Leaching is a liquid-solid operation. The two phases are in intimate contact, the solute(s) can diffuse from the solid to the liquid phase, which causes a separation of the components originally in the solid. A special leaching process, when an undesirable component is removed from a solid with water, is called washing.
Leaching is widely used in the biological and food processing industries, such as the separation of sugar from sugar beets with hot water, the extraction of oils from peanuts, soybeans, sunflower seeds, cotton seeds, and halibut livers. In pharmaceutical industry, many products are obtained by leaching plant roots, leaves, and stems. In the metals processing industry, leaching is used to remove the metals from their ores, which contains many undesirable constituents, as solute salts. A good example is gold leaching, gold is leached from its ore using an aqueous sodium cyanide solution.
Preparation of Solids for Leaching
This depends on the proportion of the soluble constituent present, its distribution throughout the original solid, the nature of the solid, and the original particle size. If the soluble material is surrounded by a matrix of insoluble matter, the solvent must diffuse inside to contact and dissolve the soluble material and then diffuse out. This is common in leaching metal salts from mineral ores. In these cases crushing and grinding of the ores is used to increase the rate of leaching since the solution portions are made more accessible to the solvent.
Biological materials are cellular in structure and the soluble constituents are inside the cells. Because the cell walls provide another resistance to diffusion, the rate of leaching may be slow. In this case the biological materials are cut into thin wedge-shaped slices to reduce the diffusion distance of solvent.