Mine
Water
Definition
Mine water consists of all
water that collects in mine workings, both surface and underground, as a
result of inflow from rain or surface water and groundwater seepage.
Characteristics
Depending on the source of the water and the
regional and hydrological conditions, mine water can be a significant
problem because of its enormous quantity and chemical composition. Mine
water may have to be pumped continuously from the mine during operations.
Potential Impact
Mine water can have high concentrations
of heavy metals, beneficiation reagents, oils, and total dissolved solids
as well as elevated temperatures and altered pH.
Mine Water
Treatment
Since mine water is often acidic, one of the most
effective treatment techniques is to raise the pH by adding lime or other
suitable basic chemicals. Dissovled solids can be effectively removed from
the water in settling ponds, adding flocculates, or "filtering" the water
with ion exchange or reverse osmosis. Wetlands have also been constructed
to treat mine water. The long-term effectiveness of this approach is still
being studied.
Waste
Rock
Definition
The primary and most prevalent
waste generated by many mining operations is waste rock, or overburden.
Waste rock, which consists of rock and target minerals in concentrations
too low for economic recovery, is removed along with the ore. Waste rock
includes granular, broken rock that ranges from fine sand to large
boulders, depending on the nature of the formation and mining methods
employed.
Characteristics
The geochemistry of waste rock
varies widely from mine to mine and may vary significantly at individual
mines over time as different lithological units are exposed. Generally,
waste rock at metal mines always contain some concentration of the target
mineral, along with other metals and often sulfidic materials.
Potential Impacts
The primary impacts associated with
waste rock depend on its geochemistry and site conditions. Waste rock can
be a source of toxic, reactive materials, such as acid rock drainage and
heavy metals such as arsenic. All of these can adversely impact aquatic
and other organisms, as well as surface and ground waters.
Constructing and Designing Waste Rock Piles
Constructing
and designing waste rock piles and/or dumps involves four major
activities:
Material Characterization: Constant and accurate
characterization of the
mine and waste rock can help identify rock with high acid generation
potential and acid-buffering potential. With this knowledge, the rock
can be
placed in the pile in such a way as to minimize acid generation or
maximize
acid treatment.
Proper Design: The design of the pile depends on the topography
of the mine
area, the anticipated amount of waste rock, the hydrology patterns,
and the
geochemistry of the waste rock. Long-term stability and acid
generation
potential are key considerations in designing the pile. In addition,
any
planned reclamation and revegetation after pile closure will need to
be
incorporated into the design.
Waste Deposition: The waste needs to be deposited in
conformance with the
pile design to maximize performance and minimize impacts.
Waste Diversion: Establishing optimum water diversion control
can reduce the
potential for acid generation and/or mitigate the potential acid
impacts by
diverting acid away from susceptible habitats.
Mill
Tailings
Definition
Mill tailings are the
coarsely and finely ground waste portions of mined material remaining
after beneficiation operations have removed the valuable constituents from
the ore.
Characteristics
The physical and chemical
characteristics of tailings vary according to the ore being mined and the
particular beneficiation operations used. Tailings generally leave the
mill as a slurry. The typical content of tailings is 50 to 70% liquid by
weight and 50 to 30% solids in the form of clay, silt, and sand-sized
particles.
Potential Impacts
Water in tailings
impoundments may be toxic to wildlife because of the chemical reagents
used in beneficiation processes and non-target heavy metals. Leakage from
tailings impoundments can also be a serious ongoing environmental problem.
Leakage can transport contaminants to groundwater or surface water.
Uncontrolled leakage can threaten the integrity of the impoundment
structure itself, which can lead to the possibility of catastrophic dam or
embankment failure. Catastrophic impoundment failure can adversely impact
downstream wildlife, aquatic organisms and their habitat, and humans.
Dry Tailings Disposal
In some cases, tailings are
de-watered prior to disposal. This is called dry tailings disposal,
although the tailings may still contain some water. Unlike wet tailings,
which are disposed in impoundments, dry tailings are disposed primarily on
large piles. These piles are non-impounding structures that make use of a
variety of configurations and reduce land needs and impoundment
reclamation.
The major pile configurations for dry tailings are:
Valley-fill: tailings are dumped to fill in a valley
Side hill disposal: tailings are disposed on a side of a hill
on a series of piles
Level piles: can grow as lifts are added throughout the life of
the mine
Detoxification of Tailings Slurry
The procedure
for detoxifying tailings depends on the contaminants of concern that
necessitate treatment. Processes used to detoxify or stabilize cyanide and
some metals include ion exchange, pH adjustment, biological degradation,
alkaline chlorination, and hydrogen peroxide. The gold industry usually
uses chlorination and peroxide.
Spent Ore
Definition
Spent ore
is the material remaining after leaching. For example, copper or gold ore
subjected to heap or dump leaching will become spent ore when recovery of
the target metals is no longer economical. Usually left and reclaimed in
place, a small portion of gold mines dispose of spent ore in piles or
dumps. The remaining materials usually range in size from sand particles
to pebbles.
Characteristics
Spent ore will always
contain some portion of the target ore in very low concentrations, as well
as other metals. Sulfide minerals are most commonly found in copper ore
but are being found more frequently in gold as oxide deposits are mined
out. Selenium, mercury, and arsenic are concerns in some spent gold ores.
Other chemicals of concern are cyanide and reaction chemicals from
oxidation and cyanidation.
Potential Impacts
The
environmental impacts associated with spent ore depend on its geochemistry
and site conditions, including the final management methods. Spent ore can
be a source of acid drainage and/or of heavy metals including mercury,
copper, and arsenic. Residual leaching solutions – typically cyanide in
the case of gold ore and sulfuric acid in the case of copper ore – may
also be present in spent ore. Releases of any of these substances can
adversely affect aquatic life and other organisms as well as groundwater
and surface water.
Spent Ore Management Methods
Off-loading: Off-loading of spent ore is the removal of
leached ore from the leached pad to a final disposal area, such as in
a
valley-fill, waste piles, or an exhausted portion of an open pit.
Off-loaded
spent ore may contain low levels of contaminants, target minerals, and
sulfur-bearing minerals. Waste minerals can be capped with clay to
reduce
infiltration and run-off.
Detoxification: Active neutralization is the primary method for
enhancing
cyanide destruction in spent ore. Various rinsing liquids are used for
this
method, including fresh water, re-circulation of untreated process
solution,
alkaline chlorination, hydrogen peroxide, and biological treatment.
Capping: Capping a spent ore pile can reduce the potential for
infiltration of
precipitation, which can mobilize and transport contaminants. Capping
can
be done with a geotextile cap or heavy compaction of soils with high
silt or
clay content. Both options are followed by the addition of soil and
revegetation with shallow-rooted plants to increase stability.
The information contained on these pages is a general
statement of policy. It does not establish or affect legal rights or
obligations. It does not establish a binding norm and is not finally
determinative of the issues addressed. Agency decisions in any particular
case will be made by applying the law and regulations to the specific
facts of the case.
