Coarse ground material is placed in the body of the extractor. This may be jacketed to give control of the extraction temperature.
LEACHING BY PERCOLATION
Coarse
ground material is placed in the body of the extractor. This may be jacketed to
give control of the extraction temperature. The packing must be even or else
the solvent will preferentially flow through a limited volume of the bed and
leaching will be inefficient. In large extractors, channeling is prevented or
reduced by horizontal, perforated plates placed at intervals in the bed. These
redistribute the percolating liquid.
Solvent
inhibition will swell dried materials, and the permeability of the bed will be
reduced. This is most marked with aqueous solvents. If swelling occurs, it is
necessary to moisten the material with water or with the solvent before it is
packed into the extractor.
Once
the extractor is packed, leaching may be conducted in a number of ways. The
body of the extractor may be completely filled with the solvent. Liquid is then
withdrawn from the body through the false bottom, and more solvent is added.
This is continued until the marc is exhausted. Alternatively, the solution
issuing from the bottom may be returned to the top. After a period of
recircula-tion, the liquid is completely withdrawn, and fresh solvent is
admitted. In both processes, a period of steeping or soaking may precede the
movement of liquid.
In
beds of high permeability, adequate movement of liquid is obtained by simple
gravity operation in an open vessel. If the material forms a dense bed,
however, the liquid must be pumped through if suitable flow rates are to be
secured. A closed extraction vessel must then be used. Closed extraction
vessels are also necessary for high-temperature extraction and extraction with
volatile solvents. In alternative methods, the liquid is forced upward through
the bed. Possible migration of fine material downward and the formation of a
region of low permeability at the bottom of the bed are prevented in this way.
In other processes, the bed may not be immersed in the menstruum. This is
simply sprinkled on to the upper surface and allowed to trickle through the
bed, the voids of which are mainly filled with air.
Simple
extractions of this type will, if carried to completion, require large amounts
of solvent and yield dilute extracts. These disadvantages can be overcome if
extraction is followed by evaporation. These operations are often integrated in
extraction plant. The leach liquids leaving the extractor enter an evaporator
heated, for example, by a calandria. Since most materials encoun-tered are heat
sensitive, this will be operated at reduced pressure. The vapor leaving the
evaporator is condensed and returned to the extractor. When extraction is
carried out with water-immiscible solvents, any water derived from the feed
material and present in the condensate would be separated and rejected. The
extraction is stopped when the leach liquid is free from wanted constituents. A
concentrated extract remains in the evaporator.
Leaching
by percolation provides a simple method of separating leach liquid and solid
during the extraction. When this is complete, the permeable bed will largely
drain, permitting extensive solvent recovery. Further recovery can be gained by
mechanical expression.
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