GB625962A - Process and apparatus for the treatment of cellulose - Google Patents

Abstract

<PICT:0625962/IV (a)/1> <PICT:0625962/IV (a)/2> Cellulose or alkali-cellulose of high purity is obtained from sulphite pulp by a process in which the pulp is treated in three or more stages in caustic soda solution on the counter-current principle and fresh caustic soda solution is employed in the final stage. The pulp is treated as a suspension or slurry in the caustic soda solution and after each stage of treatment the suspension is pressed so as to remove the greater part of the solution from the pulp, this solution then being employed in the treatment of the pulp in the preceding stage or, after being used in the first stage, being removed from the system, and the pressed pulp, except after the last stage, being again formed into a suspension or slurry with caustic soda solution for the next treatment stage. The pulp may be agitated while in suspension during each stage of the caustic soda treatment. The pulp may be bleached and the wet bleached pulp may be fed without intermediate drying to the first stage of the caustic soda treatment. The fresh caustic soda introduced in the final stage may have a concentration of 14 1/2 per cent or more. After the final stage the cellulose may be allowed to retain as much caustic soda as will render it suitable for xanthation and it may then be shredded. In each stage of the caustic soda treatment, the pulp may be in the form of a suspension of concentration 6-10 per cent. The apparatus used comprises three or more treating units, each unit comprising a press wherein the wet pulp is thickened and compacted, a receiver for liquor removed from the pulp in the press, a treatment tank arranged to receive the thickened and compacted pulp from the press, means for introducing into the treatment tank a treating liquor from the receiver of the following unit or, in the case of the final unit, means for introducing fresh treating liquor, and means for removing the treating liquor and the cellulosic pulp together from the treating tank and feeding them to the press of the succeeding unit or to a final press. Sheets of dry unbleached sulphite pulp may be fed to a hydrapulper where they are formed into a slurry with water. The slurry is pumped through a chlorinating tower and then fed to a suction-drum washer and thickener. The pulp then passes to a mixer where it is treated with bleaching agents and pH control agents, and it then passes to a bleaching engine. The pulp passes on to a suction-drum washer, to further bleaching apparatus and then to another suction-drum washer. As shown in Fig. 1, the thickened pulp passes from washer 50, along conveyor 52 to a press comprising cylinder 55 and piston 56. The compacted pulp is discharged into a caustic soda treatment tank 60 and the expressed water passing through perforations in the piston is discharged into tank 61. The pulp is agitated and formed into a slurry with used caustic soda solution from the following treatment stage. From tank 60 the mixture is pumped to a screw press 67 or other thickener, and the pulp is discharged into press 71 and thence into the next treatment tank 74. Liquor from the press 84 is pumped along pipe 62 to tank 60. Similarly, the caustic soda solution from press 96, Fig. 2, is pumped along pipe 77 to tank 74. The solution expressed from the pulp in the press 111 is pumped along pipe 91 to tank 90. The pulp is discharged from press 111 into the final caustic soda treatment tank 115, which is supplied with fresh solution from tank 120. After this treatment, the slurry passes to press 126. The compacted pulp is discharged into a shredder 130 and the expressed liquid is pumped through pipe 105 to the treatment tank 100.