Simple and environmentally friendly metal recovery from waste printed circuit boards by using deep eutectic solvents

Increased environmental awareness has necessitated the development of a simple and environmentally friendly method for metal recovery from waste printed circuit boards (WPCBs). Deep eutectic solvents (DESs) are environmentally friendly and non-aqueous lixiviants that can leach metal oxides to replace mineral acids. In this study, three choline chloride (ChCl)-based DESs were synthesized, using ethylene glycol, oxalic acid (OA), and glycolic acid (GA) as the hydrogen bond donors in the eutectic mixtures. The time-dependent leaching yields and saturated loading capacities of the DESs to typical metal oxides and Ag were evaluated. Considering the leaching performances, a simple and environmentally friendly method for metal recovery from WPCBs was established, based on two-stage DES leaching processes. The research object was the mixed metal powder collected from the mechanical processing of WPCBs. The results indicated that 90.35% of Zn, 87.47% of Pb, and 16.77% of Cu were removed from the mixed metal powder after calcination by leaching of the ChCl-GA DES and precipitation of the oxalic acid solution. When the residue was subjected to ChCl-OA DES leaching, Cu was recovered by diluting ChCl-OA DES with water. Specifically, 74.93% of Cu was separated in the form of CuC2O4·2H2O with a purity of >98 wt%. The Sn remaining in the aqueous solution was efficiently recovered by the addition of reduced Fe powder, and the recovery yield of Sn in the recovery product was 51.29%. Finally, the advantages of a simplified and environmentally friendly process framework, are highlighted by a comparison of the recovery strategy for WPCB proposed in this study with conventional pyro- and hydro-metallurgical approaches.