Extractive Metallurgy of Iron
Metal And Non-metals of Class 10
EXTRACTVE METALLURGY OF IRON
Extraction of iron from its ores involves two processes.
- Smelting to obtain the curde metal.
- Refining of the crude metal
The iron so obtained by reduction method contains carbon and other impurities and it is known as pig iron or cast iron. The pig iron is then converted to wrought iron or steel according to the requirement.
Smelting is carried out in a furnace, it is almost cylindrical furnace, lined with fire bricks. Iron ore (haematite - Fe2O3) is calcined to remove volatile impurities like sulphur and arsenic. The calcined ore (8 parts) along with limestone (1 part) and coke (4 parts) are lifted to the top of the furnace and released when needed. Preheated air at a temperature of about 873 K is passed into the furnace through a number of nozzles. Near the bottom, preheated air comes in contact with the falling coke and combustion of coke take place and region is known as combustion zone .
C + O2 → CO2
(i) CO2 so produced goes upwards and comes into contact with layers of coke and gets reduced to CO.
(ii) CO acts as a reducing agent and reduces iron oxide to iron.
FeO3 + 3CO → 2Fe + 3CO2
Fe 2O3 + CO → 2FeO + CO2
Fe3O4 + CO → 3FeO + CO2
FeO + C → Fe + CO.
Molten iron thus formed is collected at the bottom from where it is tapped off.
Limestone which acts as a flux decomposes at about 1073 K. The quicklime reacts with sandy impurities (SiO2) to form a slag of calcium silicate.
CaCO3 → CaO + CO2
CaO + SiO2 → CaSiO3(slag)
The molten slag is less dense than iron and floats on the top. When both molten iron and slag are drawn off, coke and limestone are added. The gases leaving the furnace contain CO and are used to heat the incoming air - blast. The whole blast furnace process is continuous.
The molten iron tapped off from the furnace is solidified into blocks known as 'pigs'. This form of iron is called 'pig iron' or cast iron. It contains about 3-5% carbon and varying amounts of Mn, Si, S and P.
Ores of Copper
- Copper pyrites or chalcopyrites CuFeS2 or Cu2S. Fe2S3
- Copper glance or Chalcocite Cu2S
- Basic copper carbonate or malachite CuCO3. Cu(OH)2
- Cuprite Cu2O
- Bornite or peacock ore* Cu5FeS4
- Azurite 2 CuCO3. Cu(OH)2
(* The name peacock ore is given because it has a mixture of irridescent colours like peacock's feather.)
Extractive Metallurgy Of Copper
The ore is crushed and concentrated by froth floatation process, this is then roasted in a limited supply of air in a reverberatory furnace to convert iron into iron(II) oxide. Arsenic and antimony present as impurites are removed as volatile oxides.
2CuFeS2 + O2 → Cu2S + 2FeS + SO2
Sand is added to remove the iron as iron silicate slag FeSiO3 which floats on the surface. Molten mass collected from the bottom of furnace contains largely cuprous sulphide and a little ferrous sulphide. This molten mass is known as matte. Air is blown through the liquid matte causing partial oxidation.
2FeS + 3O2 → 2FeO + 2SO2
FeO + SiO2 → FeSiO3 (slag)
Iron (II) silicate
Cu2S + O2 → Cu2O + SO2
After some time the air is turned off and self reduction of the oxide and sulphide occurs, giving impure blister copper which is 98% - 99% pure.
Cu2S + 2Cu2O → 6Cu + SO2
Impure copper is further refined electrolytically to obtain 99.95 - 99.99% pure copper by using a solution of Cu (II) sulphate as an electrolyte.
Ore of Lead
- Galena PbS
- Cerussite PbCO3
- Anglesite PbSO4
Extractive Metallurgy Of Lead
The principal ore of lead isGalena, PbS. The ore is first concentrated by froth floatation process. The concentrated ore is roasted in air to convert it into lead oxide PbO and lead sulphate. Some galena is also left unchanged. If the air supply is now reduced, the unreacted PbS reacts with PbO and PbSO4 to produce metal.
3PbS + 5O2 → 2PbO + PbSO4 + SO2
2PbO + PbS → 3Pb + SO2
PbSO4 + PbS → 2Pb + 2SO2
The obtained lead contains impurities such as Cu, Ag, Bi, Sb and Sn. Silver is removed by Parke's process where molten zinc is added to molten impure lead the former is immiscible with the latter. Silver is more soluble in molten zinc than in molten lead. Zinc−silver alloy solidifies earlier than molten lead and thus can be separated. After this, crude lead is refined electrolytically (Electrolyte; lead silicofluoride, PbSiF6 and hyhrofluosilicic acid, H2SiF6 with a little gelatine, anode; curde lead and cathode; pure lead)
Ores of Magnesium
(i) Magnesite MgCO3
(ii) Dolomite MgCO3.CaCO3
(iii) Epsomite or Epsom salt MgSO4.7H2O
(iv) Caranallite MgCl2.KCl.6H2O
(v) Kieserite MgSO4.H2O
(vi) Kainite K2SO4.MgSO4.MgCl2.6H2O
Extractive Metallurgy of Magnesium
Magnesium from sea-water is obtained by the Dow's process. It consists of electrolysis of molten magnesium chloride using an iron cathode and a graphite anode. Following are the steps that involved during the process.
1. Mg+2 is precipitated as magnesium hydroxide by the addition of slaked lime, Ca(OH)2 to the sea water.
Mg2+ + Ca(OH)2 → Mg(OH)2 + Ca2+
2. Magnesium hydroxide on treatment with hydrochloric acid is converted to chloride which is crystallised as MgCl2.6H2O .
Mg(OH)2 + 2HCl ⎯→ MgCl2 + 2H2O
3. Now for electrolysis, magnesium chloride is fused as follows
On passing a current of dry HCl gas it get partially dehydrated and the chloride thus obtained is added to a molten mix. of sodium chloride and calcium chloride (temp. range 973 - 1023 K).Under this condition MgCl2 melt with the loss of water.
If magnesium chloride hydrate is heated strongly, it hydrolyses to yield magnesia (magnesium oxide ) which is a refractory.
MgCl2.6 H2O → MgO + 2HCl + 5H2O
4. Molten mixture of MgCl2 , NaCl and CaCl2 is electrolysed. Magnesium is formed at the cathode and chlorine is evolved at the anode. The chlorine is used to make HCl acid which in turn is required for making magnesium chloride.
Cathode : Mg2+ + 2e-→ Mg
Anode : 2Cl-→ Cl2 + 2e-
Ores of Aluminium
(i) Bauxite Al2O3.2H2O
(ii) Kaolin Al2O3.2SiO2.2H2O
(iii) Cryolite Na3AlF6
(iv) Felspar K2O. Al2O3. 6SiO2
Extractive Metallurgy of Aluminium
Aluminium is extracted from Bauxite and it involves two steps:
(i) Purification of bauxite
(ii) Electrolysis of pure alumina
(i) Purification of bauxite: Bauxite is treated with a hot conc. solution (45%) of sodium hydroxide. Aluminium dissolves to form sodium aluminate NaAlO2 , leaving behind iron oxide which is present as impurity.
Actually the chemistry of aluminates is complex. Aluminium hydroxide which is formed redissolves in excess of sodium hydroxide to form sodium aluminate, NaAl(OH)4 which is not stable and on heating gets converted to NaAlO2 which is called sodium meta-aluminate. Silica SiO2 also dissolves in sodium hydroxide to form soluble sodium silicate, Na2SiO3.
Al2O3.2H2O + 2H2O + 2NaOH → 2NaAlO2 + 3H2O
SiO2 + 2NaOH → Na2SiO3 + H2O
The impurities are filtered out and the solution containing sodium meta- aluminate and sodium silicate is seeded with freshly precipitated aluminium hydroxide. Aluminium hydroxide precipitates leaving behind sodium silicate in solution. This is filtered and heated at 1473 K to yield pure alumina.
NaAlO2 + 2H2O → NaOH + Al(OH)3↓
2Al(OH)3→ Al2O3 + 3H2O
(ii) Electrolysis of pure alumina : Alumina is dissolved in a fused mixture of cryolite
(Na3AlF6) with a little fluorspar ( CaF2), which lowers the temp. of the melt and electrolysed at about 1173 K. The oxygen evolved at the anode burns the carbon anode producing carbon dioxide and carbon monoxide, due to this anodes should be replaced periodically.
Cathode: Al3+ + 3e-→ Al
Anode : C + O2-→ CO + 2e-
C + 2O2-→ CO2 + 4e-