Study of gold leaching from pre-treated waste printed circuit boards by thiosulfate-cobalt-glycine system and separation by solvent extraction

The recovery of gold (Au) from waste printed circuit boards (WPCBs) derived from discarded mobile phones is crucial for the resource preservation and environmental protection. In this context, thiosulfate has emerged as a viable alternative to the toxic compound cyanide for Au leaching. However, the conventional thiosulfate leaching system has a main problem that is large thiosulfate consumption, which limits its commercial application. In this study, we developed a novel thiosulfate leaching system that utilizes a cobalt (Co(II))-glycine (gly) complex as the catalyst. Our findings demonstrate that the Co(II)-glycine-thiosulfate system exhibits excellent Au leaching performance coupled with low thiosulfate consumption. The effects of several parameters on Au leaching efficiencies and thiosulfate consumption were investigated. Increasing the concentrations of Co(II), thiosulfate, or glycine was beneficial to the Au leaching. However, increasing temperature promoted the oxidative decomposition of thiosulfate and reduced cobalt stability, negatively impacting Au leaching. Additionally, small amounts of base metals remained after pretreatment of WPCBs using HCl system and H2SO4-H2O2 system. Therefore, the roasting-assisted leaching process eliminated the adverse effects of these impurity metals and further improved Au leaching efficiency to 97.8% while also decreased thiosulfate consumption. The recovery of Au from the pregnant leach solution was also studied, and our findings demonstrated that N1923 can efficiently extract Au from the solution through anion exchange. Our study developed a novel and efficient Au thiosulfate leaching system catalyzed by the Co(gly)3/Co(S2O3)34- redox couple, and put forward a promising approach to enhance Au recyclability from WPCBs while significantly reducing thiosulfate consumption.