Leaching

1. LEACHING

2. Leaching
 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.

3. Industrial Application
 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.

4. Shank System
 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used, wherein the final withdrawn solution is taken from contact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.

5. Shank System

6. Continue….
 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.

7. Leaching equipments
 Kennedy extractor
 Continuous Countercurrent Decantation
 Bollman extractor
 Rotocell extractor

8. Kennedy Extractor

9. Kennedy 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.

10. Continue….
 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.

11. Continuous Countercurrent Decantaton
 A simple arrangement is shown in figure.

12. Contiue…
 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 thickener.
 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

13. Continue…
 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 mill.
 Many variations in the flowsheet are regularly made.

14. Bollman Extractor

15.  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.

16. Continue…
 The baskets are provided with perforations at the bottom.
 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

17. Continue…
 As the solids and solvent flow co-currently down the right hand
side of the machine, the solvent extracts more oil.
 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 chamber.

18. Continue…
 This pure solvent percolates counter-currently through them and
collects in the left hand sump which is then passed to the Half
Miscella storage tank.
 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 hour.

19. Rotocell Extracter
 In the Rotocel extractor,illusrated in figure b,
 A horizontal basket is divided into walled compartments with a
floor that is permeable to the liquid.The basket rotates slowly
about a vertical axis.

20. Continue…
 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

21. Single stage leaching
Process flow
Feed Slurry
L0, N0, y0, B
Overflow solution
V1, x1
Underflow solution
L1, N1, y1, B
V2, x2
Solvent Feed
V Mass of overflow solution xA Composition of A at overflow solution
L Mass of liquid in slurry solution yA Composition of A at slurry solution
B Mass of dry, solute – free solid.
N Mass of dry,solute (B)/Mass of solution retained (L)
Material balance is divided into 3 parts:
 
 
 balanceSolid
balanceAComp.
balancesolutionTotal
MNLNLNB
MxxVyLxVyL
MVLVL
M
AMAAAA



00 1100
11112200
1120

22. Multi – stage counter
current Leaching
Process flow
Feed Slurry
L0, N0, y0, B
Overflow solution
V1, x1
Underflow solution
LN, NN, yN, B
VN+1, xN+1
Solvent Feed
V Mass of overflow solution xA Composition of A at overflow solution
L Mass of liquid in slurry solution yA Composition of A at slurry solution
B Mass of dry, solute – free solid.

23. Multi – stage counter
current Leaching
The ideal stages are numbered in the direction of the solids or underflow stream.
The solvent (C) – solute (A) phase or V phase is the liquid phase that overflows
continuously from stage to stage countercurrently to the solid phase, and it
dissolves solute as it moves along.
The slurry phase L composed of inert solid (B) and liquid phase of A and C is the
continuous underflow from each stage.
Composition of V – denoted by x
Composition of L – denoted by y
Assumption: The solid B is insoluble and is not lost in the liquid V phase.
The flow rate of solid is constant throughout the process

24. Multi – stage counter
current Leaching
 
 
 balanceSolid
balanceAComp.
balancesolutionTotal
MNLNLNB
MxxVyLxVyL
MVLVL
MNN
AMAANNANNA
NN





00
111100
110