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

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Rare Earth Elements Review Section 2 – Introduction to Rare Earth Elements Table 2-1. Abundance of Elements in the Earth’s Crust (Wedepohl, 1995) Elements Nickel ( 28Ni) 90 Zinc ( 30Zn) 79 Copper ( 29Cu) 68 Crustal Abundance (parts per million) Cerium ( 58Ce) a 60.0 Lanthanum ( 57La) 30.0 Cobalt ( 27Co) 30 Neodymium ( 60Nd) 27.0 Yttrium ( 39Y) 24.0 Scandium ( 21Sc) 16.0 Lead ( 82Pb) 10 Praseodymium ( 59Pr) 6.7 Thorium ( 90Th) 6 Samarium ( 62Sm) 5.3 Gadolinium ( 64Gd) 4.0 Dysprosium ( 66Dy) 3.8 Tin ( 50Tn) 2.2 Erbium ( 68Er) 2.1 Ytterbium ( 70Yb) 2.0 Europium ( 63Eu) 1.3 Holmium ( 67Ho) 0.8 Terbium ( 65Tb) 0.7 Lutetium ( 71Lu) 0.4 Thulium ( 69Tm) 0.3 Silver ( 47Ag) 0.08 Gold ( 79Au) 0.0031 Promethium ( 61Pm) 10 -18 a Lanthanides (lanthanoids), scandium, and yttrium are presented in boldface type. Following a common pattern within the periodic table, the lanthanides with even atomic numbers are more common in nature. 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. These observed trends in crustal abundance among the geochemical models suggest a divide between light and heavy lanthanide-enriched minerals. Although variation exists, one example of an REE classification is presented in Figure 2-1 and shows the division between light REEs (LREEs) and heavy REEs (HREEs) (Schuler et al., 2011). Some investigators classify gadolinium and dysprosium as medium-weight lanthanides due to their physiochemical properties. Additional information on REE chemistry and other classifications is provided in Appendix A. 2-2
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
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Page 3 and 4: Foreword The U.S. Environmental Pro
Page 5 and 6: Abstract Rare earth elements (REEs)
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