Comprehensive studies on third phase behavior during solvent extraction of U(vi)/Th(iv) mixtures
Résumé
The third phase formation during liquid/liquid extraction has been studied for decades by
several authors[1-3] starting in the 50’s. Those studies were focused on the determination of the
maximum organic loading that can be reached for a metal in order to avoid phase splitting during the
industrial applications like the PUREX process. More recent studies were driven using a physical
approach[4-5] to propose models accounting for phase segregation phenomenon. A new interest on
third phase studies appeared in the early 2000’s with the investigations released for the Th-cycle [6-8].
Most of the models and results rely on SANS/SAXS or FTIR data acquired on metal-free or singlemetal
systems, and concentrations were often determined through analysis.
Despite the huge amount of data concerning LOC (Limiting Organic Concentration) of
actinides (IV) for different phosphate based solvent, a lack of data remains in the literature for U(VI)
alone and in the presence of Th(IV) or Pu(IV) for TBP/dodecane solvent. Besides, there are few values
concerning the composition of both heavy and light organic phases after splitting, and when
available, the data covers only a thin range of the 3rd phase existence.
Therefore, we have determined LOC value for U(VI)/HNO3/TBP/dodecane system at different
HNO3 and TBP concentrations which have not been yet reported in the literature. In addition to
these, the behavior of U(VI) and Th(IV) after phase segregation and the composition of each phase was
characterized using X ray fluorescence spectroscopy (XFS), allowing metal and TBP direct titration in
organic medium for the first time ever according to our knowledge. Nitric acid and water contents
were also estimated using acid/base and Karl Fischer titration respectively. Trends were established
for U(VI) and Th(IV) independently in the third phase, thus a better understanding of demixing behavior
of bimetallic U(VI)/Th(IV) systems was achieved.
A parallel has also been drawn by studying transition metals behavior in HCl/TBP/dodecane
systems (analyzed by XFS and ICP-OES) in order to determine contributions of metal and acid to
phase segregation phenomena. Moreover, we showed that LOC values can vary considerably from a
metal to another even when working in the same conditions of extraction; we are currently
investigating molecular species that may be involved in phase splitting.