Chapter 13.2 - Industrial Production of Iron

In the previous section, we saw the various stages in the production of metals. In this section, we will see how these stages are applied in the case of iron.

Industrial production of iron

We will write it in steps:
1. Haematite is the principal ore of iron.
• As we have seen in the previous sections, which ever be the metal, the ore has to be powdered.
• The 'concentration of the powdered ore' is the first stage.
2. In the case of haematite, we use the levigation method for the concentration. 
• As iron is heavier than the impurities, it will settle to the bottom of the tank. 
• The impurities will be washed away.
• So first stage is over
3. The second stage is the extraction of the metal from the concentrated ore. 
• We have seen that there are two steps to be completed for this. They are:
(a) Conversion of the ore into it's oxide.
(b) Reduction of the oxide
4. Let us see how they are accomplished in the case of iron:
(a) First we have to convert the iron into iron oxide. There are two methods available.
(i) Calcination and (ii) roasting. 
• In the case of iron, roasting is used. 
• Haematite is Fe₂O₃. So it already contains the oxide of iron. Even then, it is subjected to roasting.
• During roasting, impurities like sulphur, arsenic, phosphorus etc., are removed as their gaseous oxides. Water is also expelled. 
5. Next, we have to reduce the oxide.
• The haematite obtained after roasting will contain large quantities of silicon dioxide (sand). So we have to remove it to get the pure iron. The following procedure is used:
6. A mixture of roasted haematite, coke and lime stone (CaCO₃) is fed into a blast furnace. See fig.13.7 below:

Fig.13.7

• Blast furnace is a huge steel furnace. 
• Steel cannot withstand the high temperature produced inside the furnace. 
• So the inner side of the furnace is lined with a refractory material.
7. A blast of hot air is sent into the furnace from the bottom. 
• At the same time, the mixture containing haematite falls from the top.
• Because of the heat, chemical reactions will take place. Let us see each of those reactions:
• The calcium carbonate (in the form of limestone) which is supplied along with the haematite, will decompose into calcium oxide and carbon dioxide. 
The equation is: CaCO₃ (s) ➙ CaO (s) + CO₂ (g)
8. The calcium oxide (CaO) thus formed will combine with silicon dioxide (SiO₂). 
• This SiO₂ is the main impurity. It is sand. 
• The reaction between sand and CaO converts the sand into calcium silicate.
• The equation is: CaO (s) + SiO₂ (s) ➙ CaSiO₃ (s)
• Thus the sand is removed. 
9. Now we are left with the iron oxide Fe₂O₃
• It will soon be reduced to iron. Let us see:
10. At the bottom of the furnace, coke combines with the oxygen. This oxygen is obtained from the hot blast of air. 
• The equation of the reaction is: C (s) + O₂ (g) ➙ CO₂ (g) + heat
• From the equation, we can see that it is an exothermic reaction. So the temperature rises upto 1800^o C. 
• The CO₂ thus produced rises to the upper parts of the furnace. It will get  reduced by coke. 
• The equation is: CO₂ (g) + C (s) + heat ➙ 2CO (g)
11. The temperature in these upper parts is lower than the temperature at the bottom parts. This is because, the reaction here is endothermic. 
• Note that, the 'heat' is on the left side of the arrow. So there is an 'absorption of heat'
• The fig.13.7 above indicates that the lower parts are 'white hot'. While upper parts are 'red hot'. 
    ♦ 'White hot' is much hotter than 'red hot'.
    ♦ There is a gradation from white to red as we go upwards  
• The carbon monoxide reaches the middle portion of the furnace and reacts with the iron oxide.
• The iron oxide gets reduced to iron. 
• The equation is:  Fe₂O₃  (s) +3CO (g) ➙ 2Fe (s) + 3CO₂ (g)
12. The iron thus formed moves downwards. 
• Due to the high temperatures in the lower parts, it will melt into liquid form. 
• This liquid gets collected at the bottom of the furnace. From there it can be taken out. This is shown in fig.13.7 above

---

Thus we successfully produce iron. Now we have to learn some technical terms involved in the above process.
■ The impurity in the ore is called gangue.
• See step (5) above. In our present case, sand (SiO₂) is the gangue.
■ The chemical that is used to remove the gangue is called the flux.
• In step (8), we see that the calcium oxide (CaO) combines with sand to form calcium silicate.
• So in our present case, CaO is the flux
■ The product formed as a result of the reaction between gangue and flux is called the slag. 
• The only way to remove the sand is by converting it to 'another material'.   
• In step (8) we see that this 'another material' is calcium silicate (CaSiO₃). It is formed as a result of the reaction between the gangue and the flux.
• So in our present case, CaSiO₃ is the slag.
■ CaO is chosen as the flux because it is basic in nature. It will react with the gangue (SiO₂) which is acidic in nature

---

13. The slag formed in our present case will melt due to the high temperature. So like the molten iron, it also becomes a liquid and gets collected at the bottom of the furnace
• But the two liquids do not mix together. This is because, the slag is lighter. 
• So it floats above the molten iron. So it can be removed easily. This is shown in the fig.13.7 above
• The formation of the separate 'slag layer' above the 'molten iron layer' has the following advantages:
    ♦ The two can be removed from the furnace easily
    ♦  Contact between molten iron and oxygen is avoided, thus preventing the formation of iron oxide.
14. The iron produced in this way has 4% carbon.
• That is., if we take 100 grams of this iron, 4 grams will be carbon
• This much carbon is not allowable. It will prevent the iron from acquiring it's unique properties. 
• This iron has other impurities also like manganese, silicon, phosphorus etc., 
• This iron is called pig iron. Images can be seen here. It has only limited use and is an intermediate product of the iron industry.
15. The pig iron is taken to another special furnace (The electric arc furnace is used in modern days).
• There, it is mixed with scrap iron and coke and again melted.
• The product obtained is called cast iron. It contains 3% carbon
• When molten cast iron cools and solidifies, there will be an increase in volume. 
• So the molten cast iron is poured into moulds to make various shapes. Some products can be seen here.
• The shapes thus obtained are very hard. But they will break if subjected to bending.
16. The cast iron can be purified further. 
• The product obtained will contain only 0.2 - 0.5% carbon. 
• It will also contain some traces of phosphorus and silicon. This is called wrought iron.
• Some products using wrought iron can be seen here.
17. So we can write:
Pig iron ➙ Cast iron ➙ Wrought iron

---

• We often hear the word steel. Words 'iron' and 'steel' are sometimes used interchangeably. 

• But they are not the same. Let us see the difference:

■ Iron is an element in the periodic table. It's symbol is Fe

• We try to make pure iron in the blast furnace. 

• But the product obtained contain some carbon and other impurities. 

• It is not possible to completely avoid the 'presence of some impurities'.

• However pure iron can be prepared in specially equipped labs

■ Steel is an alloy. 

• We know that alloy is:

    ♦ a combination of two or more metals

    ♦ or a combination of a metal and another element

• Steel is an alloy of 'the metal iron' and 'the element carbon'. 

• It should not contain any other element

• Also, the amount of carbon should not exceed 2%

• By varying the content of carbon within the 2%, steels having various properties can be obtained.

    ♦ Low carbon steels have carbon less than 0.3%

    ♦ Medium carbon steels have carbon from 0.3% to 0.6%

    ♦ High carbon steels have carbon more than 0.6%

---

Following are some of the processes done on steel:
■ Annealing:
• Heat the steel to high temperature
• And then allow it to cool slowly. 
• This will make the steel ready to be shaped into different forms
■ Hardening:
• Heat the steel to high temperature
• And then cool it rapidly by dipping in water or oil
• This will make the steel very hard. But it will be brittle
■ Tempering:
• Heat the hardened steel until it becomes blue in colour
• And then allow it to cool slowly.
• This will make the hardened steel less brittle, while maintaining the hardness 

---

We will now see 3 alloys in which iron is a component:
1. Name: Stainless steel
Components: Iron (Fe), Chromium (Cr), Nickel (Ni) and Carbon (C)
Properties: Strong, will not rust.
Uses: For the manufacture of utensils, parts of vehicles
2. Name: Alnico
Components: Iron (Fe), Nickel (Ni), Aluminium (Al) and Cobalt (Co)
Properties: Magnetic in nature
Uses: For the manufacture of permanent magnets
3. Name: Nichrome
Components: Iron (Fe), Nickel (Ni),  Chromium (Cr), and Carbon (C)
Properties: High resistance
Uses: For the manufacture of heating coils
■ If we examine the above alloys, we can note an interesting point:
• Stainless steel and nichrome have the same components. But their properties are different. 
• Why is that so?
Ans: The difference in properties is achieved by increasing or decreasing the quantity of each component of the alloys. For example:
• Take 100 grams of stainless steel
Let the amount of chromium contained in it be x₁ grams
• Take 100 grams of nichrome
Let the amount of chromium contained in it be x₂ grams
• Then x₁ will not be equal to x₂
• In other words, the 'percentage of chromium' in stainless steel and nichrome are different
• Same is applicable for the other components iron, nickel and carbon
■ We can make different alloys by:
• Combining appropriate metals and also by 
• Changing the percentage content of each metal or element

---

In the next section, we will see the industrial production of aluminium and copper. 

PREVIOUS      CONTENTS       NEXT

                        Copyright©2018 High school Chemistry lessons. blogspot.in - All Rights Reserved

Chapter 13 - Production of Metals

In the previous section, we completed a discussion on Electrochemistry. In this chapter, we will see production of metals.
■ If we examine the history of human civilisation, we can see that:
• Gold and copper were the early metals to be discovered by man. They were discovered in the stone age itself. 
• Isn't it amazing that those expensive metals were discovered in very early stages of human civilisation?
• In fact it is not so amazing because, gold, though scarce, is found in pure state in nature. 
• They are found in pure state because, the reactivity of gold is very low. 
    ♦ It does not react with other elements. So it is often found as gold nuggets.
■ Similar is the case of copper. 
• Though reactivity of copper is greater than gold, it has low reactivity when compared to other metals. 
• We have seen the reactivity series in the previous chapter.
■As human civilisation progressed, methods were discovered to combine copper with other metals like tin and zinc. 
• The alloy of copper and zinc is called brass. 
• The alloy of copper and tin is called bronze.
• The use of 'tools made of bronze' became widespread. Thus began the 'bronze age'
■ As the civilisation progressed further, methods were discovered to isolate a much more reactive metal – Iron. 
• Soon the 'bronze age' came to an end and 'iron age' began.
■ The metals which are more reactive than iron (aluminium, potassium, sodium etc.,) could be discovered only much later, with the application of electricity.
• So we see that, the metals were discovered in the order of their reactivity.

---

• Today various metals are widely used in our day to day life
• The unique properties pocessed by each metal makes it useful for particular applications. 
For example:
• Copper and aluminium have high electrical conductivity.
    ♦ High electrical conductivity means, less resistance to the flow of elecricity.
    ♦ So the wastage of electric energy in the form of heat energy will be very low
    ♦ So copper and aluminium are used for making electric wires.
• Aluminium has high thermal conductivity
    ♦ High thermal conductivity means, less resistance to the flow of heat.
    ♦ So aluminium is used for making cooking utensils. 
    ♦ Because, heat will be readily transferred to the food to be cooked.
• Some metals are malleable. That is., they can be pressed or hammered into different shapes with out breaking.  For example, a piece of hot iron can be hammered into a thin sheet.
• Some metals are ductile. That is., they can be lengthened into thin wires by the application of a tensile force. Copper, aluminium and steel are examples of metals pocessing ductility. 

---

So we find that metals are a very important part of our day to day life. We will now try to find out how they are produced.
We have seen that:
• 'Less reactive metals' are found in pure form. Examples are paltinum and gold 
• 'More reactive metals' are found along with impurities. Examples are copper, aluminium, iron etc.,
■ So we have to learn about the 'more reactive metals' in some detail. We will write it in steps:
1. We know that, in certain localities, there will be an abundance of certain metals.
• Those metals and the impurities will be in a 'combined state'.
• The combination of 'pure metal' and 'impurities' is called mineral. 
2. We have to remove the impurities from the mineral to obtain the pure metal. 
• It may not be easy to remove those impurities. 
• We will have to take the mineral to a lab or an industrial factory.
3. But that is not all. Some times, it will involve very expensive processes to remove those impurities.
Consider an example: 
(i) A certain 'quantity of pure metal' is obtained after removing all impurities from a mineral sample. 
(ii) The total money spent for 'obtaining the mineral sample' and the 'purification process' is Rs. 'x'
(iii) The value of the 'quantity of pure metal' obtained is Rs.'y'
(iv) If y is greater than x, there is profit
(v) If y is less than x, then there is loss
4. We would not want to lose money. 
• So it is important to make the 'selection of the mineral' carefully.
Let us see an example:
    ♦ Bauxite (Al₂O₃.2H₂O) is a mineral containing aluminium 
    ♦ Cryolite (Na₃AlF₆) is a mineral containing aluminium 
    ♦ Clay (Al₂O₃.2SiO₂.2H₂O) is a mineral containing aluminium 
5. Which one of them would we use to obtain aluminium?
• Based on theoretical and experimental results, scientists have given us clear instructions about which one to use.
• The mineral that we would use is bauxite. Because it is easier to obtain aluminium from it.
■ A mineral from which a metal is economically, easily and quickly extracted is called the ore of the metal
• Thus we see that all minerals of a metal cannot be it's ores
6. So, for a mineral should possess the following properties to acquire the status of an ore:
• The mineral should be available in abundance in any particular locality
    ♦ That locality should be easily accessible
• It must be easy to separate the metal from the mineral
• The mineral should have a high metal content. 
    ♦ That is., if we take a sample of the mineral, a greater portion of that sample must be the metal.
■ If the mineral possess the above properties, it will be called an ore

---

Following are some of the metals and their ores:
• Metal: Aluminium
    ♦ Ore: Bauxite
    ♦ Chemical formula: Al₂O₃.2H₂O 
• Metal: Iron
    ♦ Ores: Haematite, Magnetite
    ♦ Chemical formula: Fe₂O₃, Fe₃O₄ 
• Metal: Copper
    ♦ Ores: Copper pyrites, Cuprite
    ♦ Chemical formula: CuFeS₂, Cu₂O 
• Metal: Zinc
    ♦ Ores: Zinc blende, Calamine
    ♦ Chemical formula: ZnS, ZnCO₃ 

---

Metallurgy is a branch of science and technology which deals with the properties of metals, their production and purification.
There are three important stages in the production of metals. They are:
1. Concentration of ores
2. Extraction of the metal from concentrated ore
3. Refining of the metal  
We will now see each stage in detail:

1. Concentration of ores

• The process of removing the impurities from the ore obtained from the earth's crust is termed concentration of ore. 
• The impurities are called gangue.
The method used for concentration will depend on:
(a)  Nature of the ore
(b) Nature of the impurities
• What ever be the method used for concentration, the first step is to powder the ore. 
    ♦ This is known as pulverisation. 
• The ore obtained from the mine will be in the form of lumps of various sizes. 
• But when pulverisation is done, the ore will be turned into smaller particles of uniform size. 
• This makes it easier to remove the impurities.

Levigation or Hydraulic washing

• When the impurities are lighter and the metal particles are heavier, we use Levigation method.
• Levigation is also known as Hydraulic washing. 
• Consider the fig.13.1 below:

Fig.13.1

We can write the steps:
1. The powdered ore is supplied from the top into a conical tank. 
2. The conical tank is full of water which is supplied from the bottom. 
3. The lighter impurities will float at the top portion while the heavier metal particles will settle down. 
4. The impurities are collected from the top and the metal particles are collected from the bottom. 
• Large quantities of impurities can be removed by this method. We can write a summary:
When the impurities are lighter and the ore particles are heavier, the lighter impurities are removed by washing in a current of water. 
• Examples:
    ♦ If the impurities are oxides, this method can be used
    ♦ Some ores of gold can be concentrated by this method

Froth floatation

• This method is used when impurities are heavier and ore particles are lighter. 
• Usually, sulphide ores are concentrated by this method. 
• Consider fig.13.2 below. We can write the steps:

Fig.13.2

1. A mixture of the powdered ore, water and pine oil is supplied to the bottom portion of a tank
• Pine oil is an oil obtained from pine trees
2. This mixture is agitated by rotating a paddle. Compressed air is also supplied to the bottom of the tank at the same time
• As a result, froth is formed. 
3. The sulphide impurities are wetted by water. They are heavier and so sinks to the bottom.
• The ore particles are wetted by pine oil. They stick to the froth and floats on the top. 
4. The froth can be skimmed off from the top of the tank. 
• From this froth, the ore particles can be easily obtained. 
• Copper pyrites is concentrated by this method.

Magnetic separation

• This method is used when any one (and only one) of the following two conditions are satisfied:
    ♦ The ore is magnetic
    ♦ The impurity is magnetic
• Note that, for this method, only one should be magnetic. If both the ore and impurities are magnetic, the method will not work.
We will now write the steps:
1. In fig.13.3 below, the powdered ore is fed onto a conveyor belt. 
• The roller on the left side is magnetic.

Fig.13.3

2. When the powdered ore falls over, the magnetic component (which may be either the ore or the impurity), will be attracted towards the roller. 
• So it get separated from the other component. 
3. This method is used in the following cases:
(i) Powdered magnetite. It is an iron ore
The iron, which is the magnetic component can be easily separated
(ii) Powdered tin stone. It is an ore of tin
• Tin is non magnetic. 
• But the impurity is tin tungstate. It is magnetic.

Leaching

• In this method, the powdered ore is added to a solvent. The solvent should satisfy both the conditions given below:
    ♦ The ore must dissolve in it
    ♦ The impurity must not dissolve in it
Let us write the steps:
1. The ore dissolves and forms a solution. 
2. The insoluble impurities are filtered off.
3. Now we have the ore in dissolved form. 
• So next step is to obtain the ore from the solution. 
• For this, a suitable chemical reaction is used
4. Bauxite, the ore of aluminium is concentrated by this method

---

So we have seen the basic details about 'concentration of ore'. We will now see some solved examples:
Solved example 13.1
(a) In a powdered ore, the ore is of high density and the impurities present is of low density. Which method would you use for concentration of the powdered ore?
(b) In a powdered ore, the ore is magnetic in nature and the impurities present is non magnetic in nature. Which method would you use for concentration of the powdered ore? 
(c) In a powdered ore, the ore is of low density and the impurities present is of high density. Which method would you use for concentration of the powdered ore?
(d) In a powdered ore of aluminium, the aluminium gets dissolved in a solution and the impurities is insoluble in the same solution. Which method would you use for concentration of the powdered ore?
Solution:
(a) Levigation (hydraulic washing)
• Because, the denser ore will settle down in water and the lighter impurities will float above the water
(b) Magnetic separation
• Because, the ore, which is the magnetic component, will get separated from the non magnetic impurities
(c) Froth flotation
• Because, the ore which is lighter, will rise to the top along with the froth. The heavier impurities will sink to the bottom. Thus they will get separated.
(d) Leaching
• Because aluminium will dissolve while the impurities will not. So the impurities can be filtered off. The useful aluminium can be recovered from the solution using a suitable chemical reaction.

---

In the next section, we will see the second stage after 'concentration of ore'. 

PREVIOUS      CONTENTS       NEXT

                        Copyright©2018 High school Chemistry lessons. blogspot.in - All Rights Reserved