Removal of calcium and magnesium from lithium brine concentrate via continuous counter-current solvent extraction

Abstract

In this research project, a process for purifying Li brine concentrate with a typical composition of Li 30 g/L, Ca 1.36 g/L, Mg 0.049 g/L via solvent extraction was studied. The goal was to remove Ca to below 20 mg/L and Mg to the ppm level while keeping the co-extraction of Li below 10%. Laboratory-scale batch experiments showed that conventional cation-exchange reagents D2EHPA and Versatic 10 could be used for the task in pH ranges of 3.5–4.0 and 6.5–8.0, respectively. Of these reagents, Versatic 10 had better selectivity for the target metals and better phase disengagement properties, while D2EHPA had a higher capacity. However, with either of these reagents, the organic phase cannot be loaded to a very high extent, because Ca, which has the highest affinity, then replaces Mg.

The effect of the operating parameters (pH, temperature, phase ratio, and residence time) were studied in a bench-scale two-stage continuous counter-current setup with both reagents. The overall performance was good, yielding ppm impurity levels in terms of Ca and Mg and, typically, 3–5% Li co-extraction. While the Mg extraction could be increased by increasing the pH in the mixers and decreasing the A/O phase ratio, the Li co-extraction would also be increased. A compromise must be made between purity and Li yield. Decreasing the temperature or residence time did not have a significant effect on performance. It was demonstrated that a high throughput can be achieved because a mixer residence time of 2 min or even less can be used.