Potential of Ligand-Promoted Dissolution at Mild pH for the Selective Recovery of Rare Earth Elements in Bauxite Residues

In a context of overexploitation of natural resources, a circular economy and particularly the extraction of resources from secondary sources are essential to sustain a number of key technologies including renewable energies. Among secondary sources, the bauxite residue contains critical elements including rare earth elements (REEs) (712 mg/kg). We investigated the use of soft and selective dissolution protocols at mild pH values (2-6) as an alternative to pyro- and hydrometallurgy for the recovery of REEs through ligand-promoted dissolution. This approach depends on the detailed characterization of the waste and the speciation of targeted elements. We assessed dissolution using low-molecular-weight organic acids and their conjugate bases. Citric acid/citrate showed satisfactory dissolution of REEs (up to 50% of light REEs) up to a pH of nearly 5, while tartaric acid/tartrate showed the best dissolution selectivity (enrichment factor up to 21.5 compared to Fe, Al, and Ti). Almost no heavy REEs were dissolved in any of the conditions tested, probably due to the high chemical stability of their bearing phases. Indeed, heavy REEs were found as discrete phosphate particles.