Platinum group

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Platinum group metals (PGMs) in the periodic table
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   Platinum group metals
   Other noble metals

The platinum-group metals [a] (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). [1]

Contents

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. [2] 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. [3]

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), [4] 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. [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 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"). [5]

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. [5] [6]

By 1815, rhodium and palladium had been discovered by William Hyde Wollaston, and iridium and osmium by his close friend and collaborator Smithson Tennant. [7]

Properties and uses

Replica of the NIST national prototype kilogram standard, made in 90% platinum, 10% iridium alloy National prototype kilogram K20 replica.jpg
Replica of the NIST national prototype kilogram standard, made in 90% platinum, 10% iridium alloy
Significant uses of selected PGMs, 1996 [1]
PGMUseThousand Toz
Palladium autocatalysts4470
electronics2070
dental1830
chemical reagents230
Platinum jewelry2370
autocatalysts1830
Rhodium autocatalysts490

The platinum metals have many useful 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, high mechanical strength, good ductility, and stable electrical properties. [8] Apart from their application in jewellery, platinum metals are also used in anticancer drugs, industries, dentistry, electronics, and vehicle 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 as carbon monoxide (CO), by converting them into less harmful emissions. [10]

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. [11] [12] [13] [14] Other economically significant PGE deposits include mafic intrusions related to flood basalts, and ultramafic complexes of the Alaska, Urals type. [12] : 230 

PGM minerals

Typical ores for PGMs contain ca. 10 g PGM/ton ore, thus the identity of the particular mineral is unknown. [15]

Platinum

Platinum can occur as a native metal, but it can also occur in various different minerals and alloys. [16] [17] That said, Sperrylite (platinum arsenide, PtAs2) ore is by far the most significant source of this metal. [18] 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. [19] [20] 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. [21]

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. [20] [22]

Iridium

Metallic iridium is found with platinum and other platinum group metals in alluvial deposits. [23] 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. [20]

Ruthenium

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. [20]

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 tons and there are very few rhodium minerals. [24]

Palladium

Palladium is preferentially hosted in sulphide minerals, primarily in pyrrhotite. [12] 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. [24]

Production

Process flow diagram for the separation of the platinum group metals. PtMetalExtraction II.jpg
Process flow diagram for the separation of the platinum group metals.

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. [25] Classical purification methods exploit differences in chemical reactivity and solubility of several compounds of the metals under extraction. [26] 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 [RhCl(NH3)5]Cl2. [27]

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. [28] 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. [29] [30] [31]

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 successfully reduce harmful emission from automobile exhausts. [32] However, even with all the positives of platinum metal use, the negative effects of their use need to be considered in how it might impact the future. For example, metallic Pt are considered to not be chemically reactive and non-allergenic, so when Pt is emitted from VECs it is in metallic and oxide forms it is considered relatively safe. [33] However, Pt can solubilise in road dust, enter water sources, the ground, and increase dose rates in animals through bioaccumulation. [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] Future research is needed to fully grasp the threat of platinum metals, especially since as more internal combustion cars are driven, the more platinum metal emissions there are.

The bioaccumulation of Pt metals in animals can pose a significant health risk to both humans and biodiversity. Species will tend to get more toxic if their food source is contaminated by these hazardous Pt metals emitted from VECs. This can potentiality harm other species, including humans if we eat these hazardous animals, such as fish. [34]

Cisplatin is a platinum based drug used in therapy of human neoplasms. The medical success of cisplatin is conflicted as a result of severe side effects. Cisplatin-2D.png
Cisplatin is a platinum based drug used in therapy of human neoplasms. The medical success of cisplatin is conflicted as a result of severe side effects.

Platinum metals extracted during the mining and smelting process can also cause significant environmental impacts. In Zimbabwe, a study showed that platinum group mining caused significant environmental risks, such as pollution in water sources, acidic water drainage, and environmental degradation. [35]

Another hazard of Pt is being exposed to halogenated Pt salts, which can cause allergic reactions in high rates of asthma and dermatitis. This is a hazard that can sometimes be seen in the production of industrial catalysts, causing workers to have reactions. [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 professionals, such as nurses, have also resulted in some side effects including chromosome aberrations and hair loss. Therefore, the long term effects of platinum drug use and exposure need to be evaluated and considered to determine if they are safe to use in medical care.

While exposure of relatively low volumes of platinum group metal emissions may not have any long-term health effects, there is considerable concern for how the accumulation of Pt metal emissions will impact the environment as well as human health. This is a threat that will need more research to determine the safe levels of risk, as well as ways to mitigate potential hazards from platinum group metals. [36]

See also

Notes

  1. ↑ Also known as the platinoids, platinides, platidises, platinum group, platinum metals, platinum family, or platinum-group elements (PGEs).

Related Research Articles

<span class="mw-page-title-main">Iridium</span> Chemical element with atomic number 77 (Ir)

Iridium is a chemical element; it has symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal with a density of 22.56 g/cm3 (0.815 lb/cu in) as defined by experimental X-ray crystallography. 191Ir and 193Ir are the only two naturally occurring isotopes of iridium, as well as the only stable isotopes; the latter is the more abundant. It is one of the most corrosion-resistant metals, even at temperatures as high as 2,000 °C (3,630 °F).

<span class="mw-page-title-main">Osmium</span> Chemical element with atomic number 76 (Os)

Osmium is a chemical element; it has symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mostly in platinum ores. Osmium is the densest naturally occurring element. When experimentally measured using X-ray crystallography, it has a density of 22.59 g/cm3. Manufacturers use its alloys with platinum, iridium, and other platinum-group metals to make fountain pen nib tipping, electrical contacts, and in other applications that require extreme durability and hardness.

<span class="mw-page-title-main">Palladium</span> Chemical element with atomic number 46 (Pd)

Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired by her when she slew Pallas. Palladium, platinum, rhodium, ruthenium, iridium and osmium form a group of elements referred to as the platinum group metals (PGMs). They have similar chemical properties, but palladium has the lowest melting point and is the least dense of them.

<span class="mw-page-title-main">Platinum</span> Chemical element with atomic number 78 (Pt)

Platinum is a chemical element; it has symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish platina, a diminutive of plata "silver".

<span class="mw-page-title-main">Ruthenium</span> Chemical element with atomic number 44 (Ru)

Ruthenium is a chemical element; it has symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is unreactive to most chemicals. Karl Ernst Claus, a Russian scientist of Baltic-German ancestry, discovered the element in 1844 at Kazan State University and named it in honor of Russia, using the Latin name Ruthenia. Ruthenium is usually found as a minor component of platinum ores; the annual production has risen from about 19 tonnes in 2009 to some 35.5 tonnes in 2017. Most ruthenium produced is used in wear-resistant electrical contacts and thick-film resistors. A minor application for ruthenium is in platinum alloys and as a chemical catalyst. A new application of ruthenium is as the capping layer for extreme ultraviolet photomasks. 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.

<span class="mw-page-title-main">Rhodium</span> Chemical element with atomic number 45 (Rh)

Rhodium is a chemical element; it has symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring isotope, which is 103Rh. Naturally occurring rhodium is usually found as a free metal or as an alloy with similar metals and rarely as a chemical compound in minerals such as bowieite and rhodplumsite. It is one of the rarest and most valuable precious metals. Rhodium is a group 9 element.

A period 5 element is one of the chemical elements in the fifth row of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The fifth period contains 18 elements, beginning with rubidium and ending with xenon. As a rule, period 5 elements fill their 5s shells first, then their 4d, and 5p shells, in that order; however, there are exceptions, such as rhodium.

<span class="mw-page-title-main">Noble metal</span> Metallic elements that are nearly chemically inert

A noble metal is ordinarily regarded as a metallic element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals are most often so classified. Silver, copper, and mercury are sometimes included as noble metals, but each of these usually occurs in nature combined with sulfur.

<span class="mw-page-title-main">Group 9 element</span> Group of chemical elements

Group 9, by modern IUPAC numbering, is a group (column) of chemical elements in the d-block of the periodic table. Members of Group 9 include cobalt (Co), rhodium (Rh), iridium (Ir) and meitnerium (Mt). These elements are among the rarest of the transition metals.

<span class="mw-page-title-main">Group 10 element</span> Group of chemical elements

Group 10, numbered by current IUPAC style, is the group of chemical elements in the periodic table that consists of nickel (Ni), palladium (Pd), platinum (Pt), and darmstadtium (Ds). All are d-block transition metals. All known isotopes of darmstadtium are radioactive with short half-lives, and are not known to occur in nature; only minute quantities have been synthesized in laboratories.

<span class="mw-page-title-main">Sperrylite</span> Platinum arsenide mineral

Sperrylite is a platinum arsenide mineral with the chemical formula PtAs2 and is an opaque metallic tin white mineral which crystallizes in the isometric system with the pyrite group structure. It forms cubic, octahedral or pyritohedral crystals in addition to massive and reniform habits. It has a Mohs hardness of 6–7 and a very high specific gravity of 10.6.

<span class="mw-page-title-main">Bushveld Igneous Complex</span> Large early layered igneous intrusion

The Bushveld Igneous Complex (BIC) is the largest layered igneous intrusion within the Earth's crust. It has been tilted and eroded forming the outcrops around what appears to be the edge of a great geological basin: the Transvaal Basin. It is approximately two billion years old and is divided into four limbs or lobes: northern, eastern, southern and western. It comprises the Rustenburg Layered suite, the Lebowa Granites and the Rooiberg Felsics, that are overlain by the Karoo sediments. The site was first publicised around 1897 by Gustaaf Molengraaff who found the native South African tribes residing in and around the area.

<span class="mw-page-title-main">Layered intrusion</span>

A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around 100 km2 (39 sq mi) to over 50,000 km2 (19,000 sq mi) and several hundred metres to over one kilometre (3,300 ft) in thickness. While most layered intrusions are Archean to Proterozoic in age, they may be any age such as the Cenozoic Skaergaard intrusion of east Greenland or the Rum layered intrusion in Scotland. Although most are ultramafic to mafic in composition, the Ilimaussaq intrusive complex of Greenland is an alkalic intrusion.

<span class="mw-page-title-main">Native metal</span> Form of metal

A native metal is any metal that is found pure in its metallic form in nature. Metals that can be found as native deposits singly or in alloys include antimony, arsenic, bismuth, cadmium, chromium, cobalt, indium, iron, manganese, molybdenum, nickel, niobium, rhenium, tantalum, tellurium, tin, titanium, tungsten, vanadium, and zinc, as well as the gold group and the platinum group. Among the alloys found in native state have been brass, bronze, pewter, German silver, osmiridium, electrum, white gold, silver-mercury amalgam, and gold-mercury amalgam.

<span class="mw-page-title-main">Merensky Reef</span> Layer of igneous rock in the Bushveld igneous complex, South Africa

The Merensky Reef is a layer of igneous rock in the Bushveld Igneous Complex (BIC) in the North West, Limpopo, Gauteng and Mpumalanga provinces of South Africa which together with an underlying layer, the Upper Group 2 Reef (UG2), contains most of the world's known reserves of platinum group metals (PGMs) or platinum group elements (PGEs)—platinum, palladium, rhodium, ruthenium, iridium and osmium. The Reef is 46 cm thick and bounded by thin chromite seams or stringers. The composition consists predominantly of cumulate rocks, including leuconorite, anorthosite, chromitite, and melanorite.

The mineral industry of Russia is one of the world's leading mineral industries and accounts for a large percentage of the Commonwealth of Independent States' production of a range of mineral products, including metals, industrial minerals, and mineral fuels. In 2005, Russia ranked among the leading world producers or was a significant producer of a vast range of mineral commodities, including aluminum, arsenic, cement, copper, magnesium compounds and metals, nitrogen, palladium, silicon, nickel and vanadium.

<span class="mw-page-title-main">Braggite</span>

Braggite is a sulfide mineral of platinum, palladium and nickel with chemical formula: S. It is a dense, steel grey, opaque mineral which crystallizes in the tetragonal crystal system. It is the central member in the platinum group end-members cooperite and vysotskite.

<span class="mw-page-title-main">Native element mineral</span> Elements that occur in nature as minerals in uncombined form

Native element minerals are those elements that occur in nature in uncombined form with a distinct mineral structure. The elemental class includes metals, intermetallic compounds, alloys, metalloids, and nonmetals. The Nickel–Strunz classification system also includes the naturally occurring phosphides, silicides, nitrides, carbides, and arsenides.

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<span class="mw-page-title-main">Ferronickel platinum</span> Rare occurring mineral

Ferronickel platinum is a very rarely occurring minerals from the mineral class of elements (including natural alloys, intermetallic compounds, carbides, nitrides, phosphides and silicides) with the chemical composition Pt2FeNi and thus is chemically seen as a natural alloy, more precisely an intermetallic compound of platinum, nickel and iron in a ratio of 2:1:1.

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  35. ↑ Meck, Maideyi; Love, David; Mapani, Benjamin (2006). "Zimbabwean mine dumps and their impacts on river water quality – a reconnaissance study". Physics and Chemistry of the Earth, Parts A/B/C. 31 (15–16): 797–803. Bibcode:2006PCE....31..797M. doi:10.1016/j.pce.2006.08.029.
  36. ↑ Hunt, L. B.; Lever, F. M. (1969). "Platinum Metals: A Survey of Productive Resources to industrial Uses" (PDF). Platinum Metals Review. 13 (4): 126–138. doi:10.1595/003214069X134126138 . Retrieved 2009-10-02.
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