Chemistry of Rare Earth Elements

Rare earth elements (REEs) are a group of 17 chemically similar metallic elements in Group 3 of the periodic table that include Scandium, Yttrium and the 15 Lanthanides, lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu).The rare earths were so named because of their low concentration in minerals which were scarce. 

Even though the rare earths have existed since Earth’s origin, it wasn’t until the late 18th century that their presence was discovered. Carl Axel Arrhenius, a lieutenant in the Swedish army, found a unique black mineral in a small quarry in Ytterby, a small village close to Stockholm, in 1787. Cerium was the first distinct element to be separated from that combination of rare earths in 1803.The last naturally occurring rare-earth element, lutetium, was found in 1907. However, because the number of actual rare-earth elements was unknown, it was challenging to do chemistry research on these elements.

Rare earth elements are divided into two categories based on the atomic weights:

Light REEs- those with atomic numbers from 57 to 63 (La, Ce, Pr, Nd, Pm, Sm and Eu).

i.e., lanthanum, cerium, praseodymium, neodymium, promethium, samarium, and europium

Heavy REEs– those with atomic numbers from 64 to 71 (Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu

i.e., gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium, plus yttrium

Scandium and Yttrium, although light, have been placed with the heavy REEs group due to their similar chemical and physical properties. Secondly, they tend to occur in the same ore deposits as the lanthanides but have different electrical and magnetic properties.

REEs are characterised by high density, high melting point, high conductivity and high thermal conductance.The rare earths are generally trivalent elements, but a few have other valences. Cerium, praseodymium, and terbium can be tetravalent; samarium, europium and ytterbium, on the other hand, can be divalent.

Deposits of rare-earth ore can be found worldwide. The major ores are in China, the United States, Australia, and Russia, while other viable ore bodies are found in Canada, India, South Africa, and southeast Asia. Of the approximately 160 minerals that are known to contain rare earths, only four are currently mined for their rare earths: Bastnasite (fluorocarbonate), monazite (phosphate), loparite [(R,Na,Sr,Ca)(Ti,Nb,Ta,Fe3+)O3], and laterite clays (SiO2, Al2O3, and Fe2O3) are the principal minerals found in these ore deposits.

Several rare-earth minerals contain thorium and uranium in variable amounts, but they do not constitute essential components in the composition of the minerals.

Cerium is the most abundant rare earth, approximately the same as that of copper (0.0068%)

Relative Abundance in Earth’s Crust (Light REEs)

Despite their high relative abundance, rare-earth minerals are more difficult to mine and extract than equivalent sources of transition metals, due in part to their similar chemical properties, making the rare-earth elements relatively expensive.This is mainly due to the fact that their concentration levels in many ores are relatively low (less than 5 percent by weight).

The separation chemistry of REEs involves various methods to isolate and purify these essential metals, which are crucial for modern technology. Since rare earth elements have close physical and chemical properties due to their similarly sized stable trivalent ions; their separation is more challenging and expensive. Extracting rare earth metals often involves mining, crushing, and grinding the mineral ore. Chemical processes then separate the metals from other elements. This can include using acids or bases in a series of steps to refine the desired rare earth elements. Key approaches include: Solvent extraction, Ion exchange, Membrane technologies, and Bio extraction methods.

Key areas where REEs are used:

                                                                                                –Dr Subramanian S Iyer