Electrochemical and Thermodynamic Properties of Lanthanum in a Chloride Melt – Liquid Metal System

Abstract

Separation of spent nuclear fuel (SNF) components in a “molten salt – liquid metal” system is one of prospective directions in developing pyrochemical technology of SNF reprocessing. Modeling separation processes requires basic thermodynamic data for the elements present in SNF, both in the salts and metallic phases. In particular, values of the electrode potentials in the salt melt and activity coefficients in the metallic alloy are required. Present work was aimed at determining electrochemical and thermodynamic properties of lanthanum in alkali chloride melts (based on a low melting ternary LiCl–KCl–CsCl eutectic mixture) and gallium containing alloys. Lanthanum electrode potentials were measured in the melts containing 2.5–5.0 wt. % La. The following galvanic cell was employed: Cl2(C) │ LiCl–KCl–CsCl ║(LiCl–KCl–CsCl)–LaCl3│ La(Mo). Potential values were obtained using the potential-time transient curves recorded after a short time cathodic polarization of a molybdenum electrode. Molybdenum was selected as the working electrode material because it does not form alloys with lanthanum. Chlorine electrode was used as the reference. Typical potentiograms are presented in the Figure. Average potential value on the plateau of the curve was taken as lanthanum electrode potential. From the analysis of the experimental results the following temperature dependence of lanthanum electrode potential in the ternary lithium-potassium-cesium chloride eutectic melt was obtained: E * La(III)/La = –3.46 + 5.1·10-4 · T, V (668–976 K). Gibbs free energy change of lanthanum trichloride formation in LiCl–KCl–CsCl eutectic calculated from the results of electrochemical measurements is described by the following temperature dependence: ΔG*LaCl3 = –1001 + 0.148 · T, kJ/mol. Low melting gallium-aluminum (1.6 wt. % Al) and gallium-zinc (3.64 wt. % Zn) eutectic alloys were considered as liquid metal media. Activity coefficients of lanthanum, required for calculating separation factors, can be obtained from the difference of activity and solubility at a given temperature. Activity of lanthanum was determined employing electromotive force (EMF) measurements using the following galvanic cell: La(Ga–Me) │ (LiCl–KCl–CsCl)–LaCl3│ La(In), where Me = Al or Zn, and La(Ga–Me) and La(In) are saturated two-phase alloys. Supercooled liquid lanthanum was taken as the standard state for calculating thermodynamic values. Temperature dependencies of lanthanum activity in the studied alloys are adequately approximated by the following equations: lga La(Ga–Al) = –16068 · T -1 + 6.139, (573–1074 K), lga La(Ga–Zn) = –15346 · T -1 + 5.638, (573–1073 K). Solubility of lanthanum in the studied alloys was determined using direct physical measurements (sedimentation or filtration of saturated two-phase alloys) or EMF measurements of dilute homogeneous La(Ga–Me) alloys. Results obtained using different techniques showed a very good agreement. Lanthanum solubility in Ga–Zn eutectic at 389–1034 K can be described by the following linear equation: lgX La(Ga–Zn) = –2466 · T -1+ 1.018. Similar dependence for Ga–Al eutectic based alloys has an inflection around 480 K and can be described by two equations: lgX La(Ga–Al) = –4529 · T -1 + 3.487, (480–936 K), lgX La(Ga–Al) = –481 · T -1 + 4.947, (299–480 K). Temperature dependencies of lanthanum activity coefficients were obtained by subtracting solubility from activity, and the following equations were derived: lgγLa(Ga–Al) = –13602 · T -1 + 2.652, (573–936 K), lgγLa(Ga–Zn) = –12880 · T -1 + 4.619, (573–1034 K). The obtained data allow calculating separation factors of lanthanum and other elements present in SNF in the system “LiCl–KCl–CsCl eutectic melt – Ga–Al or Ga–Zn eutectic alloy”. Figure 1