Leaching is the process of extracting a substance from a solid by dissolving it in a liquid. In the chemical processing industry, leaching is known as extraction. Leaching has a variety of commercial applications, including separation of metal from ore using acid, and sugar from beets using hot water. Chloride can also be leached from food.

In agriculture, leaching may refer to the loss of water-soluble plant nutrients from the soil, due to rain and irrigation. Soil structure, crop planning, type and application rates of fertilizers, and other factors are taken into account to avoid excessive nutrient loss. Leaching may also refer to the practice of applying a small amount of excess irrigation where the water has a high salt content to avoid salts from building up in the soil. Where this is practiced, drainage must also usually be employed, to carry away the excess water.

In a typical leaching operation, the solid mixture to be separated consists of particles, inert insoluble carrier A and solute B. The solvent, C, is added to the mixture to selectively dissolve B. The overflow from the stage is free of solids and consists of only solvent C and dissolved B. The underflow consists of slurry of liquid of similar composition in the liquid overflow and solid carrier A. In an ideal leaching equilibrium stage, all the solute is dissolved by the solvent; none of the carrier is dissolved. The mass ratio of the solid to liquid in the underflow is dependant on the type of equipment used and properties of the two phase.

Leaching is an environmental concern when it contributes to groundwater contamination. As water, from rain, flooding or other sources, seeps into the ground, it can dissolve chemicals and carry them into the underground water supply. Of particular concern are hazardous waste dumps and landfills, and, in agriculture, excess fertilizer and improperly stored animal manure.

Metallurgical application of leaching

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Leaching is widely used in extractive metallurgy since many metals can form soluble salts in aqueous media. Compared to pyrometallurgical operations, leaching is easier to perform and much less harmful, because no gaseous pollution occurs. The only drawback of leaching is its lower efficiency caused by the low temperatures of the operation, which dramatically affect chemical reaction rates.

There is great variety of leaching processes. Ususally they are classified by the types of the reagents used in the operation. The type of the reagents required depends on the type of ores (or some pretreated material) processed. A typical feed for leaching is either oxide or sulfide. In case, it is oxide, a simple acid leaching reaction can be illustrated by the zinc oxide leaching reaction :

ZnO + H₂SO₄ → ZnSO₄ + H₂O

In this reaction solid ZnO dissolves, forming soluble zinc sulfate.

In many cases other reagents are used to leach oxides. For example, in the metallurgy of aluminium, aluminium oxide is subject to leaching by alkali solutions:

Al₂O₃ + 3H₂O + 2NaOH → 2NaAl(OH)₄

Leaching of sulfides is a more complex process due to the refractory nature of sulfide ores. It often involves the use of pressurized vessels, called autoclaves. A good example of the autoclave leach process can be found in the metallurgy of zinc. It is best described by the following chemical reaction:

2ZnS + O₂ + 2H₂SO₄ → 2ZnSO₄ + 2H₂O + 2S

This reaction proceeds at temperatures above the boiling point of water, thus creating a vapor pressure inside the vessel. Oxygen is injected under pressure, making the total pressure in the autoclave more than 0.6 MPa.

Chemical processes | Environmental chemistry | Soil science

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