Effect of [Na+]/[Li+] concentration ratios in brines on lithium carbonate production through membrane electrolysis

Abstract

Lithium is a fundamental raw material for the production of rechargeable batteries. The technology currently in use for lithium salts recovery from continental brines entails the evaporation of huge water volumes in desert environments. It also requires that the native brines reside for not less than a year in open air ponds, and is only applicable to selected compositions, not allowing its application to more diluted brines such as those geothermally sourced or waters produced from the oil industry. We have proposed an alternative technology based on membrane electrolysis. In three consecutive water electrolyzers, fitted alternately with anion and cation permselective membranes, we have shown, at proof-of-concept level, that it is possible to sequentially recover lithium carbonate and several by-products, including magnesium and calcium hydroxide, sodium bicarbonate, H₂ and HCl. The big challenge is to bring this technology closer to practical implementation. Thus, the issue is how to apply relatively well-known electrochemical technology principles to large volumes and to a highly complex and saline broth. We have studied the application of this new methodology to ternary mixtures (NaCl, LiCl and KCl) with constant LiCl and KCl composition and increasing NaCl content. Results showed very similar behaviour for systems containing [Na⁺]/[Li⁺] concentration ratios ranging from 1.24 to 4.80. The voltage developed between the anode and cathode is almost the same in all systems at roughly 3.5 V when a constant current density of 50 A m⁻² is applied. The three monovalent cations migrate with different rates across the cation exchange membrane, with Li⁺ being the most sluggish and thus crystallization of Li₂CO₃ only occurs close to completion of the electrolysis. The dimensionless concentration profiles are almost indistinguishable despite the changes in total salinity. The solids crystallized from different feeds showed higher Na⁺ and K⁺ contents as the initial Na⁺ concentration was increased. However, solids with over 99.9% purity in Li₂CO₃ could be obtained after a simple re-suspension treatment in hot water. The electrochemical energy consumption greatly increases with higher Na⁺ concentrations, and the amount of fresh water that can be recovered is diminished.