The topic of the PhD thesis of Johannes was the mineralogy of microelements and their distribution in aluminium production. The study focused mainly on such microelements that are considered as critical raw materials. Certain types of bauxites, which are raw materials for producing aluminium, may contain up to five critical metals or groups of them, that is: light and heavy rare-earth elements (HMM) or lanthanides, scandium (Sc), vanadium (V) and gallium (Ga).
One of the conclusions of the study was that at the end of the industrial process almost the whole mass of HMM and Sc is directed into the bauxite waste, which is the principal by-product of the production. Thus, their concentration more than doubles in the waste. It was demonstrated for the first time that in bauxite, Sc is mainly bound to iron minerals, which remain practically in the same form during the production. However, HMM minerals alter as bauxite is processed under high pressure with a high-temperature sodium hydroxide solution. After undergoing the process, HMM minerals contain various amounts of titanium, iron and sodium instead of original comparatively clean forms (oxides and carbonates). Ga and V behave differently during aluminium production. They are partly soluble, gather in solutions of the industrial process over time and exit the circulation after reaching the concentration of saturation in the solutions. Because of the mentioned critical elements, the bauxite waste should be considered as other raw material. The accomplished study helps in developing technologies for separating these components.
A longer version of the issue has earlier been published in the blog Eesti Geoloog (in Estonian): https://eestigeoloog.ee/kategooriad/teadusgeoloogia/uhe-jaatme-lugu-puna...
The following scientific articles have been published during the doctoral studies:
1. J. Vind, A. Malfliet, B. Blanpain, P.E. Tsakiridis, A.H. Tkaczyk, V. Vassiliadou, D. Panias, Rare Earth Element Phases in Bauxite Residue. Minerals 8, 77 (2018) https://doi.org/10.3390/min8020077
2. J. Vind, A. Malfliet, C. Bonomi, P. Paiste, I. Sajó, B. Blanpain, A.H. Tkaczyk, V. Vassiliadou, D. Panias, Modes of Occurrences of Scandium in Greek Bauxite and Bauxite Residue, Minerals Engineering 123, 35-48 (2018), https://doi.org/10.1016/j.mineng.2018.04.025.
3. J. Vind, A. Alexandri, V. Vassiliadou, D. Panias, Distribution of selected trace elements in the Bayer rocess, Metals 8(5), 327 (2018), https://doi.org/10.3390/met8050327
