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Metals such as iron, zinc, lead, copper, etc., are in the middle of the reactivity series. These are moderately reactive metals and are usually present as sulphides or carbonates.
A metal is obtained from its ore by the processes of reduction or by electrolysis. In the reduction process, it is the oxide ore that is reduced.
It is easier to reduce an oxide ore as compared to its sulphides and carbonates. If the ore is not an oxide ore, it is first converted to the oxide by the process of calcination or by roasting.
Sulphide ores are converted into oxides by heating strongly in the presence of excess air, so that oxygen gets added to form the corresponding oxides. Sulphur impurities escape as gas. This process is known as roasting.
In this process the ore is heated to a high temperature in the absence of air, or where air does not take part in the reaction. Usually, carbonate ores or ores containing water are calcined to drive out carbonate and moisture impurities.
The differences between roasting and calcination are given below:
| Heating in the presence of air
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Heating in the absence of air
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| Sulphide ores are roasted
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Carbonate ores are calcined
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Sulphur dioxide is released
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Carbon dioxide is released
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| Not used for dehydrating an ore
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Can be used to drive out moisture from an ore
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