Patent GB-868703-A (GB)
Title: Improvements in or relating to the manufacture of paper pulp
IPC: D21C11/04
Filed: 1957-08-07
Abstract: <PICT:0868703/IV (a)/1> <PICT:0868703/IV (a)/2> In making paper-pulp, grasses and like raw materials are digested with weak solutions of sodium hydroxide to form, as a by-product, a black liquor containing at least one part by weight of sodium carbonate and other sodium salts (expressed as the anhydrous form) to four parts by weight of sodium hydroxide, and sodium hydroxide is continuously regenerated in the black liquor by continuously introducing the black liquor and lime into a recausticizing zone and continuously withdrawing the regenerated black liquor from the zone.  "Grasses and like raw materials" include all vegetable matter having a high bulk to weight ratio relative to wood and from which paper is customarily made, e.g. esparto grass, straw, bamboo and especially bagasse.  Preferably quicklime is used, and the operation may be batch or continuous.  The raw material may be passed into a soaking zone containing the sodium hydroxide solution, impregnated raw material being discharged from the soaking zone into a digestion zone and black liquor discharged from the soaking zone being introduced, together with lime, into a recausticizing zone, from which regenerated black liquor is continuously returned to the soaking zone.  The digestion zone is preferably held at an elevated temperature.  Black liquor may also be separated from the impregnated raw material in the digestion zone, continuously introduced into the recausticizing zone together with lime, and regenerated black liquor continuously removed from the recausticizing zone and returned to the soaking zone.  The black liquor may be filtered to remove raw material before being introduced into the recausticizing zone, and barium chloride may be added to the regenerated black liquor, to precipitate colouring matter, before it is returned to the soaking zone.  In the continuous process of Fig. 1, raw material is impregnated with weak sodium hydroxide in the steamjacketed vat 1, and heated to about 80 DEG C.  The impregnated raw material is passed to drainer 6, drained, and conveyed to digester 8.  After cooking, the pulp is freed from black liquor in pulp screw 13 and further treated in known manner.  Black liquor from 6 is passed to a lagged recausticizing vat 9, where about 60 parts by weight of quicklime are added for every 100 parts by weight of sodium carbonate and other sodium salts.  The regenerated liquor is passed to sump 11, and is returned to vat 1 when the ratio of sodium carbonate and other salts to sodium hydroxide in the black liquor in the vat 1 has reached at least 1 : 4 by weight. Impregnated raw material passing from vat 1 to digester 8 carries with it lime particles, which will regenerate sodium hydroxide in the black liquor in the digester.  Black liquor is also collected, in sump 14, from pulp screw 13, and from the overflow pipe of drainer 6, and is passed back to vat 9.  This process is suitable only for low grade pulp. For high grade pulp a further treatment of black liquor from digester 8 is described, involving filtration, recausticizing, further filtration, and, optionally, barium chloride treatment, prior to returning to vat 1.  In the batch process of Fig. 2, raw material is cooked in 24, liquor being continuously withdrawn and re-circulated via pump 25 and pipe line 26.  A proportion of this liquor is tapped off via pipe 27 and re-causticized as above in vat 28, and regenerated liquor returned via sump 29 and pump 30.  Black liquor is not withdrawn from 26 until an at least 1 : 4 ratio of sodium carbonate and other salts to sodium hydroxide has been attained in 24.  After cooking, black liquor is removed from 24 by draining or via a pulp screw, and may be filtered, regenerated, and treated with barium chloride as previously described.