Scrubbing the extract

W scrubs the extract free from the unwanted second solute. The second solute leaves the contac tor in the raffinate stream. 

Where a high-purity product is sought, the same philosophy of saturating the aqueous phase with the unwanted component is used in scrubbing the extract. A recycle stream of the pure product is used to scrub the impure extract. In the process, the extract is saturated with the desired product, and the impurities are removed. The final raffinate from scrubbing is then recycled to the aqueous feed so that the now-contaminated desired product can be recovered. This is shown in Fig. 8.1. 

As no solvent is perfectly selective for a given solute, the extract often contains higher concentrations of impurities that should be removed. This is done by "scrubbing" the extract in a second stage, usually involving the original solvent under such conditions that the desired solute is largely retained in the extract while the impurities are washed out. 

The residue may have a smell that is considered impleasant both within the site and in the neighbourhood. Transfer by vacuum from the still kettle to residue tank and scrubbing the extracted air in a liquid ring pump can be a solution to this problem. In this case the residue tank must be able to withstand full vacuum and should be a horizontal cylinder to reduce the static head involved. Normally the residue tank should be constructed of the same material as the kettle. 

Catacarb process An extraction process used to remove carbon dioxide from process gases by scrubbing the hot gases with potassium carbonate solution containing additives which increase the hydration rate of the gas in the solution. The Vetrocoke process is similar. See Benfield process. 

In both cases, the extraction process includes washing (scrubbing) of the extract, a stage that is not shown separately. The organic phase that results from the stripping process is returned to the beginning of the extraction process for reuse as an extractant. 

The composition of solutions that are used for washing and stripping of extracts is also different and is adjusted for each specific case however, some common conditions can be noted. It is recommended that the extract obtained following collective extraction, which contains both tantalum and niobium, be washed (scrubbed) using sulfuric acid solutions that contain at least 6 mol per liter of H2SO4. Collective stripping of the elements, on the other hand, can be... 

Scrubbing the loaded organic phase with water or ammonia removes HC1, transferring the copper to the pH-swing extractant, e.g., as in,... 

The nomenclature used in solvent extraction has been defined in Chapter 1 and is illustrated in Fig. 8.1. Not all of the steps shown in this figure will be found in every extraction process, but equally there may be occasions where it is necessary to add additional steps for example, to recover the extractant from the scrub raffinate. So while Fig. 8.1 is not a completely general flow diagram it covers most of the processes likely to be found in practice. Variations of this flow sheet will become apparent during the remaining chapters. 

The cascade is fed with an extract of the same composition as that resulting from the extraction shown in Table 8.5, with = 0.332 and Eg = 0.058. In four stages, using a concentrated solution of at 0 = 10, Eji = 0.350 and Eb = 0.00014, the purity of the product is increased from 85-99.96%. Note that the composition of the scrub solution is the same as... 

Techniques for achieving high-purity products by countercurrent extraction and scrubbing of the extract have proved essential for the production of nuclear-grade uranium. They have also found application in the separation of the rare earths and a number of other difficult separations. A feature of the operation of these systems is the need for close control of fiow rates and even temperature in order to achieve a consistent product quality. The product quality is a very nonlinear function of the operating parameters. However, with modern control systems this disadvantage can be overcome. 

The purity of the product is determined not only by the inherent selectivity of the solvent system for the component sought, but also on the phase ratio, the concentration of the contaminants, and the performance of any scrubbing of the extract. 

This led to the discussion of scrubbing as an essential adjunct to countercurrent extraction where purity was important, and it was shown that washing the extract with a small amount of aqueous phase could improve purity markedly. Stripping was shown to follow the same underlying principles as extraction for achieving efficient removal of extracted species, and the need to choose phase ratios carefully to maximize the concentration of the desired species in the strip solution was stressed. 

The Flow Sheet - Extraction and Scrubbing. The amount of solute that can be recovered from a particular feed solution by equilibration with a solvent depends on both the distribution coefficient and the volumetric ratio of extract to raffinate phase. 

Water vapor from the evaporator passes to a water-cooled condenser, and the weakly acidic condensate is discharged as liquid effluent. The extractive distillation section of the plant is maintained under slight negative pressure, and the condenser off-gases are scrubbed with water and discharged as gaseous effluent. 

Concentration/purification by solvent extraction usually involves four steps (a) extraction of uranium from the leach liquor in a solvent, (b) scrubbing to remove impurities from the solvent, (c) stripping to remove uranium from the solvent, and (d) regeneration of the solvent. The solvent phase in solvent extraction will contain the extractant that complexes uranium to make it soluble in the organic phase, a diluent, an inexpensive material to dilute the extractant, and a modifier to improve the solubility of the extractant in the diluent. Typical extractants are amines with isode-canol acting as a modifier to improve the amine solubility in a diluent such as kerosene. The typical chemistry of the extraction would involve the reactions... 

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