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Multicomponent Model for the Prediction of Nuclear Waste/Rare-Earth Extraction Processes
journal contribution
posted on 2018-08-06, 00:00 authored by Mario Špadina, Klemen Bohinc, Thomas Zemb, Jean-François DufrêcheWe
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.
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CAC decreasesearth extraction systemsurfactant naturephasechemical engineersresults pointwater-poor microemulsionsmonomeric extractant moleculeschemical speciesaccount competition complexationcomplexed speciesequilibrium concentrationcomplexing extractantextractant chainsextraction efficiencydistribution coefficientextractant concentrationMulticomponent Modelextractant moleculesextractant contentsalt concentrationsiron nitratesaggregatemodel
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