A novel synergistic extraction system for the recovery of scandium (III) from sulfuric acid medium with mixed Cyanex923 and N1923

High extraction capability and outstanding selectivity are very attractive in the solvent extraction process. Scandium (Sc(III)), as a primary rare earth element (REE), generally lacks independent large-scale deposits and is mainly recovered as a by-product during the processing of mineral. However, the difficulty of stripping has been always a crucial problem to be solved in the scandium extraction system. This study aims to improve stripping performance while enhancing extraction efficiency for scandium. Herein, a novel synergistic extraction system for the recovery of scandium with a mixture of Cyanex923 (A, a mixture of four trialkylphosphine oxides) and N1923 (B, primary amine R1R2CHNH2) in sulfuric acid medium was put forward. The effect of the molar fraction (X) of Cyanex923 on the extraction of scandium was systematically investigated. The experimental results demonstrated that the coupled Cyanex923-N1923 exhibited an obvious synergistic effect on Sc(III). The maximum synergistic enhancement coefficient (Rmax) was calculated to be 2.20 when the molar fraction (X) of Cyanex923 was 0.4, and the H2SO4 concentration was 0.25 mol·L−1. Moreover, the synergistic extraction mechanism was discussed combined with multiple techniques including slope analysis method, FT-IR and NMR characterizations, to which the structure of extracted complexed was deduced to be Sc(SO4)1.5∙(A)0.5(B). Thermodynamic experiment was also explored in detail. More importantly, stripping with only 2 mol·L−1 HNO3 produced a relatively high stripping rate of 95%. The synergistic system still presented excellent reusability even after five treatment cycles. Finally, this study pointed out a potential industrial application value for the separation and recovery of Sc(III) from the sulfuric acid leaching liquor of nickel laterite ores.