**4. Conclusions**

In this study, the effects of two different chemical decomposition methods, namely sulfuric acid baking and caustic digestion, to enhance the REE recovery from a refractory ore were investigated. The REE-containing particles were scattered throughout the ore, and some of them were even trapped in other minerals such as Fe oxide, although the total REE concentration was relatively high at 3.4% TREO. From the preliminary leaching tests using nitric acid, the maximum REE recovery only reached 61–73%; thus, a significant amount of REEs was still left in the residue.

Using sulfuric acid baking–water-leaching, the REE recovery was improved to 97–100% after the ore was baked at 2.0 acid/solid ratio and 200 ◦C for 2 h. The REE recovery reduced with increasing leaching temperature because of the CSD solid formation at elevated temperatures and the accompanying REE co-precipitation.

During the caustic digestion reaction, the REEs were concentrated in the solid phase as Al and P were removed to the solution phase by the decomposition reaction. In the succeeding acid-leaching, all REE-leaching efficiencies reached the maximum after 30 wt% NaOH digestion for 3 h and 3.0 M HCl-leaching at 80 ◦C for 3 h. With increasing NaOH concentration, Ce became oxidized to Ce(IV) species of Ce(OH)4, and its leaching efficiency from the acid-leaching step decreased.

From the REE–Fe correlation analysis, a stronger correlation between the REEs and Fe was observed for the sulfuric acid baking–water-leaching results because of not only Fe-bearing mineral decomposition to iron sulfate, but also CSD precipitation with REEs and Fe. In addition, comparing the sulfuric acid baking–water-leaching and caustic digestion–acid-leaching, the caustic digestion showed a more powerful effect on enhancing the REE-leaching as it resulted to a higher REE-leaching efficiency at the same Fe-leaching level than that of sulfuric acid baking. It can be concluded that in terms of REE-leaching efficiency, as the Fe-leaching reached the maximum, the REE-leaching efficiency also attained its highest value. Furthermore, the Fe-bearing mineral decomposition significantly affected the REE-leaching of the studied ore.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2075-163X/10/6/532/s1, Table S1. Mass balance of sulfuric acid baking–water-leaching process; Table S2. Mass balance of caustic digestion–acid-leaching process.

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

**Funding:** This research was supported by the Energy Efficiency and Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry and Energy (MOTIE) of the Korean governmen<sup>t</sup> (No. 20122010300041). This research was also supported by the R&D Center for Valuable Recycling (Global-Top R&BD Program) of the Ministry of Environment of the Korean governmen<sup>t</sup> (No. 2019002220001/KIGAM 20-9894).

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