Optimized Process for Separating NaOH from Sodium Aluminate Solution: Coupling of Electrodialysis and Electro-Electrodialysis

Abstract

Most bauxite-produced alumina is obtained by the Bayer process, but the production efficiency is limited by the slow gibbsite crystal growth due to the high concentration of NaOH in sodium aluminate solution. Here electrodialysis (ED) and electro-electrodialysis (EED) are coupled to separate NaOH from the sodium aluminate solution so as to enhance the gibbsite crystal growth rate and achieve high alumina production efficiency. The ED or EED process is also investigated before the coupling process to find the optimal operating conditions. The ED process indicates that the optimized current density is in the range of 45–60 mA cm–2 and the optimal membranes are CMV/AMV. The current density of 60 mA cm–2 can achieve a high recovery ratio (ηOH– 92.6%), low energy consumption (2.38 kW h kg–1), but a relatively high Al(OH)4– leakage ratio (ηAl(OH)4– 15.1%). The EED process indicates that with the optimized current density of 30 mA cm–2 and membrane CMV, the ηAl(OH)4– can be “zero” and the energy consumption can be as low as 2.07 kW h kg–1, but the treatment capability is low since OH– ions cannot be recovered directly and a single cation exchange membrane is used. The coupling process can combine the advantages of ED and EED, so that the ηOH– can keep a high value of 90.9%, the ηAl(OH)4– decreases to a low value of ∼5%, and the energy consumption remains low at 2.25 kW h kg–1. Overall, the coupling process of ED and EED is an excellent method to separate NaOH from the sodium aluminate solution.