Calcination is the process in which:

Calcination is a process used in metallurgy to prepare ores for further processing. It involves heating the ore strongly in the absence of air or in a limited supply of air. The key characteristics of calcination are:

First, the ore is heated below its melting point. This ensures that the ore does not melt but remains in a solid state. Heating above the melting point would cause melting, which is not part of calcination.

Second, the purpose of calcination is to remove volatile impurities such as water ($$H_2O$$), carbon dioxide ($$CO_2$$), or other gases. For example, carbonate ores lose $$CO_2$$, and hydrated ores lose water vapor. The chemical reactions include:

For carbonate ores: $$ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 $$

For hydrated ores: $$ \text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O} \rightarrow \text{Al}_2\text{O}_3 + 2\text{H}_2\text{O} $$

Now, let's evaluate the options:

Option A states that the ore is heated above its melting point to expel $$H_2O$$, $$CO_2$$, or $$SO_2$$. This is incorrect because calcination occurs below the melting point, and expelling $$SO_2$$ is typical of roasting, not calcination.

Option B states that the ore is heated below its melting point to expel volatile impurities. While calcination does occur below the melting point and removes volatile impurities, this description is too vague. Calcination specifically targets impurities like $$H_2O$$ and $$CO_2$$, whereas roasting removes other volatiles like sulphur and arsenic compounds.

Option C states that the ore is heated above its melting point to remove S, As, and Sb as $$SO_2$$, $$As_2O_3$$, and $$Sb_2O_3$$. This is incorrect because calcination is done below the melting point, and removing sulphur, arsenic, or antimony is characteristic of roasting, which involves oxidation.

Option D states that the ore is heated below its melting point to expel $$H_2O$$ or $$CO_2$$. This matches the definition of calcination exactly: heating below the melting point to remove specific volatile impurities like water and carbon dioxide.

Therefore, Option D is the correct description of calcination.

Hence, the correct answer is Option D.