Abstract
The high-density sludge (HDS) process is traditionally used to treat acid rock drainage (ARD), with membrane systems used to further treat the HDS supernatant in applications that require lower treated sulfate concentrations. There are benefits to pre-concentrate ARD with membrane systems, followed by HDS to treat the reverse osmosis (RO) concentrate. Concentrating the ARD stream provides more driving force for precipitation reactions, and the reduced hydraulic load leads to smaller HDS reactors and lower capital costs. Treating the RO concentrate directly, instead of in a recycle loop, removes the risk of increasing concentrations of solutes that cannot be removed by lime precipitation. Despite high rejections and low salt passage achievable in a low pH RO, the permeate may contain elevated concentrations of arsenic, manganese and iron, which will have to be removed in post-treatment systems such as manganese greensand filters.
Recent pilot studies on a Peruvian minewater have shown it is viable to treat ARD directly using ultrafiltration (UF) and RO. The feed pH and electrical potential should be in the correct range to take advantage of the increased solubility of the sparingly soluble minerals of iron, aluminium and copper. The recovery in the RO is also limited by the sulfate and calcium concentrations. The pilot test work showed that the concentrate gypsum (CaSO4.2H2O) saturation ratio (SR) should be kept below 1.5 to prevent gypsum scaling. High concentrations of aluminium and iron interfere with crystal growth inhibitors in antiscalants designed for low pH waters, resulting in high antiscalant dosages that are not economically sustainable. Furthermore, the UF and RO cleaning regimes are influenced by numerous factors associated with the feed waters, such as unstable iron(III) species and biological activity.