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

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. ΣLREEs (292.27 ppm) exhibit greater amount and dispersion with respect to ΣHREEs (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, Alhydroxides, 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 Ceferrotitanates