GB1145105A - Alumina production - Google Patents

Abstract

1,145,105. Bayer process. REYNOLDS METALS CO. 24 May, 1966, No. 23072/66. Heading C1A. In the Bayer process a slurry of aluminous ore in a quantity of caustic soda between that stoichiometrically equivalent to the silicon and that equivalent to both the silicon and the alumina in the ore is predigested at 150-490‹ F. for a time sufficient to precipitate dissolved silica as an insoluble desilication product, which remains suspended in the slurry and does not scale the heat exchange and other plant, and thereafter the digestion is completed with additional caustic alkali. As shown in Fig. 2, bauxite is slurried with 12-35% of the caustic liquor (i.e. recycle liquor plus make-up caustic); and this slurry, containing 25-50 wt. per cent solids, is heated by indirect heat exchange, with live steam and/or flash steam from flash cooling the digested slurry, to the predigestion temperature. After predigestion for ¢ to 12 hrs. at a temperature close to its atmospheric boiling point, the slurry and the rest of the caustic digestion liquor are separately heated to the digestion temperature before being combined and passed to the digester. After digestion at 290-480‹ F. the digested slurry is flash cooled in the stages 13, 14 and 15 and passed to the usual Bayer process stages 20 to 28. According to Fig. 1 (not shown), the predigested slurry may be combined with the major part of the caustic before heating to the digestion temperature.