BigPatents India

This invention relates to a continuous process for producing alumina trihydrate comprising the step of subjecting an alumina monohydrate based bauxite to high temperature caustic digestion to form a slurry which is settled and filtered in order to separate an insoluble residue and a supersaturated sodium aluminate liquor to be crystallized in the presence of seeds, wherein (a) digesting the bauxite at a temperature higher than 2000C, by contact with a liquor with a caustic concentration above or equal to 160 g Na2O/liter and a A/C weight ratio between 0.45 and 0.75; (b) cooling and flashing the resulting slurry, then introducing alumina trihydrate in said slurry, at a temperature of less than 2000C, in order to obtain A/C of the aluminate liquor between 1.20 and 1.30 when at atmospheric pressure; (c) first diluting the slurry giving a caustic concentration between 160 and 175 g Na2O/liter; (d) settling of said slurry in order to separate the insoluble residue; (e) filtering the aluminate liquor; (f) lowering the aluminate liquor concentration to less than 160 g Na2O/liter, via a complementary dilution carried out before or during crystallization; (g) after separation by recycling the spent liquor as digestion liquor.