PRINCIPLE OF LEACHING
FACTORS AFFECTING RATE OF LEACHING
METHODS OF LEACHING PROCESS
Leaching is a solid-liquid separation.
Leaching is a process of mass transfer that occurs by extracting a
substance from a solid material that has come into contact with a liquid.
a) Making of a green tea.
b) Extraction of coconut milk
c) Extraction of oil from soybean
• Leaching occurs in two steps:
1. Contacting the solvent and solid matrix to effect a transfer
2. Separation of the undesirable component from the
remaining solid with water.( washing)
Principle of leaching
• Leaching can refer to various fields:
Application of Leaching
2. Chemistry 4. Food processing
• Leaching agents are chosen based
on a numbers of factors:-
Chemical and physical properties.
Selectivity of the reagent towards
Must be cheap and readily
available in large quantities.
If possible; should be regenerated
Water can be used as leaching agents if the material contains water soluble
These compounds can be naturally occurring or can be made through pre-
Naturally occurring materials- Sulphoxy compounds like sulphite,
thiosulphate and sulphates are produced due to natural oxidation of
some sulphide material.
Pre-treated material- chloride can be leached in water.
MgO(s) + Cl2(g) + C(s) MgCl2 + CO(g)
2. Acids :
Mineral acids like H2SO4, HCl and HNO3 are generally used for leaching either
alone or in combination.
Acid leaching is not suitable for resources containing acid consuming gangue
like CaCO3, MgCO3, CaO, MgO etc.
H2SO4 is the most commonly used acid because it is cheap, abundantly
available and lower corrosion problem, more suitable if metal is produced by
electrowinning because it regenerates acid.
Some applications of H2SO4
•Dilute H2SO4 CuO, ZnO, sludges, metal scrap
•Dilute H2SO4 + oxidant sulphides of Cu, Ni, Zn;
•Concentrated H2SO4 Cu-sulphide concentrate, laterites
NaOH, NH4OH, Na2CO3 are generally used for leaching
More selectivity compared to acid leaching.
Less corrosion problem in comparison to acid.
Regeneration of reagent mostly difficult.
More expensive reagent.
Example: Bayer process for alumina
Al2O3(s) + 2NaOH (aq.) 2NaAlO2(aq.) + H2O (aq.)
NH3 --leaching of Cu, Ni, Co due to their tendency to form stable ammonia complexes.
Ammonia is less corrosive than sodium hydroxide.
It requires less mild conditions for leaching.
Non-reactive to silica unlike NaOH.
4. Aqueous Salt Solutions
Some base metal salts like Fe2(SO4)3, FeCl3, CuCl2 can be used for leaching
of sulphide materials.
Cyanide salts like NaCN, KCN are used for leaching of precious metals like
gold and silver.
1. Leaching of covellite
CuS + Fe2(SO4)3 CuSO4 + 2 FeSO4 + S0
Regeneration: 2FeSO4 + 1/2O2 + H2SO4 Fe2(SO4)3 + H2O
4Au + 8 NaCN + O2 + 2H2O 4 NaAu(CN)2 + 4 NaOH
• Solution mining process.
• Generally used for Cu and U ores.
• Open pathways are created by explosive or hydraulic fracturing
• Leaching is effected by alternate and intermittent circulation of air,
water, spent solution etc.
• For rejected off-grade material put aside in big dumps during
• Ore is directly taken from mines without crushing hence particle
sizes are big
• Leach solution percolates through the dump and collected in ditches
at the base.
Industrially practiced for recovery of Ag/Au, Cu and Ni from ore.
• Mine ore is crushed into small chunks and heaped on an plastic and/or
• Leach solution ( e.g. dilute cyanide for Au or dilute H2SO4 for Cu) is
sprinkled from top and percolates through the heap.
Vat units are usually rectangular container very big in size.
• Are made of wood or concrete lined with acid or alkali resistant
• Has been practiced industrially for gold and copper.
• Floor of the vat is made with false bed inert material with holes to
facilitate collection of leach solution.
Agitation can be performed in two
ways. By air or by mechanical
Suitable for fine ore & can be
operated in batch or continuous
Carried out at high temperature/
pressure unlike percolation
High mass transfer is achieved.
Requires solid-liquid separation
• Lower capital investment.
• Lower operating costs.
• Lower energy and water requirements.
• potentially much less harmful.
• Lesser air pollution because no gas is generated.
• Simple setup and operation.
Reagents are more costly.
These are usually either highly acidic or alkali as well as toxic (e.g. Red
May create environmental problems.
Advantages & Disadvantages
• Leaching is widely used in extractive metallurgy.
• It is highly used in the handling of solids to recover important solutes.
• Its study has increased during the last few years due to a large number
of applications and new methods of the optimization process.
• Leaching is an important operation in different fields like agriculture,
metallurgy, and chemistry that is necessary to follow its tendency of
research so that future projects follow the optimum ways to obtain the
highest contribution possible.
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7. www. Google.com