Selective separation of Sc(III) and Zr(IV) from the leaching of bauxite residue using trialkylphosphine acids, tertiary amine, tri-butyl phosphate and their mixtures

The separation of scandium from the solution generated in acid leaching is one of the main challenges for the hydrometallurgical processing of bauxite residue. Several organic extractants were evaluated, being the most prominent phosphine acids, while amine-based compounds have shown great results for separating metallic ions. The present study aims at the separation of scandium and zirconium from the Brazilian bauxite residue. The acid leaching was carried out using 20% of H2SO4, S/L ratio equals 1/10, for 8 h at 90 °C. Further, solvent extraction experiments were carried out using D2EHPA, Cyanex 923, and Alamine 336 diluted in kerosene, evaluating the effect of pH, temperature, extractant concentration, synergism with TBP and A/O ratio. After optimizing various process parameters, Alamine 336 has obtained separation factors of Zr/Al, Zr/Fe, and Zr/Ti equal to 15150, 45054, and 19713, respectively, at pH 1.0, A/O ratio equals to 1:1 and 25 °C. The stripping rate achieved 92% using Na2CO3 0.25 mol/L. The scandium separation ratio reached higher values for 10% of Cyanex Cyanex 923 than D2EHPA 10% + TBP 5%. Scrubbing for contaminants removal may be carried out using HCl 5 mol/L with 0.1% of scandium losses, and all scandium was stripped by H3PO4 5 mol/L. Despite the works reported in the literature, there a few of them exploring the recovery of scandium and zirconium by the leaching-solvent extraction process. The flowchart proposed is strictly connected to the sustainable development goals 7, 8, 9, and 12.