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Overview of extraction: Extraction is a unit operation used to separate liquid-liquid mixtures by taking advantage of the differences in the solubility of various components within a mixture in a certain solvent. The extraction process consists of the following three steps: (1) Mixing process: The feed solution and solvent come into full contact, resulting in varying degrees of interphase transfer among the various components, thereby enabling mass transfer. (2) Clarification process: The dispersed droplets coalesce, and the resulting two-phase extraction phase and raffinate phase separate due to differences in density. (3) Solvent removal process: The extract phase is subjected to solvent removal to obtain the extract, while the raffinate phase is also subjected to solvent removal to yield the raffinate; distillation is commonly used for this solvent removal process. Changes in the composition of components A and B in the extract phase and the raffinate phase obtained through extraction: Changes in the composition of components A and B in the extract and the raffinate resulting from the extraction process: The extraction operation can separate liquid mixtures to a certain extent. Application scope of extraction operations: (1) When the volatility differences among the components in a liquid mixture are very small, that is, their relative volatilities are close to 1, distillation is not an economical choice. (2) Azeotropes are formed during the distillation of liquid mixtures. (3) The substance to be recovered is a heat-sensitive material, or it tends to decompose, polymerize, or undergo other changes during distillation. (4) If the liquid mixture contains a large amount of volatile components with high vaporization latent heat, and especially if such components are not the target components, distillation processes require significant energy consumption. Extraction operations are widely used in industry, especially in the petrochemical industry. It plays an important role in the pharmaceutical industry, food industry, hydrometallurgical industry, nuclear industry for material extraction, and environmental protection and pollution control. Basic principle: It takes advantage of the differences in solubility or distribution coefficients of a compound in two immiscible (or slightly soluble) solvents to transfer the compound from one solvent to another. After repeated extractions, the vast majority of the compounds were extracted. The distribution law is the fundamental basis of the theory of extraction methods; substances have different solubilities in various solvents. Meanwhile, when a soluble substance is added to two immiscible solvents, it can dissolve in each of these solvents. Experiments have shown that, at a certain temperature, when this compound does not undergo decomposition, electrolysis, association, or solvation in reaction with these two solvents, the ratio of this compound in the two liquid layers remains constant. This is true regardless of the amount of substance added. Express it in terms of formulas. CA/CB=K CA and CB represent the concentration of a compound in two immiscible solvents, respectively. K is a constant, known as the “distribution coefficient”. Organic compounds generally have higher solubility in organic solvents than in water. Extracting water-soluble compounds using organic solvents is a typical example of extraction. During extraction, adding a certain amount of electrolyte (such as sodium chloride) to the aqueous solution takes advantage of the \"salting-out effect\" to reduce the solubility of organic substances and the extraction solvent in the aqueous solution, which often improves the efficiency of extraction. To completely extract the desired compound from the solution, one extraction is usually not sufficient; several extractions must be repeated. Using the relationship of the distribution law, it is possible to calculate the remaining amount of the compound after extraction. Let: V be the volume of the original solution; w0 be the total amount of the compound before extraction; w1 be the remaining amount of the compound after one extraction; w2 be the remaining amount after two extractions; w3 be the remaining amount after n extractions. S is the volume of the extraction solution. After one extraction, the concentration of the compound in the original solution is w1/V ； And the concentration of this compound in the extraction solvent is (w0-w1)/S ； The ratio of the two is equal to K, that is: w1/V = K → w1 = w0 KV and (w0 – w1)/S = KV + S. Similarly, after two extractions, we have w2/V = K, which means (w1 – w2)/S = KV + S and w2 = w1 KV = w0 KV. Therefore, after n extractions: wn = w0 (KV)^n / (KV + S). When a certain amount of solvent is used, it is desirable for as little residue to remain in water as possible. And the expression KV/(KV+S) is always less than 1; therefore, the larger n is, the smaller wn becomes. In other words, it is better to divide the solvent into several portions for multiple extractions rather than using the entire amount of solvent in a single extraction. It should be noted, however, that the formula above applies to solvents that are almost immiscible with water, such as benzene and carbon tetrachloride. For solvents such as diethyl ether, which are only slightly miscible with water, the formula above is merely approximate. However, it is still possible to qualitatively indicate the expected results. Extraction can be divided into the following types: 1. Two-aqueous phase extraction. Two-aqueous phase extraction (abbreviated as ATPS) refers to the phenomenon where an aqueous solution of a hydrophilic polymer can form two aqueous phases under certain conditions; separation can be achieved due to the different distribution of the substances to be separated between these two phases. This technique is widely used for the separation and extraction of compounds in fields such as biochemistry, cell biology, and bioprocessing. ATPS requires little investment in equipment and is simple to operate. The two-aqueous phase systems used in this technique are mostly based on polyethylene glycol-glucose or polyethylene glycol-inorganic salts. Since water-soluble polymers are difficult to evaporate, back-extraction becomes necessary, and the presence of salts in the back-extraction agent has a significant impact on subsequent analysis. Additionally, most water-soluble polymers have high viscosity, which makes quantitative operations difficult and poses challenges for further research. In fact, ordinary organic solvents that are miscible with water can also form two-aqueous phase systems in the presence of inorganic salts, and this approach has been used for the analysis of copper in serum and chromium in plasma. Two-aqueous phase extraction systems based on water-miscible organic solvents and saline phases offer advantages such as low cost, low toxicity, ease of evaporation, elimination of the need for back-extraction, and avoidance of the use of viscous water-soluble polymers. II. Organic solvent extraction: The water washing and separation method involves using water to remove water-soluble impurities from the organic phase, thereby purifying the organic phase. Organic solvent extraction is what is commonly referred to as extraction; it is a method in which organic solvents are used to separate the components that are soluble in those solvents from the aqueous phase, solid phase (or other phases that are insoluble in the solvent). For the theoretical part, see Afeastforeye. In typical extraction experiments, the organic phase obtained after extraction (containing the desired compound) is further purified by washing it with water or saturated saline solution. Both methods require a separatory funnel, and the procedure is essentially the same; one only needs to determine which layer (phase) should be retained. III. Supercritical extraction: The extractant used in supercritical extraction is a supercritical fluid, which is a state of matter that lies between gas and liquid – it is neither gaseous nor liquid. Such a substance can exist only when its temperature and pressure exceed the critical points. Supercritical fluids have a high density, similar to that of liquids, while their viscosity is closer to that of gases. Therefore, supercritical fluids are an extremely ideal extractant. The solvating power of supercritical fluids depends on the temperature and pressure of extraction. Taking advantage of this property, by simply changing the pressure and temperature of the extractant fluid, different components in the sample can be extracted one after another based on their solubility in the fluid. At low pressures, less polar substances are extracted first; as the pressure increases, more polar substances and those with higher molecular weights are extracted. Thus, supercritical extraction at progressively increasing pressures enables the extraction of different components, while also serving a separation function. Temperature changes affect the density of the extractant and the vapor pressure of the solute. In the low-temperature range (still above the critical temperature), an increase in temperature reduces the density of the fluid, while the vapor pressure of the solute increases only slightly. Therefore, raising the temperature enhances the solubility of the solute, allowing it to be extracted from the fluid-extractant mixture. When the temperature rises further into the high-temperature range, although the density of the extractant decreases further, the vapor pressure of the solute increases, resulting in higher volatility; as a result, the extraction efficiency not only does not decrease but actually tends to increase. In addition to pressure and temperature, adding a small amount of other solvents to supercritical fluids can also alter their ability to dissolve solutes. Its mechanism of action is not yet fully understood. The addition amount is usually no more than 10%, with polar solvents such as methanol and isopropanol being the most common. Adding a small amount of polar solvent can further expand the applicability of supercritical extraction technology to compounds with higher polarity. Introduction to the supercritical fluid extraction process: The material to be extracted is placed into the extraction vessel. Carbon dioxide is used as the supercritical solvent. Carbon dioxide gas is condensed into a liquid in a heat exchanger; a pressure pump is then used to raise the pressure to that required for the process (which must be higher than the critical pressure of carbon dioxide), while the temperature is adjusted to turn it into a supercritical carbon dioxide fluid. The carbon dioxide fluid enters from the bottom of the extraction tank as a solvent, comes into full contact with the material to be extracted, and selectively dissolves the desired chemical components. The high-pressure carbon dioxide fluid containing dissolved extract is depressurized via a throttle valve to a pressure below the critical pressure of carbon dioxide, and then enters the separation tank (also known as a stripping tank). Due to the sharp decrease in carbon dioxide solubility, the solute precipitates, resulting in automatic separation into two components: the solute, which is the product of the process and is removed periodically from the bottom of the separation tank, and carbon dioxide gas, which is recycled after being condensed into liquid carbon dioxide in a heat exchanger. The entire separation process takes advantage of the fact that carbon dioxide fluid has a significantly increased solubility for organic compounds in its supercritical state, whereas it is practically insoluble in organic compounds at temperatures below the critical point. By continuously circulating this carbon dioxide fluid between the extraction tank and the separation tank, it is possible to effectively separate the components that need to be extracted from the raw material. IV. Liquid membrane extraction is a new extraction technique. With water as the continuous phase, droplets encapsulating an aqueous core surrounded by surfactants and an organic phase are dispersed to form an emulsion. In the external aqueous phase, certain components are extracted by the organic phase outside the droplet and enter the aqueous phase inside the droplet, thereby achieving extraction separation. Since the diameter of the droplets is only a few micrometers, the specific surface area of the liquid film is large. Moreover, the component to be extracted rapidly transfers from the organic phase into the inner aqueous phase, resulting in a high mass transfer driving force. The mass transfer is not restricted by the equilibrium concentration between the outer aqueous phase and the organic phase, hence the extraction efficiency is very high. The technical challenge is demulsification. Currently, demulsification under high-voltage electrostatic fields is the most effective method. It can be used in metal ion separation, biological product separation, and wastewater treatment, among other applications. V. Solid-phase extraction Solid-phase extraction is an important application of chromatography. In this method, a certain volume of the sample solution is passed through a column containing a solid adsorbent; the components in the sample that have a strong interaction with the adsorbent are completely adsorbed ； Then, the adsorbed components are eluted using a strong eluting solvent, and the volume is adjusted to yield a small volume of the sample solution to be analyzed. Using solid-phase extraction, the components in the sample can be concentrated, and at the same time, components that interfere with the components of interest can be removed preliminarily, thereby improving the sensitivity of the analysis. Solid-phase extraction can be used not only for sample pretreatment in chromatographic analysis, but also for sample pretreatment in various analytical methods such as infrared spectroscopy, mass spectrometry, nuclear magnetic resonance, ultraviolet spectroscopy, and atomic absorption spectroscopy. C18 solid-phase extraction cartridges have hydrophobic properties and can adsorb non-polar components; therefore, they can extract polycyclic aromatic hydrocarbons from water, thereby concentrating the sample. There are other types of solid-phase extraction cartridges, such as polar and ion exchange types. VI. Liquid-Solid Extraction: The method of liquid-solid extraction (LSE), which uses columns filled with fine-particle adsorbents, has quickly surpassed the liquid-liquid extraction method; it has established its own significance in terms of simplifying the sample matrix and enriching trace amounts of samples. Liquid-liquid extraction has such problems: it is labor-intensive ； Frequently troubled by practical issues such as emulsification ； There is a tendency to consume large amounts of high-purity solvents, which often pose hazards to the health of operators and the environment ； It incurs additional costs at the time of emission. Liquid-solid extraction has the advantages of being inexpensive, time-saving, and having simple solvent consumption and processing steps. The liquid-solid extraction step can be easily carried out using specialized process unit sets to automatically extract samples simultaneously in multiple channels and prepare the samples for automatic injection ； Or use a centrifugal analyzer to process large numbers of samples in batches, thereby increasing the sample throughput and reducing labor costs. Liquid-solid extraction is convenient for field sampling, as it eliminates the need to send large amounts of samples to the laboratory for processing, thereby minimizing problems related to sample transportation and storage. Liquid-solid extraction does have its problems, but these problems are different from those encountered in liquid-liquid extraction; the two techniques can be considered complementary to each other.