Chapter 13.3 - Production of Aluminium and Copper

In the previous section, we saw the industrial production of steel. In this section, we will see the industrial production of aluminium and copper.

Industrial production of Aluminium

Extraction of Aluminium
• Aluminium is the most abundant metal on the earth's crust. 
• But it's reactivity is more than gold and silver. 
• So it is found in a combined state in nature. 
• A 'cost effective method to separate pure aluminium from it's ore' was discovered only in the nineteenth century. Until then aluminium was costlier than gold.
• The method is known as the Hall-Heroult process. It was discovered by two scientists Charles Martin Hall and Paul Heroult.
• Let us see the details of this process. We will write it in steps:
1. Bauxite (Al₂O₃.2H₂O) is the ore of aluminium. 
• The chief impurity in it, is the silicon dioxide (SiO₂) 
2. We know that any ore is to be powdered before it enters the first stage. It is true for bauxite also. So the bauxite is finely powdered.
• Stage 1 is the concentration of the powdered ore. 
• We have seen the different methods used for concentration. In the case of bauxite, leaching is used.
3. For that, the powdered bauxite is treated with hot concentrated NaOH solution. 
• Then the Al₂O₃ in the bauxite dissolves in the solution. 
• The impurities will not dissolve. So they are filtered off. 
4. But the aluminium that we want, is now in a dissolved form inside the solution. We want it back.
• Inside the solution, the aluminium exists as sodium aluminate (NaAlO₂). 
5. So, to this solution, a little aluminium hydroxide (Al(OH)₃) is added. 
• After that, the solution is diluted with water. 
• Then the Al gets separated from NaAlO₂ and will precipitate as Al(OH)₃. 
• This precipitate is filtered and washed. 
• Once we get this Al(OH)₃, leaching is complete. 
6. Next step is to convert this hydroxide into the oxide. 
• For that, calcination is used. 
• The hydroxide is heated strongly. It decomposes to give the oxide. 
• The equation is: 2Al(OH)₃ (s) ➙ Al₂O₃ (s) + H₂O (l)
• Al₂O₃ is called alumina
7. So the second stage is also over. The third and final stage is refining. 
• We have to reduce the oxide Al₂O₃ into pure aluminium. 
8. The Al₂O₃ is so stable that carbon or carbon monoxide is not sufficient to reduce it. 
• We will need the 'most powerful reducing agent', which is electricity. 
• That is., we will need to conduct electrolysis to obtain pure aluminium from Al₂O₃.
9. We will have to melt Al₂O₃ into a liquid form. 
• This molten Al₂O₃ will be the electrolyte. 
10. But Al₂O₃ will not melt easily. Because it's melting point is very high. 
• But when heated after adding cryolite, the cryolite melts and Al₂O₃ dissolves in it. 
• Cryolite helps to achieve two things:
    ♦ Bring down the melting point of Al₂O₃ 
    ♦ Increase the electrical conductivity of the molten mixture.
11. Consider the fig.13.8 below:

Fig.13.8

• The molten mixture is inside a steel tank 
    ♦ The steel tank has a carbon lining on the inner sides. 
    ♦ This carbon lining is the negative electrode. 
• Carbon rods act as positive electrodes. 
12. The Al₂O₃ exists as ions. Al³⁺ ions and O^2- ions. 
• When electricity is passed through the electrodes, the positive Al³⁺ ions gets separated from the O^2- ions and move towards the negative electrode. 
• There they accept electrons and get reduced to Al atoms. 
13. The negative O^2- atoms move towards the positive electrodes. 
• There they donate the electrons and get oxidised to Oxygen atoms. 
• Thus oxygen is produced at the positive electrodes. 
• The positive electrodes are carbon rods. The newly produced oxygen reacts with the carbon rods. So these carbon rods will have to be replaced from time to time. 
14. The newly produced pure aluminium is in a molten state. 
• It is denser than the electrolyte. So it is collected at the bottom of the tank. 
• It is taken out through the plug.
• Thus the refining process is complete

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Aluminium is a metal with many special properties:
• It is a light metal
• It has good thermal conductivity. So it is used for making cooking vessels  
• It has good electrical conductivity. So it is used for making electrical wires
• It has good metallic lustre. So it is used for making reflectors. See images here.
• It has great malleability. So it can be made into thin sheets.

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Refining of copper

We have seen that there are 3 stages in the production of metals:
1. Concentration of ore 
2. Extraction of metals from the concentrated ore
3. Refining of the metals
• But in this section we are discussing only the refining of copper. Why is that? What about the first two stages?
Ans: We have already mentioned the first two stages related to copper.
• When we discussed froth floatation, we mentioned that copper pyrites is concentrated by that method
• When we discussed roasting, we mentioned that Cu₂S is extracted by that method.
• So now we will see refining of copper
• Copper is a metal that is widely used in electrical appliances. 
• For such purposes, copper used must be very pure. Electrolytic method is used to obtain pure copper. Consider the fig.13.9 below:

Fig.13.9

1. 'Copper obtained during extraction', which contains many impurities is used as the positive electrode. We have seen the reason here. 
2. Pure copper is used as the negative electrode. 
3. Copper sulphate solution to which a little H₂SO₄ added is used as the electrolyte. 
4.When the switch is turned on:
• At the positive electrode, the Cu²⁺ ions separates themselves from the impurities and go into the solution. 
• These positive ions are attracted towards the negative electrode. There they receive two electrons and become pure copper atoms. 
• The reaction taking place is: Cu²⁺ + 2e^- ➙ Cu
5. This is a reduction reaction. 
• In any type of cell (galvanic cell or electrolytic cell), cathode is where reduction takes place.  So in our present case:
• The original pure metal piece used as the negative electrode is the cathode. 
• And the impure metal piece is the anode.
6. As more and more Cu²⁺ ions begin to leave the anode, the impurities cannot be bound together. 
• So The impurities disintegrate into smaller pieces and gets deposited below the anode. 
• This is called anode mud. Anode mud may contain valuable metals like gold or silver. 
7. The pure copper is removed from the cathode after a few days

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Now we will see some solved examples:
Solved example 13.2
During the extraction of iron, a mixture of coke, limestone and haematite is added from the top into the blast furnace. What is the significance of adding limestone in the mixture?
Solution:
1. 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)
2. 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.
3. Sand (SiO₂) is the gangue. It is acidic
• Calcium oxide (CaO) is the flux. It is basic
• The above two react to form the slag calcium silicate (CaSiO₃).
4. We can remove the gangue only by converting it into slag
• To convert the gangue into slag, we need the flux CaO
• To obtain CaO, we need CaCO₃. That is why we add limestone

Solved example 13.3
Which one does not belong to the group? Why?
(a) Leaching, Distillation, Froth floatation, Hydraulic washing
(b) Bauxite, Haematite, Clay, Cryolite 
Solution:
(a) Distillation does not belong to the group because it is a refining process
All others are concentration processes
(b) Haematite does not belong to the group because it is an ore of iron
All others are minerals of aluminium

Solved example 13.4
Match the following

	A	B	C
1	Sulphide ore	Bauxite	Electrolysis
2	Ore of iron	Zinc blende	Calcination
3	Aluminium ore	Calamine	Blast furnace
4	Carbonate ore	Haematite	Froth floatation process

Solution:

	A	B	C
1	Sulphide ore	Zinc blende	Froth floatation process
2	Ore of iron	Haematite	Blast furnace
3	Aluminium ore	Bauxite	Electrolysis
4	Carbonate ore	Calamine	Calcination

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In the next chapter, we will see Nomenclature of organic compounds

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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.

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• 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. 

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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

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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 

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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

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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.

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

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