EFFECT OF NITRIC-ACID EXTRACTION ON PHASE-BEHAVIOR, MICROSTRUCTURE AND INTERACTIONS BETWEEN PRIMARY AGGREGATES IN THE SYSTEM DIMETHYLDIBUTYLTETRADECYLMALONAMIDE (DMDBTDMA) N-DODECANE WATER - A PHASE-ANALYSIS AND SMALL-ANGLE X-RAY-SCATTERING (SAXS) CHARACTERIZATION STUDY

Among the different problems to be solved when designing a liquid-liqu id solvent extraction process, third phase formation, i.e. the splitti ng of the organic phase into two layers when extracting high concentra tion of solutes, like nitric acid or metallic nitrates (here trivalent f ions), is one of the most important to address. In some conditions the formation of a ''third phase'' is observed with dimethyldibutyltet radecylmalonamide (DMDBTDMA), a potential extractant used in the DIAME X process. We have investigated the phase behavior of the system DMDBT DMA/n-dodecane/water/HNO3, in the acceptable concentration limits for the DIAMEX process. The composition of the different phases and the su rface properties of the two-phase system were measured. The maximum in corporation of water in the two-phase system corresponds to approximat ely 0.75 water molecule per DMDBTDMA molecule, whereas at saturation i n the three-phase system it is about 1.25 water molecules per extracta nt molecule. At 0.22 M and 0.46 M DMDBTDMA concentrations, the transit ions from the two-phase to the three-phase domain takes place in a reg ion where the [HNO3](extr.)/[DMDBTDMA](init) ratio is around 0.8. We o bserve that the two-phase to three-phase transition occurs when the wa ter/acid ratio in the organic phase approaches 1. A sharp change of sl ope of the interfacial tension versus extractant concentration is attr ibuted to aggregate formation in the organic phase. Assuming a neutral form of the molecule in the absence of HNO3, the interfacial area is in this case 112 Angstrom(2). The microstructure of mixtures DMDBTDMA, water and nitric acid in n-dodecane has also been studied using small angle X-ray scattering (SAXS) in order to determine the size and shap e of the primary aggregates of DMDBTDMA as well as the interactions be tween them in the midst of the organic phase. The complexation of nitr ic acid at constant diamide concentration, strongly favours attractive interactions between the aggregates. On the contrary, the increase of the aggregates volume fraction, at a constant ratio of nitric acid an d diamide concentrations in order to control the attractions, force th e aggregates to repel each other, and repulsive hard sphere interactio ns are pointed out. The information obtained in the present work from the SAXS study, and from the interfacial tension measurements, appear to be consistent since they both evidence the onset of an aggregation process at the approach of the organic phase splitting. The simple sho rt range attractive potential defined by Baxter, describing a complex fluid of sticky spheres, is self-consistent to model the experimental data. In the organic phase, the extractant molecules of DMDBTDMA self- assemble into small reversed micelles with a polar cord of similar to 6 - 7 Angstrom radius when the organic phase is contacted with an aque ous phase (acidic or not). Within the organic phase, the aggregates ar e submitted to three major interactions : (i) the destabilizing van de r Waals interaction and (ii) the stabilizing hard sphere repulsion and (iii) a repulsive steric contribution from the remaining aliphatic ch ains of the extractant molecules. The observable macroscopic effect wh ich is the phase split of the organic phase with third phase formation is the macroscopic translation of the effect of these three interacti ons acting at the microscopic level.