The development of a caustic pre-leaching step for the recovery of Au from a refractory ore tailings heap

Several low temperature pre-leaching experiments in acidic and alkaline solutions, followed by cyanidation were performed which ultimately led to the further investigation of a caustic pre-leaching step. The Au recovery were found to be a balance between the Au being released for cyanidation through gangue mineral dissolution and the Au being leached/lost in the pre-leaching step. Depending on the NaOH concentration, the major elements (excluding Na) in the pre-leach solution were found to be Si (500-8500 mg/L), S (2400-1800 mg/L), Al (350-50 mg/L), Zn (10-100 mg/L), Fe (60-75 mg/L) and As (20-55 mg/L). Au recovery in the subsequent cyanide and elution step increased with pre-treatment temperature and NaOH concentration, reaching a peak before either stabilising or decreasing. Longer pre-leaching residence time and a lower solids percentage resulted in less Au being recovered.