7.4 Iron
🎯 Learning Outcomes
Candidates should be able to:
- Describe the occurrence of iron
- Describe the properties of iron
- State the uses of iron
- Describe the manufacture of steel by the basic oxygen method and the open hearth process
- Explain the corrosion of iron and its prevention
1 Iron — Occurrence
Iron is a reactive metal, so it does not exist in its free state. It is the fourth most abundant element in the Earth’s crust. The major ores of iron are:
| Ore | Formula |
|---|---|
| Haematite | Fe₂O₃ |
| Magnetite | Fe₃O₄ |
| Limonite | (Fe₂O₃)₂·3H₂O |
| Iron pyrites | FeS₂ |
| Siderite | FeCO₃ |
In Nepal, iron ore deposits are found in Phulchowki, Those (Ramechhap), Duwakot (Parbat), Jirwang (Chitwan), and many other places.
2 Extraction — Concentration & Roasting
Iron is mainly extracted from haematite ore. The extraction of iron involves the following steps:
Haematite is first crushed into small pieces by a jaw crusher and then concentrated by a gravity separation process to remove earthly impurities. Then it is concentrated by using a magnetic separator.
The concentrated ore is roasted with a little coke in the presence of excess air in a reverberatory furnace. During roasting, the following changes take place:
- Moisture and volatile impurities are removed.
- Impurities such as sulphur, phosphorus, arsenic, etc. are removed as their respective volatile oxides:
- If some ferrous carbonate or ferrous oxide is present, it changes to ferric oxide:
3 Extraction — Smelting & Varieties of Iron
- The roasted ore is smelted in a blast furnace.
- A blast furnace is a tall tower-like structure made of steel and iron.
- The furnace is lined from the inside with fire bricks to withstand high temperatures.
- The furnace is also provided with small pipes called tuyers to admit a blast of hot air.
- There are two outlets at the bottom to collect molten metal and slag, and one outlet at the top for waste gases.
- The roasted ore is mixed with limestone and coke in a ratio of 8:1:4 and fed into the furnace through the cup and cone arrangement.
- At the same time, a blast of air is sent upward through the tuyers.
Various reactions take place in different zones of the furnace:
(a) Zone of Combustion
The lower portion of the furnace. Here, coke burns with oxygen to form carbon dioxide.
This reaction is exothermic; therefore, the temperature rises to about 1500°C.
(b) Zone of Fusion
Carbon dioxide gas thus produced rises upwards; it comes in contact with layers of coke and gets reduced to carbon monoxide.
This reaction is endothermic, so the temperature is lowered to 1200–1300°C.
(c) Zone of Slag Formation
The middle part of the furnace, where the temperature ranges from 800 to 1000°C. Here, limestone (CaCO₃) decomposes to give CaO and CO₂.
Calcium oxide (lime) acts as a flux, which combines with silica impurities to form calcium silicate as slag.
(d) Zone of Reduction
The upper part of the blast furnace, with a temperature ranging from 300°C to 800°C. CO gas coming from the zone of fusion reduces iron oxide into iron.
- The spongy iron formed falls down slowly and melts (fuses) when it reaches the zone of fusion.
- The slag formed at the middle part of the furnace also falls downwards and forms the upper layer over the molten iron.
- They are collected from two different outlets.
- The waste gases are removed from the top of the furnace.
- Molten metal is solidified into blocks called pigs, so the iron obtained from this process is called pig iron or cast iron.
Varieties of Iron
There are three varieties of iron, based on their carbon content.
Cast Iron 2.5–4.5% C
The least pure form of iron, containing the highest percentage of carbon along with other impurities like sulphur, phosphorous, manganese, and silicon.
It is hard and brittle, therefore not used in structures — it has a low expansion tendency. Used for making stoves, pipes, radiators, toys, etc.
Wrought Iron 0.12–0.25% C
The purest form of iron, containing the lowest percentage of carbon and less than 0.25% of other impurities like sulphur, phosphorus, silicon, manganese, etc.
It is soft and malleable. Used in the manufacture of chain cores of electromagnets, nails, etc.
Steel 0.1–1.5% C
Intermediate between cast iron and wrought iron, along with other impurities like sulphur, phosphorous, and special constituents like tungsten, chromium, vanadium, molybdenum, etc.
Used in the construction of railways, cranes, springs, etc.
4 Properties of Iron
- Pure iron is a grey-white lustrous metal.
- It is malleable and ductile.
- It melts at 1536°C and boils at 2750°C.
4. Action of Air
At ordinary temperature, iron is not attacked by dry air, but with moist air it gets converted into a reddish-brown layer known as rust.
When iron is heated with oxygen, it forms ferroso-ferric oxide.
5. Action with Water
Pure iron does not react with water at ordinary temperature, but when steam is passed over heated iron, hydrogen gas is evolved.
6. Action with Acids
(a) With HCl: Iron reacts with dilute and concentrated HCl to give hydrogen gas (reaction is fast with conc. HCl).
(b) With H₂SO₄: Iron reacts with dilute H₂SO₄ to give H₂ gas, and with conc. H₂SO₄ to give SO₂ gas.
FeSO₄ can be crystallized into ferrous sulphate hepta-hydrate, popularly known as Green vitriol.
(c) With HNO₃: Iron gives different products depending on the concentration of the acid.
Iron becomes passive with concentrated HNO₃, due to the formation of a thin layer of ferroso-ferric oxide on its surface.
7. Action with Halogen
When iron is heated with a halogen, ferrous or ferric halide is formed.
8. Action with Sulphur
When iron is heated with sulphur, ferrous sulphide is formed.
9. Action with Carbon Monoxide
When iron is heated with carbon monoxide at 120°C, it gives iron pentacarbonyl complex.
10. Action with Cupric Sulphate Solution
Iron is more electropositive than copper, so copper is displaced by iron from the solution of copper sulphate.
5 Uses of Iron
- Iron and steel (bars and rods) are used as construction materials for making roads, bridges, dams, and buildings.
- Used for making ships, motor vehicles, pipes, domestic articles, and instruments.
- Biological uses — e.g., haemoglobin, a compound of iron present in the blood of humans, animals, fish, and birds, which carries oxygen.
6 Manufacture of Steel — Open Hearth Process
Steel is manufactured from pig iron or cast iron by removing impurities like carbon, silicon, manganese, phosphorous, etc. by oxidation, and adding a requisite amount of carbon to get a steel of desired properties. Steel is a product of pure iron containing 0.1 to 1.5% carbon and a small amount of manganese, chromium, and other elements.
- Developed by Siemens and Martin, therefore often called the Siemens-Martin process.
- A special type of furnace is used. Depending upon the nature of impurities present in cast iron, the hearth of the furnace is lined with acidic material (like silica) or basic material (like dolomite).
- The furnace is heated to about 1500°C by means of producer gas.
- A mixture of cast iron or pig iron, scrap iron, low-grade wrought iron, and haematite is taken in the open hearth furnace at 1500°C.
- The impurities present in the cast iron are oxidised by haematite ore and removed as slag or gases:
A small amount of molten mass is drawn out to check the carbon content. Finally, a calculated amount of Spiegeleisen is added to the obtained steel to reach the desired carbon content.
- Could use cheap, low-grade raw materials (scrap, wrought iron)
- Allowed good control over carbon content
- Could produce large batches (100–200 tonnes)
- Removed phosphorus effectively when using a basic lining
- Very slow — each batch took 8–12 hours (vs. 40 minutes for BOP)
- High fuel consumption — needed constant heating with producer gas
- High labour cost
- Polluting — released CO, CO₂, and SO₂
Replaced by faster, cleaner methods — the Basic Oxygen Process (1950s) and Electric Arc Furnaces (growing from the 1960s) made it uneconomical.
7 Manufacture of Steel — Basic Oxygen Process
The Basic Oxygen Process (BOP), also known as the Linz–Donawitz (LD) process or oxygen top-blowing process, is a modern method used to produce about 70% of the world’s steel. It is an improved version of the older Bessemer process.
In this process, high-purity oxygen is blown onto molten pig iron to remove impurities by oxidation. The result is high-quality steel.
Step-by-Step
Molten pig iron (high in carbon) is poured into a large refractory-lined vessel called an LD converter.
A water-cooled lance is lowered into the converter, and pure oxygen is blown at high pressure onto the surface of the molten metal.
Carbon removal:
Silicon removal:
Phosphorus removal:
A basic flux (calcium oxide, CaO) is added to react with the acidic oxides and form slag:
Sulphur impurities are also absorbed into the slag.
Once the desired carbon content is achieved:
- The converter is tilted.
- Molten steel is tapped from below the slag layer.
- Slag is removed separately by tilting in the opposite direction.
Advantages of the Basic Oxygen Process
- Produces high-quality steel rapidly (within 30–45 minutes per batch).
- Suitable for large-scale industrial production.
- Produces better quality steel than the open hearth process.
- Mostly autogenous — heat is generated internally by oxidation reactions.
- Lower environmental pollution compared to older methods (though still a major CO₂ emitter).
8 Heat Treatment of Steel
The quality of steel depends not only on its composition but also on the heat treatment. The hardness of steel can be modified by heat treatment in different ways.
1. Quenching (Hardening)
Steel is heated to red-hot and cooled suddenly by plunging it into cold water or oil. The steel becomes hard and brittle due to the formation of iron carbide (Fe₃C).
2. Annealing
Hard steel can be softened by first heating it to red-hot and then cooling it slowly. This causes partial decomposition of Fe₃C, so the steel becomes soft and ductile.
3. Tempering
Hardened steel is heated to a temperature much below red-hot and cooled slowly. This makes the steel neither too hard nor too brittle.
9 Rusting of Iron
If iron is left in a moist atmosphere, it is covered with a loose brown coating called rust. Rust is a hydrated form of ferric oxide, Fe₂O₃·xH₂O. A huge amount of iron is spoiled every year by rusting. It is known that moisture, oxygen, and carbon dioxide present in the atmosphere are all required for rusting.
There are many theories of rusting. Among them, the electrochemical theory of rusting is the most convincing one. According to this theory, the rusting of iron is electrochemical in nature — the attack on iron is basically a chemical reaction accompanied by the flow of a current.
Iron is usually non-homogeneous due to the heterogeneous distribution of impurities or strains in the crystal. Owing to this, some portion of iron is at a lower electrical potential compared to another. The portion of iron at a lower potential acts as an anode, where oxidation occurs and dissolution of the metal takes place.
The electrons thus liberated migrate through the metal to the portion at a higher potential, the cathode, where they are utilised in the reduction of oxygen or other cations.
The hydrogen ions necessary for reaction (3) are produced by the self-dissociation of water or by the interaction of atmospheric carbon dioxide with water.
Neutral-medium reduction of oxygen (reaction 2) and acidic-medium reduction of H⁺ (reaction 3) are the dominant reactions at the cathode. In natural water, reduction of oxygen is the dominant cathode reaction during the rusting of iron.
The anodic and cathodic products migrate and form ferrous hydroxide:
Ferrous hydroxide thus formed is rapidly oxidised to ferric hydroxide in the presence of oxygen and water:
Ferric hydroxide is unstable and subsequently loses water to give ferric oxide:
Ferric oxide absorbs moisture from the atmosphere and forms a non-coherent brown hydrated ferric oxide, called rust:
📘 Chapter Summary
- Iron does not occur free in nature — haematite (Fe₂O₃) is its most important ore.
- Extracted in a blast furnace: roasted ore is reduced by CO across the zones of combustion, fusion, slag formation, and reduction, yielding pig/cast iron.
- Cast iron, wrought iron, and steel differ mainly in their carbon content — high, lowest, and intermediate respectively.
- Steel is manufactured by oxidising impurities out of pig iron — historically by the open hearth process, now almost entirely by the Basic Oxygen Process.
- Heat treatment (quenching, annealing, tempering) controls the hardness of steel.
- Rusting is an electrochemical process — iron oxidises at anodic spots and oxygen is reduced at cathodic spots, ultimately forming hydrated ferric oxide (Fe₂O₃·xH₂O).
***This is not a complete note. It is meant to guide you. It is recommended to study the prescribed textbooks alongside this material.***