Extraction of Aluminium, Iron, copper and zinc from their ores

1) Extraction of Aluminium:

 Occurrence: Aluminium is quite reactive element and hence do not occur in nature in free state.  Aluminium is the most abundant metal. The important ores of aluminium are:

1) Bauxite- Al₂O₃.2H₂O

2) Cryolite- Na₃AlF₆

3) Feldspar- KAlSi₃O₈

4) Mica- KAlSiO₁₀ (OH)₂

Extraction of Aluminium form Bauxite: Aluminium metal is extracted from bauxite involves the following steps:

1) Purification of bauxite:

Bauxite contains SiO₂, iron oxide and titanium oxide as impurities. Bauxite is digested with a hot concentrated solution of sodium hydroxide (45%) at about 473- 523K and 35-36 atmosphere pressure.

Aluminium dissolved to form sodium meta-aluminate, NaAlO₂ leaving behind iron oxide and oxides of titanium.

Al₂O₃   +   2 NaOH ——-> NaAlO₂  +   H₂O

Silica also dissolves in sodium hydroxide to form soluble sodium silicate, Na₂SiO₃.

SiO₂  +   NaOH ———->  Na₂SiO₃ +   H₂O

The impurities are filtered out and the solution containing sodium meta-aluminate, NaAlO₂ and sodium silicate, Na₂SiO₃ are seeded with freshly precipitate aluminium hydroxide leaving behind sodium silicate in solution.  The solution is filtered and heated at 1473 K to get pure alumina.

NaAlO₂  +   H₂O ———> NaOH  +   Al(OH)₃

Al(OH)₃  ———–> Al₂O₃  +   H₂O

2) Electrolysis of pure alumina:

Aluminium is obtained from alumina by the process of electrolysis. Pure alumina is dissolved in molten cryolite and is electrolysed in an iron tank lined inside with carbon which is act as a cathode while a number of carbon rod dipping in the fused electrolyte serves as anode. Cryolite improves the electrical conductivity of alumina as it is a poor conductor. The temperature of the electrolyte is maintained between 1200 – 1300 K.

On passing electric current, aluminium is deposited at the cathode and gets collected at the bottom of the tank from where it is removed. Oxygen is evolved at the anode; combine with carbon to produce CO₂ which either burns or escapes out.

                                    Fig: Electrolytic Tank for the extraction of aluminium

The reactions taking place during electrolysis are as follows:

Al₂O₃  ——–>  Al⁺³   +   O^-2

At Cathode,   Al⁺³  +  3e^– ——–> Al (metal)

At anode,       O^-2 –  2e  ——-> O

C (s) + O  ———> CO

CO + O   ———-> CO₂ (g)

3) Purification: The aluminium thus obtained above purified by electro-refining.

 2) Extraction of Iron:

Occurrence: Iron is the second most abundant metal occurring in the earth’s crust. The important ores of iron are-

1. Haematite, Fe₂O₃ (red oxide of iron)
2. Magnetite, Fe₃O₄ (magnetic oxide of iron)
3. Limonite, Fe₂O₃. 3H₂O(hydrated oxide of iron)
4. Iron pyrites, FeS₂
5. Siderite or Spathic ore, FeCO₃

Extraction of cast iron or pig iron: The extraction of Iron involves the following steps-

(i) Concentration:

The ore is first crushed with jaw crushers into small pieces (1” — 2” in size) and then washed with a stream of water to remove earthy and siliceous impurities. Thus, concentrated ore is obtained.

(ii) Roasting:

The concentrated ore is roasted with a little coke and excess of air in a reverberatory furnace. Main advantages of roasting are given below:

(a) Ferrous oxide changes to ferric oxide and thus the conversion of ferrous silicate (slag) is avoided.

4FeO + O_{2    ———>} 2Fe₂O₃

(b) Moisture, arsenic, sulphur, phosphorus etc. are removed.

2Fe₂O₃. 3H₂O ———> 2Fe₂O₃ + 3H₂O

4As + 5O_{2 ———->} 4 2As₂O₅

P₄ + 5O₂  ———–> 2 P₂O₅

S+O_{2  ———–>} SO₂

(c) Ore becomes porous and hence easily reduced.

(d) Carbonate ore (ii present) change to iron oxide.

FeCO _{3  —————>} FeO + CO₂

4FeO + O_{2  ———–>}  2 Fe₂O₃

(iii) Smelting:

The roasted ore is mixed with desulphurised coke and lime stone in a steel blast furnace lined inside with fireclay bricks.

Details:

The various reactions taking place in the furnace at different temperatures are as follows-

                                                                Fig: Blast Furnace

Base of the furnace:

(i) Zone of combustion.

Here coke burns in oxygen to form carbon dioxide. The reaction is exothermic and temperature rises to about 1500°C.

C + O_{2  ———->} CO₂ + 395.4 kJ

(ii) Zone of fusion.

Here CO₂ (formed above) reacts with coke to form carbon monoxide. The reaction is endothermic and temperature lowers to about 1200-1300°C.

CO₂    +   C ———>  CO

The carbon monoxide formed above reduces Fe₂O₃ (if present) to iron and the fusion of iron occurs here.

3 Fe₂O₃   +  CO  ——–>  2Fe₃O₄    +  CO₂

Fe₃O₄    +  CO  ——–>  3FeO + CO₂

3FeO + CO  _———->  Fe    + CO₂

Molten

Middle of the furnace (Zone of slag formation):

In the middle of the furnace (temp. 800-1000°C), the lime stone decomposes to give lime (CaO) and CO₂. This CaO reacts with the impurities (SiO₂) to form CaSiO₃ (Slag).

CaCO_{3  ——->} CaO + CO₂

CaO     +   SiO_{2 ——–>}  CaSiO₃

Flux        Gangue             Slag

Top of the furnace (Zone of reduction):

In this part of the furnace (temperature about 400°C), the upcoming carbon monoxide reduces iron oxide to iron (Spongy). This iron moves down the furnace and melts in the zone of fusion. It dissolves elements like carbon, phosphorus, silica and collects at the base of the furnace from where it is removed. Slag, which floats on the molten iron are removed from the outlet (S).

The molten iron which is tapped out from the furnace is called Pig iron. It contains          2.6- 4.3% of carbon along with manganese, silicon, sulphur and phosphorus. It is cast into moulds, which on remelting and cooling produce cast iron.

The composition of the cast iron includes carbon = 0.7-3% along with varying quantities of phosphorus, sulphur and manganese. A part of carbon is in combination with iron as cementite, (Fe₃C). The two forms of cast iron are white cast iron and grey cast iron.                                          3) Extraction of Copper:

Occurrence: Copper is found in combined state as well as in free state. The common ores of copper are-

• Copper Pyrites, CuFeS₂
• Copper Glance, Cu₂S
• Malachite, Cu(OH)₂.CuCO₃
• Rubby Copper, Cu₂O
• Cuprite, Cu₂O
• Azurite, Cu(OH)₂.2CuCO₃

Extraction:

The metallic copper is mainly extracted from copper pyrites. The various steps involved during extraction of copper are-

Concentration: The ore is first crushed into fine powder and then concentrated by froth floatation process.

Roasting: The concentrated ore is heated in the presence of excess of air in a reverberatory furnace. During heating temperature is kept below the melting point of the ore. Changes that take place during roasting are:

(i) Moisture is expelled.

(ii) Sulphur, arsenic and phosphorus impurities are expelled as their volatile oxides.                                                                       S+O₂  ———> SO₂

4As + 5O₂ ——-> 2As₂O₅

4P + 5 O₂ ——–>  2P₂O₅

(iii) Copper pyrite ore is converted to ferrous sulphide and cuprous sulphide.

2CuFeS₂ + O_{2 —————->} Cu₂S + 2FeS + SO₂

Smelting: Mixture of roasted ore, powdered coke and sand (flux) is heated in blast furnace. In this furnace two changes take place:

(i) Oxidation of ferrous sulphide takes place and ferrous oxide which is formed as a result of this combines with silica (flux) to form fusible slag, FeSiO₃.

2FeS + 3O₂ ———–> 2FeO+2SO₂

FeO +  SiO₂ ———-> FeSiO₃.

Impurity     Flux                    Slag

(ii) Cuprous oxide which is formed as a result of oxidation is partially converted back into cuprous sulphide.

Cu₂S + 3 O₂ ——–>  2Cu₂O + 2SO₂

Cu₂O + FeS  ———->  Cu₂S + FeO

From the base of furnace molten mass called matte is removed which is nothing but cuprous sulphide containing some ferrous sulphide.

 Bessemerisation: The matte is transferred to a Bessemer converter which is a pear-shaped furnace, lined from inside with magnesium oxide. It is fitted with pipes known as tuyeres through which air blast can be admitted. The converter can be tilted so that it can be emptied and the mass poured out when the process is over. A blast of hot air and sand is blown through molten matte.

The following reactions take place in the Bessemer converter:

2Cu₂S + 3O_{2 ———>}   2Cu₂O + 2SO₂

2FeS + 3O₂ ——–> 2FeO + 2SO₂

The iron oxide forms the slag with silica.

FeO + SiO_{2   ———->} FeSiO₃

Slag

The cuprous oxide reacts with more of Cu₂S to form copper.

2Cu₂O + Cu₂S  ———>  6Cu + SO₂.

After the reaction has been completed, the molten copper is poured off. As it cools, it gives up the sulphur dioxide dissolved in it which comes out in the form of bubbles thus giving the shape of blisters to the surface of copper which is, therefore, known as Blister copper.

Fig: Bessemer converter of copper.

Electrolytic purification:

The crude copper thus obtained (about 95%) is then purified by electrolysis. Impure copper is made anode and a thin strip of pure copper is made cathode. An aqueous solution of copper sulphate, acidified with dil. H₂SO₄ is used as electrolyte. During electrolysis anode starts dissolving due to oxidation of copper whereas cathode becomes thicker due to deposition of copper.

At anode:        Cu ——–>  Cu²⁺ + 2e^–

At cathode:      Cu²⁺ + 2e- ———-> Cu

Impurities either dissolve in the solution or fall under anode as anode mud.

 4) Extraction of Zinc:

Occurrence:

Zinc has been known from very early times especially in the form of its alloys (brass and bronze). It does not occur in native form. In combined state it occurs as:

(i) Zinc blende- ZnS

(ii) Calamine – ZnCO₃

(iii) Zincite- ZnO

(iv) Franklinite – ZnO.Fe₂O₃

The first two ores are more abundant and are generally employed for the extraction of the metal.

Extraction: The various steps involved in the extraction of zinc from zinc blende are:

(i) The ore is concentrated by froth floatation process.

(ii) The concentrated ore is roasted in air to convert it into oxide.

2ZnS+ 3O_{2   ———–>} 2ZnO+2SO₂

Sulphur dioxide so formed is used in the manufacture of sulphuric acid.

(iii) The oxide obtained is mixed with crushed coke and heated to 1673 K in a fire clay retort where it is reduced to zinc metal.

ZnO + C  ———> Zn + CO

The CO evolved burns at the mouth of retort. When the flame changes colour from blue to greenish white, condenser is fitted to the retort. Being volatile at this temperature, the metal distills over and is condensed.

(iv) The metal is further purified by fractional distillation or by electrolysis.

Note:

In case of calamine, instead of roasting the ore is calcined and subsequently reduced with coke. In order to prepare zinc dust the molten metal is atomised with a blast of air. Granulated zinc is prepared by pouring molten zinc into cold water.