Distinct Extraction Behaviors of La/Ce and Sc/Y in the Phosphoric Acidic Leachate of Bauxite Residues and Their Sequential Extraction with Di-(2-Ethylhexyl) Phosphoric Acid as Extractant

Abstract

Bauxite residue is a hazardous solid waste produced in the alumina production process and has also become a significant rare earth resource. The extraction behaviors of La, Ce, Sc and Y solubilized in the phosphoric acidic leachate of bauxite residue were investigated in this study with di-(2-ethylhexyl) phosphoric acid as the extractant. With a relatively low concentration of 2% at an aqueous solution pH of 1.5, 90% Sc and 98% Y were extracted by di-(2-ethylhexyl) phosphoric acid. Less than 5% La and Ce and impurities of Fe, Al, Ti and Ca were extracted in this situation. As the concentration of di-(2-ethylhexyl) phosphoric acid increased to 20%, almost all the Sc and Y were extracted and the extraction ratios of La and Ce were 87% and 95%, respectively. A good separation of REEs against impurities was simultaneously obtained in the solvent extraction process and their separation coefficients were much higher than 1. A stepwise extraction process was proposed and established to extract Sc/Y and La/Ce sequentially from the phosphoric acidic leachate. It was further revealed that the Sc and Y in the acidic leachate were extracted by di-(2-ethylhexyl) phosphoric acid through an ion exchange process, and that the extraction of La and Ce was due to an antagonistic process where both an ion exchange reaction and a solvation reaction occurred.