The platinum group metals (abbreviated as the PGMs; alternatively, the platinoids, platidises, platinum group or platinum metals) sometimes collectively refers to six metallic elements clustered together in the periodic table. These elements are all transition metals, lying in the d-block (groups 8, 9, and 10, periods 5 and 6).
The six platinum group metals are ruthenium, rhodium, palladium, osmium, iridium, and platinum. They have similar physical and chemical properties, and tend to occur together in the same mineral deposits. They commonly occur together in nature and are among the scarcest of the metallic elements. Platinum is used principally as catalysts for the control of automobile and industrial plant emissions, as catalysts to produce acids, organic chemicals, and pharmaceuticals. PGMs are used in bushings for making glass fibers used in fiber-reinforced plastic and other advanced materials, in electrical contacts, in capacitors, in conductive and resistive films used in electronic circuits, in dental alloys used for making crowns and bridges and in jewelry. Russia and South Africa have nearly all the world's reserves.
The platinum metals have outstanding catalytic properties. They are highly resistant to wear and tarnish, making platinum, in particular, well suited for fine jewellery. 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.
Most platinum is used to produce catalytic converters in automobile exhaust systems. The goal is to limit the smog-producing chemicals that come from burning gasoline. When an internal combustion engine burns gasoline, nitrogen oxides (NOx) are produced. The exhaust passes through the catalytic converter that contains platinum and iridium. The gases are in the converter for 0.1 to 0.4 seconds and in that very short time, 75% of the nitrogen oxide is converted into nitrogen and oxygen. In addition, more than 95% of carbon monoxide and other hydrocarbons in the exhaust are oxidized. The platinum works by lowering the energy needed to cause these chemical changes. The result is a dramatic reduction in pollution.
Although about one-third of all platinum is used by the automotive industry, there are various other uses. It is alloyed with gold, silver and copper for dental uses. PGM is used in chemotherapy, particularly to fight leukemia. Platinum-iridium compounds are used to make biomedical devices. An alloy of platinum and osmium is used in pacemakers to regulate heart function and in heart replacement valves.
The production of pure platinum group metals normally starts from residues of the production of other metals with a mixture of several of those metals. One typical starting product is the anode residue of gold or nickel production. The differences in chemical reactivity and solubility of several compounds of the metals under extraction are used to separate them.
A first step is to dissolve all the metals in aqua regia forming their respective nitrates. If silver is still present, this is then separated by forming insoluble silver chloride. Rhodium sulfate is separated after the salts have been melted together with sodium hydrogensulfate and leached with water. The residue is then melted together with sodium peroxide, which dissolves all the metals and leaves the iridium. The two remaining metals, ruthenium and osmium, form ruthenium and osmium tetroxides after chlorine has been added to solution. The osmium tetroxide is then dissolved in alcoholic sodium hydroxide and separated from the ruthenium tetroxides. All of these metals' final chemical compounds can ultimately be reduced to the elemental metal using hydrogen.
As a mineral, platinum occurs in dark silicate rocks with minerals containing iron and magnesium. It is usually found as fine grains or flakes scattered throughout the rock and rarely as large nuggets. Sperrylite (platinum arsenide, PtAs2) ore is also a major source of this metal. A naturally occurring platinum-iridium alloy, platiniridium, is found in the mineral cooperite (platinum sulfide, PtS). Platinum in a native state, often accompanied by small amounts of other platinum metals, is found in alluvial and placer 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. South Africa, with vast platinum ore deposits in the Merensky Reef of the Bushveld complex, is the world's largest producer of platinum, followed by Russia. Platinum and palladium are also mined commercially from the Stillwater igneous complex in Montana, USA.
Production in nuclear reactors
Significant quantities of platinum group metals – Ruthenium, Rhodium and Palladium are formed as fission products in nuclear reactors.With escalating prices and increasing global demand, reactor produced noble metals are emerging as an alternative source. Various reports are available on the possibility of recovering fission noble metals from spent nuclear fuel.
Recently there is an upsurge in the recovery of valuable fission products which reflects in the form of articles in leading scientific journals. Palladium has been of special interest due to its less complex behaviour when compared to rhodium and ruthenium. The special interest in palladium may be also due to its widespread application in chemical catalysis and the electronics industry. Several research groups are exploring the possibility of recovering palladium by various methods like direct electrolysis of high-level liquid waste, using room temperature ionic liquids (RTILs) as electrolyte for nuclear fuel dissolution and recovery, solvent extraction, ion exchange, etc. Room temperature ionic liquids have been employed to recover rhodium, and ruthenium also recently.
Origin of the Name Platinum
Platinum has been known and used since antiquity in South America, where the use of platinum by the natives was discovered by Antonio de Ulloa of Spain in 1735. Because it is silver-gray in color, it was named after the Spanish word for silver, plata.
Other Platinum Group Metals
Iridiosmium is a naturally occurring alloy of iridium and osmium found in platinum-bearing river sands in the Ural Mountains and in North and South America. Trace amounts of osmium also exist in nickel-bearing ores found in the Sudbury, Ontario region along 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.
Metallic iridium is found 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 pyroxenite deposits in South Africa.
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 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 Basin 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 in 2003 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 of Eurasia, Australia, Ethiopia, South and North America. However it is commercially produced from nickel-copper deposits found in South Africa and Ontario, Canada. The huge volume of nickel-copper ore processed makes this extraction profitable in spite of its low concentration in these ores.
Substitutes and Alternative Sources
Some manufacturers are using less expensive palladium in place of platinum in catalytic converters. As a catalytic converter component in diesel engines, palladium is proving to be a better than platinum.
For more information About Platinum