During the industrial extraction of iron from hematite in the blast furnace, distinct chem… — JAMB UTME

During the industrial extraction of iron from hematite in the blast furnace, distinct chemical reactions occur across different temperature zones. Arrange the following key reaction stages in sequential order from the top of the furnace (coolest zone, approx. $250^\circ\text{C}-400^\circ\text{C}$ ) down to the hearth/tuyere region at the bottom (hottest zone, approx. $1500^\circ\text{C}-1900^\circ\text{C}$ ):

1Reduction of hematite ( $\text{Fe}_2\text{O}_3$ ) by carbon(II) oxide ( $\text{CO}$ ) to yield triiron tetroxide ( $\text{Fe}_3\text{O}_4$ ).
2Reduction of triiron tetroxide ( $\text{Fe}_3\text{O}_4$ ) by carbon(II) oxide ( $\text{CO}$ ) to produce iron(II) oxide ( $\text{FeO}$ ).
3Reduction of iron(II) oxide ( $\text{FeO}$ ) by carbon(II) oxide ( $\text{CO}$ ) to form spongy metallic iron ( $\text{Fe}$ ).
4Combination of calcium oxide ( $\text{CaO}$ ) with silica ( $\text{SiO}_2$ ) to form molten slag ( $\text{CaSiO}_3$ ).
5Exothermic combustion of coke ( $\text{C}$ ) in blasts of preheated air ( $\text{O}_2$ ) to form carbon(IV) oxide ( $\text{CO}_2$ ).

Answer

The correct sequential order from top (coolest zone) to bottom (hottest zone) is: (1) Reduction of hematite to triiron tetroxide, (2) Reduction of triiron tetroxide to iron(II) oxide, (3) Reduction of iron(II) oxide to metallic iron, (4) Combination of calcium oxide with silica to form slag, and (5) Exothermic combustion of coke with preheated air.

The blast furnace operates with a temperature gradient rising from top to bottom. Near the top (

200^\circ\text{C}-400^\circ\text{C}

), hematite (

\text{Fe}_2\text{O}_3

) is reduced to

\text{Fe}_3\text{O}_4

. As the mixture descends to warmer regions (

500^\circ\text{C}-700^\circ\text{C}

\text{Fe}_3\text{O}_4

is reduced to

\text{FeO}

. Deeper down (

800^\circ\text{C}-1000^\circ\text{C}

\text{FeO}

is reduced to spongy iron (

\text{Fe}

). At around

1000^\circ\text{C}-1200^\circ\text{C}

, limestone-derived

\text{CaO}

reacts with

\text{SiO}_2

to form molten slag (

\text{CaSiO}_3

). Finally, at the base near the tuyeres (

1500^\circ\text{C}-1900^\circ\text{C}

), coke reacts exothermically with oxygen to fuel the entire process.

Step-by-Step Solution

Analyze the thermal gradient inside the blast furnace

Temperatures increase from top (

200^\circ\text{C}

) to bottom hearth region (

>1500^\circ\text{C}

Cold raw materials enter from the top while hot air blasts enter from the tuyeres at the bottom.

Identify top-zone reactions (

200^\circ\text{C}-400^\circ\text{C}

)

3\text{Fe}_2\text{O}_3 + \text{CO} \rightarrow 2\text{Fe}_3\text{O}_4 + \text{CO}_2

Hematite is initially converted to magnetite at relatively low temperatures by upflowing carbon(II) oxide.

Identify upper-middle zone reactions (

500^\circ\text{C}-700^\circ\text{C}

)

\text{Fe}_3\text{O}_4 + \text{CO} \rightarrow 3\text{FeO} + \text{CO}_2

Further reduction converts magnetite into iron(II) oxide as the charge descends.

Identify lower-middle zone reactions (

800^\circ\text{C}-1000^\circ\text{C}

)

\text{FeO} + \text{CO} \rightarrow \text{Fe} + \text{CO}_2

Complete reduction of iron(II) oxide to spongy metallic iron takes place here.

Identify slag formation zone (

1000^\circ\text{C}-1200^\circ\text{C}

)

\text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3

Limestone decomposes into calcium oxide, which reacts with acidic silicon dioxide impurities to form molten calcium trioxosilicate(IV) slag.

Identify bottom tuyere region reactions (

1500^\circ\text{C}-1900^\circ\text{C}

)

\text{C} + \text{O}_2 \rightarrow \text{CO}_2

Preheated air blown in through tuyeres reacts violently with coke to generate heat and carbon dioxide, which is subsequently reduced by hot coke to carbon monoxide.

Key Concept

Blast furnace temperature zones and sequential chemical reduction stages of iron ore

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