GB1041995A - Production of cellulosic pulp of increased brightness - Google Patents

Abstract

Cellulosic pulps having high brightness are produced by impregnating cellulosic fibres either in oriented or disoriented form with an aqueous solution containing bisulphite ion and having a pH of 2-7 to provide at least 0.04% by weight of the bisulphite ion on the fibres, contacting the bisulphite ion-impregnated fibres with 0.01-0.5% by weight of the fibres of an alkali metal borohydride and permitting the reagents to act on the fibres to increase their brightness. The bisulphite ion may be introduced as sodium, potassium, magnesium, calcium or ammonium bisulphite or as sulphur dioxide or sulphurous acid. The pH of the solution used in the first stage of the process may be adjusted to 5-7 by the addition of an alkali metal sulphite and the solution may also contain sodium carbonate, bicarbonate or hydroxide and magnesium hydroxide. The process is applicable to the pulping of pines, spruces, firs, hemlocks, beeches, maples, oaks and birches. Wood chips may be impregnated with a bisulphite-containing pulping liquor, drained and introduced into a fibre-separation device, e.g. a disc refined, hammer mill or rod mill operating at 140 DEG -210 DEG F. at which stage the borohydride is added, removed from the fibre-separating device and the water removed. Alternatively the borohydride may be added after the wood has been divided into fibres or the fibres may be impregnated with the bisulphite ion in a grinder and the borohydride subsequently added. The fibres may also be pulped without bisulphite and the bisulphite added to the preformed pulp followed by the addition of the borohydride. The process is also applicable to the treatment of pulps which have been bleached with hydrogen peroxide or hypochlorite. The pulp may subsequently be bleached with a hydrogen peroxide solution containing sodium silicate and sodium hydroxide.