Investigation on the Separation of Zr Using Trialkyl Phosphate based Solvents: A Combined Experimental and Theoretical Studies with insights from EXAFS Analysis

Metallic fuels such as U–Zr and U–Pu–Zr alloys can be reprocessed by the aqueous-based PUREX process. Zr which is present in large quantities (60 kg/ton of fuel) is transferred to high level waste (HLW) solutions during the aqueous reprocessing of metal fuel. It is important to recover and reuse Zr from the HLW solutions. The recovered Zr can be recycled for the production of fresh metallic fuel. In this context, batch solvent extraction studies were carried out with tri-isoamyl phosphate (TiAP) and tri-n-butyl phosphate (TBP) in cross-current mode to evaluate the number of stages required for the quantitative extraction and stripping of Zr. Based on batch solvent extraction studies, continuous counter-current mixer-settler runs were demonstrated for the recovery of zirconium with 1.47 M TBP/n-dodecane (n-DD) from 6 M HNO3 using an ejector mixer-settler facility. The extraction mechanism of Zr by TiAP and TBP was evaluated using density functional theory (DFT) calculations in order to gain insight into the nature of interactions operating between Zr(IV) and the extractants. Complexation energies of Zr complexes with TiAP and TBP were computed using DFT and found to be in good agreement with the experimentally measured distribution ratios. The coordination behavior of zirconium nitrate with TiAP and TBP was investigated by using extended X-ray absorption fine structure (EXAFS) studies. The coordination number and bond distances of Zr(NO3)4 complexes with TiAP and TBP were obtained from EXAFS analysis. These studies demonstrate the quantitative extraction and stripping of Zr from nitric acid and organic solutions, respectively.