Separation of magnesium and lithium from brine with high Mg2+/Li+ ratio by a two-stage nanofiltration process

A two-stage nanofiltration (NF) process was developed to separate magnesium (Mg2+) and lithium (Li+) from brine with a high Mg2+/Li+ ratio, and the effects of transmembrane pressure, pH and Mg2+/Li+ ratio on the volume permeation flux, ion rejection, separation factor (SF) and Mg2+/Li+ ratio of permeate were investigated. As the transmembrane pressure increases, the rejection rates of Mg2+ (R(Mg2+)) and Li+ (R(Li+)) vary slightly for the first NF process; whereas R(Mg2+) remains almost constant, but R(Li+) increases sharply at first and then levels off for the second NF process. pH is a crucial factor for the twostage NF process, and Mg2+ and Li+ can be better separated at lower pH value. The SF of Mg2+ and Li+ increases with the increase of the Mg2+/Li+ ratio of the feed solution. The Mg2+/Li+ ratios of permeate of West Taijnar salt lake brines can be reduced from the initial 13.25 to 0.17 at pH 3.5. The long-term filtration test confirms that the two-stage NF process is stable. Thus, the two-stage NF process proposed in this study can be a promising approach to separate Mg2+ and Li+ from brine with a high Mg2+/Li+ ratio.