Exploring the potential of red muds from Portovesme (SW Sardinia, Italy): Multiscale analysis of REEs content and mineralogy

<h2>Abstract</h2> Red muds (RMs) from three storage basins at Portovesme (Sardinia, Italy) were investigated combining inductively coupled plasma optical emission spectrometry (ICP-OES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), X-ray Powder Diffraction (XRPD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The oxide composition (wt%), in terms of medians, is as follows: Fe₂O₃ 25.63 ± 3.06, Al₂O₃ 20.43 ± 2.30, SiO₂ 15.65 ± 2.53, Na₂O 8.59 ± 2.48, TiO₂ 5.33 ± 1.10, CaO 4.13 ± 0.33. Ba, Ga, Ni, Sr and Zn show significant variation among the trace elements. ∑LREEs (292.27 ppm) exhibit greater amount and dispersion with respect to ∑HREEs (134.30 ppm). Yttrium (83.50 ± 17.77 ppm) and cerium (231.50 ± 56.80 ppm) were the most abundant among HREEs and LREEs respectively. Quantitative phase analysis (QPA) by Rietveld method indicates the occurrence in the X-ray diffraction patterns of up to 20 different mineral phases out of which the main phases are Fe-oxides, Al-hydroxides, silicates, Ti-oxides, halite and carbonates whereas different combination of minor phases are detected from sample to sample. SEM and TEM investigations show particles with different morphology corresponding to the main phases detected by XRD. SEM also indicates the occurrence of Ba-sulphates, sulphides, Ca-phosphates, as well as HREE-phosphates and Ce-ferrotitanates.