Leaching is a process that extracts soluble components from solids using a liquid solvent. It involves two phases - an overflow phase containing the solute and solvent, and an underflow phase containing the solid and solvent. There are three main stages: dissolution of the solute in the selective solvent, diffusion of solutes through particle pores, and transfer of the solute into the bulk solvent. Common leaching equipment includes agitated vessels, Dorr thickeners, Kennedy extractors, Bollman extractors, and Rotocel extractors. Leaching can be classified as physical, chemical, or bacterial and can operate in batch, semi-batch, or continuous modes.

1. Leaching and its equipment
❖ It is defined as the extraction of soluble constituent from solid
material using selective liquid solvent.
❖ It is also defined as the separation of solute from solid
mixture by dissolving it in a liquid phase.
❖ There are two phases involved in Leaching:
1.Overflow phase-solute + solvent
2.Underflow phase-solid +solvent

2. Process(or)Stages involved in Leaching:
There are three major stages involved in the leaching
process.They are:
❏ Dissolution of solute in selective solvent.
❏ Diffusion of solutes through pores of particles.
❏ Transfer of solute into bulk solvent.

3. Example of Leaching:

4. Difference between Leaching & Extraction:

5. Factors influencing the rate of Leaching:
1.Particle size.
2.Solvent.
3.Temperature.
4.Agitation.

6. ➔ Basically leaching is simply known as the solid-liquid
extraction.
➔ Another example for leaching (or) solid-liquid extraction is:
1.Separation of sugar from sugar beets with hot water.
In general leaching is classified into:
a)Physical leaching.
b)Chemical leaching.
c)Bacterial leaching.

7. Classification of Leaching:
★ Fixed bed leaching.
★ Moving bed leaching.
★ Agitated leaching.
Leaching is further classified into continuous leaching and
batch leaching. The continuous leaching is classified into
co-current and countercurrent leaching. The
countercurrent leaching is further divided into variable
underflow and constant underflow. But the most used
leaching is Batch leaching.

8. Principle of leaching and its uses:
➢ Leaching can be batch,semi batch or continuous.
➢ It usually operates at an elevated temperature to increase
the solubility of solute in the solvent.
➢ Feed to a leaching system typically is solid,consisting of
basically insoluble career material and a (usually
desirable)solid compound.The feed usually must be
prepared by grinding or chopping.It is then mixed with
liquid solvent.The desired material dissolves and leaves
when liquid is drawn off as overflow.

9. Equipments of Leaching:
Some of the equipments of leaching are:
1. Agitated vessels.
2. Dorr thickeners.
3. Kennedy extractor.
4. Bollman extractor.
5. Rotocel extractor.

10. Agitated vessels& dorr thickeners:

11. Rotating extractor:
● It is actually a horizontally revolving cylindrical vessel.
● It is provided with large hatch for loading and unloading
the solids to be extracted and a drain line for loading the
extractant and draining off the extract.
● The vessel provided with baffle plates perpendicular on
interfacial surface.

13. Kennedy extractor:
1. It is originally used for leaching tannins from tanbark. It is
now used for oilseed.
2. The solids are leached in a series of tubs and are pushed
from one to next in cascade by paddles, while the solvent
flows in counter-current .
3. Perforation in paddles permit drainage of solids between
stages.

15. Bollman extractor:
❏ It is mainly used for the recovery of additional oil from the
residues obtained after mechanical pressing of solids.
❏ The bollman extractor essentially consists of vapour tight
vertical chamber in which a series of perforated baskets
are attached to chain conveyor.
❏ As the solids and solvent flow co-currently down the right
hand side of machine, the solvent extracts more oil.
❏ Simultaneously fine solids are filtered out of solvent,so
that clean full miscella pumped at outlet provided at right
hand bottom.

16. ❏ The liquid percolates down through moving baskets and is
collected at bottom as strong solution or full miscella.
❏ The baskets moving upward on left are leached counter
currently by fresh solvent sprayed on top basket.
❏ The wet flakes are dumped and are removed continuously
using spray conveyor.
❏ Usually the conveyor speed is one revolution per hour and
solvent to fresh meal ratio is 1.

18. Rotocel extractor:
★ A horizontal basket is divided into walled compartments
with floor that is permeable to liquids rotates slowly about
vertical axis.
★ The solids are admitted to each compartment at the feed
point.
★ It is mainly made up of stainless steel.