GB736300A - Improvements in the preparation of semi-chemical pulp - Google Patents

Abstract

Semi-chemical pulp is obtained by treating a cellulose-containing material at 100-152 DEG C. with a bisulphite cooking liquor containing 1.3-3.0 per cent of sulphur dioxide and a base, the pH at the start of the reaction being 3.0 to 6.2, for a time sufficient to remove only a part of the lignin binding the cellulose fibres and to attack the cellulose and hemi-cellulose only slightly, whereafter the bundles of cellulose fibres are mechanically subdivided. The base may be an oxide or hydroxide of calcium, magnesium, sodium or aluminium or ammonium hydroxide. The cooking liquor may be buffered by the addition of bases such as sodium, potassium, calcium, magnesium, aluminium or ammonium hydroxides, or organic amines, or buffers such as sodium carbonate or bicarbonate may be added. The possible precipitation of calcium sulphite may be prevented by adding complex-forming agents such as metaphosphates, imino-diacetic acid, nitrilo-triacetic acid, ethylene diamine tetra-acetic acid, aliphatic amines, polyphosphates, or sodium pyrophosphate. In an example, birch wood chips are charged to a rotary digester with cooking liquor at a pH of 4.3 containing 1.57 per cent of sulphur dioxide, 46 per cent of the sulphur dioxide being bound to lime and 27 per cent to ammonia, and containing also 50 mg./litre of sodium hexametaphosphate. The temperature is raised to 105 DEG C. in 1 hour, maintained at this temperature for 6 hours and kept for a further 4 hours at 115 DEG C. and then for 4 hours at 125 DEG C. The semi-chemical pulp is then defibrated in known manner in a disc mill. Other examples are given.