Rare Earths

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Rare Earths elements or metals are the elements belonging to the Lanthanides group and also include Yttrium (39) and Scandium (21). Their atomic numbers range from 57-71 and all have similar proprieties. The Rare Earths are not rare in nature, in fact they are found in low concentrations through out the earths crust and in high concentrations in a number of minerals. Rare Earths are also found in meteorites on the moon and in the spectra of many stars. The spectra of these stars show that the Rare Earths are more abundant in these systems than in our own. In general, it has been found that the more basic(alkaline) rocks contain smaller amounts of Rare Earths than do the more acidic rocks. Most of the Rare Earth elements are "trivalents" (Capable of combining with three atoms of hydrogen or their equivalent). Cerium is the 25th most abundant element in the earths crust at 60 parts per million, and Lutetium is the least abundant of the Rare Earths at 0.5 parts per million. The top three producers of Rare Earths are respectively China, India and the United States of America.

History

Rare earths were discovered in the 1780's by an Swedish Army Lieutenant named Karl Axel Arrhenius. He collected Ytterbite (Gadolinite) from a Quartz mine in Sweden. It was not until 1794 that the first element, an impure Yttrium oxide, was isolated from Ytterbite by Finnish Chemist Johann Gadolin
(taken from: Weeks and Leicester, 1968)

Proprieties

The Rare Earth metals are mostly bright and silvery in colour. They are very sensitive to the presence of impurities. Light Lanthanide metals will corrode much more rapidly if Calcium, Magnesium, and/or other Rare Earths are present in the metal. The melting points and transition temperatures of Rare Earths change drastically (by several hundred degrees) when alloyed with other elements. Most Rare Earths are malleable and will oxidize at room temperature.

Lanthanum, Cerium, Praseodymium, Neodymium corrode readily in air. Bars of these metals will become incrusted with a thick layer of oxide in several weeks. Europium will tarnish almost immediately when exposed to air and will convert to a powdered oxide in a couple of days. Metallic Yttrium, Gadolinium, and Lutetium will remain bright and shiny for years. Pure Cerium is likely to ignite if scratched with a knife.

When testing for Rare Earths, they will generally contain Neodymium which can be recognized by the spectrographic test. Handling Rare Earths ore or minerals, should be done in an environment where dust is at an absolute minimum. Finely divided Rare Earth metals can ignite spontaneously, somewhat like Magnesium does. Most Rare Earths are reactive at high temperatures. The melting points for rare Earths range from 798 °C for Cerium up to 1,663 °C for Lutetium.

Usage

The Rare Earths are used primarily in the television manufacturing industry. They are used in the fabrication of television tubes to generate the red, blue and green colours. Rare Earths are also used as permanent magnets. Alloys of Cobalt with Rare Earths produce more superior magnets then the conventional magnets. Rare Earths are also used as catalysts for:

the cracking of crude petroleum,
organic reactors,
hydrogenation of key tones to form secondary alcohols,
hydrogenation of olefine to form alkanes,
dehydrogenation of alcohols and butanes, and
the formation of polyesters.

Lanthanum is used in the manufacturing of glass, battery electrodes and catalytic lenses. It is also used in the medical field in testing cancer treatments.

Minerals

The Rare Earths can be found in a couple of minerals such as: Bastnaesite, Monazite, Allanite, Loparite, and Ytterbite(Gadolinite). Bastnaesite has a hexagonal crystal system (figure 4) and is a wax yellow to light reddish-brown colour. Monazite on the other hand has more of a Monoclinic crystal system and has a more yellowish brown to dark brown colour. It occurs as crystals (figure 5) or as an accessorial mineral in Granite, Syenite and Gneiss. Allanite has the same crystal system as Monazite but is usually brown or black in colour (figure 6). The mineral Allanite can sometimes be radioactive and is quite easy to ignite. It is also soluble in Hydrochloric acid.

Some interesting Rare Earth related links

http://minerals.usgs.gov/minerals/pubs/commodity/rare_earths/
http://www.britannica.com/bcom/eb/article/3/0,5616,119793+1+110615.html
http://www.geology.neab.net/