**6. Conclusions**

Many REE mineral deposits are marked by a diverse set of REE bearing minerals. These REE-bearing minerals are distributed across several mineral classes, representing complex solid solutions with diverse substitutions, and crystallise in numerous hydrous species. All of these factors result in considerable problems in labelling the energy-dispersive X-ray spectra and quantitative element data from REE bearing minerals with the corresponding mineral names.

For the e fficient examination of automated SEM mineral liberation analysis data of REE ores, a generic system of labelling the reference EDX spectra from REE-bearing minerals is proposed. This generic labelling is based on quantitative elemental EDS analyses of the REE-bearing minerals, placing particular emphasis on the elements Si, Ca, F and P. EDS spectra are assigned to the following groups (1) REE-P-monazite; (2) REE-Ca-Si-P (represented by britholithe); (3) REE-Ca-F (represented by synchysite); (4) REE-F (represented by bastnaesite and parisite), and a further group (5) REE-Low-Mix. The latter comprises of spectra with low counts in the energy range of the LREE. In case studies, this classification approach has been applied to classify automated SEM-MLA measurements on REE syenite and REE carbonatite ores.

In a REE syenite ore the fine-grained REE minerals in aggregates up to 1 mm in size are closely intergrown with Fe-Ti phases and phyllosilicates. This causes a hard mineability of the REE ores, although the bulk rock REE concentrations are convenient. In comminution tests of a REE carbonatite ore, a longer grinding time of 90 min, compared to 45 min, leads to no significant improvement of the liberation of the dominant REE mineral monazite. A successful concentration of monazite and bastnaesite to >50 wt % in a REE carbonatite ore requires a multi-stage flotation process with regrinding and de-sliming. A considerable proportion of well liberated ye<sup>t</sup> fine REE mineral particles did not float and were lost in the tailings caused by insu fficiently optimized flotation conditions and generally known problems with flotation slimes.

The datasets were classified with the spectra list involving generically labelled REE spectra with high (1℮<sup>−</sup>10) and fair (1℮<sup>−</sup>25) reliability of match. When classified at a reliability value of <sup>1</sup>℮<sup>−</sup>10, one can state an increase of the *unknowns* by a factor 2, however, the mode proportions of *unknowns* still remains below 1.0 wt %. A high reliability of match (1℮<sup>−</sup>10) induces a reduction of the mode proportions of the REE-Low-Mix group and other REE spectra groups in favour of the *unknowns*. At mode proportions of >5 wt% the e ffects of the reliability values are marginal and almost negligible.

The case studies illustrate the generic characters of the classification approach, as it is found to be highly applicable to di fferent types of REE ores and mineral-processing products.

**Author Contributions:** Conceptualization, B.S., G.M., J.G.; methodology, B.S.; investigation, G.M., B.S.; resources, J.G.; data curation, B.S., G.M.; Writing—Original Draft preparation, B.S.; Writing—Review and Editing, B.S., G.M., J.G.; visualization, B.S. and G.M.; project administration, G.M.; funding acquisition, G.M. and J.G.

**Funding:** This research was supported by TU Bergakademie Freiberg and the Helmholtz Institute Freiberg for Resource Technology.

**Acknowledgments:** The authors acknowledge the grea<sup>t</sup> expertise of A. Bartzsch, R. Wuerkert and M. Stoll at the di fficult preparation of numerous epoxy grain mount blocks from REE carbonite ores at Helmholtz Institute Freiberg for Resource Technology. S. Gilbricht is thanked for her untiring support during the automated SEM-MLA measurements at the instruments of the Geometallurgy Laboratory at the TU Bergakademie Freiberg. The authors acknowledge also the constructive comments of three reviewers to the manuscript.

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