Recovery of Alkaline-Earths into Liquid Bi in Ternary LiCl-KCl-SrCl2/BaCl2 Electrolytes at 500 °C

Abstract

Electrochemical reduction of Sr2+ and Ba2+ into liquid Bi was investigated in dilute concentrations of SrCl2/BaCl2 (0‒5 mol%) in LiCl-KCl electrolytes at 500 °C to ascertain the limit of liquid Bi electrodes for alkaline-earth recovery. Analysis of the electrodes after constant current electrolysis to the specific charge of 270 C g−1 showed Sr2+ ions consuming 29% of charge at 5 mol% before dropping to 8%‒10% of the total charge at 0.45‒0.72 mol% SrCl2. Ba2+ ions consumed 54% at 5 mol% BaCl2 before decreasing to 22%‒24% at 0.42‒0.89 mol% BaCl2; substantial co-deposition of Li was observed in all chemistries, consuming up to 53% of charge. Considering only 1% of the total charge was consumed for depositing Ba2+ and Sr2+ ions in ∼0.1 mol% SrCl2/BaCl2 electrolyte, the lower recovery limit of Bi for alkaline-earth elements is suggested to be at ∼0.4 mol% SrCl2/BaCl2 to achieve appreciable deposition of alkaline-earths (>1.5 mol% Ba/Sr in liquid Bi). The overpotentials of liquid Bi at 5 mol% of SrCl2/BaCl2 were evaluated by electrochemical impedance spectroscopy. The co-deposition of Sr and Li exhibited the largest increase in charge transfer resistances implying sluggish charge transfer kinetics whereas the co-deposition of Ba and Li exhibited a large increase in mass transport resistances due to the slow diffusion of Ba2+ ions in the electrolyte.