Method of removing silica from alumina-containing substances having high silica contents

摘要

Silica-rich alumina - containing substances such as clays or inferior bauxite calcined at 900-1100 DEG C. are treated with a caustic alkali solution containing by weight more than 20 per cent of alkali metal hydroxide at between 70 DEG C. and the boiling point at atmospheric pressure for 15-30 mins., silica being thereby removed as alkali metal silicate. The product may be treated under the same conditions with a 2-10 per cent alkali metal hydroxide solution further to reduce the silica content and if desired either or both of the resultant solutions may be treated with lime and/or magnesia, hydrated if desired, separated from the precipitate and recycled. About 0.5-2 mols, preferably 1 mol of Na2O (&c.) is required per mol of SiO2 for the two stages. The product of the invention may be used in the Bayer or sinter decomposition process or combinations thereof. Examples are given in which the preferred conditions-calcination at 1000 DEG C. for 1 hour, and leaching with 30 per cent caustic soda-are employed as a result of which products containing about 70 per cent alumina and 20 per cent silica (from clays) or 55 per cent alumina and 5 per cent silica (from bauxite) are obtained. It is stated that by precalcination at 1000 DEG C. followed by treatment with 10 per cent caustic soda solution, it is only possible to reduce the silica content of clays to that of sillimanite, i.e. to a molar ratio of 1:1.