Optimization of metals and rare earth elements leaching from spent Ni-MH batteries by response surface methodology

The rechargeable battery market has almost doubled in 15 years. Regardless of the type of batteries, their limited lifespan means that sooner or later they will constitute a mass of waste whose management is problematic as their content is high in elements and metals of high economic interest, but also toxic to the environment. This project is to optimize the solubilization conditions for rare earth elements (REEs) and other metals from waste nickel-metal hydride (Ni-MH) batteries. The Ni-MH battery powder used contained the following main elements: Ni (548 g/kg), La (45 g/kg), Co (32 g/kg), Zn (22 g/kg), Nd (15 g/kg), Sm (12 g/kg), and Ce (11 g/kg). The metals were solubilized in the presence of sulfuric acid. Acid concentration, solids concentration, leaching time, and temperature were optimized using the Box-Behnken design methodology. The optimal conditions identified are an H₂SO₄ concentration of 2 M, a S:L ratio of 10% (w:v), a leaching temperature of 60°C and a reaction time of 90 min. These conditions make it possible to solubilize 81% Ni, 99% Co, and 70% REEs, while the mathematical model predicted 83% Ni, 100% Co, and 80% REEs respectively. The process was also operated in counter-current leaching mode with the optimal parameters. The high solubilized yields obtained after five loops for all metals, REE and the significant reduction of water consumption confirm that this process leaching can be apply for industrial application.