Patent ID: 7217408

Claim:
Alumina production process from bauxite containing a diaspore or boehmite alumina monohydrate with more than 1.5% by weight of calcium carbonate, comprising the steps of: a) digesting the ore at a temperature of more than 220° C. using an aluminate liquor with a soda concentration of more than 200 g of Na 2 O/liter; b) diluting the aluminate slurry thus obtained; c) separating insoluble residues obtained by settling and filtering; d) crystallizing the liquor by seeding with aluminum trihydroxide particles in order to cause precipitation of alumina in solution in the form of aluminum trihydroxide, to obtain thereby a spent liquor; and e) restoring the soda concentration of the spent liquor and recycling the spent liquor to the bauxite digestion step; wherein the ore digestion step comprises the steps of: a1) transferring the slurry comprising ground bauxite ore and concentrated alumina liquor to an installation comprising a series of n digesters (A 1 , . . . , A k−1 , A k , A k+1 , . . . , A n ) and a series of m flash tanks (D 1 , . . . , D i−1 , D i , D i+1 , . . . , D m ), where m is less than n, while supplying tubular heaters (R m , . . . , R k+1 , R k , R k−1 . . . , R l ) with steam passing through m digesters; a2) passing the slurry first through the series of digesters (A 1 , . . . , A k−1 , A k , A k+1 , . . . , A n ), with a duration defined such that ore digestion is substantially complete at upon exit from final digester (A n ); a3) passing the slurry through the series of flash tanks (D 1 , . . . , D i−1 , D i , D i+1 , . . . , D m ), the steam (Vi) output from each flash tank (D i ) supplying the heater (R k ) passing through the digester (A k ); and a4) directing the slurry to stations for said dilution (C) and said separating (F); and wherein the slurry (S i−1 ) entering a flash tank (D i ) is diluted in at least one location in series of said flash tanks (D 1 , . . . , D i−1 , D i , D i+1 , . . . , D m ), using water obtained from condensation of steam originating from at least one upstream flash tank (D 1 , . . . , D i+1 ).