Cyanide and copper cyanide recovery by activated carbon

A process is proposed to recover the cyanide and copper cyanides following the leaching and recovery of gold from copper-containing gold ores. Metallic copper is dissolved into the tailings stream to convert the residual free cyanide to copper cyanides and to concurrently reduce the overall cyanide-to-copper ratio to facilitate copper and cyanide recovery by activated carbon. Metallic copper readily dissolves in both free cyanide and Cu(CN)32- solutions to achieve a final cyanide-to-copper ratio below 3. The theoretical critical CN− and Cu(CN)32- concentration for copper dissolution in air saturated solutions are 2.9 and 2.7 mM, respectively, matching well with the measured values of 3 mM. The copper dissolution rate decreases proportionately with concentration below these critical concentrations. The increase in pH due to oxygen reduction during copper dissolution may cause copper to precipitate as Cu(OH)2. The pH of precipitation decreases with increasing copper concentration and with decreasing cyanide-to-copper ratio. However, precipitation is not observed in the presence of carbon due to the simultaneous adsorption of copper onto carbon, which reduces the solution copper concentration.