Calcination and Roasting

In metallurgy, we need to extract the metals from the ores. The ores may be of carbonates or sulphides. These ores are converted into oxides of the metals, and then the metal oxides are converted into metals using the electrolysis or reduction process. But it is not easy to reduce the sulphide ores or carbonates ores compared to oxides. Then how is this process done? This is done by using the methods of calcination and roasting. 

Let’s look at these methods and their uses, the difference between calcination and roasting, and why they are used. 

Definition of calcination

Calcination is defined as converting the ores into oxides under an insufficient supply of air or a limited air supply. High heating is done to melt the ores. Hydroxides and carbonates are converted into oxides using this method.

It can also be stated as a thermal process in which thermal decomposition is used to convert the ores and other solid materials to oxides. During this process, all the impurities of volatile and moisture are removed. Dehydration of ore may take place.

An example of calcination is as follows:

CaCO3→CaO+CO2

The CaO derived in this reaction is a product, and it is called calcining.

Definition of roasting

Roasting is usually used in metallurgy to extract metal oxides mainly from sulphide ores. In this process, the ores are heated at a high temperature below the melting point in excess of air to get the oxides of the ores, which are later used to extract the metals. The impurities like moisture and non-metallic impurities are released in volatile gases. 

The roasting method consists of oxidation, reduction, sulfation, chlorination, and pyro hydrolysis. These reactions are called solid-gas thermal reactions.

Take a look at an example of roasting:

2ZnS+3O2 —> 2ZnO + 2SO2  

Why is calcination done?

Calcination is the process of obtaining the oxides from the ores using the process of heating in limited air. This process thus helps remove unwanted impurities in the form of moisture or any volatile substances. They exit from the heating mixture in volatile gases, and then the oxides remain behind. It is widely used in the oxidation of carbonate compounds.

Why is roasting done?

Roasting is the process in which the ores are heated at a high temperature below the melting point in excess of air to get the oxides of the ores, which are later used to extract the metals. The impurities like volatile substances and moisture get removed in this process. The roasting method is widely used to convert sulphide ores into oxides. 

Uses of calcination

• Calcination of limestone – decomposition of carbonate ores to remove carbon dioxide;
• Calcination of bauxite and gypsum – decomposition of hydrated minerals to get rid of water of crystallisation as water vapour;
• In raw petroleum coke – decomposition of volatile matter.
• In the conversion of anatase to rutile or denitrification of glass materials.
• Zeolites Production – To remove ammonium ions.

Uses of roasting

• Roasting of ZnS to get the oxide of ZnO.
• Roasting of HgS to get the oxide of free Hg metal.
• It is also used in the fish-making, cashew nut industry. 

Difference between calcination and roasting

Conclusion

Thus, we can conclude from the above description of calcination and roasting that both methods are used to extract metals. Calcination is done in limited air, while roasting is done in excess air. The roasting process does not lead to ore dehydration, whereas this is not in the case of calcination, as the whole moisture gets driven out. During calcination, carbon dioxide is released, whereas there is no release of carbon dioxide during roasting. Calcination is widely used for carbonate ores, whereas roasting is done for sulphide ores. These are the differences between calcination and roasting jee notes you must note down.