Use of TBP and N,N-dialkylamides for uranium extraction with 25 mm annular centrifugal contactors: effect of residence time on extraction performance

Tri-n-butyl phosphate (TBP) is used as solvating extractant in the PUREX process, but N,N- dialkylamides can also extractant uranium from HNO₃ and have the particular advantage that they comprise only the elements carbon, hydrogen, oxygen and nitrogen (CHON). Annular centrifugal contactors (ACCs) have interesting characteristics and advantages in continuous solvent extraction processes compared to e.g. mixer-settlers. Besides their small footprint, very short contact times in the annular mixing zone can be achieved, limiting solvent degradation. An experimental setup consisting of 16 stages of 25 mm diameter, ACCs made from 316 L stainless steel was used for SX R and D at SCK CEN (Mol, Belgium). The housing in which the rotor is spinning, is made from acrylic glass, which allows direct visual observation of the annular mixing zones. The R and D setup can be easily converted from an extraction-scrubbing process to a back-extraction configuration. It was investigated how the efficiency of solvent extraction processes can be optimized by changing the contact time in the centrifugal contactor. The SX Process software package was used to create a model for U extraction by 1.1 M TBP as a function of the HNO₃ and free TBP concentrations. Distribution isotherms were constructed from available literature data and data fitting was performed in MATLAB. Flow sheets with high saturation of the organic phase were modelled to maximize decontamination of fission products. Several experiments were performed to assess the TBP benchmark system. Ru and Zr decontaminations were less efficient than predicted by the model, in agreement with literature reports. Increasing the residence time in the mixing zone by increasing the mixer volume improved the efficiency of the scrubbing process, indicating that a kinetic effect is responsible. Batch extraction experiments were performed to determine distribution isotherms of 1.1 M D2EHBA and 1.1 M D2EHiBA for U and HNO₃ at different HNO₃ concentrations. The multi-stage extraction experiments with D2EHiBA confirmed a better performance of Ru and Zr decontamination compared to TBP. Where it was found that for the TBP process, only 4 extraction stages are required to achieve a decontamination factor of >10⁵ for U, for D2EHiBA 8 stages are required, while comparable throughput was obtained. (authors)