Platinum metals are found in their native state in alluvial deposits in Colombia and the Ural mountains in the U.S.S.R., and in gold-bearing rock at Witwatersrand in South Africa. Platinum occurs as its sulfide, PtS (cooperite), and a mixed sulfide containing palladium, platinum and nickel at Rustenburg in South Africa, and is found as platinum diarsenide, PtAs2 (sperrylite), in the huge copper and nickel sulfide deposits in Ontario.
Where the metals are gathered in their native state they are concentrated by methods similar to those employed in the gold mines. Where, however, they are obtained as by-products in the large-scale extraction of copper and nickel from their sulfides, the concentration is effected by adding slightly less than sufficient sulfur at the matte-forming stage so as to preserve small regions of nickel or copper metal in which the platinum metals dissolve. These enriched alloys are removed by crushing and magnetic separation and further concentrated by heating with more sulfur and repeating the cycle. Electrorefining of the enriched concentrate leaves the platinum metals as insoluble anode slimes.
The separation and refining of the various members of the platinum family involves a series of complex chemical operations which may vary in detail but are basically similar. The first stage of the process is treatment with hot aqua regia (a mixture of nitric and hydrochloric acids) which dissolves only the platinum and palladium. These metals are precipitated separately from the solution as ammonium salts which are reduced to the metal at high temperature.
The fraction which is insoluble in aqua regia is reacted with molten sodium chloride at 1290° F (700° C) in the presence of chlorine to form double chlorides which are water soluble. Osmium, which is not dissolved at any stage, is recovered from the final residue by forming its volatile oxide. Ruthenium and rhodium are precipitated from the double chloride solution as sulfides which are subsequently separated by redissolution, followed by individual precipitation. Iridium is finally removed from the double chloride solution as its oxide and reduced to pure metal by hydrogen.