Critical metals distribution in bauxite residues: a multivariate statistical analysis approach

Bauxite residues (RM), the byproduct of the Bayer process for aluminium extraction from bauxite ore, pose significant environmental challenges, while containing valuable critical metals (CMs). A preliminary analysis of red mud (RM) samples from the disposal sites in Portovesme, Sardinia, Italy was conducted to understand the geochemical and mineralogical features influencing the accumulation of CM in these matrices. The major elements-CM relationships were assessed by Compositional Data Analysis (CoDa). The analytical characterization was performed through a combination of X-ray powder diffraction (XRPD) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). XRPD reveals that the main minerals are hematite, gibbsite, boehmite, anatase, cancrinite, sodalite, and quartz, consistent with what depicted in previous studies for RM from the same site (Castaldi et al., 2008; Mombelli et al., 2019). ICP-MS assessment includes the following major oxides composition (wt.%): SiO2 (11.9-22.6), Al2O3 (17.4-24.9), Fe2O3 (22.2-30.3), MgO (0.7-4.7), CaO (2.6-5.9), Na2O (3.5-11.5), K2O (0.2-0.7). The average iron percentage is 25.8 wt.%, underscoring the considerable presence of iron in bauxite residues, whereas the average alumina percentage is 20.75 wt.%, consistent with the large amount of Al-hydroxides detected by XRPD. Among the LREE, the Ce is the most abundant (93-258 ppm). CoDa was performed through Principal Component Analysis after a transformation of raw data into centred log ratio. Significant variability pertains to SiO2, Al2O3, Fe2O3, followed by TiO2 and MgO. Similarly, elements like Sc, V, As, Ni, Ga, Hf, and Sr exhibit variable concentration ranges, whereas Co, LREE, HREE, Ta, and W have substantially constant low concentrations within this dataset. The PC1 and PC2 association reveals that TiO2 and Na2O have a significant impact on the distribution of HREE, LREE, Sc, Nb, and Co, likely through mineral interactions and complex formation. MgO and Al2O3 play a role in controlling V, As, Ni, and Ga distribution. This preliminary study highlights the potential of red muds as a geo-material for the recovery of critical metals, providing an environmentally friendly solution to the challenges posed by their disposal. Further research could facilitate the efficient utilization of this secondary byproduct, aligning with sustainability goals (Liu & Naidu, 2014).