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.