Selectively leaching the iron-removed bauxite residues with phosphoric acid for enrichment of rare earth elements

Bauxite residue (BR) is a hazardous industrial waste and also becomes an important reserve of rare earth elements (REE). An elegant process for achieving separation of REEs against silica from an iron-removed BR was established in this paper through selective leaching of silica with simultaneous enrichment of REEs. It was found that the phosphoric acid performed better than other acids (HNO3, HCl) for multiple REEs enrichment. The ideal conditions for the leaching process was determined through response surface methodology (RSM) to be H3PO4 concentration of 1.2 mol/L, L/S ratio of 11 mL/g and leaching temperature of 40 degrees C. 82.3% SiO2 in iron-removed BR was leached out while the REEs (La2O3, Ce2O3, Sc2O3 and Y2O3) remained in the leached residue with recovery of more than 98%. La2O3 and Ce2O3 were proved to be in lattice replacement within the acid-resistant pervoskite and thus remained in the leached residue. Only part of Sc2O3 and Y2O3 entered into pervoskite and the remainder was trapped in the aluminosilicate minerals. The enrichment of Sc2O3 and Y2O3 in phosphoric acid leaching process was related to the low acidity of H3PO4 solution.