Abstract
Scandium (Sc) is classified as a critical and strategic raw material due to its applications in the aerospace industry and in modern energy systems. As scandium rarely forms independent minerals, it accumulates in secondary streams. This article describes a conceptual process, which can be integrated into existing titanium processes, for the selective recovery of scandium from chloride-rich hydrolysis solutions from titanium dioxide production (approx. 25 mg/l Sc). The greatest challenge lies in the similar chemical behaviour of scandium compared to iron and titanium, which complicates selective separation. The key point is that selectivity depends less on the extractants than on the control of metal species, particularly iron, within the complex multi-metal system. The reduction of Fe 3 ⁺ to Fe 2 ⁺ (e.g., using ascorbic acid or iron powder) significantly suppresses the co-extraction of iron in organophosphorus-based SX systems but, at the same time, requires the control of reduced titanium species (e.g., Ti 3 ⁺). Solvent extraction with D2EHPA serves as the main separation step. Synergistic systems with defined TBP fractions improve phase stability and reduce emulsion formation during high Sc extraction. The process route comprises the following steps: (i) redox pre-treatment, (ii) solvent extraction or, alternatively, ion exchange at low pH, (iii) HCl scrubbing, alkaline stripping, redissolution and precipitation (e.g., with oxalate), followed by calcination to produce Sc 2 O 3 , and (iv) utilisation of the by-products (Fe, Ca). Selective iron precipitation is not possible as Sc precipitates completely at a pH of around 5, and therefore redox control takes priority. Overall, the approach demonstrates that an efficient value creation from scandium is possible within existing titanium production routes. However, the validation of the key mechanisms and scalability still need to be confirmed experimentally.