Extraction phase transfer processes

If a neutral chelate formed from a ligand such as acetylacetone is sufficiently soluble in water not to precipitate, it may stiH be extracted into an immiscible solvent and thus separated from the other constituents of the water phase. Metal recovery processes (see Mineral recovery and processing), such as from dilute leach dump Hquors, and analytical procedures are based on this phase-transfer process, as with precipitation. Solvent extraction theory and many separation systems have been reviewed (42). 

Extensive studies have been carried out concerning ion transfers, electron transfers and combinations of ion and electron transfers at liquid-liquid interfaces. Po-larography and voltammetry at liquid-liquid interfaces are of analytical importance, because they are applicable to ionic species that are neither reducible nor oxidizable at conventional electrodes. They are also usefid in studying charge-transfer processes at liquid-liquid interfaces or at membranes solvent extractions, phase transfer catalyses, ion transport at biological membranes, etc. are included among such processes. 

The pigments are formed in the fish scales as platelet-shaped crystals (0.05 pm x 1-10 pm x 20-50 pm). A commercial synthetic process for producing purines with this crystal shape has not been found. An aqueous suspension of fish scales is, therefore, extracted with organic solvents to dissolve and remove the proteins. The remaining dispersion contains purine crystals and scale which are separated from one another by a complicated washing and phase-transfer process [5.216]. 

The possibility of developing an efficient phase-transfer process under liquid liquid conditions relies on the selective extraction of a quaternary salt from an aqueous into an organic phase. The hydration sphere, electronegativity, and volume and structure of anions play a fundamental role in determining the relative ease of their extraction into... 

Droplets in emulsions have large specific interfacial areas for mass transfer, but they are often difficult to break once formed. In most cases, the phases must be separated after the phase transfer processes. To overcome this disadvantage, the system consists of a small, magnetically stirred vessel (300 pL) for extraction and an electrocoalescence device after this vessel was developed (Figure 12.10) [33]. Phenol... 

Phase transfer consists of the transfer of a solute in a mixture from one phase to another. An important type of phase transfer process is solvent extraction (Figure 2.12), a process in which a substance is transferred from solution in one solvent (usually water) to another (usually an organic solvent) without any chemical change taking place. When solvents are used to leach substances from solids or... 

Describe solvent extraction as an example of a phase transfer process. 

Phase transfer consists of the transfer of a solute in a mixture from one phase to another. An important type of phase transfer process is solvent extraction, a process... 

Many phenomena of interest in science and technology take place at the interface between a liquid and a second phase. Corrosion, the operation of solar cells, and the water splitting reaction are examples of chemical processes that take place at the liquid/solid interface. Electron transfer, ion transfer, and proton transfer reactions at the interface between two immiscible liquids are important for understanding processes such as ion extraction, " phase transfer catalysis, drug delivery, and ion channel dynamics in membrane biophysics. The study of reactions at the water liquid/vapor interface is of crucial importance in atmospheric chemistry. Understanding the behavior of solute molecules adsorbed at these interfaces and their reactivity is also of fundamental theoretical interest. The surface region is an inhomogeneous environment where the asymmetry in the intermolecular forces may produce unique behavior. 

Although phosphine [7803-51-2] was discovered over 200 years ago ia 1783 by the French chemist Gingembre, derivatives of this toxic and pyrophoric gas were not manufactured on an industrial scale until the mid- to late 1970s. Commercial production was only possible after the development of practical, economic processes for phosphine manufacture which were patented in 1961 (1) and 1962 (2). This article describes both of these processes briefly but more focus is given to the preparation of a number of novel phosphine derivatives used in a wide variety of important commercial appHcations, for example, as flame retardants (qv), flotation collectors, biocides, solvent extraction reagents, phase-transfer catalysts, and uv photoinitiators. 

Dissociation extraction is the process of using chemical reac tion to force a solute to transfer from one liquid phase to another. One example is the use of a sodium hydroxide solution to extract phenolics, acids, or mercaptans from a hydrocarbon stream. The opposite transfer can be forced by adding an acid to a sodium phenate stream to spring the phenolic back to a free phenol that can be extrac ted into an organic solvent. Similarly, primary, secondary, and tertiary amines can be protonated with a strong acid to transfer the amine into a water solution, for example, as an amine hydrochloride salt. Conversely, a strong base can be added to convert the amine salt back to free base, which can be extracted into a solvent. This procedure is quite common in pharmaceutical production. 

The main objective for calculating the number of theoretical stages (or mass-transfer units) in the design of a hquid-liquid extraction process is to evaluate the compromise between the size of the equipment, or number of contactors required, and the ratio of extraction solvent to feed flow rates required to achieve the desired transfer of mass from one phase to the other. In any mass-transfer process there can be an infinite number of combinations of flow rates, number of stages, and degrees of solute transfer. The optimum is governed by economic considerations. 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

Classical solvent extraction is a phase transfer of solute from the solid phase to solution. All analytical extractions from solid matrices undergo three processes ... 

This technology lends itself to continuous, rather than batch, processing and has been proven robust since first applied for the extraction of uranium in the early 1940s.10 The types of extractant used can be classified according to the types of reactions involved in phase transfer.111... 

The phase transfer catalysis not only promotes the reactions between the reagents which are mutually insoluble in immiscible phases, but also offers a number of process advantages such as, increase in rate of reactions, increase in product specificity, lowering of energy requirement, use of inexpensive solvents and catalysts, extraction of cations or even neutral molecules from one phase to another etc. 

In many reactions, transfer of the anion across the interface and subsequent diffusion into the bulk of the organic phase will not be the rate-determining step when lipophilic catalysts are used, but the effect of less lipophilic catalysts may be influenced more by the anion and the mechanism of the transfer process. Thus, for example, the reactive anion is frequently produced in base-initiated reactions by proton extraction from the substrate at the two-phase interface and diffusion of the ion-pair contributes to the overall kinetics of the reaction. Additionally, the reactivity of the anion depends on its degree of hydration and on its association with the quaternary ammonium cation. In most situations, the activity of the transferred anion is enhanced, compared with its reactivity in aqueous media, as its degree of hydration is reduced, whereas a relatively weak electrostatic interaction between the two ions resulting from the bulkiness of the cation enhances the reactivity of the anion by making it more available for reaction and will be a major factor in the ratedetermining step. 

In the main, the original extractive alkylation procedures of the late 1960s, which used stoichiometric amounts of the quaternary ammonium salt, have now been superseded by solid-liquid phase-transfer catalytic processes [e.g. 9-13]. Combined soliddiquid phase-transfer catalysis and microwave irradiation [e.g. 14-17], or ultrasound [13], reduces reaction times while retaining the high yields. Polymer-supported catalysts have also been used [e.g. 18] and it has been noted that not only are such reactions slower but the order in which the reagents are added is important in order to promote diffusion into the polymer. 

The previous chapters have demonstrated that liquid-liquid extraction is a mass transfer unit operation involving two liquid phases, the raffinate and the extract phase, which have very small mutual solubihty. Let us assume that the raffinate phase is wastewater from a coke plant polluted with phenol. To separate the phenol from the water, there must be close contact with the extract phase, toluene in this case. Water and toluene are not mutually soluble, but toluene is a better solvent for phenol and can extract it from water. Thus, toluene and phenol together are the extract phase. If the solvent reacts with the extracted substance during the extraction, the whole process is called reactive extraction. The reaction is usually used to alter the properties of inorganic cations and anions so they can be extracted from an aqueous solution into the nonpolar organic phase. The mechanisms for these reactions involve ion pah-formation, solvation of an ionic compound, or formation of covalent metal-extractant complexes (see Chapters 3 and 4). Often formation of these new species is a slow process and, in many cases, it is not possible to use columns for this type of extraction mixer-settlers are used instead (Chapter 8). 

To obtain a large transfer area between raffinate and extract phases, one of the two liquids must be dispersed into drops. Figure 9.2 demonstrates this process schematically at a single nozzle. Similar to a dripping water tap, individual drops periodically leave the nozzle when the volumetric flow rate of the dispersed phase is low. When the flow rate is higher, however, the liquid forms a continuous jet from the nozzle that breaks into droplets. Because of stochastic mechanisms, uniform droplets are not formed. If the polydispersed droplet swarm is characterized by a suitable mean drop... 

The most important step in any aqueous extraction scheme is facilitation of the transfer of mineral particles from the bituminous matrix to the aqueous phase (Fig. 1). This process appears to be more favorable in the Athabasca tar sands than in other tar sands due to a postulated, but unobserved, film of absorbed water present on the mineral particles. The transfer process may be visualized in 2 stages 1) the transition from complete immersion in the bulk bituminous phase to partial contact with both phases, and 2) the transition from partial contact with both phases to complete immersion in the aqueous phase (Fig. la,b). 

Phase-transfer oxidation is a technology for destruction of organic contaminants. It was developed to treat contaminated liquid streams using adsorption for contaminant removal and advanced oxidation processes for spent adsorbent regeneration. It was used in testing to treat the contaminated effluent from groundwater extraction technologies. 

An alternative to the injection method for importing enzymes into a microemulsion is the phase transfer method. In this method, a layer of an aqueous enzyme solution is located under a mixture of surfactant and oil. Upon gentle shaking, the enzyme is transferred into the reverse micelles of the hydrocarbon phase. Finally, the excess of water is removed and the hydrophobic substrates can be added. The main advantage of this method is that it ensures thermodynamically stable micro emulsions with maximum water concentrations. However, the method is very time consuming. The method is often applied in order to purify, concentrate or renaturate enzymes in the reverse micellar extraction process [54-58]. 

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