Patent GB-318868-A (GB)
Title: Improvements in or relating to the treatment of cellulose
IPC: D21C9/08
Filed: 1928-03-08
Abstract: 318,868. Thiriet, A. March 8, 1928. Pulp, preparation of.-Wood cellulose prepared by the soda or bisulphite processes is purified to give a product containing 97 to 98 per cent of alpha cellulose by a process which comprises a physical purification of the cellulose pulp as it flows down an inclined gauze by means of jets of water under pressure, a washing treatment of the pulp by suspending it in water free from alkaline earth salts preceded if desired by a treatment with dilute sulphuric or hydrochloric acids, and then a chemical treatment with alkalis, characterized in that the liquids used in this treatment are either free from alkaline earth salts or have only a very low content thereof, for example, less than 0.001 per cent and that the cellulose pulp comes into contact with alkali liquors which are the purer the more the treatment progresses. The purified cellulose is then washed with water free from alkaline earth salts so that the alkali is recovered. Soda cellulose may be subjected to a thorough oxidation, for example with a solution of calcium hypochlorite after the physical purification but prior to the treatment with alkalis. The chemically purified cellulose after the alkali treatment may be bleached for example with calcium hypochlorite and may also be treated with a dilute acid, for example 0.1 per cent sulphuric acid. The alkali solution employed for treating soda cellulose is preferably cold concentrated sodium hydroxide lye of about 10 per cent strength and that for treating sulphite cellulose hot dilute sodium hydroxide lye of about 4 per cent strength preferably at 70-80 C. In the apparatus shown, alkaline earth salts are removed from the water employed by heating the water, after the addition of about 0.2 per cent of caustic soda calculated on the weight of liquid, in vats 20 heated by tubular coils 21 in which steam is circulated from pipe 30. The vat 23 receives purified water from the vats 20 through pipes 22 and the water is delivered by the apparatus 24 to the alkali treatment vats 1 through pipes 14. The caustic soda solution is prepared in vat 25, which receives purified water through a pipe 22 and the alkali solution is delivered through the apparatus 26, mechanically connected to the apparatus 24, and the pipes 15 into the treatment vats 1 in amounts proportioned to the weight of pulp to be treated. Each of the apparatus employed in the alkali treatment comprises a cylindrical vat 1 and a pump 6 for circulating the alkali lyes. The pulp is charged into the vats, in which it remains stationary, through the pipe 16 and is discharged through the plug 5. Each vat is provided with a strainer 4 which only allows the treatment liquid to pass through. The pump 6 draws the liquid into the pipe 2 fitted with a draining valve 3 at a point 7 below which the level of the liquid in the vat cannot fall and delivers the liquid through a heater 8 to a device 9 from which the liquid can be discharged through pipe 10 for recovery if it is spent or into pipe 11 leading to the vat from which the liquid was withdrawn, thus forming a closed circuit, or into the pipe 12 leading to the next vat. The pipe 12 has an overflow 13 by which lye can be returned to the vat from which it was withdrawn. After the alkali treatment, the pump is washed with water free from alkaline earth salts by the same displacement method. The apparatus employed for the bleaching process and subsequent acid treatments comprises high-walled vats of a capacity slightly greater than that of the vats employed for the alkali treatment and lined or made of a material capable of withstanding the action of the bleaching solution or the acid. Each of the vats has a circulating pump for circulating the bleaching or acid liquid in closed circuit. The treatment with dilute acids and the final washing with ordinary water is conducted in the same vats.