In metallurgy (the process of extracting metals from rocks), ores must often be converted into metal oxides before the pure metal can be extracted. The two primary thermal (heating) processes used for this are Calcination and Roasting.
The Sulfur dioxide (SO₂) gas released during the Roasting of massive amounts of sulphide ores is not wasted. Factories capture this gas and use it to manufacture Sulfuric Acid (H₂SO₄) through the Contact Process.
Calcination is the process of heating an ore to a high temperature (below its melting point) in the absence of air (oxygen) or in a severely limited supply of air.
Chemical Example: Zinc carbonate (Calamine ore) is heated without oxygen to give zinc oxide and carbon dioxide. ZnCO₃ (s) + Heat → ZnO (s) + CO₂ (g)
Roasting is the process of heating an ore to a high temperature (below its melting point) in the regular presence of excess air (oxygen).
Chemical Example: Zinc sulphide (Zinc blende ore) is roasted with oxygen to give zinc oxide and sulfur dioxide. 2ZnS (s) + 3O₂ (g) + Heat → 2ZnO (s) + 2SO₂ (g)
| Feature | Calcination | Roasting |
|---|---|---|
| Air Supply | Absence or limited supply of air. | Presence of excess air. |
| Type of Ore | Used for Carbonate ores (CO₃). | Used for Sulphide ores (S). |
| Gas Released | Releases Carbon Dioxide (CO₂) or water vapor. | Releases Sulfur Dioxide (SO₂). |
You might ask, why convert them to oxides at all? Why not just extract the metal straight from the sulphide or carbonate rock?
The answer is simple: It is chemically much easier (and cheaper) to extract a pure metal from its Oxide form using a reducing agent like carbon, than it is to extract it from a carbonate or sulphide form.
No. Both roasting and calcination are done at temperatures strictly *below* the melting point of the ore. The goal is to dry it out and change its chemical structure, not melt it into a liquid puddle.
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