Q.Differentiate between the following:

Roasting and Calcination: Turning Ores into Oxides

Most metals are not found pure in nature. They are locked inside compounds — usually sulphides, carbonates, or oxides. To get the metal out, you first need to break that compound down into a metal oxide, because oxides are much easier to reduce to the pure metal. Roasting and calcination are the two main ways to do this conversion.

The Intuition

Think of an ore as a metal atom that is "holding hands" with something else — sulphur or a carbonate group. You want to make it let go and hold hands with oxygen instead, because oxygen is easier to kick out later.

• If the ore is a sulphide (metal + sulphur), you need to burn the sulphur away. That requires a lot of oxygen — you roast it in excess air.
• If the ore is a carbonate (metal + carbonate group), the carbonate group breaks apart when heated, releasing carbon dioxide. You don't need extra oxygen for this; you just need heat. Too much air would be wasteful or could cause unwanted side reactions, so you use limited air.

That is the core difference: roasting needs oxygen to react with sulphur; calcination just needs heat to decompose the carbonate.

The Precise Definitions

Roasting — The Sulphide Route

A typical sulphide ore is zinc blende, $\text{ZnS}$. When roasted in excess air:

The sulphur atom leaves as $\text{SO}_2$ gas, and the zinc is left behind as zinc oxide. The reaction is exothermic — it gives off heat, which helps sustain the process.

Roasting is not just about oxide formation. It also removes volatile impurities like arsenic and antimony, which burn off as their oxides. Sometimes, if the temperature is not controlled, a small amount of sulphate can form instead of the oxide — that is undesirable and is avoided by careful heating.

Calcination — The Carbonate Route

Take limestone, $\text{CaCO}_3$, which is a carbonate ore of calcium. When calcined:

No oxygen from the air is needed. The carbonate ion ($\text{CO}_3^{2-}$) is unstable at high temperature and simply falls apart. The calcium oxide (quicklime) is left behind, and carbon dioxide escapes.

Calcination is also used for hydrated ores — like removing water from bauxite ($\text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O}$) before electrolysis. In that case, the water is driven off as steam. …