Extractive Metallurgy of Iron

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.

Copper

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.

Lead

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)

Magnesium

Ores of Magnesium

(i) Magnesite MgCO₃

(ii) Dolomite MgCO₃.CaCO₃

(iii) Epsomite or Epsom salt MgSO_4.7H₂O

(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-

Aluminium

Ores of Aluminium

(i) Bauxite Al2O_3.2H₂O

(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-