Highly Selective Recovery of Scandium over Aluminum from Sulfuric Acid Leaching Solution of Red Mud Using Ionic Liquids

Extracting scandium (Sc) from red mud facilitates the comprehensive management of this solid waste, mitigating its environmental impact, while securing a sustainable supply of this critical metal. Two ionic liquids (ILs), [lauryl betaine][bis(trifluoromethanesulfonyl)imide] ([HLaur][Tf2N]) and [cocamidopropyl betaine][bis(trifluoromethanesulfonyl)imide] ([HCoca][Tf2N]) were used to separate Sc3+ from Al3+ in H2SO4 aqueous solution. At pH 3.0, [HLaur][Tf2N] showed higher Sc3+ extraction efficiency (44.4 vs 36.4%) and better selectivity over Al3+ compared to [HCoca][Tf2N]. The optimal conditions for [HLaur][Tf2N] extraction were determined as follows: a [HLaur][Tf2N]-to-Sc3+ molar ratio of 5.8:1, an extraction time of 40 min, 0.5 mol·L–1 Na2SO4, and pH ≥ 3.0. The extraction process was found to follow a cation exchange mechanism involving sulfate ions. Quantitative stripping of the loaded Sc3+ was achieved using a 0.035 mol·L–1 H2SO4 solution. FT-IR analysis and recycling tests indicated that [HLaur][Tf2N] maintained its structural integrity over six cycles. Even in a simulated red mud leachate with high Al3+ and low Sc3+ concentrations, [HLaur][Tf2N] exhibited strong Sc3+ selectivity, and a total extraction efficiency of 94.4% was achieved after five extraction stages, with the residual Sc3+ concentration as low as 0.26 mg·L–1.