Rare-earth element

Rare earth ore

According to IUPAC, rare earth elements or rare earth metals are a collection of seventeen chemical elements in the periodic table, namely scandium, yttrium, and the fifteen lanthanoids. Scandium and yttrium are considered rare earths since they tend to occur in the same ore deposits as the lanthanoids and exhibit similar chemical properties.

The term "rare earth" arises from the minerals from which they were first isolated, which were uncommon oxide-type minerals (earths) found in Gadolinite extracted from one mine in the village of Ytterby, Sweden. However, with the exception of the highly-unstable promethium, rare earth elements are found in relatively high concentrations in the earth's crust, with cerium being the 25th most abundant element in the earth's crust at 68 parts per million.

Discovery and early history

Rare earth elements became known to the world with the discovery of the black mineral ytterbite (also known as gadolinite) by Lieutenant Carl Axel Arrhenius in the year 1787, in a quarry in the village of Ytterby, Sweden. Many of the rare earths are named for the scientists who discovered or elucidated the elemental properties, or for their geographical discovery, or for Latin or Greek references, or for mythical references:

Name	Etymology
Lanthanum	from the Greek "lanthanon," meaning I am hidden.
Cerium	for the Roman deity of fertility Ceres.
Praseodymium	from the Greek "praso," meaning leek-green, and "didymos," meaning twin.
Neodymium	from the Greek "neo," meaning new-one, and "didymos," meaning twin.
Promethium	for the Titan Prometheus, who brought fire to mortals.
Samarium	for Vasili Samarsky-Bykhovets, who discovered the rare earth ore samarskite.
Europium	for the continent of Europe.
Gadolinium	for Johan Gadolin (1760-1852), to honor his investigation of rare earths.
Terbium	for the village of Ytterby, Sweden, where the first rare earth ore was discovered.
Dysprosium	from the Greek "dysprositos," meaning hard to get.
Holmium	for Stockholm (in Latin, "Holmia"), native city of one of its discoverers.
Erbium	for the village of Ytterby, Sweden.
Thulium	for the mythological land of Thule.
Ytterbium	for the village of Ytterby, Sweden.
Lutetium	for Lutetia, the city which later became Paris.

The ytterbite, renamed to gadolinite in 1800, of Lt Arrhenius reached Johann Gadolin, a University 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 oxydes: pure ytteria, terbia and erbia (all the names are parts of the 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, lanthanium, didyium '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 technique of optical flame spectroscopy and found new spectral lines in didymia, and 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 samarskithe. They named the element gadolinium after Johan Gadolin, and the oxide was gadolinia.

Further spectroscopic analysis between 1886 and 1901 of samaria, ytteria and samarskithe by William Crookes, Lecoq de Boisbaudran and Eugène-Anatole Demarçay yielded new spectroscopic lines indicating an unknown element. The fractionate crystallization yielded europium in 1901.

In 1839 the third source for rare eaths became available, a mineral similar to gadolinite, uranotantalum (now samarskite). This mineral from Miass in the southern Ural Mountains was described by Gustave Rose. The russian chemist R. Harmann postulated the new element ilmenium must be present in the mineral, but later Christian Wilhelm Blomstrand, Jean-Charles Galissard de Marignac, and Heinrich Rose only found tantalum and niobium.

The exact number of rare earth elements was unclear and a maximum number of 25 was estimated. The use of x-ray spectra obtained by diffraction in crystals of Henry Moseley made it possible to determine the atomic numbers. The absolute number of Lanthanoides had to be 15, with a still missing element 61.

Using this technique Moseley proved that hafnium was not a rare earth element and that the claims of Georges Urbain of having discovered element 72 were false.

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.

Abbreviations

The following abbreviations are often used:

• RE = rare earth
• REM = rare earth metals
• REE = rare earth elements
• LREE = light rare earth elements (La-Sm)
• HREE = heavy rare earth elements (Eu-Lu)

Technological applications

Rare earth elements are incorporated into many modern technological devices, including superconductors, miniaturized magnets, electronic polishers, refining catalysts and hybrid car components. Rare earth ions are used as the active ions in luminescent materials used in optoelectronics applications, most notably the Nd:YAG laser. Phosphors with rare earth dopants are also widely used in cathode ray tube technology such as television sets.

Global rare earth production

Until 1948, most of the world's Rare Earths were sourced from placer sand deposits in India and Brazil. Through the 1950s, South Africa took the status as the world's Rare Earth source, after large Rare Earth bearing veins were discovered in Monazite. Today, those 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 95% of the world's Rare Earth supply.

The use of rare earth elements in modern technology has increased dramatically over the past years. For example, dysprosium has gained significant importance for its use in the construction of hybrid car motors. Unfortunately, this new demand has strained supply, and there is growing concern that the world may soon face a shortage of the materials. All of the world's heavy rare earths (such as dysprosium) are sourced from Chinese Rare Earth sources such as the polymetallic Bayan Obo deposit. High Rare Earth prices have wreaked havoc on many rural Chinese villages, as many illegal rare earth mines have been spewing toxic waste into the general water supply.

Chinese export quotas have also resulted in a dramatic shift in the world's Rare Earth knowledge base. For example, the division of General Motors which deals with miniaturized magnet research recently shut down its US office and moved all of its staff to China.

Geologic distribution

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 orebodies of the cerium earths are known around the world, and are being actively 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 of these 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 that contain 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, also 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 has never been nearly as abundant as the similarly recovered monazite (which typically contains a few percent of yttrium). Uranium ores processed in Ontario have occasionally yielded yttrium as a byproduct.

Well-known minerals that contain cerium and the light lanthanoids include bastnaesite, monazite, allanite, loparite, ancylite, parisite, lanthanite, chevkinite, cerite, stillwellite, britholite, fluocerite, and cerianite. Over the years, 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 lanthanoids.

A few sites are under development outside of China, the most significant of which are the Nolans Project in Central Australia, the remote Hoidas Lake project in northern Canada and the Mt. Weld project in Australia. 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.