Effect of Iron Oxidation State on Solvent Extraction Scandium Extraction Process from Bauxite Residue and Life Cycle Assessment

Abstract

The extraction of Sc from bauxite residue (also known as red mud) is a promising but challenging secondary source due to the high Fe content, reducing efficiency. This study investigated the impact of Fe on Sc recovery by solvent extraction and evaluated the environmental impact of the process. A hydrometallurgical route was chosen for Sc extraction involving leaching with H2SO4 followed by solvent extraction with Cyanex 923 and Alamine 336. Synergistic combination of these extractants was tested to increase selectivity. Results showed that Cyanex 923 extracted nearly 100% of the Sc, but the co-extraction of Fe (25–80%) remained a significant challenge. A combination of Cyanex 923 and Alamine 336 improved Sc selectivity by minimizing Fe extraction at pH 0.5–1.0 (< 20%). LCA indicated that leaching had the greatest environmental impact due to high energy consumption, while solvent extraction also contributed considerably because of kerosene use for dilution. The highest environmental impact is on ozone depletion in all steps of the process (leaching and solvent extraction). Synergistic use of Cyanex 923 and Alamine 336 is an efficient strategy for Sc extraction with low Fe co-extraction. Further optimizations are needed for the industrial scale, particularly concerning environmental impacts.