Leaching is a solid-liquid extraction process where one or more constituents of a solid mixture are separated by contact with a liquid solvent. It works on the principles of solvent penetrating the solid, the solute then diffusing through the solid-solvent mixture to the particle surface until equilibrium is established. There are two types - unsteady state processes like in situ leaching and heap leaching, and steady state processes using agitated vessels or continuous countercurrent decantation. Various equipment is used for leaching like percolation tanks, shank systems, agitated column vessels, thickeners, hydrocyclones, and various extractors.

1. LEACHING

2. LEACHING
• Solid-Liquid Extraction process
• Mass transfer operation
• One or more constituents of solid mixture by
contact with a liquid solvent.
• Widely used in the biological and food
processing industries
• The separation of sugar from sugar beets with
hot water

3. Principle
• Solvent penetrate or diffuse into the solid
• Solute then diffuse through the solid solvent
mixture to surface of particle
• Until the equilibrium is established
• Rate of solvent transfer to surface of solids is
faster
• Rate of solvent transfer into solids is slower.

4. Types of leaching
• Unsteady state operation
• Steady state operation

5. Unsteady state operation
• In situ leaching
• Heap leaching
• Percolation tank
• Shank system
• Filter press leaching
• Agitated vessel
Mining industries

6. Steady state operation
• Agitated vessels
• Thickeners
• Continuous countercurrent decantation
• Hydrocyclone
• Filter leaching

7. Percolation tank leaching

8. • Closed tanks are necessary to prevent
evaporation losses when the solvent is volatile.
• Examples: Sugar from sugar-beet slice
Percolation tank leaching

9. Shank system

10. • Used in Metallurgical industries.
• Recovery of tannins from tree barks and
woods
Shank system

11. Agitated column vessel

12. • Used for coarse solids.
• Closed cylindrical vessels containing paddles
or stirrers on vertical shafts, as well as false
bottoms for leach solution removal.
• Finely divided solids can be suspended in
leaching solvents by agitation
Agitated column vessel

13. Agitated column vessel steady state

14. • 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.
• Arms attached to the shaft bottom moves the
Agitated column vessel steady state

15. Thickeners
• It increases the ratio of solid to liquid in a
dilute suspension of finely sized particles by
settling and decanting, producing clear liquid
and thickened sludge.
• It reduces filtering costs, readily transportable

16. Dorr thickners

17. Continuous Countercurrent
Decantation

18. Hydrocyclones
• It is same as of size of classifications of solids.
• Used for solid-liquid separation in place of
thickeners in countercurrent washings of
solids.

19. Hydrocyclones

20. Bollman Extractor

21. • Contains a bucket elevator in a closed casing.
• The buckets are loaded with flaky solids such
as soybeans.
• The solids are sprayed with appropriate
amount of half miscella as they travel
downward.
• Half miscella is the intermediate solvent
containing some extracted oil and some small
solid particles.
• As solids and solvent flow concurrently down
Bollman Extractor

22. Rotocel Extractorm

23. • 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.
• Solid are admitted to each compartment at feed
point.
• The compartments then pass a number of
solvent sprays, a drainage section and a
discharge point.
• To give countercurrent extraction, the fresh
Rotocel Extractorm

24. Kennedy Extractor

25. Continuous Horizontal Extractor