to the pulp or the bleaching solution, and the process may form the last stage of a multi-stage bleaching process.