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Saturday, December 8, 2012

Logam Nadir Bumi - Rare earth element

As defined by IUPAC, rare earth elements ("REEs") 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.

Rare earth ore, shown with a United States penny for size comparison

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 concentrated as rare earth minerals in economically exploitable ore deposits.[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; several of the rare earth elements bear names derived from this location.

Discovery and early history
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.[8]

Arrhenius' "ytterbite" reached Johan Gadolin, a Royal Academy of Turku professor, and his analysis yielded an unknown oxide (earth) which he called yttria. 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 yttria (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 yttria into three oxides: pure yttria, 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 yttria 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.

Global rare earth production
Until 1948, most of the world's rare earths were sourced from placer sand deposits in India and Brazil.[13] 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.[13] 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 had produced over 95% of the world's rare earth supply, mostly in Inner Mongolia,[3][14] even though it had only 37% of proven reserves,[15] although these numbers have since been reported to have slipped to 90% and 23%, respectively, by 2012.[16] All of the world's heavy rare earths (such as dysprosium) come from Chinese rare earth sources such as the polymetallic Bayan Obo deposit.[14][17] In 2010, the United States Geological Survey (USGS) released a study which found that the United States had 13 million metric tons of rare earth elements.[18]

New demand has recently strained supply, and there is growing concern that the world may soon face a shortage of the rare earths.[19] 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.

Environmental considerations
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.[52] Additionally, toxic acids are required during the refining process.[15] 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,[53] where mines – commonly small, rural, and illegal operations – are particularly prone to releasing toxic wastes into the general water supply.[14][54] 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.[15]

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.[42] 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."[55]

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."[55] 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.[56] New restrictions were announced by the Malaysian government in late June.[43]

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 on Thursday June 2011 said it did not find any instance of "any non-compliance with international radiation safety standards" in the project

Promethium (one of element of rare earth)
Promethium, originally prometheum, is a chemical element with the symbol Pm and atomic number 61. All of its isotopes are radioactive; it is one of only two such elements that are followed in the periodic table by elements with stable forms, a distinction shared with technetium. Chemically, promethium is a lanthanide, which forms salts when combined with other elements. Promethium shows only one stable oxidation state of +3; however, a few +2 compounds may exist.




source : http://en.wikipedia.org/wiki/Rare_earth_element
              http://en.wikipedia.org/wiki/Promethium
ber IKLAN PERCUMA DI 
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