Methods of Leaching

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Chapter: Pharmaceutical Engineering: Solid-Liquid Extraction

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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