Synergistic extraction systems

These equations do not provide complete definition of the reactions that may be of significance in particular solvent extraction systems. For example, HTTA can exist as a keto, an enol, and a keto-hydrate species. The metal combines with the enol form, which usually is the dominant one in organic solvents (e.g., K = [HTTA]en i/[HTTA]]jet = 6 in wet benzene). The kinetics of the keto -> enol reaction are not fast although it seems to be catalyzed by the presence of a reagent such as TBP or TOPO. Such reagents react with the enol form in drier solvents but cannot compete with water in wetter ones. HTTA TBP and TBP H2O species also are present in these synergistic systems. However, if extraction into only one solvent (e.g., benzene) is considered, these effects are constant and need not be considered in a simple analysis. 

Watanabe, M., Mirvaliev, R., Tachimori, S. et al. 2004. Selective extraction of americium(III) over macroscopic concentration of lanthanides(III) by synergistic system of TPEN and D2EHPA in 1-octanol. Solvent Extr. Ion Exch. 22 (3) 377-390. 

The choice is wide for the formulation of the chemical system developed to separate the target species (e.g., number and nature of the extracting agent(s), including the use of binary synergistic systems or phase modifiers, concentration of species, nature of the organic diluent). 

Cordier, P.Y., Francois, N., Boubals, N., Madic, C., Hudson, M.J., Liljenzin, J.O. 1999. Synergistic systems for the selective extraction of trivalent actinides from mixtures of trivalent actinides and lanthanides. ISEC 99 Conference on Solvent Extraction for the 21st Century, July, Barcelona, Spain. 

Andersson, S., Ekberg, C., Foreman, M.R.S., Hudson, M.J., Liljenzin, J.O., Nilsson, M., Skamemark, G., Saphiu, K. 2003. Extraction behavior of the synergistic system 2,6-bis-(benzoxazolyl)-4-dodecyloxylpyridine and 2-bromodecanoic acid using Am and Eu as radioactive tracers. Solvent Extraction and Ion Exchange 21(5) 621-636. 

The new features are detailed in this review most using diamide as a model system and illustrated with published data on other extractant systems. Figure 7.1 summarizes the structure of the different extractants cited in this chapter. The mixture of extractants as synergistic systems and extractants, such as the N-polydentate ligands BTP (39, 40), BTBP (41, 42), or bis-malonamide (43), will not be treated here. These extractants have low solubility in alkane and are generally used in chlorinated solvent or octanol or diamide/alkane solution to enhance their solubility. 

There are a number of practical synergistic systems that make use of organophosphorus compounds. A classic example is the combination of HDEHP and TOPO. The TOPO is thought to replace water or HDEHP in the coordination sphere of the metal. Some early work in this area included a study of the extraction of uranium in such systems (82). Commercial processes now exist for the recovery of uranium from wet-process phosphoric acid utilizing synergistic systems (83,84). Descriptive studies of such systems have also been made (85,86). 

Synergistic systems have also been reported in which one of the adducts is in the aqueous phase. The combination of TOPO (in CCI4) and benzoic acid (in aqueous and organic) was shown to extract uranium with a synergistic effect. The organic-phase adduct was reported to be 1102(0104) C COO TOPO. An optimum benzoic acid/TOPO ratio exists, above which compound formation between TOPO and benzoic acid decreases the synergistic effect (91). 

As explained above, in this section, we review results concerning extraction systems in which the molecular solvent has been replaced by an IL phase, using a molecular well-known ligand for the extraction, and we include synergistic systems of the form M /HX/ZOL+LO/DL. 

In this section, we review the yet to be explored field of ILs used as additives to traditional extraction systems, that is, M " /HX//(L + lL)/Org. The situation is different from the one described in Section 5, where ILs are used as ligands, without any other solute and resembles more that of a traditional synergistic system (see Section 3). 

C. Y. Cheng, Solvent extraction of nickel and cobalt with synergistic systems consisting of carboxylic acid and aliphatic hydroxy oxime . Hydrometallurgy, 84 (2006), 109-117. 

The solvent extraction reaction chemistry in specific medium, extractants, pH, diluents, and in synergistic systems are discussed in relation to the transfer of the rare earth extractable complex from the aqueous phase to the organic phase. 

Extraction behaviour of the synergistic system 2,6-bis-(benzoxazolyl)-4-dodecyloxylpyridine and 2-bromodecanoic acid using Am and Eu as radioactive tracers. Solvent Extr. Ion Exch., 21, 621 -636. 

In addition to systems of the above type, i.e. involving adduct formation, various other types of synergistic extraction systems are recognised and have been reviewed.4 An example is the synergistic influence of zinc in the extraction and A AS determination of trace cadmium in water.5... 

Synergistic extraction systems include various new types of interesting interfacial reactions. These systems should be studied more extensively. 

In the simplest cases, the solvent may consist of one specified component, although in fact in a steady-state cyclic process it is highly unlikely that the solvent will ever come back to the initial composition at time zero. Rather, perhaps, one can say that make-up will entail addition of one material only. Again, clearly this need not be a pure compound, but its composition should be consistent. The single solvent offers limited scope for manipulating the system since it alone must meet all process and operational requirements. In other words, it must satisfy all aspects that will lead to an overall viable system. These aspects include selectivity, capacity, solubility, mass transfer, phase separation, costs, among others. The solvent is, therefore, a mixture components. The solvent components are extractant, (ii) diluent, (iii) modifier, and (iv) synergist. 

Preston, J. S. du Preez, A. C. Solvent extraction of nickel from acidic solutions using synergistic mixtures containing pyridinecarboxylate esters. 3. Systems based on arylsulphonic acids. J. Chem. Technol. Biotechnol. 1998, 71, 43-50. 

The selectivity of extractive separations of metal ions M" is commonly described by the separation factor (SF), defined as the ratio of the distribution ratios, [Eq. (4.3)], of the ions in the same system. In the case of two metal ions A" and Z" extracted from the same aqueous solution by an acidic bidentate extractant HE, with no synergist in the system, one obtains ... 

However, even this simplified formula does not justify the use of the ratio of stability constants of the extracted complexes as the only measure of selectivity of extractive separations. Such a widely used approach is obviously based on an implicit assumption that the partition constants of neutral complexes ML of similar metal ions are similar, so that their ratio should be close to unity. This is, however, an oversimplification because we have shown that the ifoM values significantly differ even in a series of coordi-natively saturated complexes of similar metals [92,93]. Still stronger differences in the values have been observed in the series of lanthanide acetylacetonates, due to different inner-sphere hydration of the complexes (shown earlier), but in this case, self-adduct formation acts in the opposite direction [100,101] and partly compensates the effect of the differences in. Tdm on S T(see also Fig. 4.15). Such compensation should also be observed in extraction systems containing coordinatively unsaturated complexes and a neutral lipophilic coextractant (synergist). 

Synergistic extraction, in the system DEHPA and TOPO, is quite interesting for its ability to extract uranium from high concentrations of phosphoric acid (38). This finds application in the recovery of uranium from dilute phosphoric acid medium in... 

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