In-line mixer-settler

Fig. 4.18. In-line mixer-settler. (Lowes, L. and Tanner, M. C. U.K. Patent 835,282. By permission of the Controller of H.M. Stationery Office.)...

Tanner, M. C. In-line Mixer-Settler having Double Mixed Phase Ports. U.K. Patent Specification 835,282 (application 1957). 

Density. The difference in density between the two hquid phases in eqiiilibrium affects the countercurrent flow rates that can be achieved in extrac tion equipment as well as the coalescence rates. The density difference decreases to zero at a plait point, but in some systems it can become zero at an intermediate solute concentration (isopycnic, or twin-density tie line) and can invert the phases at higher concentrations. Differential types of extractors cannot cross such a solute concentration, but mixer-settlers can. 

The mixer-settler used at the Vitro uranium recovery operation near Salt Lake City, Utah, is shown schematically in Fig. 4.25. The contactor is a rubber-lined tank 6.1 m in diameter and... 

Materials of construction. It is necessary to use materials of construction for this separation that resist corrosion at the high concentrations of HCl used in the process. The Bureau of Mines plant [Ml] used glass columns, ass- or rubber-lined equipment, and rubber tubing connectors. The British plant [Jl] used polythene mixer-settlers. 

These are the devices in which fine dispersion occur because of high flow rates or static internals. Three types of mixers are available mixer-settler [4, 5], cyclone [6], and interdigital mixer [3] (see Figure 7.2). The principle of fluid contacting in micromixer-settler is almost similar to the conventional mixer-settler assembly. However, because of the reduced size of the equipment, the moving part of the mixer is replaced by the static. Two-fluid streams are introduced from the top of mixer and biphasic mixture is taken out from the top central line, which is further introduced to the mini-settler where the phases disengage based on their density... 

Hydrogen Sulfide. Extraction of the petroleum fraction with an aqueous solution of NaOH (5 to 15 per cent concentration) is usually used to remove H2S, frequently prior to other sulfur-removal operations. This will remove thiophenols as well. A line mixer (Chap. 9) followed by a settler in a one-stage contact, with recirculation of the caustic solution until its ability to remove H2S is spent, is usually the practice. The fouled extract, containing NaHS and Na2S, cannot conveniently be regenerated by heating, since the acidity of H2S increases with increased temperature and the caustic thus retains the H2S more firmly. Oxidation by aeration is an unsatisfactory alternative, and usually the fouled solution is discarded. 

In general, satisfactory separation is obtained from iron, manganese, titanium and tin impurities in the TBP processes. Mixer-settler plant can be used, but is best constructed of, or lined with, polythene, because of the high concentrations of hydrofluoric acid which are necessary. 

Columns are useful for processing low flow rates and for systems that exhibit a tendency to form emulsions. An important benefit of a column contactor is the large number of possible theoretical stages and the ability to operate closer to the operating line rather than the equilibrium curve, thereby maximising mass-transfer kinetics. The settling volume is considerably lower than for the corresponding mixer-settler, so columns are preferred for systems in which solution lock-up and low solvent inventories are important (such as in precious metal extraction systems). Columns take up very little floor space, but require considerable headroom mixer-settler requirements are the opposite (Movsowitz et al. 2001 Fox et al. 1998). 

Treybtd Extractor [73] This is in reality a vertical stack of mixer-settlers. The mixers are in a vertical line, and the impellers for the stages on a common shaft. They not only mix but also pump, so that throu put rates are high. Since there is no axial mixing, mass-transfer rates are high. 

Figure 6.4.8. (a) Mixer - a well-mixed vessel with one phase dispersed in the other, (b) Settler-the two phases have settled into two separate layers, (c) Equilibrium diagram, operating line and stage efficiencies in solvent extraction, (d) Well-mixed continuous mixer-settler. 

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