Extractant

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Extractants are very important in industry, and choosing the right extractant improves the efficiency of extraction processes; this decision is made based on several factors. 1. Selectivity of the extractant: The selectivity of an extractant refers to the difference in its ability to dissolve component A, the solute, and component B, the original solvent, in the feed solution. If the extractant S has a much greater ability to dissolve solute A than the original solvent B, the amount of extractant required can be reduced, and the quality of the product improves as well. The selectivity of an extractant is usually measured by the selectivity coefficient β. For extraction processes, the larger the β value, the better the separation effect; therefore, an extractant with a β value much greater than 1 should be selected. 2. Physical properties of the extractant: Physical properties such as the density, viscosity, and interfacial tension of the extractant also have an impact on the extraction process. The extractant must ensure a certain density difference between the extract phase and the raffinate phase under operating conditions; after thorough mixing and mass transfer in the extractor, this density difference enables rapid separation of the two liquid phases, thereby increasing the production capacity of the extraction equipment. A low viscosity of the extractant facilitates the mixing and separation of the two phases, as well as flow and mass transfer; therefore, a low viscosity is advantageous for extraction. When the interfacial tension of the extract system is high, small liquid droplets tend to coalesce more easily, which is advantageous for the separation of the two phases. However, if the interfacial tension is too high, the liquid does not disperse easily, making it difficult for the two phases to mix and requiring more external energy. The interfacial tension is low, allowing the liquid to disperse easily; however, this also facilitates emulsification, making it difficult to separate the two phases. Therefore, an appropriate interfacial tension should be selected by considering its effect on both the mixing and separation of the two liquid phases. The interfacial tension of certain systems is shown in the table. The smaller the mutual solubility between the extractant S and the original solvent B, the wider the range of feasible extraction conditions and the better the separation efficiency. For the B,S completely insoluble system, the selectivity coefficient reaches infinity, resulting in the best selectivity. 3. Difficulty of extracting agent recovery: To obtain a pure product and to recycle the extracting agent S, it is necessary to recover the extracting agent S present in the new mixture formed as a result of extraction – namely, in the extract phase and the raffinate phase. Common methods for extracting agent recovery include distillation, evaporation, back-extraction, etc. When the solute being extracted is non-volatile or has a very low volatility, the extractant can be recovered using conventional evaporation or flash distillation methods ； If the relative volatility α of the separated system is close to 1, the temperature of the extraction phase can be lowered to cause the solute to crystallize out, or chemical methods can be used for separation. The ease of extractant recovery directly affects the cost of the extraction process and largely determines its economic viability. Some extractants possess many desirable properties; however, they are not used due to the difficulty in recovering them. 4. Chemical properties of the extractant: The extractant should possess good chemical stability, be resistant to decomposition and polymerization, have sufficient thermal and antioxidant stability, and cause little corrosion to the equipment. 5. Other factors: When selecting an extractant, other factors should also be taken into consideration, such as the extractant being non-toxic or having low toxicity, being non-irritating, being difficult to vaporize, being readily available, being inexpensive, and experiencing minimal loss during recycling. Generally, it is difficult to find an extractant that can meet all of the above requirements simultaneously; therefore, it is necessary to consider the characteristics of the system and the actual production conditions, compare various options, weigh the pros and cons, and select an appropriate solvent. 6. Commonly used extractants in industry There are a wide variety of extractants used in industry, most of which are organic solvents. Common industrial extractants include: alcohols: isopentanol, sec-octanol, substituted primary alcohols, etc ； Ethers: diisopropyl ether, ethylhexyl ether, etc ； Similar ones: Methyl isobutyl ketone, **, etc ； Esters: ethyl acetate, amyl acetate, butyl acetate, etc ； Carboxylic acids: cinnamic acid, fatty acids, lauric acid, naphthenic acid, etc ； Sulfonic acids: dodecylbenzenesulfonic acid, trinonylnaphthalenesulfonic acid, etc ； Phosphates: di(2-ethylhexyl) hexyl phosphate, octyl dioctyl phosphate, tributyl phosphate, etc ； Sulfoxides: dioctyl sulfoxide, diphenyl sulfoxide, hydrocarbon sulfoxides, etc ； Organic amines: trialkylmethylamine, didecylamine, trioctylamine, tri-nonylamine, etc. Image