Enhanced cementation of gold via galvanic interactions using activated carbon and zero-valent aluminum: A novel approach to recover gold ions from ammonium thiosulfate medium

In gold (Au) hydrometallurgy, ammonium thiosulfate is the most attractive and promising alternative to conventional lixiviants like cyanide or the halides because it is non-toxic, less corrosive and has high selectivity for Au. However, its industrial-scale application for Au extraction is still severely limited because the recovery of dissolved Au from pregnant solutions remains challenging. The present study proposes a novel, simple and effective technique to address this difficulty using zero-valent aluminum (ZVAl) and activated carbon. Gold recovery was investigated by mixing 0.15 g of ZVAl and/or activated carbon and 10 ml of a solution containing 1 M Na2S2O3, 0.5 M NH3, 0.25 M (NH4)(2)SO4 and 10 mM CuSO4 with 100 mg/l of dissolved Au in a constant temperature water bath shaker at 25 degrees C for 24 h. Gold recovery from the ammonium thiosulfate solution was negligible when only ZVAl or activated carbon was added. When ZVAl and activated carbon were both present in the ammonium thiosulfate solution, however, over 99% of Au ions were successfully recovered. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) observations of the solid residues indicate that Au was predominantly deposited on activated carbon attached to ZVAl. Based on the rest potentials of these two materials, their synergistic effect on Au recovery could be attributed to the formation of numerous galvanic cells in solution where ZVAl acted as anodes (i.e., primary electron donor) while activated carbon acted as the cathode onto which Au ions are reduced via cementation to elemental Au.