Salt-Partitioning Processes

SALT-PARTITIONING PROCESSES A. Water-Immiscible Solvents 

If an intermediate situation prevails, which is typical for dielectric constants between approximately 5 and 20, Eqs. (33) and (34) both apply. The solvent-phase ion-pair association reaction may then be obtained by subtraction of Eq. (33) and (34) to give  

Of course, other equilibria may occur in a given system of interest, requiring a more complex treatment, and it is worthwhile at least to mention some of the possibilities. Beyond simple ion pairs, species formed by aggregation of ions and ion pairs may exist, especially when ion solvation is weak, the solvent-phase salt concentration is high, and the anion has amphiphilic properties. For example, ion pairs may aggregate  

As for cation selectivity, Eq. (33) suggests that the nature of the anion should have no effect when the extracted salt is completely dissociated in the solvent phase, since the environment around the cation consists only of solvent molecules. In general, for high-dielectric solvents such as nitrobenzene, this has been shown to be true experimentally [4l]. However, in ion-paired systems as defined by Eq. (34), the anion can very well influence selectivity. In this regard, one may refer to the electrostatic nature of ion pairing but may also invoke coordination concepts. Although these issues surpass the scope of this review, we will briefly cover some of the major issues later. 

Lrini Irophcnyldinilro-naphihylaminaic iciranilro phenol hiazinc S Oxidc mcLhylorangc 

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IV. SALT-PARTITIONING PROCESSES A. Water-Immiscible Solvents... 

Upon making an extrathermodynamic assumption such as the TATB assumption, the standard Gibbs energy of the salt partitioning process given by Eq. (33) becomes simply... 

Early experimental work in electrorefining at Los Alamos by Mullins et-all ) demonstrated that americium could be partitioned between molten plutonium and a molten NaCl-KCl salt containing Pu+3 ions, and Knighton et-al(8), working at ANL on molten salt separation processes for fuel reprocessing, demonstrated that americium could be extracted from Mg-Zn-Pu-Am alloys with immiscible molten magnesium chloride salts. Work... 

Americium is separated from plutonium by a liquid-liquid extraction process involving immiscible molten salt and molten plutonium metal phases. The molten salt extraction process is based upon equilibrium partitioning (by oxidation-reduction reactions) of americium and plutonium between the molten chloride salt and molten plutonium metal phases. 

Salt Transport Processing (8, 9, 10, 11) The selective transfer of spent fuel constitutents between liquid metals and/or molten salts is being studied for both thorium-uranium and uranium-plutonium oxide and metal fuels. The chemical basis for the separation is the selective partitioning of actinide and fission-product elements between molten salt and liquid alloy phases as determined by the values of the standard free energy of formation of the chlorides of actinide elements and the fission products. Elements to be partitioned are dissolved in one alloy (the donor... 

The solvent effect on extraction constants is a combination of the influence on salt partition and association. Both processes are influenced by the polarity of the solvent [97]. The Kirkwood function describes the relation between polarity, , and extraction constant a more polar membrane solvent promotes extraction. In general, polar solvents favor salt partition, but the tendency toward complexation diminishes. Since the overall effect of solvent polarity on the extraction is positive, the polarity appears to affect the partition coefficient to a higher degree. 

Whereas salt partitioning depends equally on the natures of the cation and the anion, the exchange process has no formal dependence on the anion and describes only the cation selectivity. By studying Eqs. (33) and (42) separately, we can infer the exchange given by Eq. (43). If we label the exchange constant Xs-. n-m, it may readily be seen that Xs. n-m = Xs (NX)/X,(MX). 

Metal extraction by carboxylic acids is a stepwise process in which the acid partitions between the organic phase and the aqueous phase and ionizes in the latter. Then the anion reacts with the metal cation and the neutral salt partitions between the organic and aqueous phases. Although this description is well-known in extraction, an ysis of transport through liquid membranes with the same chemistry is usually based for the sake of simplicity, on the assumption that metal ions in the aqueous phases are present in well-stirred reservoirs and the ion-exchange itself takes place only at the membrane surface. The carrier in this case is able to move only inside the membrane from one of its surfaces to the other. These mechanisms where the carrier is or is not able to leave the membrane could be called, a " big and small carrouser, respectively (6). 

As shown in Table 3.1.2, some authors have proposed adding different reagents to the mobile phase to reduce the peak tailing of some componnds, like for example acetic acid (AcOH) in the event of BZ3 (DiNunzio and Gadde, 1990), or ethylenediaminetetraacetic acid (EDTA) for BDM determination (Schakel et al, 2004). Salts, snch as citrate, phosphate and ammonium or sodium acetates, have often been nsed for bnffering purposes. On the other hand, different authors have proposed the use of voluminous ions like tetram-ethylammonium chloride (TMAC) and/or sodinm perchlorate (Gagliardi et al, 1986, 1987, 1989 De Orsi et al, 1995), or stearyltrimethylammonium chloride (STAC) (Ohba et al, 1991) to establish an ion-pair partition process. Moreover, the nse of cyclodextrines has helped to solve unresolved peaks, as performed by Chisvert et al (2001d) who employed hydroxypropyl-jS-cyclodextrine (HP-j -CD) as mobile-phase modifier. 

Solvent extraction—purification of wet-process phosphoric acid is based on preferential extraction of H PO by an organic solvent vs the cationic impurities present in the acid. Because selectivity of acid over anionic impurities is usually not sufficient, precipitation or evaporation steps are included in the purification process for removal. Cmde wet-process acid is typically concentrated and clarified prior to extraction to remove post-precipitated sludge and improve partition of the acid into the solvent. Concentration also partially eliminates fluoride by evaporation of HF and/or SiF. Chemical precipitation of sulfate (as Ba or Ca salts), fluorosiUcates (as Na salt), and arsenic (as sulfides) may also be used as a prepurification step preceding solvent extraction. 

Silver occurs naturally in several oxidation states, the most common being elemental silver (Ag°) and the monovalent ion (Ag+). Soluble silver salts are, in general, more toxic than insoluble salts. In natural waters, the soluble monovalent species is the form of environmental concern. Sorption is the dominant process that controls silver partitioning in water and its movements in soils and sediments. As discussed later, silver enters the animal body through inhalation, ingestion, mucous membranes, and broken skin. The interspecies differences in the ability of animals to accumulate, retain, and eliminate silver are large. Almost all of the total silver intake is usually... 

Other Onium Salt Catalysts. We have examined several quaternary ammonium and phosphonium salts in this process. And while they vary in their effectiveness, most seem to have at least some activity. In the case of symmetrical onium salts, substituents having four or more carbon atoms are preferred (Table III). This likely originates from a more favorable partitioning into the organic phase as the size of the substitutents increase. Herriott has reported that there is a... 

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