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Mixing Guide >> Liquid - Liquid >> Extraction |
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Extraction
The main advantage of using mixing or some type of mechanical energy, compared to packed plate or spray columns, is the ability to get a smaller volume for the same degree of extraction. However, if an attempt is made to use too much energy, then problems of settling characteristics are encountered, and this negates the advantages of the mixed system many times.
In the mining industry, it is quite typical to use mixer settlers. These usually involve an extraction step, a scrubbing step, and then a stripping step. Usually the requirement is for only one or two stages in each of these are with the use of very selective ion exchange chemicals in the system. To eliminate interstage pumps a pump-mixer is used in which some of the head component of the impeller is converted to a static head so that fluids can be pumped against small static heads in the mixers and settlers of the whole train. This has worked well in many applications, although there is a potential problem that the conditions required for effective pumping are not optimum for the mixing that is required in the mixing stage, and there may be some design parameters that are difficult to satisfy in the systems.
 
The other area is the countercurrent liquid- liquid extraction column.Using mixer stages separated by stationary horizontal discs. These have the advantage of only one interface for settling to occur, plus the fact that solids can be handled in one or both phases. Also, all the principals of fluid mixing can be used to design an effective transfer system.
One of the key variables to be studied in the pilot plant is the effect of turndown ratio, which is the ratio of flow to the design flow through the column, so that predictions can be made of performance during reduced throughput during certain parts of the plant processing startup.
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