Geochemistry and mineralogy of bauxite residues (red muds) from Porto Vesme (Sardinia): from disposal material to new resource

In last decades, there is a growing and relevant interest for bauxites since they represent the main source of aluminum and concentrate several minor critical metals (Mongelli et al., 2017; 2021). The aluminum production is based on the Bayer process which generates high amount of residual material and great interest is now being focused on “red muds” the bauxite processed waste material. Until now, the production of red muds involved a major problem related to recovery and disposal as they also contain elements that are potentially harmful to the environment. However, it has widely reported that red muds can concentrate large amounts of valuable critical metals which can be used for several industrial, electronic and nano-technological purposes (Liu & Naidu, 2014; Borra et al., 2016). The compositional features of red muds stored in Porto Vesme (southern Sardinia) disposal sites have been performed. The aim of this work is to highlight and further detail the processes promoting the concentration of critical metals in red muds. The analytical characterization was performed in order to assess geochemical and mineralogical composition by using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and X-ray powder diffraction (XRPD) while micromorphological and micro-chemical features were provided using a Scanning Electron Microscope (SEM-EDS) at the Department of Sciences of University of Basilicata. XRPD revealed that the main mineral phases in red muds are hematite, α-Fe2O3; gibbsite, Al(OH)3; boehmite, AlO(OH); anatase, TiO2; cancrinite, (Na,Ca)8(Al6Si6)O24(CO3,SO4)2*2H2O); sodalite, Na4(Si3Al3)O12Cl and quartz, SiO2. Geochemical analysis with ICP-MS showed that major oxide composition (wt.%) is dominated by SiO2 (11.9 ÷ 22.6), Al2O3 (17.4 ÷ 24.9), Fe2O3 (22.2 ÷ 30.3) with less abundances of MgO (0.7 ÷ 4.7), CaO (2.6 ÷ 5.9), Na2O (3.5 ÷ 11.5) and K2O (0.2 ÷ 0.7). Regarding trace elements (ppm) high concentrations of Zr, Cr, Ce, V, Sr, Ni, Zn, Ga and Y were detected. It has been observed that, among rare earth elements, Ce is the most abundant element showing abundances in the 93 to 258 ppm range. Since red muds contain high concentrations of critical metals, their reuse can be an important achievement for the valorization of these materials as opposed to their disposal.