Extraction of Copper and Aluminium

Extraction of Copper

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.

1. Ores of Copper

2. Extraction of Metallic Copper

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)

Self reduction in Bessemer Converter

• Reactions involved are,

      2FeS + 3O₂ →  2FeO + 2SO₂

      FeO + SiO₂ →  FeSiO_­3 (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

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.

4. Refining of 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

Extraction of Aluminium

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.

• Purification of Bauxite

• Electrolytic reduction of Alumina, (AI₂O₃)

• Purification of AI.

1. Ores of Aluminium

Aluminium is mainly extracted from bauxite ore.

2. Extraction of Aluminium    

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

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

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

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 900^oC 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 900^oC 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 1^st 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

3. Refining of Aluminium

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.

4. Uses of Aluminium

• Aluminium, being very light, is used in householf utensils, aeroplane parts, precision and surgical instruments etc.

• Since it is unattached by nitric acid, is used in chemical plants and also for transporting nitric acid.

• Aluminium foil is used for packing chocolates, cigarettes etc.

• Alums are used as mordents in dyeing and points.

• Mixed with oil, it is used in steam piped and other metal objects.

• It is used as a reducing agent for the production of certain metals such as chromium, iron, manganese etc.

• Alumina is used for making refractory bricks and ultramarine.

5. Alloys of Aluminium

Aluminium forms a number of useful alloys, which are given as follow;

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

(D)  None

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