Multicomponent Model for the Prediction of Nuclear Waste/Rare-Earth Extraction Processes

Abstract

We develop a minimal model for the prediction of solvent extraction. We consider a rare earth extraction system for which the solvent phase is similar to water-poor microemulsions. All physical molecular quantities used in the calculation can be measured separately. The model takes into account competition complexation, mixing entropy of complexed species, differences of salt concentrations between the two phases, and the surfactant nature of extractant molecules. We consider the practical case where rare earths are extracted from iron nitrates in the presence of acids with a common neutral complexing extractant. The solvent wetting of the reverse aggregates is taken into account via the spontaneous packing. All the water-in-oil reverse aggregates are supposed to be spherical on average. The minimal model captures several features observed in practice: reverse aggregates with different water and extractant content coexist dynamically with monomeric extractant molecules at and above a critical aggregate concentration (CAC). The CAC decreases upon the addition of electrolytes in the aqueous phase. The free energy of transfer of an ion to the organic phase is lower than the driving complexation. The commonly observed log–log relation used to determine the apparent stoichiometry of complexation is valid as a guideline but should be used with care. The results point to the fact that stoichiometry, as well as the probabilities of a particular aggregate, is dependent on the composition of the entire system, namely the extractant and the target solutes’ concentrations. Moreover, the experimentally observed dependence of the extraction efficiency on branching of the extractant chains in a given solvent can be quantified. The evolution of the distribution coefficient of particular rare earth, acid, or other different metallic cations can be studied as a function of initial extractant concentration through the whole region that is typically used by chemical engineers. For every chemical species involved in the calculation, the model is able to predict the exact equilibrium concentration in both the aqueous and the solvent phases at a given thermodynamic temperature.

Effects of porous media on extraction kinetics: Is the membrane really a limiting factor?

“…However, if pH is too low, it induces the co-extraction of large quantities of water, which limits the selectivity versus iron. [50] These known trends are all found in results presented in figure 4: iron is not extracted. When quenching mechanisms are in minority, all amounts for all dissolved lanthanide ions are extracted.…”

“…In classical view of extraction, this is a surprising observation, corresponding to an apparent entropic term in the free energy linked to structural reorganisation of the solvent phase upon complexation. However, once complexation, bulk and electrostatic free energy terms are identified [50], it is clear that this term is related to entropy of the complexing molecules, that participate to a highly bent interfacial film [54]. Figure 6: The system was studied at two different temperatures, 19°C and 35°C, pH 2.3 using both microfluidic, (full symbols).…”

Separation of neodymium and dysprosium by solvent extraction using ionic liquids combined with neutral extractants: batch and mixer-settler experiments

“…Different models to study and understand the complexation and aggregation in both the aqueous and the organic phases have been proposed for the extraction of rare earths using different types of extractants and diluents. [60][61][62] Recycling of the organic phase…”

A microfluidic study of synergic liquid–liquid extraction of rare earth elements

“…Such an influence of the pH on extraction kinetics has previously been explained as being linked to the ion exchange mechanism: at low pH, the concentration of H + in extractant aggregates's polar cores is high which facilitates cation exchange. 20 Although observed curves can show some irregularities, which are likely to be attributed to measurment artefacts, it should be noted that the difference in kinetics for the considered ions could be exploited to enhance selectivity by using a contact time that maximises it (instead of working at thermodynamic equilibrium): for example in the absence of DMDOHEMA, fast extraction allows targetting of heavy rare earths. Such an observation cannot be obtained using the standard batch approach in the case of fast kinetics.…”

“…13,15 Ion transfer between two phases and its driving force have been the subject of many kinetic, thermodynamic, theoretical, and modeling studies which require years of research with yet many remaining unknowns. 11,[16][17][18][19][20][21][22] In addition to this complexity, even when promising hydrometallurgy processes are described, their use by the recycling industry is hindered by the cost associated with the use of very low pH and the variability in the elemental composition from one waste lot to the other. The industry is therefore not only in need of more efficient approaches that can work at higher pH and show higher performances but also of methods enabling a faster speed of process development or adjustment.…”