Question

Explain calcination and roasting with example.

Answer 1

Hint: At first think about the two terms calcination and roasting. Generally, metals are obtained from oxide ores after going through a reduction or electrolysis process. It is easier to obtain metal from oxides compared to its sulphides and carbonates.

Complete step by step answer:
Calcination is defined as the process of converting ore into an oxide by heating it strongly. The ore is heated below its melting point either in the absence in the air or in limited supply. This method is commonly used for converting carbonates and hydroxides to their respective oxides. Calcination is derived from the Latin word which means to burn lime. So calcination is mostly used in the decomposition of limestone to lime and carbon dioxide. The reaction is as follows
$CaC{O_3} \to CaO + C{O_2}$
Roasting is a process of metallurgy where ore is converted into its oxide by heating it above its melting point in the presence of excess air. It is a method used for converting sulphide ores to their respective oxides. During roasting, moisture and non-metallic impurities in the form of volatile gases are released. In roasting which involves sulphides are a major source of air pollution and the main problem with this process is that it releases a large amount of metallic as well as toxic and acidic compounds which causes harm to the environment.
For example, zinc sulphide is converted to zinc oxide by the method of roasting
$2ZnS + 3{O_2} \to 2ZnO + C{O_2}$
Note:
The calcination process is carried out in the absence of air or limited supply of air whereas the roasting process is carried out in the presence of air. Calcination involves thermal decomposition of carbonate ores and roasting does not involve dehydrating an ore.