Platinum Metals

Occurrence.

The platinum metals appear to be widely dis tributed in nature, although generally in very small quantities. Localities in which they occur are Brazil, Colombia, Peru, Russia, Australia, Borneo, Tasmania, California and South Africa. Allu vial deposits have hitherto been the usual sources, but platinifer ous reefs have now been found in the Transvaal and Natal. Alluvial platinum was reported in 1927 from Sierra Leone over an area of 4osq. miles. The first definite recognition of crude platinum as a separate metal occurs in the 18th century, when chemists obtained samples of platinum ore from Carthagena.

In 1803 and 1804 four new metals were obtained from plati num. Osmium and iridium were discovered and isolated by Smith son Tennant and palladium and rhodium by W. H. Wollaston. The remaining platinum metal, ruthenium, was not discovered till 1828 when Osann succeeded in isolating it from the platiniferous ore of the Ural mountains, a deposit discovered in 1819. The platinum ores obtained from various localities differ greatly in composition. Platinum is generally found as the chief metal and the other metals of the group occur in relatively small and variable quanti ties. Occasionally, however, and especially in Australia and South Africa, small nuggets or grains of osmiridium are found, and recently considerable amounts have been found in Tasmania which is now its chief source. This mineral consists chiefly of the metals osmium and iridium, and also usually of a certain amount of ruthenium, the other platinum metals occurring to a small extent only. In addition to the six platinum metals, platinum ores contain other elements, the most common being gold, copper, silver, iron, chromium and nickel.

Separation.

The methods in general use for the separation of the six platinum metals are here outlined, detailed accounts of their purification being shown under the headings of the several metals. A platinum ore which has been freed from base metals and other impurities is treated with aqua regia (q.v.) ; osmium, most of the iridium and a small amount of ruthenium remain un dissolved. The solution, containing platinum, palladium, rhodium and small amounts of ruthenium and iridium, is evaporated to dryness, when the metals are obtained as chlorides. These chlo rides are then heated to about 150° C whereby the chlorides of palladium and iridium are partly decomposed giving lower chlo rides. The residual mass is dissolved in very dilute hydrochloric acid, and ammonium chloride is added to precipitate most of the platinum and iridium as double chlorides, the other metals re maining in solution. This solution is then treated with metallic iron (technically termed "footing"), which throws all the metals completely out of solution; this precipitate, like all others similarly obtained, is technically called "foots." The precipitate

of ter thorough washing is again dissolved in aqua regia, the solution is concentrated and again treated with ammonium chlo ride to remove further amounts of platinum and iridium and then treated with ammonium hydrate, when nearly all the palladium is thrown out as a difficultly soluble ammine, The solution now containing rhodium and ruthenium is evaporated to dryness and strongly ignited, leaving the metals themselves. These are fused with potassium bisulphate, when the rhodium forms a soluble double sulphate leaving the ruthenium un attacked.

The foregoing residues containing osmium, iridium and some ruthenium are heated in a tube in a stream of air or preferably oxygen. Osmium and ruthenium form volatile oxides which are deposited in the cold parts of the tube. The residue, iridium with some ruthenium and small amounts of other platinum metals, is purified by suitable methods. A method of treating crude platinum ores, particularly useful in concentrating the platiniferous portion, is to heat the ores with metallic lead or to make a mixture of the ores with galena, scrap iron, glass and borax, the last two materials serving as a flux. The platinum metals, except most of the osmiridium, are dissolved in the lead which is removed by ordinary cupellation, the platinum metals remaining ready for subsequent treatment. In modern reef-mining, the platiniferous ores are concentrated after grinding by methods of oil flotation.

Another method of opening up platinum ores is to treat a heated mixture of the substance and common salt with chlorine. All the platinum metals are attacked by this process giving double chlorides of the metal with sodium. As these double chlorides differ in stability, variations of the temperature of treatment give different products. Even if the double chloride is decom posed at or below the temperature of formation, the metal resulting from this decomposition remains in a finely divided con dition and subsequent treatment is facilitated, and at the same time some of the stable double chlorides, especially that of irid ium, are soluble in water and are easy to treat subsequently. It is obvious that with ores of different compositions some of the foregoing methods are more suitable than others in particular cases. Furthermore, all firms engaged in the refining of these metals have their own special secret methods. (F. E. M.) BIBLIOGRAPHY.-E. A. Smith, Platinum Metals (1925) ; A. D. Lumb, The Platinum Metals (1920).