A conceptual dft study of the Solvent Extraction of Zirconium and Hafnium

A Conceptual DFT Study of the Solvent Extraction of Zirconium and Hafnium

D.B. Jansen van Vuuren*, C.G.C.E. van Sittert, D.J. van der Westhuizen, H.M. Krieg

Focus Area: Chemical Resources Benefication, North-West University, Potchefstroom Campus, Potchefstroom, 2531, South Africa. *E-mail: [e-mail]

In this study density functional theory (DFT) and conceptual DFT were applied to study the mechanisms underpinning the solvent extraction of zirconium and hafnium. Pure zirconium is crucial to the nuclear energy industry, where it is used in coating material for uranium fuel rods.¹ The most abundant source of zirconium is zircon ore, which also contains 3% hafnium.¹ Since nuclear grade zirconium has to be purified of hafnium, the two metals need to be separated. Solvent extraction has been suggested as an economically efficient and environmentally friendly way to separate zirconium and hafnium.² The majority of research on the solvent extraction of zirconium and hafnium is done by a screening approach.² In such an approach experimental conditions, such as extractant type and concentration, are varied across a given range to find suitable conditions for separation.

This study is part of a broader initiative to use a rational approach towards the design of optimal experimental conditions for the solvent extraction of zirconium and hafnium. A rational design approach requires understanding of the mechanism by which the solvent extraction of zirconium and hafnium occurs. The mechanisms underpinning solvent extraction reactions are governed by the principles of chemical reactivity.³ Conceptual DFT provides a means by which the reactivity of the solvent extraction of zirconium and hafnium can be investigated at the atomistic level.

Conceptual DFT is a modern area of DFT in which notions of chemical reactivity, such as hardness and regioselectivity, are expressed in terms of electron densities. Starting in 1977, Robert Parr and co-workers published a series of article in which the notions of electron chemical potential⁴, chemical hardness⁵, and frontier orbitals⁶ are all defined as changes to a system’s energy with respect to changes in the system’s electron density.

In this work the solvent extraction of zirconium and hafnium, by phosphorus based extractants, was investigated. Fukui function were calculated on the extractants to predict how the extractant bind to the zirconium and hafnium complexes. Hirshfeld charges and frontier orbitals energies were calculated on the extractants to predict the local hardness of various binding sites on the extractants. Calculating the extractant’s chemical hardnesses, allowed us to investigate the relationship between extractant chemical hardness and extractant selectivity. Furthermore, various possible mechanisms by which the extractants bind to the metal complexes were suggested. The energies of these mechanisms were compared to extraction results from experimental work.

References

1. Harper, E. M.; Diao, Z.; Panousi, S.; Nuss, P.; Eckelman, M. J.; Graedel, T. E., The criticality of four nuclear energy metals. Resources, Conservation and Recycling 2015, 95, 193-201.

2. Banda, R.; Lee, M. S., Solvent extraction for the separation of Zr and Hf from aqueous solutions. Separation and Purification Reviews 2015, 44 (3), 199-215.

3. Wilson, A. M.; Bailey, P. J.; Tasker, P. A.; Turkington, J. R.; Grant, R. A.; Love, J. B., Solvent extraction: The coordination chemistry behind extractive metallurgy. Chemical Society Reviews 2014, 43 (1), 123-134.

4. Parr, R. G.; Donnelly, R. A.; Levy, M.; Palke, W. E., Electronegativity: The density functional viewpoint. The Journal of Chemical Physics 1977, 68 (8), 3801-3807.

5. Parr, R. G.; Pearson, R. G., Absolute hardness: Companion parameter to absolute electronegativity. Journal of the American Chemical Society 1983, 105 (26), 7512-7516.

6. Parr, R. G.; Yang, W., Density functional approach to the frontier-electron theory of chemical reactivity. Journal of the American Chemical Society 1984, 106 (14), 4049-4050.