Extraction and Uses of the Lanthanide Elements

World production of rare earth mineral was 80,000 tonnes in 1992, containing 47,900 tonnes of lanthanide oxides Ln2O3. The main sources of minerals are the USA 27%, China 20%, Australia 8% and India 6%. These minerals contain a substantial percentage of the lanthanide elements.

1. Monazite sand is the most important and most widespread mineral.

Before 1960 monazite was the only source of lanthanides. It is a mixture containing mostly La phosphate and trivalent phosphates of the lighter lanthanide elements {Ce, Pr and Nd). In addition, it contains smaller amounts of Y and heavier lanthanides, and thorium phosphate. This is weakly radioactive and traces its daughter product Ra are also present, and are more radioactive.

2. Bastnaesite is a mixed flourocarbonate M^IIICO₃F where M is La or the lanthanides. Large amounts are mined in the USA, and it provides 22% of the total supply of lanthanides. It is only found in the USA and Madagascar.

3. Very small amounts of another mineral, xenotime, are also mined.

Monazite is treated with hot concentrated H₂SO₄. The La and the lanthanides dissolve as sulphates, and are separated from insoluble material. This is precipitated as ThO₂ by partial neutralization with NH₄OH. Na₂SO₄ is used to salt out La and the light lanthanides as sulphates, leaving the heavy lanthanides in solution. The light lanthanides are oxidized with bleaching powder Ca(OCl)₂. Ce³⁺ is oxidized to Ce⁴⁺ which is precipitated as Ce(IO₃)⁴ and removed La³⁺ may be removed by solvent extraction with tri-n-butyphosphate. The individual elements can be obtained by ion exchange if required. The treatment of bastnaemite is slightly simpler as it does not contain Th.

Once the different lanthanide elements have been separated completely or partially, the metal may be obtained as follows:

1. By electrolysis of the fused LnCl₃, with NaCl or CaCl₂ added to low the melting point.
2. La and the lighter metals Ce to Eu are obtained by reducing anhydrous LnCl₃ with Ca at 1,000 – 1,100^oC in an argon-filled vessel. The heavier elements have higher melting points and so require a temperature of 1,400^o At this temperature, CaCl₂ boils, so LnF₃ are used instead, and in some cases Li is used instead of Ca.

About 5,000 tonnes of La and 13,000 tonnes of the lanthanides are produced annually. The metals are of little use on their own. The main use is for an unseparated mixture of La and the lanthanides called Mischmetal (50% Ce, 40% La, 7% Fe, 3% other metals). This is added to steel to improve its strength and workability. It is also used in Mg alloys. Mischmetal is also used in small amounts as ‘lighter flints’. La₂O₃ is used in Crooke’s lenses, which give protection from UV light by absorbing it. CeO₂ is used to polish glass and as a coating in ‘self cleaning’ ovens. Ce^IV(SO₄)₂ is used as an oxidizing agent in volumetric analysis. Gas mantles are treated with a mixture of 1% CeO₂ and 99% ThO₂ to increase the amount of light emitted by coal gas flames. Other lanthanide oxides are used as phosphors in TV tubes, ‘Didymium oxide’ (a mixture of praseodymium and neodymium oxides) is used with CuCl₂ from HCl. Nd2O3 is used dissolved in SeOCl₂ as a liquid laser. (Selenium oxochloride is used as the solvent because it contains no light atoms which convert the input energy into heat). Lanthanide elements are present in warm superconductors such as La_{(2 – x)}Ba_xCuO_{(4 – y)} and YBa₂Cu₃O_{7 –  x}, and others (Sm, Eu, Nb, Dy and Yb) have been substituted.