Patent GB-478943-A (GB)
Title: System of obtaining and treating cellulose and product thereof
IPC: D21C1/10
Filed: 1936-04-23
Abstract: In a process for preparing highly reactive and permeable wood cellulose for paper or for the manufacture of cellulose derivatives comprising impregnating pieces of wood of suitable size with a delignifying liquor under non-delignifying conditions and subjecting the wood to delignification, the wood is first brought into contact with an aqueous liquid heated to or above the boiling point of water to displace the air in the wood by steam, the steam is then condensed to draw liquid into the interstices of the wood which is then subjected to uniform delignification with a delignifying liquor.  The aqueous liquid may be the delignifying solution itself, or particularly in the case of an alkaline cooking process, a non-delignifying solution of an electrolyte of specific concentration, e.g. sodium chloride of 10<-2> molar strength or aluminium sulphate of 10<-5> molar, which permits subsequent uniform dispersion of delignifying reagent throughout the chips.  In a modification employing longer chips, the chips are first submerged and boiled in water for about 30 minutes, the water, maintained at a vigorous boil being then displaced from the bottom of the digester by means of super-heated steam added above the liquor level. Condensation of the steam by addition of treating liquor at a temperature below 100 DEG  C. then creates a vacuum which draws the delignifying liquor into the chips. Boiling with the aqueous liquor is conducted under freely vented conditions; ebullition throughout the mass being obtained by closing the digester, heating to above the boiling point of the liquor at the bottom of the digester, and releasing pressure through a vent.  Condensation of the steam is thereupon effected by closing the digester and, while keeping the chips submerged, raising the pressure or lowering the temperature. In a modification, the chips may be impregnated with ammonium or other sulphite solution and SO2 subsequently added.  The process of the invention permits reduction of cooking time, temperature, concentration of reagent and/or ratio of reagent to wood.  All these factors may be varied to suit the material being treated and to obtain the desired product.  In an example, freshly cut spruce wood is debarked, washed, cut to chips \ba1/4\be" long, or into flakes 1" long and ,02--,03" thick, and charged into a rotary, chrome-nickel steel digester.  The temperature is raised to about 60 DEG  C. with steam, pressure due to expanded air being released.  Ammonium bisulphite cooking liquor heated to 90 DEG  C. in a separate accumulator is added and the whole heated to 90 DEG  C. for 1 hour, and then raised during 2 hours to 122 DEG  C.  Short pressure reliefs are made to the atmosphere at 103--106 DEG  C. to remove air. These reliefs cause boiling of the entire contents of the digester which may cause a lowering of the temperature.  Increase of pressure on closing of the relief valve causes condensation of the steam within the chips.  The temperature is maintained at 122 DEG  C. for 4 hours and then raised to 135 DEG  C. during 1\ba1/2\be hours and maintained there for 2\ba1/4\be hours.  Reliefs may be practised during the step from 122 to 135 DEG  C. to prevent excessive pressure. Reliefs during the cooking process are practised through the accumulator tank which is at lower pressure than the digester and has a valve set at a desired pressure, e.g. 40 pounds.  Release of pressure through the accumulator after cooling permits recovery of SO2.  Used treating liquor is also drawn off and conveyed to a liquor recovery system. Water at 100--150 DEG  C. is injected into the digester which is rotated.  The washing operation is repeated, air being excluded.  The pulp is then discharged to a dump pit and violently agitated to break down fibre bundles.  It is then treated with chlorine water at 20 DEG C., washed, boiled with 1 per cent NaOH, again washed, bleached with hypochlorite at 20--25 DEG  C., again boiled with alkali, and again bleached.  It is then treated with 13--18 per cent NaOH at 20--25 DEG C., drained, washed and again mercerized.  Finally, it is classified through an Alleghany metal screen against which the pulp in a rubber lined tank is thrown with a pulsating action by an agitator.  The long fibred fraction of the pulp is treated with dilute hydrochloric acid at 20--23 DEG  C., washed at 60--70 DEG  C. in purified water and again at 20 DEG  C. and then dewatered to the desired moisture content.  All these operations are carried out with exclusion of air and with at least 50 per cent moisture in the pulp.  In a second example, flakes or chips of yellow birch, maple, aspen, tulip or poplar are filled into a digester, covered and boiled at atmospheric pressure with 10<-5> M. aluminium sulphate solution.  The liquid is then cooled to 97--98 DEG  C. and allowed to stand for one hour, drained and caustic soda added to the pulp at 70 DEG  C.  The alkaline liquor is circulated by drawing off at the bottom and spraying in at the top and steam at 200 DEG  C. is added. Relief of pressure during initial steaming allows air above the liquid to escape.  The temperature is maintained at 96--98 DEG  C. for 1 hour, and then raised to 170 DEG  C. during 1\ba1/2\be--2 hours, and held there for 1 hour.  The pulp is then discharged to a blow pit, being directed at a steel target to break it down.  Waste black liquor from a previous digestion aids the discharge.  The pulp is vigorously agitated in water, and then chlorinated, treated with lime, washed and treated with bleaching powder, again washed and again bleached. Preferably, when pulping mixed woods, woods with similar absorptive capacities are employed, and the differing absorptive capacities of heartwood and sapwood from the same species are also taken into account.  The Specification as open to inspection under Sect. 91 comprises also matter relating to selection of wood for desirable pulping properties to yield cellulose for making cellulose derivatives, to the desirability of pure water supply for treating pulp, to the material of the vessels employed for various treatments and to the subsequent conversion of the purified cellulose into cellulose acetate, this conversion involving replacement of the water content of the pulp with an organic swelling agent, e.g. acetic acid, without drying of the pulp.  Apparatus for effecting this dehydration is described either as a continuous or a discontinuous operation, the material being treated with the swelling agent either as a cake or as a sheet between two screen wire belts passing over suction boxes and through drying chambers to remove some of the moisture.  The preparation and ripening of the cellulose acetate are also described. An example is also given of a treatment of a commercial wood pulp to bring it to suitable condition for esterification with repeated mercerizing and washing treatments followed by an acid treatment. Definitions are given of permeability, reactivity, viscosity measurements, and colour and haze measurements.  This subject-matter does not appear in the Specification as accepted.  Most of it is to be found in Specification 478,944 which is referred to.