Determination of boundary conditions for highly efficient separation of magnesium and lithium from salt lake brine by reaction-coupled separation technology

The high Mg/Li ratio in salt lake brine in China remains a challenge for the efficient separation of Mg and Li and the Li extraction. A reaction-coupled separation technology we proposed has been successfully applied to the highly efficient separation of Mg and Li from the brine. In this work, we conducted intensive investigations both experimentally and theoretically on the boundary conditions of Mg/Li separation from the brine in association with the initial Mg/Al ratio and Mg/Li ratio. We analyzed the relationship between the composition and the formation of precipitates by changing the Mg/Al molar ratio and the Mg/Li mass ratio in feedstock solution to determine the boundary conditions. The experimental results show that the Mg/Al molar ratio in the initial solution has a great influence on the separation of magnesium and lithium. It is evident that lithium enters the solid as a LiAl-layered double hydroxides (LiAl-LDHs) phase when the Mg/Al molar ratio is less than 2. It has also been found that the precipitate is independent of the Mg/Li mass ratio. From the theoretical results, it has been determined that an increase in the number of Li+ clusters results in a decrease of the binding energy in the same system. Al3+ prefers to combine with Mg2+ to form MgAl-LDH clusters. However, when the Mg/Al ratio is lower than 2:1, Li+ can form a cluster of LiAl-LDH layer. It can also be seen that different Li+ numbers do not change the formation of clusters. Good agreement was found between the theoretical simulation results and the experimental data. This work provides a key criterion for the efficient separation of magnesium and lithium from the brine by a reaction-coupled separation technology.