Selective and efficient separation of scandium(III) from titanium dioxide waste acid by synergistic extraction

The coexistence of scandium (Sc) and titanium (Ti) in titanium dioxide waste acid impedes sustainable Sc recovery due to their analogous physicochemical behaviors in acidic solution. In this study, a ternary synergistic extraction system of Di-(2-ethylhexyl) phosphoric acid (P204), 2-Ethylhexyl phosphonic acid mono-2-ethylhexyl ester (P507) and Tributyl phosphate (TBP) is employed to selectively separate Sc and Ti from the titanium dioxide waste acid. By systematically optimizing key parameters (including pH, extractant concentration, phase ratio, and extraction time), a maximum Sc/Ti separation factor of 167 is achieved. The synergistic extraction system demonstrated exceptional Sc selectivity, achieving 96.8% Sc extraction efficiency with only 15.5% co-extraction of Ti. Slope method, infrared spectroscopy, proton nuclear magnetic resonance/phosphorus nuclear magnetic resonance, and density functional theory calculations, are used to elucidate the mechanism in depth. Mechanistic studies reveal that Sc(III) is efficiently extracted by forming a 1:1:1 mixed cation exchange complex; while Ti(IV), is difficult to effectively dissociate and can only be co-extracted through an inefficient ion association/solvation path. These findings provide a theoretical foundation for optimizing Sc/Ti separation processes in complex titanium dioxide waste acid and offer novel insights into extraction mechanisms, which are of great significance for the green recycling and utilization of strategic rare metal resources.