Engineering Chemistry S Datta

318 ENGINEERING CHEMISTRY
(i) Pyrometallurgy: It means reactions at high temperature often above 1000°C.
(a) Electrolytic reduction at high temperatures of the fused salts especially halides
(for example Na, Al).
(b) Chemical reduction by coke at high temperatures (for example, iron, lead)
MO + C ⎯⎯→
∆
M + CO
(c) Chemical reduction by a more reactive i.e., electropositive metal (for example,
chromium and titanium extraction).
Cr 2
O 3
+ 2Al ⎯⎯→
∆
Al 2
O 3
+ 2Cr (Goldschmidt Thermit process)
(ii) Hydrometallurgy: It means, reactions at low temperature in solution in water.
(a) Electrowinning at low temperature in solution in water (for example, Zn extraction
for ZnSO 4
solution).
(b) Cementation using a displacement reaction (for example, using iron to displace
copper from a solution of CuSO 4
)
Goldschmidt Thermit Process
CuSO 4
+ Fe = Cu + FeSO 4
The metallic aluminium has a strong affinity for oxygen at 1000°C. The combination of
oxygen with aluminium at that temperature evolves much heat, which is utilised in Goldschmidt
Thermit process or Aluminothermic process for the reduction of metallic oxides e.g., MnO 2
or Cr 2
O 3
and for the separation of molten metals.
Goldschmidt Thermit process is nowadays mostly used for welding rails and
machine parts (Fig. 14.10). Thermite, a mixture of aluminium powder and iron oxide in the
ratio 3:1 by weight, is ignited in a crucible by a ribbon of magnesium, when a violent reaction
takes place producing molten iron at 2500°C.
2Al + Fe 2
O 3
= Al 2
O 3
+ 2Fe + 199 kcal
The liquid iron is dropped on the two pieces directly to the joint to be welded.
Fig. 14.10 Goldschmidt Thermit process.