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Overview of Acid Generation, Acid Drainage, and Treatment

Acid drainage occurs from the oxidation of sulfide minerals, predominantly pyrite and marcasite (FeS₂), as a result of mining activities at base and precious metal, uranium, diamond, and coal mines. Upon exposure to oxygen and water, sulfide minerals oxidize to form highly-acidic, sulfate-rich drainage. High concentrations of trace metals, mobilized by decreases in pH, are often associated with acid drainage. Releases of acid drainage in the environment occur as runoff or seepage from waste rock stockpiles, tailings impoundments, and open pit walls or as groundwater discharge from mine adits or shafts.

The chemical reactions governing the oxidation of pyrite and subsequent acid generation were originally developed by Singer and Stumm (1970).

Upon review of these equations, it is apparent that oxygen and Fe⁺³ are the major oxidants of pyrite with water acting as a necessary reactant. In equation 1, the iron and sulfur in pyrite are oxidized in the presence of oxygen and water to ferrous iron (Fe II), sulfate, and acidity. The reaction shown in equation 1 produces two moles of acidity for each mole of pyrite oxidized.

Ferrous iron released from equation 1 is further oxidized in equation 2 to form ferric iron (Fe III). The reaction in equation 2 is the rate limiting step in the oxidation of pyrite. This reaction is pH dependant and undergoes slower reaction rates at low pH values (2-3) as microbial catalysts cannot survive at these pH values. At pH values greater than 3.5, Thiobacillus ferroxidans and possibly other bacteria act as catalysts in the oxidation of Fe⁺²to Fe⁺³, increasing the overall oxidation rates of ferrous iron by several orders of magnitude. Other microbes such as Thiobacillus thiooxidans enhance reaction rates by assisting in the oxidation of sulfur (equations 1 and 4). For further discussion on microbial catalysts in pyrite reduction refer to Parker and Robertson (1999).

The next step in the oxidation of pyrite is pH dependant. If pH values are less than 3.5, ferric iron reacts with water in equation 3 and precipitates as ferric hydroxide (yellowboy). If pH is greater than 3.5, the reaction proceeds to equation 4 where ferric iron oxidizes pyrite and produces greater amounts of ferrous iron, sulfate, and acidity than equation 1 where oxygen is the oxidizing reagent. In equation 4, 16 moles of acidity are released for each mole of pyrite. Equation 4 rapidly continues until either ferric iron or pyrite is depleted.

Reclamation of mine sites with acid generation and drainage is a three step process:
1) Control of acid generation at its source
2) Control of the migration of acid-rich drainage
3) Collection and treatment of acidic drainage

Control of acid generation involves using covers to minimize the limiting reagents in the formation of acid, primarily oxygen and water. Bactericides applied to waste rock and tailings piles can also be used to inhibit Thiobacillus ferroxidans , minimizing pyrite oxidation rates. Additionally, alkali materials can be used to control pH in waste rock piles and mill tailings to augment metal precipitation and limiting metal mobility.

Covers also minimize the mobility and transport of acid drainage by limiting water infiltration. Other methods used to control acid drainage include controlled placement of waste to minimize potential infiltration and surface and groundwater interception and diversion.

Acid drainage from groundwater and surface water sources can be collected and treated using either active or passive treatment methods. Active treatment of acid-drainage include neutralization through the addition of chemical amendments such as limestone, quicklime, hydrated lime, and caustic soda and passive treatment through the use of aerobic and anaerobic wetlands, successive alkalinity producing systems, diversion wells, open limestone channels, and anoxic limestone drains.

References:

Environment Australia, 1997, Managing Sulphidic Mine Wastes and Acid Drainage, Commonwealth of Australia.

Singer, P.C., and Stumm, W., 1970, Acid mine drainage: the rate determining step: Science, v. 167, p. 1121-1123.

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