Consider two chemical reactions (A) and (B) that take place during metallurgical process :
(A) ZnCO3

Q: Why do we even need to convert these ores into oxides first? Can't we just extract the metal directly?

Great question! It is thermodynamically much easier to reduce a metal oxide to its pure metal using reducing agents like carbon or carbon monoxide, compared to reducing a metal carbonate or sulphide directly. Oxides are the perfect stepping stones in metallurgy.

Q: Can we perform roasting on a carbonate ore?

There's no need to! Carbonate ores already contain oxygen within their chemical structure. Heating them simply decomposes them into the metal oxide and carbon dioxide. Adding more air won't help the process and is just a waste of resources.

Q: What happens to the sulphur dioxide gas produced during roasting? Isn't it harmful?

Yes, it is a severe pollutant that can cause acid rain. However, in modern metallurgical plants, this sulphur dioxide is captured and used as a raw material in the Contact Process to manufacture sulphuric acid, turning a toxic byproduct into a highly valuable industrial chemical.

Q: Do the ores melt during calcination and roasting?

No, they don't. Both calcination and roasting are carried out at temperatures strictly below the melting point of the metal ore. We want to keep the ore in a solid, porous state because a porous solid has a higher surface area, making subsequent reduction reactions much faster and more efficient.

Q: Why is the air supply 'limited' in calcination? Why not just say 'no air'?

While calcination doesn't require oxygen for the chemical decomposition itself, in practical industrial furnaces (like reverberatory furnaces), it's incredibly difficult and expensive to create a perfect vacuum. So, we say 'absence or limited supply' to reflect realistic industrial conditions.

Question

Consider two chemical reactions (A) and (B) that take place during metallurgical process :
(A) $	ext{ZnCO}_3(s) \xrightarrow{\Delta} 	ext{ZnO}(s) + 	ext{CO}_2(g)$
(B) $2	ext{ZnS}(s) + 3	ext{O}_2(g) \xrightarrow{\Delta} 2	ext{ZnO}(s) + 2	ext{SO}_2(g)$
The correct option of names given to them respectively is

Options:

(A) (A) is calcination and (B) is roasting

(B) Both (A) and (B) are producing same product so both are roasting

(C) Both (A) and (B) are producing same product so both are calcination

(D) (A) is roasting and (B) is calcination

Sigma Percentile · Accepted Answer

### Demystifying Calcination and Roasting: The Art of Purifying Ores

Imagine you are an ancient metallurgist, tasked with extracting pure, shining zinc from a pile of dull, rocky ores. You can't just melt the rock and expect pure metal to flow out. The metal is chemically bound to other elements, often as carbonates or sulphides.

Before we can extract the pure metal, we must first convert these stubborn ores into a more cooperative form: **metal oxides**. Why oxides? Because thermodynamically, it is significantly easier to reduce a metal oxide to its pure metallic form than it is to reduce a carbonate or a sulphide. To achieve this transformation, we rely on two fundamental pyrometallurgical processes: **Calcination** and **Roasting**.

### Analyzing Reaction A: The Calcination Process

Let's look at the first reaction provided in our problem:

$$ 	ext{ZnCO}_3(s) \xrightarrow{\Delta} 	ext{ZnO}(s) + 	ext{CO}_2(g) $$

Here, we are dealing with Zinc Carbonate ($	ext{ZnCO}_3$), commonly known as calamine ore. Notice the conditions of the reaction. We are applying heat ($\Delta$), but there is no oxygen gas ($	ext{O}_2$) on the reactant side.

This is the hallmark of **Calcination**. Calcination is defined as the process of heating an ore strictly below its melting point in the **absence or limited supply of air**.

When we heat a carbonate ore like this, it undergoes thermal decomposition. The chemical bonds break, releasing carbon dioxide gas and leaving behind the desired solid zinc oxide. Because the ore already contains oxygen within its carbonate group, we don't need to pump in external air to form the oxide.

### Analyzing Reaction B: The Roasting Process

Now, let's shift our focus to the second reaction:

$$ 2	ext{ZnS}(s) + 3	ext{O}_2(g) \xrightarrow{\Delta} 2	ext{ZnO}(s) + 2	ext{SO}_2(g) $$

This time, our starting material is Zinc Sulphide ($	ext{ZnS}$), known as zinc blende or sphalerite. Look closely at the reactants. We are not just heating the ore; we are actively reacting it with oxygen gas ($	ext{O}_2$).

This process is called **Roasting**. Roasting involves heating the ore strongly, again below its melting point, but this time in the **presence of a regular and excess supply of air**.

Sulphide ores do not contain oxygen. Therefore, to convert them into metal oxides, we must force them to react with atmospheric oxygen. During this intense heating, the sulphur in the ore combines with the oxygen to form sulphur dioxide gas ($	ext{SO}_2$), which escapes, leaving behind the solid zinc oxide.

### The Core Takeaway

The distinction between these two processes is a classic, high-yield concept in chemistry.

**Calcination** is for ores that already contain oxygen (like carbonates or hydrated oxides) and is done *without* air.

**Roasting** is for ores that lack oxygen (like sulphides) and requires a *continuous blast of air* to force the oxidation.

By simply observing the presence or absence of oxygen gas as a reactant in the given chemical equations, we can confidently conclude that Reaction (A) is calcination and Reaction (B) is roasting.

Animated Solution for Chemistry - Metallurgy: Consider two chemical reactions (A) and (B) that take place during metallurgical process : (A) $ZnCO_{3} (s) Δ ZnO (s) + CO_{2} (g)$ (B) $2 ZnS (s) + 3 O_{2} (g) Δ 2 ZnO (s) + 2 SO_{2} (g)$ The correct option of names given to them respectively is

Select Answer:

Visualized Solution

$Reaction (A): Heating of Carbonate Ore$

$Identifying Process A$

$Reaction (B): Heating of Sulphide Ore$

$Identifying Process B$

$Final Conclusion$

The Sigma Insight: Principles of Metallurgy and Extraction

Demystifying Calcination and Roasting

Analyzing Reaction A

Analyzing Reaction B

The Core Takeaway

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