What is Leaching?

The word ‘leaching’ in reference to the chemical industry corresponds to the removal of chemicals. It is quite a traditional method that involves solid-liquid operation.

Traditionally it was regarded as percolation. Leaching is a process where a mixture is diffused or dissolved in liquid in order to separate the solute. More simply, diffusion of solid into liquid takes place.

One should note here that washing is known to be one of the special leaching processes where water is used to remove undesirable element from solid substances.


We have already mentioned that earlier it was named as percolation. However, there are some other similar techniques as well which are given different names. Like when alkali is leached from ashes then it is referred to as Lixiviation.
Another technique is elutriation or elution which corresponds to the method of washing off the solute present on the surface of insoluble solid by making use of solvent.

Not only this, it finds applications in metallurgical industries when metals are extracted from ores of aluminium, nickel, cobalt, zinc, etc. Other than these,  sugar is also extracted from sugar beets using hot water, by making use of organic solvents oil is extracted from oil seeds.
Various pharmaceutical products are also extracted from roots of various plants and leaves.

It is important to note here that preparation of the solid is a crucial aspect of this operation. Furthermore, according to the type of solid, crushing is performed and is grounded to a certain size so that leaching can be accelerated.

Factors Affecting Leaching

If we proceed to talk about the factors that influence the rate of dissolution of the material in liquid for leaching to take place then there are number of factors that can be distinguished as physical-chemical and biological.

The physical factor is associated with the way solid is exposed to liquid for leaching to occur. The degree of leachability can be varied by checking for the dissolved constituents at the time of retention. On the contrary, chemical changes are somewhat associated with the chemical reactions such as carbonation or oxidation that take place when mixture is exposed to solvent.

Thus, leaching shows dependency on:

  1. Nature possessed by the structure of solid.
  2. The size of the particle and how it is distributed
  3. The degree of solubility if solute in the solvent.
  4. The operating temperature.
  5. The degree of diffusion of solute in the solvent.

One should note here that the leaching technique can be determined by observing the leaching behaviour.

How leaching is performed?

The principle of leaching is such that initially some solvent is put up over the surface of the solid material. Afterwards, penetration or diffusion time within the solid must be provided to the solvent. This time is required so that the solute can diffuse properly into the solid solvent mixture along with each and every particle of the surface. Once this is done then the solute is provided to the solvent in bulk quantity.

One should note here that the transferring rate of solvent to solid surface is quite fast. ­­­

The rate with which diffusion of solute into the solid occurs is the controlling element and it depends on various factors. Suppose if the solid is porous then it is said that the diffusivity of the solid is effective. In this case the resistance offered by the solid to the solvent is very small.

Applications of Leaching

The various applications of leaching are as follows:

  • The food processing industries make use of leaching. One of the examples is the extraction of sugar from sugar beets.
  • This technique is also used in metal processing industries where metals are removed from its ores. Like in order to extract gold from its ore, sodium cyanide solution is used.
  • Pharmaceutical industries also make use of leaching where we get various products by performing leaching of roots, leaves, stems of plants.

This is all about leaching.

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