Comparison of the Mg²⁺-Li⁺ Separation of Different Nanofiltration Membranes

Nanofiltration application for the separation of Mg²⁺-Li⁺ from salt-lake brines was attempted in the present work. Four different nanofiltration membranes identified in the manuscript as DL, DK, NF-270, and NF-90 were used to treat salt brine with a magnesium to lithium ratio (MLR) of 61, additionally contaminated by the other ions such as Na⁺, K⁺, Ca²⁺, etc. The effect of the dilution factor, operating pressure, circulation rate, and feed pH were assessed to identify the optimal operating conditions for each membrane based on the retention efficiency of each ion. The results showed an insignificant effect of Ca²⁺ on the retention performance of Mg²⁺-Li⁺. Na⁺ and K⁺ had a smaller hydration radius and larger diffusion coefficient, which competed with Li⁺ and altered the separation of Mg²⁺-Li⁺. Under the optimal conditions (dilution factor: 40; operating pressure: 1.2 MPa; circulation flow rate: 500 L/h; pH: 7), the retention efficiency of lithium was as low as 5.17%, separation factor (SF) was as low as 0.074, and the MLR in the permeate reduced to 0.088.