Why Metal Sulphides and Carbonates are Converted to Oxides:
- Reason: Metal oxides are generally more stable and easier to reduce than metal sulphides or carbonates. Oxides are converted to metals more readily through reduction processes.
Chemical Reactions:
- Roasting of Sulphide Ores:
- Purpose: To convert metal sulphides into metal oxides and sulfur dioxide.
- Reaction: 2ZnS+3O2→2ZnO+2SO2
- Calcination of Carbonate Ores:
- Purpose: To convert metal carbonates into metal oxides and carbon dioxide.
- Reaction: ZnCO3→ZnO+CO2​
Calcination vs Roasting
Calcination and roasting are two important thermal processes used in metallurgy to prepare ores for further extraction of metals. They involve heating ores, but they differ in their methods and objectives.
| Aspect | Calcination | Roasting |
| Definition | Heating of ores in the absence of air or with limited air. | Heating of ores in the presence of excess air. |
| Objective | To drive off volatile components (like COâ‚‚ or water) and convert metal carbonates into metal oxides. | To oxidize impurities and convert metal sulphides or other compounds into metal oxides. |
| Ores Treated | Typically metal carbonates (e.g., limestone, zinc carbonate). | Typically metal sulphides (e.g., copper pyrite, zinc blende). |
| Process | Heating the ore to high temperatures to decompose carbonates and leave behind metal oxides. | Heating the ore in excess oxygen to oxidize sulphides or other compounds, producing metal oxides and gases. |
| Example Reactions | ZnCO3​→ZnO+CO2​ (for zinc carbonate) | 2ZnS+3O2​→2ZnO+2SO2​ (for zinc sulphide) |
| Temperature Range | Generally lower temperatures compared to roasting. | Higher temperatures due to the need for oxidation. |
| End Products | Metal oxides and gases like COâ‚‚. | Metal oxides and gases like SOâ‚‚. |