Rare Earth Research Paper

Rare earth elements (REEs) are a group of 15 chemical elements in the periodic table, specifically the lanthanides. The lanthanides are elements spanning atomic numbers 57(lanthanum, La) to 71 (lutetium, Lu). From them only promethium (Pm) does not occur naturally in the Earth’s crust. Two other elements, scandium and yttrium, have similar physicochemical properties to the lanthanides, are commonly found in the same mineral assemblages, and are often referred to as REEs. The term rare earth is a misnomer arising from the rarity of the minerals from which they were originally isolated. In contrast, REEs are relatively plentiful in the Earth’s crust having an overall crushing abundance of 9,2 ppm (UK report). However, REEs rarely occur in concentrated

Abundance

Occurrence and distribution of rare earths are not only of interest because of their usefulness for industry but also because they have raised a discussion whether rare earths might be essential for plants, animals and even humans. The most plentiful rare earth element, cerium, occur more often in the earth’s crust than lead, molybdenum or arsenic. Lanthanum and neodymium occur in similar quantities as lead. But even thulium, the rarest of the rare earth elements, can be found more often than gold, platinum or iodine. On this fate, REEs are not “rare” at all. Table 3.1 provides the abundance of REEs in the earth’s crust. Moreover, since rare earths are fractionated during weathering processes, ores tend to be rich in either heavy or light rare earth elements. Rare earth elements are mined from a great variety of ores, principally bastnasite and monazite.

Table 3.1 Abundance of REEs in Earth’s Crust

Abundance in Earth's crust, mg/kg

Atomic Number Element Evans Taylor and McLennan Wedepohl,EPA [50] [51]

21 Sc - 30 16 - -

39 Y 28 20 24 - -

57 La 18 16 30 8,4 – 1,4 33

58 Ce 46 33 60 15,8 –64,4 60

59 Pr 5,5 3,9 6,7 1,5 – 5,0 7,7

60 Nd 24 16 27 7,6 –28,6 29

61 Pm - - - -

This can be noted and from the previous table. So, according to the Oddon-Harkins rule, the element with an even atomic number is more abundant than the next element with an odd atomic number. This rule also governs the distribution of all elements in the universe, but is much less pronounced than in the case of lanthanides. Additionally, early on, geochemists observed a pattern in the occurrence and crustal abundance of some lanthanides. Lanthanides with lower atomic numbers were noted to be more common ionic constituents in REE mineral ores and, in general, occurred in greater abundance than the lanthanide elements with higher atomic numbers. A graphical display of the Oddon-Harkins rule is shown at Figure