Electric phosphate smelting furnace in a TVA chemical plant (1942)
For other uses, see Smelt (disambiguation).
Smelting is a form of extractive metallurgy; its main use 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 metals from their ores. Smelting uses heat and a chemical reducing agent, commonly a fuel that is a source of carbon such as coke, or in earlier times charcoal, to change the oxidation state of the metal ore. The carbon or carbon monoxide derived from it removes oxygen from the ore to leave the metal. The carbon is thus oxidized, producing carbon dioxide and carbon monoxide. As most ores are impure, it is often necessary to use flux, such as limestone, to remove the accompanying rock gangue as slag.
Plants for the electrolytic reduction of aluminium, while not using carbon, are also generally referred to as smelters.
Contents
1 Smelting basics
2 First smelting: campfires
3 Copper smelting: kilns
4 Bronze smelting
5 Iron smelting
5.1 Early iron smelting
5.2 Later iron smelting
6 Base metals
7 See also
8 References
9 Bibliography
10 External links
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Smelting basics
The seven metals that were known in ancient times (mercury, tin, lead, copper, silver, gold, and iron) can in principle be smelted through similar chemical reactions from their ores:
Mercury oxide to mercury
Cassiterite to tin
Minium to lead
Silver oxide to silver
Cuprite to copper
Hematite to iron
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 in this case, 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 (932), and the oxide will decompose into mercury and oxygen; as mercury boils at 357 (675), 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. Such a combination 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 ?atal H?y site in Anatolia (Turkey), and were dated of 6500BC. 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 short of the temperature needed to smelt copper. A more probable path may have been through pottery kilns, invented in Persia by 6000BC. 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 5000BC.
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
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 at the time, 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 4200BC from Asia Minor, and were used for a long time until replaced by the modern copper/tin bronzes by 1500BC. 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 3200BC, 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 2000BC 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 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 2000BC.
Iron smelting
Main article: History of ferrous metallurgy
Early iron smelting
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