Solvent extraction stage efficiency

The separation of components by liquid-liquid extraction depends primarily on the thermodynamic equilibrium partition of those components between the two liquid phases. Knowledge of these partition relationships is essential for selecting the ratio or extraction solvent to feed that enters an extraction process and for evaluating the mass-transfer rates or theoretical stage efficiencies achieved in process equipment. Since two liquid phases that are immiscible are used, the thermodynamic equilibrium involves considerable evaluation of nonideal solutions. In the simplest case a feed solvent F contains a solute that is to be transferred into an extraction solvent S. 

Mass Transfer As mentioned earlier, spray columns rarely develop more than 1 theoretical stage due to the axial mixing in the column. Nevertheless, it is necesary to determine what column height will give this theoretical stage. It is recommended by Cavers in Lo et al. Handbook of Solvent Extraction p. 323 and p. 327, John Wiley Sons, New York, 1983 that the following equation be used to estimate the overall efficiency coefficient ... 

FIG. 15-47 Correlation of mixing-stage efficiency with power input and liquid flow rates. [Scheihel in Lo, Baird, Hanson, Handbook of Solvent Extraction, p. 428 John Wiley h- Sons, NY, 1983. Used with peimission.]... 

The actual stage can be a mixing vessel, as in a mixer-settler used for solvent extraction applications, or a plate of a distillation or gas absorption column. In order to allow for non-ideal conditions in which the compositions of the two exit streams do not achieve full equilibrium, an actual number of stages can be related to the number of theoretical stages, via the use of a stage-efficiency factor. 

The results indicate that the process is robust and can tolerate and recover from disruptions in operation. In addition, hydraulic stage efficiencies in the extraction and strip sections were greater than the 80% design requirement. Test results showed that the loaded solvent could be stripped of cesium and recycled to the process with... 

Lanthanides form many types of complexes in both aqueous and non-aqueous solutions and have been studied extensively. In the early stages the main interest was in the development of efficient ligands for the separation of lanthanides by the ion-exchange technique. The second thrust was the study of complexes in non-aqueous solutions which can be used in the solvent extraction separation of lanthanides. 

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