Leaching of Gold from the Waste Mobile Phone Printed Circuit Boards (PCBs) with Ammonium Thiosulphate

The rapid growth in the use of electronic equipments, combined with early obsolescence has contributed enormously to the generation of large quantity of electronic (e) waste. One such e-waste, the mobile phone printed circuit boards (PCBs) contain various precious metals which can be extracted by different hydrometallurgical routes. The present work deals with the recovery of gold using ammonium thiosulfate as a leaching agent from waste mobile PCBs containing 0.021% Au, 0.1% Ag, 56.68% Cu, 1.61% Ca, 1.42% Al, 1.40% Sn, 0.24% Fe, 0.22% Zn, 0.01% Pd etc.. The cutting granules of 0.5 -3.0 mm PCBs were used for leaching in a 500 mL glass beaker in open atmosphere. The effect of various parameters viz. ammonium thiosulfate concentration, copper sulfate concentration, pH and pulp density was studied. A leaching of 56.7% gold was obtained under the optimum condition of 0.1M ammonium thiosulfate, 40 mM copper sulfate, pH: 10-10.5, pulp density: 10 g/L at room temperature and stirring speed of 250 rpm in 8h duration. The maximum leaching of gold in the pH range 10-10.5 may be attributed to the higher stability of the ammonium thiosulfate. The decomposition of ammonium thiosulfate in the different pH ranges was chemically analysed by iodometric method. The ammonium thiosulfate contents in the leach liquors were in agreement with the quantity of gold leached in the respective pH ranges. In this process the copper sulfate worked as a catalyst. The experiment conducted with complete PCBs scrap exhibited a maximum leaching of 78.8% gold at the above optimised condition.