Mixer settler units

Ignoring the quite distinct functions and hydrodynamic conditions which exist in the actual mixer and settler items of the combined mixer-settler unit, it is possible, in principle, to treat the combined unit simply as a well-mixed equilibrium stage. This is done in exactly the way, as considered previously in Secs. 3.2.1 to 3.2.6. A schematic representation of an actual mixer-settler... 

Figure 3.43. Schematic representation of a mixer-settler unit.

The notation for separate mixer-settler units is shown in Fig. 3.45, for stage n of the cascade. 

The extraction is represented by a single perfectly mixed constant volume, continuous flow equilibrium stage. This may actually consist of separate mixer-settler units. 

If separation is difficult in a mixer-settler unit, a centrifugal extractor may be used in which the mixing and the separation stages are contained in the same unit which operates as a differential contactor. 

Fig. 1.5 The principle of a mixer-settler unit, e.g., for separation of iron(III) from nickel and cobalt.

The initial bench-scale experimental investigations into solvent extraction processes are conducted with small apparatus, such as separating funnels. Following the successful completion of these tests, when the best reagent and other conditions for the system have been established, small-scale continuous operations are run, such as in a small mixer-settler unit. The data so obtained are used to determine scale-up factors for pilot plant or plant design and operation (see Chapters 7 and 8). 

This process offers a definite advantage in terms of economics, and is highly automated, thus requiring only a few skilled operators. In our LaRochelle plant, we operate more than one thousand extraction stages based on the mixing/settling principle, (Figure 18) A typical mixer settler unit is shown... 

Fig. 7. Flow sheet of combined forward and back extraction processes in continuous mode using two mixer/settler units with reverse micellar phase recirculation. Wl feed (first) aqueous solution, W2 stripping (second) aqueous phase, RM reverse micellar phase. (Reproduced from [139] with permission of Elsevier Science)...

Extraction can be enhanced by the application of a dc or pulsed electric field, typically on the order of 1 kV/cm. This requires that the aqueous phase be dispersed and the organic phase be of low conductivity. The improvement in mass transfer rate is due to the breakup of large drops by the action of the field and to the increase of drop velocity resulting in increased mass transfer coefficients. It has also been found that low-frequency pulsed fields are effective in breaking up emulsions in the settler stage of mixer-settler units. 

The mixer-settler unit shown in Fig. 26a is usually considered to be approximately equivalent to one theoretical stage. Some of the typical characteristics of the mixer-settler are shown in Table XIX. For many applications outlined in Table XVIII, a large number of stages (more than five)... 

To meet the supercritical conditions more suitable a mixer-settler unit has been developed by Schaffner [2], A regenerative pump was used as mixer to achieve an intense contact between the two phases and to create a large mass transfer surface area. This type of pump is capable of conveying gases as well as liquids and creates a high pressure difference which is used to separate the phases in the following fluid cyclone (fig. 1). 

In 1968, an electrolytic reduction process was proposed by A. Schneider and A. L. Ayers (6) to circumvent the above disadvantages. A research program was carried out in the Allied Chemical Corporation s laboratories during the years 1968 to 1972 to develop the process and equipment. The work resulted in the development of the Electropulse Column ( 7) for the continuous (differential) electrolytic uranium-plutonium partition process, which was later scaled up, fabricated, and installed in the Allied-General Nuclear Services reprocessing plant at Barnwell, South Carolina. About the same time, a stagewise electrolytic uranium-plutonium partition process was tested on a mini mixer-settler unit in Germany. (8)... 

Laboratory Scale Contactor Tests. The feasibility of each part of the flow sheet was tested in small, commercially available mixer-settler units which had 12 or 16 stages with individual mixer and settler volumes of 15 mL and 49 mL, respectively. 

Acetic acid is to be extracted from a dilute aqueous solution with isopropyl ether at 298 K in a countercurrent cascade of mixer-settler units. In one of the units, the following conditions apply ... 

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