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AssigneeAluminiumipari Troszt
InventorsGyorgy Banvolgyi, Jozsef Zoldi, Peter Siklosi, Tibor Ferenczi, T. Anna Csordas, Ivan Feher, Ildiko Tassy nee Varju, Istvan Sajo
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An improvement in the process for extracting alumina from gibbsitic bauxites, wherein the basic, known process comprises mixing the bauxites with a sodium hydroxide-sodium aluminate digestion liquor, digesting the bauxites at a temperature in the range of 100.degree.-180.degree. C., diluting the resulting slurry, separating the red mud therefrom, precipitating alumina hydrate by cooling and agitation of the sodium aluminate liquor, and calcining the alumina hydrate precipitate to obtain alumina, wherein the improvement is characterized by so controlling the process that, in the liquid phase of the slurry leaving the digestion reactor, the reactive hydroxide ion concentration is maintained below 6 g/l, and preferably between 0 and 2 g/l, where the reactive hydroxide ion concentration is defined by the following formula: ##EQU1## where: C.sub.Al.sbsb.2 O.sub.3,gi.sup.eq is the equilibrium solubility of Al.sub.2 O.sub.3 for gibbsite, in g/l. C.sub.Al.sbsb.2 O.sub.3 is the actual concentration of Al.sub.2 O.sub.3, in g/l and C.sub.SiO.sbsb.2 is the actual concentration of SiO.sub.2, in g/l. Desirably, control of the process is maintained by regulating the ratio of bauxite to digestion liquor.
A process for producing alumina from gibbsitic bauxite by mixing the bauxite with a sodium hydroxide-sodium aluminate digestion liquor, having an A/C ratio digesting in a digestion reactor the bauxite in said liquor at a temperature of 100.degree.-180.degree. C. to form a resulting slurry having a liquid phase, of sodium aluminate liquor and red mud, diluting the slurry, separating the red mud therefrom, precipitating alumina hydrate from the thus-obtained sodium aluminate liquor through cooling and agitation, and calcining the precipitated alumina hydrate into alumina, the improvement characterized by so controlling the process that, in the liquid phase of the slurry leaving the digestion reactor, maintaining a reactive hydroxide ion concentration below about 6 g/l, said reactive hydroxide ion concentration, AH, being defined by the following formula ##EQU3## where C.sub.Al.sbsb.2.spsb.O.sub.3,gi is the equilibrium solubility of Al.sub.2 O.sub.3 for gibbsite, in g/l.
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