In-Situ-Generated Fluoride-Assisted Rapid Dissolution of Uranium Oxides by Ionic Liquids

A quantitative, rapid, endothermic dissolution of U₃O₈ in C₄mim·PF₆ (1-alkyl-3-methyl imidazolium hexafluorophosphate) has been achieved within 2 h at 65 °C by in situ generated fluoride ions by pre-equilibrating the ionic liquid with suitable concentrations of nitric acid. The efficiency of the dissolution followed the trend: UO₃ > UO₂ > U₃O₈. The fluoride generation was found to increase with the concentration of nitric acid being equilibrated, the water content of the ionic liquid, and also the time of equilibration. The rate of dissolution of U₃O₈ followed the trend: C₄mim·PF₆> C₆mim·PF₆ > C₈mim·PF₆. The maximum loading observed for the present case was 200 mg mL^-1 which is considered to be quite high with an ionic liquid. The effects of different acid pre-equilibration (HClO₄, HCl) on F^- generation and subsequent dissolution characteristics have also been investigated. The in situ F^- generation, as well as U₃O8 dissolution, were found to predominantly follow a pseudo-second-order rate kinetics, while the rate constants for U₃O₈ dissolution were found to be higher than that of F^- generation. The dissolved uranium was successfully electrodeposited on a Cu plate, as confirmed by EDXRF, while the formation of UO₂ was revealed from the XRD pattern of the deposit.