Selective ion-exchange separation of scandium(III) over iron(III) by crystalline α-zirconium phosphate platelets under acidic conditions

A continuous worldwide increase in scandium (Sc) criticality leads to a quest for secondary scandium resources. Among them, bauxite residue (BR) - a waste product from alumina refineries - often contains substantial amounts of scandium. However, the complexity in BR composition drives the need for developing a selective, efficient and cost-effective process to achieve the separation and purification of scandium. Insoluble salts of tetravalent metal ions are inorganic, acid-resistant ion exchangers with well-established preparation procedures, but their potential use in rare-earth recovery and purification has not been extensively explored yet. Zirconium and titanium phosphates, both in amorphous and a-layered crystalline forms, were screened for Sc(III)/Fe(III) separation, as Fe(111) is one of the base elements in BR that is the most challenging to separate from Scalp. The studied alpha-zirconium phosphate (alpha-ZrP, Zr(HPO4)(2)center dot H2O) exhibited the highest Sc(III)/Fe(III) separation factors (up to approximately 23) from HCl solutions. The metal selectivity of alpha-ZrP was considered to be affected by the solution pH, and the size and hydration enthalpy of the metal cations. Breakthrough curves for a binary Sc(III)/ Fe(III) solution, composed of metal concentrations realistic to a typical BR leachate, revealed the selectivity of alpha-ZrP for Sc(III). Furthermore, chromatographic separation of Sc(In) from a real HCl leachate of BR was successfully achieved on an alpha-ZrP column. After a two-step elution with HCl about 60% of Sc(III) was collected in fractions without measurable Fe(III), Al(III) or other rare-earth impurities. Overall, this study highlights the possibility for direct and simplified separation of Sc(UI) from a much higher concentration of Fe(III) in BR, without the need of using reducing agents.