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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the final withdrawn solution is taken from contact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
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
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
The solids to be leached, together with solution from
the second thickner are introduced into the leaching
agitators at the left and the strong solution thus
produced is decanted from the solids by the first
The agitators together with the first thickener then
constitute a single stage.
The sludge is passed through the cascade to be
washed by the solvent in true counter-current fashion
and the washed solids are discharge at the right
There may, of course be more or fewer than the four
stages shown, and the agitators may be replaced by
any continuous leaching device, such as a grinding
Many variations in the flowsheet are regularly made.
Bollman Extractor is mainly used for the recovery of
additional oil from the residues obtained after
mechanical pressing of solids.
The Bollman Extractor essentially consists of a
vapour tight vertical chamber in which a series of
perforated baskets are attached to a chain conveyor.
The baskets are provided with perforations at the
At the top right hand corner of the extractor solids
are conveyed in to the perforated baskets. The
baskets are loaded with flaky solids.
These fully loaded baskets are then sprayed with
Half Micelle as they travel downward through the
right hand side.
Half Miscella is the intermediate solvent containing
some extracted oil and some small solid particles.
As the solids and solvent flow co-currently down the
right hand side of the machine, the solvent extracts
Simultaneously the fine solids are filtered out of the
solvent, so that the clean full miscella can be pumped
from the outlet provided at the right hand bottom.
Then as these partially extracted solids rise through
the left hand side of the machine a stream of pure
solvent is sprayed on them to obtain a dilute solution
of the oil (half miscella) at the bottom of the
This pure solvent percolates counter-currently
through them and collects in the left hand sump
which is then passed to the Half Miscella storage
The fully extracted solids are then dumped from the
baskets at the top of the elevator to the hoppers from
which they are removed by Screw Conveyors.
Usually the conveyor speed is one revolution per
In the Rotocel
extractor,illusrated in figure
A horizontal basket is
divided into walled
compartments with a floor
that is permeable to the
liquid.The basket rotates
slowly about a vertical
Solids are admitted to each compartment at the feed
point;the compartments then successively pass a
number of solvent sprays, a drainage section, and a
discharge point at which the floor of the
compartment opens to discharge the extracted
solids.The empty compartment moves to the feed to
point to receive its next load of solids.
To give countercurrent extraction, fresh solvent is
fed only to the last compartment before the discharge
point, and the solids in each preceeding compartment
are washed with the effluent from the succeeding