Extraction, also known as solvent extraction or liquid-liquid extraction, also known as extraction, is a unit operation that uses the different solubilities of the components in the system in the solvent to separate the mixture. That is, it is a method of transferring solute substances from one solvent to another by taking advantage of the difference in solubility or distribution coefficient of substances in two mutually immiscible (or slightly soluble) solvents. It is widely used in chemical, metallurgy, food and other industries, and is generally used in the petroleum refining industry. In addition, the operation of separating two immiscible liquids after extraction is called liquid separation.
Solid-liquid extraction, also called leaching, uses solvents to separate components in a solid mixture, such as leaching sugars in beets with water; leaching soybean oil in soybeans with alcohol to increase oil yield; Leaching active ingredients from traditional Chinese medicine with water to prepare liquid extract is called "leaking" or "leaching".
Although extraction is often used in chemical experiments, its operation process does not cause changes in the chemical composition of the extracted substances (or chemical reactions), so the extraction operation is a physical process.
Extraction is one of the methods used in organic chemistry laboratories to purify and purify compounds. Through extraction, the desired substance can be extracted from a solid or liquid mixture. Basic introduction Chinese name: Extraction Foreign name: Extraction Also known as: Solvent extraction Also known as: Extraction Principle: Different applications of solubility: Chemistry, metallurgy, food, medical principles, evolution, methods, application, the principle uses substances in two mutually exclusive The difference in solubility or partition coefficient in miscible (or slightly soluble) solvents allows substances to be transferred from one solvent to another. After repeated extractions, most of the compounds are extracted. Principle diagram The solvent extraction process generally consists of extraction, washing and back-extraction. Generally, the process of extracting solutes from the aqueous phase by the organic phase is called extraction, the process of removing other solutes or inclusions in the aqueous phase from the loaded organic phase is called scrubbing, and the process of analyzing the solutes in the organic phase by the aqueous phase is called scrubbing. Stripping. The distribution law is the main basis for the theory of extraction methods. Substances have different solubilities in different solvents. At the same time, when a certain soluble substance is added to two mutually immiscible solvents, it can be dissolved in the two solvents respectively. Experiments have proved that at a certain temperature, the compound does not decompose with the two solvents. During electrolysis, association and solvation, the ratio of this compound in the two liquid layers is a constant value. This is true regardless of the amount of substance added. It is a physical change. Express it with a formula. CA/CB=K CA.CB respectively represent the concentration of a substance in two mutually immiscible solvents. K is a constant called the "partition coefficient". Organic compounds are generally more soluble in organic solvents than in water. The use of organic solvents to extract compounds dissolved in water is a typical example of extraction. During extraction, if a certain amount of electrolyte (such as sodium chloride) is added to the aqueous solution and the "salting out effect" is used to reduce the solubility of organic matter and extraction solvent in the aqueous solution, the extraction effect can often be improved. Separating funnel To completely extract the required solute from the solution, usually one extraction is not enough and the extraction must be repeated several times. Using the relationship of the distribution law, the remaining amount of the compound after extraction can be calculated. Suppose: V is the volume of the original solution w0 is the total amount of the compound before extraction w1 is the remaining amount of the compound after one extraction w2 is the remaining amount of the compound after two extractions wn is the remaining amount of the compound after n extractions S is the extraction solution After the volume is extracted once, the concentration of the compound in the original solution is w1/V; while the concentration of the compound in the extraction solvent is (w0-w1)/S; the ratio between the two is equal to K, that is: w1/V =K w1= w0 KV (w0-w1)/S KV+S In the same way, after secondary extraction, w2/V =K is (w1-w2)/S w2=w1 KV =w0 KV KV+S KV+S Therefore , after n extractions: wn=w0 (KV) KV+S When using a certain amount of solvent, it is hoped that the remaining amount in the water will be as small as possible. The above formula KV/(KV+S) is always less than 1, so the larger n is, the smaller wn is. In other words, it is better to divide the solvent into several times for multiple extractions than to use the entire amount of solvent for one extraction.
However, it should be noted that the above formula applies to solvents that are almost immiscible with water, such as benzene, carbon tetrachloride, etc. For solvents such as ether that are miscible with water in a small amount, the above formula is only approximate. However, it is possible to indicate qualitatively the expected results. Instrument: Separating funnel Use carbon tetrachloride to extract iodine in iodine water. Common extraction agents: toluene, dichloromethane, chloroform, gasoline, ether, straight-run gasoline, n-butanol, carbon tetrachloride. Requirements: The extraction agent and the original solvent are immiscible with each other. The extractant and solute do not react with each other. The solubility of the solute in the extraction agent is much greater than the solubility in the original solvent. Related rules: Organic solvents are easily soluble in organic solvents, polar solvents are easily soluble in polar solvents, and vice versa. History 1842 E.-M. Pelliro studied the extraction of uranyl nitrate from nitric acid solution with ether. In 1903, L. Ediranu used liquid sulfur dioxide to extract aromatic hydrocarbons from kerosene. This was the first industrial application of extraction. In the late 1940s, the need to produce nuclear fuel spurred research and development on extraction. Today's extraction is commonly used in the petroleum refining industry and is widely used in chemical, metallurgical, food and atomic energy industries. For example, extraction has been applied to the separation and refining of petroleum fractions, the extraction and purification of uranium, thorium, and plutonium, the extraction and separation of non-ferrous metals, rare metals, and precious metals, the extraction of antibiotics, organic acids, and alkaloids, and wastewater treatment, etc. . Method: Add an extractant that is not mutually soluble (at most partially soluble) into the solution to be separated (feed liquid) to form two remaining liquid phases. The difference in the solubility of each component (including dissolution after chemical reaction) of the original solvent and the extraction agent is used to make them unequally distributed in the two liquid phases, and then the separation of the components is achieved through the separation of the two liquid phases. . For example, if an aqueous solution of iodine is extracted with carbon tetrachloride, almost all the iodine will move to the carbon tetrachloride, and the iodine can be separated from a large amount of water. The most basic operation is single-stage extraction. It brings the feed liquid and the extraction agent into close contact during the mixing process, allowing the extracted components to enter the extraction agent through the phase interface until the distribution of the components between the two phases basically reaches equilibrium. Then it is left to settle and separated into two layers of liquid, namely the extraction liquid converted from the extraction agent and the raffinate liquid converted from the feed liquid. When the single-stage extraction reaches phase equilibrium, the phase equilibrium ratio of the extracted component B is called the distribution coefficient K, that is: K = y B/ x B where y B and x B are the sum of component B in the extraction solution respectively. Concentration in the raffinate. The expression method of concentration needs to consider the various forms of existence of the components and calculate according to the same chemical formula. If another component D in the feed liquid is also extracted, the ratio of the distribution coefficient of component B to the distribution coefficient of component D, that is, the separation factor of B versus D, is called the selectivity coefficient α, that is: α = K B ·K D= y B· Separation of two components. The extraction rate that single-stage extraction can achieve for a given component (the ratio of the amount of the extracted component in the extraction liquid to the initial amount in the raw material liquid) is low and often cannot meet the process requirements. In order to improve the extraction rate, you can A variety of methods are used: ① Multi-stage cross-flow extraction. The feed liquid and raffinate at all levels are in contact with fresh extraction agent, which can achieve a higher extraction rate. However, the amount of extraction agent is large and the average concentration of the extraction solution is low. ②Multi-stage countercurrent extraction. The feed liquid and extraction agent are added from both ends of the cascade (or plate tower) respectively, and flow countercurrently between stages. Finally, they become raffinate and extraction liquid, and each leave from the other end. The feed liquid and the extraction agent have been extracted multiple times each, so the extraction rate is high and the concentration of the extracted components in the extraction liquid is also high. This is a commonly used process in industrial extraction. ③Continuous countercurrent extraction. In the differential contact extraction tower (see extraction equipment), the feed liquid and the extraction agent are in contact and mass transfer during the counterflow process, which is also a commonly used industrial extraction method. Among the feed liquid and extraction agent, the one with higher density is called heavy phase, and the one with lower density is called light phase. The light phase enters from the bottom of the tower and overflows from the top; the heavy phase joins from the top of the tower and exits from the bottom of the tower. When the extraction tower is operating, one liquid phase that fills the entire tower is called the continuous phase; the other liquid phase is usually dispersed in it in the form of droplets, which is called the dispersed phase. The dispersed phase liquid is dispersed when it enters the tower, condenses and stratifies before leaving the tower and is exported. Which of the feed liquid and extraction agent is the dispersed phase must be selected taking into account the operation and process requirements of the tower. In addition, there are reflux extractions and partial extractions that can achieve higher degrees of separation.
Compared with other methods of separating solution components, the advantages of extraction are that it operates at room temperature, saves energy, does not involve solids or gases, and is easy to operate. It is usually advantageous to apply extraction in the following situations: ① The boiling points of the components of the feed liquid are similar, and even form super boiling substances, which is not easy for distillation to work, such as the separation of alkanes and aromatic hydrocarbons in petroleum fractions , Dephenolization of coal tar; ② The separation of low-concentration and high-boiling components requires a lot of energy by distillation, such as the dehydration of dilute acetic acid; ③ The separation of multiple ions, such as the separation and purification of mineral leaching liquid, if Adding chemicals for partial precipitation not only results in poor separation quality, but also involves filtration operations and high losses; ④ Separation of unstable substances (such as heat-sensitive substances), such as preparing penicillin from fermentation broth. The application of multi-stage liquid-liquid extractor extraction is still under development. Most of the elements in the periodic table can be extracted and separated by extraction methods. The selection and development of extraction agents, determination of process and operating conditions, and design calculations of processes and equipment are all topics in developing extraction operations. For example, extraction experiment: mix iodine water with carbon tetrachloride or benzene, shake well, and then distill iodine crystals.