Aluminium Pechiney

Jean Deville

Jean-Michel Lamerant

The invention concerns a process for the production of alumina for ceramics by calcining alumina trihydrate in the presence of a mineralizer such as a halogenated compound and/or a boron compound, the calcined alumina being constituted by friable agglomerates of elementary particles, or crystallites, of alpha alumina, the average size of which can be fixed as required, with a unimodal narrow distribution of crystallite sizes, characterized in that calcining of the alumina trihydrate is carried out in an industrial kiln in a renewed oxidising atmosphere at a temperature in the range 800.degree. C. to 1300.degree. C. for a period of 0.5 hour to 4 hours in the simultaneous presence of a halogenated compound acting as a mineralizer and a silica based alumina recrystallisation modifier which is uniformly and intimately distributed in the alumina trihydrate charge.

Production of calcined alumina with a crystallite size which can be

A process for eliminating iron from supersaturated sodium aluminate solutions obtained from the alkaline attack according to the Bayer process of the monohydrate bauxite including successively the steps of grinding and eventually of desilicatation in the presence of an aliquot of decomposed liquor, then attack at a temperature higher than 200.degree. C., usually between 240.degree. and 270.degree. C., of the said crushed and eventually desilicatated bauxite by the remaining fraction of the decomposed liquor, to make the alumina hydrate soluble and to form at the end of the attack an insoluble suspension of the residue or red mud in the supersaturated sodium aluminate liquor which, after cooling and dilution, is decanted to separate the mud or insoluble residues from the supersaturated liquor destined to be decomposed in the presence of seeds after filtration, characterized by the fact that at the end of the attack, during the cooling of the suspension achieved by decreasing the pressure in the reactors, one injects in the said suspension at a temperature higher than 140.degree. C. a small quantity of a mixture containing calcium.

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Aix-en-Provence, France

Jean Deville