**9. Conclusions**

The chemical precipitation of REEs with precipitants including carbonate, fluoride, phosphate, sulfate, and oxalate was studied over a wide range of pH such as 1, 2, 4, and 6. The concentration of precipitants was fixed at 10−<sup>4</sup> M chosen as the standard concentration for comparison. Such precipitation occurs in practice after primary leaching as a part of the purification process.

With all of the five precipitants studied, Ligjt REEs tend to precipitate much easier than Heavy REEs. The effect of anions associated with acids used in the leaching process was found to have had a significant impact on the overall precipitation process. In general, the complexed REEs with these anions tend to provide a conducive environment in precipitation resulting in an order of magnitude difference in the precipitation extent. The nitrate system was found to be most effective followed by sulfate and chloride systems. The pH of the system has another significant factor in determining the precipitation in that the difference in pKa values of the precipitants introduces precipitant moiety of the precipitants, which has a pronounced effect on the overall precipitation.

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

**Acknowledgments:** This paper was written in preparation of a lecture to be given at the 7th International Conference on Hydrometallurgy to be held in Ganzhou, China in May of 2020.

**Conflicts of Interest:** This author declares no conflict of interest in relation to this paper.
