Separation performance and fouling analyses of nanofiltration membrane for lithium extraction from salt lake brine

In this study, the separation performance of NF membrane for magnesium and lithium in salt lake brine with a high magnesium-to‑lithium mass ratio and membrane fouling properties were investigated. The results showed that the retention rates for magnesium and lithium were higher than 81 % and less than −69 %, respectively. The mass ratio of magnesium and lithium in the permeate reduced 8.5 times and the yield of lithium was 83 % after separation indicating the effective lithium recovery for DK NF membrane. However, the polyamide functional groups of the DK NF membrane showed redshift after fouling, and inorganic ions (Mg and Ca) and carboxyl groups were increased on the membrane surface. Furthermore, functional layer and spongy porous structures of the NF membrane were damaged after pollution decreasing the effective membrane area and pore size. Moreover, the hydrophilicities of the fouled membranes decreased, while their electronegativities increased. Therefore, the NF membrane exhibited a decreased separation performance after long-term recovery of lithium from brine. Nanofiltration fouling showed that trace calcium in salt lake brine with a high magnesium-to‑lithium mass ratio was the crucial factors on membrane fouling, and thus calcium could be reduced in the recovery of lithium from brine via nanofiltration.