DiaNanofiltration-based process for effective separation of Li⁺ from the high Mg²⁺/Li⁺ ratio aqueous solution

Lithium separation from salt lake brines with a high concentration of magnesium is challenging. In this work, separation efficiencies for Li⁺ and Mg²⁺ with three commercial NF membranes (NF-270, Desal-5DL, and Desal-5DK) were systematically investigated by DiaNanofiltration method. Experimental results showed that DiaNanofiltration was an effective technique to separate Li⁺ and reduce the high Mg²⁺/Li⁺ ratio from ternary salt solutions of LiCl and MgCl₂. The solution-diffusion transport model was also used to calculate the real retention of Li⁺ and Mg²⁺, which predicted good agreement with the experimental results of the diafiltration process based on steric hindrance and the Donnan exclusion mechanism. NF270 membrane exhibited high permeation to Li⁺, about 90% for the ternary solution of LiCl + MgCl₂ + H₂O at a flux of 21.33 L·m⁻²·h⁻¹. The real Mg²⁺ retention was attained around 96% with DK membrane, which was higher as compared to NF270 due to the steric hindrance effect. Moreover, the Mg²⁺/Li⁺ ratio was also reduced from 30 to 1.7 in the single-stage DiaNanofiltration process.