(479d) Modeling Internal Recycle within a Mixer-Settler to Aid Extraction Performance

Precise models for solvent extraction have become necessary as the demand for rare earth elements (REE) continues to rise. Similarly, improved solvent extraction methods are required to ensure the supply of REE is maintained in the future. This study employed and modeled internal recycle – remixing the organic phase during extraction – to develop a precise model of and an improved method for solvent extraction. The recycled organic phase consisted of the cationic extractant (bis[2-ethylhexyl] phosphate [HDEHP]) in Isopar-L and the aqueous phase consisted of yttrium (Y) in dilute sulfuric acid.

Although internal recycle within a mixer-settler has been shown to minimize emulsions (Ryon et al, 1963) and reduce entrainment (Rowden et al, 1974), it has the added benefit of increasing extraction efficiency when performed in the correct proportions. Treybal indicated that only by recycling the dispersed organic phase during solvent extraction would the mixer-settler efficiency increase (Treybal, 1964), whereas Hoh determined that recycling either the dispersed or continuous phase was beneficial as long as the organic to aqueous phase ratio (O:A) in the mixer remained equal to 2:1 for their system (Hoh et al, 1989). Although Hoh's model suggests that the change in mixer O:A caused the change in efficiency, a thorough model relating the internal recycle parameters, mixer O:A, organic phase residence time and mixer efficiency is needed.

The current study modeled the effect of internal recycle on mixer O:A and mixer residence time, then determined the corresponding efficiency for each of the internal recycle rates. To model the mixer O:A and residence time, a mathematical relation was developed. To determine the efficiency for a specific recycle rate, Y was extracted from a surrogate phosphate-derived byproduct stream. The quantity of Y extracted in a single stage employing recycle was compared against batch equilibrium data. Using both the mathematical relation and the extraction results, the final model related extraction efficiency with mixer residence time and mixer O:A. When internal recycle was not employed, the mixer O:A was equal to the external O:A (1:10), the mixer residence time was approximately 45 seconds, and the efficiency was 55%. When 80% of the of the organic phase was internally recycled, the mixer O:A was 1:2, the mixer residence time was approximately 155 seconds, and the efficiency was 85%. Experimental results confirmed the trends predicted by the model and show a strong correlation between mixer O:A, mixer residence time and mixer-settler efficiency.

References

1. Ryon, A. D. and Lowrie, R.S., 1963. U.S. Atomic Energy Comm., ORNL-3381

2. Rowden, G.A., Scuffham, J.B. and Warwich, G.G.I., 1974. The effect of change in operating organic/aqueous ratio on the operation of a mixer-settler. Proc. ISEC'74, 1:81

3. Treybal, R.E., 1964. Recycle in liquid extraction. I&EC Fundamentals, 3(3):185

4. Hoh, Y.C., Ju, S.J., Chiu, T.M., 1989. Effect of internal recycle on mixer-settler performance. Hydrometallurgy, (23): 105-118

Acknowledgements

This research is supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. This research is also supported by the Graduate Assistance in Areas of National Need Fellowship funded by the U.S. Department of Education. Special thanks to Natasha Ghezawi and Samantha Thorpe for their contributions and Dr. David DePaoli of Oak Ridge National Laboratory for his expertise.