Patent GB-625961-A (GB)
Title: Method of purifying cellulose
IPC: C08B1/08
Filed: 1947-03-28
Abstract: <PICT:0625961/IV (a)/1> Cellulosic material is purified or converted to alkali-cellulose by mixing it with caustic soda solution so as to form a suspension or slurry and subsequently separating the solution from the material by compression, segregating the solution expressed in the first part of the compressing operation from that expressed in the remainder of the operation, and employing all or part of the solution first expressed for the treatment of further cellulosic material.  The purified cellulose or alkali-cellulose may be used for making viscose or other cellulose derivatives. The cellulosic material may be sulphite pulp and it may be subjected to a multi-stage treatment with caustic soda solution, the material being subjected after each stage to compression to remove the solution. Fresh caustic soda solution may be employed in the last stage only, solution expressed from one stage being used for the treatment in the next preceding stage.  After any stage, the expressed solution may be collected in two portions, the first of which is used for treatment in the next preceding stage while the second portion, being more contaminated, is passed to a dialyser or other purifying apparatus. The caustic soda treatment may be followed or preceded by one or more bleaching treatments.  The concentration of the caustic soda solution used in the last stage may be about 11 per cent when cellulose suitable for acetylation is required, and about 16 per cent for alkali-cellulose.  The amount of water introduced with the pulp may be such that the concentration of caustic soda in the first stage is reduced to 9 per cent.  As shown in Fig. 1, sulphite pulp in the form of a slurry is received in tank 10 and then pumped into chlorination tower 12.  It then passes to a washer and thickener 16.  The pulp is fed to plunger press 20 wherein most of its water content is removed. The press may comprise an inner cylinder formed by numerous spaced rings through which the pulp is forced by a plunger.  The pulp becomes compacted and the liquid p  passes to a discharge port 21. The thickened pulp is mixed with caustic soda solution in the mixer 22, and the suspension is fed to plunger press 24.  The liquid expressed during the first part of the pressing passes through pipe 25 to tank 26 and a portion of it is returned by pipe 29 to tank 23 and mixer 22.  The remainder of expressed liquid passes through pipe 27 to a dialyser.  The compacted pulp is fed to a second mixer 22a, where it is treated with caustic soda from tank 23a.  The liquid expressed in press 24a is collected in tank 26a and returned by pipe 29 to tank 23. The pulp is passed in succession to further similar treatment stages, expressed liquid from one stage being passed back for use at the next preceding stage.  Tank 23d is supplied with fresh caustic soda solution and with solution which has been purified by the dialysers.  The pressed pulp from press 24d passes to vacuum washers and thickeners 33, and thence to a mixer 36 supplied with bleach liquor.  The pulp passes into a bleaching cell and is then conveyed to washer and thickener 41 and drying chest 42.  For producing bleached alkali-cellulose, the apparatus may be modified by arranging one or more bleaching installations between the chlorinating tower 12 and the alkali-treatment sections. Specification 625,962 is referred to.