| H | He | ||||||||||||||||||
| Li | Be | B | C | N | O | F | Ne | ||||||||||||
| Na | Mg | Al | Si | P | S | Cl | Ar | ||||||||||||
| K | Ca | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn | Ga | Ge | As | Se | Br | Kr | ||
| Rb | Sr | Y | Zr | Nb | Mo | Tc | Ru | Rh | Pd | Ag | Cd | In | Sn | Sb | Te | I | Xe | ||
| Cs | Ba | * | Lu | Hf | Ta | W | Re | Os | Ir | Pt | Au | Hg | Tl | Pb | Bi | Po | At | Rn | |
| Fr | Ra | ** | Lr | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Cn | Nh | Fl | Mc | Lv | Ts | Og | |
| * | La | Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | |||||
| ** | Ac | Th | Pa | U | Np | Pu | Am | Cm | Bk | Cf | Es | Fm | Md | No | |||||
Platinum group metals | |||||||||||||||||||
Othernoble metals | |||||||||||||||||||
Theplatinum-group metals[a] (PGMs) are sixnoble,preciousmetallicelements clustered together in theperiodic table. These elements are alltransition metals in thed-block (groups8,9, and10, periods5 and6).[1]
The six platinum-group metals areruthenium,rhodium,palladium,osmium,iridium, andplatinum. They have similar physical and chemical properties, and tend to occur together in the same mineral deposits.[2] However, they can be further subdivided into theiridium-group platinum-group elements (IPGEs: Os, Ir, Ru) and thepalladium-group platinum-group elements (PPGEs: Rh, Pt, Pd) based on their behaviour in geological systems.[3]
The three elements above the platinum group in the periodic table (iron,nickel andcobalt) are allferromagnetic; these, together with the lanthanide elementgadolinium (at temperatures below 20 °C),[4] are the only known transition metals that display ferromagnetism near room temperature.
Naturally occurring platinum and platinum-rich alloys were known bypre-Columbian Americans for many years.[5] 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 humanistJulius 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").[5]
The name platinum is derived from the Spanish wordplatina ("little silver"), the name given to the metal by Spanish settlers inColombia. They regarded platinum as an unwanted impurity in the silver they were mining.[5][6]
By 1815, rhodium and palladium had been discovered byWilliam Hyde Wollaston, and iridium and osmium by his close friend and collaboratorSmithson Tennant.[7]
| PGM | Use | ThousandToz |
|---|---|---|
| Palladium | autocatalysts | 4470 |
| electronics | 2070 | |
| dental | 1830 | |
| chemical reagents | 230 | |
| Platinum | jewelry | 2370 |
| autocatalysts | 1830 | |
| Rhodium | autocatalysts | 490 |
The platinum metals have many usefulcatalytic properties. They are highly resistant to wear and tarnish, making platinum, in particular, well suited for finejewellery. Other distinctive properties include resistance to chemical attack, excellent high-temperature characteristics, high mechanical strength, good ductility, and stableelectrical properties.[8] Apart from their application in jewellery, platinum metals are also used in anticancer drugs, industries, dentistry, electronics, andvehicle exhaust catalysts (VECs).[9] VECs contain solid platinum (Pt), palladium (Pd), and rhodium (Rh) and are installed in the exhaust system of vehicles to reduce harmful emissions, such ascarbon monoxide (CO), by converting them into less harmful emissions.[10]
Generally,ultramafic andmaficigneous rocks have relatively high, andgranites low, PGE trace content. Geochemically anomalous traces occur predominantly in chromianspinels and sulfides. Mafic and ultramafic igneous rocks host practically all primary PGM ore of the world. Maficlayered intrusions, including theBushveld Complex, outweigh by far all other geological settings of platinum deposits.[11][12][13][14] Other economically significant PGE deposits include mafic intrusions related toflood basalts, and ultramafic complexes of the Alaska, Urals type.[12]: 230
Typical ores for PGMs contain ca. 10 g PGM/ton ore, thus the identity of the particular mineral is unknown.[15]
Platinum can occur as a native metal, but it can also occur in various different minerals and alloys.[16][17] That said,Sperrylite (platinumarsenide, PtAs2)ore is by far the most significant source of this metal.[18] A naturally occurring platinum-iridium alloy, platiniridium, is found in themineralcooperite (platinumsulfide, PtS). Platinum in a native state, often accompanied by small amounts of other platinum metals, is found inalluvial andplacer deposits inColombia,Ontario, theUral Mountains, and in certain westernAmerican states. Platinum is also produced commercially as a by-product ofnickel 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 theMerensky Reef of theBushveld complex, is the world's largest producer of platinum, followed byRussia.[19][20] Platinum and palladium are also mined commercially from theStillwater igneous complex in Montana, USA. Leaders of primary platinum production are South Africa and Russia, followed by Canada, Zimbabwe and USA.[21]
Osmiridium is a naturally occurring alloy of iridium and osmium found in platinum-bearing river sands in theUral Mountains and inNorth andSouth America. Trace amounts of osmium also exist in nickel-bearing ores found in theSudbury,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.[20][22]
Metalliciridium is found with platinum and other platinum group metals in alluvial deposits.[23] Naturally occurring iridium alloys includeosmiridium andiridosmine, both of which are mixtures of iridium and osmium. It is recovered commercially as a by-product from nickel mining and processing.[20]
Ruthenium is generally found in ores with the other platinum group metals in the Ural Mountains and inNorth andSouth America. Small but commercially important quantities are also found inpentlandite extracted fromSudbury, Ontario, and inpyroxenite deposits inSouth Africa.[20]
The industrial extraction ofrhodium is complex, because it occurs in ores mixed with other metals such as palladium,silver, platinum, andgold. 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 theUral Mountains, North and South America, and also in the copper-nickel sulfide mining area of theSudbury 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 8tons and there are very few rhodium minerals.[24]
Palladium is preferentially hosted in sulfide minerals, primarily inpyrrhotite.[12] Palladium is found as a free metal and alloyed with platinum and gold with platinum group metals inplacer deposits of theUral Mountains ofEurasia,Australia,Ethiopia,South andNorth America. However it is commercially produced from nickel-copper deposits found inSouth Africa andOntario, Canada. The huge volume of nickel-copper ore processed makes this extraction profitable in spite of its low concentration in these ores.[24]
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 theBushveld Igneous Complex because of this. Further research is needed to determine the environmental impacts.[25] Classical purification methods exploit differences inchemical reactivity andsolubility of several compounds of the metals under extraction.[26] These approaches have yielded to new technologies that utilizesolvent extraction.
Separation begins with dissolution of the sample. Ifaqua 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[RhCl(NH3)5]Cl2.[27]
Significant quantities of the three light platinum group metals—ruthenium, rhodium and palladium—are formed asfission products in nuclear reactors.[28] With escalating prices and increasing global demand, reactor-producednoble metals are emerging as an alternative source. Various reports are available on the possibility of recovering fission noble metals fromspent nuclear fuel.[29][30][31]
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.[32] 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.[33] However, Pt can solubilise in road dust, enter water sources, the ground, and increase dose rates in animals throughbioaccumulation.[33] These impacts from platinum groups were previously not considered, however[34] over time the accumulation of platinum group metals in the environment may actually pose more of a risk than previously thought.[34] 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.[34]
Platinum metals extracted during the mining and smelting process can also cause environmental damage. In Zimbabwe, platinum-group mining caused pollution in water sources, acidic water drainage, andenvironmental degradation.[35]
Another hazard of Pt is being exposed tohalogenated 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.[33] Workers removed immediately from further contact with Pt salts showed no evidence of long-term effects, however continued exposure could lead to health effects.[33]
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.[33] Handling of these drugs by medical personal also led to side effects including chromosome aberrations and hair loss. The long-term medical effects of platinum drug use and exposure await evaluation.
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.[36]
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