Platinum group

The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) 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]

Platinum group metals (PGMs) in the periodic table
H He
LiBe BCNOFNe
NaMg AlSiPSClAr
KCaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKr
RbSrYZrNbMoTcRuRhPdAgCdInSnSbTeIXe
CsBaLa*HfTaWReOsIrPtAuHgTlPbBiPoAtRn
FrRaAc**RfDbSgBhHsMtDsRgCnNhFlMcLvTsOg
*CePrNdPmSmEuGdTbDyHoErTmYbLu
**ThPaUNpPuAmCmBkCfEsFmMdNoLr
   Platinum group metals
   Other noble metals

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 being the only known transition metals with this property.

With the platinum group metals having many desirable properties, they have a wide application of uses. This leads to an increased demand for these metals as well as an increase in their production for use.

The increase in platinum group use as well as production activity could cause environmental and human health risks not previously considered where more research is needed to determine the risks associated with platinum group metal use and production.

History

Naturally occurring platinum and platinum-rich alloys were known by pre-Columbian Americans for many years.[4] 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").[4]

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.[4][5]

Properties and uses
Replica of the NIST national prototype kilogram standard, made in 90% platinum - 10% iridium alloy

As of 1996, the largest applications of platinum metals were, in thousands of troy ounces/year: Pd for autocatalysts (4470), Pt for jewelry (2370), Pd for electronics (2070), Pt for autocatalysts (1830), Pd for dental (1230), Rh for autocatalysts (490), and Pd for chemical reagents (230).[1]

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.[6] Apart from their application in jewellery, platinum metals are also used in anticancer drugs, industries, dentistry, electronics, and vehicle exhaust catalysts (VECs).[7] 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.[8]

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.[9] Other economically significant PGE deposits include mafic intrusions related to flood basalts, and ultramafic complexes of the Alaska, Urals type.[10]

PGM minerals

Typical ores for PGMs contain ca. 10 g PGM/ton ore, thus the identity of the particular mineral is unknown.[11]

Platinum

Platinum can occur as a native metal, but it can also occur in various different minerals and alloys.[12][13] That said, Sperrylite (platinum arsenide, PtAs2) ore is by far the most significant source of this metal.[14] 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.[15][16] 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.[16][17]

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.[16]

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.[16]

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.[18]

Palladium

Palladium is preferentially hosted in sulphide minerals, primarily in pyrrhotite.[9] 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.[18]

Production
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 environmental consequences. With Pt emissions expecting to rise as a result of increased demand for platinum metals as well as expanded mining activity in the Bushveld Igneous Complex, further research is needed to determine the environmental impacts.[19] Classical purification methods exploit differences in chemical reactivity and solubility of several compounds of the metals under extraction.[20] 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.[21]

Production in nuclear reactors
Main article: Synthesis of precious metals

Significant quantities of the three light platinum group metals—ruthenium, rhodium and palladium—are formed as fission products in nuclear reactors.[22] 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.[23][24][25]

Environmental problems

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.[26] 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.[27] However, Pt can solubilise in road dust, enter water sources, the ground, and in animals through bioaccumulation.[27] These impacts from platinum groups were previously not considered, however[28] over time the accumulation of platinum group metals in the environment may actually pose more of a risk then previously thought.[28] Future research is needed to fully grasp the threat of platinum metals, especially since as more 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.[28]

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.[29]

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.[27] Workers removed immediately from further contact with Pt salts showed no evidence of long-term effects, however continued exposure could lead to health effects.[27]

Platinum drugs use also needs to be reevaluated, as some of the side effects to these drugs include nausea, hearing loss, and nephrotoxicity.[27] 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.[30]

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See also

Notes
  1. Renner, H.; Schlamp, G.; Kleinwächter, I.; Drost, E.; Lüschow, H. M.; Tews, P.; Panster, P.; Diehl, M.; et al. (2002). "Platinum group metals and compounds". Ullmann's Encyclopedia of Industrial Chemistry. Wiley. doi:10.1002/14356007.a21_075.
  2. Harris, D. C.; Cabri L. J. (1991). "Nomenclature of platinum-group-element alloys; review and revision". The Canadian Mineralogist. 29 (2): 231–237.
  3. Rollinson, Hugh (1993). Using Geochemical Data: Evaluation, Presentation, Interpretation. Longman Scientific and Technical. ISBN 0-582-06701-4.
  4. Weeks, M. E. (1968). Discovery of the Elements (7 ed.). Journal of Chemical Education. pp. 385–407. ISBN 0-8486-8579-2. OCLC 23991202.
  5. Woods, Ian (2004). The Elements: Platinum. Benchmark Books. ISBN 978-0-7614-1550-3.
  6. 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. Retrieved 2009-10-02.
  7. Khaiwal Ravindra,László Bencs,René Van Grieken (5 January 2004). "Platinum group elements in the environment and their health risk". ScienceDirect. Retrieved 2020-02-28.CS1 maint: multiple names: authors list (link)
  8. Deborah M. Aruguete, Adam Wallace, Terry Blakney, Rose Kerr, Galen Gerber, Jacob Ferko, (2019). "Palladium release from catalytic converter materials induced by road de-icer components chloride and ferrocyanide". ScienceDirect. Retrieved 2020-02-28.CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
  9. Walter L. Pohl, Economic Geology Principles and Practice 2011
  10. Walter L. Pohl, Economic Geology Principles and Practice 2011, P 230
  11. 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. doi:10.1016/j.reactfunctpolym.2005.05.011
  12. "Mineral Profile: Platinum". British Geological Survey. September 2009. Retrieved 6 February 2018.
  13. "Search Minerals By Chemistry - Platinum". www.mindat.org. Retrieved 2018-02-08.
  14. Feick, Kathy. "Platinum | Earth Sciences Museum | University of Waterloo". University of Waterloo. Retrieved 6 February 2018.
  15. Xiao, Z.; Laplante, A. R. (2004). "Characterizing and recovering the platinum group minerals—a review". Minerals Engineering. 17 (9–10): 961–979. doi:10.1016/j.mineng.2004.04.001.
  16. "Platinum–Group Metals" (PDF). U.S. Geological Survey, Mineral Commodity Summaries. January 2007. Retrieved 2008-09-09.
  17. Emsley, J. (2003). "Iridium". Nature's Building Blocks: An A-Z Guide to the Elements. Oxford, England, UK: Oxford University Press. pp. 201–204. ISBN 0-19-850340-7.
  18. Chevalier, Patrick. "Platinum Group Metals" (PDF). Natural Resources Canada. Archived from the original (PDF) on 2011-08-11. Retrieved 2008-10-17.
  19. Sebastien, Rauch (November 2012). "Anthropogenic Platinum Enrichment in the Vicinity of Mines in the Bushveld Igneous Complex, South Africa". Retrieved 14 February 2020.
  20. 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. Retrieved 2009-10-02.
  21. 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. doi:10.1016/j.reactfunctpolym.2005.05.011
  22. R. J. Newman, F. J. Smith (1970). "Platinum Metals from Nuclear Fission – an evaluation of their possible use by the industry" (PDF). Platinum Metals Review. 14 (3): 88.
  23. Zdenek Kolarik, Edouard V. Renard (2003). "Recovery of Value Fission Platinoids from Spent Nuclear Fuel; PART I: general considerations and basic chemistry" (PDF). Platinum Metals Review. 47 (2): 74.
  24. Kolarik, Zdenek; Renard, Edouard V. (2005). "Potential Applications of Fission Platinoids in Industry" (PDF). Platinum Metals Review. 49 (2): 79. doi:10.1595/147106705X35263.
  25. Zdenek Kolarik, Edouard V. Renard (2003). "Recovery of Value Fission Platinoids from Spent Nuclear Fuel; PART II: Separation process" (PDF). Platinum Metals Review. 47 (3): 123.
  26. Gao, Bo; Yu, Yanke; Zhou, Huaidong; Lu, Jin (2012). "Accumulation and distribution characteristics of platinum group elements in roadside dusts in Beijing, China". Environmental Toxicology and Chemistry. 31 (6): 1231–1238. doi:10.1002/etc.1833. PMID 22505271.
  27. Khaiwal Ravindra,László Bencs,René Van Grieken (5 January 2004). "Platinum group elements in the environment and their health risk". Science of the Total Environment. 318 (1–3): 1–43. Bibcode:2004ScTEn.318....1R. doi:10.1016/S0048-9697(03)00372-3. hdl:2299/2030. PMID 14654273.CS1 maint: multiple names: authors list (link)
  28. Clare L.S. Wiseman, Fathi Zereini (2012). "Airborne particulate matter, platinum group elements and human health: A review of recent evidence". Science of the Total Environment. 407 (8): 2493–2500. doi:10.1016/j.scitotenv.2008.12.057. PMID 19181366.
  29. 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.
  30. 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. Retrieved 2009-10-02.
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