As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium.[2] Scandium and yttrium are considered rare earth elements since they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties.
Despite their name, rare earth elements (with the exception of the radioactive promethium) are relatively plentiful in the Earth's crust, with cerium being the 25th most abundant element at 68 parts per million (similar to copper). However, because of their geochemical properties, rare earth elements are typically dispersed and not often found in concentrated and economically exploitable forms. The few economically exploitable deposits are known as rare earth minerals.[3] It was the very scarcity of these minerals (previously called "earths") that led to the term "rare earth". The first such mineral discovered was gadolinite, a compound of cerium, yttrium, iron, silicon and other elements. This mineral was extracted from a mine in the village of Ytterby in Sweden; many of the rare earth elements bear names derived from this location.
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A table listing the seventeen rare earth elements, their atomic number and symbol, the etymology of their names, and their main usages (see also Lanthanide#Technological_applications) is provided here. Some of the rare earths are named for the scientists who discovered or elucidated their elemental properties, and some for their geographical discovery.
| Z | Symbol | Name | Etymology | Selected applications |
|---|---|---|---|---|
| 21 | Sc | Scandium | from Latin Scandia (Scandinavia), where the first rare earth ore was discovered. | Light aluminium-scandium alloy for aerospace components, additive in Mercury-vapor lamps.[4] |
| 39 | Y | Yttrium | for the village of Ytterby, Sweden, where the first rare earth ore was discovered. | Yttrium-aluminum garnet (YAG) laser, yttrium vanadate (YVO4) as host for europium in TV red phosphor YBCO high-temperature superconductors, yttrium iron garnet (YIG) microwave filters.[4] |
| 57 | La | Lanthanum | from the Greek "lanthanein", meaning to be hidden. | High refractive index glass, flint, hydrogen storage, battery-electrodes, camera lenses, fluid catalytic cracking catalyst for oil refineries |
| 58 | Ce | Cerium | for the dwarf planet Ceres. | Chemical oxidizing agent, polishing powder, yellow colors in glass and ceramics, catalyst for self-cleaning ovens, fluid catalytic cracking catalyst for oil refineries, ferrocerium flints for lighters |
| 59 | Pr | Praseodymium | from the Greek "prasios", meaning leek-green, and "didymos", meaning twin. | Rare-earth magnets, lasers, core material for carbon arc lighting, colorant in glasses and enamels, additive in didymium glass used in welding goggles,[4] ferrocerium firesteel (flint) products. |
| 60 | Nd | Neodymium | from the Greek "neos", meaning new, and "didymos", meaning twin. | Rare-earth magnets, lasers, violet colors in glass and ceramics, ceramic capacitors |
| 61 | Pm | Promethium | for the Titan Prometheus, who brought fire to mortals. | Nuclear batteries |
| 62 | Sm | Samarium | for Vasili Samarsky-Bykhovets, who discovered the rare earth ore samarskite. | Rare-earth magnets, lasers, neutron capture, masers |
| 63 | Eu | Europium | for the continent of Europe. | Red and blue phosphors, lasers, mercury-vapor lamps, NMR relaxation agent |
| 64 | Gd | Gadolinium | for Johan Gadolin (1760–1852), to honor his investigation of rare earths. | Rare-earth magnets, high refractive index glass or garnets, lasers, X-ray tubes, computer memories, neutron capture, MRI contrast agent, NMR relaxation agent |
| 65 | Tb | Terbium | for the village of Ytterby, Sweden. | Green phosphors, lasers, fluorescent lamps |
| 66 | Dy | Dysprosium | from the Greek "dysprositos", meaning hard to get. | Rare-earth magnets, lasers |
| 67 | Ho | Holmium | for Stockholm (in Latin, "Holmia"), native city of one of its discoverers. | Lasers |
| 68 | Er | Erbium | for the village of Ytterby, Sweden. | Lasers, vanadium steel |
| 69 | Tm | Thulium | for the mythological northern land of Thule. | Portable X-ray machines |
| 70 | Yb | Ytterbium | for the village of Ytterby, Sweden. | Infrared lasers, chemical reducing agent |
| 71 | Lu | Lutetium | for Lutetia, the city which later became Paris. | PET Scan detectors, high refractive index glass |
The following abbreviations are often used:
Rare earth elements became known to the world with the discovery of the black mineral "ytterbite" (renamed to gadolinite in 1800) by Lieutenant Carl Axel Arrhenius in 1787, at a quarry in the village of Ytterby, Sweden.[5]
Arrhenius' "ytterbite" reached Johan Gadolin, a Royal Academy of Turku professor, and his analysis yielded an unknown oxide (earth) which he called Ytteria. Anders Gustav Ekeberg isolated beryllium from the gadolinite but failed to recognize other elements which the ore contained. After this discovery in 1794 a mineral from Bastnäs near Riddarhyttan, Sweden, which was believed to be an iron-tungsten mineral, was re-examined by Jöns Jacob Berzelius and Wilhelm Hisinger. In 1803 they obtained a white oxide and called it ceria. Martin Heinrich Klaproth independently discovered the same oxide and called it ochroia.
Thus by 1803 there were two known rare earth elements, yttrium and cerium, although it took another 30 years for researchers to determine that other elements were contained in the two ores ceria and ytteria (the similarity of the rare earth metals' chemical properties made their separation difficult).
In 1839 Carl Gustav Mosander, an assistant of Berzelius, separated ceria by heating the nitrate and dissolving the product in nitric acid. He called the oxide of the soluble salt lanthana. It took him three more years to separate the lanthana further into didymia and pure lanthana. Didymia, although not further separable by Mosander's techniques was a mixture of oxides.
In 1842 Mosander also separated the ytteria into three oxides: pure ytteria, terbia and erbia (all the names are derived from the town name "Ytterby"). The earth giving pink salts he called terbium; the one which yielded yellow peroxide he called erbium.
So in 1842 the number of rare earth elements had reached six: yttrium, cerium, lanthanum, didymium, erbium and terbium.
Nils Johan Berlin and Marc Delafontaine tried also to separate the crude ytteria and found the same substances that Mosander obtained, but Berlin named (1860) the substance giving pink salts erbium and Delafontaine named the substance with the yellow peroxide terbium. This confusion led to several false claims of new elements, such as the mosandrium of J. Lawrence Smith, or the philippium and decipium of Delafontaine.
There were no further discoveries for 30 years, and the element didymium was listed in the periodic table of elements with a molecular mass of 138. In 1879 Delafontaine used the new physical process of optical-flame spectroscopy, and he found several new spectral lines in didymia. Also in 1879, the new element samarium was isolated by Paul Émile Lecoq de Boisbaudran from the mineral samarskite.
The samaria earth was further separated by Lecoq de Boisbaudran in 1886 and a similar result was obtained by Jean Charles Galissard de Marignac by direct isolation from samarskite. They named the element gadolinium after Johan Gadolin, and its oxide was named "gadolinia".
Further spectroscopic analysis between 1886 and 1901 of samaria, ytteria, and samarskite by William Crookes, Lecoq de Boisbaudran and Eugène-Anatole Demarçay yielded several new spectroscopic lines that indicated the existence of an unknown element. The fractional crystallization of the oxides then yielded europium in 1901.
In 1839 the third source for rare earths became available. This is a mineral similar to gadolinite, uranotantalum (now called "samarskite"). This mineral from Miass in the southern Ural Mountains was documented by Gustave Rose. The Russian chemist R. Harmann proposed that a new element he called "ilmenium" should be present in this mineral, but later, Christian Wilhelm Blomstrand, Galissard de Marignac, and Heinrich Rose found only tantalum and niobium (columbium) in it.
The exact number of rare earth elements that existed was highly unclear, and a maximum number of 25 was estimated. The use of X-ray spectra (obtained by X-ray crystallography) by Henry Gwyn Jeffreys Moseley made it possible to assign atomic numbers to the elements. Moseley found that the exact number of lanthanides had to be 15 and that element 61 had yet to be discovered.
Using these facts about atomic numbers from X-ray crystallography, Moseley also showed that hafnium (element 72) would not be a rare earth element. Moseley was killed in World War I in 1915, years before hafnium was discovered. Hence, the claim of Georges Urbain that he had discovered element 72 was untrue. Hafnium is an element that lies in the periodic table immediately below zirconium, and hafnium and zirconium are very similar in their chemical and physical properties.
During the 1940s, Frank Spedding and others in the United States (during the Manhattan Project) developed the chemical ion exchange procedures for separating and purifying the rare earth elements. This method was first applied to the actinides for separating plutonium-239 and neptunium, from uranium, thorium, actinium, and the other actinide rare earths in the materials produced in nuclear reactors. The plutonium-239 was very desirable because it is a fissile material.
The principal sources of rare earth elements are the minerals bastnäsite, monazite, and loparite and the lateritic ion-adsorption clays. Despite their high relative abundance, rare earth minerals are more difficult to mine and extract than equivalent sources of transition metals (due in part to their similar chemical properties), making the rare earth elements relatively expensive. Their industrial use was very limited until efficient separation techniques were developed, such as ion exchange, fractional crystallization and liquid-liquid extraction during the late 1950s and early 1960s.[6]
Rare earth elements are heavier than iron and thus are produced by supernova nucleosynthesis or the s-process in asymptotic giant branch stars. In nature, spontaneous fission of uranium-238 produces trace amounts of radioactive promethium, but most promethium is synthetically produced in nuclear reactors.
Rare earth cerium is actually the 25th most abundant element in the Earth's crust, having 68 parts per million (about as common as copper). Only the highly unstable and radioactive promethium "rare earth" is quite scarce.
The rare earth elements are often found together. The longest-lived isotope of promethium has a half life of 17.7 years, so the element only exists in nature in negligible amounts (approximately 572 g in the entire Earth's crust).[7] Promethium is one of the two elements that do not have stable (non-radioactive) isotopes and are followed by (i.e. with higher atomic number) stable elements.
Due to lanthanide contraction, yttrium, which is trivalent, is of similar ionic size to dysprosium and its lanthanide neighbors. Due to the relatively gradual decrease in ionic size with increasing atomic number, the rare earth elements have always been difficult to separate. Even with eons of geological time, geochemical separation of the lanthanides has only rarely progressed much farther than a broad separation between light versus heavy lanthanides, otherwise known as the cerium and yttrium earths. This geochemical divide is reflected in the first two rare earths that were discovered, yttria in 1794 and ceria in 1803. As originally found, each comprised the entire mixture of the associated earths. Rare earth minerals, as found, usually are dominated by one group or the other, depending upon which size-range best fits the structural lattice. Thus, among the anhydrous rare earth phosphates, it is the tetragonal mineral xenotime that incorporates yttrium and the yttrium earths, whereas the monoclinic monazite phase incorporates cerium and the cerium earths preferentially. The smaller size of the yttrium group allows it a greater solid solubility in the rock-forming minerals that comprise the Earth's mantle, and thus yttrium and the yttrium earths show less enrichment in the Earth's crust relative to chondritic abundance, than does cerium and the cerium earths. This has economic consequences: large ore bodies of the cerium earths are known around the world, and are being exploited. Corresponding orebodies for yttrium tend to be rarer, smaller, and less concentrated. Most of the current supply of yttrium originates in the "ion adsorption clay" ores of Southern China. Some versions provide concentrates containing about 65% yttrium oxide, with the heavy lanthanides being present in ratios reflecting the Oddo-Harkins rule: even-numbered heavy lanthanides at abundances of about 5% each, and odd-numbered lanthanides at abundances of about 1% each. Similar compositions are found in xenotime or gadolinite.
Well-known minerals containing yttrium include gadolinite, xenotime, samarskite, euxenite, fergusonite, yttrotantalite, yttrotungstite, yttrofluorite (a variety of fluorite), thalenite, yttrialite. Small amounts occur in zircon, which derives its typical yellow fluorescence from some of the accompanying heavy lanthanides. The zirconium mineral eudialyte, such as is found in southern Greenland, contains small but potentially useful amounts of yttrium. Of the above yttrium minerals, most played a part in providing research quantities of lanthanides during the discovery days. Xenotime is occasionally recovered as a byproduct of heavy sand processing, but is not as abundant as the similarly recovered monazite (which typically contains a few percent of yttrium). Uranium ores from Ontario have occasionally yielded yttrium as a byproduct.
Well-known minerals containing cerium and the light lanthanides include bastnaesite, monazite, allanite, loparite, ancylite, parisite, lanthanite, chevkinite, cerite, stillwellite, britholite, fluocerite, and cerianite. Monazite (marine sands from Brazil, India, or Australia; rock from South Africa), bastnaesite (from Mountain Pass, California, or several localities in China), and loparite (Kola Peninsula, Russia) have been the principal ores of cerium and the light lanthanides.
In 2011, Yasuhiro Kato, a geologist at the University of Tokyo who led a study of Pacific Ocean seabed mud, published results indicating the mud could hold rich concentrations of rare earth minerals. The deposits, studied at 78 sites, came from "[h]ot plumes from hydrothermal vents pull[ing] these materials out of seawater and deposit[ing] them on the seafloor, bit by bit, over tens of millions of years. One square patch of metal-rich mud 2.3 kilometers wide might contain enough rare earths to meet most of the global demand for a year, Japanese geologists report July 3 in Nature Geoscience." "I believe that rare earth resources undersea are much more promising than on-land resources," said Kato. "[C]oncentrations of rare earths were comparable to those found in clays mined in China. Some deposits contained twice as much heavy rare earths such as dysprosium, a component of magnets in hybrid car motors."[8]
Until 1948, most of the world's rare earths were sourced from placer sand deposits in India and Brazil.[9] Through the 1950s, South Africa took the status as the world's rare earth source, after large veins of rare earth bearing monazite were discovered there.[9] Through the 1960s until the 1980s, the Mountain Pass rare earth mine in California was the leading producer. Today, the Indian and South African deposits still produce some rare earth concentrates, but they are dwarfed by the scale of Chinese production. China now produces over 97% of the world's rare earth supply, mostly in Inner Mongolia,[10][11] even though it has only 37% of proven reserves.[12] All of the world's heavy rare earths (such as dysprosium) come from Chinese rare earth sources such as the polymetallic Bayan Obo deposit.[11][13] In 2010, the USGS released a study which found that the United States had 13 million metric tons of rare earth elements.[14]
New demand has recently strained supply, and there is growing concern that the world may soon face a shortage of the rare earths.[15] In several years from 2009 worldwide demand for rare earth elements is expected to exceed supply by 40,000 tonnes annually unless major new sources are developed.[16]
These concerns have intensified due to the actions of China, the predominant supplier. Specifically, China has announced regulations on exports and a crackdown on smuggling.[17] On September 1, 2009, China announced plans to reduce its export quota to 35,000 tons per year in 2010–2015, ostensibly to conserve scarce resources and protect the environment.[18] On October 19, 2010 China Daily, citing an unnamed Ministry of Commerce official, reported that China will "further reduce quotas for rare earth exports by 30 percent at most next year to protect the precious metals from over-exploitation".[19] At the end of 2010 China announced that the first round of export quotas in 2011 for rare earths would be 14,446 tons which was a 35% decrease from the previous first round of quotas in 2010.[20] China announced further export quotas on 14 July 2011 for the second half of the year with total allocation at 30,184 tons with total production capped at 93,800 tonnes.[21] In September 2011 China announced the halt in production of three of its eight major rare earth mines, responsible for almost 40% of China's total rare earth production.[22]
As a result of the increased demand and tightening restrictions on exports of the metals from China, some countries are stockpiling rare earth resources.[23] Searches for alternative sources in Australia, Brazil, Canada, South Africa, Greenland, and the United States are ongoing.[24] Mines in these countries were closed when China undercut world prices in the 1990s, and it will take a few years to restart production as there are many barriers to entry.[17] One example is the Mountain Pass mine in California, which is projected to reopen in 2011.[11][25] Other significant sites under development outside of China include the Nolans Project in Central Australia, the remote Hoidas Lake project in northern Canada,[26] and the Mount Weld project in Australia.[11][25][27] The Hoidas Lake project has the potential to supply about 10% of the $1 billion of REE consumption that occurs in North America every year.[28] Vietnam signed an agreement in October 2010 to supply Japan with rare earths[29] from its northwestern Lai Châu Province.[30]
Also under consideration for mining are sites such as Thor Lake in the Northwest Territories, various locations in Vietnam,[11][16] and a site in southeast Nebraska in the US, where Quantum Rare Earth Development, a Canadian company, is currently conducting test drilling and economic feasibility studies toward opening a niobium mine.[31] Additionally, a large deposit of rare earth minerals was recently discovered in Kvanefjeld in southern Greenland.[32] Pre-feasibility drilling at this site has confirmed significant quantities of black lujavrite, which contains about 1% rare earth oxides (REO).[33]
In early 2011, Australian mining company, Lynas, was reported to be "hurrying to finish" a US$230 million rare earth refinery on the eastern coast of Malaysia's industrial port of Kuantan. The plant would refine "slightly radioactive" ore - Lanthanide concentrate from the Mount Weld mine in Australia. The ore would be trucked to Fremantle and transported by container ship to Kuantan. However, the Malaysian authorities confirmed that as of October 2011, Lynas was not given any permit to import any rare earth ore into Malaysia. Within two years, Lynas was said to expect the refinery to be able to meet nearly a third of the world's demand for rare earth materials, not counting China."[34] The Kuantan development brought renewed attention to the Malaysian town of Bukit Merah in Perak, where a rare-earth mine operated by a Mitsubishi Chemical subsidiary, Asian Rare Earth, closed in 1992 and left continuing environmental and health concerns.[35] In mid-2011, after protests, Malaysian government restrictions on the Lynas plant were announced. At that time, citing subscription-only Dow Jones Newswire reports, a Barrons report said the Lynas investment was $730 million, and the projected share of the global market it would fill put at "about a sixth."[36] An independent review was initiated by Malaysian Government and UN and conducted by IAEA between 29 May and 3 June 2011 to address concerns of radioactive hazards. The IAEA team was not able to identify any non-compliance with international radiation safety standards.[37]
Significant quantities of rare earth oxides are found in tailings accumulated from 50 years of uranium ore, shale and loparite mining at Sillamäe, Estonia.[38] Due to the rising prices of rare earths, extraction of these oxides has become economically viable. The country currently exports around 3,000 tonnes per year, representing around 2% of world production.[39]
Nuclear reprocessing is another potential source of rare earth or any other elements. Nuclear fission of uranium or plutonium produces a full range of elements, including all their isotopes. However, due to the radioactivity of many of these isotopes, it is unlikely that extracting them from the mixture can be done safely and economically.
Another recently developed source of rare earths is electronic waste and other wastes that have significant rare earth components. New advances in recycling technology have made extraction of rare earths from these materials more feasible, and recycling plants are currently operating in Japan, where there is an estimated 300,000 tons of rare earths stored in unused electronics.[40]. In France, the Rhodia group is setting up two factories, in La Rochelle and Saint-Fons, that will produce 200 tons a year of rare earths from used Fluorescent lamps, magnets and batteries. [41] [42]
Mining, refining, and recycling of rare earths have serious environmental consequences if not properly managed. A particular hazard is mildly radioactive slurry tailings resulting from the common occurrence of thorium and uranium in rare earth element ores.[43] Additionally, toxic acids are required during the refining process.[12] Improper handling of these substances can result in extensive environmental damage. In May 2010, China announced a major, five-month crackdown on illegal mining in order to protect the environment and its resources. This campaign is expected to be concentrated in the South,[44] where mines – commonly small, rural, and illegal operations – are particularly prone to releasing toxic wastes into the general water supply.[11][45] However, even the major operation in Baotou, in Inner Mongolia, where much of the world's rare earth supply is refined, has caused major environmental damage.[12]
The Bukit Merah mine in Malaysia has been the focus of a US$100 million cleanup which is proceeding in 2011. "Residents blamed a rare earth refinery for birth defects and eight leukemia cases within five years in a community of 11,000 — after many years with no leukemia cases." Seven of the leukemia victims died. After having accomplished the hilltop entombment of 11,000 truckloads of radioactively contaminated material, the project is expected to entail in summer, 2011, the removal of "more than 80,000 steel barrels of radioactive waste to the hilltop repository." One of Mitsubishi's contractors for the cleanup is GeoSyntec, an Atlanta-based firm.[35] Osamu Shimizu, a director of Asian Rare Earth, "said the company might have sold a few bags of calcium phosphate fertilizer on a trial basis as it sought to market byproducts" in reply to a former resident of Bukit Merah who said, "The cows that ate the grass [grown with the fertilizer] all died."[46]
In May 2011, after the Fukushima Daiichi nuclear disaster, widespread protests took place in Kuantan over the Lynas refinery and radioactive waste from it. The ore to be processed has very low levels of thorium, and Lynas founder and chief executive Nicholas Curtis said "There is absolutely no risk to public health." T. Jayabalan, a doctor who says he has been monitoring and treating patients affected by the Mitsubishi plant, "is wary of Lynas's assurances. The argument that low levels of thorium in the ore make it safer doesn't make sense, he says, because radiation exposure is cumulative."[46] Construction of the facility has been halted until an independent United Nations IAEA panel investigation is completed, which is expected by the end of June 2011.[47] New restrictions were announced by the Malaysian government in late June.[36]
IAEA panel investigation is completed and no construction has been halted. Lynas is on budget and on schedule to start producing 2011. The IAEA report has concluded in a report issued by the nuclear watchdog Thursday June 2011 said it did not find any instance of "any non-compliance with international radiation safety standards" in the project. [48]
China has officially cited resource depletion and environmental concerns as the reasons for a nationwide crackdown on its rare earth mineral production sector.[22] However, non-environmental motives have also been imputed to China's rare earth policy.[12] According to The Economist, "Slashing their exports of rare-earth metals...is all about moving Chinese manufacturers up the supply chain, so they can sell valuable finished goods to the world rather than lowly raw materials."[49] One possible example is the division of General Motors which deals with miniaturized magnet research, which shut down its US office and moved its entire staff to China in 2006.[50] (It should be noted that China's export quota only applies to the metal but not products made from these metals such as magnets.
It was reported,[51] but officially denied,[52] that China instituted an export ban on shipments of rare earth oxides (but not alloys) to Japan on 22 September 2010, in response to the detainment of a Chinese fishing boat captain by the Japanese Coast Guard.[53] On September 2, 2010, a few days before the fishing boat incident, The Economist reported that "China...in July announced the latest in a series of annual export reductions, this time by 40% to precisely 30,258 tonnes."[54]
The United States Department of Energy in its 2010 Critical Materials Strategy report identified dysprosium as the element that was most critical in terms of import reliance.[55]
A 2011 report issued by the U.S. Geological Survey and U.S. Department of the Interior, “China’s Rare-Earth Industry," outlines industry trends within China and examines national policies that may guide the future of the country's production. The report notes that China’s lead in the production of rare-earth minerals has accelerated over the past two decades. In 1990, China accounted for only 27% of such minerals. In 2009, world production was 132,000 metric tons; China produced 129,000 of those tons. According to the report, recent patterns suggest that China will slow the export of such materials to the world: “Owing to the increase in domestic demand, the Government has gradually reduced the export quota during the past several years.” In 2006, China allowed 47 domestic rare-earth producers and traders and 12 Sino-foreign rare-earth producers to export. Controls have since tightened annually; by 2011, only 22 domestic rare-earth producers and traders and 9 Sino-foreign rare-earth producers were authorized. The government’s future policies will likely keep in place strict controls: “According to China’s draft rare-earth development plan, annual rare-earth production may be limited to between 130,000 and 140,000 [metric tons] during the period from 2009 to 2015. The export quota for rare-earth products may be about 35,000 [metric tons] and the Government may allow 20 domestic rare-earth producers and traders to export rare earths.” [56]
The United States Geological Survey is actively surveying southern Afghanistan for rare earth deposits under the protection of United States military forces. Since 2009 the USGS has conducted remote sensing surveys as well as fieldwork to verify Soviet claims that volcanic rocks containing rare earth metals exist in Helmand province near the village of Khanneshin. The USGS study team has located a sizable area of rocks in the center of an extinct volcano containing light rare earth elements including cerium and neodymium. It has mapped 1.3 million metric tons of desirable rock, or about 10 years of supply at current demand levels. The Pentagon has estimated its value at about $7.4 billion. [57]
Rare earth elements are not exchange-traded in the same way that precious (for instance, gold and silver) or non-ferrous metals (such as nickel, tin, copper, and aluminum) are. Instead they are sold on the private market, which makes their prices difficult to monitor and track. However, prices are published periodically on websites such as mineralprices.com.[58] The 17 elements are not usually sold in their pure form, but instead are distributed in mixtures of varying purity, e.g. "Neodymium metal ≥ 99.5%".[58] As such, pricing can vary based on the quantity and quality required by the end user's application.
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