Whatever method is adopted, leaching consists of a number of consecutive diffusional or mass transfer processes.
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.
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