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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:
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.
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.
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:
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.
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.
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:
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 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.
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.
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