Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi
The solubilities of metal sulphides are considerably lower than those of equivalent hydroxides. Therefore, sulphide precipitation can be used in mining water treatment instead of conventional alkaline treatment when metals need to be removed to very low concentration.
Description of the technology
Metal precipitation by sulphide compounds (FeS, CaS, Na2S, NaHS, NH4S or H2S) can be used to remove metals at low pH. The reactions between sulphides (S2– and HS–) and metals occur over a broad pH range. After the precipitation step the solid precipitates are removed by suitable physical processes, such as coagulation and clarification or filtration. (e.g. Kuyucak 2006, INAP 2009)
While sulphide precipitation is not a conventional method in mine water treatment, it has been widely used in the treatment of waste waters from metal finishing industry. Sulphide precipitation is beneficial especially when water contains phosphate, ammonia, organics, surfactants, chelators and Cr6+. In some cases, sulphide precipitation is applied after conventional lime treatment, such as high density sludge treatment (HDS), to further reduce metals concentrations (e.g. Cd). (Kuyucak 2006, INAP 2009, Simate & Ndlovu 2014)
As solubilities of individual metal sulphides are different, selective sulphide precipitation can be carried out by controlling sulphide concentration by a pS electrode to achieve precipitation of the target metals. Several metals can be selectively recovered in Selective Sequential Precipitation (SSP) process that utilizes a number of successive oxidation-sulphide precipitation-filtration steps. (Jarvis and Mayes 2012, Simate & Ndlovu 2014)
An anoxic biological process with sulphate reducing bacteria (SRB) may be used to generate H2S for the metal precipitation process provided that the sulphate concentration is sufficiently high. Specific anaerobic bacteria have a metabolism where sulphates, sulphites and/or other reducible sulphur compounds are the final electron acceptors. This results in the production of biogenic H2S. There are a number of laboratory and pilot scale studies done on mine waters related to the production of biogenic sulphide and its utilization in metal precipitation process. The biogenic sulphate reduction process might be limited by the toxicity of metals and high acidity on bacterial activity. (e.g. Bowell 2000, Bowell 2004, Acheambong 2009, Lewis 2010)
Sulphide precipitation is applicable for metal containing mine waters when very low effluent concentrations are required or when selective metal removal and recovery are needed. The process can remove lead, copper, chromium (VI), silver, cadmium, zinc, mercury, nickel, thallium, antimony, and vanadium (INAP 2009).
Advantages of sulphide precipitation (e.g. Kuyucak 2006, INAP 2009, Lewis 2010) include e.g.:
- Reduces the metal concentrations to lower levels (compared to lime treatment)
- Can be operated in lower pH (compared to lime treatment)
- Shorter retention time and smaller sludge volumes (compared to lime treatment)
- Better stability of waste sludge; less susceptible to pH changes (if stored in anaerobic conditions)
- Possibility for selective metal recovery
Disadvantages of sulphide precipitation (e.g. Lewis 2010, Mokone et al. 2012) are e.g.:
- Potential of toxic gaseous H2S emissions
- Odour problems
- Corrosiveness of excess sulphide
- Potential for residual sulphide in effluent
- Sludge more difficult to separate due to fine and colloidal precipitates; may require filtration systems (compared to lime treatment)
- Need for sludge treatment and disposal
- Dosing of sulphide and process control might be challenging due to the sensitivity of the process (very low solubility of sulphides)
A wastewater treatment plant in Breckenridge in Summit Hill, Colorado, US purifies AMD from the former Wellington-Oro mine site where Pb, Zn, Cu, Ag and Au ores were mined underground between 1850 and 1960s. The drainage has high concentrations of Zn and Cd (on average 123,000 μg/l and 59 μg/l, respectively). The dissolved metals are precipitated with sulphide and recovered for commercial reuse. The reagents used are NaHS, Na2CO3 and flocculent. Removal rates of > 90% for Zn and > 99% for Cd have been obtained when the plant has been operating as designed. However, the plant has experienced problems especially in the sludge separation process. (EPA 2013, EPA 2014)
Full scale sulphide precipitation combined with lime treatment was tested at the central water treatment plant of Bunker Hill mining and metallurgical complex in US to evaluate its effectiveness in the removal of Cd, Pb and Zn. The results of a short test period showed that sulphide precipitation could reduce the Cd concentrations to 30% lower level in filtered effluent compared to lime treatment alone. However, Pb or Zn removal rates were not significantly improved. (CH2M Hill 2000)
One important aspect considering the performance of sulphide precipitation is the amorphous nature of the metal precipitates generated in most of the AMD processes. These precipitates have different (usually higher) solubilities than those of equivalent crystalline metal sulphides. The higher solubility compared to theoretical values must be taken into account also in the design phase of the system. (Lewis 2010)
A special feature of sulphide precipitation is the high reactivity of sulphide with metals and the characteristics of the precipitates. This emphasizes the need for process control and optimization as well as the choice of suitable solids/liquid separation technique. Due to possibility of toxic gaseous emissions, the occupational safety is to be carefully considered.
Acheampong, M.A., Meulepasa, R.J.W. & Lensa, P.N.L. 2009. Removal of heavy metals and cyanide from gold mine wastewater. J Chem Technol Biotechnol, 85: 590–613.
Bowell, R.J. 2000. Sulphate and salt minerals: the problem of treating mine waste. Mining Environmental Management, May 2000.
Bowell, R.J. 2004. A review of sulphate removal options for mine waters. – In: Jarvis, A.P., Dudgeon, B.A. & Younger, P.L.: Mine water 2004 – Proceedings International Mine Water Association Symposium 2. – p. 75-91, 6 Fig., 7 Tab.; Newcastle upon Tyne (University of Newcastle).
CH2M Hill 2000. Phase 2 Testing Results Bunker Hill Mine Water Treatability Study. November 2000
EPA 2013. Optimization Review. French Gulch/Wellington-Oro Mine Site Water Treatment Plant. Breckenridge, Summit County, Colorado. EPA 542-R-13-013. http://epa.gov/tio/download/remed/rse/optimizationreport_frenchgulch_may2013.pdf
EPA 2014. French Gulch site description. http://www2.epa.gov/region8/french-gulch#4
INAP 2009. The International Network for Acid Prevention. Global Acid Rock Drainage Guide (GARD Guide). http://www.gardguide.com
Jarvis, A.P. & Mayes, W.M. 2012. Prioritisation of abandoned non-coal mine impacts on the environment. SC030136/R12 Future management of abandoned non-coal mine water discharges. Environment Agency 2012.
Kuyucak, N. 2006. Selecting suitable methods for treating mining effluents. Golder Associates Ltc. PerCan Mine Closure Course , July 13-23, 2006, Lima, Peru.
Lewis, A.M. 2010. Review of metal sulphide precipitation. Hydrometallurgy 104:222–234.
Mokone, T.P., van Hille, R.P. & Lewis, A.E. 2012. Metal sulphides from wastewater: Assessing the impact of supersaturation control strategies. Water research 46:2088-2100.
Simate, G.S. & Ndlovu, S. 2014. Acid mine drainage: Challenges and opportunities. Journal of Environmental Chemical Engineering, 2:1785-1803.