Acid mine drainage formation, control and treatment: Approaches and strategies

Abstract

Acid mine drainage (AMD) occurs after mining exposes metal sulfides to oxidizing conditions. Leaching of reaction products into surface waters pollute over 20,000 km of streams in the USA alone. The coal mine permitting process requires prediction of AMD potential via overburden analysis. Where a potential exists, AMD control measures including spoil handling plans, alkaline amendment, and oxygen barriers or water covers may be required to stop or hinder AMD generation. Other AMD control technologies include injection of alkaline materials (coal ashes and limestone products) into abandoned underground mines and into buried acid material in mine backfills, remining of abandoned areas, and installation of alkaline recharge trenches. Where AMD already exists, effluent treatment is required. Active treatment includes adding alkaline chemicals such as Ca(OH)2, CaO, NaOH, Na2CO3, and NH3, but chemical treatment is costly, requires dispensing equipment and facilities, and often extends for decades. Passive treatment systems may also be employed to treat problem drainages and are effective under certain flow and acidity conditions. Such systems include aerobic and anaerobic wetlands, anoxic limestone drains, vertical flow wetlands, open limestone channels, and alkaline leach beds. This article discusses the process of AMD formation, preventative and control measures, and describes treatment methods for existing AMD discharges.