Toward high temperature and low pH gold mining effluent reclamation by different membrane separation processes

Abstract

Gold mining effluents are characterized by low pH and high concentrations of sulfate, metals and metalloids, especially arsenic, which is a toxic element that requires specific treatment. The treatment of gold mining effluents by nanofiltration (NF) and membrane distillation (MD) is a very promising alternative since these membranes can retain bivalent ions efficiently for the production of a high quality permeate that can be reused on industry. The objective of this study was to evaluate the main operational conditions for treatment of gold mining effluents by NF, vacuum MD (VMD) and direct contact MD (DCMD) conjugated or not to arsenic solvent extraction (SX) and to conduct a preliminary estimation of operating costs. The results showed that all technologies were efficient in the treatment of gold mining effluent, allowed the high permeate flux and satisfactory solids retention efficiencies up to a recovery rate of 30% (for MD systems) and 40% (for NF), and the pH of the effluent affected both the solute retention efficiency and the membrane fouling trend. For NF, higher permeate fluxes were obtained and the system can operate up to a higher recovery rate, but presents costs of effluent cooling. The retention of 85% of arsenic III and 75% of arsenic V was achieved and the permeate obtained can be reused in the industry. On the other hand, the MD systems (both VMD and DCMD) presented fairly high rejection (above 96%), relatively high permeate flux, compared to NF. However, they have a low operating cost as they allow for the recovery of the residual heat of the effluent as a driving force of MD process and produce a permeate of high quality, without restrictions for industrial reuse. Solvent extraction (SX) showed high arsenic removal efficiency (80%) for Aliquat 336, extraction bot arsenite and arsenate. The concentration of sulfate in solution must be controlled for guarantee the extraction efficiency. The application of the membrane separation processes reduces the costs with chemical precipitation, the volume of sludge generated and allows the reuse of water, while SX can allow for arsenic recovery. Thus, the use and / or conjugation the these processes represent a great economic advantage for the industry while contributing to the mining eco-efficiency.