Determination of E–pO2− diagram for lanthanum in LiCl melt at 923 K

Abstract

The potential (E)–pO2− diagram for lanthanum in LiCl melt at 923K was investigated as part of the development of an electrolytic reduction process for spent nuclear fuel. LaCl3 dissolved in LiCl melt reacted with added Li2O to give a LaOCl precipitate. As the Li2O concentration was increased, LaOCl was converted to La2O3 and then to LiLaO2. The equilibrium Li2O concentrations for LaOCl/La2O3 and La2O3/LiLaO2 systems were experimentally determined to be 1.79mol% and 10.6mol%, respectively. Moreover, the lanthanum concentration in the LiCl melt increased in proportion to the Li2O concentration, which might be due to the formation of soluble complex oxide species of lanthanum. Lanthanum metal reacted with oxide ions to give lithium metal and LaOCl when the Li2O concentration was more than 0.2mol%. The standard Gibbs free energy of formation of LiLaO2 was estimated to be −1006.7kJ/mol at 923K. The E–pO2− diagram for lanthanum constructed from the experimental results suggests that La2O3 will be reduced to metal via LaOCl if the Li2O concentration in the LiCl melt is kept sufficiently low. However, in electrolytic reduction tests on La2O3, La2O3 was easily converted to LaOCl and the reduction of LaOCl did not proceed. It was found that the addition of nickel, which forms stable intermetallic compounds with lanthanum, was effective for reducing LaOCl to the metallic form.