Chapter 3 · Question 11

Distinguish between roasting and calcination. Why are carbonate and sulphide ores converted to their oxides before reduction? Write a balanced chemical equation for the roasting of zinc sulphide and the calcination of zinc carbonate, followed by reduction to zinc metal.

Q11

Distinguish between roasting and calcination. Why are carbonate and sulphide ores converted to their oxides before reduction? Write a balanced chemical equation for the roasting of zinc sulphide and the calcination of zinc carbonate, followed by reduction to zinc metal.

Answer Revealed

Direct Answer:

Roasting is the process of heating a sulphide ore strongly in the presence of excess air to convert it into its oxide. Calcination is heating a carbonate ore strongly in limited air to convert it into its oxide. Ores are converted to oxides first because it is easier to obtain a metal from its oxide than directly from its sulphide or carbonate — oxides are more readily reduced by carbon. Roasting of ZnS:

2\text{ZnS} + 3\text{O}_2 \xrightarrow{\text{Heat}} 2\text{ZnO} + 2\text{SO}_2

. Calcination of ZnCO₃:

\text{ZnCO}_3 \xrightarrow{\text{Heat}} \text{ZnO} + \text{CO}_2

. Reduction of ZnO:

\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}

Simple Explanation

Roasting and calcination are both heating processes, but for different ores. Roasting is for sulphide ores (like ZnS) — you heat them with plenty of air, and they turn into oxides while releasing SO₂ gas. Calcination is for carbonate ores (like ZnCO₃) — you heat them with limited air and they decompose into oxides, releasing CO₂. The reason we make oxides first: it is much easier to pull the metal out of an oxide using carbon (coke) than from a sulphide or carbonate. Think of the oxide as a 'halfway stop' from which the metal is easily extracted.

Exam-Ready Structure

The terms roasting and calcination (Section 3.4.4) are critical intermediate steps in the metallurgy of moderately reactive metals, as illustrated in the NCERT flow chart (Fig. 3.10). 1. Roasting: (a) Definition: The process of converting a sulphide ore into its oxide by heating it strongly in the presence of excess air. (b) Example — zinc sulphide (zinc blende):

2\text{ZnS(s)} + 3\text{O}_2\text{(g)} \xrightarrow{\text{Heat}} 2\text{ZnO(s)} + 2\text{SO}_2\text{(g)}

. (c) During roasting, sulphur dioxide gas is released as a by-product. 2. Calcination: (a) Definition: The process of converting a carbonate ore into its oxide by heating it strongly in limited supply of air. (b) Example — zinc carbonate (calamine):

\text{ZnCO}_3\text{(s)} \xrightarrow{\text{Heat}} \text{ZnO(s)} + \text{CO}_2\text{(g)}

. (c) Carbon dioxide gas is released as the carbonate decomposes. 3. Key differences: Roasting uses excess air (oxidising environment) on sulphide ores; calcination uses limited air (decomposition) on carbonate ores. Roasting produces SO₂; calcination produces CO₂. 4. Why convert to oxides? It is easier to obtain a metal from its oxide than from its sulphide or carbonate. Metal oxides can be reduced to the corresponding metal using common reducing agents such as carbon (in the form of coke). Reduction of ZnO with carbon:

\text{ZnO(s)} + \text{C(s)} \rightarrow \text{Zn(s)} + \text{CO(g)}

. This is a redox reaction — ZnO is reduced to Zn and C is oxidised to CO. 5. The conversion step is mandatory for metals in the middle of the activity series. For metals at the top (K to Al), electrolysis of molten compounds is required because these metals have a higher affinity for oxygen than carbon — carbon cannot reduce their oxides.

Key Points

Roasting: heating sulphide ore in excess air → oxide + SO₂ ( $2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2$ )
Calcination: heating carbonate ore in limited air → oxide + CO₂ ( $\text{ZnCO}_3 \rightarrow \text{ZnO} + \text{CO}_2$ )
Ores are converted to oxides because oxides are easier to reduce to the metal
Reduction of ZnO with carbon: $\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}$ (redox reaction)
Medium-reactivity metals use carbon reduction; top-reactivity metals use electrolytic reduction

Distinguish between roasting and calcination. Why are carbonate and sulphide ores converted to their oxides before reduction? Write a balanced chemical equation for the roasting of zinc sulphide and the calcination of zinc carbonate, followed by reduction to zinc metal.