Efficient and Selective Recovery of Trace Scandium by Inorganic Titanium Phosphate Ion-Exchangers from Leachates of Waste Bauxite Residue

The research leading to these results has received funding from the European Union (EU) Horizon 2020 Programme Marie Skłodowska-Curie actions under Grant Agreement no. 636876 (MSCA-ETN REDMUD).Bauxite residue (BR) is an inevitable industrial waste generated through the classic Bayer extraction of alumina from bauxite minerals. It contains relatively significant amount of valuable rare earth elements, including scandium, and therefore, we explored the suitability of trace scandium recovery from BR acid leachate by titanium phosphate (TiP) ion exchangers. Three kinds of TiP materials (amorphous TiP, α-TiP, and γ-TiP) were synthesized through fluorine-free precursors and characterized by chemical analysis, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR), ultraviolet/visible (UV/vis) diffuse reflectance spectrometry, 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and potentiometric titration. The Sc3+ exchange capacities were determined as 1.74, 0.55, and 0.22 mequiv g–1 for amorphous, α-, and γ-TiP, respectively. Competition of major elements (Fe, Al, Ca) in BR leachate with Sc uptake were studied in batch experiments using binary equimolar mixtures, and the separation factors of Sc/Fe2+, Sc/Al, and Sc/Ca reached magnitudes of 10–1000 on amorphous TiP. The high Sc3+selectivity by amorphous TiP was suspected to be the matching of Ti4+ lattice radius with Sc3+ ionic radius (both 0.745 Å). Finally, the separation of trace scandium from the simulated BR leachate solution was demonstrated on an amorphous TiP column. The interference of Fe3+ has been partially resolved by on-column reduction using sodium sulphite. The optimized final eluate contained only Sc, Fe, and Al. The concentration ratio of Sc/Fe can be increased by a factor of 8.8 and Sc/Al by 265 through a single cycle of chromatographic separation with an Sc recovery rate of 91.1%.Peer reviewe