Solvent-extraction studies

Organic extractants facilitate the transfer of the metal ions from the aqueous phase to the organic phase in solvent extraction. Based on the nature of the organic extractant, the metal ion, and the diluent, effective separation methods can be devised. Uranium extraction into diethyl ether from nitrate medium by salting out is perhaps one of the first uses of solvent extraction for large-scale actinide processing (9). In this case, ether not only acts as the diluent, it also acts as the extractant, which works according to the solvation mechanism (discussed below). 

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. 

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The heavy metal salts, ia contrast to the alkah metal salts, have lower melting points and are more soluble ia organic solvents, eg, methylene chloride, chloroform, tetrahydrofiiran, and benzene. They are slightly soluble ia water, alcohol, ahphatic hydrocarbons, and ethyl ether (18). Their thermal decompositions have been extensively studied by dta and tga (thermal gravimetric analysis) methods. They decompose to the metal sulfides and gaseous products, which are primarily carbonyl sulfide and carbon disulfide ia varying ratios. In some cases, the dialkyl xanthate forms. Solvent extraction studies of a large number of elements as their xanthate salts have been reported (19). 

In almost all theoretical studies of AGf , it is postulated or tacitly understood that when an ion is transferred across the 0/W interface, it strips off solvated molecules completely, and hence the crystal ionic radius is usually employed for the calculation of AGfr°. Although Abraham and Liszi [17], in considering the transfer between mutually saturated solvents, were aware of the effects of hydration of ions in organic solvents in which water is quite soluble (e.g., 1-octanol, 1-pentanol, and methylisobutyl ketone), they concluded that in solvents such as NB andl,2-DCE, the solubility of water is rather small and most ions in the water-saturated solvent exist as unhydrated entities. However, even a water-immiscible organic solvent such as NB dissolves a considerable amount of water (e.g., ca. 170mM H2O in NB). In such a medium, hydrophilic ions such as Li, Na, Ca, Ba, CH, and Br are selectively solvated by water. This phenomenon has become apparent since at least 1968 by solvent extraction studies with the Karl-Fischer method [35 5]. Rais et al. [35] and Iwachido and coworkers [36-39] determined hydration numbers, i.e., the number of coextracted water molecules, for alkali and alkaline earth metal... 

Paiva, A. P. Review of recent solvent extraction studies for recovery of silver from aqueous solutions. Solvent Extr. Ion Exch. 2000, 18, 223-271. 

Halides other than fluoride form very weak complexes in aqueous solution there are no reliable equilibrium constants to be found in the literature. The solution chemistry of aqueous solutions of beryllium chloride, bromide, and iodide have been reviewed previously (9). Some evidence for the formation of thiocyanate complexes was obtained in solvent extraction studies (134). 

Table I presents an evaluation of Equation 21 based on previously reported values for log Kqw, V, and 6. The constant Cj was calculated from the log KQC of benzene estimated from Figure 2. Estimates of log KQC for the remaining compounds in Table I are presented for 6 equal to 10.3 and 11.5. The value of 10.3 corresponds to the 5p implied from solvent extraction studies (31). In this case,...

Richter, H. G., and J. Irvine jr. Solvent Extraction Studies. Mass. Inst. 

The temperature variation method is used often in solvent extraction studies. It can give reliable values of the enthalpy if care is taken to verify that the change in temperature is not causing new reactions, which perturb the system. Also, a reasonable temperature range (e.g., AT 50°C) should be used so that it is possible to ascertain the linearity of the In P vs. l/T plot upon which Eq. (3.17) is based. 

Albinsson, Y. Development of the AKUFVE-LISOL Technique. Solvent Extraction Studies of Lanthanide Acetylacetonates. Diss. Chalmers Univ. Techn., Gothenburg (1988). 

Inclusion of this practice in all solvent extraction studies will ensure that the solvent is not discarded as being unsuitable, and that problems in pilot plant or continuous operations are not the result of impurities that were present in the bench-scale test work. 

If the mobile phase is present in a significant concentration, as suggested by the results of solvent extraction studies (1,8), the practical meaning of the mobile phase to coal conversion processes may be profound. In coal liquefaction, two stage processes emphasizing the mobile phase and the macromolecular structure separately could well be most economical. In devolatilization kinetics, at least two sets of kinetic parameters are necessary to model the devolatilization phenomena associated with the mobile phase and the macromolecular structure respectively since the mobile phase components devolatilize at much lower temperatures than the macromolecular structure components 0. In addition, the mobile phase appears to have a significant influence on the thermoplastic properties of coal (0 and thereby on coke quality. 

Solvent extraction studies on both the trace (71) and milligram (IS) scale indicate the formation of a compound with two molecules of dithizone, probably PoODz2 (71). The compound is red (IS) and sublimes at about 120°C in air (82). 

Formation and subsequent extraction of a neutral species such as (C2H5)3PbCl° suggest that a combined chemical complexing-solvent extraction technique might be more effective in terms of a lower Cr/Cl ratio than direct precipitation. This is confirmed by the results of the chemical complexing solvent-extraction studies. 

Chemical complexing-solvent extraction studies were conducted using aliquots of synthetic effluents which contained varying quantities of Hs PbCl and (C2H5)3NCSSNa 3H20 with the sodium chloride concentration maintained at 0.83 m. 

In solvent extraction studies of various anions using Aliquat 336S, values of Kgx were obtained that showed a one-to-one correspondence between Kgj and electrode selectivity coefficients,... 

Taylor, R.J., May, I., Koltunov, V.S. etal. 1998. Kinetic and solvent extraction studies of the selective reduction of Np(VI) by new salt free reducing agents. Radiochim. Acta 81 149-156. 

Spjuth, L. 1999. Solvent extraction studies with substituted malonamides and oligopyri-dines. Ph.D. Thesis. Chalmers University of Technology, Gothenburg. 

Boubals, N., Drew, M.G.B., Hill, C. et al. 2002. Americium (III) and eruopium (III) solvent extraction studies of amide-substituted triazine ligands and complexes formed with ytterbium (III). J. Chem. Soc. Dalton Trans. 2002 (1) 55-62. 

Several spectroscopic techniques, namely, Ultraviolet-Visible Spectroscopy (UV-Vis), Infrared (IR), Nuclear Magnetic Resonance (NMR), etc., have been used for understanding the mechanism of solvent-extraction processes and identification of extracted species. Berthon et al. reviewed the use of NMR techniques in solvent-extraction studies for monoamides, malonamides, picolinamides, and TBP (116, 117). NMR spectroscopy was used as a tool to identify the structural parameters that control selectivity and efficiency of extraction of metal ions. 13C NMR relaxation-time data were used to determine the distances between the carbon atoms of the monoamide ligands and the actinides centers. The II, 2H, and 13C NMR spectra analysis of the solvent organic phases indicated malonamide dimer formation at low concentrations. However, at higher ligand concentrations, micelle formation was observed. NMR studies were also used to understand nitric acid extraction mechanisms. Before obtaining conformational information from 13C relaxation times, the stoichiometries of the... 

Manchanda, V.K. Chang, C.A. Solvent extraction studies of lanthanum(III) and neodymium(III) with ionizable macrocyclic ligands and thenoyltrifluoroacetone, Anal. Chem. 58 (1988) 2269-2275. 

Pathak, P.N. Veeraraghavan, R. Ruikar, P.B. Manchanda, V.K. Solvent extraction studies on Th(IV), Pa(V) and U(VI) from nitric acid medium using di-2-ethylhexyl isobutyramide (D2EHIBA), Radiochim. Acta 86 (1999) 129-134. 

Sharma, J.N., Suri, A.K., Manohar, S., Chitnis, R.R., Shah, G.J., Wattal, P.K. 2004. Solvent extraction studies of synthetic high level waste using novel extractant tetra (2-ethylhexyl) dilycolamide (TEHDGA). SESTEC-2004 Emerging Trends in Separation Science and Technology, July, Mumbai, India. 

Solvent Extraction Experiments. Solvent extraction studies were done on two feed samples representing dissolved hydroxide cake (SSA) and evaporator supernate (SSB). SSA was prepared by dissolution of hydroxide cake with slow addition of concentrated HN03, adjustment of the final acidity to 0.5 N by addition of water and/or HN03, and clarification by filtration. To prepare SSB, some supernate liquid from the evaporator was titrated, acidity adjusted to 0.5 N by NaOH and water addition, then clarified by filtration. No appreciable solids were observed on the clarification filters for either solution. Compositions of these feeds are listed in Table III. 

Solvent extraction studies for the separation of rare earths using crown ethers are summarized in Table 3.9. 

Although salts containing the anion [PoCNOsj ] " have never been isolated, solvent extraction studies with indicate that anionic nitrate complexes are formed in nitric acid solutions which contain the metal cation. 

Since protactinium(IV) is readily oxidized in the atmosphere halo-genoprotactinates(IV) must be prepared in oxygen-free solvents or by heating the component halides together in an inert atmosphere. It is reported that solvent extraction studies have established the existence of the fluoro and chloro species PaXl" " and PaX + in aqueous acid solution (81)... 

Based on Equation 31.1, pertraction rates of uranium were expected to increase with increasing HNO3 concentration. A maximum flux (7) at 2 M HNO3 indicated the formation of TBP HNO3 complexes at higher nitric acid concentration. Formation of such species is well known in the solvent extraction studies reported earlier [28]. Similarly, the increase in transport rates with increasing TBP concentration was valid up to only 30% TBP beyond which the strong viscosity effects decreased the flux. 

The species extracted into the BLM phase is Pu(N03)4 2TBP(o> which is similar to that reported in the solvent extraction studies [30]. With increase in carrier concentration in the organic membrane, the increase in the amount of plutonium that could be extracted into the membrane is countered by the increase in the viscosity of the carrier solution. These opposing effects resulted in maximum plutonium permeation with about 30% TBP in dodecane [29]. More than 85% transport was observed in about 5 h when 5 mg/L solution of Pu in 2 M HNO3 was used as the feed. Selective transport of Pu(TV) over fission product contaminants, such as Ce-144, Ru-106, and Cs-137, was also observed. 

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