Introduction and Equipment for Liquid-Solid Leaching

Inorganic and organic materials. The method of preparation of the solid depends to a large extent upon the proportion of the soluble constituent present, its distribution throughout the original solid, the nature of the solid—i.e., whether it is composed of plant cells or whether the soluble material is completely surrounded by a matrix of insoluble matter—and the original particle size. 

12 Liquid-Liquid and Fluid-Solid Separation Processes 

Introduction and general steps. In the leaching of soluble materials from inside a particle by a solvent, the following general steps can occur in the overall process. The solvent must be transferred from the bulk solvent solution to the surface of the solid. Next, the solvent must penetrate or diffuse into the solid. The solute dissolves into the solvent. The solute then diffuses through the solid solvent mixture to the surface of the particle. Finally, the solute is transferred to the bulk solution. The many different phenomena encountered make it almost impracticable or impossible to apply any one theory to the leaching action. 

In general, the rate of transfer of the solvent from the bulk solution to the solid surface is quite rapid, and the rate of transfer of the solvent into the solid can be somewhat rapid or slow. These are not, in many cases, the rate-limiting steps in the overall leaching process. This solvent transfer usually occurs initially when the particles are first contacted with the solvent. The dissolving of the solute into the solvent inside the solid may be simply a physical dissolution process or an actual chemical reaction that frees the solute for dissolution. Our knowledge of the dissolution process is limited and the mechanism may be different in each solid (Kl). 

The rate of diffusion of the solute through the solid and solvent to the surface of the solid is often the controlling resistance in the overall leaching process and can depend on a number of different factors. If the solid is made up of an inert porous solid structure with the solute and solvent in the pores in the solid, the diffusion through the porous solid can be described by an effective diffusivity. The void fraction and tortuosity are needed. This is described in Section 6.5C for diffusion in porous solids. 

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