Chapter 8 – General Principles and Processes of Isolation of Elements
1. Introduction to Metallurgy
Metallurgy is the science and process of extracting metals from their ores
and refining them for use.
Most metals are not found in free state due to their high reactivity.
2. Important Terms
| Term | Meaning |
|---|---|
| Mineral | Naturally occurring compound of metal |
| Ore | Mineral from which metal can be economically extracted |
| Gangue | Unwanted earthy impurities |
3. Steps in Extraction of Metals
The extraction of metals generally involves the following steps:
- Concentration of ore
- Conversion of ore into oxide
- Reduction of oxide to metal
- Refining of metal
4. Concentration of Ores
The process of removing gangue from ore is called concentration.
(a) Hydraulic Washing
Based on difference in densities of ore and gangue.
(b) Magnetic Separation
Used when ore or gangue is magnetic.
(c) Froth Flotation Process
Used for sulphide ores.
Ore + Water + Pine oil + Air → Froth (Ore) + Gangue
Depressants like NaCN are used to prevent froth formation of unwanted sulphides.
5. Conversion of Concentrated Ore into Oxide
(a) Calcination
Heating of ore in absence or limited supply of air.
$\text{ZnCO}_3 \xrightarrow{\Delta} \text{ZnO} + \text{CO}_2$
(b) Roasting
Heating of ore in excess of air.
$2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2$
6. Reduction of Metal Oxide
Metal oxides are reduced to metals using suitable reducing agents.
$\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$
7. Thermodynamic Principles of Metallurgy
Feasibility of reduction depends on Gibbs free energy change.
$\Delta G = \Delta H - T\Delta S$
Negative $\Delta G$ indicates spontaneous reaction.
8. Ellingham Diagram
Ellingham diagram is a plot of $\Delta G^\circ$ vs temperature
for formation of metal oxides.
Lower line → more stable oxide.
9. Refining of Metals
Refining is the process of removing impurities from crude metal.
Common Refining Methods
| Method | Used For |
|---|---|
| Electrolytic refining | Cu, Ag, Au |
| Distillation | Zn, Hg |
| Liquation | Sn, Pb |
| Zone refining | Si, Ge |
10. Electrolytic Refining
Impure metal is anode, pure metal is cathode.
$\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$
Impurities settle as anode mud.
11. Zone Refining
Based on difference in solubility of impurities in molten and solid metal.
Used for semiconductor materials.
12. Summary for Exam
To score full marks:
- Remember conditions of calcination and roasting
- Understand Ellingham diagram trends
- Learn refining methods with examples
- Practice chemical equations