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 α-layered crystalline forms, were screened for Sc(III)/Fe(III) separation, as Fe(III) is one of the base elements in BR that is the most challenging to separate from Sc(III).
The studied α-zirconium phosphate (α-ZrP, Zr(HPO4) 2 ·H2O) exhibited the highest Sc(III)/Fe(III) separation factors (up to approximately 23) from HCl solutions.
The metal selectivity of α-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 α-ZrP for Sc(III).
Furthermore, chromatographic separation of Sc(III) from a real HCl leachate of BR was successfully achieved on an α-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(III) from a much higher concentration of Fe(III) in BR,without the need of using reducing agents.
Reusability of α-ZrP
Reusability of α-ZrP (triplicate measurements). Sorption conditions:m= 0.050g, 20mL of 1.0mmol L−1 Sc(III), pH ini = 1.6, pH eq =1.5, 18h.Desorption conditions: 20mL of 1.5molL−1 equimolar mixture of HNO3 and H3PO4 , 24 h.
Conclusions
α-ZrP materials possess excellent selectivity towards Sc(III) in the presence of high concentrations of Fe(III) and other base elements.
This interesting feature indicates potential to apply these materials in the separation of Sc(III) and Fe(III) from BR leachates, as these elements exhibit similar properties and thus their separation is generally difficult.
Among the tested materials (α-ZrP, am-ZrP, α-TiP, am-TiP), α-ZrP exhibited the highest Sc(III)/Fe(III) separation factors (up to 23).
Hydration enthalpy, ionic radii, crystallinity of the material and the pH of the solution were found to be the dominant factors that have an influence on the α-ZrP selectivity for Sc(III). α-ZrP exhibited very fast kinetics for Sc(III) uptake of 15min, which indicated a practical applicability of the ion exchanger for column chromatography processes.
Furthermore, the applicability of α-ZrP was verified in a chromato-graphy column by loading BR leachate with 0.7mol L−1 HCl, without any pretreatment. Most elements were not recovered by α-ZrP, whereas the recovery of Sc(III) was complete (>99.99%).
Moreover, by elution with 2mol L−1 HCl, 60% of the recovered Sc(III) could be separated well from the base elements and other REEs in BR. High-purity Sc(III) fractions were obtained after a single chromatography separation.
In conclusion, α-ZrP is highly effective in purification of Sc(III) from very complex matrices, such as a leachate of BR.
（Article in Separation and Purification Technology ）
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