Solubilities and solubilisation enthalpies of alkali metal uranates(VI) in carbonate melts

Abstract

A detailed investigation of the dissolution of alkali metal uranates in carbonate melts has been performed for the first time. The solubilities are reported for three alkali metal (Li, Na and K) mono- and diuranates, between 723 and 1073 K in alkali metal carbonate mixtures, and ranged from 20 to 275 wt ppm. The solvents used were the carbonate eutectics Li–Na, Li–K and Li–Na–K, and the low melting carbonate liquid Na–K. The well-known V-shaped behaviour of the solubilities of binary oxides in molten sulfates and carbonates with basicity increase was here observed for mono- and diuranates(VI). The explanation in terms of acid–base behaviour holds, the acidity and solubilities decreasing initially with temperature increase and then reversing as the melt becomes more basic. Uranate dissolution is thus both a physical and chemical process and hence from these data the solubilisation enthalpy in both acidic and basic melts was obtained for all six uranates(VI) in the four melts. Further, the oxidation of UO₂ to form uranates in molten carbonates must proceed by mass transfer of an oxidising species through a layer of uranates formed on the urania surface.