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.
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 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.
- J. Viñals, E. Juan, M. Ruiz, E. Ferrando, M. Cruells, A. Roca and J. Casado (February 2006). "Leaching of gold and palladium with aqueous ozone in dilute chloride media". Hydrometallurgy 81 (2): 142–151. doi:10.1016/j.hydromet.2005.12.004.