An alternative to cyanide leaching of waste activated carbon ash for gold and silver recovery via synergistic dual-lixiviant treatment

This study reports the development of a hydrometallurgical treatment for activated carbon ash (ACA); a waste product of the carbon-in-pulp (CIP) process used in the gold mining industry, rich in adsorbed precious metals. After an initial screening of known leaching chemistries, the research focusses on dual-lixiviant (thiourea and thiocyanate) and thiourea systems, both of which have lower environmental impact than traditional cyanide leaches. Comparing ferric sulfate and hydrogen peroxide as oxidants showed that a thiourea leach is more suited to ferric sulfate whereas the dual-lixiviant leach achieved greater extraction with hydrogen peroxide. This was believed to be due to a more favourable effective [thiourea]:[thiocyanate] molar ratio in solution. The latter demonstrated faster kinetics and improved efficiency for dissolution of gold and silver. However, both leaches had issues with silver extraction due to formation of passivating layers on the surface of the silver nanoparticles. Kinetic modelling showed both systems tended towards a mixed-controlled process. Gold extraction of 89%, with leaching reaching equilibrium within 100 min, using the dual-lixiviant process demonstrated that there is an alternative to cyanide leaches employed in gold recovery.