Organic extractant phases

Solvent extraction has also been used to enhance the selectivity of polaro-graphic determinations. Such measurements are normally carried out in aqueous solutions, and extraction followed by back-extraction has been widely used. However, it may be unnecessary to perform a back-extraction if the organic extractant phase has a sufficiently high dielectric constant to dissolve sufficient background electrolyte for a voltammetric determination or if the organic phase can be diluted with suitable polar solvents, such as methanol or acetonitrile [26]. 

The Winsor II microemulsion is the configuration that has attracted most attention in solvent extraction from aqueous feeds, as it does not affect the structure of the aqueous phase the organic extracting phase, on the other hand, is now a W/0 microemulsion instead of a single phase. The main reason for the interest in W/0 microemulsions is that the presence of the aqueous microphase in the extracting phase may enhance the extraction of hydrophilic solutes by solubilizing them in the reverse micellar cores. However, this is not always the case and it seems to vary with the characteristics of the system and the type of solute. Furthermore, in many instances the mechanism of extraction enhancement is not simply solubilization into the reverse micellar cores. Four solubilization sites are possible in a reverse micelle, as illustrated in Fig. 15.6 [19]. An important point is that the term solubilization does not apply only to solute transfer into the reverse micelle cores, but also to insertion into the micellar boundary region called the palisade. The problem faced by researchers is that the exact location of the solute in the microemulsion phase is difficult to determine with most of the available analytical tools, and thus it has to be inferred. 

The principle Zr ore, zircon (Zr silicate) is processed by caustic fusion or by direct chlorination of milled coke and zircon mixts. Washing of the Na fusion cake leave an acid soluble hydrated Zr oxide, whereas chlorination yields mixed Si and Zr tetrachlorides which are separated by distillation. Removal of the Hf from the Zr takes place through counter current liq-liq extraction (Ref 33), For this purpose the oxide or the tetrachloride is dissolved in dil hydrochloric acid to which ammonium thiocyanate is added as a complexing agent. The organic extracting phase is methyl isobutylketone... 

In this review, we summarize recent progress in understanding the aggregation states of organic extractant phases in relation to third-phase formation. The organic extractant phase is described from a colloidal standpoint to interpret the phase-separation boundary. The aim is to provide the basis for developing a theoretical approach to predict third-phase formation. Finally, the structure of the third phase is described. 

Another method for obtaining the extension domain of the organic extractant phase before third-phase formation is to prepare a solution under the conditions of formation of a third phase, and allow it to equilibrate. Chemical analysis of the third phase and the dilute organic phase in equilibrium with the aqueous phase identifies tie-lines in the phase diagrams (Figure 7.5) (a tie-line joins the composition of the two phases (dilute and concentrated) in equilibrium after the splitting of the organic phase into two phases). 

Here again, a general trend is observed in all these studies the organic extractant phase is stabilized by a long-chain extractant this is related to the steric stabilization component of the intermicellar potential. 

The phase stability of organic extractant phases and classical reverse microemulsions are thus governed by the same rule. This general conclusion would not have been valid for extractant systems in the case of an emulsification failure mechanism, namely rejection of the internal phase, but this was not observed in liquid/liquid extraction systems when salt is extracted. 

Regarding the liquid/liquid extraction from the metal standpoint is rather different. This is the classical approach of coordination chemistry (most of the publications in this area). Today, it is still difficult to establish a direct link between the two descriptions of the organic extractant phases. To better understand liquid/ liquid extraction, the aggregation number and coordination number must be measured separately for each system and set of initial conditions. This is the only way to determine the role of the aggregates in the extraction efficiency. This important point was emphasized by Yaita et al. (61). In this way, Gannaz et al. has used an approach combining studies on both supramolecular and molecular speciation of extractant systems of the DIAMEX-SANEX process (36). 

Although the process proved satisfactory from the chemical standpoint, practical problems emerged in that the hydraulic operation of the mixer-settler batteries was extremely poor. In effect, as soon as the aqueous solutions from the dissolution of irradiated targets were placed in contact with the organic extraction phases, a stable emulsion was formed, produced by the appearance of extensive precipitates at the aqueous solution/organic solution interface. As no chemical remedy was found to solve this problem, we attempted to adapt this type of process to extraction chromatographic techniques. 

The commercially obtained extractants were purified by a modification of a method previously described (13) for the purification of TBP. The undiluted extractant was stirred with an equal volume 6 M HC1 at room temperature for 1 hour and scrubbed with two equal volume portions of water. The aqueous phases were discarded, and the organic phase was stirred for 1 hour with 1 M NaOH at room temperature. The aqueous phase was discarded, and the organic (extractant) phase was diluted to 30 percent by volume with n-heptane. The n-heptane solution was scrubbed with 6 one-half volume portions of distilled water. The water phases were discarded, and the ii-heptane was removed from the organic phase by evaporation in an open beaker at room temperature. The purified product was freed from traces of n-heptane, H2O and alcohols (formed in the purification step) by pumping for eight hours at 10 2 mm pressure and at room temperature. 

Figure 9.17 The effect of complexing agent concentration in the organic extraction phase on the amount of uranium extracted from an aqueous solution.17 (Organic Phase Alamine 336 dissolved in Aromatic 150. Aqueous Phase 0.2% uranium, pH 1.0).

In view of industrial applications, after extraction from water, the metallic cation has to be transferred from the organic extraction phase into a fresh aqueous phase (possibly acidified, to avoid precipitation) from which various hydrometallurgical processes can be easily implemented. This operation is called either back-extraction or stripping. This is of course an important industrial aspect which is nevertheless often neglected in academic studies, assuming reversibility of processes. 

The bright blue color of the organic extractant phase confirms that the extracted cobalt(n) species is indeed tetrahedral. 

Here also we are not exact since the volumetric flow rate ratio is not equivalent to the mass flow rate ratio due to the aqueous phase being in general somewhat heavier than the organic extract phase.)... 

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