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Electrochemical Principles of Metallurgy:
1. For simple electrolysis when a salt in molten form is electrolysed using suitable electrodes, metal ion Mn+ is deposited at cathode. Sometimes a suitable electrolyte is also used to decrease the melting point and to increase the conductivity of the melt.
2. Electrolysis of molten NaCl gives Na metal and Cl2 gas. Electrolysis of aqueous NaCl using Hg cathode and graphite anode gives Na at cathode and Cl2 at anode.
Extraction of Aluminium (Electrometallurgy): Extraction of highly reactive metals such as Na, K, Ca, Mg, Al, etc, by carrying electrolysis of one of the suitable compound in fused or molten state.
Bauxite Al2O3.2H2O is converted to alumina (purified Al2O3) by leaching. A mixture of 3-5% Al2O3, 3-8% CaF2 (Flux), 80-85% cryolite, Na3AlF6 (main electrolyte) and 3-8% AlF3 is electrolysed using steel cathode lined with carbon and graphite anode. Molten Al (99.5%pure) is tapped from the bottom. The method is commonly known as Hall method. The reaction can be written as
Cathode: Al3+ (melt) + 3e- ------> Al (l)
Anode: C (s) + O2- (melt) -----> CO (g) + 2e-
C (s) + 2O2- (melt) ------> CO2 (g) + 4e-
The overall reaction is
2Al2O3 + 3C ------> 4Al + 3CO2
For the production of about 1 kg of Al, 0.5 kg of carbon of anode is used up.
Hydrometallurgy: When metal is collected from the material obtained by the application of leaching by using some reducing agent through displacement reaction, the process is called hydrometallurgy.
1. Extraction of Au or Ag from native ore: Leaching is carried out with 0.2 to 0.5% NaCN and zinc dust is used as reducing agent.
4Au(s) + 8CN-(aq) + 2H2O (ag) + O2 (g) -----> 4[Au(CN)2]-(ag) + 4OH-(ag)
Zn(s) + 2[Au(CN)2]-(ag) -----> 2Au(s) + [Zn(CN)4]2-(ag)
2. Extraction of Ag from argentite or silver glance (Ag2S): The same method as mentioned above is used. In the absence of O2, the reaction is reversible.
Ag2S + 4NaCN + 2O2 -----> Na2SO4 + 2Na[Ag(CN)2]
2Na[Ag(CN)2] + Zn -----> Na2[Zn(CN)4] + 2Ag
The process is commonly known as Mac Arthur Forrest cyanide method.
Thermodynamical Principle of Metallurgy:
Gibbs free energy for a reaction is given by the equation ΔG = ΔH – TΔS and
ΔG° = -RT lnK. Negative value of ΔG and positive value of K favour the formation of products.
The reaction Cr2O3(s) + 2Al(s) -----> Al2O3 + 2Cr(s); ΔG = -421 KJ is the thermodynamically feasible but does not take place at room temperature, because unless the temperature is so high to melt Cr(s) to liquid form, TΔS will not overcome ΔH.
If any product of a reaction having positive value of ΔG1 can be used up by another reactant to carry out a reaction having negative value of ΔG2, the two reactions can occur together (coupling of reactions) only if the numerical value of ΔG2 is greater than ΔG1.
2FeO ------> 2Fe + O2 ; ΔG1250 k = 320 KJ mole-1
2C(coke) + O2 -----> 2CO ; ΔG1250 k = -430 KJ mole-1
Net reaction, 2FeO + 2C -----> 2Fe + CO2 ; ΔGNet = -110 KJ
