Abstract
Scandium (Sc) is a critical element with applications in high-performance alloys and energy technologies. Yet, its recovery from secondary sources such as coal ash remains challenging due to low concentrations and strong association with refractory aluminosilicates. This study evaluated the leaching behavior of Sc from coal ash using mineral (HCl, H2SO4) and organic (citric, malic, oxalic, tartaric) acids under identical conditions (80 °C, 0.5 M, solid-to-liquid ratio 1:20). Mineral acids achieved the highest extraction yields (82-90%). Still, they exhibited poor selectivity, with extensive co-dissolution of Fe and Al. Among the organic acids, citric acid provided the best balance between recovery and selectivity, yielding up to 68% Sc with Fe and Al dissolution below 26%. A complete factorial design varying citric acid concentration (0.1-1.0 M) and pulp density (25-100 g/L) revealed that both efficiency and selectivity could be tuned, with optimal conditions (0.5 M, 25 g/L) resulting in 64% Sc recovery and selectivity factors Sc/Fe = 6.40, Sc/Al = 3.76.