Leaching in the pharmaceutical and allied industries is operated as a batch process.
METHODS OF LEACHING
Leaching
in the pharmaceutical and allied industries is operated as a batch process.
This is because high-cost materials are processed in relatively small
quantities. Frequent changes of material may be made, creating problems of
cleaning and contamination. For these reasons, continuous extraction, which is
characterized by a large throughput and the mechanical movement of the solid
counter to the flow of solvent, is not applicable to pharmaceutical extraction
and is not described in this text.
Whatever
the scale of the extraction, however, leaching is performed in one of two ways.
In the first, the raw material is placed in a vessel, forming a permeable bed
through which the solvent or menstruum percolates. Dissolution of the wanted
constituents occurs, and the solution issues from the bottom of the bed. This
liquid is sometimes called the miscella, and the exhausted solids are called
the marc. This process is called leaching by percolation. The alter-native
process is leaching by immersion and consists of immersing the solid in the
solvent and stirring. After a suitable period of time, solid and liquid are
separated.
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