A Study of the Occurrence of Selected Rare-Earth Elements in Neutralized-Leached Bauxite Residue and Comparison with Untreated Bauxite Residue

This study investigates the chemical associations of the selected rare-earth elements (Sc, Y, Ce, La, and Nd) with major element phases in the postprocessed bauxite residues and compares them with the untreated bauxite residue. The treatment of bauxite residue considers our previous published process which involved the neutralization with CO2, followed by leaching with H2SO4. Neutralized bauxite residue resulted with larger aggregates than the untreated bauxite residue after making contact with CO2 as the consequence of additional CaCO3 formation. Neutralization with CO2, however, has a negligible effect on the distribution of the rare-earth elements (REEs) with respect to the untreated bauxite residue, but a large amount of rare earths remained unreacted after acid leaching. Electron probe microanalysis (EPMA) confirmed the chemical associations of Sc(III) and Ce(IV) with Fe(III)- and Al(III)-containing minerals in the postprocessed bauxite residues, i.e., bauxite residues subjected to CO2-neutralization and neutralization-acid leaching processes. The occurrence of Nd(III) is positively correlated to that of La(III) in the untreated bauxite residue, but both of them may be associated with the same mineralogical phase as Ce(IV) after processing. Y(III) may remain associated with the Al/Si-minerals, cancrinite and chamosite. Ergo, the extractability of Sc, Y, Ce, La, and Nd from neutralized bauxite residue is more difficult in H2SO4 media due to the presence of coarser particles compared to those of the untreated bauxite residue, but also due to the formation of a solid product layer (i.e., CaSO4) that is presumably adsorbed on the surface of Fe(III)-rich phases (hematite and goethite) and Al(III)-containing minerals (diaspore, gibbsite, boehmite, and chamosite).