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

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 degrees 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 degrees 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.