Animated Solution for Chemistry - Metallurgy: The pair that does not require calcination is

The journey of extracting a pure metal from its raw ore is a fascinating sequence of chemical transformations. One of the most crucial early steps in this journey is converting the concentrated ore into a metal oxide. Why an oxide? Because metal oxides are generally much easier to reduce to pure metals compared to other compounds like carbonates or sulphides.

To achieve this conversion, metallurgists rely on two primary thermal processes: Calcination and Roasting. In this problem, we are specifically investigating calcination to determine which pair of compounds does not require it.

Imagine you have a sponge soaked in water, and you want to dry it out without melting the sponge itself. Calcination is somewhat similar. It is the process of heating an ore to a high temperature—but strictly below its melting point—in the absence or very limited supply of air.

The primary objective of calcination is to drive off volatile impurities. This includes: 1. Expelling water of crystallization from hydrated ores. 2. Decomposing carbonate ores to release carbon dioxide gas. 3. Removing organic matter and moisture.

Mathematically, the thermal decomposition during calcination looks like this:

For a carbonate ore like Zinc Carbonate (Calamine):

For a hydrated oxide ore like Limonite:

In both cases, the solid residue left behind is a porous metal oxide, which is perfectly primed for the next step: reduction.

Now, let's put on our detective hats and examine the pairs given in the question. We are looking for a pair where neither compound needs to undergo calcination. This means we are looking for compounds that are already simple, anhydrous oxides.

Option (b): $\text{ZnO}$ and ${\text{Fe}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}$ While Zinc Oxide ($\text{ZnO}$) is already an oxide, the second compound is hydrated iron(III) oxide. The "$x{\text{H}}_2\text{O}$" indicates trapped water molecules. To remove this water and obtain pure ${\text{Fe}}_2{\text{O}}_3$, calcination is absolutely necessary.

Option (c): ${\text{ZnCO}}_3$ and $\text{CaO}$ Here, Calcium Oxide ($\text{CaO}$) is a simple oxide. However, Zinc Carbonate (${\text{ZnCO}}_3$) is a carbonate ore. As we saw in our earlier equation, heating it will release ${\text{CO}}_2$. Thus, this pair requires calcination.

Option (d): ${\text{Fe}}_2{\text{O}}_3$ and ${\text{CaCO}}_3\cdot {\text{MgCO}}_3$ Iron(III) oxide (${\text{Fe}}_2{\text{O}}_3$) is good to go. But the second compound is Dolomite, a mixed carbonate of calcium and magnesium. Heating Dolomite will cause it to decompose and release carbon dioxide: Clearly, calcination is required here.

Option (a)

Finally, let's look at Option (a): $\text{ZnO}$ and $\text{MgO}$.

Both Zinc Oxide ($\text{ZnO}$) and Magnesium Oxide ($\text{MgO}$) are already in their simplest, anhydrous oxide forms. They do not contain any water of crystallization to be vaporized, nor do they contain carbonate groups to be decomposed into carbon dioxide.

Heating these compounds in the absence of air will not result in any chemical decomposition or release of volatile gases. They are already in the exact chemical state that calcination aims to achieve!

Therefore, the pair that does not require calcination is $\text{ZnO}$ and $\text{MgO}$.

It is always helpful to contrast calcination with its sister process, Roasting. While calcination is for carbonates and hydrated ores in the absence of air, roasting is used primarily for sulphide ores (like $\text{ZnS}$ or $\text{PbS}$) and involves heating in a regular supply of air. The oxygen from the air reacts with the sulphur to form sulphur dioxide gas (${\text{SO}}_2$), leaving the metal oxide behind. Remembering this distinction is a high-yield concept for competitive exams!