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Introduction to the Rare Earths
The so-called rare-earth elements (REEs) are a unique group of chemical elements that exhibit a range of special electronic, magnetic, optical and catalytic properties. These elements are enablers; their use in components manufactured from a wide range of alloys and compounds, can have a profound effect on the performance of complex engineered systems.
The International Union of Pure and Applied Chemistry defines the rare-earth metals as the 15 lanthanoid elements (with atomic numbers of 57 through to 71) in addition to scandium (Sc) and yttrium (Y). One of the lanthanoid elements, promethium (Pm), is radioactive and does not occur naturally; Sc is also seldom found together with the other rare earths. When referring to the rare earths, therefore, the industry is generally talking about the 15 elements shown in the table below:
REEs are frequently grouped into the so-called light REEs (LREEs) and heavy REEs (HREEs). The distinction between LREEs and HREEs varies depending on who is making that distinction. Strictly speaking, the definition should be based on the specific configurations of electrons within each REE atom. As such, scientists would usually define the LREEs as La-Ce-Pr-Nd-Sm-Eu-Gd, with the HREEs as Tb-Dy-Ho-Er-Tm-Yb-Lu-Y. However, many industry players refer only to La-Ce-Pr-Nd-Sm as the LREEs, with Eu-Gd-Tb-Dy-Ho-Er-Tm-Yb-Lu-Y being referred to as the HREEs.
A further distinction is frequently made, when an additional grouping, the medium REEs (MREEs) is defined based on the way in which groups of elements are processed. In this scenario, the LREEs would be La-Ce-Pr-Nd, the MREEs would be Sm-Eu-Gd, with the rest defined as HREEs. IMC uses the term L/MREE to refer to a combined grouping of the aforementioned LREEs and MREEs.
The lesson here? The reader should determine which definitions have been used, by the particular entity that refers to them.
REEs are frequently separated and sold in their oxide form and thus it is customary to render mineral-deposit data in terms of rare-earth oxide (REO) equivalents. The corresponding terms light rare-earth oxide (LREO) and heavy rare-earth oxide (HREO) are used as appropriate, particularly when assessing the supply and demand characteristics of the overall rare-earths market. Exploration-stage projects may report REE concentrations as elemental parts-per-million (ppm) values, which will be slightly lower than their oxide equivalents. Total rare-earth oxide(s) (TREO(s)) refers to the sum total of rare earths present in a deposit.
HREEs are generally (though not always) much scarcer than LREEs. The majority of REE-bearing mineral deposits are dominated, in tonnage terms, by the presence of LREEs. So-called HREE mineral deposits are thus generally designated as such on the basis of the potential value of that deposit by virtue of the presence of HREEs, not their overall tonnage, because HREEs tend to be more valuable due to their scarcity, but also occur in lower material grades within the mineral resource.
HREEs frequently occur in minerals that are significantly more challenging to process than the more common LREE-rich minerals, which have a well-established history of processing.