Extraction process selection

Separation of a fat or oil from its source material can be accompHshed by several different methods. Selection of an extraction process is based on (/) obtaining oil substantially undamaged and relatively free of undesirable impurities, (2) achieving the highest practical yield, and (J) obtaining the maximum economic return on the oil and coproducts. 

Liquid—Liquid Extraction. The tiquid—tiquid extraction process for the rare-earth separation was discovered by Fischer (14). Extraction of REE using an alcohol, ether, or ketone gives separation factors of up to 1.5. The selectivity of the distribution of two rare-earth elements, REI and RE2, between two nonmiscible tiquid phases is given by the ratio of the distribution coefficients DI and D2 ... 

Extraction and Extractive Distillation. The choice of an extraction or extractive distillation solvent depends upon its boiling point, polarity, thermal stabiUty, selectivity, aromatics capacity, and upon the feed aromatic content (see Extraction). Capacity, defined as the quantity of material that is extracted from the feed by a given quantity of solvent, must be balanced against selectivity, defined as the degree to which the solvent extracts the aromatics in the feed in preference to paraffins and other materials. Most high capacity solvents have low selectivity. The ultimate choice of solvent is deterrnined by economics. The most important extraction processes use either sulfolane or glycols as the polar extraction solvent. 

Selectivity for a single metal of a group is the basis of a solvent extraction process for the recovery of copper (qv) from low concentration ore leach solutions containing high levels of iron (qv) and other interfering metals (16). 

The separation of components by liquid-liquid extraction depends primarily on the thermodynamic equilibrium partition of those components between the two liquid phases. Knowledge of these partition relationships is essential for selecting the ratio or extraction solvent to feed that enters an extraction process and for evaluating the mass-transfer rates or theoretical stage efficiencies achieved in process equipment. Since two liquid phases that are immiscible are used, the thermodynamic equilibrium involves considerable evaluation of nonideal solutions. In the simplest case a feed solvent F contains a solute that is to be transferred into an extraction solvent S. 

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

The copper(II) flux is directly proportional to the cuiTent density up to 10 mPJcrcf. The extraction degree of platinum(IV) into the strip solution is less than 0.1 % per hour of electrodialysis. About 55% of copper(II) is removed from the feed solution under optimal conditions. The copper(II) extraction process is characterized by high selectivity. Maximum separation factor exceeds 900 in the studied system. 

Removal of diluent by an extraction process To obtain the final stable macroporous structure, the liquid organic diluents and the linear polymer are removed from the crosslinked structure by extraction with a good solvent for the inert diluents and particularly for the linear polymer. Toluene or methylene chloride are usually preferred for the removal of linear polystyrene from the divinylbenzene crosslinked macroporous polystyrene particles [125,128]. The extraction is carried out within a Soxhelet apparatus at the boiling point of the selected solvent over a period usually more than 24 h. 

The raffinate from the selective extraction process contains mostly niobium. The tantalum extract is treated by steam stripping to obtain a tantalum strip solution. The method results in the effective separation and relatively high concentration of tantalum and niobium in the respective strip solutions. 

We are not aware of any previous studies of the removal of plutonium or americium from (NH )2ZrF6-NHltF-NH N03 solutions. For ready plant-scale application, precipitation, sorption on inorganic materials, or batch solvent extraction processes may all be satisfactory. An inexpensive inorganic material with great selectivity and capacity for sorbing actinides, and with suitable hydraulic properties, would be especially attractive. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

The extraction procedure begins the process to separate the analytes from the matrix and present the material in a form that can be more easily analyzed. The type of extraction step that is used for a particular matrix depends on the nature of the matrix and analytes. There are two competing views in the extraction process among chemists. Some prefer to extract the analytes exhaustively from the matrix and rely on extensive cleanup to remove matrix co-extractives. Others prefer the just enough extraction concept, in which the selectivity of the extraction process is honed as much as possible... 

The polarity index is a measure of the polarity of the solvent, which is often the most important factor in the solvent choice for the particular application. In extraction processes, the tenet that like dissolves like (and conversely, opposites do not attract ) is the primary consideration in choosing the solvent for extraction, partitioning, and/or analytical conditions. For example, hexane often provides a selective extraction for nonpolar analytes, and toluene may provide more selectivity for aromatic analytes. 

O. Winkler and R. Bakish (ed.), Vacuum Metallurgy, Elsevier, Amsterdam, 1971. P. C. Hayes, Process Selection in Extractive Metallurgy, Hayes Publishing Co., Brisbane, 1985. 

MIBK is a valuable industrial solvent used primarily in the paint and coating industry and in metallurgical extraction processes. It is also used as a precursor in the production of specialty chemicals such as pesticides, rubber anti-oxidants as well as antibiotics and pharmaceuticals (1). Historically, MIBK has been produced commercially from acetone and hydrogen feedstock in three stages. First, acetone is dimerized to produce diacetone alcohol (DAA). Second, DAA is dehydrated to produce MO and water. Finally, the carbon-carbon double bond of MO is selectively hydrogenated to produce MIBK. These consecutive reactions are outlined in equations (1-3). 

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