**5. Conclusions**

From this study, it is established REF can be produced from end-of-life Nd magne<sup>t</sup> from hard disk drive by hydrometallurgical techniques using NH4F·HF. The final product (REF) can also be used as part of the salt bath for molten salt electrolysis using fluoride salt [39]. This process is preferred over those reported in the literature because the use of hazardous HF was avoided. Employing statistical design of experiments (DOE), process parameters such as volume of deionized water, amount of NH4F·HF and degree of stirring were defined in detail, and a conducive process regime for the recovery of the REFs was identified. It was established that at higher volumes of water, NH4F·HF addition, and degree of stirring, optimum REF recovery with reasonably high purity can be achieved.

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

**Funding:** Research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-15-2-0020.

**Acknowledgments:** The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the o fficial policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein. The authors acknowledge Grant Wallace and Jan Chorney for their support.

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