The Platinum Group Metals
An Industrially Important Family of Very Rare and Expensive Elements
Oct 14, 2008
Taking their name from the best-known member they are known as the platinum group. The natives of South America used platinum long before the conquistadors arrived. The Spaniards, however, were interested only in gold, and failed to appreciate platinum's value. The only known example from the ancient Mediterranean world is an Egyptian casket dating from the seventh century BC. Though prices fluctuate, platinum is normally twice as expensive as gold.
Platinum
About 150 tonnes of platinum are produced each year, half of which is used in jewellery. The rest is employed as a catalyst, in industry or in car exhaust pipes.
A catalytic converter uses about two grammes of platinum supported on a ceramic base. This is finely powdered to give it an effective surface area of about the size of a football field. Exhaust gases are adsorbed onto the surface, where the pollutants, carbon monoxide, unburned hydrocarbons and nitrogen oxides, are converted into less harmful carbon dioxide, nitrogen and water vapour.
About 10% of platinum is used, along with cobalt, as a magnetic alloy, in the hard disc drives of computers. Fuel cells, which could eventually power the cars of the 21st century, have electrodes made from platinum alloys.
Palladium
Palladium is also present in catalytic converters, and is better than platinum at removing unburned hydrocarbons from cold engines. Tiny capacitors made from palladium are used increasingly in televisions, mobile phones and computers. It can absorb 900 times its own volume of hydrogen and is used to purify the gas. This also suggests potential for storage of the gas in a future hydrogen-based economy.
Rhodium
Rhodium is particularly good at converting the acidic nitrogen oxides from car engines into harmless nitrogen. 80% of the 16 tonnes of rhodium manufactured each year is used in catalytic converters, with most of the rest going into industrial catalysts. It is about four times as expensive as platinum.
Ruthenium
Ruthenium is used, along with iron, to speed the production of ammonia. It withstands highly corrosive conditions and is alloyed with titanium to make electrodes for the manufacture of chlorine and for use in undersea pipes. It is non-toxic, but traces of a radioactive isotope from Sellafield nuclear power station, in West Cumbria, have been found to concentrate in an edible seaweed harvested in South Wales. Among the many dangers of the nuclear industry, this problem was unforeseen.
Osmium
Osmium is the densest of all metals, being nearly three times as dense as iron. It is extremely hard, and was used to make clock bearings, record player needles and pen nibs. Changes in technology have led to a decrease in demand, so that less than 100 kilograms are now extracted each year. Osmium oxide is used in forensic science as a staining agent for fingerprints and DNA.
Iridium
Iridium is among the ten rarest elements on earth. It is the most corrosion-resistant of all metals, and like ruthenium, is alloyed with titanium for use in corrosive environments and in aircraft engine parts. Iridium occurs in high concentration in a thin layer of rocks, found worldwide, marking what geologists refer to as the Cretaceous-Tertiary boundary. This layer was formed in the period during which the dinosaurs became extinct. As iridium is found in high abundance in meteorites, this supports the theory that the dinosaurs died out, at least partly as a result of a meteorite impact with the earth about 65 million years ago.
Reference: John Emsley, Nature's Building Blocks, Oxford University Press 2001
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