Membrane Contactors as a Cost-Effective Cyanide Recovery Technology for Sustainable Gold Mining

Abstract

Cyanide is the ligand of choice for large-scale gold extraction plants and is increasingly replacing amalgamation for small-scale gold extraction processes. Although cyanide is less recalcitrant than mercury, it is still acutely toxic. Current absorption column-based recovery and oxidation processes have high capital and chemical costs, respectively, and are therefore rarely implemented and only at large-scale operations. Membrane contactors with a pH gradient can be used for selective separation of cyanide into a distillate solution. This study evaluated cyanide recovery using polypropylene (PP) and polytetrafluoroethylene (PTFE) membranes and the impact of operating conditions with synthetic and real mining wastewater. The results show the successful recovery of cyanide with the PTFE and PP membranes, as well as a significant impact of distillate pH, temperature, and feed cyanide concentration on process efficiency. Conversely, distillate cyanide concentrations had a very limited impact on process performance, suggesting limited freshwater resources are required for cyanide recovery. The results of a techno-economic analysis indicate that membrane contactors are cost-effective on large and small scales. While the PTFE membrane system is slightly less expensive at a small scale and the PP membrane system is more advantageous at a large scale, both membrane systems can be considered cost-competitive with current cyanide recovery methods.