Leaching by Percolation

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 Liquid is then withdrawn from the body through the false bottom, and more solvent is added.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 After a period of recircula-tion, the liquid is completely withdrawn, and fresh solvent is admitted.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.