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

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.