The six platinum group metals (PGMs) are elements that possess unique chemical and physical characteristics. Members of the group have been in high demand by jewellers since their discovery in ancient times, and industrial consumption has been rising ever since their unique catalytic properties were recognized, late in the 19th century. Currently, three of the six members of the group are more valuable than gold (see the accompanying table entitled “Basic PGM Data”). The characteristics of the PGMs that make them so valuable include inertness to attack by acids, molten salts, and so on; their high melting points (1,769 c for platinum); high density (22.6 for osmium); and uniquely strong catalytic properties. These attributes are not possessed to the same degree by all members of the group of six, and this is the primary reason for their differing values.
The usage pattern for the PGMs is changing (see table entitled “End Use Data”). There is considerable interest in PGM development in Canada and other countries. This interest is partly the result of increasing demand for bullion and coin (including the new Canadian platinum coin) as a hedge against inflation. Interest also stems from increased demand for catalysts associated primarily with the automobile industry.
However, the interest also includes a measure of speculation which is justified when it is realized that more than 90% of world PGM supply arises in South Africa and Russia, and that there are no comparably priced substitutes. It is obvious why PGM prices surged in August, 1987, when South African miners went on strike and again in May, 1988, when there was talk in South Africa of suspending shipments of strategic materials, such as platinum, to the U.S. The
future demand for these metals is expected to remain strong. There will be increasing use of platinum, palladium and rhodium catalysts to reduce auto emissions and platinum/ ruthenium and platinum/iridium catalysts to produce high-octane, lead- free fuels required in the future. New uses are expected and will probably include the phosphoric acid fuel cell (pafc) which, according to predictions, will account for 70 tonnes per year of new platinum demand within a decade.
Production from several PGM sources varies from year to year. South African PGM production is primary and, therefore, quite respon sive to normal supply and demand. However, most other PGM production comes as byproducts of copper/ nickel operations, so quantities vary with base metal demand.
The several mine sources of PGM contain a wide range of PGM distributions, from the predominantly platinum deposits of South Africa to the palladium-rich Stillwater orebody, in Montana. (See the accompanying table for distribution details of existing and certain potential Canadian producers.) The exact distribution of the PGM in the ore has a profound effect on the ore value because of the widespread in prices of the individual PGM units. Thus a sample of Stillwater ore containing 20 g per tonne of total PGM has a contained metal value of $133 (us) per tonne, while a sample of the relatively low-grade Merensky Reef ore containing only 9 g per tonne has almost the same value at $127 (us) per tonne. The values of the copper, nickel and any contained gold are to be added in order to arrive at the total metal value.
Because of the inertness of the PGM, they are not destroyed and so eventually become available as scrap — especially those used as catalysts. Most industrial catalysts are reprocessed and serve as an important secondary source of PGM. An increasingly signif icant source of PGM will be the recycling of spent automobile catalytic converters. PGM converters were first used in 1975 and are becoming progressively more available as older cars are scrapped. Potential North American production from this source could soon be as high as 28 tonnes per year (70% platinum, 29% palladium and 1% ruthenium. Several companies are now reprocessing scrap or developing processes. Actual PGM recovery is probably less than one-third of potential because of difficulties in collection.
The placer deposits of Colombia, Alaska, the Ural Mountains and the Witwatersrand are processed for the recovery of the contained PGM alloys using classical placer techniques. On the Witwatersrand gold plants, osmiridium concentrates, consisting of an alloy of osmium and iridium, are re covered from behind the mill liners. However, in all other deposits, the PGMs are found associated with cop per and/or nickel sulphides. Initial concentration by flotation (sometimes complemented by gravity flotation) produces sulphide concentrates con taining copper, nickel and PGMs. This same primary recovery process is em ployed on ores from the Merensky and UG2 reefs of South Africa, the Noril’sk deposit in the USSR, the Sudbury mines, and the Stillwater in Montana.
The majority of the copper/nickel sulphide concentrates follow a route including an initial smelting step followed by hydrometallurgical refin ing to finished product. Recovery from South African PGM concentrates involves submerged-arc smelting and subsequent production of a converter matte. Matte is hydrometallurgically treated by pressure leaching to remove base metals and to concentrate the PGMs in an insoluble residue. PGM-bearing residue is processed in South Africa using either the Mintek or Johnson Matthey process for pure metal recovery. Both processes employ a series of chemical precipitation, solvent extraction, distillation and ion exchange steps to separate and refine the individual PGMs.
Canadian and Russian PGMs are byproducts of copper/nickel recovery. At Inco Ltd., the metals are concentrated in the insoluble material re maining after pressure leaching the carbonylation residue of the nickel refinery. Additional PGMs are present in the slimes of the copper refinery. Both types of material are pre-treated at Port Colborne, Ont., before final PGM isolation and refining using precipitation, solvent extraction and distillation techniques at Acton in the United Kingdom. Falconbridge’s PGMs are contained in the copper/nickel matte which is shipped to Kristiansand in Norway for copper and nickel recovery. Pgm-bearing sludges gen erated at Kristiansand are shipped to the U.S. for refining.
Soviet PGMs are refined at Krasnoyarsk, some 1,400 km south of the mines at Noril’sk. Stillwater flota tion concentrates are trucked to the Eastern seaboard, shipped to Europe and smelted and hydrometallurgically refined in Belgium by Metallurgie Hoboken Overpelt. The several companies considering PGM produc tion in Canada can either emulate Stillwater and arrange for custom smelting of their flotation concentrates or they can build their own smelting/ refining facilities.
The burgeoning nickel demand, long PGM process transit times (six months is not uncommon), and sulphur abatement programs appear to have combined to make the smelting of custom ores unattractive to current Canadian nickel smelters and potential PGM producers. Thus Madeleine Mines, which is developing the Lac des Iles PGM deposit in Ontario, has elected to build its own facilities.
Despite higher prices for several of the PGMs than for gold, a Canadian deposit with 7 g per tonne (0.2 oz) of PGMs may be difficult to bring to production. Potential domestic producers face several penalties: the high cost of a smelter/refinery dedicated to a single, small mine and unfavorable custom smelting contracts and transportation costs. A centrally located, custom PGM smelter and refinery would do much to encourage the projects now under investigation. John Goode, a metallurgist for the past 25 years, is manager of metallurgy at Kilborn Ltd.
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