Pressure leaching

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Pressurised tank leaching is one of several leaching methods. Chemical dissolution of soluble minerals within a solid ore or concentrate carried out at elevated pressures and giving rise to a solution containing metals to be recovered. The increased pressure used in the pressure leaching process improves the solubility rate of solids and increases the speed of dissolution into the leach solution. Pressure leaching is carried out in closed autoclaves which permit higher temperatures than are possible with open tanks, this also increases the dissolution rate.

Pressure leaching is carried out in autoclaves (Figure …) at high temperatures (>220&deg) and pressures (>20 atm). It is usually characterized by very fast kinetics and the duration of the leaching is from 30 minutes up to 24 hours depending on concentrate being leached and conditions applied. The high pressure makes the method ideal for oxidative leaching of sulphides since the content of dissolved oxygen in the leaching solution is directly proportional to the pressure. Also in acid leaching of sulphides it can be chosen if the sulphide should be converted to elemental sulphur (S) or sulphate SO42-). At temperatures below 120-150C elemental sulphur is formed which in many cases is a preferred form and also require less oxidant to be added. Pressure leaching is applied in leaching Co and Ni sulphides with ammonia as leaching reagent, in the so-called Sherrit Gordon process. Another big application of pressure leaching is the Bayer process where bauxite is leached with sodium hydroxide to produce alumina (Al2O3), which is used as raw material for aluminium production by smelt electrolysis.

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