-->

Κυριακή, 9 Δεκεμβρίου 2018

REE Potential of Greek Bauxite Active Mines and Nano-mineralogy of Greek Bauxites

Polished-section of Fe-depleted
 diasporic Greek industrial bauxite sample
 (upper) and detail of pisolith in the optical
microscope (lower) prior to the XRF
measurement.
Greece is the 12th largest bauxite mine producer worldwide, but also the largest in the EU with an annual production of 1800 thousand metric dry tons in 2017. Nowadays, the Greek bauxite reserves, which could be produced economically, are estimated to be approximately 250,000 thousand metric dry tons . Most of the raw material is processed by the ‘‘Aluminium of Greece S.A.’’ at its industrial complex at Aghios Nikolaos (Antikyra, Corinth gulf) for the production of alumina (800,000 metric dry tons in 2017) and metallic aluminum for use in the industry and constructions. According to the U.S. Geological Survey, Greece is being considered to be the 16th alumina producer in the globe, and the 4th among the EU member-states. The Greek bauxites are currently mined in the Parnassos-Ghiona area, where there is also a metallurgical plant for Al metal production, while there is no process in the Greekbauxite industry to extract REEs from the raw Al-ore.
Thus, most of REEs are transferred in the bauxite metallurgical residue (the so-called red mud), along with certain actinides, such as Th. The exploitation of REEs from red mud is rather challenging, although there is evidence that REEs can be extracted easier directly from theraw Al-ore. Thus, the mineralogy and the geochemistry of REEs in Greek bauxites are rather critical. Previous studies indicated the presence of Th-bearing REE-minerals and detrital zircon grains fundamentally containing REE and actinides. A concurrent publication to the current study also revealed that Sc is present in Fe-oxides (mainly hematite and goethite) and zircons. Concerning the bulk REE geochemistry of Parnassos- Journal of Sustainable Metallurgy Ghiona bauxites, some data have sporadically been reported by previous authors. However, the REE geochemistry in Greek bauxites has not been systematically investigated, in comparison to relevant karst-type deposits from the Mediterranean (including Balkans), Irano-Himalayan, and East Asian bauxite belts.

The first paper presented here to our knowledge is a first attempt to contribute to the future REE exploration and exploitation from base metal deposits, with an emphasis on the REE-containing bauxites, in the light of the sustainable REE demand in the EU and the globe. Moreover, it is worthy to mention that—among other REEs—Sc is of special interest due to its several high-tech promising applications, such as solid oxide fuel cells (SOFCs) and in Sc–Al alloys, due to specific qualifications (in aerospace industry and in other applications with high-performance demands, e.g., in 3D printing), as well as in ceramics, electronics, lighting, and TV and computer monitors. According to a latest EU report (December 2017), Sc is among the critical raw materials. This means that it has a high economic impact while also having a high supply risk. In this respect, the geochemical aspects of Sc and Y are also considered in the current study, except for the lanthanides series (La to Lu).
In the second work presented here a  Typical red–brown (Fe-rich) and high-quality white–grey (Fe-depleted) bauxite samples from active mines of the Parnassos-Ghiona area, central Greece, were investigated.

In the third  work presented here, a combination of various techniques is utilized for the study of nano-mineralogy and -geochemistry of high-grade karst-type bauxite (Al-rich and Fe-depleted samples; Al2O3 ca. 80 wt.%) from the Parnassos-Ghiona mines located in Greece. Initial characterization using PXRD and electron microscopy in microscale and mesoscale (SEM-EDS including STEM mode), proved the presence of “Fe-Cr-Ti-containing diaspore”, anatase and minor rutile.
The study by means of 57Fe Mössbauer spectroscopy, in correlation with magnetic susceptibility measurements and, complemented, with Synchrotron-based spectroscopies at the microscale (SR micro-XRF and micro-XANES/-EXAFS), indicated that Fe3+, in contrast to [6]Cr3+, is not exclusively a componentof the diaspore structure.While Cr3+ substitutes Al3+ in octahedral sites of diaspore ([6]Cr3+↔[6]Al3+), the electron microscopy in nanoscale (TEM-EDS & EELS) revealed that Fe exists in the form of peculiar Fe3+-bearingnanominerals (most likely maghemite-type phases) between 25 and 45 nm in size, in addition to the Fe3+ ions substituting Al3+ in the diaspore structure.  Moreover, it was proven that TiO2 polymorph mineral nanoparticles, particularly rounded anatase mesocrystals and nanocrystals and individual needle-shaped rutiles, are dispersed into the diaspore matrix.Thus, diaspore in the studied bauxite concerns -in fact- a distinct Fe3+-Cr3+-AlOOH low-T authigenic phase, demonstrated for the first time in literature. On the other hand, the observed TiO2 mineral nanoparticles (formed, together with diaspore, during diagenesis) and Fe nanominerals (formed during epigenesis) were hitherto unknown not only for the allochthonous karst-type bauxite deposits of Greece, but also for the overall bauxite deposits, worldwide.

[Επιμέλεια: Π. Τζεφέρης]