Solvent extraction cascade

Figure 4.9 Stage concentration diagram for solvent extraction cascade.

In a simple solvent extraction cascade without a scrubbing section, a feed stream containing an extractable component at concentration is contacted countercurrently by an organic stream containing a concentration yo of extractable component. The distribution coefficient D and extraction factor /3 are constant. 

The conventional separation scheme is to leach the primary ore or concentrates and use the resulting solution containing the rare earth mixtures as the feedstock to the solvent extraction plant. Solvent extraction of the rare earth mixture in the leached solution separates them into bulk concentrates of light (La, Ce, Pr, Nd, etc.), middle (Sm, Eu, Gd, etc.) and heavy (Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) rare earths. A typical solvent extraction of rare earths in a HCl medium is with di-2-ethylhexyl phosphoric acid, HDEHP, in a kerosene diluent. The individual rare earth is separated from the bulk light, middle, and heavy rare earth solution mixtures by additional individual rare earth solvent extraction streams. The number of stages for solvent extraction cascades or batteries increases with the increase in purity of each individual rare earth produced. Further purification... 

Figure 8.1.37. (a) Continuous countercurrent multistage solvent extraction cascade of N stages (b) graphical determination of stage numbers in such a cascade of equilibrium extraction stages. 

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. 

Dual solvent fractional extraction (Fig. 7b) makes use of the selectivity of two solvents (A and B) with respect to consolute components C and D, as defined in equation 7. The two solvents enter the extractor at opposite ends of the cascade and the two consolute components enter at some point within the cascade. Solvent recovery is usually an important feature of dual solvent fractional extraction and provision may also be made for reflux of part of the product streams containing C or D. Simplified graphical and analytical procedures for calculation of stages for dual solvent extraction are available (5) for the cases where is constant and the two solvents A and B are not significantly miscible. In general, the accurate calculation of stages is time-consuming (28) but a computer technique has been developed (56). 

The graphical construction of an extraction isotherm, an operating line, and the stepwise evaluation of the number of stages in this manner is known as a McCabe-Thiele diagram. Flistorically, it found great application in a variety of mass transfer operations, from gas adsorption through distillation to solvent extraction. Flowever, the advent of modern computational techniques has made it largely redundant, as it is often easier and certainly more accurate to calculate the cascade directly. 

Plutonium purification proceeds by reducing the aqueous phase pH that oxidizes the plutonium to Pu" +, which then extracts into the TBP phase. Impurities stay in the aqueous phase. The TBP phase strip-ping/extraction cycle is repeated to complete the plutonium purification. The uranium is purified using the same TBP/nitric acid extraction/stripping cycle. Careful control of the each element s oxidation state in the extraction cascade produces the plant-scale separations of uranium from plutonium of 10 . Fission product decontamination factor was 10. The plutonium and uranium recovery is about 99.9% with 95% of the nitric acid values and 99.7 /o of the organic solvent recycled. ... 

Modify the above program to model the case of a discontinuous five-stage extraction cascade, in which a continuous flow of aqueous phase L is passed through the cascade. The solvent is continuously recycled through the cascade and also through a solvent holding tank, of volume Vs, as shown below. This problem of a stagewise discontinuous extraction process has been solved analytically by Lelli (1966). 

The calculation of the concentration of extractable components in a countercurrent cascade of equilibrium solvent extraction stages is first developed for the simple countercurrent extraction section of Fig. 4.3. The theory is then extended to the extracting-scrubbing system of Fig. 4.4 for fractional extraction and is illustrated by a numerical calculation for the separation of zirconium from hafnium, using TBP in kerosene as solvent. 

Figure 4.8 Nomenclature for cascade of solvent extraction stages.

The stage holdup time h and separation factor a of a solvent extraction column for uranium enrichment are = 10 s, a = 1.0010. What is the minimum equilibrium time of an ideal cascade fed with natural uranium, stripping to 0.2 w/o and enriching to 3 w/o product Repeat for 90 w/o product. 

Extract flow in cascade V Raffinate flow in cascade L Extract product D Raffinate product B Solvent separator Raffinate solvent stripper Extract solvent stripper Solvent to cascade s ... 

---