Pilot-Scale Lanthanide Precipitation from Sulfate-Based Spent Ni-MH Battery Leachates: Thermodynamic-Based Choice of Operating Conditions

Access to critical metals required for high-performance technologies, particularly, the light rare earth elements (REEs = La, Ce, Nd, Pr), has become a major challenge for import-dependent economies such as the European Union. In this regard, the recycling of spent nickel metal hydride (Ni-MH) batteries by hydrometallurgical processes can serve as an attractive secondary source of REEs. In such processes, precipitation of REEs from pregnant leach solutions (PLS) in sulfate media using Na2SO4 is often reported. However, little consideration is given as to whether and how sodium ions influence the precipitation efficiency and selectivity. This work focuses on a better understanding of the precipitation process by coupling pilot-scale (2 L) experiments on industrially sourced PLS containing 50 g/L of Ni and 17 g/L of REEs, with thermodynamic modeling, to assess the influence of temperature (25 °C < T < 60 °C) and the Na/REEs molar ratio (0.8:1 < Na/REEs < 3.2:1). Equilibria calculations were performed using OLI Systems Inc. software whose database covers rare earth sulfate compound properties and an accurate description of the aqueous electrolytes. Highly selective precipitation was obtained at 60 °C and for a Na/REEs molar ratio of 4:1. A lanthanide-alkali solid solution was identified by multianalytical characterization.