Third-phase formation

The amount of modifier required to prevent third phase formation can be determined in the following way. The aqueous and solvent phases are first contacted, and once the three phases have separated, the lower aqueous phase is drawn off and discarded. The modifier to be considered is then added from a burette in small increments to the two organic phases, and the mixture shaken after each addition. The amount of modifier required to produce a single organic phase is then used to calculate the amount required to be added to the solvent. Generally, about 2-5 vol% of modifier is needed, but more may be necessary if high concentrations of extractant are used in the solvent. Any effects of modifiers on the kinetics and equilibria of metal extraction and stripping can be determined by shakeout tests. 

The Modifiers. Additional chemicals are sometimes added to solvents to prevent a third phase formation. In the past, long chain alcohols or tri-n-butyl phosphate (TBP) have been used. Certain modifiers enhance the rate of extraction and the final equilibirum position. It is known for example, that tributylphosphate acts synerglstically with dialkyIphosphoric acids. 

Chiarizia, R. 2006. Mechanism and energetics of third phase formation in TBP solvent extraction of metal salts. IPNS Progress Report 2001-2006. In Celebration of the 25th Anniversary of IPNS. ANL-06/54. Argonne National Laboratory. 

Rao, P.R.V., Kolarik, Z. 1996. Areview of third phase formation in extraction of actinides by neutral organophosphorus extractants. Solvent Extr. Ion Exch. 14 (6) 955-993. 

Osseo-Asare, K. 2002. Microemulsions and third phase formation. In Proc. Int. Solv. Extr. Conf. ISEC 2002. Sole, K.C., Cole, P.M., Preston, J.S., Robinson, D.J. Eds. South African Institute of Mining and Metallurgy, Cape Town, South Africa, March 18-21, Chris Van Rensburg Pub. Ltd., Melville, Johanesburg, pp. 118-124. 

Chiarizia, R., Jensen, M.P., Borkowski, M. et al. 2003. Third phase formation revisited The U(VI), HNO3-TBP, n-dodccane system. Solvent Extr. Ion Exch. 21 (1) 1-27. 

Chiarizia, R., Jensen, M.P., Rickert, P.G. et al. 2004. Extraction of zirconium nitrate by TBP in n-octanc Influence of cation type on third phase formation according to the sticky spheres model. Langmuir 20 (25) 10798-10808. 

Plaue, J., Gelis, A., Czerwinski, K., Thiyagarajan, P., Chiarizia, R. 2006. Small-angle neutron scattering study of plutonium third phase formation in 30% TBP/HN03/alkane diluent systems. Solvent Extr. Ion Exch. 24 (3) 283-298. 

Chiarizia, R., Briand, A. 2007. Third phase formation in the extraction of inorganic acids by TBP in n-octane. Solvent Extr. Ion Exch. 25 (3) 351-371. 

Berthon, L., Martinet, L., Testard, F., Madic, C., Zemb, T. 2007. Solvent penetration and sterical stabilization of reverse aggregates based on the DIAMEX process extracting molecules Consequences for the third phase formation. Solvent Extr. Ion Exch. 25 (5) 545-576. 

Kumar, Sh., Koganti, S.B. 1998. A Setchenow type model for Pu(IV) third phase formation in nitric acid-bis(2-ethylhexyl)sulphoxide/dodecane biphasic system at 298.15K. Solvent Extr. Ion Exch. 16 (3) 829-841. 

Erlinger, C.V. 1998. Towards a physical interpretation of third phase formation in liquid-liquid extraction. Application to the DIAMEX process for the treatment of high radioactive nuclear wastes. Thesis. University Paris XI, Paris. 

Lefrancois, L., Belnet, F., Noel, D., Tondre, C. 1999. An attempt to theoretically predict third-phase formation in the dimethyldibutyltetradecylmalonamide (DMDBTDMA)/ dodecane/water/nitric acid extraction system. Sep. Sci. Technol. 34 (5) 755-770. 

The term Third-Phase Formation in solvent extraction refers to a phenomenon in which the organic phase splits into two phases (126). One of the two phases is diluent rich, whereas the other is rich in extractant and also contains the metal solvate. Third-... 

Suresh et al. investigated the extraction of uranium and thorium by TsBP and TiBP (isomers of TBP with branched carbon chain) as an alternative choice for TBP (47). Higher homologues of TBP, for example, THP and TEHP, were reported to have higher extraction ability with reduced tendency toward third-phase formation (50, 51). The esters with bulkier substituents in place of the butyl group were proposed to be of practical value for the process applications in uranium and thorium separation (54). The LOC of thorium in equilibrium with aqueous nitric acid-thorium nitrate was reported to decrease in the order THP > TAP > TBP. Pathak et al. showed that TEHP can be a better choice for U/Th separation compared to TBP and TsBP (55). 

Vidyalakshmi, V. Subramanian, M.S. Srinivasan, T.G. Vasudeva Rao, P.R. Effect of extractant structure on third phase formation in the extraction of uranium and nitric acid by n-dialkyl amides, Solvent Extr. Ion Exch. 19 (2001) 37—49. 

Srinivasan, T.G. Ahmed, M.K. Shakila, A.M. Dhamodaran, R. Vasudeva Rao, P.R. Mathews, C.K. Third phase formation in the extraction of plutonium by tri-normal-butyl phosphate, Radiochim. Acta 40 (1986) 151-154. 

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