Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi
Membranes can separate both organic and inorganic contaminants from different water streams. The technology is considered as established treatment method for mining influenced waters.
Description of the technology
The separation of substances in membrane process is based on sieving effects as well as electrorepulsive forces due to membrane surface charge. The membrane is semipermeable, enabling the movement of water molecules through it to permeate and at the same time the rejection of dissolved/colloidal constituents to concentrate. The most common membrane processes applied to mining water treatment include reverse osmosis (RO) and nanofiltration (NF). (Mullet et al. 2014)
Membrane processes are used in the removal/concentration of metals, metalloids (e.g. arsenic), cations, anions (e.g. sulphate) and organic compounds from mining water. The technology is mainly utilized as final effluent treatment, but it possesses potential also for metal and water recovery purposes. (García et al. 2013)
The goal in membrane processes is to reduce the formation of brine e.g. by concentrate circulation. Options for final brine treatment include evaporation in pond (not applicable in cold climate) and mechanical evaporation + crystallization. (DWA 2013)
Specific membrane process variants that can handle e.g. high sulphate concentrations have been developed. A commercial process SPARRO introduces seed crystals (usually gypsum) into the slurry with high calcium sulphate content prior to RO treatment. The concentration increases close to the membrane, the solubility products of gypsum, silicates, and other scaling salts are exceeded and they are precipitated on the seed crystals enabling better membrane filtration. (Bowell 2000, Simate & Ndlovu 2014)
RO process always requires pretreatment such as pH adjustment and NF or HDS of mine water in order to reduce metals, solids and concentration of other scalants to acceptable level considering the lifetime of membranes. (DWA 2013)
Advantages of membrane processes (e.g. García et al. 2013) include e.g.:
- Considered as established technology in mine water treatment applications
- Wide range of pollutants can be removed, both organic and inorganic
- Can remove monovalent ions unlike other treatment methods
- Possibility for recovery of valuable resources
- Produces high quality discharge water for further use
- Low chemical consumption
- Small footprint area
- Modular configuration possible
- Ease of operation
Disadvantages of membrane processes:
- Need for pretreatment
- High operating (electricity, chemicals) and maintenance cost
- Disposal costs for brine and pretreatment sludges
- Risk of membrane fouling, especially by gypsum formation
Nanofiltration (NF) has been considered as a preferred membrane process for the treatment of AMD at low temperatures because, compared to other membrane processes such as RO, it presents higher fluxes at lower pressure leading to lower capital and operational costs. In addition to producing high quality effluent, NF can be used to concentrate and recover metals and sulphuric acid. High metal rejection rates (up to 99%) have been obtained. (Al-Zoubi et al. 2010, Fornarelli et al. 2013)
Conventional RO process is usually applicable for water with low calcium (< 100 mg/l) and sulphate (< 700 mg/l) concentrations. Process variants such as SPARRO can be applied to waters with high gypsum formation potential. (Bowell 2000)
A single stage RO can reach water recovery between 50% and 80% if metals removal and/or gypsum softening are applied as pre-treatment. Thus the formation of concentrate is high and further treatment is typically needed to reduce the brine volume. Therefore membrane process, such as RO, can be applied as a pretreatment step to concentrate the recoverable metals for further processing or gypsum removal processes. Ca. 99% water recovery to permeate can be achieved by multistage RO, depending on the number of RO units. (DWA 2013)
An industrial scale HiPRO process (High Recovery Precipitating Reverse Osmosis) with a treatment capacity of 25-30 Ml/d is used to purify coal mining influenced water in Witbank coalfields, South Africa. Most of the treated water is used as potable water in the municipality of eMalahleni and a portion in the current mining operations. The process includes neutralization, primary ultrafiltration, RO in three phases, chlorination and treatment for sludge and brine. The water recovery rate is > 99% and the produced water complies with the quality criteria for potable water. The waste streams of the process include brine (100 m3/d) and gypsiferous waste (91 t/d). (Golder Associates 2010, Bhagwan 2012, WCA 2013)
The membrane type applied should be tubular to prevent fouling (Bowell 2000). With proper pretreatment and maintenance membranes typically last two to five years. Control of water temperature may be needed in cold/hot climates to minimize water viscocity. (EPA 2014)
Operating pressure has an impact on the metal separation efficiency. However, according to recent studies, moderate pressures (e.g. 15 bar) are adequate for effective separation by NF. (Sierra et al. 2013)
Solution pH has significant impact on membrane performance and metal separation. Important parameter is the membrane iso-electric point (IEP, pH at which the net membrane charge is zero). At pH values below IEP the membrane is positively charged, and negatively charged elsewhere. Metal rejection has been indicated highest when operating at pH below the IEP. For anions, such as sulphur, the pattern is reversed: increased retention when feed pH is raised. (Fornarelli et al. 2013, Mullett et al. 2014)
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.
Bhagwan, J. 2012. Turning Acid Mine Drainage Water into Drinking Water: The eMalahleni Water Recycling Project. Water Research Commission. 2012 Guidelines for Water Reuse. Appendix E | International Case Studies.
Bowell, R.J. 2000. Sulphate and salt minerals: the problem of treating mine waste. Mining Environmental Management, May 2000.
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.
EPA 2014. Reference Guide to Treatment Technologies for Mining-Influenced Water. EPA 542-R-14-001.
Fornarelli, R., Mullett, M. & Ralph, D. 2013. Factors influencing nanofiltration of acid mine drainage. Reliable Mine Water Technoly. Golden CO, USA. IMWA 2013. pp. 563-568.
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.
Golder Associates 2010. Environmental impact assessment (EIA) for the Anglo American Thermal coal proposed expansion of the eMalahleni mine water reclamation scheme. Mdedet reference number 17/2/2/1(e) nk-5. October 2010.
Mullett, M., Fornarelli, R. & Ralph, D. 2014. Nanofiltration of Mine Water: Impact of Feed pH and Membrane Charge on Resource Recovery and Water Discharge Membranes 4:163-180.
Sierra, C., Álvarez Saiz, J.S. & Gallego, J.R.L. 2013. Nanofiltration of Acid Mine Drainage in abandoned Mercury Mining Area. Water Air Soil Pollut 224:1734.
Simate, G.S. & Ndlovu, S. 2014. Acid mine drainage: Challenges and opportunities. Journal of Environmental Chemical Engineering, 2:1785-1803.
WCA 2013. South Africa Anglo American eMalahleni Water Reclamation Plant – Winner of WCA Award for Excellence in Environmental Practice 2013. WCA Case Study