GB883311A - Improvements in and relating to the production of alumina - Google Patents

Abstract

<PICT:0883311/III/1> Alumina is obtained from solutions produced by dissolving aluminium-containing materials, for example bauxite, in sulphuric acid by so adjusting the solution that the SO3 : Al2O3 weight ratio is less than 2,35 (which value corresponds to Al2(SO4)3), hydrolysing the adjusted solution at 180 DEG -300 DEG C. to precipitate a basic aluminium sulphate or hydrated alumina, and decomposing this precipitate to leave alumina. The hydrolysis is preferably carried out under pressure. The SO3 : Al2O3 weight ratio in the solution may be adjusted by dissolving aluminium metal in the solution, or by digesting the solution with a source of alumina, for example bauxite, preferably at 100 DEG C. to 140 DEG C., or by electrolysing the solution in a cell provided with a suitable membrane, for example a selective anion exchange membrane, paper, or parchment, thereby removing sulphate ions from the solution in the cathode compartment, or by adding to the solution, which may have been partially adjusted, some of the hydrolysis precipitate from a previous batch or a basic aluminium sulphate from any source. In examples, crushed bauxite is digested with sulphuric acid under pressure in digestion stage 1. If sufficient air or oxygen is provided, at least part of the sulphuric acid may be replaced by pyrite. The liquor from stage 1 is passed to modification stage 2, where the SO3 : AL2O3 ratio is adjusted by any of the methods previously indicated. Any iron in the solution may be reduced to the ferrous state by aluminium metal or other reducing agent. The adjusted liquor is passed to precipitation stage 3, where it is maintained at 180 DEG -300 DEG C. to permit hydrolysis to take place. The precipitate is recovered, and part is recycled to stage 3 and the remainder is calcined in stage 4 to give alumina product and sulphur dioxide, which is passed to digestion stage 1. The liquor from stage 3 is similarly recycled to stage 1.