Selection of extraction solvents for treating organic wastewater using centrifugal extractors

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The key to using centrifugal extractors for treating organic wastewater lies in properly selecting the extractant suitable for different types of organic wastewater, choosing appropriate methods for recovering the organic substances and regenerating the extraction solvents, as well as selecting a suitable process flow and extraction equipment. We use a centrifugal extractor as an example to illustrate. According to the separation requirements of different systems, a range of industrial extractants have been developed, and new types of extractants are also being continuously created. Extractants are usually organic solvents. Generally speaking, the following requirements should be met for an industrial extractant. (1) Strong extraction capacity and large extraction volume: The extractant can provide a relatively high extraction equilibrium distribution coefficient for the substances to be separated. The extraction capacity of the extractant per unit volume or per unit mass for the substances to be separated is high. The centrifugal separation effect achieved with a centrifugal extractor holds great promise for application. (2) High selectivity: The separation factor β for the several substances to be separated should be relatively large. If the substance to be separated is a single solute, the separation factor of the extractant for that solute and water should be high, thereby reducing the amount of water required by the extractant and lessening the workload associated with its regeneration. Due to the good phase separation performance of the centrifugal extractor, there is no need for a high separation factor β. (3) Strong chemical stability: The extractant is not prone to hydrolysis and does not decompose easily when heated; it can resist the chemical effects of acids, bases, salts, oxidizing agents, and reducing agents, possessing sufficient chemical and radiation stability. Furthermore, the extractant has low corrosivity to the equipment. Today, with the development of centrifugal extractors, corrosion resistance and high-temperature resistance have long been incorporated into modern separation equipment. (4) Low solvent loss: Under various operating conditions, the dissolution loss of the extractant in the aqueous phase of the feed solution is low; it is easy for the extraction phase and the feed phase to separate, no third phase is formed during the extraction process, and emulsification does not occur. (5) Appropriate basic properties of the extractant: The density, viscosity, and interfacial tension of the extractant, among other basic properties, are suitable, ensuring a fast mass transfer rate during extraction and back-extraction, as well as good phase separation and flow properties. (6) Easy back-extraction and solute recovery: The extractant should provide a relatively high extraction equilibrium distribution coefficient for the substances to be separated, while also allowing control over its binding capacity to those substances. This enables back-extraction of the substances to be separated when operating conditions are changed, thus facilitating solute recovery and the reuse of the extractant. For extraction and back-extraction, centrifugal extractors can perform single-stage extraction, multi-stage back-extraction, as well as multi-stage counter-current extraction. (7) Safe operation: The extractant has a high flash point, ignition point, and boiling point, low volatility, is non-toxic or only slightly toxic, and non-irritating, making it easy to use safely. (8) High cost efficiency: The extractants are readily available, their synthesis methods are relatively simple, and they are inexpensive. It should be noted that the conditions for selecting an extractant are difficult to meet simultaneously; generally, it is necessary to take into account these factors based on the actual conditions of industrial applications, leverage the specific advantages of a particular extraction system, and find ways to overcome its shortcomings. For large-scale industrial applications, the efficiency and cost-effectiveness of the extractant are two key criteria for selecting it. For the extractants used in the extraction treatment of industrial organic wastewater, in addition to meeting the requirements for extractants in general industrial extraction processes, special attention must be paid to the loss of the extractant due to dissolution, in order to avoid secondary pollution. Extractants with low toxicity and biodegradability should be chosen to ensure the successful implementation of the extraction treatment process for organic wastewater. The CWL centrifugal extractor developed by our company achieves an extraction efficiency of over 95% for the materials being extracted. Thanks to its sealed operation, the entire extraction and separation process does not cause any secondary pollution to the outside environment. Additionally, it has very low power consumption and generates minimal noise, making it a truly efficient, energy-saving, and environmentally friendly centrifugal extraction device.