Separation of lithium chloride from ammonium chloride by an electrodialysis-based integrated process

Abstract

Li+ and NH4+ have the same charge, valence, and similar ionic hydration radii. Hence, it is challenging to separate them through individual electrodialysis. Herein, an electrodialysis (ED)-based integrated process is proposed to separate Li+ from the mixed solution of LiCl and NH4Cl and simultaneously generate a concentrated LiOH solution. First, bipolar membrane electrodialysis (BMED) converts the mixed solution into the alkali solution of LiOH and NH3·H2O. The BMED performance was investigated by varying the operating parameters. The results show that the BMED membrane stack with the BP-A-C configuration performs better than the one with the BP-C configuration. A solution of 2.16 mmol/L LiOH and 0.05 mol/L NH3·H2O, selected as the initial alkali solution for the alkali tank, can reduce the energy consumption from 88.51 to 83.45 kW·h/kg, and improve the current efficiency from 53.45% to 56.69% slightly. Reducing the initial volume ratio of the alkali solution to the feed can concentrate the generated alkali solution and increase the LiOH conversion ratio simultaneously. Furthermore, reducing the initial feed concentration can efficiently improve the BMED performance. A maximum LiOH conversion ratio of 97.10% can be achieved in the BMED process. Second, ammonia removal with direct air stripping (AR-DAS) process is performed on the mixed alkali solution of LiOH and NH3·H2O to remove the ammonia and obtain the pure LiOH solution. The results show an ammonia removal ratio of more than 99%. Third, conventional electrodialysis (CED) concentrates the LiOH solution. Effects of the applied voltages and initial LiOH solution concentrations on the CED performance were investigated. The results show that a higher applied voltage increases energy consumption, while a higher initial LiOH solution concentration improves the LiOH concentration ratio. A maximum LiOH concentration ratio of 2.19 can be achieved in the CED process. Moreover, through analysis, the final solid products, generated by evaporation and crystallization, are LiOH·H2O crystals with the purity of 94.3 ± 3.56%. This work suggests that the ED-based integrated process is a highly effective approach to separating the mixed salt solutions of Li+ and NH4+.