Smelting
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[[Image:TVA phosphate smelting furnace.jpg|thumb|Electric phosphate smelting furnace in a [[Tennessee Valley Authority|TVA]] [[chemical plant]] (1942)]]
[[Chemical compound|Chemical]] [[redox|reduction]], or '''smelting''', is a form of [[extractive metallurgy]]. The main use of smelting is to produce a metal from its ore. This includes [[iron]] extraction (for the production of [[steel]]) from iron [[ore]], and [[copper extraction]] and other base [[metal]]s from their [[ore]]s. It makes use of a chemical reducing agent, commonly a fuel that is a source of [[carbon]] such as [[coke (fuel)|coke]], or in earlier times [[charcoal]], to change the [[oxidation state]] of the metal ore; however, plants for the electrolytic reduction of [[aluminium]] are also generally referred to as smelters. The carbon or [[carbon monoxide]] derived from it removes [[oxygen]] from the ore to leave the metal. The carbon is oxidized, producing [[carbon dioxide]] and [[carbon monoxide]]. As most ores are impure, it is often necessary to use [[Flux (metallurgy)|flux]], such as [[limestone]] to remove the accompanying rock [[tailings|gangue]] as [[slag]] (also called [[scoria]] or [[cinder]]).
== Smelting basics ==
The 7 metals that were known in ancient times ([[mercury (element)|mercury]], [[tin]], [[lead]], [[copper]], [[silver]], [[gold]], and [[iron]]) can in principle be smelted through similar chemical reactions from their ores:
;Mercury Oxide: <math>2\,HgO + C \rarr 2\,Hg + CO_2</math>
;Cassiterite: <math>2\,SnO_2 + 2\,C \rarr 2\,Sn + 2\,CO_2</math>
;Minium: <math>2\,PbO + C \rarr 2\,Pb + CO_2</math>
;Silver oxide: <math>2\,Ag_2O + C \rarr 4\,Ag + CO_2</math>
;Cuprite: <math>2\,CuO + C \rarr 2\,Cu + CO_2</math>
;Hematite: <math>2\,Fe_2O_3 + 3\,C \rarr 4\,Fe + 3\,CO_2</math>
Different ores require different reactions at different temperatures, but almost always the reducing agent is carbon. The list above is sorted in increasing temperature order, so iron is the most difficult metal to smelt from the ones in the list (that is why historically iron smelting was the last to be discovered).
A common mistake is to think that the metal is obtained from the ore because at high temperature the metal just melts out of the ore. That is incorrect: if a blacksmith just heats up the ore without the proper [[reducing agent]] (carbon), they will just obtain molten ore. Also, one can smelt some ores at a temperature lower than the temperature required to melt the metal. Usually, though, these reactions happen at temperatures high enough to melt the resulting metal, so the metal can just be cast directly out of the furnace.
The exception is that some metal oxides just decompose at relatively low temperatures, so instead of trying to smelt mercury out of mercury oxide, one can just heat up mercury oxide to about 500 °C, and the oxide will decompose into mercury and oxygen; as mercury boils at 357 °C, this will cause the oxide to decompose and boil out, producing the highly toxic gaseous mercury. This is possible only for mercury and a handful of other metal oxides; most metal oxides must be smelted with carbon as the reducing agent.
== First smelting: campfires ==
Smelting is a chemical reaction that requires a particular ore (and many ores look just like any other common sedimentary rock), a particular content of carbon and a particular temperature in order to produce the metal. Without knowledge of chemistry, it is impossible to predict if a given rock can be smelted or not, and what it will produce. Therefore, there is continuous debate to understand how the ancient people learned how to smelt.
Probably the first smelting was done by accident by making a campfire on top of tin or lead ores. That may accidentally produce metallic tin and lead at the bottom of the campfire, as the temperatures to smelt tin and lead are easily obtained by an ordinary fire.
The earliest cast lead beads known today were found in the [[Çatal Höyük]] site in [[Anatolia]] ([[Turkey]]), and were dated of [[7th millennium BC|6500 BC]]. It is unclear when the earliest cast tin artifacts were made, given that tin is much less common than lead, and earlier tin artifacts may have been reused to make [[bronze]].
Although lead is a relatively common metal, the first smelting of lead had less impact in the ancient world. It is soft compared with bronze and steel, but is easy to cast and shape, so became important in the classical world of [[Ancient Greece]] and [[Ancient Rome]] for piping and storage of water.
== Copper smelting: kilns ==
There were in the past some arguments that copper was first smelted by accident also in campfires, but that seems improbable as campfires are about 200°C short of the temperature needed to smelt copper. A more probable path may have been through pottery kilns, invented in Iran by 6000 BC. Pottery kilns produce ceramics that can be glazed with colorful earths (mostly metallic oxides) to produce colorful vases; it happens that malachite (copper oxide) is a colorful green stone, so a potter that encrusts malachite in a ceramic vase in a coal-fired kiln will produce a few droplets of metallic copper (ruining the vase). That may have set the way to smelt copper.
The first known cast copper artifact is a mace head found in Can Hasan, Turkey from [[6th millennium BC|5000 BC]].
Copper created some impact on the ancient world, as it produces good blunt weapons and reasonable armor, but it is still too soft to produce useful blade weapons. Therefore, the smelting of copper did not replace the manufacture of stone weapons, which still produced superior blades.
== Bronze smelting ==
[[Image:Tiangong Kaiwu Tripod Casting.jpg|thumb|right|Casting bronze ding-tripods, from the Chinese ''Tiangong Kaiwu'' encyclopedia of [[Song Yingxing]], published in 1637.]]
[[Bronze]] is a [[copper]]/[[arsenic]] or copper/[[tin]] [[alloy]]. The presence of arsenic and tin dramatically increased the hardness of copper and produced war-winning weapons, as a bronze mace or hammer seemed indestructible by then, as compared to stone maces and hammers that frequently shattered and flaked on impact. When smiths learned to make bronze daggers and swords they found that they kept their edge much longer compared to the existing stone and volcanic glass daggers. Moreover, while one cannot make stone armor (and therefore warriors had to rely on leather armor), bronze can be readily made into a body armor which is impervious to all weapons of the period. Therefore, knowledge of the smelting of bronze allowed kings to overcome their enemies, and caused such a revolution that it marked the end of the Stone Age and the beginning of the Bronze Age. It would be millennia, though, until bronze could be used by common soldiers and townsfolk, and for a long time they were luxury items used by nobility.
The first copper/arsenic bronzes date of [[5th millennium BC|4200 BC]] from [[Asia Minor]], and were used for a long time until replaced by the modern copper/tin bronzes by [[16th century BC|1500 BC]]. It is unclear whether at some point in time the smiths that produced copper/arsenic bronze added arsenic oxides on purpose, or if they explored some copper lodes that happened to have arsenic as a lucky contamination.
The first copper/tin bronzes date of [[32nd century BC|3200 BC]], again from Asia Minor. Copper/tin bronzes are harder and more durable than copper/arsenic ones, and made these obsolete. The process through which the smiths learned to produce copper/tin bronzes is once again a mystery. The first such bronzes were probably a lucky accident from tin contamination of copper ores, but by [[20th century BC|2000 BC]] we know that tin was being mined on purpose for the production of bronze. This is amazing, given that tin is a semi-rare metal, and even a rich [[cassiterite]] ore only has 5% tin. Also, cassiterite looks like any common rock, and it takes special skills (or special instruments) to find it and locate the richer lodes. But, whatever steps were taken to learn about tin, these were fully understood by 2000 BC.
== Iron smelting ==
{{Main|History of ferrous metallurgy}}
=== Early iron smelting ===
The earliest evidence to date for the [[bloomery]] smelting of iron is found at [[Tell Hammeh]], Jordan ([http://www.ironsmelting.net/www/smelting/ see also external link]), and dates to 930 CalBC ([[C14 dating]]). However, based on the archaeological record of iron artifacts, it is clear that intentional reduction of iron metal from terrestrial ores (in the case of [[Hammeh]] a [[Haematite]] ore), must have started near the end of the [[List of archaeological periods|Late Bronze Age]] (ca. 1600–1150 BC). Where and how iron smelting was discovered is widely debated, and remains uncertain due to the significant lack of production finds. Nevertheless, there is some consensus{{Fact|date=July 2008}} that iron technology originated in the Near East, perhaps in Eastern [[Anatolia]].
In [[Ancient Egypt]] somewhere between the [[Third Intermediate Period]] and [[Twenty-third dynasty of Egypt|23rd Dynasty]] (ca. 1100–750 BC) there are indications of iron working. Significantly though, no evidence for the smelting of iron from ore has been attested to Egypt in any period. There is a further possibility of iron smelting and working in [[West Africa]] by [[1200 BC]]<ref>[http://www.homestead.com/wysinger/ironage.html How Old is the Iron Age in Sub-Saharan Africa?] - by Roderick J. McIntosh, Archaeological Institute of America (1999)</ref>. In addition, very early instances of [[carbon steel]] was found to be in production around 2000 years before the present in northwest [[Tanzania]], based on complex preheating principles. These discoveries are significant for the history of metallurgy.<ref>Peter Schmidt, Donald H. Avery. [http://www.sciencemag.org/cgi/content/abstract/201/4361/1085 Complex Iron Smelting and Prehistoric Culture in Tanzania], Science 22 September 1978: Vol. 201. no. 4361, pp. 1085 - 1089</ref>
Most early processes in Europe and Africa involved smelting iron ore in a [[bloomery]], where the temperature is kept low enough so that the iron does not melt. This produces a spongy mass of iron called a bloom, which then has to be consolidated with a hammer.
=== Later iron smelting ===
From the medieval period, the process of direct reduction in bloomeries began to be replaced by an indirect process. In this a [[blast furnace]] was used to make [[pig iron]], which then had to undergo a further process to make forgeable bar iron. Further details of this will be found in the article on the [[blast furnace]]. Processes for the second stage include fining in a [[finery forge]] and from the [[Industrial Revolution]] [[puddling]]. However both processes are now obsolete, and wrought iron is now hardly made. Instead, mild [[steel]] is produced from a [[bessemer converter]] or by other means.
== Base metals ==
[[Image: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|date=1905|url=http://books.google.com/books?id=g14wAAAAMAAJ&pg=PA244|pages=244 (Minet speaking) +116 (Héroult speaking)|publisher=John Wiley and Sons, Chapman & Hall, via Google Books scan of University of Wisconsin - Madison copy|location=New York, London|accessdate=2007-10-28}}</ref>]]
The ores of base metals are often sulfides. In recent centuries, [[reverberatory furnace|reverberatory smelters]] (sometimes called cupolas) have been used. These keep the fuel and the charge being smelted separate. Traditionally these were used for carrying out the first step: formation of two liquids, one an oxide [[slag]] containing most of the impurity elements, and the other a sulfide [[matte (metallurgy)|matte]] containing the valuable metal sulfide and some impurities. Such "reverb" [[furnace]]s are today about 40 m long, 3 m high and 10 m wide. Fuel is burned at one end and the heat melts the dry sulfide concentrates (usually after partial roasting), which is fed through the openings in the roof of the furnace. The slag floats on top of the heavier matte, and is removed and discarded or recycled. The sulfide matte is then sent to the [[converter (Metallurgical)|converter]]. However the precise details of the process will vary for one metal to another.
==References==
{{reflist|2}}
==Bibliography==
<div class="references-small">
*'''Pleiner, R.''' (2000) ''Iron in Archaeology. The European Bloomery Smelters'', Praha, Archeologický Ústav Av Cr.
*'''Veldhuijzen, H.A.''' (2005) Technical Ceramics in Early Iron Smelting. The Role of Ceramics in the Early First Millennium Bc Iron Production at Tell Hammeh (Az-Zarqa), Jordan. In: Prudêncio, I.Dias, I. and Waerenborgh, J.C. (Eds.) ''Understanding People through Their Pottery; Proceedings of the 7th European Meeting on Ancient Ceramics (Emac '03)''. Lisboa, Instituto Português de Arqueologia (IPA).
*'''Veldhuijzen, H.A. and Rehren, Th.''' (2006) Iron Smelting Slag Formation at Tell Hammeh (Az-Zarqa), Jordan. In: Pérez-Arantegui, J. (Ed.) ''Proceedings of the 34th International Symposium on Archaeometry, Zaragoza, 3-7 May 2004''. Zaragoza, Institución «Fernando el Católico» (C.S.I.C.) Excma. Diputación de Zaragoza.
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==See also==
{{Wiktionary}}
*[[Blast furnace]]
*[[Bloomery]]
*[[Cast iron]]
*[[Clinker (waste)]]
*[[Copper extraction]]
*[[Furnace]]
*[[Iron Age]]
*[[Metallurgy]]
*[[Pyrometallurgy]]
*[[Slag]]
*[[Wrought iron]]
*[[Zinc smelting]]
[[Category:Dutch loanwords]]
[[Category:Metallurgy]]
[[Category:Metals processes]]
[[Category:Steelmaking]]
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[[ja:製錬]]
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[[pl:Wytapianie metalu]]
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[[sv:Smältverk]]