Patent GB-928024-A (GB)
Title: Improvements in pulp refining
IPC: D21C9/08
Filed: 1961-10-23
Abstract: In the cold-caustic refining of wood pulp to produce high alpha cellulose, the pulp is slurried with dilute cold caustic solution of less than 3% concentration at a consistency of less than 5%, concentrated to a consistency of 20-35%, the residual caustic solution of the pulp displaced with 3-6% caustic soda solution, the pulp concentrated to a consistency of 20-35%, the residual caustic solution displaced with a caustic solution not exceeding 10% concentration, this caustic solution removed to a consistency of from 20-35%, the pulp slurried in a caustic solution not exceeding 10% concentration at a consistency of 2-5%, and the caustic solution then displaced through a series of displacement extractions each of which is followed by displacement with a caustic solution of lower concentration, and then washing the pulp with pure water, thereby removing impurities without reprecipitation of the same on the pulp fibres and without clumping or "fish eye" formation resulting in a highly purified alpha cellulose.  Desirably, refining is carried out at 5 DEG -40 DEG C., especially at 15 DEG -30 DEG C., the consistency increased in each stage by mechanical extraction, and the pulp slurried in the caustic solution not exceeding 10% concentration for from 5 to 30 minutes. Residual 10% caustic solution may be removed from the 2-5% consistency pulp by displacement with a solution of about 6% caustic, and the 6% caustic solution may be removed by displacement with a solution of about 3% caustic.  Extraction in each stage may be effected by compressing the pulp to a consistency of up to about 33%.  The apparatus of Fig. 1 comprises two segmented vacuum washer units P and P1 each of which has a screened cylinder C rotatable over a fixed segmented structure S or S1 comprising a vacuum pickup section and vacuum sections 1, 2, 3, 4, 5, the whole providing a solubilizing stage, an extraction stage and a flushing and washing stage.  Each press roll unit is partially immersed in a tank T in which pulp is slurried, and cylinder rollers R apply pressure on the pulp picked up on cylinder C.  The initial pulp is slurried in solution A (1-3% NaOH) and a layer of this pulp is picked up on the screen over the vacuum pickup section.  As screen C <PICT:0928024/IV(a)/1> <PICT:0928024/IV(a)/2> rotates clockwise, weak caustic solution is sucked out of the pulp in the vacuum pickup section and in section 1.  In section 2 pulp is compressed by roller R to 20-35% consistency and solution A removed, and pulp is then sprayed with solution B (3-6%NaOH).  In sections 3, 4 and 5 pulp is alternately compressed and treated with solutions C (6-9% NaOH), and D (9-10% NaOH), compressed and the pulp sent to slurry vessel V for slurrying with solution E (10% NaOH).  The resulting slurry passes to tank T of unit P1 and, as shown, is picked up and alternately compressed and treated with solutions G (9-6% NaOH), H (6-3% NaOH), and with water, and is finally removed.  The apparatus of Fig. 2 similarly provides a solubilizing, an extraction stage, and a flushing and washing stage.  In the solubilizing and flushing and washing stages it comprises multi-stage continuous centrifugal extractors E and E1 each of which has concentric perforated liners or baskets 10, 11, 12 and 13 and scraper rims 14, 15 and 16 for engaging the liner above to push the pulp upwards in stages and a top rim 17 on liner 13.  Liner 10 is in the form of a vessel and has a ram in the bottom which periodically pushes the pulp upward a small increment toward the next stage 11.  The ram is also connected with means not shown for moving liners 11 and 12 first downward and then upward to scrape off and push upward the pulp from one liner to the next one above.  Pulp slurry passes into the extractor via duct 18. Rotation of the unit removes a major portion of the solution A through the perforations of the liner, while movement of the ram carries the pulp upwardly in stages to be sprayed by nozzles 20, 21, 22 with the solutions B, C, D.  Finally pulp passes over the top edge of lip 17 to the vessel V and hence to the extraction and flushing and washing stages effected in vessel E1.