Leaching by Immersion

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Chapter: Pharmaceutical Engineering: Solid-Liquid Extraction

In pharmaceutical processes, leaching by immersion is carried out in simple tanks, which may be agitated by a turbine or paddle.


LEACHING BY IMMERSION

In pharmaceutical processes, leaching by immersion is carried out in simple tanks, which may be agitated by a turbine or paddle. If the solids are adequately suspended, intimate contact between the phases promotes efficient extraction. Incomplete extraction due to channeling is avoided, and difficulties due to swelling do not arise. Problems arise, however, in the subsequent separation of the phases. The materials to which leaching by immersion is applied are nor-mally either finely divided or coarse and compressible. When agitation ceases, the solids will settle and the leach liquid can be siphoned or pumped off by lines suitably placed in the tank. The sediment will, however, contain a large volume of the leach liquid, which must be recovered by resuspending the solids in fresh solvent, allowing the solids to sediment and decanting the supernatant liquid. Cake filtration provides an alternative method of separation. The leach liquid remaining in the cake is displaced by passing a wash liquid. In some cases, a filter press may be used for both extraction and separation.

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