Continuous countercurrent emulsion

Solvent Extraction. Solvent extraction has widespread appHcation for uranium recovery from ores. In contrast to ion exchange, which is a batch process, solvent extraction can be operated in a continuous countercurrent-fiow manner. However, solvent extraction has a large disadvantage, owing to incomplete phase separation because of solubihty and the formation of emulsions. These effects, as well as solvent losses, result in financial losses and a potential pollution problem inherent in the disposal of spent leach solutions. For leach solutions with a concentration greater than 1 g U/L, solvent extraction is preferred. For low grade solutions with <1 g U/L and carbonate leach solutions, ion exchange is preferred (23). Solvent extraction has not proven economically useful for carbonate solutions. 

Co and Ci are the continuous phase solute concentrations in teed and inside the mixer, respectively, and are the feed rates of the continuous and emulsion phases, respectively, Cir is the internal reagent concentration (based on volume of emulsion), R is the emulsion globule radius, Kr is the emulsion phase holdup volume in the mixer, a is the partition coefficient for solute between external phase and emulsion, is the effective solute diffusivity in the emulsion, and % is the dimensionless reaction front position. Hatton and Wardius [51] also extended their analysis to develop simple graphical and numerical procedures for the prediction of multistage extraction performance of mixer-settler trains operating either cocurrendy or countercurrently without any external recycle over individual stages. For a typical stage i in a cocurrent mixer-settler, they defined the parameter 6 as... 

In order to develop a continuous separation process, Kataoka et al. [54] simulated permeation of metal ion in continuous countercurrent column. They developed the material balance equation considering back mixing only in the continuous phase and steady-state diffusion in the dispersed emulsion drops which is similar to the Hquid extraction situation. Bart et al. [55] also modeled the extraction of copper in a continuous countercurrent column. They considered only the continuous phase back mixing in the model and assumed that the reaction between copper ions and carrier is slow, so that the differential mass balance equation for external phase in their model is... 

In the devolatilization of emulsion polymers, the mass transfer between the aqueous phase and the gas phase is frequently the rate-determining step. Consequently, the agitation, the gas flow rate, and other process variables or design characteristics that increase the interfadal area between the aqueous phase and the gas phase will improve the devolatilization. However, care must be taken to avoid foaming and coagulation. Column continuous countercurrent strippers are often used in devolatilization of suspension polymers, while stripping in tank reactors is more convenient for low-Tg film-forming latexes. 

Oil and emulsion t low through a spreader into (lie back, or coalescing, section ot the vessel that is lluid packed. Tlic spreader distributes flow evenly throughout the length ol this section Treated oil is collected at the top by means of a collection device sized to maintain a uniform vertical oil flow Coalescing water droplets fall countercurrent to the rising oil continuous phase. The oil-water interface is maintained by a level controller and dump valve for this section of the vessel. 

Figure 8.1.48. Emulsion liquid membrane (a) a large emulsion globule containing many tiny droplets (b) continuous ELM process schematic (c) countercurrent ELM column for removing Zn from wastewater. (After Ho and Sirkar (2001), chaps. 36, 39.)...

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