Chemical treatment of acid mine drainage Anna Gulkova, Water and Environmental Engineering, Aalto University 1
Contents Acid mine drainage formation Problems associated with acid mine drainage Treatment methods Neutralization Neutralization with polishing treatment Sulfide precipitation Conclusions 2/20
Acid mine drainage formation Mining of pyrite-rich ores or their exposure to water and air causes formation of acid mine drainage (AMD) as follows: FeS 2 + 3.75O 2 + 3.5H 2 O Fe(OH) 3 (s) + 2SO 4 2- + 4H + net reaction of pyrite oxidation Inlet water of Kristineberg s water treatment plant in northern Sweden Courtesy of Boliden Mineral AB 3/20
Problems associated with AMD Formed acid promotes release of metals. Environmental pollution Heavy metals bioaccumulation Inaccessible substrate Unavailable plants nutrients Reduced activity of soil microorganisms Pollution of drinking water supply Corrosion of infrastructure AMD collection in Kristineberg s tailings pond Courtesy of Boliden Mineral AB 4/20
Treatment methods Possibilities: membrane filtration, use of adsorbents, wetlands, ionexchange, evaporation or crystallization. Chemical treatment: Neutralization Fenton reagent Manganese oxidation Sulfide precipitation Sedimentation ponds of Kristineberg s WTP Courtesy of Boliden Mineral AB 5/20
Neutralization 6/20
Neutralization theory Addition of alkali to increase ph and precipitate metal hydroxides. Reagents: Ca(OH) 2, NaOH, CaCO 3, or Mg(OH) 2. Pros: Most common Simple Cheap Cons: High solubility of metal hydroxides Large sludge volumes Sludge is hard to dewater Sludge stability issues Sludge after AMD neutralization Courtesy of Boliden Mineral AB 7/20
Neutralization - High Density Sludge process Courtesy of Boliden Mineral AB 8/20
Polishing treatment 9/20
Polishing treatment: co-precipitation and adsorption Unreacted lime particles have adsorptive properties. At acidic ph: Ferric hydroxides adsorb As, Pb, Cr and Cu. Al hydroxides adsorbs As, Cu, Si, and Cr. At alkaline ph: Mn hydroxides adsorb trace metals, such as Zn and Cd. Fe(OH) 3 precipitation on limestone @https://en.wikipedia.org/wiki/ File:Mineral_CSA_Science.jpg Coated limestone particle 10/20
Polishing treatment theory Fenton reagent oxidation of FeSO 4 with H 2 O 2 at ph 3-5. Precipitation of Fe(OH) 3. Removal of As, Pb, Cr, Cd as well as Cu and Zn. Manganese(II) oxidation oxidation of MnSO 4 and precipitation of MnO 2 at ph 8-9. Removal of Ni, Co, Cu, Zn, as well as As and Cr. Fenton experiments with 25, 50, and 100 mg/l FeSO 4 11/20 Manganese oxidation experiments with 50 and 125 mg/l MnSO 4 Courtesy of Boliden Mineral AB
Sulfide precipitation 12/20
Sulfide precipitation theory Reagents: NaHS, Na 2 S, NH 4 S, H 2 S(g), FeS(s), organosulfides, green liquor. Bioreactors. Pros: Cons: Low solubility of metal sulfides Potential to recover metals Fast reaction rates Low sludge volumes Operational difficulties (dose control) Toxicity, H 2 S gas hazard High costs Sulfide precipitation black copper sulfide Courtesy of Boliden Mineral AB 13/20
Sulfide precipitation process - chemical BQE ChemSulphide plant block diagram Parameter Influent ChemSulphide effluent ph 2.2-2.6 3.0 Copper, mg/l 146 <3 Ref.: M. Nodwell and D. Kratochvil, Sulphide Precipitation and Ion Exchange Technologies to Treat Acid Mine Drainage, 9th Int. Conf. Acid Rock Drain., May 2012. 14/20
Sulfide precipitation process - biological BQE BioSulphide plant block diagram Feed water H 2 S(g) Bioreactor Contactor Tank Treated Water Elemental S, acetic acid Filter Metal product Ref.:P. Littlejohn, et al., 2015. Using novel technology for residue management and sustainable mine closure. 15/20
Sulfide precipitation process - biological Feed water with SO4 2- (up to 25 g/l) and metals Paques Sulfateq TM plant block diagram Anaerobic Sulphide Bioreactor Recycling Dissolved Sulphide Aerobic Sulphur Bioreactor Treated water SO4 2- <300 mg/l decreased hardness E-donor (H 2, organics) Metal Sulphides Air Elemental Sulphur Ref.:Paques, SULFATEQ TM - PAQUES. [Online]. Available: http://en.paques.nl/products/other/sulfateq. 16/20
Conclusions Large volumes of AMD need to be treated Integrated approach is essential: prevention, treatment, sludge management Site-specific research is required Outflow from liming station of the Kristineberg s tailings pond Courtesy of Boliden Mineral AB 17/20
Conclusions Goals of AMD treatment: Decrease pollution Increase water reuse Recover metals Outflow from liming station of the Kristineberg s tailings pond Courtesy of Boliden Mineral AB 18/20
Thank you for your attention! Q&A 19/20
Acknowledgements Boliden Mineral AB BQE Water PAQUES 20/20