Editing Smelting (section)

==Process==
[[File:Karabash copper smelter.jpg|thumb|Copper smelter, Chelyabinsk Oblast, Russia]]
[[File:Série de cuves d'électrolyse.jpg|thumb|[[Electrolysis|Electrolytic cells]] at an aluminum smelter in Saint-Jean-de-Maurienne, France]]
Smelting involves more than just melting the metal out of its ore. Most ores are the chemical compound of the metal and other elements, such as oxygen (as an [[oxide]]), sulfur (as a [[sulfide]]), or carbon and oxygen together (as a [[carbonate]]). To extract the metal, workers must make these compounds undergo a [[chemical reaction]]. Smelting, therefore, consists of using suitable [[reduction (chemistry)|reducing substances]] that combine with those [[oxidation (chemistry)|oxidizing]] elements to free the metal.

===Roasting===
In the case of sulfides and carbonates, a process called "[[Roasting (metallurgy)|roasting]]" removes the unwanted carbon or sulfur, leaving an oxide, which can be directly reduced. Roasting is usually carried out in an oxidizing environment. A few practical examples:

* [[Malachite]], a common ore of [[copper]] is primarily copper carbonate hydroxide Cu<sub>2</sub>(CO<sub>3</sub>)(OH)<sub>2</sub>.<ref name="mindat">{{cite web|url=http://www.mindat.org/min-2550.html|title=Malachite: Malachite mineral information and data.|publisher=mindat.org|access-date=26 August 2015|url-status=live|archive-url=https://web.archive.org/web/20150908005239/http://www.mindat.org/min-2550.html|archive-date=8 September 2015}}</ref> This [[mineral]] undergoes [[thermal decomposition]] to 2CuO, CO<sub>2</sub>, and H<sub>2</sub>O<ref name="asminternational">{{cite web|url=http://www.asminternational.org/documents/10192/1942082/coppermetal.pdf/86992a2a-61db-4628-b01d-c70c0338b756|title=Copper Metal from Malachite &#124; Earth Resources|publisher=asminternational.org|access-date=26 August 2015|url-status=live|archive-url=https://web.archive.org/web/20150923175352/http://www.asminternational.org/documents/10192/1942082/coppermetal.pdf/86992a2a-61db-4628-b01d-c70c0338b756|archive-date=23 September 2015}}</ref> in several stages between 250&nbsp;°C and 350&nbsp;°C. The carbon dioxide and [[water]] are expelled into the atmosphere, leaving [[copper(II) oxide]], which can be directly reduced to copper as described in the following section titled ''Reduction''.
* [[Galena]], the most common mineral of lead, is primarily lead sulfide (PbS). The sulfide is oxidized to a sulfite (PbSO<sub>3</sub>), which thermally decomposes into lead oxide and sulfur dioxide gas (PbO and SO<sub>2</sub>). The [[sulfur dioxide]] is expelled (like the [[carbon dioxide]] in the previous example), and the lead oxide is reduced as below.

===Reduction===
Reduction is the final, high-temperature step in smelting, in which the oxide becomes the elemental metal. A reducing environment (often provided by carbon monoxide, made by incomplete [[combustion]] in an air-starved furnace) pulls the final [[oxygen]] atoms from the raw metal. The carbon source acts as a chemical reactant to remove oxygen from the ore, yielding the purified metal [[Chemical element|element]] as a product. The carbon source is oxidized in two stages. First, carbon (C) combusts with oxygen (O<sub>2</sub>) in the air to produce [[carbon monoxide]] (CO). Second, the carbon monoxide reacts with the ore (e.g. Fe<sub>2</sub>O<sub>3</sub>) and removes one of its oxygen atoms, releasing carbon dioxide ({{CO2}}). After successive interactions with carbon monoxide, all of the oxygen in the ore will be removed, leaving the raw metal element (e.g. Fe).<ref>{{cite web | title=Blast Furnace | website=Science Aid | url=https://scienceaid.co.uk/chemistry/applied/blastfurnace.html | ref={{sfnref | Science Aid}} | access-date=2021-10-13}}</ref> As most ores are impure, it is often necessary to use [[Flux (metallurgy)|flux]], such as [[limestone]] (or [[Dolomite (mineral)|dolomite]]), to remove the accompanying rock [[gangue]] as slag. This [[calcination]] reaction emits carbon dioxide.

The required temperature varies both in absolute terms and in terms of the melting point of the base metal. Examples:

* [[Iron oxide]] becomes metallic iron at roughly 1250&nbsp;°C (2282&nbsp;°F or 1523&nbsp;K), almost 300 degrees ''below'' iron's melting point of 1538&nbsp;°C (2800&nbsp;°F or 1811&nbsp;K).<ref>{{Cite book |last=Eisele |first=T.C. |title=Direct Biohydrometallurgical Extraction of Iron from Ore |year=2005 |doi=10.2172/877695|url=https://digital.library.unt.edu/ark:/67531/metadc880305/ }}</ref>
* [[Mercuric oxide]] becomes vaporous mercury near 550&nbsp;°C (1022&nbsp;°F or 823&nbsp;K), almost 600 degrees ''above'' mercury's melting point of -38&nbsp;°C (-36.4&nbsp;°F or 235&nbsp;K), and also above mercury's ''boiling'' point.<ref>{{Cite web|title=Mercury processing - Extraction and refining|url=https://www.britannica.com/technology/mercury-processing|access-date=2021-02-23|website=Encyclopedia Britannica|language=en}}</ref>

===Fluxes===
Fluxes are materials added to the ore during smelting to catalyze the desired reactions and to chemically bind to unwanted impurities or reaction products. [[Calcium carbonate]] or [[calcium oxide]] in the form of [[Lime (material)|lime]] are often used for this purpose, since they react with sulfur, phosphorus, and silicon impurities to allow them to be readily separated and discarded, in the form of slag. Fluxes may also serve to control the viscosity and neutralize unwanted acids.

Flux and slag can provide a secondary service after the reduction step is complete; they provide a molten cover on the purified metal, preventing contact with oxygen while still hot enough to readily oxidize. This prevents impurities from forming in the metal.

===Sulfide ores===
[[File:Cowles furnace-2.jpg|thumb|[[Electric Smelting and Aluminum Company|Cowles Syndicate]] of [[Ohio]] in [[Stoke-upon-Trent]] [[England]], late 1880s. [[British Aluminium]] used the process of [[Paul Héroult]] about this time.<ref name=Minet>{{cite book|author=Minet, Adolphe|others=Leonard Waldo (translator, additions)|title=The Production of Aluminum and Its Industrial Use|url=https://archive.org/details/productionalumi01minegoog|year=1905|page=[https://archive.org/details/productionalumi01minegoog/page/n254 244] (Minet speaking) +116 (Héroult speaking)|publisher=John Wiley and Sons, Chapman & Hall|location=New York, London|ol=234319W}}</ref>]]

The ores of base metals are often sulfides. In recent centuries, [[reverberatory furnace]]s have been used to keep the charge being smelted separately from the fuel. Traditionally, they were used for the first step of smelting: forming two liquids, one an oxide slag containing most of the impurities, and the other a sulfide [[matte (metallurgy)|matte]] containing the valuable metal sulfide and some impurities. Such "reverb" [[Reverberatory furnace|furnace]]s are today about 40&nbsp;meters long, 3&nbsp;meters high, and 10&nbsp;meters wide. Fuel is burned at one end to melt the dry sulfide concentrates (usually after partial roasting) which are fed through openings in the roof of the furnace. The slag floats over the heavier matte and is removed and discarded or recycled. The sulfide matte is then sent to the [[converter (Metallurgical)|converter]]. The precise details of the process vary from one furnace to another depending on the mineralogy of the ore body.

While reverberatory furnaces produced slags containing very little copper, they were relatively energy inefficient and off-gassed a low concentration of [[sulfur dioxide]] that was difficult to capture; a new generation of copper smelting technologies has supplanted them.<ref>{{cite book|author=W. G. Davenport |contribution=Copper extraction from the 60s into the 21st century |title=Proceedings of the Copper 99–Cobre 99 International Conference |volume=I—Plenary Lectures/Movement of Copper and Industry Outlook/Copper Applications and Fabrication|editor1=G. A. Eltringham |editor2=N. L. Piret |editor3=M. Sahoo |publisher=The Minerals, Metals and Materials Society |location=Warrendale, Pennsylvania |year=1999 |pages=55–79 |oclc=42774618}}</ref> More recent furnaces exploit bath smelting, top-jetting lance smelting, [[flash smelting]], and blast furnaces. Some examples of bath smelters include the Noranda furnace, the [[Isasmelt]] furnace, the Teniente reactor, the Vunyukov smelter, and the SKS technology. Top-jetting lance smelters include the Mitsubishi smelting reactor. Flash smelters account for over 50% of the world's copper smelters. There are many more varieties of smelting processes, including the Kivset, Ausmelt, Tamano, EAF, and BF.