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Extraction of Copper
Ores of Copper
Extraction of Metallic Copper
Refining of Copper
Extraction of Aluminium
Ores of Aluminium
Refining of Aluminium
Uses of Aluminium
Alloys of Aluminium
Related Resources
The chief and important ore of copper from which the metal is most isolated is copper pyrites (CuFeS2). Froth floatation process is used for concentrating the ore. The powdered ore is suspended in water and after adding little pine oil is stirred by means of air. The sulphide ore particles come to the surface & gangue remains at the bottom is rejected.
Name of Ore
Formula of Ore
Copper glance or Chalcocite:
Cu2S
Chalcopyrites(copper pyrites):
Cu2S×Fe2S3
Cuprite
Cu2O
Malachite
[CuCO3.Cu(OH)2]
Azurite
[2CuCO3.Cu(OH)2]
Concentration of the ore by froth floatation process:
Copper pyrites contains only (2-3)% of copper. The rest of the ore contains iron or sulphide, silica, silicious materials, sulphur, arsenic etc. as impurities. Froth flotation by Xanthate and pine oil. The froth is collected and dried when concentrated ore is obtained which contains 25-30% of Cu.
Roasting
Cu2S.Fe2S3 + O2 → Cu2S + 2FeS + SO2
Cu2S.Fe2S3 + 4O2 → Cu2S + 2FeO + 3SO2
2Cu2S + 3O2 → 2Cu2O + 2SO2
Cu2O + FeS → Cu2S + FeO
Smelting of the roasted ore in blast furnace: material required
Roasted ore
Lime stone
Coke (used as fuel)
Silica (used as flux)
Lime stone (used to remove excess silica)
Reactions occurring given as follows
2FeS + 3O2 → 2FeO + 2SO2 Cu2O + FeS → Cu2S + FeO FeO + SiO2 → FeSiO3 (removed as slag) CaO + SiO2 → CaSiO3 (removal as slag)
2FeS + 3O2 → 2FeO + 2SO2
FeO + SiO2 → FeSiO3 (removed as slag)
CaO + SiO2 → CaSiO3 (removal as slag)
Self reduction in Bessemer Converter
2FeS + 3O2 → 2FeO + 2SO2 FeO + SiO2 → FeSiO3 (slag) 2Cu2S + 3O2 → 2Cu2O + 2SO2 Cu2S + 2O2 → Cu2SO4
2FeS + 3O2 → 2FeO + 2SO2
FeO + SiO2 → FeSiO3 (slag)
2Cu2S + 3O2 → 2Cu2O + 2SO2
Cu2S + 2O2 → Cu2SO4
When 2/3 of the cuprous sulphide is oxiidsed, the balst is stopped. The produced Cu2O and Cu2SO4 are reduced by the rest of cuprous sulphide to produce metallic copper with the evolution of SO2.
Cu2S + 2Cu2O → 6Cu + SO2 Cu2SO4 + Cu2S → 4Cu + 2SO2
Cu2S + 2Cu2O → 6Cu + SO2
Cu2SO4 + Cu2S → 4Cu + 2SO2
As the molten copper cools, it gives off the dissolved of SO2. The SO2 gas escaping in the form of bubbles, leaves the surface of the metal with full of cavities which gives the metal a blistered appearnace. This is why the metal thus obtained is by blister copper.
Copper is refined using electrorefining method. Anode: Impure copper obtained above Cathode: Pure copper Electrolyte: 15% CuSO4 solution + 5% H2SO4 When electric current is passed through the electrolyte, the anodes gradually dissolve and pure copper is deposited on the cathodes which gradually grow in size. The impurities like Fe, Zn, Ni etc., dissolved in the solution as sulphates while gold, silver, platinum settle down below the anode as anode mud. Reactions coming are as follows
CuSO4 Cu+2 + SO4–2 At anode: Cu – 2e Cu+2 At cathode: Cu+2 + 2e Cu
CuSO4 Cu+2 + SO4–2
At anode: Cu – 2e Cu+2
At cathode: Cu+2 + 2e Cu
Aluminium is the most abundant metal in the earth’s crust. Aluminium does not occur free in nature, but its compounds are numerous and widely distributed.
The chief and important ore from which aluminium is exclusively and profitably obtained is Bauxite, AI2O3.2H2O. The extraction of the metal from bauxite involves the three main steps.
Bauxite
Al2O3×2H2O
Cryolite
Na3AlF6
Feldspar
K2Oal2O3×6SiO2 or KalSi3O8
Mica
K2O×3Al2O3×6SiO2×2H2O
Corundum
Al2O3
Aluminium is mainly extracted from bauxite ore.
Purification of Bauxite
By Bayer’s process comercially it is being carried out (for red bauxite not for the white bauxite).
Flow sheet of Bayer’s process for the preparation of pure Al2O3
Hall’s process
Crude bauxite at 1100°C reacts with Na2CO3, little CaCO3 when CaSiO3, NaSiO2, NaFeO2 etc. form
Al2O3 + Na2CO3 → 2NaAlO2 + CO2 Fe2O3 + Na2CO3 → 2NaFeO2 + CO2 SiO2 + Na2CO3 → Na2SiO3 + CO2 CaO + SiO2 → CaSiO3
Al2O3 + Na2CO3 → 2NaAlO2 + CO2
Fe2O3 + Na2CO3 → 2NaFeO2 + CO2
SiO2 + Na2CO3 → Na2SiO3 + CO2
CaO + SiO2 → CaSiO3
Then at 50° – 60°C CO2is passed through NaAlO2 solution and produces thereby Al(OH)3
2NaAlO2 + CO2 + 3H2O ¾® 2Al(OH)3¯ + Na2CO3 2Al(OH)3 Al2O3 + 3H2O
2NaAlO2 + CO2 + 3H2O ¾® 2Al(OH)3¯ + Na2CO3
2Al(OH)3 Al2O3 + 3H2O
Serpeck’s Process
Bauxite containing high percentage of silica can be purified by Serpeck’s process. In this process finely powdered bauxite is mixedf with coke and the mixture is heated to 1800°C in a current of nitrogen. The AlN thus obtained is reacted with hot and dilute NaOH, produced NaAlO2 and excess AlN is hydrolysed and Al(OH)3 is formed.
Al2O3 + 3C + N2 → 3AlN + 3CO SiO2 + 2C → Si + 2CO AlN +NaOH → NaAlO2 + NH2+ NaAlO2 + 2H2O → Al(OH)3¯ + NaOH AlN + 3H2O → Al(OH)3¯ + NH3 2Al(OH)3 Al2O3 + 3H2O
Al2O3 + 3C + N2 → 3AlN + 3CO
SiO2 + 2C → Si + 2CO
AlN +NaOH → NaAlO2 + NH2+
NaAlO2 + 2H2O → Al(OH)3¯ + NaOH
AlN + 3H2O → Al(OH)3¯ + NH3
Electrolytic Reduction of Al2O3
Pure alumina melts at about 2000°C and is a bad conductor of electricity. If fused cryolite AlF3.3NaF and CaF2 (Fluorspar) is added the mixture melts at 900°C and Al2O3 becomes a good conductor of electricity. Metallic Al is liberated at the cathode
Alumina is mixed with cryolite (Na3AIF3), fluorspar (CaF2) in the ratio 20 : 60 whereby, it not only becomes good conductor but also fuses at about 900oC which is much below the b.p. of aluminium. The electrolysis of the fused mass is carried out in an iron box, which lined with gas carbon. The lining serves as the cathode, the anode consists of carbon rods dipped in the fused mass. The fused electrolyte is kept covered with a layer of powdered coke to prevent any action of air. The voltage employed in the electrolysis is 5.3 volts. The current passed (about 50,000 amperes) serves to purposes: (i) heating and (ii) electrolysis. Thus the fused mass is automatically kept at 900oC during electrolysis.
Aluminium is obtained at the cathode and being heavier than the electrolyte sinks to the bottom and is tapped off periodically from the tap hole. Oxygen liberated at the anode attacks carbon rods and forms CO and CO2. During electrolysis the concentration of the electrolyte goes on falling thereby increasing the resistance of the cell which is indicated by the glowing of a lamp placed parallel. Much of the alumina is then added and the process is made continuous.
Electrolysis of molten mixture
Cathode: Carbon
Anode: Graphite rods
Electrolyte: 60 parts cryolite + 20 parts fluorspar + 20 parts pure Al2O3
Temperature: 900°C
Reactions
According to the 1st theory the following reaction occurs
Al2O3 2Al+3 + 3O–2
At cathode : 2Al+3 + 6e → 2Al
At anode : 3O–2 – 6e → 3O2
As cryolite has greater electrochemical stability it does not dissociate. It only increases the dissociation of Al2O3
But the second theory states that, cryolite undergoes electrolytic dissociation first then Al+3 goes to the cathode, produced F2 at anode then reacts with Al2O3 produces AlF3.
AlF3.3NaF Al+3 + 3Na+ + 6F–
At cathode : Al+3 + 3e → Al
At anode : 6F– – 6e → 3F2
Overall Reaction : Al2O3 + 6F2 → 4AlF3 + 3O2
Solved Problem
Question 1:
Aluminium is not extracted directly from bauxite, instead, bauxite is first purified to produce pure alumina from which aluminium is extracted by electrolytic reduction – Explain why?
Solution
Aluminium is not directly extracted from bauxite. This is because of the fact that, bauxite is always associated with impurities like ferric oxide and silica. If bauxite is used directly for the extraction of aluminium, the iron and silica present in if would deposit at the cathode during its electrolytic reduction. The aluminium thus obtained at the cathode, becomes contaminated with iron and silica. As a result, the produced aluminium becomes brittle and is readily attacked by air and water. ___________________________________________
Question 2:
In moist air copper corrodes to produce a green layer on the surface. Explain
Solution:
In presence of moist air a thin film of green basic copper carbonate is formed on its surface and hence copper corrodes
2Cu + O2 + H2O + CO2 → CuCO3.Cu(OH)2
The aluminium metal obtained by the electrolysis of fused almina is about 99.5% pure. It can be further refined by Hoope’s electrolytic process
Aluminium as produced by the electrolysis of AI2O3 is 90% pure. It can be refined further up to 99.9% purity by Hoope’s process.
The electrolytic cell consists of an iron tank lined with carbon. It is filled with three liquids differing in specific gravity. The upper layer is of pure fused aluminium and serves as cathode.
The bottom layer is that of impure metal in the fused state and serves as anode. The central layer is that of molten mixture of the fluorides of AI, Ba and Na and serves as an electrolyte
On passing electric current, pure aluminium goes to the top layer from the central layer and an equivalent amount of the metal from the bottom layer passes into the central layer. There is thus gradual transference of aluminium from bottom layer to the top and the impurities are left behind. Crude aluminium is added from time to time.
Aluminium forms a number of useful alloys, which are given as follow;
Alloy
Approximate composition
Uses
(1) Aluminium bronze
AI 10%, Cu 90%
For hard, non-corrodible vessels
(2) Duralumin
AI 95%, Cu3% Mn1% Mg1%
Aeroplanes and automobile parts
(3) Magnalium
AI 90%, Mg10%
Balance beams
(4) Y-alloy
AI 92.5%, Cu 4%, Ni 2%, Mg 1.5%
Aeroplan
Question 1: In Serpek’s process, by product obtained in the purification of bauxite is
(A) Al2O3
(B) N2
(C) NH3
(D) None
Question 2: Which of the following metal is thrown as anode mud during electrolytic refining of copper ?
(A) Zn
(B) Fe
(C) Ag
(D) Ni
Question 3: In the electrorefining, the impure metal is made
(A) Cathode
(B) Anode
(C) Both
Q.1
Q.2
Q.3
c
b
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