Question
Distinguish between roasting and calcination. Give one example of each and explain why each process is carried out before the reduction of the ore.
Solution — Step by Step
Most metal ores are not pure metals — they are sulphides, oxides, carbonates, or other compounds. Before we can reduce the ore to get the metal, we often need to convert it into a form that is easier to reduce, typically an oxide.
Both roasting and calcination are pyrometallurgical processes (involving high heat) that convert ores into oxides. This is step 2 in the general metallurgy flow: Ore → Concentration → Roasting/Calcination → Reduction → Refining.
Roasting is the process of heating a sulphide or carbonate ore in the presence of excess air (oxygen) at a temperature below the melting point of the metal.
Purpose: Convert sulphide ores to oxides. Sulphides are difficult to reduce directly; oxides are much easier to reduce with carbon or CO.
Reaction example — roasting of zinc blende (ZnS):
The ZnO can then be reduced:
Other examples:
- Galena (PbS):
- Copper pyrites (Cu₂S):
Note: The SO₂ released is a major air pollutant — modern smelters capture it to make sulphuric acid.
Calcination is the process of heating a carbonate, hydroxide, or hydrated ore in a limited supply of air (or in the absence of air) at a temperature below the melting point of the metal oxide produced.
Purpose: Drive off volatile components — CO₂ from carbonates, water from hydrated ores — to leave behind the oxide.
Reaction example — calcination of calamine (ZnCO₃):
Reaction example — calcination of bauxite (Al₂O₃·2H₂O, gibbsite):
Reaction example — calcination of malachite (Cu₂(OH)₂CO₃):
| Feature | Roasting | Calcination |
|---|---|---|
| Ore type | Sulphide (or carbonate) | Carbonate or hydrated ore |
| Air supply | Excess air (oxidising conditions) | Limited/no air |
| What is removed | Sulphur (as SO₂) | CO₂ and/or H₂O |
| Product | Metal oxide | Metal oxide |
| Example | ZnS → ZnO + SO₂ | ZnCO₃ → ZnO + CO₂ |
| Temperature | High (but below melting point) | High (but below melting point) |
Why This Works
The goal of both processes is the same — produce a metal oxide that can be reduced. Metal oxides are reducible by carbon, carbon monoxide, or aluminium (aluminothermy). Metal sulphides cannot be reduced this way efficiently.
The choice between roasting and calcination is determined by the ore type. You cannot calcine a sulphide (there’s no CO₂ or H₂O to drive off). You should not roast a carbonate in excess air (there’s no sulphur to oxidise; it simply calcines).
Alternative Method
A quick memory rule: Sulphide ore → Roast (oxidise in air). Carbonate/hydroxide ore → Calcine (decompose by heat). Both give oxide. Both require high temperature. The difference is only in the starting ore type and the gas evolved.
Common Mistake
Students often say “roasting uses air, calcination doesn’t use air.” While broadly correct, be careful: calcination of carbonates and hydroxides doesn’t need air, but the ore is still heated at high temperature. The precise statement is: roasting is oxidation (requires O₂); calcination is thermal decomposition (requires only heat, not O₂). The distinction is chemical, not just about air supply.
In CBSE Class 12 board exams and JEE Main, this is a standard 2-mark question requiring one definition, one example reaction, and a difference. Include the chemical equation for both examples — reactions earn 1 mark each. The equation for roasting (ZnS or Cu₂S) and calcination (ZnCO₃ or CaCO₃) are the most commonly tested.