Solvent extraction emulsions

Solvent Extraction. Solvent extraction has widespread appHcation for uranium recovery from ores. In contrast to ion exchange, which is a batch process, solvent extraction can be operated in a continuous countercurrent-fiow manner. However, solvent extraction has a large disadvantage, owing to incomplete phase separation because of solubihty and the formation of emulsions. These effects, as well as solvent losses, result in financial losses and a potential pollution problem inherent in the disposal of spent leach solutions. For leach solutions with a concentration greater than 1 g U/L, solvent extraction is preferred. For low grade solutions with <1 g U/L and carbonate leach solutions, ion exchange is preferred (23). Solvent extraction has not proven economically useful for carbonate solutions. 

Lippi et. al (87) and Dirstine (88) circumvented titration by converting the liberated fatty acids into copper salts, which after extraction in chloroform are reacted with diethyldithio-carbamate to form a colored complex which is measured photometrically. While the end point appears to be more sensitive than the pH end point determination, the advantages are outweighed by the additional steps of solvent extraction, centrifugation and incomplete extraction when low concentrations of copper salts are present. Other substrates used for the measurement of lipase activity have been tributyrin ( ), phenyl laurate (90), p-nit ro-pheny1-stearate and 3-naphthyl laurate (91). It has been shown that these substrates are hydrolyzed by esterases and thus lack specificity for lipase. Studies on patients with pancreatitis indicate olive oil emulsion is definitely superior to water soluble esters as substrates for measuring serum lipase activity. 

Solvent extraction carried out in conventional contactors like mixer-settlers and columns has certain limitations, including (a) controlling optimum dispersion and coalescence, (b) purifying both phases to ensure that stable emulsions are avoided (c) temperature control within a narrow band (d) high entrained solvent losses and related environmental and process economic effects and (e) large equipment dimensions and energy requirements when the density differential or selectivity is low. 

The solvent/water emulsion is heated in its subsystem and flows through a Micro Motion mass flow meter. The emulsion is fed to the POD, where the water and organic phases are separated. Through this contact and separation the impurities are extracted into the aqueous phase. This results in a relatively clean solvent. 

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As noted earlier (see section 12.3.1), the Amex process, which uses long-chain amines, is preferred over the Dapex process, which uses HDEHP, for solvent extraction of uranium from H2SO4 leach solutions. Because the surfactant properties of amine sulfates are conducive to formation of objectionable emulsions, the Amex process is very sensitive to the presence of solids in the H2SO4 leachate. For acceptable phase coalescence in the Amex process, feeds should contain no more than 20 ppm solids. The Dapex process can tolerate feeds containing as much as 100 ppm solids. 

All the novel separation techniques discussed in this chapter offer some advantages over conventional solvent extraction for particular types of feed, such as dilute solutions and the separation of biomolecules. Some of them, such as the emulsion liquid membrane and nondispersive solvent extraction, have been investigated at pilot plant scale and have shown good potential for industrial application. However, despite their advantages, many industries are slow to take up novel approaches to solvent extraction unless substantial economic advantages can be gained. Nevertheless, in the future it is probable that some of these techniques will be taken up at full scale in industry. 

The basic process involves three steps a pretreatment solvent application, actual solvent extraction, and a final drying phase. Pretreatment is done to break the emulsions. The drying phase involves steam heating to recover residual solvent. In some cases a biofilter is used to further treat liquid effluents if present. 

The reaction in a homogeneous solution with a polar organic solvent in which the enzymes and substrates are both soluble, occurs often at the expense of the enzyme stability [4, 5]. Besides immobilised enzymes in organic solvents [6], emulsion reactors, especially enzyme-membrane-reactors coupled with a product separation by membrane based extractive processes [7-9] and two-phase membrane reactors [10-12], are already established on a production scale. 

Liquid-liquid partitioning constitutes tlie most common form of solvent extraction. It is a relatively simple, rapid, and flexible procedure that is readily applicable to all types of matrices and a wide range of analytes ranging from fairly polar to nonpolar compounds (54). Despite the fact that they are simple and rapid, liquid-liquid extractions may result in highly selective isolation (55, 56). However, they also necessitate use of toxic and inflammable solvents, favor formation of emulsions, may cause sample losses by occlusions or adsorption onto glass surfaces, and are often laborious and costly. 

In solvent extraction, this same contact is achieved either slowly over extended time periods of mild shaking or more rapidly through generation of a forced emulsion by violent agitation that consumes large amounts of energy. 

EMULSION Free. The solvent extraction of many water solutions, particularly waste waters and biofluids, is a prolonged and often frustrating procedure because of the formation of emulsions. This problem is obviated when solid adsorbents are used because phase separation is inherent to the procedure. 

While benzene is the most satisfactory solvent for this extraction, emulsions are sometimes produced when it is used for the extraction of other mercaptans. To obviate this difficulty the same amount of ether may be used, provided that the alcohol is first removed by distillation on a steam bath. 

Occasionally emulsions are formed in the extraction of aqueous solution by organic solvents, thus rendering a clean separation impossible. Emulsion formation is particularly liable to occur when the aqueous solution is alkaline, and when dichloromethane is the extracting solvent. The emulsion may be broken by any of the following devices, but in general its occurrence may be minimised... 

Birdwell, Jr., J. F., K. K. Anderson, and D. E. Hobson. 2002. Investigation of Emulsion Formation in Solvent Washing in the Caustic-side Solvent Extraction (CSSX) Process. Oak Ridge National Laboratory Report ORNL/TM-2002/126, Oak Ridge, TN. 

Berrama T, Pareau D, Stambouli M, Durand G. Purification and concentration of lactic acid by emulsion liquid membrane extraction. In Cox, Hidalgo M, Valiente M, eds. Solvent Extraction for the 21st Century, Proceedings of ISEC 99, Barcelona. London Society of Chemical Industry, 2001 983-987. 

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