Leaching countercurrent, continuous

Since the 1960s the commercial development of continuous countercurrent processes has been almost entirely accompHshed by using a flow scheme that simulates the continuous countercurrent flow of adsorbent and process Hquid without the actual movement of the adsorbent. The idea of a simulated moving bed (SMB) can be traced back to the Shanks system for leaching soda ash (58). 

Solvent Extraction. Solvent extraction has widespread appHcation for uranium recovery from ores. In contrast to ion exchange, which is a batch process, solvent extraction can be operated in a continuous countercurrent-fiow manner. However, solvent extraction has a large disadvantage, owing to incomplete phase separation because of solubihty and the formation of emulsions. These effects, as well as solvent losses, result in financial losses and a potential pollution problem inherent in the disposal of spent leach solutions. For leach solutions with a concentration greater than 1 g U/L, solvent extraction is preferred. For low grade solutions with <1 g U/L and carbonate leach solutions, ion exchange is preferred (23). Solvent extraction has not proven economically useful for carbonate solutions. 

By comparing this simple diagram with the conventional continuous countercurrent decantation circuit, shown in Fig. lb (Coulson and Richardson, 1968, 1978 Treybal, 1955, 1968, 1980) for carrying out the same duty, it is easy to deduce the following characteristics for fluidized leaching and washing ... 

The variety of extractors used in liquid-solid extraction is diverse, ranging from batchwise dump or heap leaching for the extraction of low grade ores to continuous countercurrent extractors to extract materials such as oilseeds and sugar beets where problems of solids transport have dominated equipment and development. 

By comparing this simple diagram with the conventional continuous countercurrent decantation circuit, it is easy to deduce the following characteristics for fluidized leaching and washing ... 

The uranium industry more than any other founded the commercial realization of Continuous Countercurrent Ion Exchange (CIX) technology which has resulted in being able to treat unclarified leach liquors in a near ideal continuous manner. Several modern CIX plants in the uranium industry are based on the successful Multistage... 

Because of its importance, only the continuous countercurrent method is discussed here. Also, since the stage method is normally used, the differential-contact method is not considered. In common with other stage cascade operations, leaching may be considered, first, from the standpoint of ideal stages and, second, from that of stage efficiencies. 

If, as is usual, several contact stages are required, a train of mixer-settlers is operated with countercurrent flow, as shown in Fig. 20.5. The raffinate from each settler becomes the feed to the next mixer, where it meets intermediate extract or fresh solvent. The principle is identical with that of the continuous countercurrent stage leaching system shown in Fig. 17.3. 

Marked axial dispersion in both the liquid and solid phases has been observed in continuous countercurrent leaching systems. The solid-phase dispersion is probably caused by nonuniform conveying and by backmixing caused by the baffles which are used to prevent solid beds from turning en masse. VnliD n values of 16.1 m and 20 m, respectively, have b n repotted for sugar beet extraction in tower and slope extractors. Local fiow nonuniformity and larger-scale flow maldistribution are the primary factors that cause axial dispersion in the extract. 

R as a parameter for spherical particles in a continuous countercurrent leaching qrstem. 

Finite Bi heat transfer solutions corresponding to the infinite Bi mass transfer solutions listed for continuous countercurrent leaching are available. In these solutions a/, corresponds to a, to r, 7 s to T or y/M, and Tx to X at corresponding locations in the leaching system. When either Bi or Bi is finite the equations used to predict g, and Q are more complex than thi listed in Tables 10.8-1, 10.13-1, and 10.13-2. For example, for spherical particles when a > 1.0 (the usual case) and Bi is finite. 

EX A MPLE 12.10-1. Countercurrent Leaching of Oilfrom Meal A continuous countercurrent multistage system is to be used to leach oil from meal by benzene solvent (B3). The process is to treat 2000 kg/h of inert solid meal (B) containing 800 kg oil (/4) and also 50 kg benzene (C). The inlet flow per hour of fresh solvent mixture contains 1310 kg benzene and 20 kg oil. The leached solids are to contain 120 kg oil. Settling experiments similar to those in the actual extractor show that the solution retained depends upon the concentration of oil in the solution. The data (B3) are tabulated below as N kg inert solid B/kg solution and kg oil zl/kg solution. 

Example 13-6 Benzene is used to leach oil from meal in a continuous countercurrent unit. The system is to process 909 kg of solids per hour (based on completely exhausted solids). Meal fed to the train has 22.7 kg of benzene and 363.6 kg of oil. Solvent mixture fed to the system is made up of 595.5 kg of benzene and 9.09 kg of oil. Solids leaving the unit will have 54.5 kg of oil. 

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