Countercurrent Multistage Leaching

SCFs may be used in the same way as other ordinary solvents taking into account their different properties and behaviors. Supercritical fluids can replace liquids solvents in many processes, such as extractions from solids (leaching), countercurrent multistage separations, chromatographic separations, and others, provided the solvent properties of the SCFs are adequate. 

Countercurrent Multistage Leaching. Countercurrent extraction offers the most economical use of solvent, permitting high concentrations in the hnal extract and high recovery from the initial solid hul utilizing the least amount of solvent. When the amount of solvent removed with the insoluble solid in the underflow is constant, it is convenient to define the ratio... 

Fiji. 2- Flciw diagram for countercurrent multistage leaching... 

To illustrate the principle of an equilibrium-stage cascade, two typical countercurrent multistage devices, one for distillation and one for leaching, are described here. Other types of mass-transfer equipment are discussed in later chapters. 

A process flow sheet for countercurrent multistage leaching is shown in Fig. 12.10-1 and is similar to Fig. 12.7-1 for liquid-liquid extraction. The ideal stages are numbered in the direction of the solids or underflow stream. The solvent (C)-solute (A) phase or V phase is the liquid phase that overflows continuously from stage to stage countercurrently to the solid phase, and it dissolves solute as it moves along. The slurry phase L composed of inert solids (B) and a liquid phase of A and C is the continuous underflow from each stage. Note that the composition of the V phase is denoted by x and the composition of the L phase by y, which is the reverse of that for liquid-liquid extraction. 

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. 

Countercurrent Multistage Leaching of Halibut Livers. Fresh halibut livers containing 25.7 wt % oil are to be extracted with pure ethyl ether to remove 95% of the oil in a countercurrent multistage leaching process. The feed rate is 1000 kg of fresh livers per hour. The final exit overflow solution is to contain 70 wt % oil. The retention of solution by the inert solids (oil-free liver) of the liver varies as follows (Cl), where N is kg inert solid/kg solution retained and is kg oil/kg solution. 

Countercurrent Leaching of Flaked Soybeans. Soybean flakes containing 22 wt % oil are to be leached in a countercurrent multistage process to contain 0.8 kg oil/100 kg inert solid using fresh and pure hexane solvent. For every 1000 kg soybeans, 1000 kg hexane is used. Experiments (SI) give the following retention of solution with the solids in the underflow, where N is kg inert solid/kg solution retained and y is wt fraction of oil in solution. 

In leaching processes, finely divided solids are contacted with solvents to remove soluble constituents. Usually some kind of multistage and countercurrent operation is desirable. The most bothersome aspect is handling of the wet solids. 

Needless to say, kinetic parameters established in this way are empirical factors dependent on the definition of other variables,e.g., averaging of particle size distribution to determine R. Nevertheless, the model provides a rational approach to the complex physical and chemical phenomena of multistage, multireaction leaching. It is intended to expand the model to include countercurrent operations. 

FIGURE 9.6 Stage diagram for multistage countercurrent leaching (Example 9.2). 

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... 

Ion exchange and extraction with organic solvents have proved effective in separating uranium from the leaching solutions. Combinations of the ion exchange proce.ss with solvent extraction are also known. This is normally preceded by separation of the leaching solution from solids by multistage filtration or countercurrent decantation followed by clarification e.g. over a bed of sand. 

M. Desai, "Analysis of Multistage Nonequilibrium Countercurrent Leaching System," Ph.D. thesis. University of Massachusetts, Amherst, 1977. 

Figure 12.10-2. N umber of stages for multistage countercurrent leaching.

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