Rare Earth Elements: A Review of Production, Processing ...
Rare Earth Elements: A Review of Production, Processing ...
Rare Earth Elements: A Review of Production, Processing ...
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
<strong>Rare</strong> <strong>Earth</strong> <strong>Elements</strong> <strong>Review</strong> Section 3 – Life-Cycle Stages <strong>of</strong> <strong>Rare</strong> <strong>Earth</strong> <strong>Elements</strong> Mines<br />
feldspar, fluorspar, and uranium (BLM, 1999b). Usually refers to rock types or mining<br />
environments where the rocks are hard and strong and where blasting is needed to break<br />
them for effective mining."<br />
Placer mining and placer deposits are generally defined (U.S. EPA, 2011d) as<br />
“The extraction <strong>of</strong> a mineral deposit that has achieved its present distribution through the<br />
prior action <strong>of</strong> moving water or wind. Placers are usually in poorly consolidated materials<br />
and are the sources <strong>of</strong> much, but not all, tin, titanium, rare earths, diamonds, and<br />
zirconium, and some gold.”<br />
Hardrock mines are typically either aboveground or underground operations. Solution mining techniques<br />
could also be used to extract REM oxides from the subsurface; however, the use <strong>of</strong> solution mining is<br />
unlikely in the United States due to the potential for environment impacts and the absence <strong>of</strong> suitable<br />
deposits. Solution mining is one <strong>of</strong> the principal mining methods currently used in China to recover REEs<br />
from clay deposits. Placer deposits are typically near-surface deposits that extend over greater areas rather<br />
than to great depths.<br />
The method <strong>of</strong> extraction is dependent on the grade <strong>of</strong> the ore; size <strong>of</strong> the deposit; ore body position (i.e.,<br />
the size, shape, dip, continuity, and depth); geology <strong>of</strong> the deposit; topography; tonnage, ore reserves; and<br />
geographic location and is based on maximizing ore recovery within economic constraints. A low-grade<br />
cut<strong>of</strong>f point is established on a site-specific basis and depends on recovery costs at the site, the market<br />
price <strong>of</strong> the ore, and feed requirements at the mill. It is not uncommon for open-pit and underground<br />
mining to occur at the same site. Mine pits at aboveground mines are generally conical in shape, with the<br />
diameter decreasing with increasing depth; however, a conical pit is not always developed, and the<br />
advance <strong>of</strong> some aboveground mines are controlled by topographic and/or geologic features. Eventually,<br />
there may no longer be adequate room for equipment to work safely in the pit, and it may not be feasible<br />
or practical to remove additional waste rock to widen the pit. After that point, an underground mine may<br />
be developed inside the lower reaches <strong>of</strong> the mine pit to reach deeper parts <strong>of</strong> the deposit. The economics<br />
<strong>of</strong> widening the pit may also make mining <strong>of</strong> deeper sections <strong>of</strong> the ore body using underground methods<br />
more advantageous. The more common mining methods are described below.<br />
In Situ Leach Mining<br />
It is unlikely that in situ leach (ISL) mining methods would be used in the United States to recover REEs.<br />
Limited geologic conditions suitable for ISL, environmental concerns associated with the strong acids that<br />
would likely be needed, and the low solubility <strong>of</strong> REEs likely preclude ISL from being considered as a<br />
viable method for recovering REEs from deep ore deposits. However, this method is presented here<br />
because ISL has been used in China and in the United States for other metals.<br />
Very simply, ISL mining injects a fluid (i.e., water, acids, or other chemicals) into a circuit <strong>of</strong> drilled<br />
boreholes intersecting the mineralized zone, and then an enriched or saturated solution is pumped from<br />
the ore containing the target metals. The enriched or saturated solution is then processed, typically with a<br />
solvent extraction method, to recover the metals. This method has been used for mining copper and<br />
uranium in the United States on a limited scale. Copper ore dumps around formally active underground<br />
copper mines have also been mined using similar techniques, and REEs are known to occur along with<br />
some copper deposits (e.g., Bear Lodge, Wyoming). Although it is thought to be unlikely, use <strong>of</strong> ISL<br />
might be considered as an alternative to recover REEs from abandoned shafts backfilled with<br />
subeconomic ore, dredge wastes <strong>of</strong> former mines, or from the waste <strong>of</strong> active mines producing other<br />
commodities from ores containing REE. The hydrogeologic conditions must be suitable for in situ mining<br />
to be successful and also environmentally safe. Because in situ mining does not cause the level <strong>of</strong> ground<br />
disturbance that either aboveground or underground mining does, due to the lack <strong>of</strong> waste piles and ore<br />
3-14