Extraction of Iron

Q: How is iron extracted from haematite?

Haematite is crushed, concentrated and roasted in air, then charged with coke and limestone into a blast furnace. Coke burns in the hot air blast to carbon monoxide, which reduces the iron oxide in the upper zone, while coke reduces the rest lower down. Limestone removes silica as slag, and molten pig iron is tapped from the hearth.

Q: What happens in each zone of the blast furnace?

At the bottom, near the tuyeres, coke burns to carbon dioxide and then carbon monoxide at about 2170 K. Above it, iron melts and absorbs carbon in the fusion zone. In the middle, limestone forms calcium silicate slag. Near the top, at 500 to 900 K, carbon monoxide reduces the iron oxides.

Q: Why is limestone added to the blast furnace?

Limestone is the flux. It decomposes to lime and carbon dioxide at about 1270 K, and the lime combines with the silica gangue to form fusible calcium silicate slag. The slag floats on the molten iron, protects it from oxidation and is run off through the slag notch.

Q: What is the difference between pig iron, cast iron and wrought iron?

Pig iron comes straight from the blast furnace and has about 4 percent carbon. Cast iron is pig iron remelted with scrap and coke, with about 3 percent carbon; it is hard and brittle. Wrought iron, made by puddling with haematite, has only 0.1 to 0.25 percent carbon and is the purest, most malleable form.

Q: Why is carbon monoxide the main reducing agent in the upper blast furnace?

At 500 to 900 K the oxidation of carbon monoxide to carbon dioxide releases more Gibbs energy than the formation of the iron oxides, so it reduces them. Coke is a better reducing agent only at higher temperatures, above about 1073 K, where the carbon to carbon monoxide Ellingham line has fallen lower.

Q: How is steel made from pig iron?

The impurities in molten pig iron are oxidised by air in a Bessemer converter, by haematite in an open-hearth furnace or by pure oxygen in the LD process. The oxides leave as gases or slag, and the exact carbon is added back, often as spiegeleisen. Alloy steels are made from scrap in electric arc or induction furnaces.

Q: Is the blast furnace important for JEE Advanced?

Yes. JEE Advanced lists iron as an example of the thermodynamic principles of metallurgy. Questions ask which zone a reaction belongs to, why carbon monoxide works in the upper furnace and coke lower down, the role of limestone, and the Ellingham crossing of the carbon and iron oxide lines.

Q: Is the extraction of iron in the NEET syllabus?

No. NEET dropped metallurgy in 2024, so the blast furnace and steel making are not examined. NEET students still use related ideas elsewhere: the oxidation states of iron in the d-block chapter, and Gibbs energy and equilibrium in thermodynamics, which explain why carbon monoxide reduces iron oxides.

ChemistryGeneral Principles And Processes Of Isolation Of ElementsFor JEE aspirants

The extraction of iron starts from haematite or magnetite. The ore is concentrated, calcined and roasted, then smelted with coke and limestone in a blast furnace, where carbon monoxide and coke reduce the iron oxides to molten pig iron. Pig iron is then turned into cast iron, wrought iron or steel. The extraction of iron is the standard example of the thermodynamic (Ellingham) principles of metallurgy in JEE Advanced.

On this page1Ores2Outline3Roasting4Blast furnace5Forms of iron6Steel making7Heat treatment8Alloy steels9Mind map10Examples

Key Formulas - Quick Reference

Ores: haematite $F e_{2} O_{3}$ , magnetite $F e_{3} O_{4}$ (richest, 72.4 % Fe), limonite $F e_{2} O_{3} \cdot 3 H_{2} O$ , siderite $FeC O_{3}$
Charge: roasted ore + coke + limestone (about 8 : 4 : 1); hot air blown in through the tuyeres
★ Must learnCombustion zone: $C + O_{2} \to C O_{2}$ ; $C O_{2} + C \to 2CO$ (endothermic)
★ Must learnUpper furnace, 500-800 K: $3F e_{2} O_{3} + CO \to 2F e_{3} O_{4} + C O_{2}$ ; $F e_{3} O_{4} + 4CO \to 3Fe + 4C O_{2}$
Lower furnace, 900-1500 K: $FeO + CO \to Fe + C O_{2}$ ; $FeO + C \to Fe + CO$
★ Must learnSlag: $CaC O_{3} \to CaO + C O_{2}$ ; $CaO + Si O_{2} \to CaSi O_{3}$
★ Must learnCarbon content: pig iron about 4 %, cast iron about 3 %, steel 0.25-2 %, wrought iron 0.1-0.25 %
Wrought iron (puddling): $F e_{2} O_{3} + 3C \to 2Fe + 3CO$
★ Must learnBasic Bessemer: $4P + 5 O_{2} \to 2 P_{2} O_{5}$ ; $3CaO + P_{2} O_{5} \to C a_{3} (P O_{4})_{2}$ (Thomas slag)

1. Occurrence and Ores of Iron

Iron is the fourth most abundant element in the Earth's crust and the second most abundant metal, after aluminium. Being fairly reactive, it occurs in the combined state, mainly as oxides.

Ore	Formula	Colour and remarks	Fe (% by mass)
Magnetite	$F e_{3} O_{4}$ ( $FeO \cdot F e_{2} O_{3}$ )	black; magnetic; richest ore	72.4
Haematite	$F e_{2} O_{3}$	red; the chief ore	69.9
Limonite	$F e_{2} O_{3} \cdot 3 H_{2} O$	yellow, brown or red (hydrated oxide)	52.3
Siderite (spathic iron ore)	$FeC O_{3}$	called clay-ironstone when mixed with clay	48.2
Iron pyrites	$Fe S_{2}$	used to make $S O_{2}$ and sulphuric acid, not iron	-
Copper pyrites	$CuFe S_{2}$	an ore of copper, not of iron	-

The percentages are for the pure minerals. Pyrites is not used for iron because any sulphur left in the metal makes iron brittle; instead it is roasted to make $S O_{2}$ for sulphuric acid.

2. Outline of the Extraction

The extraction of iron has three stages, after which the pig iron from the blast furnace is converted into the commercial forms of iron (Figure 1):

Concentration of the ore.
Calcination and roasting to give porous ferric oxide.
Smelting in a blast furnace with coke and limestone.

Figure 1: Route map for iron. The blast furnace always gives pig iron; the three commercial forms differ mainly in how much carbon is left in them.

3. Concentration, Calcination and Roasting

3.1 Dressing and concentration

The ore is crushed into pieces about 3-5 cm across. Large lumps of stony impurity are removed by hand picking. The ore is then concentrated by gravity separation (hydraulic washing), and magnetite is separated from non-magnetic gangue by magnetic separation.

3.2 Calcination and roasting

The concentrated ore is heated strongly in a regular supply of air. Water, $C O_{2}$ , sulphur and arsenic are driven off, and ferrous oxide is oxidised to ferric oxide:

F e_{2} O_{3} \cdot 3 H_{2} O Δ F e_{2} O_{3} + 3 H_{2} O ↑

FeC O_{3} Δ FeO + C O_{2} ↑

S + O_{2} S O_{2} ↑

4As + 3 O_{2} 2A s_{2} O_{3} ↑

4FeO + O_{2} 2F e_{2} O_{3}

4F e_{3} O_{4} + O_{2} 6F e_{2} O_{3}

Converting FeO to $F e_{2} O_{3}$ matters. Ferrous oxide is basic enough to combine with silica at high temperature, $FeO + Si O_{2} \to FeSi O_{3}$ , so iron would be lost in the slag. Ferric oxide does not do this. Roasting also makes the ore porous, which lets the reducing gases reach its interior.

Exam Trick

Ferrous goes to slag, ferric goes to iron. Roasting in excess air turns every FeO into $F e_{2} O_{3}$ so that no iron is wasted as $FeSi O_{3}$ slag.

4. Smelting in the Blast Furnace

4.1 The furnace and the charge

The blast furnace is a tall steel shaft, about 25-30 m high, lined inside with fire bricks. Its main parts are:

Cup and cone (double bell) at the top, through which the charge is fed without letting the furnace gases escape. The furnace is kept filled to about four-fifths of its height.
Stack, which widens downwards so that the descending charge does not jam, and waste gas outlets near the top.
Bosh, the widest part, just above the tuyeres.
Tuyeres: nozzles near the bottom through which a blast of hot air (about 1000 K, preheated in stoves heated by the waste gases) is blown in.
Hearth at the bottom, where molten iron collects under molten slag. The slag notch is placed higher than the tap hole for iron.

The charge is roasted ore, coke and limestone, roughly in the ratio 8 : 4 : 1 by mass. Coke is both the fuel and the reducing agent; limestone is the flux. The charge moves down against the rising stream of hot gases, and the temperature rises from about 500 K at the top to about 2170 K near the tuyeres (Figure 2).

Figure 2: Blast furnace. The charge moves down against rising hot CO. Oxides are reduced by CO near the top, coke takes over lower down, and molten iron collects under a layer of slag.

4.2 Reactions zone by zone

Zone of combustion (near the tuyeres, about 2170 K). Coke burns in the hot air blast and supplies most of the heat. The $C O_{2}$ formed is at once reduced by more hot coke to CO, the main reducing agent:

C + O_{2} C O_{2} Δ H^{\circ} = - 393.5 kJ

C O_{2} + C 2CO Δ H^{\circ} = + 172.5 kJ

The second reaction is endothermic, so the temperature falls a little just above the combustion zone.

Zone of fusion (about 1570 K). The spongy iron formed higher up melts and dissolves carbon, silicon, manganese, phosphorus and sulphur. FeO still present is reduced directly by coke, and some other oxides are reduced too:

FeO + C Fe + CO

Si O_{2} + 2C Si + 2CO

Mn O_{2} + 2C Mn + 2CO

2C a_{3} (P O_{4})_{2} + 6Si O_{2} 6CaSi O_{3} + P_{4} O_{10}

P_{4} O_{10} + 10C P_{4} + 10CO

Zone of slag formation (about 1270 K). Limestone decomposes and the lime combines with silica to form fusible calcium silicate slag, which trickles down and floats on the molten iron. Here CO also reduces FeO:

CaC O_{3} CaO + C O_{2}

CaO + Si O_{2} CaSi O_{3}

FeO + CO Fe + C O_{2}

Zone of reduction (upper part, 500-900 K). The rising CO reduces the iron oxides step by step to spongy iron:

3F e_{2} O_{3} + CO 2F e_{3} O_{4} + C O_{2}

F e_{3} O_{4} + 4CO 3Fe + 4C O_{2}

F e_{2} O_{3} + CO 2FeO + C O_{2}

The overall change is $F e_{2} O_{3} + 3CO \to 2Fe + 3C O_{2}$ . In the coolest part near the top, some CO also splits into fine carbon and $C O_{2}$ ( $2CO \to C + C O_{2}$ ), because this reverse change is favoured at low temperature.

Zone	Temperature	Main reactions
Reduction (top)	500-900 K	$3F e_{2} O_{3} + CO \to 2F e_{3} O_{4} + C O_{2}$ ; $F e_{3} O_{4} + 4CO \to 3Fe + 4C O_{2}$
Slag formation (middle)	about 1270 K	$CaC O_{3} \to CaO + C O_{2}$ ; $CaO + Si O_{2} \to CaSi O_{3}$
Fusion (lower middle)	about 1570 K	iron melts; $FeO + C \to Fe + CO$ ; Si, Mn, P reduced
Combustion (bottom)	about 2170 K	$C + O_{2} \to C O_{2}$ ; $C O_{2} + C \to 2CO$

4.3 The thermodynamics behind the zones

The Ellingham diagram explains why the work is shared. At the lower temperatures of the upper furnace, the CO/ $C O_{2}$ line lies below the lines for $F e_{2} O_{3}$ and $F e_{3} O_{4}$ , so CO reduces them. The C/CO line slopes downward and falls below the Fe/FeO line above about 1073 K, so in the hotter lower part coke itself reduces FeO. For $FeO + C \to Fe + CO$ (per mole of $O_{2}$ ):

Δ_{r} G^{\circ} = Δ_{f} G_{(C, CO)}^{\circ} - Δ_{f} G_{(Fe, FeO)}^{\circ} < 0 above about 1073 K

Figure 3 shows the three lines that matter.

Figure 3: The Ellingham lines behind the blast furnace. Coke's C to CO line falls below Fe/FeO near 1000 K, so coke reduces FeO in the hot lower furnace; the CO line lies below Fe/FeO only near the cool top.

JEE Advanced

How can CO reduce FeO at 900-1500 K? For $FeO + CO \to Fe + C O_{2}$ , $Δ H^{\circ} \approx - 11$ kJ and $Δ S^{\circ} \approx - 17$ J K^-1, so $Δ G^{\circ}$ is slightly positive above about 630 K. At 1000 K, $Δ G^{\circ} \approx + 6.4$ kJ and $K = p_{C O_{2}} / p_{CO} \approx 0.47$ . The reaction still goes because the furnace gas is kept very rich in CO: as long as the CO : $C O_{2}$ ratio is above about 2.2, $Δ G = Δ G^{\circ} + R T ln Q$ is negative. Hot coke keeps converting $C O_{2}$ back to CO, which is why the gas leaving the top still contains a lot of CO.

Figure 4 plots this minimum ratio against temperature.

Figure 4: How much CO the furnace gas must hold. Above the curve FeO is reduced; below it iron would be re-oxidised. Hot coke keeps the gas above the curve.

4.4 Products of the blast furnace

Molten iron is denser than slag and collects at the bottom. It is tapped off and cast into moulds called pigs, giving pig iron.
Slag ( $CaSi O_{3}$ ) floats on the iron, protects it from oxidation, and is run off through the slag notch. It is used to make cement and for road building.
Waste gases (mainly $N_{2}$ , with CO and $C O_{2}$ ) leave at the top. Their CO is burnt to heat the hot-air stoves.

Quick Recall: tap to check

Which zone of the blast furnace is hottest, and why?

The combustion zone near the tuyeres (about 2170 K), where coke burns in the hot air blast.

What reduces

F e_{2} O_{3}

in the upper furnace?

Carbon monoxide.

Why is the slag notch above the tap hole?

Slag is lighter and floats on the molten iron.

Key idea

CO reduces the iron oxides in the cool upper furnace; hot coke takes over lower down and keeps regenerating CO, and limestone carries silica away as slag.

5. Commercial Forms of Iron

5.1 Pig iron and cast iron

Pig iron from the blast furnace contains about 4 % carbon and smaller amounts of Si, Mn, P and S. Cast iron is made by remelting pig iron with scrap iron and coke in a hot air blast. It has slightly less carbon (about 3 %), is very hard and brittle, and expands slightly on solidifying, so it gives sharp castings. It cannot be welded or forged.

5.2 Wrought iron (malleable iron)

Wrought iron is the purest commercial form of iron (0.1-0.25 % C, other impurities below 0.3 %). It is made by puddling: cast iron is melted in a reverberatory (puddling) furnace lined with haematite, in a hot blast of air. The haematite and the air oxidise the impurities:

F e_{2} O_{3} + 3C 2Fe + 3CO ↑

2F e_{2} O_{3} + 3S 4Fe + 3S O_{2} ↑

2F e_{2} O_{3} + 3Si 4Fe + 3Si O_{2}

F e_{2} O_{3} + 3Mn 2Fe + 3MnO

4P + 5 O_{2} 2 P_{2} O_{5}

The oxides of Mn, Si and P then form slags of manganese silicate and ferric phosphate. Limestone may be added as flux:

MnO + Si O_{2} MnSi O_{3}

F e_{2} O_{3} + P_{2} O_{5} 2FeP O_{4}

As the impurities leave, the melting point of the iron rises and it becomes a pasty mass. It is gathered into balls with iron rods (rabbles), taken out and hammered or passed through rollers to squeeze out the slag. The product is fibrous, tough and malleable; it is used for chains, anchors, wires and the cores of electromagnets.

5.3 Steel

Steel contains 0.25-2 % carbon. Its properties can be tuned by the carbon content, by alloying metals and by heat treatment, which makes it the most useful form of iron. Steel can be made by lowering the carbon content of cast iron (the usual way), by adding carbon to wrought iron, by mixing cast and wrought iron in the right proportion, or directly from the ore.

Figure 5: The commercial forms of iron differ mainly in carbon content. Removing carbon makes iron softer and more malleable; adding it makes iron harder and more brittle.

Cast iron

About 3 % carbon.
Hard and brittle.
Expands on setting: sharp castings.
Cannot be welded or forged.

Wrought iron

0.1-0.25 % carbon (purest form).
Soft, malleable, fibrous.
Made by puddling with haematite.
Can be welded and forged.

Form	Carbon (%)	Properties	Uses
Pig iron	about 4 (2.3-4.6)	impure, hard, brittle	raw material for cast iron and steel
Cast iron	about 3 (2.6-4.3)	hard, brittle, cannot be welded; expands on setting	pipes, stoves, railings, machine bases
Steel	0.25-2	tough, elastic; properties adjustable	construction, machines, tools
Wrought iron	0.1-0.25	purest, soft, malleable, fibrous, weldable	chains, anchors, wires, electromagnet cores

Key idea

The commercial forms of iron differ mainly in carbon: pig iron about 4 %, cast iron about 3 %, steel 0.25-2 %, wrought iron below 0.25 %.

6. Manufacture of Steel

All methods remove C, Si, Mn, P and S from pig iron by oxidation, then add back exactly the carbon (and any alloying metals) needed (Figure 6).

6.1 Bessemer process

Molten pig iron is poured into a pear-shaped steel Bessemer converter, and a blast of air is blown through it from tuyeres at the bottom. The impurities burn, and the heat released keeps the iron molten without any fuel:

2Mn + O_{2} 2MnO

Si + O_{2} Si O_{2}

2C + O_{2} 2CO ↑

MnO + Si O_{2} MnSi O_{3}

CO burns with a blue flame at the mouth of the converter. When all the carbon has gone the flame suddenly dies, and the blast is stopped. The exact amount of carbon is then added as spiegeleisen, an alloy of Fe, Mn and C; the manganese also removes dissolved oxygen. The manganese silicate formed above leaves as slag.

Acid Bessemer process: the converter is lined with silica. It suits pig iron low in phosphorus.
Basic Bessemer process (Thomas process): the lining is lime or magnesia (dolomite) and limestone is added. It is used for pig iron rich in phosphorus, which is removed as calcium phosphate (Thomas slag):

4P + 5 O_{2} 2 P_{2} O_{5}

3CaO + P_{2} O_{5} C a_{3} (P O_{4})_{2}

Thomas slag is ground and sold as a phosphate fertiliser.

6.2 Siemens-Martin (open-hearth) process

Pig iron, scrap steel and haematite are melted on the shallow hearth of a regenerative furnace; the hot waste gases preheat the incoming air and fuel gas, so very high temperatures are reached. Scrap dilutes the carbon, and haematite oxidises the impurities. The hearth is lined with silica or dolomite depending on the impurities. The process is slow, which allows the composition to be checked and controlled.

F e_{2} O_{3} + 3C 2Fe + 3CO ↑

2F e_{2} O_{3} + 3S 4Fe + 3S O_{2} ↑

10F e_{2} O_{3} + 3 P_{4} 20Fe + 6 P_{2} O_{5}

2F e_{2} O_{3} + 3Si 4Fe + 3Si O_{2}

F e_{2} O_{3} + 3Mn 2Fe + 3MnO

The oxides then form slags: $MnSi O_{3}$ , $CaSi O_{3}$ and $C a_{3} (P O_{4})_{2}$ .

6.3 Oxygen top-blowing (LD) process

Liquid iron from the blast furnace and some scrap steel are charged into a converter, and a jet of pure oxygen is blown onto the surface through a retractable, water-cooled steel lance. The impurities are oxidised and, with lime added, form a slag that is removed by tilting the converter. It is fast, and because no air is used, no nitrogen dissolves in the steel. Most steel is now made this way.

6.4 Electric arc process

Scrap steel and turnings are melted by an electric arc struck between adjustable carbon electrodes. Acid or basic linings are chosen according to the phosphorus content. With no fuel to contaminate the metal, it is used for alloy steels, stainless steel and high-speed cutting steel.

6.5 High-frequency induction process

Alloy scrap of known composition, with iron, is placed in a crucible surrounded by water-cooled copper coils carrying alternating current (500-2000 Hz). The changing magnetic field sets up eddy currents in the metal, which heat it and stir it strongly. High-quality alloy steels containing W, V, Cr, Mn, Mo, Co and Ni, for ball bearings, magnets, dies and tools, are made this way.

Figure 6: Steel-making furnaces. The first two oxidise impurities with air or oxygen; the electric furnaces melt scrap cleanly for alloy and stainless steels.

Process	Oxidising agent	Lining or feature	Best for
Acid Bessemer	air	silica lining	low-phosphorus pig iron
Basic Bessemer	air	lime or dolomite lining	high-phosphorus pig iron; gives Thomas slag
Open hearth	haematite + air	regenerative heating; silica or dolomite	carefully controlled steel
LD (top-blowing)	pure oxygen	water-cooled lance	most modern bulk steel
Electric arc	(melting of scrap)	carbon electrodes	alloy and stainless steels
Induction	(melting of scrap)	eddy-current heating	high-grade tool and alloy steels

Figure 7 turns the choice into three questions.

Figure 7: Choosing a steel-making process. The charge, the phosphorus content and the scale of production decide the furnace and its lining.

Acid Bessemer

Silica lining.
For low-phosphorus pig iron.
Removes C, Si, Mn.
Slag: $MnSi O_{3}$ .

Basic Bessemer

Lime or dolomite lining; limestone added.
For phosphorus-rich pig iron.
Also removes P.
Slag: Thomas slag, a fertiliser.

Exam Trick

Phosphorus needs a basic lining. $P_{2} O_{5}$ is an acidic oxide, so it can only be held as slag by a base (CaO). A silica lining would be eaten away by that basic slag, so high-phosphorus iron goes to the basic Bessemer or basic open-hearth furnace.

Quick Recall: tap to check

What is spiegeleisen?

An Fe-Mn-C alloy added at the end of the Bessemer process.

Why does the LD process give steel with less nitrogen?

It blows pure oxygen, not air.

Which furnace makes stainless steel from scrap?

The electric arc furnace.

Key idea

Steel making is controlled oxidation: burn out C, Si, Mn and P, choose a lining that matches the slag, then add back exactly the carbon needed.

7. Heat Treatment of Steel

The hardness and elasticity of steel depend strongly on how it is heated and cooled.

Treatment	What is done	Result
Annealing	heated to redness, then cooled slowly	soft, malleable and pliable
Hardening (quenching)	heated to redness, then plunged into water or oil	very hard and brittle
Tempering	hardened steel reheated to about 470-570 K, then cooled slowly	less hard, much less brittle
Case hardening	heated in contact with charcoal (carbon)	hard carbon-rich surface over a tough core
Nitriding	heated in ammonia	very hard surface layer of iron nitride

Figure 8 compares the three schedules.

Figure 8: The same steel, three schedules. Cooling rate decides hardness; tempering trades a little hardness for toughness.

8. Alloy Steels

Alloy steel	Added metal (%)	Special property and use
Chrome steel	Cr 2-4	very hard; ball bearings, cutting tools
Stainless steel	Cr 12-18, Ni 2-8 (common 18/8: Cr 18, Ni 8)	resists rusting; utensils, surgical tools
Tungsten steel	W 10-20	stays hard when hot; high-speed tools
Manganese steel	Mn 10-18	very hard and tough; rock crushers, rails, safes
Nickel steel	Ni 3-5	hard, elastic, resists corrosion; cables, gears
Vanadium steel	V 0.2-1	high tensile strength; springs, axles
Invar	Ni 36 (Fe 64)	almost no expansion on heating; pendulums, measuring tapes

9. Summary Mind Map

The whole extraction at a glance (Figure 9).

Figure 9: Mind map of iron: prepare the ore, smelt it, then tune the carbon.

10. Solved Examples

Solved Example 1

The iron ore magnetite is concentrated mainly by
(A) froth flotation
(B) magnetic separation
(C) hand picking
(D) all of the above

Solution:

Answer: (B). Magnetite is strongly magnetic, so it is separated from non-magnetic gangue on a magnetic roller. Hand picking only removes large stony lumps before concentration, and froth flotation is meant for sulphide ores, not iron oxides.

Solved Example 2

The flux used in the extraction of iron is
(A) limestone
(B) silica
(C) flint
(D) feldspar

Solution:

Answer: (A). The gangue of iron ore is mainly silica, an acidic oxide, so a basic flux is needed. Limestone gives lime, which forms slag: $CaO + Si O_{2} \to CaSi O_{3}$ .

Solved Example 3

Why is the iron ore roasted in excess air before smelting, rather than just calcined?

Solution:

Roasting in air oxidises FeO to $F e_{2} O_{3}$ ( $4FeO + O_{2} \to 2F e_{2} O_{3}$ ). If FeO reached the furnace it would combine with silica to form $FeSi O_{3}$ slag and the iron would be lost. Roasting also removes S and As as volatile oxides and makes the ore porous for the reducing gases.

Solved Example 4

In the blast furnace, the reaction

CaO + Si O_{2} \to CaSi O_{3}

takes place in the
(A) zone of combustion
(B) zone of fusion
(C) zone of slag formation
(D) zone of reduction

Solution:

Answer: (C). Limestone decomposes near 1270 K in the middle of the furnace, and the lime at once combines with silica to form fusible calcium silicate.

Solved Example 5

Calculate the percentage of iron in magnetite and in haematite. Which is the richer ore? (Fe = 55.8, O = 16.0)

Solution:

Magnetite, $F e_{3} O_{4}$ : $\frac{3 \times 55.8}{3 \times 55.8 + 4 \times 16.0} \times 100 = \frac{167.4}{231.4} \times 100 = 72.3 %$ .

Haematite, $F e_{2} O_{3}$ : $\frac{2 \times 55.8}{2 \times 55.8 + 3 \times 16.0} \times 100 = \frac{111.6}{159.6} \times 100 = 69.9 %$ .

Magnetite is the richer ore of iron.

Solved Example 6

What mass of carbon (as coke) is needed to reduce 1.00 tonne of

F e_{2} O_{3}

completely, if all the reduction is done by CO made from coke? What mass of iron is formed? (Fe = 55.8, C = 12.0, O = 16.0)

Solution:

F e_{2} O_{3} + 3CO 2Fe + 3C O_{2}

2C + O_{2} 2CO

Each $F e_{2} O_{3}$ needs 3 CO, which needs 3 C. Moles of $F e_{2} O_{3}$ $= \frac{1.00 \times 1 0 ^{6}}{159.6} = 6.27 \times 1 0^{3}$ mol.

Carbon $= 3 \times 6.27 \times 1 0^{3} \times 12.0 = 2.26 \times 1 0^{5}$ g $\approx 226$ kg. Iron $= 2 \times 6.27 \times 1 0^{3} \times 55.8 = 7.00 \times 1 0^{5}$ g $= 700$ kg.

In practice much more coke is charged, because coke is also the fuel.

Solved Example 7

For

FeO + CO \to Fe + C O_{2}

Δ H^{\circ} = - 11.0

kJ and

Δ S^{\circ} = - 17.4

J K^-1. Find the minimum CO :

C O_{2}

ratio needed for this reaction to proceed at 1000 K.

Solution:

$Δ G^{\circ} = - 11.0 - 1000 \times (- 0.0174) = + 6.4$ kJ.

K = e^{- Δ G^{\circ} / R T} = e^{- 6400/ (8.314 \times 1000)} = 0.46

At equilibrium $p_{C O_{2}} / p_{CO} = 0.46$ , so the reaction goes forward only while $p_{CO} / p_{C O_{2}} > 1/0.46 \approx 2.2$ . The blast furnace keeps the gas this rich in CO by passing $C O_{2}$ over hot coke.

Solved Example 8

Steel heated to redness and suddenly plunged into cold water becomes
(A) soft and malleable
(B) hard and brittle
(C) tough and elastic
(D) coated with nitride

Solution:

Answer: (B). This is hardening (quenching). Slow cooling (annealing) would make it soft; reheating hardened steel to about 470-570 K (tempering) reduces the brittleness.

Practice Questions

Why is iron pyrites not used as an ore of iron?Answer: Its sulphur would make the iron brittle and is hard to remove completely; the sulphur is more valuable for making sulphuric acid.
Name the four zones of the blast furnace from top to bottom, with one reaction in each.Answer: Reduction: $F e_{2} O_{3} + 3CO \to 2Fe + 3C O_{2}$ ; slag formation: $CaO + Si O_{2} \to CaSi O_{3}$ ; fusion: $FeO + C \to Fe + CO$ ; combustion: $C + O_{2} \to C O_{2}$ .
Why is the slag notch placed higher than the tap hole?Answer: Slag is lighter and floats on the molten iron, so it is drawn off from a higher outlet while iron is tapped from the bottom.
What is spiegeleisen and why is it added in the Bessemer process?Answer: An alloy of iron, manganese and carbon. It adds back the required carbon, and its manganese removes dissolved oxygen.
Arrange pig iron, steel, cast iron and wrought iron in increasing order of carbon content.Answer: Wrought iron < steel < cast iron < pig iron.
What mass of limestone removes 100 kg of silica as slag? (Ca = 40, C = 12, O = 16, Si = 28)Answer: 1 mol $CaC O_{3}$ (100 g) removes 1 mol $Si O_{2}$ (60 g): $100 \times \frac{100}{60} \approx 167$ kg.
Which converter lining suits phosphorus-rich pig iron, and what happens to the phosphorus?Answer: A basic lining (lime or dolomite); P is oxidised to $P_{2} O_{5}$ and leaves as Thomas slag, $C a_{3} (P O_{4})_{2}$ .

Common Mistakes to Avoid

Watch out

Saying coke is the only reducing agent. In the upper furnace the iron oxides are reduced mainly by CO; coke reduces FeO only in the hot lower part.
Writing the Boudouard step the wrong way. At high temperature it is $C O_{2} + C \to 2CO$ ; the reverse happens only in the cool upper part.
Calling limestone a reducing agent. It is the flux; it removes silica as $CaSi O_{3}$ .
Thinking cast iron is the purest form. Wrought iron is the purest; cast iron has about 3 % carbon.
Mixing up pig iron (about 4 % C, straight from the furnace) and cast iron (about 3 % C, remelted with scrap).
Using an acid (silica) lining for phosphorus-rich pig iron. Phosphorus needs a basic lining and lime.
Calling spiegeleisen a process. It is an Fe-Mn-C alloy added at the end.
Confusing annealing (slow cooling, soft) with hardening (quenching, hard and brittle).

Frequently Asked Questions

How is iron extracted from haematite?

Haematite is crushed, concentrated and roasted in air, then charged with coke and limestone into a blast furnace. Coke burns in the hot air blast to carbon monoxide, which reduces the iron oxide in the upper zone, while coke reduces the rest lower down. Limestone removes silica as slag, and molten pig iron is tapped from the hearth.

What happens in each zone of the blast furnace?

At the bottom, near the tuyeres, coke burns to carbon dioxide and then carbon monoxide at about 2170 K. Above it, iron melts and absorbs carbon in the fusion zone. In the middle, limestone forms calcium silicate slag. Near the top, at 500 to 900 K, carbon monoxide reduces the iron oxides.

Why is limestone added to the blast furnace?

Limestone is the flux. It decomposes to lime and carbon dioxide at about 1270 K, and the lime combines with the silica gangue to form fusible calcium silicate slag. The slag floats on the molten iron, protects it from oxidation and is run off through the slag notch.

What is the difference between pig iron, cast iron and wrought iron?

Pig iron comes straight from the blast furnace and has about 4 percent carbon. Cast iron is pig iron remelted with scrap and coke, with about 3 percent carbon; it is hard and brittle. Wrought iron, made by puddling with haematite, has only 0.1 to 0.25 percent carbon and is the purest, most malleable form.

Why is carbon monoxide the main reducing agent in the upper blast furnace?

At 500 to 900 K the oxidation of carbon monoxide to carbon dioxide releases more Gibbs energy than the formation of the iron oxides, so it reduces them. Coke is a better reducing agent only at higher temperatures, above about 1073 K, where the carbon to carbon monoxide Ellingham line has fallen lower.

How is steel made from pig iron?

The impurities in molten pig iron are oxidised by air in a Bessemer converter, by haematite in an open-hearth furnace or by pure oxygen in the LD process. The oxides leave as gases or slag, and the exact carbon is added back, often as spiegeleisen. Alloy steels are made from scrap in electric arc or induction furnaces.

Is the blast furnace important for JEE Advanced?

Yes. JEE Advanced lists iron as an example of the thermodynamic principles of metallurgy. Questions ask which zone a reaction belongs to, why carbon monoxide works in the upper furnace and coke lower down, the role of limestone, and the Ellingham crossing of the carbon and iron oxide lines.

Is the extraction of iron in the NEET syllabus?

No. NEET dropped metallurgy in 2024, so the blast furnace and steel making are not examined. NEET students still use related ideas elsewhere: the oxidation states of iron in the d-block chapter, and Gibbs energy and equilibrium in thermodynamics, which explain why carbon monoxide reduces iron oxides.

Previous year questions on Extraction of Iron

1 question from past papers, each with a step-by-step solution.

JEE Advanced 2022 Paper 2, Chemistry Section 2 Q4

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