**4. Conclusions**

Good separation of zinc, cobalt, and nickel from each other can be achieved with Cyanex 272 and Ionquest 801 systems, and both extractants performed similar selectivity for zinc over cobalt with the latter performing slightly better. Although Cyanex 272 has much higher selectivity for cobalt over nickel than that of Ionquest 801, separation factors over thousands were also obtained when low Ionquest 801 concentration or a high *A/O* ratio was used, which were very similar to that of Cyanex 272, indicating that high cobalt loaded in organic can significantly improve the separation of cobalt from nickel with Ionqest 801.

Slope analysis showed that one molecule metal extraction requires two dimer extractant molecules for zinc and cobalt extraction, but three for nickel extraction with both Cyanex 272 and Ionquest 801. The mixture of Cyanex 272 and Ionquest 801 has a slightly synergistic effect for the extraction of zinc and cobalt, particularly with the equal concentration of each in the mixture. A significant synergistic effect was observed for nickel extraction when Cyanex 272 concentration was higher than the Ionquest 801 concentration. The synergistic species was determined to have the form of *M(AB).*

**Author Contributions:** Conceptualization, Z.Z. and J.L. Methodology, Z.Z. Software, W.L. Validation, J.Z. and Z.X. Investigation, Z.Z. Resources, Z.Z. Data curation, J.Z. and Z.X. Writing-original draft preparation, W.L. and Z.Z. Writing-review and editing, W.L., J.L., and Z.Z. Project administration, J.Z. Funding acquisition, Z.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (51774260, 51804289, 51904286, 21908231), Beijing Natural Science Foundation (2202053), Key Program of Innovation Academy for Green Manufacture, and Chinese Academy of Sciences (Grant No. IAGM-2019-A15), Key Research Program of Frontier Sciences of Chinese Academy of Sciences (Grant No. QYZDJ-SSW-JSC021), and CAS Interdisciplinary Innovation Team.

**Institutional Review Board Statement:** The study did not involve humans or animals.

**Informed Consent Statement:** The study did not involve humans or animals.

**Data Availability Statement:** This study did not report any data and the data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors would like to acknowledge the financial support by all the funders.

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

#### **References**

