Chemical interactions between La(iii) and O2− in molten fluoride salts and La(iii)/U(iv) separation by oxide precipitation method

The chemical interactions between La(iii) and O²⁻ in 66.7LiF-33.3BeF₂ (FLiBe) and 46.5LiF-11.5NaF-42KF (FLiNaK) molten salt systems at 873 K were studied by dissolution and oxide titration methods. In the FLiBe, the precipitation-dissolution behavior of La₂O₃ is a simple equilibrium mechanism between La(iii) and O²⁻ ions. The solubility of La₂O₃ in FLiBe melt was 0.078 mol/kg with the dissolution equilibrium time of 5 h, and the corresponding apparent solubility product ($K_{\mathrm{sp}}^{\prime }$) of La₂O₃ was (3.43 ± 0.75) × 10⁻⁴ mol⁵/kg⁵. The oxide titration experiment showed that the product of the interaction between La(iii) and O²⁻ in FLiBe is La₂O₃ precipitate, and the $K_{\mathrm{sp}}^{\prime }$ was (3.45 ± 0.37) × 10⁻⁴ mol⁵/kg⁵, which was highly consistent with that obtained by the dissolution method. Based on the $K_{\mathrm{sp}}^{\prime }$ value, the oxide tolerance for La₂O₃ precipitation was then evaluated. However, the chemical reaction between La(iii) and O²⁻ in FLiNaK was more complicated. The dissolution of La₂O₃ would produce oxyfluoride LaOF, and addition of Li₂O into the FLiNaK-La(iii) molten salt could cause precipitation of equimolar solid compounds La₂O₃ and LaOF. The oxyfluoride species LaOF was correlated with a high content of free fluoride ions (F⁻) in FLiNaK. At last, an oxide precipitation method was proposed for La(iii)/U(iv) separation based on the analysis of $K_{\mathrm{sp}}^{\prime }$(La₂O₃) and $K_{\mathrm{sp}}^{\prime }$(UO₂), and this method achieved a good La(iii)/U(iv) separation efficiency in the FLiBe-LaF₃-UF₄ melt.