Acid mine drainage (AMD) is typically characterized by low pH and high dissolved iron. The acid mine drainage may also contain high amounts of CO2, which forms carbonic acid and further depresses the pH.
There are four chemical reactions that represent the chemistry of pyrite weathering to form acid mine drainage. An overall summary reaction is as follows:
2 FeS2+7 O2+2 H2O → 2 FeSO4-+2 H2SO4
Pyrite + Oxygen + Water → Ferrous Sulfate + Sulfuric Acid
The acid mine drainage waste is characterized by red water. The simplest treatment is neutralization and clarification. The ideal neutralization first combines one of the reactants with previously precipitated solids. This blend is then mixed with the other reactant. This seeding provides the opportunity for crystal growth.
It also significantly reduces the reaction time. The final pH range of most neutralization reactions is 6-9. Many heavy metals precipitate as hydroxides within this pH range. However, if these heavy metal hydroxides are subjected to a pH > 11.5 for a few minutes, they convert to a crystal-like particle that clarifies, thickens, and filters more effectively than the original hydroxide.
The most commonly used neutralization agent is lime. Lime is added to previously precipitated solids in a blend tank, commonly called the densification tank. The neutralization flowsheet with this high pH feature is a high-density sludge (HDS) flowsheet.
CO2 Stripping
Excess CO2 dissolved in the acid mine drainage stream can be stripped out using a surface aerator. Lowering the CO2 levels can raise the pH as much as one point and lower the amount of lime required for pH adjustment. This step also begins to oxidize iron and manganese and assists in their precipitation.
Aeration
After stripping, the HDS slurry from the densification tank and the acid mine drainage stream are mixed in the reaction/aeration tank(s). The combination of aeration, high pH, and mixing causes the iron, manganese, and other heavy metals (if present) to precipitate to the fullest extent possible at the set pH level.
Thickening/Clarification
Treated water flows to a thickener for sludge thickening and clarification of the water. The metal precipitates as sludge, and a portion of the sludge is recycled to the sludge densification tank. The remainder of the sludge goes to disposal. Generally, the sludge will also contain gypsum and unreacted lime, which enhance the resistance to re-acidification and metal mobilization. A gravity sand filter may be used to polish the stream prior to discharge, depending on permit limits.
Sludge Disposal
Depending on the site conditions, the thickened waste sludge may be redirected to another portion of the mine, dewatered and deposited prior to disposal in a landfill, or concentrated to paste and stacked.
Since acid mine drainage comes from abandoned mines these sites are often in mountainous, uninhabited areas where access to the site may be difficult. Many systems have been built with ease of operation being paramount. In fact, the systems are often built for operation with no onsite operator. This has led to the use of caustic for neutralization. Caustic is a liquid and is much easier to feed than lime. However, it does not make the same crystals that lime does, so the precipitate is difficult to settle. Caustic-fed systems are much more liable to upsets and the precipitation tanks should be designed for longer holding times. Also, caustic is much more expensive than lime.
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