GB293392A - Improvements in process for manufacturing and purifying alumina - Google Patents

Abstract

293,392. Seailles, J. C. July 5, 1927, [Convention date]. Sodium aluminate; alumina.-In the process for the manufacture of alumina, as described in Specifications 277,697 and 283,509, from non-halogenated aluminium ores, the proportion of silica in the alkaline-earth aluminate obtained by the use of an autoclave, and capable of dissolving on the subsequent treatment with sodium carbonate, thus forming an undesirable impurity, may be reduced by regulating the temperature, pressure and duration of heating in the autoclave, without reducing the quantity of aluminate extracted from the bauxite by an excessive amount. A further improvement is gained, when lime is used for treatment of the bauxite, by the employment of catalysts forming soluble compounds with lime, which are decomposed by alumina. Such catalysts are sulphonated organic derivatives, such as benzene sulphonic acid, sulpho-ricinoleic acid, and the like. The bauxite may also be treated previous to extraction in a colloid mill or similar apparatus. The sodium aluminate bath, containing some dissolved silica, may be further purified by preventing all excess of free alkali when the alkaline-earth aluminate is treated with sodium carbonate. This may be carried out in several ways. For example, when tetracalcium aluminate is produced in the autoclave, the subsequent reaction with sodium carbonate is as follows : 2(Al2O3.4CaO)+8Na2CO3+5H2O.=2(Al2O3.3Na2O.) +8CaCO3+10NaOH. The free alkali may be diminished by washing the crude aluminate with water, thus dissolving some of the lime, so that the amount of free alkali will be reduced in proportion to the amount of calcium carbonate to be produced, equilibrium being reached when 2¢ molecules of lime have been removed from the calcium aluminate. A similar effect is produced by treating the crude aluminate with sufficient carbonic acid to convert 2¢ molecules of lime to the carbonate. Either or both of the above processes may be further combined with treatment by sodium bicarbonate to obviate or reduce the free alkali in the solution. A further purifying process, which may be used in conjunction with those previously described, consists in the addition of calcium aluminates to the sodium aluminate solution containing dissolved silica. A double decomposition occurs, with the precipitation of the greater part of the silica. About 10 per cent of calcium aluminate is added to the solution and may consist of the crude aluminate discharged from the autoclave. By this means the amount of silica present may be reduced from 3.02 to 0.16 per cent. In practice, known methods of exhaustion may be used, systematically employing this purifying effect, or alternatively, a cyclic or continuous process may be used. The proportions of the compounds employed may be modified during the first operations, but subsequently an equilibrium is established from which there are no appreciable variations.