Extraction of Lanthanides(III) and Am(III) by Mixtures of Malonamide and Dialkylphosphoric Acid
Résumé
The N,N--dimethyl-N,N--dioctylhexylethoxymalonamide, DMDOHEMA, and the di-n-hexylphosphoric acid, HDHP, are the extractants of reference for the French DIAMEXSANEX process for the separation of trivalent actinide ions from the lanthanide ions. In this work, the extraction of Eu3+ and Am3+ by the two extractants, alone or in mixtures, has been investigated under a variety of experimental conditions.The two cations are extracted by HDHP as the M(DHP.HDHP)3 complexes with an Eu-Am separation factor of ~10. The extraction is driven by the favorable enthalpy change. With DMDOHEMA, Eu3+ and Am3+ are extracted as the M(NO3)3(DMDOHEMA)2 disolvate species with an Am-Eu separation factor of ~2. The metal distribution ratios measured with HDHP increase sharply along the lanthanide series. DMDOHEMA exhibits a moderate decrease of the D values for the heaviest lanthanides. As a consequence of these opposite tendencies, a mixture of the two reagents extracts almost all lanthanides equally well.The extraction of Eu3+ and Am3+ by HDHP-DMDOHEMA mixtures exhibits a change of extraction mechanism and a reversal of selectivity taking place at ~1 M HNO3 in the aqueous phase. Below this aqueous acidity, HDHP dominates the metal extraction by the mixture, whereas DMDOHEMA is the predominant extractant at higher aqueous acidities. Continuous variation experiments provided no indication of synergism between HDHP and DMDOHEMA. Other measurements, however, indicated apparent modest antagonism in the extraction of Eu3+ and synergism in the extraction of Am3+. These data were interpreted as resulting from the formation in the organic phase of mixed HDHP-DMDOHEMA species containing two HDHP and five DMDOHEMA molecules.