Gold recovery by microwave augmented ashing of waste activated carbon

Gold ore processing plants that utilize carbon adsorption technologies generate a waste activated carbon, which contains very high gold values, and the recovery of this gold represents a significant source of extra revenue. In this research, microwave energy was utilized to combust the waste activated carbon and the resulting ash was treated by conventional cyanide leaching to recover the gold. Firstly, the real and the imaginary permittivities of the activated carbon were measured by the cavity perturbation technique and it was found that they were extremely high and thus the carbon would be expected to be a good microwave absorber. Secondly, the microwave heating behaviour of the activated carbon was studied as a function of incident microwave power, processing time, sample mass and particle size. The results showed that the sample temperature increased with increasing incident microwave power, processing time, sample mass and decreasing particle size. For a 3 g sample with a particle size of −90 μm, temperatures of over 1000 °C could be attained at an incident microwave power of 1000 W and for 5 min processing time. Finally, the effects of air flowrate, microwave power and processing time on the combustion of the activated carbon were studied. By introducing air into the microwave combustion chamber, the carbon could be completely oxidized and the residue was a gold-containing ash, which was treated for gold recovery by the conventional cyanide leaching process. Over 95% gold extraction could be achieved within 8 h. The results of this research demonstrate that the microwave combustion of gold-containing waste activated carbon is a technically viable and novel process for gold recovery.