Platinum group
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The platinum group or platinum family is a group of six metal elements with similar physical and chemical properties. The family consists of ruthenium, rhodium, palladium, osmium, iridium, and platinum. These transition metals occupy the rectangular area in the periodic table at the intersection of periodic table groups, 8, 9, and 10 with periods 5 and 6.
Strictly taken the elements of Period 4 (iron, cobalt and nickel) have lots of similar physical (high density and melting points) and chemical properties (e.g. catalysts and complex compounds) as well so it is unclear why these elements do not belong to this Platinum group.
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History
Naturally occurring platinum and platinum-rich alloys have been known for a long time. Though the metal was used by pre-Columbian Indians, the first European reference to platinum appears in 1557 in the writings of the Italian humanist Julius Caesar Scaliger (1484-1558) as a description of a mysterious metal found in Central American mines between Darién (Panama) and Mexico ("up until now impossible to melt by any of the Spanish arts").
The Spaniards named the metal platina, or "little silver", when they first encountered it in Colombia. They regarded platinum as an unwanted impurity in the silver they were mining.
Properties
The catalytic properties of the six platinum group metals are outstanding. Platinum's wear and tarnish resistance characteristics are well suited for making fine jewelry. Other distinctive properties include resistance to chemical attack, excellent high-temperature characteristics, and stable electrical properties. All these properties have been exploited for industrial applications.
Applications
Platinum, platinum alloys, and iridium are used as crucible materials for the growth of single crystals, especially oxides. The chemical industry uses a significant amount of either platinum or a platinum-rhodium alloy catalyst in the form of gauze to catalyze the partial oxidation of ammonia to yield nitric oxide, which is the raw material for fertilizers, explosives, and nitric acid.
In recent years, a number of platinum-group metals have become important as catalysts in synthetic organic chemistry. Ruthenium dioxide is used as coatings on dimensionally stable titanium anodes used in the production of chlorine and caustic.
Platinum supported catalysts are used in the refining of crude oil, reforming, and other processes used in the production of high-octane gasoline and aromatic compounds for the petrochemical industry. Since 1979, the automotive industry has emerged as the principal consumer of platinum-group metals. Palladium, platinum, and rhodium have been used as oxidation catalyst in catalytic converters to treat automobile exhaust emissions. A wide range of platinum-group metal alloy compositions are used in low-voltage and low-energy contacts, thick- and thin-film circuits, thermocouples and furnace components, and electrodes.
Sources of platinum group metals
- Platinum is often found in a native state and the ore sperrylite (platinum arsenide, PtAs2) is a major source of this metal. A naturally occurring platinum/iridium alloy is platiniridium and this metal is also found in the mineral cooperite (platinum sulfide, PtS). Platinum is often accompanied by small amounts of other platinum family metals which are found in alluvial deposits in Colombia, Ontario, the Ural Mountains, and in certain western American states. Platinum is also produced commercially as a by-product of nickel ore processing. The huge quantities of nickel ore processed makes up for the fact that platinum makes up only two parts per million of the ore.
- Osmium is found in iridiosmium a naturally occurring alloy of iridium and osmium and in platinum-bearing river sands in the Ural Mountains, North, and South America. It also occurs in nickel-bearing ores found in the Sudbury, Ontario region with other platinum group metals. Even though the quantity of platinum metals found in these ores is small, the large volume of nickel ores processed makes commercial recovery possible.
- Iridium is found uncombined in nature with platinum and other platinum group metals in alluvial deposits. Naturally occurring iridium alloys include osmiridium and iridiosmium, both of which are mixtures of iridium and osmium. It is recovered commercially as a by-product from nickel mining and processing.
- Ruthenium is generally found in ores with the other platinum group metals in the Ural Mountains and in North and South America. Small but commercially important quantities are also found in pentlandite extracted from Sudbury, Ontario and in pyroxinite deposits in South Africa. Ruthenium is commercially isolated through a complex chemical process in which hydrogen is used to reduce ammonium ruthenium chloride yielding a powder. The powder is then consolidated by powder metallurgy techniques or by argon-arc welding.
- The industrial extraction of rhodium is complex as the metal occurs in ores mixed with other metals such as palladium, silver, platinum, and gold. It is found in platinum ores and obtained free as a white inert metal which it is very difficult to fuse. Principal sources of this element are located in river sands of the Ural Mountains, in North and South America and also in the copper-nickel sulfide mining area of the Sudbury, Ontario region. Although the quantity at Sudbury is very small, the large amount of nickel ore processed makes rhodium recovery cost effective. However, the annual world production of this element is only 7 or 8 tons and there are very few rhodium minerals.
- Palladium is found as a free metal and alloyed with platinum and gold with platinum group metals in placer deposits of the Ural Mountains, Australia, Ethiopia, South and North America. However it is commercially produced from nickel-copper deposits found in South Africa and Ontario (the huge volume of ore processed makes this extraction profitable in spite of its low concention in these ores).
An earlier version of this article was adapted from the public domain USGS Minerals Information publication "Platinum-Group Metals Statistics and Information", available online at http://minerals.usgs.gov/minerals/pubs/commodity/platinum/ Please update as necessary.