Leaching (metallurgy)

Leaching is a widely used extractive metallurgy technique which converts metals into soluble salts in aqueous media. Compared to pyrometallurgical operations, leaching is easier to perform and much less harmful, because no gaseous pollution occurs. Drawbacks of leaching are the highly acidic and in some cases toxic residual effluent, and its lower efficiency caused by the low temperatures of the operation, which dramatically affect chemical reaction rates.

Chemistry

Leaching is done in long pressure vessels which are cylindrical (horizontal or vertical) or of horizontal tube form known as autoclaves. A good example of the autoclave leach process can also 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.

See also

• Heap leaching
• In-situ leaching