Extraction of Iron

Extraction of Iron

Iron, atomic number 26, is the most abundant element on earth by mass. It belongs to the 3d series or first transition series of the periodic table. Pure metallic iron is not usually found in the earth’s crust. Therefore, extracting metal from ore is the only way to obtain usable iron. 

Extraction of iron from its ore is a lengthy process and requires smelters and furnaces. These furnaces should be capable of reaching a temperature of 1500 °C. Let us study the iron extraction process in detail. 

Ores of Iron

Being a reactive metal, iron is not found in a free state. It occurs as sulphides, oxides and carbonates. Given below are some common ores of iron –

1. Magnetite Fe3O4

2. Haematite Fe2O3

3. Iron pyrite FeS2

4. Limonite Fe2O3.3H2O

5. Siderite FeCO3

Extraction of Iron

A mixture of oxide ores of iron (Fe3O4 and Fe2O3) is used to extract pure iron.

1. Concentrating the ores

Jaw crushers are used, and the ore is broken into smaller pieces. Concentration is carried out by the gravity separation process. Clay, sand, etc., are removed by washing it with water. Magnetic separation is also used as Iron is a magnetic ore.

2. Calcination

The concentrated ores are then heated in a limited supply of air. This process is called calcination. This process is used for oxide and carbonate ores.

During this process – 

1. Moisture is removed

2. The impurities of sulphur, phosphorus and arsenic convert into their respective gaseous oxides

3. These oxides escape easily as they are volatile

3. Ferrous carbonate, if present, also converts into oxide (Fe2O3)

4. The mixture becomes porous

5. This is helpful in the further reduction steps

3. Smelting

The calcined ore is reduced by carbon, i.e., it is smelted in the blast furnace.

A blast furnace is tall and cylindrical. It is made of steel and lined with fire bricks.

It involves a number of steps – 

1. The calcined ore is mixed with coke and limestone in an 8:4:1 ratio. The mixture is added through the cup and cone arrangement at the top. 

The coke serves as a fuel, and lime acts as a reducing agent.

2. A blast of air (1000K) is blown upwards using the tuyeres arrangement. 

3. The burning of the coke supplies the heat required in this process. The temperature at the bottom of the furnace is 2200K.

4. The temperature falls gradually as we move towards the top of the furnace. It is 1300K in the middle and 700-900K at the top.

5. Carbon monoxide (CO) is the reducing agent at the top, and carbon is the reducing agent towards the bottom part.

4. Reactions in the blast furnace

1. Combustion zone

At the base of the furnace, coke burns to produce carbon dioxide. The CO2  produced rises upwards. This is an exothermic reaction. Therefore the temperature rises up to 2200K. This part of the furnace is called the combustion zone.

C + O2  → CO2  (ΔH = -393.4 kJ)

2. Fusion zone

This involves the reduction of carbon dioxide to carbon monoxide. It happens when CO2 comes in contact with the layers of coke.

CO2 + C → 2 CO  (ΔH = +163.2 kJ)

Positive ΔH is an indication that this is an endothermic reaction. The temperature reduces to 1570K. 

The iron which was produced in the upper part melts here. Fe2O3  reduces to iron by hot coke. This region is known as the fusion zone.

Fe2O3  + 3 C → 2Fe + 3 CO + heat

3. Slag formation zone 

Limestone decomposes and forms lime (CaO). The lime reacts with silica to form slag. Carbon dioxide is also produced. The temperature here is about 1270K.

CaCO3 → Cao + CO2

CaO + SiO2 → CaSiO3

The molten slag is lighter than molten iron and forms a separate layer. This region is known as the slag formation zone.

4. Reduction zone

This is near the top of the furnace, where the temperature is around 875K. The reduction of iron oxides to iron (Fe)  is made by carbon monoxide (CO).

Fe2O3  is first reduced to Fe3O4. Further reduction leads to the formation of FeO and ultimately Fe.

3 Fe2O3 + CO → 2 Fe3O4 + CO2

Fe3O4  + 4 CO →  3 Fe + 4 CO2

Fe2O3  + CO → 2 FeO + CO2

The following reaction occurs at the lower part, where the temperature is 900-1500K.

FeO + CO → Fe + CO2

The iron obtained from the blast furnace has a 4% carbon concentration. It has other impurities of S, P, Si, Mn. This iron is known as pig iron. It is possible to mould pig iron into various shapes.

Cast iron has lower carbon content than pig iron. It is made by melting scrap iron, pig iron and coke using a hot air blast. Cast iron is brittle. It is also tough.

Wrought Iron

Wrought iron is the purest form of commercial iron. It contains less than 0.5% impurities. It is highly malleable and is prepared from cast iron. Impurities are oxidised in a reverberatory furnace lined with hematite. Carbon is oxidised to carbon monoxide.

Fe2O3 + 3 C → 2 Fe + 3CO

Limestone is added as a flux, and sulphur, silicon, and phosphorus are oxidised. They are then passed into the slag. It is freed from slag and passed through rollers to remove the metal.

Conclusion 

Here we learned the principles of extraction of iron in detail, including step by step procedure to complete the process. The topic is important for exams, so we advise you to carefully read the definition, formula and methods regarding the extraction of iron given above in the article.