Chemistry · Ch 8 — Transition and Inner Transition Elements

Extraction of Iron from Haematite ore using Blast furnace

8.9.2

Extraction of Iron from Haematite ore using Blast furnace

Note

Do you know ? Iron is the fourth most abundant element in the earth's crust.

Iron is extracted from haematite by its reduction using coke and limestone. Carbon in the limestone is reduced to carbon monoxide. Carbon and carbon monoxide together reduce $\mathrm{Fe_2O_3}$ to metallic iron. Limestone acts as flux: it combines with the gangue material to form molten slag.

Composition of Haematite ore :

Figure 8.9.2aComposition of haematite ore written as iron(III) oxide plus silica plus alumina plus phosphates, with a curly brace grouping the silica, alumina and phosphate impurities under the label Gangue.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

What this line shows (uncaptioned in the book). Mined haematite is never pure $\mathrm{Fe_2O_3}$ - it comes mixed with silica ( $\mathrm{SiO_2}$ ), alumina ( $\mathrm{Al_2O_3}$ ) and phosphates. The brace gathers those three under the name Gangue - the unwanted impurities that concentration (and later the flux in the blast furnace) must remove, while the $\mathrm{Fe_2O_3}$ itself is the ore being extracted. *(In the print the heavy brace sits loosely centred under the whole line; it is drawn here under the …

Extraction of iron from haematite ore involves the following steps.

i. Concentration : The powdered ore is washed in a powerful current of water introduced into the hydraulic classifier. The lighter gangue particles are separated, and the concentrated ore is collected at the bottom.

ii. Roasting : The concentrated ore is heated in a current of air. The sulfur and arsenic impurities present in the ore get converted into their oxides and escape as vapour. Ferrous oxide in the ore is converted to $\mathrm{Fe_2O_3}$ .

\mathrm{4FeO + O_2 \longrightarrow 2Fe_2O_3}

The roasted ore is converted into lumps by sintering.

iii. Reduction (Smelting) : This step is carried out in a blast furnace. The blast furnace is a tall cylindrical steel tower which is lined with refractory bricks. The height of a typical blast furnace is 25 m, and its diameter varies between 5 and 10 m. The furnace works on the counter current principle, where the charge comes down and hot gases move up the tower. The furnace is comprised of 3 parts - 1. Hearth, 2. Bosh and 3. Stack.

The charge containing ore and limestone is introduced into the furnace through a cup and cone arrangement. In this arrangement the cone enables uniform distribution of charge, and the cup prevents the loss of gases. A blast of preheated air is introduced into the furnace below the bosh (the book prints "furnance"). The charge and hot air come in contact with each other, and various reactions take place.

Figure 8.9.2bFlow chart of the extraction of iron: iron ore through crushing and grinding, concentration, reduction and refining to pure iron, with the sub-methods of concentration (magnetic separation, gravity separation, leaching, froth flotation), of reduction (by coke, by carbon monoxide, by heat, by aluminium, by electrolysis) and of refining (liquification, distillation, oxidation, electro-refining) fanned out beneath their steps.

Fig. 8.9.2b — Flow chart of the extraction of iron: iron ore through crushing and grinding, concentration, reduction and refining to pure iron, with the sub-methods of concentration (magnetic separation, gravity separation, leaching, froth flotation), of reduction (by coke, by carbon monoxide, by heat, by aluminium, by electrolysis) and of refining (liquification, distillation, oxidation, electro-refining) fanned out beneath their steps.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

What this flow chart shows (uncaptioned in the book). The pink pipeline is the whole journey: Iron ore → Crushing and grinding → Concentration → Reduction → Refining → Pure iron. Under each middle stage fan out the methods to choose from - concentration by Magnetic separation, Gravity separation, Leaching or Froth Flotation; reduction by coke, by CO, by heat, by Al or by electrolysis (the blast furnace of Fig. 8.5 is the coke/CO route); refining by Liquification, Distillation, Oxidation or Electro-refining *("Liquification" as the book …

Figure 8.5Fig. 8.5 - labelled cross-section of a blast furnace: burden charging at the top, the stack, bosh, tuyeres and hearth zones marked inside the furnace profile with their temperatures of 250, 1200, 1700, 1500 and 1350 degrees celsius, the reduction reactions of iron oxide by carbon monoxide and carbon listed alongside, the hot blast air inlet, the slag tap hole with the slag composition, and the pig iron tap hole delivering 93 percent iron.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Fig. 8.5 : Blast furnace. Follow the counter-current: the charge (Burden charging) descends from 250⁰C at the throat through the Stack, the 1200⁰C Bosh and the 1700⁰C Tuyeres belt to the 1350⁰C Hearth, while the hot reducing gas ( $\mathrm{CO + N_2}$ , arrow) climbs the other way. The left-hand ladder of equations is the descent chemistry - $\mathrm{3Fe_2O_3 + CO \rightarrow 2Fe_3O_4 + CO_2}$ , $\mathrm{Fe_3O_4 + CO \rightarrow 3FeO + CO_2}$ , $\mathrm{FeO + C \rightarrow Fe + CO}$ , with MnO, $\mathrm{P_2O_5}$ and $\mathrm{SiO_2}$ reduced lower down - fed by the Hot blast air burning coke ( $\mathrm{C + O_2 \rightarrow CO_2}$ , then $\mathrm{CO_2 + C \rightarrow 2CO}$ ). Slag (roughly 1/2 SiO₂, 1/3 CaO, 1/3 Al₂O₃ with MgO and MnO) leaves by the Slag tap hole; molten pig iron - Fe with C, Si, S, P, Mn, about 93% Iron - taps from the hearth. The right-hand labels name what each level is doing, from Charge distribution down t …

Reactions in the blast furnace : There are different temperature zones in the blast furnace. The temperature goes on increasing from top to bottom in the furnace. At the top the temperature is 500 K; the maximum temperature of the furnace is 2000 K, above the tuyers (the figure labels them "Tuyeres", the prose "tuyers" — both the book's own spellings). There are 3 temperature zones in the furnace.

Zone of combustion - Combustion of coke with $\mathrm{O_2}$ in the air.
Zone of reduction - Reduction of $\mathrm{Fe_2O_3}$ to metallic iron
Zone of slag formation - Formation of slag by reaction of gangue with limestone

Chemical reactions taking place in different zones of the blast furnace

1. Zone of combustion : This is 5 - 10 m from the bottom. The hot air blown through the tuyers reacts with coke from the charge to form CO.

\mathrm{C + \tfrac{1}{2}O_2 \longrightarrow CO,\ \Delta H = -220\ kJ}

The reaction is highly exothermic; thus the temperature of this zone is around 2000 K. Some of the CO formed dissociates to form finely divided carbon.

\mathrm{2\,CO \longrightarrow 2\,C + O_2}

The hot gas rich in CO rises upwards in the blast furnace. The charge coming down gets heated and reacts with CO. Thus CO acts as a fuel and also a reducing agent.

2. Zone of Reduction (22-25 m near the top) (no colon in the print's heading) — Here the temperature is around 900 K. $\mathrm{Fe_2O_3}$ is reduced to spongy iron by CO.

\mathrm{Fe_2O_3 + 3\,CO \longrightarrow 2Fe + 3\,CO_2}

Some amount of $\mathrm{Fe_2O_3}$ is reduced to iron by carbon.

\mathrm{Fe_2O_3 + 3C \longrightarrow 2\,Fe + 3\,CO}

3. Zone of slag formation (20 m unit) : The gangue present in the ore is converted to slag. This slag can be used for making road foundation. The temperature of this zone is 1200 K. The gangue contains silica, alumina and phosphates. Removal of this gangue is effected by adding lime-stone in the charge, which acts as flux. Limestone decomposes to give CaO (quick lime).

\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}

CaO combines with gangue to form molten slag of calcium silicate and calcium aluminate.

\mathrm{CaO + SiO_2 \longrightarrow CaSiO_3}

\mathrm{12\,CaO + 2Al_2O_3 \longrightarrow 4Ca_3AlO_3 + 3\,O_2}

4. Zone of fusion (15 m ht) : $\mathrm{MnO_2}$ and $\mathrm{Ca_3(PO_4)_2}$ present in the iron ore are reduced to Mn and P. Some of the silica is also reduced to Si. The spongy iron coming down in the furnace melt absorbs impurities like C, Si, Mn, P and S. This molten iron collects at the bottom of the furnace. The slag, which is lighter, floats on the surface of the molten iron. Molten slag and iron are collected through separate outlets. Molten iron is poured into moulds; these solid blocks are called pigs. This iron contains about 4% of carbon. When pig iron is remelted, run into moulds and cooled, it becomes cast iron. The waste gases containing $\mathrm{N_2}$ , CO and $\mathrm{CO_2}$ escape through the outlet at the top; these hot gases are used for preheating the blast of air.

Table 8.10 : Summary of reactions taking place in blast furnace at different temperature zones

Temp K	Changes taking place	Reaction
500	loss of moisture from ore	—
900	Reduction of ore by CO	$\mathrm{Fe_2O_3 + 3CO \longrightarrow 2\,Fe + 3CO_2}$
1200	Decomposition of lime	$\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}$
1500	Reduction of ore by C	$\mathrm{Fe_2O_3 + 3C \longrightarrow 2Fe + 3CO}$
2000	Slag formation	$\mathrm{CaO + SiO_2 \longrightarrow CaSiO_3}$ ; $\mathrm{12CaO + 2Al_2O_3 \longrightarrow 4Ca_3AlO_3 + 3\,O_2}$

5. Refining : Pure iron can be obtained by electrolytic refining of impure iron, or other methods given in the flow chart. The choice of extraction technique is governed by the following factors. 1. Nature of ore. 2. Availability and cost of reducing agent — generally cheap coke is used. 3. Availability of hydraulic power. 4. Purity of product (metal) required. 5. Value of byproducts — for example, $\mathrm{SO_2}$ obtained during roasting of sulphide ores is vital for manufacture of $\mathrm{H_2SO_4}$ . Knowledge of the electrochemical series provides solutions to many problems. …

Figure 8.9.2cClassification tree of the commercial forms of iron: the word Iron branching through a bracket into its three commercial forms - cast iron, wrought iron and steel.

Fig. 8.9.2c — Classification tree of the commercial forms of iron: the word Iron branching through a bracket into its three commercial forms - cast iron, wrought iron and steel.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

What this tree shows (uncaptioned in the book). Commercial iron comes in three forms - Cast iron (hard, brittle, about 4% carbon - pig iron remelted and moulded), Wrought iron (very soft, under 0.2% carbon) and Steel (0.2-2% carbon, neither too hard nor too soft). The comparison table just below this tree in the …

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