In the previous section, we saw the industrial production of steel. In this section, we will see the industrial production of aluminium and copper.
• 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:
• 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 O2- ions.
• When electricity is passed through the electrodes, the positive Al3+ ions gets separated from the O2- ions and move towards the negative electrode.
• There they accept electrons and get reduced to Al atoms.
13. The negative O2- 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
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.
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:
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
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
Solution:
In the next chapter, we will see Nomenclature of organic compounds
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 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 O2- ions.
• When electricity is passed through the electrodes, the positive Al3+ ions gets separated from the O2- ions and move towards the negative electrode.
• There they accept electrons and get reduced to Al atoms.
13. The negative O2- 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
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.
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 |
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
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 |
In the next chapter, we will see Nomenclature of organic compounds
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