Critical metals distribution in Bauxite Residues: a Compositional Data Analysis approach

This research explores the major element control on the distribution of critical metals (CM) in red muds (RM), the residues of the Bayer process developed on bauxite ore, sampled at the Porto Vesme disposal site, Sardinia, Italy. RM represent an environmental challenge due to their high alkalinity and storage issues. The relationships between major elements and CM were assessed by Compositional Data Analysis (CoDa).

X-ray powder diffraction (XRPD) revealed predominant minerals including hematite, gibbsite, boehmite, anatase, cancrinite, sodalite, and quartz, consistent with previous studies on RM from the same site (Castaldi et al., 2008; Mombelli et al., 2019). Inductively Coupled Plasma Mass Spectrometry (ICP-MS) assessment reported the 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). Iron content averaged at 25.8 wt%, underscoring i considerable presence of iron in bauxite residues, while alumina averaged at 20.75 wt% is consistent with the large amount of Al-hydroxides detected by XRPD. Among CM, specifically the LREE, the Ce abundance (93 – 258 ppm) is a notable feature.

CoDa involved Principal Component Analysis after a transformation of raw data into centred log ratio. The PC1 and PC2 association revealed significant influence from TiO₂ and Na₂O on critical metals such as HREE, LREE, Sc, and Co. Noteworthy is the Bayer process involving high-temperature pressure leaching of bauxite ores with sodium hydroxide solution for the Al recovery, leaving TiO₂ in the resulting residue. Other CM, such as V and Ga, are predominantly controlled by MgO.

This preliminary study suggests that RM could be a promising geo-material for a strategic CM recovery and efficiently reusing the secondary byproduct, providing a sustainable and environmentally friendly alternative to its mere disposal (Liu and Naidu, 2014).