Mineralogical and geochemical characterization of red muds from Portovesme (SW Sardinia, Italy)

Red mud (RM) residues produced by Bayer process for alumina refining is a hazardous material which may be valorized as a source of secondary raw material with a potential for REE extraction (Pietrantonio et al., 2021). The mineralogic composition of RM depends on the mineral composition of the source material, i.e. bauxite and on the specific processing parameters of the Bayer cycle. In turn, bauxite is a complex ore that can contain up to one hundred mineral phases (Ostojić et al., 2014). In this study, ten RM samples supplied by the Eurallumina company (Portovesme, Sardinia, Italy) and randomly collected from three storage basins were investigated combining X-ray powder diffraction (XRPD), Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Transmission Electron Microscopy (TEM). XRPD results evidence that the common phases to all samples are hematite, gibbsite and boehmite, anatase, cancrinite, sodalite and quartz as also found in previous investigations (e.g. Snars & Gilkes, 2009; Castaldi et al., 2011 and references therein). Oxide composition (wt.%) from ICP-MS analysis indicates significant amounts of Al2O3 (20.8) and Fe2O3 (25.8) as major components. Elements like Sc, V, As, Ni, Ga, Hf and Sr exhibit high variability whereas Co, light rare elements (LREE), heavy rare elements (HREE), Ta and W have low variability. Abundance of Ce (93 ÷ 258 ppm) is a notable feature of the analysed samples. TEM investigation was carried out on the sample with the highest concentration of Ce. From a morphological viewpoint, the selected sample shows particles with variable morphology, with occurrence of lamellar or acicular crystals, mostly forming globular nano-aggregates. SAED patterns show peaks, corresponding to the main phases detected by XRD. Chemical mapping reveals an elemental distribution consistent with the main phases detected by XRD and SAED and suggests the occurrence of a second Ti-bearing (titanite) and a Ca-phosphate phase (apatite). At the present stage of investigation, there is no clear evidence of a Ce-bearing phase (i.e. Ce may be dispersed in small amount in several different minerals).