The fate of the organic phase beyond third phase formation

Abstract

Third phase formation is an undesirable event in a solvent extraction procedure. It is the splitting of the organic phase into two phases during the course of solvent extraction at a particular concentration of metal ions in the organic phase characterised by the limiting organic phase concentration (LOC). The occurrence of the third phase is due to the extensive aggregation of strongly polar reverse micelles formed beyond the LOC, leading to the de-mixing of polar aggregates from a non-polar diluent present in the solvent phase. In this work, the extraction behaviour of nitric acid and Nd(III) was studied in a solution of tetra-2-ethylhexyl diglycolamide (TEHDGA) in n-dodecane (n-DD) and was probed by dynamic light scattering, ATR-FTIR and NMR spectroscopy to understand the fate of the organic phase beyond the LOC. The studies revealed that the aggregation of reverse micelles in the organic phase increased gradually with an increase in the extraction of Nd(III) and nitric acid before the LOC, and remarkably increased in the third phase beyond the LOC. A significant increase in the aggregate size of the third phase beyond the LOC was due to the high loading of nitric acid and Nd(III) in the third phase, eventually leading to polydispersion of reverse micelles in third phase. Before poly-dispersion, the TEHDGA molecules present in the organic phase were completely transferred to the third phase beyond the LOC, facilitating a mode of recovery of the diluent, n-dodecane, from the solvent phase, even though the third phase formation was regarded as an undesirable event.