Factors Affecting the Rate of Leaching

FACTORS AFFECTING THE RATE OF LEACHING

Whatever method is adopted, leaching consists of a number of consecutive diffusional or mass transfer processes. The solvent first penetrates the raw material and dissolution of the soluble elements occurs. These diffuse in the opposite direction to the surface of the solid matrix and then through the liquid layers at its surface to reach the bulk solution. These processes proceed under the influence of an overall concentration gradient, the concentration being least in the bulk solution. Any of these processes may be responsible for limiting the rate at which leaching proceeds. In pharmaceutical leaching, however, the solid matrix is usually cellular and this structure will normally offer the highest diffusional resistance. The complexity of such structures does not permit a strict analysis of the processes of mass transfer. Nevertheless, the simple diffusional concepts expressed in Fick’s law suggest that the following factors will influence the rate of leaching: the size distribution of the leached particles, the tempera-ture of leaching, the physical properties of solvent, and the relative movement imposed on the solids and the liquid.