Chemical reflux isotope exchange

Because of the very large enrichments required in heavy water production, cascades taper markedly. In the upper stages the relative advantage of chemical exchange over water distillation vanishes. Most heavy water plants carry out the last portion of the enrichment by distillation (from 20% or 30% D to 99.85%). Accordingly both exchange and distillation will be briefly treated below. First, however, to clarify the important distinction between chemical and thermal reflux we treat an example of isotope separation using chemical reflux. 

An Aside Monothermal Isotope Exchange with Chemical Reflux 15 N Enrichment... 

To avoid the high cost of chemical reflux the dual temperature H2S/H20 exchange was independently suggested by Geib (1946) and Spevack (1957) (GS). The method exploits the fact that the equilibrium constant for isotope exchange is temperature dependent. The scheme is illustrated in Fig. 8.13. To carry out the exchange... 

Chemical reflux of a chemical exchange reaction accomplishes reflux by chemical inter-conversion of the two species. The conversion process supplies a countercurrent stream of enriched or depleted isotope of the appropriate isotopic composition. The use of a hot tower leads to a back transfer of enriched isotope from the enriching phase to the phase being depleted in the cold tower. The hot tower requires a number of plates comparable with that in the cold tower. The effective separation factor is, therefore. 

The domain of the search for an improved separation process was defined by certain criteria (a) isotopic fractionation should be achieved by means of a two-phase, chemical exchange reaction which was amenable to countercurrent operation in a multistage contactor at ambient temperature and pressure (b) the single-stage isotopic fractionation factor for the reaction should be appreciably larger than that for the distillation of Me20 BF3 (c) the molecular species in each process stream should be thermally refluxable—i,e, convertible from one species to the other by the addition or removal of heat alone (d) process materials should be more stable with respect to irreversible decomposition than those used in the (CH3)20 process and (e) the chemical form of the product should permit a ready, quantitative conversion of the separated isotopes to the elemental state. 

In chemical exchange systems, reflux consists of converting the chemical forms of the isotopic species from that of one reactant to that of the other. In the systems under consideration in this paper, these reactions are ... 

As another example, the lithium isotope Li (used as LiOH for pH control in some nuclear power plants because of its small neutron cross-section) is produced in 99.99% purity by countercurrent chemical exchange of lithium between an aqueous solution of LiOH and lithium amalgam. A separation factor of 1.06 to 1.07 is reported. Reflux of lithium is obtained at one end by electrolytic reduction of LiOH to Li(Hg) at a mercury cathode and at the other end by spontaneous oxidation of Li(Hg) on graphite by water producing hydrogen gas and LiOH. 

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