The rare earth elements (REE) consist of scandium, yttrium, lanthanum and the lanthanides. The lanthanides are the elements from cerium to lutetium. They are not as rare as the name indicates, most of them are far more abundant than a familiar element such as silver. Cerium, yttrium and lanthanum are even as abundant as copper. The term «rare earth elements» reflects the fact that their chemical behavior is quite similar, due to the special characteristics of the properties of «4f-electrons». This makes them almost solely tri-valent and with almost equal ionic radii. The ionic radii vary from 103 pm for lanthanum (La) to 86 pm for lutetium (Lu), but two neighboring elements may only differ with one pm (10-12m). A consequence of this is that the rare earth bearing minerals usually also contain other elements and other minerals, e.g. calcite and zircon, may contain REE.
The main rare earth bearing minerals are bastnäsite ((Ce,La)CO3F), monazite ((Ce,La,Th)PO4), xenotime (YPO4), and eudialyte, a zirconium silicate where the rare earth elements are substituting portions of Ca, Fe, or Mn. No pure rare earth ore is known. The deposits with the highest contents of REE are Bayan Obo in inner Mongolia, Steenkamskraal in South Africa, Moutain Pass in California, and Mount Weld in the inner Australia. Today an abundance of 2% may be economical feasible to operate depending on the site infrastructure and the mineralogy.
200 years since the discovery, but only 60 years of high purity compounds.
To separate the individual elements is a comprehensive task. Two centuries have passed since Johan Gadolin discovered yttria (yttrium oxide) in a mineral found in Ytterby in Sweden, but only since World War II have chemical means been available enabling us to produce large amounts of pure elements. The challenge today is not to find resources and to recover them, but to find ways to
i) by physical means separate the REE-bearing minerals from the rest (gangue minerals),
ii) to find low cost ways of dissolving the REEs, and
iii) to separate and concentrate the REEs of interest.
Many companies are in the market for supply of concentrates for REE, but only a few have the know-how and competence of producing high purity rare earths, either as metals, oxides or salts. The major company here is Rhodia of France. The rare earth elements are all very electropositive and, contrary to similar elements like aluminium and titanium, most of them do not form protective oxide layers on the surface. Instead, they turn to powders. Thus, most REE are produced and sold in the oxide form. Not surprisingly, most uses are based on oxides.