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 Figure 3-3. Aerial image of Pea Ridge magnetite mine, Missouri. Open-pit mines use a variety of drilling, earth-moving equipment, and explosives to terrace into the subsurface to extract overburden, waste rock, and ore. Underground mine operations are more targeted, but also use drilling, excavation equipment, and explosives to construct a series of shafts that advance to and then remove ore to the surface. Because of their typically elongated nature, geochemical deposit models suggest that both thorium-rare earth element veins and carbonatite dikes could be developed by underground mining techniques (Armbrustmacher et al., 1995). The depth and extent to which open-pit or underground works are advanced depends upon the ore grade, nature of the overburden (e.g., thickness and hardness, or difficulty of removal), and the stripping ratio. Stripping ratios of overburden to ore can be large (e.g., 80:1) for aboveground mines and much smaller for underground mines (e.g., 1:1 to 1:20). The stripping ratios also illustrate that the potential of environmental impact from rock waste is generally much less from underground mining operations. Both methods produce overburden piles, piles of waste rock, ore and subgrade-ore stockpiles, and sediment. Overburden piles that contain soil and non-ore-bearing (or non-mineralized) rock, removed to expose the mineralized ore body, are produced during aboveground and underground mining operations. Overburden is typically stored separately from other stockpiles and wastes because it can contain less of the deeper mineralized rock of the ore body. Overburden may also be stored from the development of underground mines or used to backfill mine shafts as mining progresses; however, the volume of overburden and waste rock is typically less from underground mines. Waste rock piles store more mineralized rock that was removed from around the ore body. The differentiation between waste rock and ore can be relative and dependent upon economic variables. 3-16
Rare Earth Elements Review Section 3 – Life-Cycle Stages of Rare Earth Elements Mines Placer Mining Placer deposits are accumulations of mineral or metal-bearing alluvial sediments that have accumulated in depressions or stream beds. Placer mines are not considered hardrock mining because the REE ore is recovered from unconsolidated alluvial or colluvial deposits; however, these mines are often grouped with hardrock mines for regulatory purposes because the REE host minerals can present the same processing challenges as those mined directly from a rock ore body. Placer mining includes the classic image of a gold miner using a pan or a sluice box. Larger and more modern operations may excavate an open-pit or tunnel into ancient riverbed sediments. Excavation might be achieved using water pressure (hydraulic mining), excavation or tunneling equipment, or dredging equipment that is used to mine ore-bearing sediments from stream beds or depressions. Native gold deposits, uranium, thorium, zirconium, and mineral ores for REMs are examples of placer ore deposits that occur in the United States. More REE mineral ores have been discovered associated with placer deposits in the United States than any other deposit type (see Table 3-1 and Appendix B). The Vero Beach mine in Florida and the Old Hickory mine in Virginia are examples of placer mines that had produced REEs as a by-product of zirconium mining in the past. The Old Hickory mine continues to operate, but currently is not producing REE-containing minerals (ILUKA Resources, Ltd, 2007). Placer deposits containing monazite REE-mineral ores are typically associated with higher concentrations of minerals containing radioactive elements, which has deterred production from these deposits in the United States. Prospective Mine Types by State A general summary indicating the number of potential rare earth mining sites (i.e., potential resources) that have been documented in the United States is summarized in Table 3-1. Data were summarized from the table in Appendix B, which was compiled from a combination of sources, but is principally from information available from the U.S. Geological Survey. The table is presented not to suggest that every known deposit will be exploited, or that there is potential viability for the rare earth resources that occur at these locations. It is provided simply to assist stakeholders in these states gain an awareness of the level of known occurrences of REE resources. Other deposits and mine sites likely exist or are being explored that were not identified or included in Appendix B; however, the following table gives a general idea of types of mines that might be found or developed in each state. It should be noted that this information is very general; no aspects of the mine sites have been verified, and the current condition of the mines listed is largely unknown. Actual data were available for some existing mines, but the mine type for a few of the mines listed was inferred from the deposit types or information found on the Internet; approximate counts are shown in parentheses. In large part, the table illustrates that the majority of discovered REE deposits, documented in the information sources found, would be mined as placer deposits or from aboveground open pit mine types. Neither the Molycorp or Pea Ridge mines, previously discussed, that are being developed and are near production stage are the more common placer deposit (i.e., dredge type mining) operations, but rather the Molycorp mine is an aboveground mine and the Pea Ridge mine is an underground mine. As previously discussed, the feasibility and viability of a deposit depend on a number of complex variables. 3-17
Page 1 and 2: Rare Earth Elements Review Table of
Page 3 and 4: Foreword The U.S. Environmental Pro
Page 5 and 6: Abstract Rare earth elements (REEs)
Page 7 and 8: Table of Contents List of Acronyms
Page 9 and 10: List of Tables 2-1. Abundance of El
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