**4. Conclusions**

Hydrometallurgy method was used to separate rubidium and cesium effectively from the desalination brine in this study. The recommended recovery process is shown in Figure 20. The brine was treated by chemical precipitation and solvent extraction to recover rubidium and cesium. Perchloric acid was added into brine to pH 2 at −5 ◦C to precipitate potassium perchlorate which could reduce the influence of potassium in the extraction procedure. After that, t-BAMBP and ammonia were used as extractant and a stripping agents. The results show that 0.1 M t-BAMBP was used as extractant to separate cesium and impurities under the optimal condition of pH 8, O/A ratio 0.1, 3 min reaction time, and 35 ◦C reaction temperature. Then, cesium was stripped by using 1 M ammonia under O/A ratio 2 and 25 ◦C reaction temperature. On the other hand, the results show that 0.5 M t-BAMBP was used as extractant to separate rubidium and potassium under the optimal condition of pH 12, O/A ratio 0.1, 15 min reaction time, and 5 ◦C reaction temperature. In the stripping process, 0.5 M ammonia under O/A ratio 2 and 35 ◦C reaction temperature are the optimal parameters. Finally, the rubidium carbonate and cesium carbonate were produced by adding ammonium carbonate. By these processes, the purity of rubidium carbonate and cesium carbonate were 98.0% and 98.9%.

**Figure 20.** Recommended recovery process of this experiment.

**Author Contributions:** Conceptualization, W.-S.C. and C.-H.L.; methodology, W.-S.C. and C.-H.L.; validation, W.-S.C. and C.-H.L.; formal analysis, C.-H.L. and Y.-F.C.; investigation, C.-H.L., Y.-F.C. and K.-W.T.; resources, C.-H.L., Y.-F.C. and Y.-J.C.; data curation, C.-H.L., K.-W.T., Y.-J.C. and Y.-A.C.; writing—original draft preparation, C.-H.L.; writing—review and editing, C.-H.L., K.-W.T., Y.-J.C. and Y.-A.C.; visualization, C.-H.L.; supervision, W.-S.C.; project administration, W.-S.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
