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When reducing iron oxide with carbon, must the substances be set alight or do they only need to be heated. Also, when the iron is formed will it immediately oxidise in the air?

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  • $\begingroup$ What is the difference between "set alight" and "be heated", really? $\endgroup$ – Ivan Neretin Apr 2 '16 at 19:22
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The direct reduction with carbon is only possible if the iron oxide ($\ce{Fe2O3}$, $\ce{Fe3O4}$, or $\ce{FeO}$) is present as a liquid.

$$\ce{FeO$_n$ + $n$C -> Fe + $n$CO}$$

Accordingly, the required temperature is very high. Note the melting points of $\ce{Fe2O3}$ (1539 °C), $\ce{Fe3O4}$ (1597 °C), $\ce{FeO}$ (1377 °C), pure $\ce{Fe}$ (1538 °C), and the eutectic of iron containing 4.3 % carbon (about 1150 °C).

In the blast furnace process, solid iron oxides at lower temperatures are mainly reduced with carbon monoxide.

$$\ce{FeO$_n$ + $n$CO -> Fe + $n$CO2}$$

The required carbon monoxide is produced from the exothermic reaction of carbon with oxygen

$$\ce{2C + O2 -> 2CO}$$

and from the endothermic Boudouard rection of carbon with carbon dioxide.

$$\ce{C + CO2 <=> 2CO}$$

A temperature well above 700 °C is required to favour the formation of carbon monoxide. Hence, a part of the carbon has to be burned in order to generate the required heat and to produce carbon monoxide.

Hot fine-dispersed iron would be quickly oxidized in air (for example, steel wool can burn in air) whereas fused iron is only oxidized at the surface of the liquid. Note that the pig iron produced in the blast furnace process has a high carbon content, which significantly lowers the melting point.

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