The platinum-group metals ( PGMs) are six Noble metal, Precious metal chemical element clustered together in the periodic table. These elements are all 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.
The three elements above the platinum group in the periodic table (iron, nickel and cobalt) are all ferromagnetic; these, together with the lanthanide element gadolinium (at temperatures below 20 °C),
History
Naturally occurring platinum and platinum-rich alloys were known by
pre-Columbian Americans for many years.
However, even though the metal was used by pre-Columbian peoples, 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 name platinum is derived from the Spanish word platina ("little silver"), the name given to the metal by Spanish settlers in Colombia. They regarded platinum as an unwanted impurity in the silver they were mining.
By 1815, rhodium and palladium had been discovered by William Hyde Wollaston, and iridium and osmium by his close friend and collaborator Smithson Tennant.[Platinum Metals Rev., 2003, 47, (4), 175. Bicentenary of Four Platinum Group Metals PART I: RHODIUM AND PALLADIUM – EVENTS SURROUNDING THEIR DISCOVERIES (W. P. Griffith)]
Properties and uses
The platinum metals have many useful catalytic properties. They are highly resistant to wear and tarnish, making platinum, in particular, well suited for fine Jewelry. Other distinctive properties include resistance to chemical attack, excellent high-temperature characteristics, high mechanical strength, good ductility, and stable Electricity properties.
Occurrence
Generally,
ultramafic rock and
mafic have relatively high, and
low, PGE trace content. Geochemically anomalous traces occur predominantly in chromian
and sulfides. Mafic and ultramafic igneous rocks host practically all primary PGM ore of the world. Mafic layered intrusions, including the
Bushveld Complex, outweigh by far all other geological settings of platinum deposits.
Other economically significant PGE deposits include mafic intrusions related to
, and ultramafic complexes of the Alaska, Urals type.
PGM minerals
Typical ores for PGMs contain ca. 10 g PGM/ton ore, thus the identity of the particular mineral is unknown.
Platinum
Platinum can occur as a native metal, but it can also occur in various different minerals and alloys.
That said,
Sperrylite (platinum
arsenide, PtAs
2)
ore is by far the most significant 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
alluvium and
placer deposit deposits in
Colombia,
Ontario, the
Ural Mountains, and in certain western
United States 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. Leaders of primary platinum production are South Africa and Russia, followed by Canada, Zimbabwe and USA.
Osmium
Osmiridium is a naturally occurring alloy of iridium and osmium found in platinum-bearing river sands in the
Ural Mountains and in
North America and
South America. Trace amounts of osmium also exist in nickel-bearing ores found in the
Greater 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.
Iridium
Metallic
iridium is found with platinum and other platinum group metals in alluvial deposits.
Naturally occurring iridium alloys include
osmiridium and
Osmiridium, both of which are mixtures of iridium and osmium. It is recovered commercially as a by-product from nickel mining and processing.
Ruthenium
Ruthenium is generally found in ores with the other platinum group metals in the Ural Mountains and in
North America and
South America. Small but commercially important quantities are also found in
pentlandite extracted from Sudbury, Ontario, and in
pyroxenite deposits in
South Africa.
Rhodium
The industrial extraction of
rhodium is complex, because it 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 South Africa, Zimbabwe, in the river sands of the
Ural Mountains, 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
and there are very few rhodium minerals.
Palladium
Palladium is preferentially hosted in sulfide minerals, primarily in
pyrrhotite.
Palladium is found as a free metal and alloyed with platinum and gold with platinum group metals in
placer mining deposits of the
Ural Mountains of
Eurasia,
Australia,
Ethiopia,
South American 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.
Production
The production of individual platinum group metals normally starts from residues of the production of other metals with a mixture of several of those metals. Purification typically starts with the anode residues of gold, copper, or nickel production. This results in a very energy intensive extraction process, which leads to environmental consequences. Carbon dioxide emissions are expected to rise as a result of increased demand for platinum metals and there is likely to be expanded mining activity in the Bushveld Igneous Complex because of this. Further research is needed to determine the environmental impacts.
Classical purification methods exploit differences in chemical reactivity and
solubility of several compounds of the metals under extraction.
These approaches have yielded to new technologies that utilize solvent extraction.
Separation begins with dissolution of the sample. If aqua regia is used, the chloride complexes are produced. Depending on the details of the process, which are often trade secrets, the individual PGMs are obtained as the following compounds: the poorly soluble (NH4)2IrCl6 and (NH4)2PtCl6, PdCl2(NH3)2, the volatile OsO4 and RuO4, and [PentaamminechlororhodiumCl2]].[Bernardis, F. L.; Grant, R. A.; Sherrington, D. C. "A review of methods of separation of the platinum-group metals through their chloro-complexes" Reactive and Functional Polymers 2005, Vol. 65,, p. 205-217. ]
Production in nuclear reactors
Significant quantities of the three light platinum group metals—ruthenium, rhodium and palladium—are formed as
in nuclear reactors.
With escalating prices and increasing global demand, reactor-produced
are emerging as an alternative source. Various reports are available on the possibility of recovering fission noble metals from spent nuclear fuel.
Environmental concerns
It was previously thought that platinum group metals had very few negative attributes in comparison to their distinctive properties and their ability to reduce harmful emission from automobile exhausts.
However, even with all the positives of platinum metal use, its possible future harm should be considered. Metallic Pt is considered not chemically reactive and non-allergenic, so that Pt emitted from VECs in metallic and oxide forms is considered relatively safe.
However, Pt can solubilise in road dust, enter water sources, the ground, and increase dose rates in animals through
bioaccumulation.
These impacts from platinum groups were previously not considered, however
over time the accumulation of platinum group metals in the environment may actually pose more of a risk than previously thought.
As more internal combustion cars are driven, platinum metal emissions increase.
The bioaccumulation of PGMs in animals can pose a health risk to both humans and biodiversity. Species whose food source is contaminated by these hazardous PGMs emitted from VECs may accumulate them, as may the species that consume them, including humans.
Another hazard of Pt is being exposed to Halogenation Pt salts, which can cause allergic reactions leading to high rates of asthma and dermatitis. This response is sometimes seen in workers employed in production of industrial catalysts.
Platinum use in drugs also may need to be reevaluated, as some of the side effects to these drugs include nausea, hearing loss, and nephrotoxicity.
While exposure to relatively low volumes of platinum group metal emissions may not have long-term health effects, it is unknown how the accumulation of Pt metal emissions will affect the environment as well as human health, what levels of risk are safe, and how potential hazards from platinum-group metals can be mitigated.
See also
-
Platinum group metals in Africa
-
Merensky Reef
-
Johnson Matthey Technology Review (formerly published as Platinum Metals Review)
Notes
External links
