Reduction of acid production potential

Päivi M. Kauppila, Geological Survey of Finland, P.O. Box 1237, FI-70211 Finland; e-mail: paivi.kauppila(at)gtk.fi

Acidity produced by the oxidation of iron sulphide minerals (e.g. pyrite FeS₂, pyrrhotite Fe_1-xS) in the mine wastes, once exposed to atmospheric oxygen and water, is the main environmental issue in the mine waste management. Oxidation of sulphides releases acidity (H⁺), Fe²⁺, SO₄^2-, and trace metals into the pore water of the waste material. The increased acidity further promotes the dissolution of other sulphides and minerals in the wastes, resulting in enhanced liberation of potential contaminants into the pore water. In general, sulphide oxidation primarily depends on the availability of oxygen and/or ferric iron [Fe(III)] (Singer and Stumm 1970, Alpers et al. 1994).

The acidity of the mine drainage largely depends on the balance between acid producing and neutralising minerals in the mine waste. The primary neutralising minerals are Ca and Mg carbonates, particularly calcite (CaCO₃) and dolomite (CaMg(CO₃)₂), which react relatively rapidly compared to the rate of acid production by sulphide weathering. However, other minerals, such as intermediate or mafic silicate minerals (e.g. Mg bearing serpentine and pyroxenes) may also contribute to the neutralization. (Jambor 2003) When the amount of acidity produced by the sulphide minerals exceeds the buffering capacity of the neutralising minerals in the mine wastes, drainage from the mine wastes will be acidic (AMD, acid mine drainage). On the other hand, if the neutralizing capacity of the minerals outnumbers the acidity production by the sulphides, mine waste effluents will be near-neutral or neutral, but may still contain elevated concentrations of heavy metals and sulphate (NMD, neutral mine drainage).

As a result, several source control waste management technologies have been developed to reduce the net acid production capacity of the waste materials during mining operations in order to decrease the acidity and metal loadings of the waste effluents and to minimize the environmental impacts of the wastes. This can be achieved e.g. by decreasing the sulphide mineral content or increasing the neutralisation capacity of the waste or by limiting the access of those components into waste that promote oxidation: e.g. oxygen and ferric iron.

Technologies aiming at the reduction of the acid production potential of mine waste during mining operations have been evaluated in detail in the following pages:

• Segregation/selective handling (acid producing and non-acid producing waste materials are separated and disposed in separate facilities)
• Reduction of the content of sulphide minerals in the waste (desulphurization)
• Addition of alkaline material during disposal to increase neutralising capacity
  • Blending
  • Layering
  • Encapsulation
  • Alkaline amendments
  • Placement of alkaline material above or below waste as liner or cover material
• Blending – mixing of waste rock types of varying acid generation and neutralisation potential (requires homogenous and thorough mixing)
• Layering
  • Grain size layering
  • Acid-producing/non-acid producing material layering
• Removal of water
  • Thickening of tailings
  • Paste disposal
  • Dry stacking of tailings (filtered tailings)
• Co-disposal of waste rock with tailings
  • Paste rock (homogenous mixture of tailings and waste rock)
  • Co-disposal of highly reactive waste rock within saturated tailings impoundment
  • Layered co-mingling of thickened tailings and waste rock
• Coating/passivation with chemical amendments (microencapsulation)
  • Potassium permanganate
  • Combined phosphate and thiocyanate treatment
  • High-pH aluminium waste
  • Silica treatment
• Mixing with organic material
  • Sewage sludge, municipal landfill waste, pulp and paper waste
• Underwater disposal
  • In-pit disposal
  • In undergound mines
  • Subaqueous disposal in waste facility
  • Disposal in artificial water bodies
  • (Disposal in natural water bodies)

References

Alpers, C.N., Blowes, D.W., Nordstrom, D.K. & Jambor, J.L. 1994. Secondary Minerals and Acid Mine-water Chemistry. In: Jambor, J.L., Blowes, D.W. (Eds) The Environmental Geochemistry of Sulfide Mine-wastes. Mineralogical Association of Canada. Short Course Handbook, Vol. 22:247-270.

Jambor, J.L. 2003. Mine-waste mineralogy and mineralogical perspectives of acid-base accounting. In: Jambor, J.L., Blowes, D.W., Ritchie, A.I.M. (Eds) Environmental aspects of mine wastes. Mineralogical Association of Canada, Short Course Series, Vol. 31:117-145.

Singer, P.C. & Stumm, W. 1970. Acid Mine Drainage: The Rate-determining Step. Science 167:1121-1123.