Smelting
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- See smelt for the style of fishing with dip nets in tributaries of the Great Lakes during the spring spawning run of that small fish.
It makes use of 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 oxidised, 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 (also called scoria or cinder).
Early iron smelting
The earliest evidence to date for the bloomery smelting of iron is found at [Tell Hammeh, Jordan], and dates to 930 CalBC (C14 dating). However, based on the archaeological record of iron artefacts, 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 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 that iron technology originated in the Near East, perhaps in Eastern Anatolia.In Ancient Egypt somewhere between the Third Intermediate Period and 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 in Egypt in any period. There are further indications of iron smelting and working in West Africa in 500 BC [link].
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 consoldated 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
The ores of base metals are often sulphides. In recent centuries, 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" containing the valuable metal sulfide and some impurities. Such "reverb" furnaces are today about 40m long, 3m high and 10m 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. However the precise details of the process will vary for one metal to another.See also
- Bloomery
- Slag
- Wrought iron
- Cast iron
- Blast furnace
- Furnace
- Metallurgy
- Pyrometallurgy
- copper extraction
- Iron Age
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