Urban mining of REEs from wastewater treatment plant ash: Process optimization of inorganic acid leaching

This study investigates the leaching of rare earth elements (REEs) from a novel secondary source, incineration ash of urban wastewater treatment plants. Hydrochloric, nitric, and sulfuric acid were selected as leaching agents. The optimal leaching conditions were determined from Central Composite Design (CCD), evaluating acid concentration, temperature, liquid-to-solid ratio, and H₂O₂ concentration. The findings suggest that HCl and HNO₃ achieved almost complete leaching of REEs, concentrations in the leachate ranging from 0.5 to 5.0 mg L⁻¹. Sulfuric acid showed lower efficiency. The optimal conditions for HCl and HNO₃ included a temperature of 85 °C, acid concentrations of 3.1 M and 5.1 M respectively, and liquid-to-solid ratio 6.0 mL g⁻¹. Under these conditions, leaching yields reached up to 100% for target REEs Nd, Pr, Gd, Tb, and Dy. As demonstrated, the potential of UWWTP ash as REEs secondary source, is highlighted, contributing to sustainable urban mining practices.