Chemistry
There are a variety of leaching processes, usually classified by the types of reagents used in the operation. The reagents required depend on the ores or pretreated material to be processed. A typical feed for leaching is either oxide or sulfide.
For material in oxide form, 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.
The leaching of precious metals such as gold can be carried out with cyanide or ozone under mild conditions.
Read more about this topic: Leaching (metallurgy)
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