Acid mine drainage
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- Mineral processing and other anthropogenic activities leave by-products exposed to oxygen and moisture in the air. If the by-products are sulfide minerals they may start to bioleach out of control and cause the environmental problem known as acid mine drainage (AMD). AMD manifests itself as acidified groundwater contaminated by heavy metals.
- Drainage flowing from or caused by surface mining or coal refuse piles that are typically highly acidic with elevated levels of dissolved metals.
Sometimes AMD is mistaken for acid rock drainage (ARD) and acid rock generation (ARG) which have slightly different meanings.
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Formation
Acid mine drainage is formed by uncontrolled bioleaching reactions - a series of complex geochemical and microbial reactions that occur when water and oxygen come into contact with pyrite (iron disulfide minerals) in for example coal, refuse or the overburden of a mine operation.
Risk factors for AMD
Very fine sulfide mineral particles are more prone to leaching since the reduced particle size will expose a greater surface area. As a consequence, leaching chemicals can more easily come into contact with the ore surface, leading to a faster leaching reaction.
Strategies to prevent acid mine drainage
- By covering waste with a sealing layer, as well as layers of water or soil, contact with oxygen is prevented. Another way to avoid oxygen contact is by backfilling the waste into exhausted underground mines. Since the microorganisms oxidise the sulfur and iron in the ore, they can't thrive without oxygen or water. Then leaching can't occur.
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Strategies to treat acid mine drainage
- Sometimes tailings from past mining are re-mined by bioleaching and other methods as economical circumstances permit. A convenient consequence is that the potential for AMD is decreased.
- High-rate sulphidogenic treatment of AMD by courtesy of Erkan Sahinkaya
- Microbiological development of sustainable technologies for mineral processing and wastewater treatment by courtesy of Barrie Johnson
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