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

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Rare Earth Elements Review Section 3 – Life-Cycle Stages of Rare Earth Elements Mines mass of overburden and waste rock that might be removed from aboveground mining operations are not provided in these models. The amount of overburden and waste rock depends on many factors, including the type of deposit, the geometry of the ore body, and the mining method. In general, overburden includes the sediments or rock materials below the soil horizon that must be removed from above and possibly around a subsurface deposit and that does not contain concentrations of the commodity mined. Overburden does not include the mass of waste rock removed from around the economically viable ore. Waste rock may represent a subeconomic ore material, removed from around or within specific regions of the ore body, that does not contain appreciable quantities or concentrations of the mined commodity relative to the target ore. Waste rock is removed to reach the more economically viable regions of the orecontaining rock. Subeconomic ore specifically refers to conventionally mined ore with a grade that is not rich enough to meet the current market condition for viable production. Mines classify their subeconomic to uneconomic material based on potential for future processing, given appropriate market conditions and environmental risks. Active mining can encounter all variations in the concentration of the commodity in the rock or sediments being extracted, depending upon the complexity of the ore body. The waste rock material is generally managed separately from the overburden piles since the waste rock can contain higher concentrations of metals that, if released, may have environmental consequences. Clearly, the efforts and resources required for management of overburden, waste rock, and ore materials alone are generally significant. Management of the soil and rock material removed from the mine is necessary since these materials may contain concentrations of metals, acid-producing sulfides, and radioactive elements. Freshly exposed rock surfaces enhance the effects of weathering and can allow release of metals as environmental contaminants, which may represent a potential human and ecological health risk to site workers, nearby residents, nearby recreational users, and aquatic and terrestrial species and habitat. However, REE ores generally contain few acid-producing sulfide minerals, so the risk of producing acids to leach metals and producing mine drainage (AMD) is considered low. Because REE deposits are associated with igneous rocks, the greatest AMD risk would probably come from these igneous bodies intruding sediments or other rocks containing sulfide minerals. 3.1 Active REE Mining As of this writing, ongoing active mining operations for extraction of REE ores are not widespread in the United States. Aboveground mining of REEs last occurred in the United States in 2002 (Long et al., 2010) at the Molycorp Minerals rare earth mine in Mountain Pass, California (Figure 3-1). Currently, the Molycorp mine is preparing to resume ore production, but initial start-up plans are for operations to begin with processing of stockpiled ore (Jim Sims, Molycorp; personal communication). 3-2
Rare Earth Elements Review Section 3 – Life-Cycle Stages of Rare Earth Elements Mines With Permission of GoogleEarth (© 2012 Google) and USDA Farm Service Agency and © 2012 GeoEye Figure 3-1. Molycorp Minerals rare earth mine, Mountain Pass, California. Other mine sites around the conterminous United States and Alaska have produced REE-containing minerals in the past; however, the status of those sites is currently undetermined. A listing of known mines that have produced REE-mineral ores or REOs and locations where REE-containing mineral deposits have been discovered was compiled from Gupta and Krishnamurthy (2004), Orris and Grauch (2002), and Schuler et al. (2011) and is provided in Appendix B. The only other significant REE mining operations are located in the Sichuan, Jiangxi, Shandong, and the Inner Mongolia provinces in China (Schuler et al., 2011); however, the Chinese REE mining operations and environmental practices are not considered applicable for comparative purposes in this report and are not discussed in any detail. In the absence of modern examples of mining activities for the extraction of REE ores in the United States, and considering the variability in the mineralogy and geology of known REE reserves, the following sections present a general discussion that introduces the life cycle of typical mining operations. The wastes generated during each mining stage that have the potential to cause environmental effects are also discussed, as applicable. 3.2 REE Ores The REE metal is an elemental constituent of mineral REO ores. Concentrations of REE minerals typically occur in igneous and metamorphic rocks and also in sedimentary placer deposits. As discussed in Section 3.3.2.1, concentrations of the target REO in the ore have implications for selection of the 3-3
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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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