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Hardrock Mining Wastes



Overview

The extraction and beneficiation of metals and non-fuel minerals necessarily lead to the generation of large quantities of waste. Four basic types of waste produced by hardrock mining are:

  • Mine Water
  • Waste Rock
  • Spent Ore
  • Mill Tailings

    Total waste (rock waste and mill tailings) produced during extraction and beneficiation of minerals can range from 10% of the total raw material removed from the earth for potash to more than 99.9% for gold. Table 1 illustrates the amount of amount of waste various mining industries produced in 1992 (Bureau of Mines 1992). To put these quantities in perspective, about 200,000,000 metric tons of municipal solids wastes are generated in the United States each year.

    Table 1: Mining Waste Produced by Various Industries in 1992
    Industry Amount of Waste
    (metic tons)
    Gold Mining 540,661,000
    Copper Mining 731,065,000
    Potash 197,000


    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.