Platinum group

{| style="float:right; margin:5px; border:1px solid grey; text-align:center;"

|+ Platinum group metals (PGMs) in the periodic table

| white;| H|| colspan="17" |  ||He

| Li||Be|| colspan="11" |  ||B || white;|C || white;|N || white;|O ||F ||Ne

| |Na||Mg|| colspan="11" |  ||Al||Si||P ||S ||Cl||Ar

| |K ||Ca|| ||Sc||Ti||V||Cr||Mn||Fe||Co||Ni|| style="background:#90ff00;"|Cu||Zn||Ga||Ge||As||Se||Br||Kr

| |Rb||Sr|| ||Y||Zr||Nb||Mo||Tc|| style="background:#00ff90;"|Ru|| style="background:#00ff90;"|Rh|| style="background:#00ff90;"|Pd|| style="background:#90ff00;"|Ag||Cd||In||Sn||Sb||Te||I ||Xe

| Cs||Ba|| * ||Lu||Hf||Ta||W|| style="background:#90ff00;"|Re|| style="background:#00ff90;"|Os|| style="background:#00ff90;"|Ir|| style="background:#00ff90;"|Pt|| style="background:#90ff00;"|Au|| style="background:#90ff00;"|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||

| colspan="19" style="line-height:110%;" |  

| || ||* ||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|| ||

| colspan=6 |

| colspan=5 |

The platinum-group metals (PGMs) are six noble, precious metallic elements clustered together in the periodic table. These elements are all transition metals 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. However, they can be further subdivided into the iridium-group platinum-group elements (IPGEs: Os, Ir, Ru) and the palladium-group platinum-group elements (PPGEs: Rh, Pt, Pd) based on their behaviour in geological systems.

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), are the only known transition metals that display ferromagnetism near room temperature.

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").

By 1815, rhodium and palladium had been discovered by William Hyde Wollaston, and iridium and osmium by his close friend and collaborator Smithson Tennant.

Properties and uses

thumb|Replica of the [[NIST national prototype kilogram standard, made in 90% platinum, 10% iridium alloy]]

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|+ Significant uses of selected PGMs, 1996 Apart from their application in jewellery, platinum metals are also used in anticancer drugs, industries, dentistry, electronics, and vehicle exhaust catalysts (VECs). VECs contain solid platinum (Pt), palladium (Pd), and rhodium (Rh) and are installed in the exhaust system of vehicles to reduce harmful emissions, such as carbon monoxide (CO), by converting them into less harmful emissions.

Occurrence

Generally, ultramafic and mafic igneous rocks have relatively high, and granites low, PGE trace content. Geochemically anomalous traces occur predominantly in chromian spinels 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 flood basalts, and ultramafic complexes of the Alaska, Urals type.

Platinum

Platinum can occur as a native metal, but it can also occur in various different minerals and alloys. That said, Sperrylite (platinum arsenide, PtAs2) 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 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. 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 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.

Iridium

Metallic iridium is found with platinum and other platinum group metals in alluvial deposits. Naturally occurring iridium alloys include osmiridium and iridosmine, both of which are mixtures of iridium and osmium. It is recovered commercially as a by-product from nickel mining and processing.

Palladium

Palladium is preferentially hosted in sulfide minerals, primarily in pyrrhotite. 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 [[Pentaamminechlororhodium dichloride|[RhCl(NH3)5]Cl2]].

Production in nuclear reactors

Significant quantities of the three light 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.

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. over time the accumulation of platinum group metals in the environment may actually pose more of a risk than previously thought.

Another hazard of Pt is being exposed to halogenated 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.

Notes

References

External links

Platinum group prices (provided by Johnson Matthey) - spot prices on the global commodities market
Platinum Group Metals (PGM) Database, created by Johnson Matthey
Platinum-Group Metals Statistics and Information from the U.S. Geological Survey's National Minerals Information Center