Reaction kinetics, solvent extraction

II. HOW TO STUDY THE INTERFACIAL REACTION IN SOLVENT EXTRACTION KINETICS... 

A growing-drop method has been reported [53] for measuring interfacial liquid-liquid reactions, in which mass transport to the growing drop was considered to be well-defined and calculable. This approach was applied to study the kinetics of the solvent extraction of cupric ions by complexing ligands. 

A significant advance was made in this field by Watarai and Freiser [58], who developed a high-speed automatic system for solvent extraction kinetic studies. The extraction vessel was a 200 mL Morton flask fitted with a high speed stirrer (0-20,000 rpm) and a teflon phase separator. The mass transport rates generated with this approach were considered to be sufficiently high to effectively outrun the kinetics of the chemical processes of interest. With the aid of the separator, the bulk organic phase was cleanly separated from a fine dispersion of the two phases in the flask, circulated through a spectrophotometric flow cell, and returned to the reaction vessel. 

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. 

The kinetics of solvent extraction is a fnnction of both the various chemical reactions occurring in the system and the rates of diffusion of the various species that control the chemistry of the extraction process. 

Equations (5.16) of Table 5.1 refer to series first-order reactions. Of interest for the solvent extraction kinetics is a special case arising when the concentration of the intermediate, [Y], may be considered essentially constant (i.e., d[Y]/dt = 0). This approximation, called the stationary state or steady-state approximation, is particularly good when the intermediate is very reactive and present at very small concentrations. This situation is often met when the intermediate [Y] is an interfacially adsorbed species. One then obtains... 

Section 5.1 describes how, in a stirred system, solvent extraction kinetics can be controlled only by slow chemical reactions or only by diffusion through the interfacial films. An intermediate situation can also occur whereby both the rates... 

When one or more of the chemical reactions is sufficiently slow in comparison with the rate of diffusion to and away from the interface of the various species taking part in an extraction reaction, such that diffusion can be considered instantaneous, the solvent extraction kinetics occur in a kinetic regime. In this case, the extraction rate can be entirely described in terms of chemical reactions. This situation may occur either when the system is very efficiently stirred and when one or more of the chemical reactions proceeds slowly, or when the chemical reactions are moderately fast, but the diffusion coefficients of the transported species are very high and the thickness of the two diffusion films is close to zero. In practice the latter situation never occurs, as diffusion coefficients in liquids generally do not exceed 10 cm s, and the depth of the diffusion films apparently is never less than 10 cm. 

Finally, it has to be emphasized that both the hydrodynamic parameters and the concentrations of the species involved in the extraction reaction simultaneously determine whether the extraction regime is of kinetic, diffusional, or mixed diffusional-kinetic type. It, therefore, is not surprising that different investigators, who studied the same chemical solvent extraction system in different hydrodynamic and concentration conditions, may have interpreted their results in terms of completely different extraction regimes. 

In a kinetic regime system, the kinetics of solvent extraction can be described in terms of chemical reactions occurring in the bulk phases or at the interface. The number of possible mechanisms is, in principle, very large, and only the specific chemical composition of the system determines the controlling mechanism. Nevertheless, some generalizations are possible on considerations based... 

In this section, we describe three simple cases of rates and mechanisms that have been found suitable for the interpretation of extraction kinetic processes in kinetic regimes. These simple cases deal with the exuaction reaction of a monovalent metal cation (solvation water molecules are omitted in the notation) with a weakly acidic solvent extraction reagent, BH. The overall extraction reaction is... 

Solvent extraction is a kinetic process. The key variables in determining the rate of extraction are (1) the displacement of the system from equilibrium, also referred to as the driving force (2) the area through which mass can be transferred, or the interfacial area and (3) specific resistances in the interfacial region, particularly any slow interfacial reactions. 

Ir(IV), Pt(IV), with the states from Rh(III) being termed inert. Thus, kinetic factors tend to be more important, and reactions that should be possible from thermodynamic considerations are less successful as a result. On the other hand, the presence of small amounts of a kinetically labile complex in the solution can completely alter the situation. This is made even more confusing in that the basic chemistry of some of the elements has not been fully investigated under the conditions in the leach solutions. Consequently, a solvent extraction process to separate the precious metals must cope with a wide range of complexes in different oxidation states, which vary, often in a poorly known fashion, both in kinetic and thermodynamic stability. Therefore, different approaches have been tried and different flow sheets produced. 

Liquid-liquid systems have been most widely used in the solvent extraction of various compounds in chemical and hydrometallurgy industries. Kinetic process of solvent extraction of metal ions depends intrinsically on the mass transfer to or across the interface and the chemical reactions in both bulk phases and at the interface. Therefore, the study of the role of the interface is very... 

Where unsaturated compounds cannot be epoxidized via in situ techniques then equilibrium peracids or solvent extracted peracids are frequently used. The equilibrium peracetic acid is often used stoichiometrically and is the favoured method for unreactive alkenes. Kinetics favour the use of equilibrium peracids over in situ techniques. Ring opening is still, however, a major side-reaction but... 

The solvent extraction process of metal ions inherendy depends on the mass transfer across the interface and the reaction that occurs at the interfacial region. Therefore, the elucidation of the kinetic role of the interface was very important in order to clarify the extraction mechanism and to control the extraction rates. In 1982, Watarai and Preiser invented the high-speed stirring (HSS) method [4,5]. Figure 10.1 shows the schematic drawing of the HSS method [6]. When a two-phase system is vigorously stirred in a... 

What is the specific feature in the reaction at the liquid/liquid interface The catalytic role of the interface is of primary importance in solvent extraction and other two-phase reaction kinetics. In solvent extraction kinetics, the adsorption of the extractant or an intermediate complex at the liquid/liquid interface significantly increased the extraction rate. Secondly, interfacial accumulation or concentration of adsorbed molecules, which very often results in interfacial aggregation, is an important role played by the interface. This is because the interface is available to be saturated by an extractant or mehd complex, even if the concentration of the extractant or metal complex in the bulk phase is very low. Molecular recognition or separation by the interfacial aggregation is the third specific feature of the interfacial reaction and is thought to be closely related to the biological functions of cell membranes. In addition, molecular diffusion of solute and solvent molecules at the liquid/liquid interface has to be elucidated in order to understand the molecular mobility at the interface. In this chapter, some examples of specific... 

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