Rare Earth Elements: A Review of Production, Processing ...

Rare Earth Elements Review Section 2 – Introduction to Rare Earth Elements
Figure 2-1. Periodic table of the elements showing
the division between LREEs and HREEs (Schuler et al., 2011).
The elemental forms of REEs that are extracted from mineral ores, as oxides (i.e., REOs), are iron-gray to
silvery lustrous metals that are typically soft, malleable, ductile, and usually reactive, especially at
elevated temperatures or when finely divided. The REEs’ unique properties are used in a wide variety of
applications. For example, magnets made with REEs are much more powerful, weigh less, and can be
made smaller than conventional magnets. Some REEs also have high electrical conductivity, can
withstand extreme heat, and give off intense white light when heated.
In aquatic systems, REEs typically occur in the trivalent state. However, cerium can be present as Ce 4+
and europium can occur in both the divalent and trivalent states. The chemical behaviors of all REEs are
very similar, but smooth variations can be attributed to their atomic number and ionic radii, which are
inversely correlated. These attributes make REEs well suited to the study of processes such as
complexation, sorption, precipitation, and the formation of colloids (Merten and Büchel, 2004).
2.2 Geologic Environments of REEs
REEs do not occur as native elemental metals in nature, only as part of the host mineral’s chemistry. For
this reason, the recovery of REMs must be accomplished through a complex processing method
(described in Section 4) to chemically break down the minerals containing the REEs.
Despite more than 200 known REE-bearing minerals, only three are considered to be the principal REE
mineral ores most feasible for the extraction of REMs: bastnasite, xenotime, and monazite (Gupta and
Krishnamurthy, 2004), as described below:
� Bastnasite, the most abundant among the three REE mineral ores, is a carbonate mineral found
mainly enriched in LREEs (e.g., cerium, lanthanum, and yttrium). Bastnasite is found in vein
deposits, contact metamorphic zones, and pegmatites. It forms in carbonate-silicate rocks
occurring with and related to alkaline intrusions (e.g., Mountain Pass mine).
� The two phosphate minerals, xenotime and monazite, can occur together, but crystallize in
different temperature and pressure regimes from a similar igneous environment. While these
2-3