Mixer settler phase separation

In a Scheibel Column phase mixing [6.60] is caused by blade mixers or blade [6.61] mixers with mounted baffles. Wire-mesh packing or filling material of one to three times the height of the mixing zone acts as the settling zone. Operation mode is like the mixer-settler principle. Separation efficiency ca. 3-5 theoretical stages/m. 

A widely used type of pump—mixer—settler, developed by IsraeH Mining Industries (IMI) (115), is shown in Figure 13a. A unit having capacity 8.3 m /min (2000 gal /min) has been used in phosphoric acid plants (116). The unique feature of this design is that the pumping device is not required to act as the mixer, and the two phases are dispersed by a separate impeller mounted on a shaft miming coaxially with the drive to the pump. 

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. 

The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

Small number Chemically stable Easy phase separation Low value A, B or mixer-settler E or F... 

In one mixer-settler design, the mixed phases flow down a shallow trough placed over the settler, which gives them an opportunity to coalesce and separate before entering the settler. In this way, the capacity of the settler is markedly increased, with a concomitant reduction in the inventory of solvent required for a given duty. 

The previous chapters have demonstrated that liquid-liquid extraction is a mass transfer unit operation involving two liquid phases, the raffinate and the extract phase, which have very small mutual solubihty. Let us assume that the raffinate phase is wastewater from a coke plant polluted with phenol. To separate the phenol from the water, there must be close contact with the extract phase, toluene in this case. Water and toluene are not mutually soluble, but toluene is a better solvent for phenol and can extract it from water. Thus, toluene and phenol together are the extract phase. If the solvent reacts with the extracted substance during the extraction, the whole process is called reactive extraction. The reaction is usually used to alter the properties of inorganic cations and anions so they can be extracted from an aqueous solution into the nonpolar organic phase. The mechanisms for these reactions involve ion pah-formation, solvation of an ionic compound, or formation of covalent metal-extractant complexes (see Chapters 3 and 4). Often formation of these new species is a slow process and, in many cases, it is not possible to use columns for this type of extraction mixer-settlers are used instead (Chapter 8). 

In this process developed by Lurgi [17], the phenolic effluent is contacted with the solvent in a multistage mixer-settler countercurrent extractor (Fig. 10.8). The extract, containing phenol, is separated into phenol and solvent by distillation and solvent is recycled to the extractor. The aqueous raffinate phase is stripped from solvent with gas, and the solvent is recovered from the stripping gas by washing with crude phenol and passed to the extract distillation column. 

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. 

The original and in concept the simplest way of accomplishing extractions is to mix the two phases thoroughly in one vessel and then to allow the phases to separate in another vessel. A series of such operations performed with series or countercurrent flows of the phases can accomplish any desired degree of separation. Mixer-settlers have several advantages and disadvantages, for instance ... 

Vessels for the separation of two immiscible liquids usually are made horizontal and operate full, although some low rate operations are handled conveniently in vertical vessels with an overflow weir for the lighter phase. The latter mode also is used for particularly large flows at near atmospheric pressures, as in the mixer-settler equipment of Figure 3.19. With the usual L/D ratio of three or more, the travel distance of droplets to the separated phase is appreciably shorter in horizontal vessels. 

The mixer and settler can be combined in one vessel or separated. In the combined case, the agitator is turned off after extraction so that the two phases can separate. For continuous operation, the mixer and settler are separated. A settler is nothing but a large tank. 

Ban et al. reviewed the reduction properties of several salt-free reagents for Np(VI) and Pu(IV) to choose selective reductants that reduce only Np(VI) to Np(V) for separating Np from U and Pu in TBP by reductive back-extraction (165). Allylhydrazine was proposed as a candidate for selective Np(VI) reduction, and it was confirmed by a batch experiment that allylhydrazine reduced almost all Np(VI) to Np(V) and back-extracted Np from the organic phase (30% TBP/n-dodecane) to the aqueous phase (3 M HN03) within 10 minutes. A continuous countercurrent experiment using a miniature mixer-settler was carried out with allylhydrazine at room temperature. At least 91% of Np(VI) fed to the mixer-settler was selectively reduced to Np(V) and separated from U and Pu. 

---