Distribution and fractionation of Rare Earths (La–Lu, Sc, Y) and other critical metals in bauxite residues: addressing the profitability of the red muds stored at the Porto Vesme disposal site, Sardinia Island, Italy

In southern Sardinia, Italy, at the Porto Vesme disposal site, about 30 million tonnes of bauxite residues deriving from Al extraction through the Bayer process are stored.The mineralogical composition includes hematite, goethite, ilmenite, gibbsite, boehmite, bayerite, anatase, rutile, quartz, sodalite, cancrinite, andradite, calcite, dolomite, halite. Lanthanides-rich phosphate minerals as well as LREE ferrotitanates were sporadically observed. The samples chemistry provides Fe2O3, Al2O3, SiO2, Na2O, TiO2 and CaO in order of abundance. The REEs are enriched relative to the parent material with LREEs > HREEs+Y and Sc. Ce is the most abundant LREE and the chondrite normalized patterns show significant positive Ce anomalies. The inter-elemental relationships among major, minor and trace elements were evaluated by R-mode factor analysis. The first factor accounts for most of the variance (52.1 %) of the dataset and explains the competition between the desilication and relict + neoformed phases with the latter hosting the LREEs and other critical metals. The second factor (variance = 15.2 %) represents the control exerted by the mineral zircon on the distribution of HREEs+Y. The HREEs+Y are enriched relative to the average Upper Continental Crust along with other critical metals such as Hf, TiO2 and W. In addition, a set of profitability indices, including the ratio (Nd + Dy+Ce + Pr + La)/ Σ(REEs+Y), specifically addressed to the electromobility market, suggest an overall significant economic potential for the bauxite residues stored at the Porto Vesme site.