Equilibrium Relations and Single-Stage Leaching

The amount of solution retained with the solids in the settling portion of each stage may depend upon the viscosity and density of the liquid in which the solid is suspended. 

Equilibrium diagrams for leaching. The equilibrium data can be plotted on the rectangular diagram as wt fraction for the three components solute A), inert or leached solid (B), and solvent (Q. The two phases are the overflow (liquid) phase and the underflow (slurry) phase. This method is discussed elsewhere (B2). Another convenient method of plotting the equilibrium data will be used, instead, which is similar to the method discussed in the enthalpy-concentration plots in Section 11.6. 

The concentration of inert or insoluble solid B in the solution mixture or the slurry mixture can be expressed in kg (lb ) units. 

There will be a value of N for the overflow where N = 0 and for the underflow N will have different values, depending on the solute concentration in the liquid. The compositions of solute A in the liquid will be expressed as wt fractions. 

In Fig. 12.9-la a typical equilibrium diagram is shown where solute A is infinitely soluble in solvent C, which would occur in the system of soybean oil (/l)-soybean inert solid meal (B)-hexane solvent (C). The upper curve of N versus for the slurry underflow represents the separated solid under experimental conditions similar to the actual stage process.The bottom line of N versusx, where N = Oon the axis, represents the overflow liquid composition where all the solid has been removed. In some cases small amounts of solid may remain in the overflow. The tie lines are vertical, and on a yx diagram, the equilibrium line is y = on the 45° line. In Fig. 12.9-lb the tie lines are not vertical, which can result from insufficient contact time, so that all the solute is not dissolved adsorption of solute A on the solid or the solute being soluble in the solid B. 

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