Countercurrent solvent extraction

We have been introduced to various aspects of solvent extraction in the following sections Section 3.3.7.2, liquid-liquid equilibria in aqueous-organic, organic-organic, aqueous-aqueous systems Section 3.4.1.2, flux expressions in liquid-liquid systems Section 3.4.3.2, solute transport in phase barrier membranes Section 4.1.3, separation achieved in a closed vessel Section 5.2.2, role of chemical reaction in liquid extraction Section 5.3.2, rate controlled aspects of chemical reaction in liquid-liquid systems Section G.3.2.2, bulk flow parallel to force Section 6.4.1.2, mixer-settler, CSTS system. 

We have described very briefly two general types of extractors those requiring mechanical agitation and those without it. For systems where the liquid viscosities or the surface tensions are high, or the liquid-phase density differences are small, or all three conditions are present, it is 

Liquid 2 is introduced through the bore of the porous hollow fibers from one end and flows out to the other side, in effective countercurrent flow to liquid However, locally on both sides of the baffle, liquid ti is in crossflow around each hollow fiber and therefore around liquid 4-Since crossflow introduces very efficient liquid-phase mass transfer at a low Reynolds number in such systems, it is preferable to introduce that liquid phase to the shell side whose resistance is likely to control the mass-transfer/ solvent extraction rate. Further, the liquid phase wetting the pores of the membrane should prefereably be the lower resistance phase. These considerations are described in detail in Prasad and Sirkar (2001) for both porous hydro-phobic as well as porous hydrophilic membranes. 

A most important additional aspect of such devices is that, as long as the phase interfaces are immobilized via appropriate pressure/wetting conditions, one can have a very wide range of flow rate ratios between the two phases. There is no need for any density difference between the phases. The issue of flooding does not arise, emulsification is unlikely to arise, and the need for coalescence is absent However, surfactant impurities, if present, could interfere with interface immobilization. Further, the solvents must not swell the membrane very much. Therefore the compatibility of the membrane with the solvents to be used should be checked. Smaller pore membranes will lead to a broader range of pressure difference between the two phases for nondispersive operation. The value of Kta for such devices can be larger than conventional devices by 5-50 times. 

2 Extraction of a dilute solute between immiscible solvents in a continuous device 

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In the early years of plutonium scrap processing operations, the CAW stream was routed to trenches(1 ) specially excavated in Hanford soil. Batch recovery of americium was started in 1965. Later (1970-1976), a continuous countercurrent solvent extraction process employing DBBP (dibutylbutyl phosphonate) as the extractant was operated to recover, at least partially, plutonium and americium values from the CAW stream. Aqueous waste from the DBBP extraction process, still containing some plutonium and americium, was blended with other Plutonium Reclamation Facility (PRF) wastes, made alkaline, and routed to underground tanks for storage. 

FIGURE 13.13 Countercurrent solvent extraction, using a copper-selective complexer in the organic phase used to raise the concentration and purity of copper ion solutions fed to elect rowinning. 

In the Purex process, irradiated UO2 is dissolved in nitric acid under such conditions that uranium is oxidized to uranyl nitrate and plutonium to Pu(N03)4. The resulting aqueous solution of uranyl, plutonium, and fission-product nitrates is fed to the center of countercurrent solvent extraction contactor I, which may be either a pulse column or a battery of mixer-settlers. This contactor is refluxed at one end by clean solvent and at the other by a dilute nitric acid scrub solution. The solvent extracts all the uranium and plutonium from the aqueous phase and some of the fission products. The fission products are removed from the solvent by the nitric acid scrub solution. Fission products leave contactor I in solution in aqueous nitric acid. 

Figure 4.2 Multistage countercurrent solvent extraction. M, mixer 5, settler.

FIG. A.3. Countercurrent solvent extraction with n extraction and m washing stages. 

Fig. 6.2-5 Scheme and graphical presentation of four stage countercurrent solvent extraction... 

Takahashi, T., Takano, A., Saitoh, T., Nagano, N., 2003. Separation and recovery of rare earth phosphor from phosphor sludge in waste fluorescent lamp by multistage countercurrent solvent extraction. In Reports of the Hokkaido Industrial Research Institute, No. 302, pp. 41-48. 

Three steps required for multistage solvent extraction, i.e., phase mixing, phase settling, and transfer of the mobile phase, are defined clearly in the discontinuous countercurrent distribution process using the Craig apparatus. These basic requirements are essentially fulfilled by the use of a coiled tube in a continuous fashion. Solvent extraction using a coiled column is most efficiently performed with a horizontally laid coil that rotates about its own axis. In this horizontal coil orientation, the rotation induces the well known Archimedean screw force, which can be utilized for performing countercurrent solvent extraction. 

Use of the mixture point M for overall material balance in countercurrent solvent extraction. 

Source Vandegrift, G.F. 2007. Demonstration by countercurrent solvent extraction of the TALSPEAK process for separation of Np, Pu, Am, and Cm from rare earth elements. 31st Annual Separations Conference, June 11-15, Las Vegas, NV. 

Figure 8.1.2. (a) Type (2) systems. Countercurrent flow of a gas stream and an absorbent liquid in a vertical column for absorption of a species from the gas countercurrent separation system without recycle or reflux, (b) Countercurrent solvent extraction column lighter liquid introduced at column bottom by dispersing it as droplets as it rises through a continuous heavier liquid, which flows downward through the column. In aqueous-organic extraction systems, usually the aqueous phase is heavier. 

Almost all countercurrent extraction devices utilize dispersion of one immiscible phase as drops in another immiscible phase we will provide a brief introduction here. At the end, we will introduce porous hollow fiber membrane based nondispersive countercurrent solvent extraction devices. The dispersive devices may involve continuous agitation or no agitation at all. Dispersive devices without any agitation as such are of three types spray towers packed towers perforated plate towers. Spray towers were illustrated in Figure 8.1.2(b). 

Figure 8.1.36. Countercurrent solvent extraction in a column with continuous contact...

Example 8.1.16 Obtain an analytical expression for the number of equilibrium stages N required in a countercurrent solvent extraction cascade in terms of the extraction factor E and the fractional solute recovery (l - (M(Ki/Mifi(w+i)))-Assume that the extracting solvent has zero solute concentration, the equilibrium distribution is linear and the two phases are immiscible. 

The previous sections considered countercurrent solvent extraction primarily in the context of aqueous-otganic systems. Section 4.1.3 illustrated solvent extraction in polar organic-organic systems (see Figure 4.1.9(a)) and aqueous two-phase systems (Figure 4.1.9(h)) as well. An additional two-phase system for solvent extraction of proteins is the reverse micelle-aqueous phase system shown in Figures 4.1.19. 1.21. 

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