Design, construction and operation of a laboratory scale electrolytic cell for sodium production using a β″-alumina based low-temperature process

Abstract

Sodium metal can be produced at low temperatures (∼523 K) by electrolysis of sodium tetrachloroaluminate (NaAlCl₄) in a cell, which employs sodium ion conducting beta-alumina as diaphragm. A laboratory-scale electrolytic cell and associated systems were designed and constructed to study the various aspects of the energy efficient process. Graphite/reticulated vitreous carbon (RVC) was used as the anode and molten sodium as the cathode. Electrolysis was carried out at ∼523 K with currents in the range 1–10 A (10–125 mA cm⁻²). The cathodic current efficiency was close to 100%, but the anodic current efficiency was very low (20–30%), probably due to the consumption of chlorine in the intercalation reaction of graphite and aluminium chloride. The sodium metal was analysed by AAS and found to have 5N purity. On prolonged electrolysis, the graphite anode disintegrated due to the formation of ‘graphite intercalation compounds’. RVC behaved as a better chlorine-evolving anode in the initial period of electrolysis, but its ability for chlorine evolution decreased on continuous electrolysis. The study indicated the need for effective stirring of the electrolyte with excess NaCl to avoid build up of aluminium chloride and the resultant complications in the cell.