Selective leaching of rare earths, base metals and precious metals from used smartphones

Discarded smartphones represent a valuable source of rare earths (REE), base metals and precious metals. This study focussed on the optimisation of three-stage selective leaching conditions for REE, copper and precious metals (Au and Ag), respectively, contained in printed circuit boards (PCBs) found in end-of-life smartphones. The effects of several leaching conditions, such as sulphuric acid and thiourea concentrations, were investigated using a statistical approach based on a design of experiments using Box-Behnken methodology. Optimum leaching efficiencies were achieved when PCB powder was contacted (solid concentration of 100 g/L) with (1) a 0.2 M H₂SO₄ solution for 30 min at a temperature of 20°C for REEs; (2) a 1 M H₂SO₄ solution with 67 g H₂O₂/L for 180 min at 80°C for Cu and (3) a solution of 42 g thiourea/L in 0.1 M H₂SO₄ and 9 g Fe₂(SO₄)₃/L for 120 min at 20°C for Au and Ag. Using these optimal conditions, a complete leaching procedure included an REE solubilisation step and a base metal leaching step, both repeated twice, and a precious metal leaching step. This procedure solubilised 91% of the REE, 100% of the copper, 98% of the gold and 87% of the silver contained in the PCB powder during their respective leaching stages.