Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi
Ion exchange (IX) is a process where ions are reversibly exchanged between a substrate and its surroundings (Simate & Ndlovu 2014, EPA 2014). It is common process used in water softening and purification as well as selective recovery of target components from solution (Nodwell et al. 2012) Ion exchange as an active treatment technique is not state-of-the-art in full-scale for mine water treatment (Al-Zoubi et al. 2010, Haanpää 2013) but possesses future potential.
Description of the technology
Ion exchange process utilizes a polymer resin packed column for the removal of target species. The resin contains active functional groups which capture target ions and release an equivalent ion to the solution. Usually the resin can exchange cations or anions, but some (amphoteric) are capable of exchanging both, depending on the pH. The loaded ions (such as metals) are removed from the resin by regeneration. The volume of regenerant solution is considerably smaller than the influent flow, thus resulting in effective concentration of target ions into regenerant. (Nodwell et al. 2012, Simate & Ndlovu 2014)
Suitable ion exchange columns can be used to remove sulphate from mine waste water. A commercial example of ion exchange that recovers uranium, sulphate and metals is the EARTH process. The GYP-CIX process utilized continuous fluidized bed ion exchange process to remove calcium and sulphate from water saturated with gypsum. The resin can be recovered by sulphuric acid and lime. The regeneration chemicals may represent relatively high share of the total operating costs. (Bowell 2000, Bowell 2004, Ackil & Koldas 2006)
Ion exchange can be applied for the recovery of different metals from AMD as a pre-concentrating step for further metal recovery. The eluated metals can be precipitated e.g. by sulphide precipitation. Especially, capture of copper, nickel and cobalt has been investigated and indicated having economic potential. (Nodwell et al. 2012) Ion exchange can be also utilized to remove hardness, alkalinity, radioactive constituents, cyanide and ammonia (García et al. 2013, EPA 2014).
Another aim for the use of ion exchange could be the extension of existing water treatment capacity by concentrating the effluents and thus reducing the hydraulic load to the subsequent treatment. (Nodwell et al. 2012)
Conventional ion exchange processes are not applicable for high CaSO4 waters due to scaling. Modified processes, such as GYP-CIX are developed to overcome these problems. GYP-CIP can treat waters with up to 2,000 mg/l sulphate and 1,000 mg/l calcium. (Bowell 2000, INAP 2009)
Ion exchange is best applicable to waters in the pH range of 4 to 8 containing low concentrations of suspended solids, iron and aluminium. (EPA 2014)
General advantages of ion exchange (e.g. Simate & Ndlovu 2014) include e.g.:
- Removal can be targeted to specific contaminants
- Suitable for dilute solutions
- Possibility of metal recovery
- Not sensitive for toxic substances
- Relatively simple treatment systems
- Modular configuration possible
Disadvantages of ion exchange (Bowell 2004, Taylor et al. 2005) are e.g.:
- Need for pretreatment (e.g. pH adjustment, oxidation, solids removal)
- Need for the treatment and/or disposal of regenerant as well as spent resin
- High capital and operating costs
- Applications for AMD still at research/development stage
- Limitations in cold conditions (EPA 2014)
- Organic compounds, oxidants or high temperatures can degrade the resin (EPA 2014)
GYP-CIX process is capable of > 95% sulphate removal (Bowell 2004). In laboratory scale test selenium removal has been up to 99% with over 90% recovery rates. In larger scale (ca. 330 m3/d) mine application at Soudan Mine in Minnesota the removal rate of copper and cobalt at pH 4 was > 99% and > 90%, respectively. Performance was observed to deteriorate with time as the resin gets loaded with metals. (EPA 2014)
Pretreatment of water is typically needed to reduce solids and competing ions to acceptable level. A typical ion exchange process cycle includes the following steps: service, backwash, regeneration and rinse. The volume of the waters needed in the resin maintenance and requiring storage can be significant, and too high influent solids content increases the need for backwash and regeneration. pH and temperature effects are other important parameters to consider and control. (EPA 2014)
The regenerant must be treated or it can be conveyed to further recovery processes. The treatment processes available for regenerant treatment include evaporation and crystallization, biological treatment or treatment by zero valent iron. The final treatment wastes may be classified as hazardous waste influencing the disposal options. (EPA 2014)
Important design considerations for ion exchange include (EPA 2014):
- type of resin
- volume and type of regenerant
- backwash water source
- backwash quantities
- need for pre-filtration of solids
- column configuration
- the need for pH adjustment before and after ion exchange
- cycle length
Akcil, A. & Koldas, S. 2006. Acid Mine Drainage (AMD): causes, treatment and case studies. Review article. Journal of Cleaner Production, 14:1139-1145
Al-Zoubi, H., Rieger, A., Steinberger, P., Pelz, W., Haseneder, R. & Härtel, G.2010. Optimization Study for Treatment of Acid Mine Drainage Using Membrane Technology. Separation Science and Technology, 45: 2004–2016.
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).
Bowell, R.J. 2000. Sulphate and salt minerals: the problem of treating mine waste. Mining Environmental Management, May 2000.
EPA 2014. Reference Guide to Treatment Technologies for Mining-Influenced Water. EPA 542-R-14-001.
García, V., Häyrynen, P., Landaburu-Aguirre, J., Pirilä, M., Keiski, R.L. & Urtiaga, A. 2013. Purification techniques for the recovery of valuable compounds from acid mine drainage and cyanide tailings: application of green engineering principles. J Chem Technol Biotechnol 89: 803–813.
Haanpää, K. 2013. Kaivosvesien hallinta ja käsittelymenetelmät. Pöyry Finland Oy. Kestävä kaivostoiminta –tutkimusseminaari. 21.11.2013, Mutku ry.
INAP 2009. The International Network for Acid Prevention. Global Acid Rock Drainage Guide (GARD Guide). Available: http://www.gardguide.com
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)