Practical Isotope Separation Some Examples

The selection of an isotope separation process including the design of the separative units and the cascade depends principally on engineering and economic considerations. One must consider the amount of product which is desired, choose the starting material, consider energy demand, etc. It comes as no surprise, then, that many different methods have been used for isotope separation. Some of these have been listed in Table 8.1 and a few are discussed in more detail in the material which follows. 

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We shall not be much concerned with differences in physical properties, as these differences are generally too small to be of much chemical consequence except for very light elements. Deu-terium oxide and ordinary water, for example, differ sufficiently in their solvent properties to affect noticeably rates and equilibria of reactions in aqueous solution. Even the small differences between isotopes of heavy elements are not devoid of all practical consequence, as witness the separation of uranium-235 from uranium-238 by diffusion processes. Indeed, some quite remarkable feats have been performed in the design of apparatus for separation of isotopes. This work, however, is more closely related to engineering than to chemistry and will not be considered further. 

Chemical reactions occur in many commonly practiced separation processes. By chemical reactions, we mean those molecular interactions in which a new species results (Prausnitz et al, 1986). In a few processes, there will he hardly any separation without a chemical reaction (e.g. isotope exchange processes). In some other processes, chemical reactions enhance the extent of separation considerably (e.g. scrubbing of acid gases with alkaline absorbent solutions, solvent extraction with complexing agents). In still others, chemical reactions happen whether intended or unintended estimation of the extent of separation requires consideration of the reaction. For example, in solvent extraction of organic acids, the extent of acid dissociation in the aqueous phase at a given pH should be taken into account (Treybal, 1963, pp. 38-41). Chemical equilibrium has a secondary role here, yet sometimes it is crucial to separation. 

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