Recovery of scandium from sulfation-roasted leachates of bauxite residue by solvent extraction with the ionic liquid betainium bis(trifluoromethylsulfonyl)imide

摘要

Bauxite residue (red mud) is a waste product of the alumina refining in the Bayer process and contains significant concentrations of critical metals, including scandium. Greek bauxite residue contains exploitable levels of scandium and is thus considered a suitable source for its production. A process was developed to recover scandium from Greek bauxite residue using a combination of sulfation-roasting-leaching and solvent extraction with the hydrophobic ionic liquid betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf2N]. Sulfation-roasting-leaching was the preferred leaching technique to dissolve scandium from bauxite residue because of the low acid consumption, good selectivity towards scandium and low co-dissolution of the major elements. The scandium concentration in the leachate was increased by applying multistage leaching, during which the obtained leachate is contacted multiple times with freshly roasted material. In a next step, scandium was selectively extracted from the obtained leachate with [Hbet][Tf2N]. To improve the separation between scandium and iron, Fe(III) was reduced to Fe(II) by addition of ascorbic acid to the sulfate leachate prior to extraction. The phase ratio and pH of the extraction were optimized to achieve high extraction and concentration of scandium in the ionic liquid phase. Co-extracted metal ions were scrubbed from the loaded ionic liquid phase by HCl and the purified scandium was removed by stripping with H2SO4. Scandium was recovered from the strip solution by precipitation together with sodium. Finally, the entire process was performed on lab scale as a proof-of-principle.