Rare Earth Elements (La-Lu, Sc, Y) and Other Critical Metals deportment in Bauxite Residues: Assessing the Economic Viability of Red Mud Deposits at the Porto Vesme Disposal Site, Sardinia, Italy

Around 30 million tonnes of bauxite residues (red muds), resulting from aluminium extraction via
Bayer process, are stored at the Porto Vesme disposal site in southern Sardinia, Italy. The primary
crystalline components of these residues include iron oxides, aluminium hydroxides, titanium
oxides, silicates, carbonates, and halides, along with several minor phases. Lanthanide-rich
phosphate minerals are occasionally detected. The Na-aluminosilicates are results of desilication,
while the aluminium hydroxides and oxyhydroxides are residual alumina minerals, and the iron
oxides, most titanium minerals, and REE-rich phosphate minerals originate from the parent
bauxite. LREE ferro-titanates have also been identified.
The residues are predominantly composed of Fe2O3, Al2O3, SiO2, Na2O, TiO2, and calcium, with the
REEs being significantly enriched compared to the parent bauxite. LREEs are the most abundant,
followed by HREE+Y and Sc. Cerium is the dominant lanthanide, and the chondrite-normalized
patterns reveal substantial positive Ce anomalies. Vanadium (V) is the most abundant critical
metal, followed by niobium (Nb), gallium (Ga), and hafnium (Hf).
Factor analysis of the major elements (SiO2, Al2O3, Fe2O3, CaO, Na2O, TiO2, P2O5), REEs, and critical
metals (V, Co, Cu, Ga, Nb, Hf, Ta, W) indicates that the first factor (52.1% variance) reflects the
competition between desilicated minerals and secondary phases that host the LREEs and critical
metals such as V, Cu, and Nb. The second factor (15.2% variance) is associated with zircon’s role in
distributing HREE+Y.
The HREE+Y and metals like Hf, TiO2, and W are enriched relative to the Upper Continental Crust,
suggesting a high supply risk and economic importance. Several profitability indices, including the
outlook coefficient, critical REEs to total REEs ratio, and the (Nd+Dy+Ce+Pr+La)/Σ(REE+Y) ratio,
indicate that the bauxite residues at Porto Vesme have significant economic potential.