Mixers-settlers purposes

Each cell in the extraction system presented in Fig. 122 is called a mixer-settler extractor and is made up of two parts. The role of the first part, the mixer, is to emulsify the incoming aqueous and organic phases and to transfer the emulsion to the second part of the extractor-settler cell. The purpose of the settler is to stratify the phases and enable the separation of the two liquids. 

In the early 1970s Li [13] proposed a method that is now called Emulsion (surfactant) Liquid Membrane (ELM) or Double Emulsion Membrane (DEM) (Fig. 3). The name reveals that the three liquid system is stabilized by an emulsifier, the amount of which reaches as much as 5 % or more with respect to the membrane liquid. The receiving phase R, which usually has a smaller volume than the donor solution, F of similar nature, is finally dispersed in the intermediate phase, M. In the next step the donor solution F is contacted with the emulsion. For this purpose, the emulsion is dispersed in the donor solution F by gentle mixing typically in a mixer-settler device. After this step, the emulsion is separated and broken. The enriched acceptor solution is further processed and the membrane liquid M is fed back for reuse. 

Mixer-settler assemblies for extraction purposes often are preferable to differential contact towers in order to obtain very high extraction yields or to handle large flow rates or when phase... 

From this knowledge, the idea of using a mixer-settler system for the extraction with supercritical fluids was derived. The purpose of this project is to build a multistage high pressure apparatus and to compare the separation efficiency and behaviour of the new system with that for a conventional column. 

For the purpose of improving the decontamination factor (DF) of FPs from U or Pu in the reprocessing of highly irradiated fuels such as those from FBR, a modified method adding inactive zirconium or hafnium ion is proposed. The feasibility of this concept has been experimentally demonstrated by both batchwise extraction and process studies with miniature mixer-settlers. 

The main purpose of the separation of Am-Cm from RE by the well known Talspeak process is to liberate the RE fraction from actinides. The acceptable resi-duel RE content in the actinide stream on the contrary will depend widely on the further destination of the latter. It is in any way not a critical point. Therefore, high A/O ratios and an efficient aqueous scrub section must provide for an as low as possible extraction of Am and Cm. For these operations mixer-settler batteries seem to offer the most convenient approach, taking into account radiation levels, flow rates and extraction (and back-extraction) kinetics. 

They do require extensive floor space, however, and this can be a disadvantage if many stages are needed. Also, mixer-settlers generally have Iaige residence times and therefore require considerable time to achieve steady state and large liquid-liquid inventories, Another di rad vantage of mixer-settlers is that most commercial units are difficult to seal, If the main purpose of the extraction step is to dehydrate an organic material, this feature can preclnde their use. 

The Purex process is presented schematically in Figure 21.11, where the solvit extraction steps are within the dotted frame. Three purification cycles for both uranium and plutonium are shown. High levels of beta and gamma activity is presort only in the first cycle, in which > 99 % of the fission products are separated. The principle of the first cycle is shown in Fig. 21.13. The two other cycles are based upon the same chemical reactions as in the first cycle the purpose is to obtain additional decontamination and overall purity of the uranium and plutonium products. Each square in Figure 21.13 indicates a number of solvent extraction stages of the particular equipment used pulsed columns, mixer-settlers, etc. (see Appoidix A). 

If a stirred tank is used for dispersion purposes, it is followed by a gravity settler (mixer-settler process) to separate the globules from the continuous phase. In a column, phase separation occurs at column top. 

The purpose of the mixer tFigure 13-311 is to disperse one of the two phases (raffinate = feed and extract = solvent) in the other phase to provide a large area for mass transfer. Most of the mass transfer occurs in the mixer. The settler s job is to undo the dispersion done in the mixer and produce two clear phases. To some extent, these two operations have opposite requirements. Mass transfer is more rapid if there are a large number of small droplets, while settling is much faster with large droplets. Thus, the design is a conpromise that balances the needs of the mixer and the settler. The best way to do the design of both... 

---