Abstract
Two nanofiltration membranes, a Dow NF 270 polyamide thin film and a TriSep TS 80 polyamide thin film, were investigated for their retention of ionic species when filtering mine influenced water streams at a range of acidic pH values. The functional iso-electric point of the membranes, characterized by changes in retention over a small pH range, were examined by filtering solutions of sodium sulphate. Both membranes showed changes in retention at pH 3, suggesting a zero net charge on the membranes at this pH. Copper mine drainage and synthetic solutions of mine influenced water were filtered using the same membranes. These solutions were characterized by pH values within 2 and 5, thus crossing the iso-electric point of both membranes. Retention of cations was maximized when the feed solution pH was less than the iso-electric point of the membrane. In these conditions, the membrane has a net positive charge, reducing the transmission rate of cations. From the recoveries of a range of cations, the suitability of nanofiltration was discussed relative to the compliance with mine water discharge criteria and the recovery of valuable commodity metals. The nanofiltration process was demonstrated to offer advantages in metal recovery from mine waste streams, concomitantly enabling discharge criteria for the filtrate disposal to be met.
Highlights
The management of water in mining operations is becoming increasingly scrutinized, with water reuse, water treatment and discharge being major issues faced by the industry [1]
Minimum rejections of sodium and sulphur were obtained at pH 3.0 when testing the NF 270 membrane, suggesting that the iso-electric point (IEP) was in the vicinity of pH 3 under these conditions
The general discharge criteria for copper and manganese were met for TS 80 at pH 2.60 only (Table 4). These results demonstrate that two factors need to be considered when treating mine influenced water by nanofiltration to meet discharge criteria
Summary
The management of water in mining operations is becoming increasingly scrutinized, with water reuse, water treatment and discharge being major issues faced by the industry [1]. Acid mine drainage (AMD) is a typical by-product of the mining industry and a specific type of mine influenced water (MIW). AMD occurs when rock containing reduced sulphur is exposed to air and water, resulting in metals and sulphate being released from a variety of rock types, and a broad range of metal concentrations and pH can result. Treatment of mine water is often seen as an end-of-pipe process aimed at producing a discharge stream that meets specified limits of acidity and concentrations of metals and sulphate. Anaerobic conditions are used to reduce the sulphate to sulphide, leading to the precipitation of metal sulphides that are incorporated in benthic organics and live biomass
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