With increasingly more stringent environmental regulations, growing concern with water quality and management and a greater focus on water recycling and minimization of water use in the mining industry, conventional treatment methods are being challenged to meet lower concentrations of contaminants in the discharge stream. Economics and costs are important factors in technology selection and have a significant weight in the selection of treatment options, unless other factors such as regulatory requirements are the drivers. In the past decade, membrane separation processes have attracted significant attention and are applied in different sectors of the industry especially in water and wastewater treatment
The applicability of different membrane separation processes for the mitigation of acidic drainage has been demonstrated in the literature. Work by various researchers has demonstrated the successful application of Reverse Osmosis (RO), Nanofiltration (NF) and charged Ultrafiltration (UF) membranes for the treatment of acidic drainage and the removal of contaminants and metal species of interest, such as selenium, from neutral drainage streams.
Membrane separation processes, such as reverse osmosis, can generate a highly concentrated retentate. This could create problems with the precipitation of different metal species and could cause scaling and fouling problems; however, these issues can be managed with the proper mode of operation, the use of additives, the implementation of cleaning cycles and module design. The membrane concentrate, in industrial applications, is usually a highly saline stream that requires treatment before disposal. However, this stream could also have a cost benefit by allowing for the economic recovery of metals or other products of interest and improve efficiency of chemical treatment due to increases in the concentration of contaminants. With proper process design and material selection, high water recoveries are possible with the added benefits of waste minimization and volume reduction.
Commercially available hybrid membrane systems that apply combinations of Microfiltration (MF) or Ultrafiltration with Reverse Osmosis or Nanofiltration are reviewed in this report. This report also addresses other membrane options such as the use of charged UF membranes or other innovative approaches such as the VSEP (Vibratory Shear Enhanced Process) filtration process for acidic drainage (AD) treatment. Although the main focus of the present report was to assess membrane separation technology to AD treatment, the application of membrane separation to other mining applications was also reviewed.
Finally, a number of case studies on the application of membrane separation in mining operations are presented, with an emphasis on the cost impacts and improvements on the environmental performance.
The information in this review shows that membrane separation is an efficient and cost-effective technology for mine water and wastewater treatment and mitigation applications. A comparison with conventional treatment technologies has also shown that membrane separation, if properly designed and operated, can provide superior treatment results and can have lower capital and operating costs. Membranes cannot completely replace conventional treatment technologies and be a stand-alone treatment option. They can be a powerful tool for volume reduction and waste minimization, allowing for the recovery and recycling of water and other potentially valuable by-products such as acid, gypsum (calcium sulphate), heavy metals and sulphur. Because of the volume reduction that membrane separation offers, the footprint and capital costs of the accompanying conventional treatment options such as clarifiers and other chemical treatments could be substantially reduced.
Due to the composition of mining streams and effluents, the most important aspects of membrane separation are membrane fouling and brine disposal. As a result, the main technology development drivers are:
• Membrane fouling – lowering membrane replacement costs, maximizing recoveries;
• Pretreatment as a means of fouling control;
• Maximizing water recoveries; and
• Brine disposal or treatment and the minimization of its associated costs.