GB596121A - Improvements in or relating to purification of cellulosic materials - Google Patents

Abstract

Material of the character of cotton linters containing non-cellulosic contaminants is suspended in water; the pH of the suspension is adjusted to 3.5 to 6.8, e.g. by addition of any acid (usually mineral) of dissociation constant greater than 3.2 X 10-2; and the material is cooked by steam under pressure; the treated material is then digested under pressure with an alkali digestion liquor followed if desired by an oxidising bleach. The steam pressure for the pre-cooking of the suspension may be from about 75 to 135 lbs. per square inch and during the alkali digestion about 30 lbs. per square inch. Owing to the variable amounts of inorganic ash in the raw linters, the addition of acid must be made after the linters are in suspension, and it follows that the amount of acid will vary, though usually it is of the order of a few thousandths of one per cent. Should the pH of the suspension be below the desired value, alkali such as caustic soda may be added to raise it above the desired value, and then acid added to adjust it to the desired value. Small amounts of wetting agents such as turkey red oil, or sodium lauryl sulphate may be added to the material in the pre-cook treatment. In addition, about 0.01 to 0.16 per cent of such salts as ammonium nitrate may also be added. In an example, the raw linters are suspended in ten times their weight of water, and the pH of the slurry adjusted to 4.7 by the addition of nitric acid. Steam at 100 lbs. per square inch pressure is passed in for one hour (corresponding to a temperature of about 340 DEG F.) and the linters washed. They are then purified by digestion with 2.5 per cent caustic soda solution under steam pressure of 30 lbs. per square inch (about 275 DEG F.) followed by multistage bleaching treatment with dilute aqueous solutions of available chlorine. The purified linters may be acetylated or treated in any other manner according to known methods.