Leaching (metallurgy)
Leaching is a process widely used in extractive metallurgy where ore is treated with chemicals to convert the valuable metals within the ore, into soluble salts while the impurity remains insoluble. These can then be washed out and processed to give the pure metal; the materials left over are commonly known as tailings. Compared to pyrometallurgy, leaching is easier to perform, requires less energy and is potentially less harmful as no gaseous pollution occurs. Drawbacks of leaching include its lower efficiency and the often significant quantities of waste effluent and tailings produced, which are usually either highly acidic or alkali as well as toxic (e.g. bauxite tailings).
There are four types of leaching:
- Cyanide leaching (e.g. gold ore)
- Ammonia leaching (e.g. crushed ore)
- Alkali leaching (e.g. bauxite ore)
- Acid leaching (e.g. sulfide ore)[1][2]
Leaching is also notable in the extraction of rare earth elements, which consists of lanthanides, yttrium and scandium.[2]
Chemistry[edit]
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:
- 2 ZnS + O2 + 2 H2SO4 → 2 ZnSO4 + 2 H2O + 2 S
This reaction proceeds at temperatures above the boiling point of water, thus creating a vapour pressure inside the vessel. Oxygen is injected under pressure, making the total pressure in the autoclave more than 0.6 MPa and temperature at 473-523 K .
The leaching of precious metals such as gold can be carried out with cyanide or ozone under mild conditions.[1]
Historical Uses[edit]
Origins[edit]
Leaching can be traced all the way back to alchemy.[3] Early examples of leaching performed by alchemists resembled mixing iron with copper sulfate, yielding a layer of metallic copper.[3] In the 8th century, Jabir Ibn Hayyan, an Arab alchemist, discovered a substance he coined "aqua regia".[3] Aqua regia, a combination of hydrochloric acid and nitric acid, was found to be effective in dissolving gold, which was previously thought to be insoluble.[3]
Heap leaching dates further back to the second century BC in China, where iron was combined with copper sulfate.[4] By the time of the Northern Song Dynasty, a copper alloy was able to be recovered by leaching.[4]
Pre-World War II[edit]
In the 16th century, heap leaching became commonly used to extract copper as well as saltpeter, from organic matter.[3] Primarily used in the Germany and Spain, pyrite would be brought to the surface and left out in the open.[3][4] The pyrite would be set outside for months at a time, where rain and air exposure would lead to chemical weathering.[3] A solution containing copper sulfide would be collected in a basin, then precipitated in a process called cementation, resulting in metallic copper.[3] Heap leaching, in this natural chemical-free form, was further developed to obtain different, more economically viable, types of ore. This was done by incorporating chemical lixiviation, which applies more chemical manipulation and technique to heap leaching.[5]
From 1767-1867, the production of potash in Quebec became an important industry to supply France's glass and soap manufacturers.[3] Potash was most frequently made from the ash remains of wood-burning stoves and fireplaces, which were agitated with water and filtered.[3] Once evaporated, the remains would be potash. 400 tons of hardwood would be required to burn to yield one ton of potash.[3]
In 1858, Adolf von Patera, an Austrian metallurgist, utilized lixiviation to separate soluble and insoluble compounds from silver in an aqueous solution.[5][6]The technique of Patera's lixiviation was further developed by American E. H. Russell around 1884, creating the "Russell Process".[7] In 1887, when the cyanidation process was patented in England, it began to be incorporated into the existing leaching process, creating the more specific cyanide leaching.[5]
In 1887, when the cyanidation process was patented in England, it began to replace the existing Russell Process.[5] Cyanidation was much more efficient and had a recovery rate of up to 90%.[5]
Leading up to World War I, many new ideas for leaching processes were proposed. This included using ammonia solutions for copper sulfides, and nitric acid for leaching sulfide ores.[3] Most of these ideas fizzled out into obscurity due to the high cost of the leaching agents required.[3]
Modern Leaching[edit]
As a result of the Manhattan Project, the United States government needed ready access to uranium.[3] Many different techniques in leaching were quickly employed at a large scale.[3] Both synthetic resins and organic solvents were used early on to extract uranium.[3] Ultimately, the use of organic solvents was less tedious compared to ion exchange through synthetic resins, and further production of uranium and other rare earth metals moved towards solvent extraction.[3]
See also[edit]
References[edit]
- ^ a b J. Viñals; E. Juan; M. Ruiz; E. Ferrando; M. Cruells; A. Roca; 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.
- ^ a b Borges de Lima, Ismar; Filho, Walter Leal (2016), "Highlights on Rare Earths", Rare Earths Industry, Elsevier, pp. 395–424, doi:10.1016/b978-0-12-802328-0.00026-7, ISBN 978-0-12-802328-0, retrieved 2024-03-01
- ^ a b c d e f g h i j k l m n o p q Habashi, Fathi (2005-09-01). "A short history of hydrometallurgy". Hydrometallurgy. Teaching and Learning Hydrometallurgical Science and Engineering-Part I. 79 (1): 15–22. doi:10.1016/j.hydromet.2004.01.008. ISSN 0304-386X.
- ^ a b c Bin, Yu; Kuangdi, Xu (2022), "Leaching Mining Method", in Xu, Kuangdi (ed.), The ECPH Encyclopedia of Mining and Metallurgy, Singapore: Springer Nature, pp. 1–3, doi:10.1007/978-981-19-0740-1_703-1, ISBN 978-981-19-0740-1, retrieved 2024-03-08
- ^ a b c d e McQueen, Robery. "Mining "Invisible" Gold: Heap Leaching and Nevada's Contribution to Twentieth- Century Gold Mining" (PDF). 2021 Mining History Journal – via Mining History Association.
- ^ Eissler, Manuel (1891). The metallurgy of silver; a practical treatise on the amalgamation, roasting, and lixiviation of silver ores including the assaying, melting, and refining of silver bullion. unknown library. London, C. Lockwood and son.
- ^ "Collection: Letters Regarding the Russell Process | Special Collections ArchivesSpace | University of Arizona Libraries". archives.library.arizona.edu. Retrieved 2024-03-22.