Separation of magnesium from lithium in salt-lake brine through struvite precipitation

In this study, a struvite-precipitation method that uses triammonium phosphate trihydrate was introduced to separate Mg2+ from Li+ in a salt-lake brine. The precipitation experiments in the simulated brines exhibited excellent Li/Mg separation efficiency. The Mg2+ recovery in the precipitates, Li(+ )recovery in the solution, and Mg/Li mass ratio in the solution reached 99.71%, 4.88%, and 0.1%, respectively, under optimum condition. The relevant separation and adsorption mechanisms were investigated using solution-chemistry calculations, first principle density functional theory (DFT) calculations, X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) analysis, Brunauer-Emmett-Teller (BET) analysis, and zeta-potential measurement. The solution-chemistry calculations, first-principle DFT calculations, and XRD analysis demonstrated selective Li/Mg precipitation separation. The XRD analysis illustrated that the precipitates were struvite and dittmarite. The SEM, BET, and zeta-potential measurement results demonstrated that Li+ in the solution could adsorb on the magnesium precipitates and suffered from a significant loss during the aging process. Therefore, performing a quick and direct filtration after the precipitation process is necessary to eliminate the Li+ loss in the precipitates. The triammonium phosphate trihydrate also exhibited excellent performance in the simulated salt-lake brines with a broad range of initial Li+ and Mg2+ concentration and actual salt-lake brines.