Extraction of Iron
Master the Blast Furnace, Temperature Zones, and Iron Types.
Iron is the most widely used metal in the world. It is extracted primarily from its oxide ores, such as Hematite ($Fe_2O_3$) and Magnetite ($Fe_3O_4$), using a massive, towering reactor known as a Blast Furnace. This process is a classic example of pyrometallurgy, relying on high temperatures and carbon as a reducing agent.
1. Raw Materials & Ore Preparation
Before feeding the ore into the blast furnace, it is concentrated (usually by gravity separation/hydraulic washing to remove lighter earthy impurities) and then Roasted/Calcined in a reverberatory furnace.
Why do we roast the iron ore?
- To remove moisture and volatile impurities (like Sulfur, Arsenic, and Phosphorus) as volatile oxides ($SO_2$, $As_2O_3$).
- To decompose carbonates into oxides: $FeCO_3 \xrightarrow{\Delta} FeO + CO_2$
- Crucial Step: To oxidize any ferrous oxide ($FeO$) to ferric oxide ($Fe_2O_3$) so it does not react with silica gangue to form iron silicate slag prematurely.
$4FeO + O_2 \xrightarrow{\Delta} 2Fe_2O_3$
1. Roasted Ore ($Fe_2O_3$)
2. Limestone ($CaCO_3$ - acts as the basic flux)
3. Coke (Carbon - acts as both fuel and reducing agent)
Usually mixed in a ratio of 8 : 1 : 4.
2. The Blast Furnace & Temperature Zones
The blast furnace is a tall, cylindrical steel tower lined with refractory fire bricks. A blast of hot air ($~1000 \text{ K}$) is blown from the bottom through pipes called tuyeres. This creates a massive temperature gradient, ranging from $~1900 \text{ K}$ at the bottom to $~500 \text{ K}$ at the top. Different chemical reactions occur in these different temperature zones.
Figure 1: Cross-section of a Blast Furnace illustrating temperature zones and material flow.
3. Chemical Reactions Zone by Zone
A. Combustion Zone (1500 K – 1900 K)
At the very bottom, hot air oxidizes the falling coke (Carbon). This reaction is highly exothermic and supplies the immense heat required for the entire furnace.
B. Heat Absorption (Fusion) Zone (1200 K – 1500 K)
As the $CO_2$ rises, it meets more falling hot coke. The carbon reduces the $CO_2$ into Carbon Monoxide ($CO$). This reaction is endothermic, causing a drop in temperature slightly higher up in the furnace. Spongy iron melts in this zone.
C. Slag Formation Zone (900 K – 1200 K)
In the middle of the furnace, the limestone ($CaCO_3$) decomposes into Calcium Oxide ($CaO$, a basic flux) and $CO_2$. The $CaO$ immediately reacts with the acidic silica ($SiO_2$) gangue to form molten Calcium Silicate Slag.
$CaO(s) + SiO_2(s) \rightarrow CaSiO_3(l) \text{ (Slag)}$
D. Reduction Zone (500 K – 800 K)
At the top of the furnace, the rising Carbon Monoxide ($CO$) gas chemically reduces the falling iron oxide ore to solid, spongy iron. It happens in a stepwise manner:
$Fe_3O_4 + 4CO \rightarrow 3Fe + 4CO_2$
$Fe_2O_3 + CO \rightarrow 2FeO + CO_2$
4. Commercial Forms of Iron
The iron obtained directly from the blast furnace is not pure. Based on the carbon and impurity content, iron is classified into three main types:
1. Pig Iron
The molten iron tapped directly from the bottom of the blast furnace. It contains about 4% Carbon and many impurities (S, P, Si, Mn). It is extremely hard and brittle, making it unsuitable for structural use directly.
2. Cast Iron
Made by melting Pig Iron with scrap iron and coke using a hot air blast. It has slightly lower carbon content (~3% Carbon). It is extremely hard and brittle, used for casting pipes, stoves, and manhole covers.
3. Wrought Iron
The purest commercial form of iron (malleable iron). It contains only 0.2% to 0.5% Carbon. It is prepared by oxidizing impurities in a reverberatory furnace lined with Hematite ($Fe_2O_3$). Very tough and malleable.
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