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NCERT Section

6.4.1 Applications

(a) Extraction of iron from its oxides

 Oxide ores of iron, after concentration through calcination/roasting (to remove water, to decompose carbonates and to oxidise sulphides) are mixed with limestone and coke and fed into a Blast furnace from its top. Here, the oxide is reduced to the metal. Thermodynamics helps us to understand how coke reduces the oxide and why this furnace is chosen. One of the main reduction steps in this process is:
FeO(s) + C(s) → Fe(s/l) + CO (g)                                                 (6.24)
It can be seen as a couple of two simpler reactions. In one, the reduction of FeO is taking place and in the other, C is being oxidised to CO:
FeOs→Fes+12O2g              ∆GFeO,Fe                   (6.25)
Cs+12O2g→COg                    ∆GC,CO                   (6.26) 
When both the reactions take place to yield the equation (6.24), the net Gibbs energy change becomes:
∆GC,CO+∆GFeO,Fe=∆rG                                               (6.27)
Naturally, the resultant reaction will take place when the right hand side in equation 6.27 is negative. In ΔG0 vs T plot representing reaction 6.25, the plot goes upward and that representing the change C→CO (C,CO) goes downward.  At temperatures above 1073K (approx.), the C,CO line comes below the Fe,FeO line [ΔG (C, CO) < ΔG(Fe, FeO)]. So in this range, coke will be reducing the FeO and will itself be oxidised to CO. In a similar way the reduction of Fe3O4 and Fe2O3 at relatively lower temperatures by CO can be explained on the basis of lower lying points of intersection of their curves with the CO, CO2 curve in Fig. 6.4.
                                
                                                                          Fig. 6.4: Gibbs energy (∆rGƟ) vs T plots (schematic) for the formation of some oxides per mole of oxygen consumed (Ellingham diagram)
 In the Blast furnace, reduction of iron oxides takes place in different temperature ranges. Hot air is blown from the bottom of the furnace and coke is burnt to give temperature upto about 2200K in the lower portion itself. The burning of coke therefore supplies most of the heat required in the process. The CO and heat moves to upper part of the furnace. In upper part, the temperature is lower and the iron oxides (Fe2O3 and Fe3O4) coming from the top are reduced in steps to FeO. Thus, the reduction reactions taking place in the lower temperature
range and in the higher temperature range, depend on the points of corresponding intersections in the ΔrG0 vs T plots. These reactions can be summarised as follows:

At 500 – 800 K (lower temperature range in the blast furnace),

Fe2O3 is first reduced to Fe3O4 and then to FeO

3 Fe2O3 + CO → 2 Fe3O4 + CO2                                                                    (6.28)

Fe3O4 + 4 CO → 3Fe + 4 CO2                                                                         (6.29)

Fe2O3 + CO → 2FeO + CO2                                                              (6.30)

At 900 – 1500 K (higher temperature range in the blast furnace):

C + CO2 → 2 CO                                                                               (6.31)

FeO + CO → Fe + CO2                                                                     (6.32)

Fig. 6.5: Blast furnace

 Limestone is also decomposed to CaO which removes silicate impurity of the ore as slag. The slag is in molten state and separates out from iron.

The iron obtained from Blast furnace contains about 4% carbon and many impurities in smaller amount (e.g., S, P, Si, Mn). This is known as pig iron and cast into variety of shapes. Cast iron is different from pig iron and is made by melting pig iron with scrap iron and coke using hot air blast. It has slightly lower carbon content (about 3%) and is extremely hard and brittle.

Further Reductions

 Wrought iron or malleable iron is the purest form of commercial iron and is prepared from cast iron by oxidising impurities in a reverberatory furnace lined with haematite. The haematite oxidises carbon to carbon monoxide:

Fe2O3 + 3 C → 2 Fe + 3 CO                                                                    (6.31)

Limestone is added as a flux and sulphur, silicon and phosphorus are oxidised and passed into the slag. The metal is removed and freed from the slag by passing through rollers.

(b) Extraction of copper from cuprous oxide [copper(I) oxide]

 In the graph of ∆rGƟ vs T for the formation of oxides (Fig. 6.4), the Cu2O line is almost at the top. So it is quite easy to reduce oxide ores of copper directly to the metal by heating with coke. The lines (C, CO) and (C, CO2) are at much lower positions in the graph particularly after 500 – 600K. However, many of the ores are sulphides and some may also contain iron. The sulphide ores are roasted/smelted to give oxides:

2Cu2S + 3O2 → 2Cu2O + 2SO2                                                              (6.32)

The oxide can then be easily reduced to metallic copper using coke:

Cu2O + C → 2 Cu + CO                                                                         (6.33)

In actual process, the ore is heated in a reverberatory furnace after mixing with silica. In the furnace, iron oxide ‘slags of’ as iron slicate is formed. Copper is produced in the form of copper matte. This contains Cu2S and FeS.

FeO + SiO2 → FeSiO3     (Slag)                                                                       (6.34)

 Copper matte is then charged into silica lined convertor. Some silica is also added and hot air blast is blown to convert the remaining FeS, FeO and Cu2S/Cu2O to the metallic copper. Following reactions take place:

2FeS + 3O2 → 2FeO + 2SO2                                                                (6.35)

FeO + SiO2 → FeSiO3                                                                                            (6.36)

2Cu2S + 3O2 → 2Cu2O + 2SO2                                                            (6.37)

2Cu2O + Cu2S → 6Cu + SO2                                                                               (6.38)

The solidified copper obtained has blistered appearance due to the evolution of SO2 and so it is called blister copper.

(c) Extraction of zinc from zinc oxide

The reduction of zinc oxide is done using coke. The temperature in this case is higher than that in the case of copper. For the purpose of heating, the oxide is made into brickettes with coke and clay.

ZnO + C Zn + CO                                                           (6.39)

The metal is distilled off and collected by rapid chilling. 

Intext Questions

6.3 The reaction,

Cr2O3+2Al → Al2O3+2Cr                       (∆GƟ= – 421kJ)

is thermodynamically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature?

6.4 Is it true that under certain conditions, Mg can reduce Al2O3 and Al can reduce MgO? What are those conditions?

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