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

Rare Earth Elements Review Section 3 – Life-Cycle Stages of Rare Earth Elements Mines
building foundations, road beds, retaining structures, or other purposes. Many of these reuse strategies
encapsulate waste-rock material, effectively separating it from atmospheric influence that would cause a
release of toxic metals, acids, or alkaline waters to the environment.
3.4.1 Soil Storage Piles
The soils removed during the construction-mining stage of an aboveground mine site are typically placed
in a storage pile and may be used during restoration at the end of the mine’s life. Soils are typically not
considered wastes but can be a potential source of pollution if not properly managed, especially for
providing a source of sediment in runoff. Humus-rich soils may be acidic, and fines that run off to streams
could affect surface water quality. Acidic soils used as cover for metal-bearing rock-waste areas could
encourage metal leaching from the rock.
3.4.2 Overburden
Overburden storage piles of sediments and rock materials that do not contain the commodity can include
other metals and constituents that may also be a source of pollution and contamination. While relatively
small in comparison to quantities of other mining waste, overburden, which is the material removed to
expose the ore during mining, may include unconsolidated materials such as alluvium, colluviums, glacial
deposits, or residuum and be a source of sediment or acid-generating minerals; however, the acidity of
these materials is usually low and they generally do not contain significant concentrations of metals.
Overburden usually does not include rock that contains the mineral ore, but it may include the very low
mineralized rock removed from around the ore from hardrock mines. These materials can contain greater
metal concentrations than typically associated with unconsolidated deposits. Without proper management,
erosion of the overburden storage piles resulting in sediment loading to streams and surface water can
occur. Ground water may be entrained with the overburden material, which can then leach through to the
base of the pile, carrying with it high concentrations of metals and other potentially harmful constituents.
Overburden piles can cover hundreds of acres and be a prominent feature in the general landscape as they
reach heights extending hundreds of feet. Large mines may have multiple overburden storage piles. Large
mine trucks and other vehicles deliver materials to the top of the pile and shape the slopes. Fugitive dusts
are a concern from overburden storage piles.
3.4.3 Ore and Subeconomic Ore Storage
Ores may be stockpiled to provide a supply, lasting 6 months or more, of ore stock feed for beneficiation
and processing plants. Subeconomic ore is often stockpiled at the mine site for future exploitation under
the appropriate economic or market demand conditions. These ore stockpiles and subeconomic ore
storage piles can include large areas. Mines may stage ores at multiple sites. Ore stockpiles can represent
a significant source of toxic metals. Runoff, flooding, or infiltration of rainwater, if not captured and
managed, can have significant effects on aquatic habitat. Ground water may be entrained with the ore
material, which can then leach out to the base of the pile, carrying with it high concentrations of metals
and other potentially harmful constituents. Large mine trucks and other vehicles deliver materials to the
top of the pile and shape the slopes. Fugitive dusts containing metals or other harmful constituents from
the ore and subeconomic ore stockpiles, even though not as much of a concern as tailings wastes or
conveyances, may still be of potential concern depending upon the material stored.
As previously discussed, AMD is uncommon in most REE deposits; however, some potential exists for
low levels of acid generation from accessory minor sulfide minerals, especially in low-Ti iron oxide Cu-
U-Au-REE deposits like Pea Ridge (Foose et al., 1995). Carbonatite ores, such as those at the Mountain
Pass mine, may cause elevated pH in streams, depending on the concentration of other acid-producing
accessory minerals in the ore-bearing rock and surrounding country rock that can serve to neutralize any
runoff. Thorium-rare earth element veins have moderately high sulfur content, but the sulfur is present
chiefly as sulfate in barite. Sulfide mineral content of these vein deposits is very low, thus potential for
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