Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi
The addition of alkaline agent, such as limestone (CaCO3), CaO, Ca(OH)2, NaOH, Na2CO3, or ammonia can be used to raise pH and to achieve precipitation of metals as hydroxides, oxyhydroxides or carbonates. Some removal of sulphate as gypsum takes place when Ca-containing chemicals are used. (Kauppila et al. 2011)
Description of the technology
The process referred to as “ODAS” includes oxidation, dosing with alkali, and sedimentation. Though, the first phase is usually dosing with alkali, followed by oxidation and sedimentation. The alkali is usually added to the reactor as water slurry. Oxidation of the reactor content, typically by mechanical aeration, allows reduced metals, especially Fe and Mn, to oxidize so that they can form precipitates. Sometimes oxidizing chemical is added. Oxidation is not needed if the metals are already present in their oxidized form or when the target metals have lower solubility in their reduced form (e.g. chromium, selenium, and uranium). In the sedimentation phase coagulants or flocculants may be used to improve settling. (Costello 2003, Johnson & Hallberg 2005)
Resulting sludge in conventional “Low density treatment” (LDS) has solids content 2-7%. So-called High Density Treatment (HDS, several commercial variants available) can reach solids content > 30% by recycling sludge back to the neutralization tanks and applying more efficient flocculation. Thus, the volume of waste sludge is reduced and also the chemical usage is reduced. HDS process with hydrated lime is the most widely adopted process for alkaline treatment. (Aube 2004, Taylor et al. 2005, DWA 2013)
Another variant of alkaline treatment is Pulsed Carbonate Reactor (PCR) where increased partial pressure of carbon dioxide (CO2) in water increases the solubility of carbonate (e.g. limestone) and thus improves the neutralizing ability of carbonate. (Taylor et al. 2005)
For example, at Wheal Jane Tin Mine in Cornwall alkaline treatment together with passive treatment techniques has been used to treat mine waters.
Alkaline treatment is usually applicable for acid mine drainage containing a mix of metals with little or no commercial value and when relatively modest effluent quality standards need to be met. Alkaline treatment can also be applied as pretreatment step when the targeted water quality is high. (Nodwell et al. 2012, DWA 2013, Simate & Ndlovu 2014)
Metals that can be precipitated by pH control include Cu, Pb, Zn, Ni, Cd, Fe, Mn, Al, Cr-III, Sb, As-V, Ag, Se, Th and Be. In contrast, some metals are virtually unaffected by pH control alone, e.g. Hg, Mo, Cr-VI and As-III. (Taylor et al. 2005)
Selective removal of certain components, such as arsenic and molybdenum, can be achieved by e.g. multiple-stepped addition of reagents accompanied by pH control. (Johnson & Hallberg 2005)
Advantages of alkaline treatment:
- Well proven, state-of-the-art technology
- Effective remediation of AMD
- Wide range of different neutralizing chemicals available
- Stable and easily controllable process
- Adaptable to changes in water flow and quality
Disadvantages of alkaline treatment:
- Non-selective process, no possibility for metal recovery
- The precipitation processes are relatively slow
- Possibility of equipment failure due to blockages
Alkaline treatment can be operated on large water volume. The operating cost can be high due to large chemical need. The actual neutralizer amount needed as well as sludge volume is difficult to predict; stoichiometric amounts of alkalinity are not enough for complete metal precipitation (Koide et al. 2012). The capital costs are directly proportional to the flow to be treated (DWA 2013).
Vast amount of sludge with low solids content and poor dewaterability requiring appropriate disposal is generated (especially in low density treatment). The long-term stability of sludge and possible re-release of metals is a specific concern. (Kalin et al. 2006, DWA 2013) Thus, the sludge treatment and disposal costs may be ca. an order of magnitude higher than the capital and chemical costs (Simate & Ndlovu 2014).
Innovative methods proposed for the utilization of sludge generated in AMD alkaline treatment include production of building materials from the inorganic content of sludge and the use of dried sludge as low-cost adsorbent to remove phosphorus from agricultural or municipal wastewaters (sludge containing iron and/or aluminium hydroxide precipitates). (Simate & Ndlovu 2014)
The choice of most suitable alkaline treatment (the reagent used and the specific process configuration) depends on the characteristics of chemical supply, such as transportation need, required storage and dosing equipment, requirements for chemical handling due to possible classification, availability and cost as well as the efficiency of the selected alkaline agent. (Taylor et al. 2005, Trumm 2010)
The properties of the mine water have an effect on the process parameters of alkaline treatment. Especially, the presence of multiple metals with different precipitation properties emphasizes the importance of pH control. (Kalin et al. 2006) Other factors related to water and site characteristics include:
- solids content,
- flow rate,
- ionic strength,
- redox potential,
- concentrations of suitable complexing agents (e.g. humic substances) and
- interactions of the precipitated solids
- available land area
Aube, B. 2004. The Science of Treating Acid Mine Drainage and Smelter Effluents. InfoMine – Your Global Mining Resource. http://www.infomine.com/publications/docs/Aube.pdf
Costello, C. 2003. Acid Mine Drainage: Innovative Treatment Technologies. U.S. Environmental Protection Agency.
DWA 2013. Feasibility Study for a Long-term Solution to Address the Acid Mine Drainage Associated with the East, Central and West Rand Underground Mining Basins. Treatment Technology Options. Study Report No.5.4. Third draft.
Johnson, D.B. & Hallberg, K.B. 2005. Acid mine drainage remediation options: a review. Science of the Total Environment 338: 3–14.
Kalin, M., Fyson, A. & Wheeler, W.N. 2006. The chemistry of conventional and alternative treatment systems for the neutralization of acid mine drainage. Science of the Total Environment 366:395–408.
Kauppila, P., Räisänen, M.L. & Myllyoja, S. 2011. Best Environmental Practices in Metal Ore Mining. Finnish Environment 29 en/2011.
Koide, R., Tokoro, C., Murakami, S., Adachi, T. & Takahashi, A. 2012. A Model for Prediction of Neutralizer Usage and Sludge Generation in the Treatment of Acid Mine Drainage from Abandoned Mines: Case Studies in Japan. Mine Water Environ 31:287–296
Nodwell, M., Kratochvil, D., Sanguinetti, D. & Consigny, A. 2012. Reduction of water treatment costs through ion exchange preconcentration of metals while maintaining strict effluent standards. 51st Annual Conference of Metallurgists (COM 2012). Niagara Falls, ON, September 30 to October 3, 2012.
Simate, G.S. & Ndlovu, S. 2014. Acid mine drainage: Challenges and opportunities. Journal of Environmental Chemical Engineering, 2:1785-1803.
Taylor, J., Pape, S. & Murphy, N. 2005. A Summary of Passive and Active Treatment Technologies for Acid and Metalliferous Drainage (AMD). Prepared for the Australian Centre for Minerals Extension and Research (ACMER)
Trumm, D. 2010. Selection of active and passive treatment systems for AMD flow charts for New Zealand conditions. New Zealand Journal of Geology and Geophysics. 53:195-210.