Extraction of Zinc: Roasting and Calcination, Class 10

Quick answer: Zinc is in the middle of the reactivity series, so its ores are first converted to zinc oxide and then reduced with carbon. A sulphide ore (zinc blende, $\mathrm{ZnS}$) is roasted in excess air: $\mathrm{2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2}$. A carbonate ore (calamine, $\mathrm{ZnCO_3}$) is calcined in limited air: $\mathrm{ZnCO_3 \rightarrow ZnO + CO_2}$. Zinc oxide is then reduced: $\mathrm{ZnO + C \rightarrow Zn + CO}$. Impure zinc can be refined by electrolysis.

Ores of zinc

Metals in the middle of the reactivity series (such as zinc, iron and lead) are usually found as sulphides or carbonates.

Why convert to the oxide first? It is easier to obtain a metal from its oxide than from its sulphide or carbonate. So the ore is first changed into the metal oxide.

Step 1: converting the ore to zinc oxide

Roasting of zinc blende

Roasting is heating a sulphide ore strongly in the presence of excess air.

$\mathrm{2ZnS(s) + 3O_2(g) \xrightarrow{heat} 2ZnO(s) + 2SO_2(g)}$

Sulphur dioxide gas is given off. It is a pollutant, so it must not be released into the air; in industry it can be used to make sulphuric acid.

Calcination of calamine

Calcination is heating a carbonate ore strongly in limited air.

$\mathrm{ZnCO_3(s) \xrightarrow{heat} ZnO(s) + CO_2(g)}$

Carbon dioxide gas is given off.

Roasting vs calcination

Step 2: reducing zinc oxide with carbon

Zinc oxide is heated with carbon (coke):

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

• Zinc oxide loses oxygen, so it is reduced.
• Carbon gains oxygen, so it is oxidised; carbon is the reducing agent.

At the high temperature used, zinc (boiling point 907 °C) is produced as a vapour, which is cooled to collect the metal.

Step 3: refining zinc

The zinc obtained contains impurities. It can be purified by electrolytic refining:

When current is passed, pure zinc is deposited on the cathode, and insoluble impurities settle below the anode as anode mud.

Summary table

Exam tips

• Say which ore you start from: blende is roasted, calamine is calcined.
• Balance the roasting equation carefully: 2, 3, 2, 2.
• When asked for the extraction of zinc from calamine, write both the calcination and the reduction equations.

Common mistakes

• Calcining zinc blende. A sulphide ore is roasted in excess air.
• Writing $\mathrm{ZnS + O_2 \rightarrow ZnO + SO_2}$ without balancing.
• Calling carbon the oxidising agent. Carbon removes oxygen, so it is the reducing agent.
• Forgetting the state of the gases given off in roasting and calcination.

Frequently Asked Questions

What is roasting?

Heating a sulphide ore strongly in excess air to convert it into the metal oxide, for example 2ZnS + 3O₂ → 2ZnO + 2SO₂.

What is calcination?

Heating a carbonate ore strongly in limited air to convert it into the metal oxide, for example ZnCO₃ → ZnO + CO₂.

What is the difference between roasting and calcination?

Roasting is for sulphide ores in excess air and gives off SO₂; calcination is for carbonate ores in limited air and gives off CO₂.

How is zinc oxide reduced to zinc?

By heating it with carbon: ZnO + C → Zn + CO.

Why are sulphide and carbonate ores converted to oxides?

It is easier to obtain a metal from its oxide than from its sulphide or carbonate.