Indium Production

Wastes of lead-zinc and tin production may serve as main raw material for Indium processing. Leaching and extraction by kerosene solution of di-2-ethylhexylphosphor acid or by ion exchange, for instance, on phosphate cation-exchange materials.

Indium is extracted by electrolysis or by Indium displacement from the solution: In³⁺ + Al⁰ => In⁰ + Al³⁺) with following smelting under alkali layer. Then electrolytic refining methods such as zone melting, vacuum volatile impurities treatment and crystals drawing from the fusion are applied.

Indium is best extracted from zinc that has been prepared from blende containing indium. The zinc, which may contain 0.1 per cent, of indium, is treated with a slight deficit of dilute sulphuric or hydrochloric acid, and the solution allowed to stand in contact with the excess of zinc for several days. From the spongy residue, which contains indium, lead, copper, iron, cadmium, arsenic, and the excess of zinc, pure indium oxide is best prepared by the method due to Bayer. The washed residue is dissolved in nitric acid and the solution evaporated with a slight excess of sulphuric acid. Iron and indium hydroxides are precipitated by the addition of ammonia to the filtered solution of the sulphates and the washed precipitate dissolved in hydrochloric acid. The nearly neutral solution when boiled with excess of sodium bisulphite yields basic indium sulphite as a fine, crystalline powder. The precipitate is dissolved in aqueous sulphurous acid and the filtered solution heated to boiling, when pure basic indium sulphite is obtained.

The basic sulphite is dissolved in sulphuric acid, and indium hydroxide thrown down by the addition of ammonia. The precipitate is converted into indium sesqui-oxide by ignition.

Indium oxide may be reduced to the metal by heating in hydrogen or by heating with sodium. In the latter case, the brittle alloy of indium and sodium obtained is decomposed by water and the residual indium fused with sodium carbonate.

Traces of iron may be removed from indium by converting it into the trichloride, adding potassium thiocyanate to the feebly acid solution, and extracting the ferric thiocyanate with ether. Pure indium trichloride is also obtained by adding pyridine to its alcoholic solution. A double compound of the chloride with pyridine is precipitated, iron and aluminium chlorides remaining in solution.

Indium is easily deposited electrolytically from a solution of the chloride or sulphate, in the presence of pyridine, hydroxylamine, or formic acid, and fractional electrolysis of the sulphate solution affords the best method of obtaining pure indium. The deposited metal is pressed together, washed, dried at 120°, and fused in a charcoal boat in a current of hydrogen.