Presentation On Leaching

1. PRESENTATION REPORT
Name : Javiya Smit A.
Enrollment : 180470105019
Department : Chemical
Subject : MassTransfer Operation
Report on Leaching

2. Leaching
• Leaching is a solid-liquid operation
• The two phases are in intimate contact, the
solute(s)can diffuse from the solid to the liquid
phase, which causes a separation of the
components originally in the solid.
• A special leaching process, when an
undesirable component is removed from a solid
with water, is called Washing

3. Application
• It is used in the biological and food processing industries.
Ex: the separation of sugar from sugar beets with hot
water, the extraction of oils from peanuts, soybeans, sunflower
seeds, cotton seeds.
• 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.
Ex: gold leaching, gold is leached from its ore using an aqueous
sodium cyanide solution.

4. Principles of Leaching
• The solvent must be transferred from the bulk solvent
solution to the surface of the solids.
• Next, the solvent must penetrate or diffuse into the solids.
• The solute then diffuses through the solid solvent mixture to
the surface of the particle.
• Finally, the solute is transferred to the bulk solution.
• The rate of the solvent transfer from the bulk solution to the
• solid surface is quite rapid.
• However, the rate of transfer of the solvent into the solid can
be rather slow or rapid.
• This solvent transfer usually occurs initially when the
particle are first contacted with the solvent.

5. • The rate of diffusion of the solute through the solid
and solvent to the surface of the solid is often the
controlling resistance in the overall leaching process
and can depend on a number of different factors.
• If the solid is made of porous the diffusion through
the porous solid can be described by an effective
diffusivity.
• The resistance to mass transfer to the solute from the
solid surface to the bulk solvent is generally quite
small compared to the resistance to the diffusion
within the solid itself.

6. Types of leaching
Unsteady state operations
• In Situ (In-Place)
Leaching
• Heap Leaching
• Percolation Tanks
• Shanks System
• Filter press leaching
Steady state (Continuous)
operations
• Agitated Vessels
• Thickeners
• Continuous Countercurren
Decantation (CCD)
• Hydrocyclones
• Filter leaching

7. Unsteady State Leaching
1) In Situ (In-Place) Leaching:
• Also called as solution mining.
• The percolation leaching of minerals in place at the
mine, by circulation of solvent over the ore.
• Removal of salts from deposits below earth surface by
the solution of salt water which pumped into the
deposit.
• Example: Leaching of Low-grade copper ores,
uranium ores.

8. 2) Heap Leaching:
• Low-grade ores whose minerals values do not warrant the
expense of crushing or grinding can be leached in the form
of run of mine lumps into huge piles.
• Examples: Copper from pyritic ores, uranium.
3) Percolation tank Leaching:
• When the pressure drop for flow of liquid is too high for
gravity flow, closed vessels must be used. (Diffusers)
• Closed tanks are necessary to prevent evaporation losses
when the solvent is volatile.
• Examples: Sugar from sugar-beet slices.

9. Percolation tank Leaching

10. 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.

11.  Applications of Shanks system (Extraction
Battery):
• Recovery of tannins from tree barks and woods.
• Leaching of Sodium-nitrate from Chilean-nitrate
bearing rock.
• Used in Metallurgical industries

12.  This shank system is operated in following
manner:
I. 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.
II. 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.
III. 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.
IV. Continue the operation in the same manner as before.

13. Leaching equipments
• Agitated vessels
• Dorr thickeners
• Continuous counter-current decantation
• Hydrocyclones
• Rotocel
• Kennedy extractor
• Bollman extractor
• Continuous horizontal extractor

14. Agitated Vessels
• Finely ground solids can be readily suspended in
liquids by agitation be continuously leached in any
type of agitated tanks or vessels.
• There is a continuous flow of liquid and solid into and
out of the tank, so that no accumulation of solid
occurs.
• Turbine type agitator is used for effective operations.
• Types: Pachuca tanks and Dorr agitators (air-lift and
mechanical principle).
• Central shaft acts as an air lift and revolves slowly.

15. • Arms attached to the shaft bottom moves the settled
solids towards the center, where they lifted by air
through the shaft to the revolving launders attached to
the top.

16. Kennedy extractor

17. • 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 leachingoperations.
• 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.

18. Continuous counter-current
decantation
A simple arrangement is shown in figure:

19. • 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
• 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.

20. Bollman Extractor

21. • 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
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.

22. • 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
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.

23. • 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.
• 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.

24. Rotocel Extractor
• 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.

25. • 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
one.