Mg2+/Li+ separation by electric field assisted nanofiltration：the impacts of membrane pore structure, electric property and other process parameters

Abstract

An electric field assisted nanofiltration (ENF) process was used for Mg2+/Li + efficient separation from a MgCl2/LiCl solution and a simulated salt-lake brine. The influence of membrane pore structure, electric property, and other process parameters on the Mg2+/Li+ separation performance in the ENF was systematically investigated. The structural coefficient (λ) of NF membrane was introduced to quantify the membrane pore structure including pore size, pore length and porosity. Results suggested that membrane pore structure, instead of membrane pore size, had a critical role on the ENF separation performance. Direct current electric field (DC-EF) markedly promoted Mg2+/Li+ separation. However, the effect of alternating current EF (AC-EF) with different frequencies was nominal. The synergistic effect between λ and DC-EF on Mg2+/Li+ separation was also demonstrated. A low-concentration feed with suitable Mg2+/Li+ mass ratio and pH value, and low transmembrane pressure were beneficial to Mg2+/Li + separation. For 2000 ppm MgCl2/LiCl feed solution with a Mg2+/Li+ mass ratio of 20, a nearly complete rejection to Mg2+ and a high Li + flux simultaneously achieved; correspondingly, a separation factor of Li + over Mg2+ (SLi,Mg) reached above 7000, which is 2-3 orders of magnitude higher than that of pure NF membrane. For a simulated 10,000 ppm brine solution, a SLi,Mg value was up to above 50. The study suggested that ENF process witnessed simple and efficient separation performance for Li + extraction from salt lakes.