Rare earth element

Rare earth elements and rare earth metals are, according to IUPAC, the 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 share similar chemical properties with them.

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 Karl Arrhenius in the year 1787, in a quarry in the village of Ytterby, Sweden. Many of the rare earths are named in honor of the scientists who discovered or elucidated the elemental properties, geographical discovery, Latin or Greek, or mythology:

Name	Etymology
Lanthanum	from the Greek "lanthanon" meaning I am hidden.
Cerium	after Roman deity of fertility, Ceres.
Praseodymium	from the Greek "praso" which means leek-green.
Neodymium	from a Greek word "neo" which means new-one.
Promethium	after Prometheus who brought fire to mortals.
Samarium	Vasili Samarsky-Bykhovets discovered the rare-earth ore called samarskite.
Europium	Demarcay is generally credited with the discovery, named in honor of Europe.
Gadolinium	after Johan Gadolin (1760-1852) to honor his investigation of rare earths.
Terbium	after Ytterby, a village in Sweden, Discovered by Mosander in 1843.
Dysprosium	from the Greek "dysprositos" meaning hard to get.
Holmium	(L. Holmia: Stockholm). Discovered in1878 by Delafontaine and Soret. Cleve, of Sweden, later independently discovered the element.The element is named after Cleve's native city. Holmia,
Erbium	after Ytterby, a town in Sweden.
Thulium	after the mythological land of Thule.
Ytterbium	named after the village of Ytterby, Sweden, where the first rare earth ore was discovered.
Lutetium	Lutetia, ancient name for Paris.

The term "Rare Earth" arises from the minerals from which they were isolated, which were uncommon oxide-type minerals (earths), only found in Gadolinite from one mine in the village of Ytterby, Sweden, However, with the exception of the highly-unstable promethium, the 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.

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 very 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 50's and early 60's.

Abbreviations

The following abbreviations are often used:

• 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 laser. Phosphors with rare earth dopants are also widely used in cathode ray tube technology such as television sets.

Global rare earth production

Up until 1948, most of the world's Rare Earths were sourced from placer sand deposits in India and Brazil. Through the 50's, South Africa then 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, however 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.

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.

References