Gold Leaching from Printed Circuit Boards Using a Novel Synergistic Effect of Glycine and Thiosulfate

Abstract

Printed circuit boards (PCBs) are a secondary source for the extraction of precious metals, such as gold (Au), silver (Ag), platinum (Pt), palladium (Pd), etc. Thiosulfate and glycine systems have recently gained a significant amount of attention for Au leaching. However, in the thiosulfate system, the stability of leached Au tends to decrease when using lower thiosulfate concentrations. In this study, a copper–ammonia–thiosulfate system (thiosulfate system) and glycine or histidine were combined to leach Au from PCBs. The glycine–thiosulfate system resulted in a higher Au leaching than the histidine–thiosulfate system. The results revealed that the glycine–thiosulfate system exhibited a synergistic effect on Au leaching (93.7%) at pH 9.3 and 40 °C, while the Au leaching percentages were 47.1% and 50.7% for the thiosulfate and glycine systems, respectively. In the dual system, Fe leaching was insignificant, although Ag and Al leaching were 95.3% and 27.0%, respectively. Compared to the thiosulfate system, the dual system maintained the stability of the leached Au. The system required 60 mM thiosulfate and 0.5 M glycine at 40 °C and pH 9.3 in order to leach Au from PCBs. The kinetic study suggested that Au and Ag leaching from PCBs in the dual system followed the diffusion-controlled model. The Au leaching rate in the initial phase of the dual system was similar to that of the glycine–cyanide system. This novel, mild approach could be applied to hydrometallurgy to leach other precious metals from sources, such as ore and spent catalysts.