Organic phase equilibrium metal concentrations

The equilibrium metal concentrations in the aqueous and organic phases and the equilibrium acidity of the aqueous phase are presented along with the calculated values for the distribution coefficient in Table IV. 

For the supported catalyst it is expected that the ligand does not leach since it is chemically bonded to the carrier. In contrast, the rhodium metal bound to the ligand is subject to leaching due to the reversible nature of the complex formation. The amount will depend on the equilibrium between rhodium dissolved in the organic phase and that bound to the ligand. When an equilibrium concentration of 10 ppb Rh is attained, the yearly loss of Rh for a 100 kton production plant will be about 1 kg Rh per year. Compared to the reactor contents of rhodium (see Table 3.9, 70 kg Rh) this would result in a loss of 1.5% of the inventory per year, which would be acceptable. 

It can be seen from Figure 11 that, by the appropriate choice of the equilibrium concentration of chloride ion in the aqueous phase, separations between certain pairs of metals can be made, for example between copper(II) and manganese(II) at a chloride concentration of 3.0 M, and between cobalt(II) and nickel(II) at a chloride concentration of 6 to 8 M. Furthermore, the metals can be stripped from the loaded organic phase by being contacted with an appropriate volume of water so that the equilibrium concentration of chloride ion in the strip liquor lies on the lower portion of the extraction curve, where substantial aquation of the extracted chlorometallate occurs... 

As stated earlier the polymeric species are often involved in the extraction of metal carboxylates. Therefore, the extraction equilibrium is sometimes more complicated than in the chelate extraction system. As is evident from the following treatment, it is advantageous and often indispensable to study the total metal concentration in the organic phase [Eq. (8)] instead of the conventionally utilized distribution ratio of the metal [Eq. (7)]. 

One would expect the organic phase of other amine extraction systems in which more than one metal anion can be formed to exhibit similar equilibria. It is fortunate that in this system not only is the solvent not present in the coordination sphere of either complex but also the equilibrium constant between the two is of an order of magnitude which allows concentration of both to be measured readily by spectrophoto-metric methods. This allows the effect of the dielectric constant of the solvent on the ratio of the species to be studied easily without the perturbing effect of specific interactions caused by differences in the tendency of the solvents to enter the coordination sphere. 

The extraction and back-extraction steps take place consecutively, connected by the concentration of the metal-extractant complex species in the organic phase. The description of the back-extraction process is carried out using similar equations to those used in the extraction process. The equilibrium of the interfacial reaction between the organic complex species and the back-extraction agent is applied in this case. 

The HNO3 dependencies of the extraction of U(VI) and Th(IV), shown in Figure 4, should be considered of operational significance only since several parameters vary simultaneously as the equilibrium aqueous HNO3 concentration is varied. For example, NOl activity and nitrato complexing of metals in the aqueous and the free extractant concentration in the organic phase may vary as the aqueous HNO3 concentration is varied. 

Solvent Extraction Relationships. In a solvent extraction system, the two phases are immiscible, but under proper conditions phase-transfer of one or more species can occur across the organic/aqueous interface. Expansion of the interface by gentle agitation of the vessel containing the two phases allows phase equilibrium to be attained quickly, usually in 1 to 2 minutes. In such an extraction system, the distribution of a metal, M, between the two phases at equilibrium is described by a distribution coefficient, Dj, where [M] represents the concentration of the metal. 

DYNAMICS OF DISTRIBUTION The natural aqueous system is a complex multiphase system which contains dissolved chemicals as well as suspended solids. The metals present in such a system are likely to distribute themselves between the various components of the solid phase and the liquid phase. Such a distribution may attain (a) a true equilibrium or (b) follow a steady state condition. If an element in a system has attained a true equilibrium, the ratio of element concentrations in two phases (solid/liquid), in principle, must remain unchanged at any given temperature. The mathematical relation of metal concentrations in these two phases is governed by the Nernst distribution law (41) commonly called the partition coefficient (1 ) and is defined as = s) /a(l) where a(s) is the activity of metal ions associated with the solid phase and a( ) is the activity of metal ions associated with the liquid phase (dissolved). This behavior of element is a direct consequence of the dynamics of ionic distribution in a multiphase system. For dilute solution, which generally obeys Raoult s law (41) activity (a) of a metal ion can be substituted by its concentration, (c) moles L l or moles Kg i. This ratio (Kd) serves as a comparison for relative affinity of metal ions for various components-exchangeable, carbonate, oxide, organic-of the solid phase. Chemical potential which is a function of several variables controls the numerical values of Kd (41). 

An example of nonideal organic-phase behavior arises with di-2-ethylhexy phosphoric acid (DEHPA) in the extraction or metal cations. Equilibrium data of Troyer9 for extraction of copper in the presenes of nickel from sulfate solution into xylene solutions of DEHPA are shown in Fig. 8.3-10. Although the organic-phase copper concentration would be expected to rise in proportion to that in the aqueous phase,... 

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