In the operation of solvent extraction systems in the laboratory, where new reagents and processes are typically developed, conditions are typically maintained in a moderately idealized state. In particular, the effects of “loading”, i. e., contacting aqueous solutions containing high concentrations of metal ions with extractant solutions targeting those metal ions, are typically not investigated until an advanced stage of process development is approached. In this limit, deviations from ideality are often observed. For example, a UO22+ extraction reaction having the following ideal stoichiometry,

UO22+ + 2 NO3- + 2 Ex = UO2(NO3)2Ex2

might adhere to this ideal stoichiometry through some range of concentra­tions, or more appropriately up to a solubility limit in the organic phase. At

higher concentrations of metal ions, the extraction stoichiometry might drop to

UO22+ + 2 NO3- + 1 Ex = UO2(NO3)2Ex

While the metal ion continues to partition, the possibility solute-molecule reorganization in the organic phase can occur leading to precipitation or to the separation of a third liquid phase (third phase formation).

In this metastable (though typically reversible) condition, the aqueous phase retains much of its essential character, while the organic phase will split into solute-poor (mainly diluent) and solute-rich (mainly extracted metal complex and excess extractant molecules) phases. This condition is at best an annoyance as a source of process upset and at worst a serious safety hazard in the operation of a reprocessing facility, as the unintentional formation of a critical mass of fissile material can occur with potentially disastrous consequences. Such phenomena are well known in solvent extrac­tion of metal ions and mineral acids. The minimization of the potential for such an event to occur is addressed through appropriate alteration of diluent structures and mixtures of diluents, effective extractant design or by carefully designing process monitoring and control procedures to prevent overloading of extractant solutions. Recent research being conducted in the US and in particular in Europe have provided unique insights into the nature and driving force for such interactions. In the design of any facility or process that incorporates fissile materials, it is quite important to always consider these possibilities.