Solvent extraction contacting equipment

Extraction (sometimes called leaching) encompasses liquid-liquid as well as liquid-solid systems. Liquid-liquid extraction involves the transfer of solutes from one liquid phase into another liquid solvent it is normally conducted in mixer settlers, plate and agitated-tower contacting equipment, or packed or spray towers. Liquid-solid extraction, in which a liquid solvent is passed over a solid phase to remove some solute, is carried out in fixed-bed, moving-bed, or agitated-solid columns. 

Solubilizing all or part of a sample matrix by contacting with liquids is one of the most widely used sample preparation techniques for gases, vapors, liquids or solids. Additional selectivity is possible by distributing the sample between pairs of immiscible liquids in which the analyte and its matrix have different solubilities. Equipment requirements are generally very simple for solvent extraction techniques. Table 8.2 [4,10], and solutions are easy to manipulate, convenient to inject into chromatographic instruments, and even small volumes of liquids can be measured accurately. Solids can be recovered from volatile solvents by evaporation. Since relatively large solvent volumes are used in most extraction procedures, solvent impurities, contaminants, etc., are always a common cause for concern [65,66]. 

Like gas absorption, liquid-liquid extraction separates a homogeneous mixture by the addition of another phase - in this case, an immiscible liquid. Liquid-liquid extraction carries out separation by contacting a liquid feed with another immiscible liquid. The equipment used for liquid-liquid extraction is the same as that used for the liquid-liquid reactions illustrated in Figure 7.4. The separation occurs as a result of components in the feed distributing themselves differently between the two liquid phases. The liquid with which the feed is contacted is known as the solvent. The solvent extracts solute from the feed. The solvent-rich stream obtained from the separation is known as the extract and the residual feed from which the solute has been extracted is known as the raffinate. 

Because of the diversity of contacting equipment available, it is unlikely that all these contactors will be available in any one laboratory or pilot plant. Consequently, unless test work is carried out on similar contactors, the system may not be optimized. Since mixer-settlers are the easiest to construct, are simple to operate, and require little room and low-flow rates, these contactors are, in many cases, the only ones used to investigate a continuous solvent extraction process. This is by no means ideal and may result in abandonment of a process that, using another type of contactor, could be found to be entirely satisfactory. 

There followed a brief discussion of equipment for carrying out solvent extraction in industrial practice, both by stagewise and differential contact. Some of the first principles for the design of differential contactors were outlined and the part played by the efficiency of extraction in continuous equipment was discussed. Finally there was an outline of methods for the control of solvent loss which forms probably the most important environmental aspect of the application of solvent extraction. 

Countercurrent extraction (Fig. 5b) is more effective than crosscurrent extraction because all the fresh solvent is contacted with the weakest raffinate. This arrangement is almost universally used in industrial extraction equipment. 

Contacting equipment used in the extracting section must have low holdup to minimize solvent degradation from the intense fission-product radioactivity. Here, centrifugal contactors or pulse columns are preferred to mixer-settlers. In the scrubbing section and in the balance of the solvent extraction plant, mixer-settlers are often used. 

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