- Leaching (metallurgy)
Leaching is a widely used
extractive metallurgy technique which convertsmetal s intosoluble salt s inaqueous media. Compared to pyrometallurgical operations, leaching is easier to perform and much less harmful, because nogas eouspollution 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 are a variety of leaching processes, usually classified by the types of
reagent s used in the operation. The reagents required depend on theore s or pretreated material to be processed. A typical feed for leaching is eitheroxide orsulfide .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 byalkali 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
autoclave s. A good example of the autoclave leach process can be found in the metallurgy ofzinc . 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 avapor 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 asgold can be carried out with cyanide orozone under mild conditions [cite journal |author=J. Viñals, E. Juan, M. Ruiz, E. Ferrando, M. Cruells, A. Roca and J. Casado |title=Leaching of gold and palladium with aqueous ozone in dilute chloride media |journal=Hydrometallurgy
volume=81 |issue=2 |pages=142-151 |year=2006 |month=February |pmid= |doi= 10.1016/j.hydromet.2005.12.004 |url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VBT-4J3WGKC-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=10&md5=5d3d5176b471f2484e2f54c5c4bdc205]References
ee also
*
Dump leaching
*Heap leaching
*Tank leaching
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