Patent GB-478944-A (GB)
Title: System of obtaining and treating cellulose and product thereof
IPC: D21C9/08
Filed: 1936-04-23
Abstract: <PICT:0478944/IV/1> A process for the preparation of highly purified reactive and permeable wood cellulose suitable for the production of paper or of cellulose derivatives, comprises producing or selecting a raw pulp obtained from wood by a relatively mild delignifying digestion process which causes no substantial degradation of the cellulose, chlorinating the pulp with no more than the amount of chlorine that will promptly react with the constituents of the pulp, removal of the chlorination products, bleaching with a hypochlorite solution and thereafter treating with an alkali solution of not less than 4 per cent strength under non-mercerizing conditions.  Wood, e.g. spruce, may be sub-divided, preimpregnated and digested with ammonium bisulphite liquor as described in Specification 478,943. The washed pulp so obtained is defibred, its consistency adjusted to 3--10 per cent and chlorine equal to 16 per cent of the bleachability added.  For pulps with cuprammonium viscosities of less than 25 centipoises, more chlorine may be added.  The chlorine treatment is continued for 45 minutes to allow acid formed to act upon the ash content of the cellulose.  Sodium hydroxide of not more than 7 per cent is then added, to bring the concentration in the slurry to 1 per cent, and the slurry heated to 100 DEG  C. or just sufficient caustic soda is added to neutralize the pulp.  It is then filtered, washed and neutralized if necessary.  Calcium hypochlorite is added and the pulp treated at 20--25 DEG  C.  Variation of the temperature of this treatment enables the cuprammonium viscosity of the pulp to be regulated.  The pulp is then washed, its consistency adjusted to 4--12 per cent and treated with 7 per cent caustic soda at 100 DEG C., presence of oxygen being avoided as far as possible.  It is then washed until neutral.  The pulp may then be mercerized with 10--18 per cent sodium hydroxide at 20--25 DEG  C. and 4--12 per cent consistency.  This mercerization treatment may replace the final alkali treatment and follow immediately after bleaching.  The pulp is finally classified to remove fines in a rubber-lined tank with Alleghany metal agitators and an 80 mesh Alleghany metal screen cover, against which the pulp is thrown by the agitation.  The wet cellulose is dehydrated in a tank 20, with detachable bottom 21, and a perforated Alleghany metal plate 22 covered with a 50 mesh Alleghany metal screen 23, water being sucked from the pulp to form a cake to which pressure is applied by a plunger 26.  A second cake may be formed on top of this first one.  When no more water can be drawn off, glacial acetic acid or some other suitable liquid employed in the manufacture of the cellulose derivative is poured on top of the cake and allowed to soak into the pulp. Partial vacuum together if desired with pressure may be applied to pass the acid through the cake, displacing the water.  By this means the pulp is maintained in wet swollen condition throughout the whole series of operations.  The acid discharged from the cake is preserved for re-use and/or regeneration.  The acid saturated cake is defibred, exposure to air being avoided, and then discharged into an acetylator and treated with glacial acetic acid and concentrated sulphuric acid.  Not more than 2,2 parts of acetic anhydride are added to 1 part of cellulose, and the pulp is acetylated.  At the desired viscosity, the acetylation is terminated by adding acetic acid of 50 per cent strength containing sulphuric acid, and the cellulose acetate is hydrolized to the desired acetic acid content.  Hydrolysis is terminated by addition of sodium acetate in acetic acid.  The solution of cellulose acetate is filtered, precipitated with water or dilute acetic acid, drained and dried.  The whole acetylation treatment only takes about 2\ba1/2\be hours, of which \ba1/2\be hour is taken for impregnation with acetic acid and catalyst.  The acetate yields brilliant acetone solutions and is therefore eminently suitable for photographic films, lacquers and non-shatterable glass. In a modification described in example B, yellow birch, maple, aspen, tulip or poplar is pre-impregnated and digested with caustic soda as described in Specification 478,943, and the pulp subjected to a multi-stage bleach comprising a chlorination washing, with addition of lime and one or two treatments with hypochlorite.  In this case, the final treatment with alkali may be omitted.  Definitions are given in the Specification of a -cellulose, soda soluble material and bleachability and the Specification also gives typical analyses of pulp produced according to the invention, which may contain less than 4 per cent soda soluble material over 94 or even over 98 per cent a -cellulose.  Specification 410,005 also is referred to.  The Specification as open to inspection under Sect. 91 includes descriptions of the properties of different types of wood and relates to the selection of wood suitable for the process of the invention. Experiments are described to illustrate the difficulties of removing all the air from the interstices of wood chips, and the Specification contains descriptions relating to desirable properties of a cellulose pulp, terms such as permeability, reactivity, fibre saturation point, colour and haze of acetate solutions are defined.  A modified dehydration procedure is described in which the pulp is formed into a sheet through which glacial acetic acid or other liquid used in the manufacture of cellulose derivatives is passed to displace the water. An example describes the reconditioning of a typical prior art commercial grade of rayon pulp to permit its esterification. The dried pulp laps are shredded, mixed into a slurry with water and agitated.  The pulp is then immersed and agitated in 10--18 per cent NaOH at 20--25 DEG  C.  The caustic soda is removed by draining or centrifuging, and the pulp washed at 18--22 DEG  C. to bring its pH to 7,0--10,0.  These treatments are repeated and the pulp then treated with dilute hydrochloric or acetic acid, washed, first with cold, and then with hot water, and then dewatered to 50 per cent moisture content and dehydrated and acetylated.  This subject-matter does not appear in the Specification as accepted.