Solvate extraction

Solvating extractants contain one or more electron donor atoms, usually oxygen, which can supplant or partially supplant the water which is attached to the metal ions. Perhaps the best known example of such an extractant is tri-( -butyl) phosphate) [126-73-8] (TBP), which forms... 

In contrast, the non-fusible coal requires the solvation (extraction) or solvolytic reaction to be liquefied. The solvation of non-polar organic compounds including the pitch may be rather limitted, so that the solvolytic reaction is necessary for the high liquefaction yield between the coal and the solvent. 

Table 6 The structures of some neutral (solvating) extractants.

Figure 8 An outline flowsheet to transport zinc from chloride to sulfate media using a mixture of an acidic (pH-swing) extractant L H and a solvating extractant L.187...

Mooiman, M. B. Miller, J. D. The chemistry of gold solvent extraction from alkaline cyanide solution by solvating extractants. Hydrometallurgy 1991, 27, 29-46. 

Solvating extractants, 10 751 Solvation energy, 23 96 Solvation/phase behavior, role of water in, 20 439... 

When the solvent is a good solvater, the determination of the solvation number b is difficult, unless the dependence of the extractant concentration on the solvent can be obtained. Solvation numbers can be obtained in mixtures of a solvating extractant and an inert diluent like hexane. Further, in these systems the extraction of the metal commonly requires high concentrations of salt or acid in the aqneons phase, so the activity coefficients of the solutes must be taken into acconnt. 

The oxidation of cobalt(II) in the organic phase can be minimized by the addition of donor molecules, such as the solvating extractants. 

In this flow sheet, the aqueous raffinate from extraction is acidified to 5-6 mol dm with hydrochloric acid to optimize platinum extraction by the solvating extractant TBP. The coextraction of iridium is prevented by reduction with sulfur dioxide, which converts the iridium(IV) to the (III) species, which is not extractable. Once again, kinetics are a factor in this reduction step because, although the redox potentials are quite similar, [Ir(IV)/(III) —0.87 V Pt(IV)/(II) —0.77 V], iridium(IV) has a relatively labile configuration, whereas platinum(IV) has the inert arrangement. The species H2PtCl6 is extracted by TBP, from which platinum can be stripped by water and recovered by precipitation as (NH3)2PtCl2. 

Solvent extraction of base metals by solvating extractants 810... 

The use of solvating extractants in the recovery of gold and platinum-group metals (PGM) was described in the previous section. These extractants have also found some specialized applications in the extractive metallurgy of base metals. For example, they have been used in the recovery of uranium, the separation of zirconium and hafnium, the separation of niobium and tantalum, the removal of iron from solutions of cobalt and nickel chlorides, and in the separation of the rare-earth metals from one another. 

The reasons for the selective extraction of hafnium over zirconium from thiocyanate solutions by solvating extractants are not well understood. Hence, a recent review of the chemistry of these metals described the separation process but offered no explanation for the observed selectivity.306 There is no evidence that differences in the stabilities of the thiocyanate complexes of hafnium(IV) and zirconium(IV) are responsible for the selective extraction of the former, since the formation constants of the respective complexes are essentially identical for both metals.307 However, there is some indication that the hafnium thiocyanates are more readily solvated by the extractant than are the corresponding complexes of zirconium. 

The organic extractants used for the separation of metal ions broadly fall into three classes, chelating extractants, solvating extractants, and ion-pair extractants. For the first two classes, usually nonpolar organic diluent is preferred. On the other hand, polar diluents are preferred in the case of ion-pair extraction. 

While pH plays an important role in the extraction of metal ions by the acidic chelating extractants, counteranions such as N03, CE, etc., significantly influence the extraction of metal ions by solvating extractants (L) like TBP, TOPO, etc. The extracted species thus forms solvating species such as MX4 n. or M02X2 nL for tetravalent and hexavalent actinide ions, respectively, where X is a representative counteranion and n is the number of ligand molecules in the extracted species. In... 

Extraction Constants of some Actinide Ions from Nitric Acid Medium Using Solvating Extractants along with the Acid Uptake Constant (KH)... 

The solvate extraction mechanism of trivalent 4/and 5/elements can be described by the following equilibrium, where A- symbolizes the anion of the aqueous phase and E is the neutral solvation agent of the organic phase (overbars account for species in the organic phase, where residual aqueous molecules might also hydrate the... 

For a given ionic strength, //, depends on the nature of the coextracted anion A-. To allow the formation and extraction of the neutral complex, the coextracted mineral anion A- has to lose part (or all) of its hydration shell. The smaller the hydration energy of the mineral anion is, the easier is its transfer to the organic phase, and thus the higher is the affinity of the solvation extractant toward trivalent 4/ and 5/ elements (29, 76), as observed in the series chloride < nitrate < perchlorate < pertechnetate, which inversely follows the anion hydration energy order AG/CI ) > AG/NO3) > AG(,(CI04) > AG/TcO/. 

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