These three metals are growing continually more valuable because their supply is not increasing while their use is, both for industrial and ornamental purposes.
The U. S. Geological Survey gives the world's production thus:
New South Wales and Tasmania
United States - crude
United States - all bullion
Borneo, Sumatra, etc.
As the production of palladium and iridium probably is not over 1 per cent of the total number of ounces, it is plain how inadequate their supply is. Practically this entire amount comes from placer deposits where the metal occurs as metallic grains containing 70 to 90 per cent of platinum and iron as the largest of a dozen other impurities.
The standard method of treating the metallic grains with aqua regia to dissolve the platinum gives a fair separation, but further treatment is necessary for the highest grade material. By this method the platinum in aqua regia is evaporated and the salt is taken up twice with hydrochloric acid to prepare for precipitating from clean chloride solution with ammonium chloride. The bright yellow ammonium-platinic chloride is dried, is heated to platinum sponge, and the sponge then is melted before the oxyhydrogen blowpipe.
A commercial separation, carried out on dore silver from copper and lead refining, consists in treating with aqua regia the residue from the sulphuric acid parting for silver. From the metals remaining, platinum and palladium will be dissolved, and iridium will be left. The platinum will be precipitated as the double chloride; palladium will be thrown down by metallic zinc and melted into cakes. The iridium can be purified further by dissolving in silver and extracting again with acids. The main course of the procedure is simple, but the more the matter is studied, the less sharp the separations are found to be, and several treatments may be required to give strictly pure products.
The wet treatment is not entirely satisfactory for recovering the gold from its bullion, so the electrolytic process already mentioned is in vogue largely now; this method, at the same time, separates palladium and platinum to go into the electrolyte and to be precipitated out, while osmium and iridium remain in the sludge and are recovered separately.
With high temperatures available, and with electrolytic separations to assist the wet treatment, the metallurgy of these metals is in a way to be used with much satisfaction, if the metals themselves only were available in quantity.