Nanofiltration membrane with crown ether as exclusive Li+ transport channels achieving efficient extraction of lithium from salt lake brine

Abstract

Lithium extraction from salt lake brines with high Mg2+/Li+ ratios represents an effective alternative for tackling the scarcity of lithium resources. In this work, nanofiltration (NF) membrane with exclusive Li+ transport channels was designed and prepared through facile interfacial polymerization, in which polyethyleneimine (PEI) bonded with 15-crown-5 ether (15C5) through hydrogen interaction was selected as the aqueous monomer (PEI@15C5) and reacted with trimesoyl chloride (TMC). The channels generated by 15C5 molecules could facilitate Li+ transport owing to their affinity to Li+. Molecular dynamics simulation was conducted to further confirm the Li+ transport through PEI@15C5-TMC membrane with exclusive Li+ transport channels provided by 15C5. The PEI@15C5-TMC membrane displayed a permeability of 8.0 L·m−2·h−1·bar−1 and stable separation factor (SLi,Mg) of 14, which could reduce Mg2+/Li+ mass ratio of a simulated brine from 20 to 1.7. Furthermore, a three-stage NF process was designed and performed using a PEI@15C5-TMC membrane to separate a simulated salt lake brine, containing multiple ions and Mg2+/Li+ mass ratio of 50. After the treatment, the ultimate permeate solution of the third stage contained 24 ppm of Li+ and 2.7 ppm of Mg2+ with Mg2+/Li+ mass ratio of 0.1. Therefore, this process could be applied to obtain Li2CO3 products with high purity. Our investigation highlights the highly efficient Mg2+/Li+ separation through NF membranes with exclusive Li+ transport channels, potentially applicable to lithium extraction from salt lake brines with high Mg2+/Li+ mass ratios.