Solvent extraction, also known as liquid-liquid extraction, is an effective method of separating and enriching useful substances from solution. It uses the different distribution of solutes between two immiscible liquids to achieve the purpose of separation and enrichment. Of course, extraction can also be carried out between Solid-liquid-liquid three phases. This is the case with slurry extraction.Solvent extraction of uranium nitrate (UO 2 (NO3) 2) from nitric acid solution began in the early 1940s with ether. By the 20th century, the world's first solvent extraction plant was built and put into operation in 1942. In 1945, tributyl phosphate (TBP) was developed to extract uranium. Since the mid-1960s, especially in the 1970s. In the 1990s, solvent extraction technology has been further developed.
There are many kinds of solvent extraction plant. The solvent extraction equipment commonly used in hydrometallurgy includes mixing clarifier, extraction tower and centrifugal extractor.
The mixing clarifier is composed of a continuous mixing chamber with feeding and stirring and a mixing chamber with liquid phase separation by gravity. The water phase and organic phase are stirred and contacted in the mixing chamber. When the mass transfer process is close to equilibrium, the mixing phase enters a large area of the clarifying chamber for two-phase separation. Mixed clarifier is widely used in the separation and purification of metals in hydrometallurgical solvent extraction process.Extraction column has many advantages, such as large capacity, high efficiency and low operation cost. It has been widely used in hydrometallurgy. For example, the selective extraction of cobalt from ammonia leaching solution of high-grade nickel ore and nickel from acid leaching solution containing calcium and magnesium have achieved good results, so the extraction tower is also a common solvent extraction equipment in hydrometallurgy.
The extraction tank (mixing clarification tank) consists of a mixing chamber with continuous feeding and stirring and a clarification chamber with liquid phase separation by gravity. The water and organic phases are stirred and contacted in the mixing chamber. When the mass transfer process is close to equilibrium, the mixed phase enters a large area of clarification chamber for two-phase separation. Multi-stage series connection is often used to achieve deep separation. At a certain stage, it is separated into two liquid layers by sedimentation, i. e. extraction phase and extraction residue phase. The dense phase enters the next mixing chamber from the dense phase outlet. The light phase flows from the outlet of the light phase to the former mixing chamber, so it belongs to the operation mode of multi-stage continuous countercurrent. Repeated mixing and clarification can make the solute highly enriched and purified.
The coagulation rate is highly dependent on the viscosity, density and interfacial tension of the liquid and the size of the droplets in the dispersed phase. When the emulsified phase overflows from the light phase weir or flows away from the heavy phase weir, the extraction tank (mixed clarification tank) achieves its maximum flow rate.The simplest mixing and clarifying tank is box mixing and clarifying tank. A square box is divided into several intervals (stages) at equal distances with partitions. Each stage has a mixing chamber and a clarifying chamber, and the mixing chamber has a mixer for two-phase mixing contact. The mixing chamber and the clarifying chamber of the same stage are connected by the mixing phase outlet. Mixing rooms and clarifying rooms at all levels are interlaced. The clarification chamber is connected with the front and rear mixing chambers by light overflow and heavy overflow respectively. The two-phase reverse flow is realized by the driving force generated by the density difference between the two phases at all levels. As shown in the figure, the multi-stage extraction tank is in series.
The application of centrifugal extractor has good prospects when hydrometallurgy adopts extraction technology, especially when there are many series or nonequilibrium extraction can be used. Of course, the solvent extraction equipment commonly used in hydrometallurgy can not be separated from centrifugal extraction plant.
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