On the Diluent and Solvent Effects in Liquid-Liquid Extraction Systems based on Bis-triazine-bipyridine (BTBP) ligands

Abstract: Used nuclear fuel is dangerous for mankind and her environment for a long time. If however, the minor actinides together with uranium and plutonium could be transmuted, i.e. transformed, into more shortlived or stable isotopes the volume of the waste could be significantly reduced together with a reduction in the radiotoxicity. In order to be able to transmute the actinides they need to be separated from fission and corrosion/activation products. One way of achieveing such a separation is through liquid-liquid extraction. One group of specially designed ligands that can achieve a separation of the trivalent actinides from the chemically similar lanthanides is the so called BTBPs (biz-triazine-bi-pyridine). By combining these ligands with tributyl phosphate, TBP, an extraction of all the actinides as one group, (Grouped ActiNide EXtraction) GANEX, can be achieved. As of today, cyclohexanone has been the diluent of choice in this solvent. The solvent has shown very promising extraction and separation results, however, both cyclohexanone and TBP have some drawbacks as parts in a process solvent why alternative diluents to cyclohexanone (hexanol and hexanoic acid) and an alternative ligand to TBP, DEHBA, have been investigated in this work. Thermodynamic extraction data and interfacial tension in BTBP extraction systems have also been investigated. Hexanoic acid did not work as a process diluent, since the extraction of americium and curium was much lower compared to that of plutonium. Hexanol would probably be a better choice, however, not as good as the cyclohexanone based solvent, due to mainly solubility issues of the ligand. The thermodynamic data for the americium extraction has been measured in three different diluents in order to investigate the diluent effect as well as compare the results with older data using C5-BTBP. For process development, the thermodynamic data shows that the extraction of americium and europium as well as their separation decreases as the temperature increases in several diluents. During the work it was also shown that short phase contact time favors the separation of americium from europium.