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.
