Pulsed mixer-settler

Stagewise contact with controlled coalescence redispersion cycles Tray column Pulsed sieve column. Pulsed Mixer-Settler-cascade, Extraction tower with controlled cycle Scheibel column, ARD-Extractor, Leisibach column, Mixer-Settler cascade ... 

In outline, reprocessing was based on disassembly of the fuel assemblies to individual pins, single pin cropping, dissolution in nitric acid and operation of a Purex-type flowsheet in a chemical separation plant equipped with pulsed mixer settlers. The modified dissolver cell and solvent extraction plant were recommissioned late in 1979 and were tested in reprocessing of the final batches of DFR fuel. The new head-end facility, to disassemble the PFR fuel assemblies and to crop the pins, and the new waste treatment facilities were commissioned in 1980 reprocessing of the first batches of PFR fuel began later that year. 

Special safety constraints apply to equipment selection, design, and operation in nuclear reprocessing (269). Equipment should be reHable and capable of remote control and operation for long periods with minimal maintenance. Pulsed columns and remotely operated mixer—settlers are commonly used (270). The control of criticaHty and extensive monitoring of contamination levels must be included in the process design. 

FIG. 23-38 Efficiency and capacity range of small-diameter extractors, 50 to 150 mm diameter. Acetone extracted from water with toluene as the disperse phase, V /V = 1.5. Code AC = agitated cell PPC = pulsed packed column PST = pulsed sieve tray RDC = rotating disk contactor PC = packed column MS = mixer-settler ST = sieve tray. (Stichlmair, Chem. Ing. Tech. 52(3), 253-255 [1980]). 

The need to use multiple extraction to achieve efficient extraction required the development of new types of continuously working extractors, especially mixer-settlers and pulsed columns, which were suitable for remotely controlled operations. These new extractors could be built for continuous flow and in multiple stages, allowing very efficient isolation of substances in high yield. A good example is the production of rare earth elements in >99.999% purity in ton amounts by mixer-settler batteries containing hundreds of stages. These topics will be further developed in Chapters 6 and 7. 

In the example shown in Fig. 7.6, using an alkylphosphoric acid, about 60 stages are required to give a Co/Ni ratio of about 100. So many stages would be too many for mixer-settler operation, and other types of contactors would have to be considered. In this particular example, a sieve plate pulsed column has been shown to be very effective [3]. Flowever, with the development of the alkylphosphonic and alkylphosphinic acids, the separation of cobalt and nickel can be achieved in very few stages, owing to the high rejection of nickel (see Chapter 11). 

Irradiated UO2 is dissolved in nitric acid, resulting in a dissolver solution with the approximate composition listed in Table 12.7. This is treated by the Purex process. The main steps in the conventional Purex process are shown schematically in Fig. 12.5. All existing plants listed in Table 12.8 use some variation of the Purex process. Typically, the extractant composition (percentage TBP, diluent) and the extraction equipment (i.e., pulse columns, mixer-settlers, etc.), vary from plant to plant. However, the upper concentration limit is 30% TBP to prevent a phase reversal due to the increased density of the fully loaded solvent phase. 

The uranium and plutonium are recovered for further use by first dissolving the spent fuel in nitric acid and subjecting the resulting solution to a solvent extraction process. Several different processes exist, the best known being the Purex process (Fig. 18), in which tributyl phosphate (TBP) (30% solution in kerosene) is the extractant. Extraction is carried out in compact mixer-settlers or air-pulsed columns fabricated of stainless steel, with about 99.9% removal of uranium and plutonium in the extract. 

Nickel and cobalt often occur with copper, and must be separated in pure form from hydrometallurgical leach liquors. Organic acid extractants can quite readily separate copper from cobalt and nickel, but the separation of cobalt from nickel is rather difficult. In one Ni/Co separation process, di-2-ethyl hexyl phosphoric acid (D2EHPA) is used as extractant, with strict control of the pH of the aqueous phase to take full advantage of the slightly different equilibrium constants for the Co and Ni reactions. Pulsed column contactors are used rather than mixer-settlers, and nickel impurity is removed from the loaded organic phase by scrubbing it with a cobalt-rich phase. 

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. 

Fig. 26. Mechanically agitated industrial contactors, (a) mixer-settler (b) rotating-disk column (c) mixco column (d) asymmetric rotating-disk column (e) pulsed packed column (f) Podbielniak centrifugal extractor. (Reprinted from Doraiswamy, L. K and Sharma, M. M., Heterogeneous Reactions Analysis, Examples and Reactor Design, Vols. I and 2, 1984, John Wiley and Sons.)...

Code AC (Agitated CeU) - PTC (Pulsed Packed Cdumn) - PST (Pulsed Sieve Tray) - RDC (Rotating-Disc Contactor - PC (Pecked Column) - MS (Mixer Settler) - ST (Sieve Tray)... 

Extraction equipment, 476-490 centrifugal, 484,487,490 comparison of types, 477 mixer-settlers, 477, 479 packed towers, 478,480,482,485 87 performance comparison, 478 pulsed towers, 481,483,487 REX2 (rotating disk contactor), 482, 487... 

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