GB681661A - Treatment of chemical pulp - Google Patents

Abstract

Sulphite pulp is bleached by adding thereto alkaline peroxide solution until the fibre density is 20-65 per cent, the peroxide content (calculated as H2O2) is 0.3-1.75 per cent and the alkali content (calculated as NaOH) is 0.75-3.25 per cent, based on dry pulp; the fibres are maintained damp at a density above 20 per cent and a temperature below 130 DEG F. until maximum brightness is obtained, e.g. 1-20 days. Typical concentrations are H2O2 0.75-1.25 per cent and NaOH 1.0-2.5 per cent, when the time needed is 2-14 days. The peroxide solution is applied to pulp crumbs or to a self-supporting lap or web by normal methods, e.g. spraying or padding. The fibres are stacked or piled in close contact during bleaching to prevent drying out. Examples are given of the process and results are compared with those obtained by prior-art methods. Graphs are given of the cuprammonium viscosity of the product when different alkali concentrations and temperatures are used in bleaching. The peroxide solution may be obtained from sodium peroxide, hydrogen peroxide or other inorganic compound which generates hydrogen peroxide (e.g. a perborate) by adding acid or alkali as necessary. A stabilizer is preferably added, e.g. sodium silicate, alkali phosphate or pyrophosphate or a soluble magnesium salt. This will contribute to the total alkali concentration of the solution. The process is applicable to soft, medium and hard cooked pulps, from soft or hard woods, whether cooked with calcium, magnesium, ammonium or mixed base liquors.