• Leaching is a preferential solution of one or more constituents of solid mixture by contact with a liquid solvent. This unit operation is one of the oldest in the chemical industries.
• Leaching is widely used in the biological and food processing industries, such as the separation of sugar from sugar beets with hot water.
• The extraction of oils from peanuts, soybeans, sunflower seeds, cotton seeds, and halibut livers.
• In pharmaceutical industry, many products are obtained by leaching plant roots, leaves, and stems.
• In the metals processing industry, leaching is used to remove the metals from their ores, which contains many undesirable constituents, as solute salts.
• In gold leaching, gold is leached from its ore using an aqueous sodium cyanide solution.
• Leaching and washing of the leached solute from percolation tank by crosscurrent methods results in weak solutions of the solute. The strongest solution will result if counter current scheme is used, wherein the final withdrawn solution is taken from contact with the freshest solid and the fresh solvent is added to solid from which most of the solute has been already leached or washed.
• This shank system is operated in following manner:
1. Assume at the time of inspecting the system at figA that it has been in operation for some time. Tank 6 is empty, tanks 1 to 5 are filed with solid and with leached liquid and tank 5 is n contact with freshest solid. Fresh solvent is added to tank 1.
2. Withdraw the concentrated solution from tank 5, transfer the liquid from tank 4 to tank 5, 3 to 4, 2 to 3, 1 to 2. Add fresh solid to tank 6.
3. Refer figB . Discard the spent sold frm tank 1. Transfer the liquid from tank 5 to 6, 4 to 5, 3 to 4, 2 to 3,. Add fresh solvent to tank 2.
4. Continue the operation in the same manner as before.
• Kennedy extractor
• Continuous Countercurrent Decantation
• Bollman extractor
• Rotocell extractor
• The kennedy extractor, a modern arrangement of which is indicated schematically in figure is another stagewise device which has been in use since 1927,originally for leaching tannins from tanbark.it is now used for oilseed and other chemical leaching operations.
• The solids are leached in a series of tubs and are pushed from one to the next in the cascade by paddles, while the solvent flows in counter-current.
• Perforation in the paddles permit drainage of the solids between stages and the solids are scraped from each paddle as shown.
• As many tubs may be placed in a cascade are as require.
Continuous Countercurrent Decantaton
• A simple arrangement is shown in figure.