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I've read that:

"During extraction of iron another impurity appear which is carbon from coke in furnace."

However, I don't understand how there will be enough unreacted carbon that would blend with iron. As reaction of carbon and oxygen is very exothermic so there must be enough energy for all carbon atoms to react with oxygen.

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  • $\begingroup$ Partly related: Required temperature for the reduction of iron oxide using carbon. Note that for the eutectic of iron containing 4.3 % carbon, the melting point is lowered from 1540 °C to 1150 °C. $\endgroup$
    – user7951
    Mar 6 '20 at 16:28
  • $\begingroup$ @Loong Good point. The blast furnace would be screwed if it weren´t for carbon, in more than one way. $\endgroup$
    – Karl
    Mar 6 '20 at 17:46
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A blast furnace is made of successive layers of charcoal and iron oxide mineral. In the beginning, the lower layer of charcoal is lit and the fire is maintained by air being pressed through the burning matter. The combustion produces first $CO_2$. But as the temperature is high and the charcoal is in excess, $CO_2$ soon reacts with $C$ producing $2$ $CO$ according to the equations :$$\ce{C + O_2 -> CO_2}$$ $$\ce{CO_2 + C -> 2 CO}$$ Then the CO gas touches the bits or pieces of mineral, which produces the following equation :$$\ce{3 CO + Fe_2O_3 -> 2 Fe + 3 CO_2}$$ This produces some metallic iron which is liquid if the temperature is high enough. This liquid flows between the bits of burning charcoal, and dissolves some charcoal during this flow. The obtained pig iron contains about 5% carbon. which must be later on burned in special furnaces, to get steel (containing about 1% dissolved carbon)

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  • $\begingroup$ No need to dissolve solid coke. $\ce{CO + Fe <-> FeO + C}$. $\endgroup$
    – Karl
    Mar 6 '20 at 17:33
  • $\begingroup$ There is no charcoal in a blast furnace. $\endgroup$ Mar 6 '20 at 22:57
  • $\begingroup$ So coke and iron are added in the same time but in layers right? I thought each one was added separately that's why I found it weird that there was enough coke left to react with iron that would be added later. $\endgroup$
    – Manar
    Mar 7 '20 at 13:04
  • $\begingroup$ And limestone..... $\endgroup$ Mar 7 '20 at 15:41
  • $\begingroup$ But if limestone is added since the very beginning why doesn't sulphur phosphorus and silicon react with the CaO produced from limestone thermal decomposition, why is it only silica that reacts while the rest of impurities are getten rid of in the steel-making furnace? $\endgroup$
    – Manar
    Mar 7 '20 at 17:33
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The carbon in the pig iron comes from the coke ; although modern blast furnaces may inject hydrocarbon gas and oil. The oxygen is controlled so carbon containing gas exiting is CO ( monoxide).

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  • $\begingroup$ Modern blast furnaces still use coke! $\endgroup$
    – Karl
    Mar 6 '20 at 17:24
  • $\begingroup$ They need the physical properties of the coke , but anthracite coke is getting more costly so they supplement it with oil and gas in a modern operation. They also use oxygen enrichment so there is less useless nitrogen getting heated in the furnace. I expect Wiki has the info. $\endgroup$ Mar 7 '20 at 15:39

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