Industrial Leaching Equipment

FIGURE 10.3-1 Six-cell diflusion battery. (Reproduced by permission, Chemical Engineering Progress, American Institute of Chemical Engineers.) 

FIGURE 10.3-2 Extract feed and dischaige and solids loading and dischaige sequences for diffusion battery. 

Crossflow extractors are used to extract many different solutes and can handle a wide variety of solid feeds. Some crossflow extractors are quite large, for example, 11m wide and 52 m long and can handle 10,(XX) tons of solid feed per day. They may contain up to 18 stages. The solid beds are usually between 0.5 and 3.0 m deep, although in the Filtrex extactor 0.05 m deep beds are used. 

In the French Oil Machinery Co. stationary basket extractor (Fig. 10.3-5), extract percolates through beds of solids contained in a circular array of sector-shaped compartments with perforated bottoms and drains into sumps positioned below the beds. Unlike crossflow extractors, the solids do not move. Instead, the solid feed spout and solids discharge zone rotate about the circle and the extract feed and discharge connections are switched periodically. These extractors are like automated diffusion batteries in which downflow is used but, because extract backmixes in each sump, the extract concentration leaving a stage is somewhat different from that entering the next stage. 

T=EXTRACT TRANSFER VALVE D EXTRACT DRAWOFF VALVE F= FEE0 WATER VALVE 

Extract feed and discharge and solids loading and discharge sequences for diffusion 

Difliision batteries are used for extracting soluble coffee, soluble tea, spices, pickling salts, and corn steep solids and fonnerty were used in very large numbers for extracting beet sugar. Coffee or spice extraction batteries usually contain four to ei columns, typically six beet sugar batteries contained 10-16 cells, typically 14. Coffee extraction columns usually have diameters ranging between 0.2S and 0.75 m and are 4.S-6.0 m tall. 

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The equipment used to contact the solids with the solvent is usually a special designs to suit the type of solid being processed, and is to an extent unique to the particular industry. General details of leaching equipment are given in Volume 2, Chapter 10 and in Perry et al. (1997). 

Many industrial processes begin with a leaching step, yielding a slurry that must be clarified before solvent extraction. The solid-liquid separation is a costly step. The solvent extraction of unclarified liquids ( solvent-in-pulp ) has been proposed to eliminate solid-liquid separation. The increased revenue and reduced energy cost make this an attractive process, but many problems remain to be solved loss of metals and extractants to the solid phase, optimization of equipment design, effluent disposal, etc. 

Equipment for extraction and leaching must be capable of providing intimate contact between two phases so as to effect transfer of solute between them and also of ultimately effecting a complete separation of the phases. For so general an operation, naturally a substantial variety of equipment has been devised. A very general classification of equipment, their main characteristics and industrial applications is in Table 14.2. A detailed table of comparisons and ratings of 20 kinds of equipment on 14 characteristics has been prepared by Pratt and Hanson (in Lo et al., 1983, p. 476). Some comparisons of required sizes and costs are in Table 14.3. 

The different systems of leaching may be listed as follows (1) Leaching by percolation which means flowing the solution past the stationary solids and separation of the solution thus obtained from the residues or undissolved material. (2) Leaching by agitation in which dissolution is obtained while the solids are held in suspension in the solvent and a certain amount of relative motion is maintained and separation of the solution later by decantation or filtration. Each type listed above will be discussed showing the equipment involved, comparative costs, and its applications in industrial problems. 

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