Bastnäsite

Bastnäsite

[bast-nuh-sahyt]

The mineral bastnäsite is one of a family of three carbonate-fluoride minerals. There is bastnäsite-(Ce) with a formula of (Ce, La)CO3F. There is bastnäsite-(La) with a formula of (La, Ce)CO3F. There is also bastnäsite-(Y) with a formula of (Y, Ce)CO3F. Most bastnäsite is bastnäsite-(Ce), and cerium is by far the most common of the rare earths in this class of minerals. Bastnäsite and the phosphate mineral monazite are the two largest sources of cerium and other rare earth elements.

Bastnäsite was first described in 1841 from and named for the Bastnäs Mine in the Riddarhyttan district, Västermanland, Sweden. Bastnäsite also occurs as very high quality specimens at the Zagi Mountains, Pakistan. Bastnäsite occurs in alkali granite and syenite and in associated pegmatites. It also occurs in carbonatites and in associated fenites and other metasomatites.

Mining history

In 1949, the huge carbonatite-hosted bastnasite deposit was discovered at Mountain Pass, San Bernardino County, California. This discovery alerted geologists as the existence of a whole new class of rare earth deposit: the rare earth containing carbonatite. Other examples were soon recognized, particularly in Africa and China. The exploitation of this deposit began in the mid-1960s after it had been purchased by Molycorp (Molybdenum Corporation of America). The lanthanide composition of the ore included 0.1% europium oxide, which was sorely needed by the burgeoning color television industry, to provide the red phosphor, so as to maximize picture brightness. The composition of the lanthanides was about 49% cerium, 33% lanthanum, 12% neodymium, and 5% praseodymium, with some samarium and gadolinium, or distinctly more lanthanum and less neodymium and heavies as compared to commercial monazite. However, the europium content was at least double that of a typical monazite. Mountain Pass bastnasite was the world's major source of lanthanides from the 1960s to the 1980s. Thereafter, China became increasingly important to world rare earth supply. Chinese deposits of bastnasite include several in Sichuan Province, and the massive deposit at Bayan Obo, Inner Mongolia, which had been discovered early in the 20th century, but not exploited until much later. Bayan Obo is currently (2008) providing the lion's share of the world's lanthanides. Bayan Obo bastnaesite occurs in association with monazite (plus enough magnetite to sustain one of the largest steel mills in China), and unlike carbonatite bastnaesites, is relatively closer to monazite lanthanide compositions, with the exception of its generous 0.2% content of europium.

Ore technology

At Mountain Pass, bastnaesite ore was finely ground, and subjected to flotation to separate the bulk of the bastnaesite from the accompanying barite, calcite and dolomite. Marketable products include each of the major intermediates of the ore dressing process: flotation concentrate, acid-washed flotation concentrate, calcined acid washed bastnasite, and finally a cerium concentrate, which was the insoluble residue left after the calcined bastnasite had been leached with hydrochloric acid. The lanthanides that dissolved as a result of the acid treatment were subjected to solvent extraction, to capture the europium, and purify the other individual components of the ore. A further product included a lanthanide mix, depleted of much of the cerium, and essentially all of samarium and heavier lanthanides. The calcination of bastnaesite had driven off the carbon dioxide content, leaving an oxide-fluoride, in which the cerium content had become oxidized to the less basic quadrivalent state. However, the high temperature of the calcination gave less-reactive oxide, and the use of hydrochloric acid, which can cause reduction of quadrivalent cerium, led to an incomplete separation of cerium and the trivalent lanthanides. By contrast, in China, processing of bastnaesite, after concentration, starts with heating with sulfuric acid.

References

  • C.K. Gupta, N. Krishnamurthy, Extractive Metallurgy of Rare Earths, CRC Press, 2005, ISBN 0-415-33340-7
  • B.T. Kilbourn, A Lanthanide Lanthology, Part I, A-L; Part II, M-Z, Molycorp, Inc. Reprinted 1997
  • B.T. Kilbourn, Cerium, A Guide to its Role in Chemical Technology, Molycorp, Inc. Reprinted 1995.

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