Application of a linear free energy relationship to lanthanide-zinc alloys

Abstract

The reductive extraction of f-elements between molten chloride and liquid metal deserves an examination as a possible alternative technique for reprocessing nuclear fuels. Particularly, its application for the group separation of actinides and lanthanides (Ln, hereafter) is promising owing to the high separation factors between lanthanides and actinides achievable in this system [1, 2]. The separation efficiency of lanthanides and actinides is greatly influenced by the standard free energy of formation of their chlorides and metallic alloys in two phases. In order to evaluate the effectiveness of the extraction and separation by this technique, the thermodynamic activities of lanthanides dissolved in liquid metal have to be determined. The standard free energies of the formation of lanthanide-metal alloys can be experimentally measured by the electromotive force measurement method. The standard free energies of few liquid Ln-Zn alloys at 873 K have been obtained [1, 3], but information on the quantitative systematic variation along the 4f-series is still insufficient. Identifying such systematic changes is very useful as it enables to predict those Ln that are difficult or impossible to measure experimentally. It is of great interest to predict the unknown formation free energies of Ln-Zn alloys from the known thermodynamic properties. Sverjensky et al. [4] have developed an empirically based linear free energy equation applicable to cations of any charge, radius or chemical types, which allows estimates of the free energy of solids with uncertainties of less than ±4.18 kJ/mol. This equation is directly analogous to the linear free energy relations of Hammett and others [5, 6] for aqueous organic reactions, but applies instead to crystalline solids of the following structure, MO, M(OH)2, MCO3, MF2, MCI2, and MSO4. Within an isostructural family of solids, the free energy change in going from one compositional endmember to another containing cations with the same (or similar) ionic radii is paralleled by the free energy change between the corresponding aqueous cations [4, 7–9]. The lanthanides have the similar ionic radii and charge; this equation has been successfully applied to predict the standard free energies of formation of liquid Ln-Bi alloys [10]. Therefore, it is desirable to apply this linear free energy relationship for Ln-Zn alloys in order to accurately predict of the standard free energies not previously amenable to pyrochemical analysis. Small amount of Ln dissolved in liquid Zn (mole fraction less than 0.01 in the reductive extraction systems)