THIRD PHASE FORMATION

When a solvent containing solute is contacted with another solvent, where both are immiscible, the solute preferentially distributes between them and this technique which is used to separate the pure solute is known as liquid-liquid extraction or solvent extraction. Third phase formation is a phenomenon in which loaded organic phase splits into two phases, third phase and diluent-rich phase, when metal loaded in organic phase increases beyond a particular concentration known as limiting organic concentration. The lighter phase contains most of the diluent and low concentration of extractant and solutes, while the heavier phase or third phase is a highly concentrated solution of extractant, metal, and acid. In the case of plutonium, the phenomenon has great significance with respect to danger of criticality. When third phase formed with plutonium is collected in some parts of contactor, its accumulation is a criticality hazard. Thus this phenomenon gains great importance in fast reactor fuel reprocessing. Moreover, accumulation of extracted metal ions in third phase makes it highly viscous and affect hydrodynamics of the plant level solvent extraction equipment like mixer settler and centrifugal extractor. Furthermore, these equipment are not designed to handle two organic phases and hence third phase has to be avoided during solvent extraction.

            Third phase formation depends upon various parameters like equilibrium aqueous phase acidity, metal ion, and its concentration in the feed, temperature, nature of diluents, extractant structure, organic to aqueous volume ratio, etc. Formation of third phase can be circumvented by increasing temperature of the solvent extraction system, reducing the metal concentration in the feed solution, utilizing long chain alcohol as modifiers and employing diluents with lesser chain length or aromatic in nature. However, these kinds of adjustments lead to adverse effects like low decontamination factor, reduction of throughput, change in physio-chemical properties of the solvent, etc. Among the various methods modification of structure of the extractant seems to be a better option to avoid third phase formation.