Rhodium Production

Rhodium is concentrated in mother solutions after platinum и palladium separation from hydrochloric solutions appeared after platinum processing or Cu and nickel electrochemical refining. Contaminating metals are extracted by nitration, after which rhodium chloride complexes pass into Na₃[Rh(NO₂)₆], and precipitated by NH₄Cl as (NH₄)₂Na[Rh(NO₂)₆]. It is dissolved by heating with hydrochloric acid and reduced by formic acid; heating in hydrogen at 1000°C yields rhodium sponge.

Rhodium affinage (refining) is processed by chlorine treatment of rhodium sponge which results into [Rh(NH₃)₅Cl]Cl₂, [Rh(NH₃)₃Cl₃], (NH₄)₃[Rh(SO₃)₃], (NH₄)₃[RhCl₆] complexes with further roasting until rhodium sponge appears. Various extraction and ion exchange methods are used in this process. Pure rhodium sponge is smelting in induction furnaces with inert atmosphere.

Rhodium is prepared from platinum ores. The solution from which platinum has been precipitated as ammonium chlorplatinate is treated with metallic iron, which precipitates the more noble metals. The last-named residue is fused with a mixture of metallic lead and litharge. On cooling, the button of lead remaining is well cleaned, and contains metals less easily oxidised than itself, any others more readily oxidised being in the regulus and thus easily removed by mechanical means. Treatment of the metallic button with nitric acid effects the solution of lead, copper, and palladium. The insoluble portion containing the rhodium and other metals is fused with excess of barium dioxide, lixiviated with water and then treated with aqua regia. This effects the liberation of osmium, which can be recovered by distillation; otherwise it may simply be boiled off in a fume-cupboard with a powerful draught, care being taken to avoid inhalation of the fumes, as these are exceedingly poisonous. When the odour of osmium has disappeared sufficient sulphuric acid is added to precipitate the barium. The filtrate is evaporated with a little nitric acid, and subsequently taken to dryness on the water-bath with excess of ammonium chloride. The residue is thoroughly washed with a concentrated solution of ammonium chloride (which effects the solution of the rhodium) until the wash waters cease to have an appreciable rose colour. The liquid is evaporated with an excess of nitric acid to destroy the ammonium chloride, the resulting rhodium salt is moistened with ammonium sulphide, mixed with excess of sulphur and heated to redness in a closed crucible. Metallic rhodium remains which may be further purified by successive prolonged boilings with aqua regia and concentrated sulphuric acid.

A still further purification of the rhodium is effected by fusing the metal obtained in the preceding manner to dull redness with metallic zinc, whereby the compound RhZn₂ is formed, with considerable heat evolution, a portion of the zinc being volatilised. On cooling, treatment of the mass with concentrated hydrochloric acid effects the solution of the excess of zinc, the insoluble crystalline alloy remaining unaffected.

The last named is dissolved in aqua regia, excess of ammonia added, and the solution evaporated to dryness, whereby the monochlor-rhodium pentammine chloride is formed. This is recrystallised to remove traces of iridium, calcined with sulphur in a crucible, and finally heated in the oxyhydrogen flame to volatilise any still remaining osmium.