Show chemical equations for roasting zinc sulfide

Zinc sulfide is roasted in air to form zinc oxide and sulfur dioxide: 2 ZnS(s) + 3 O₂(g) → 2 ZnO(s) + 2 SO₂(g).

What roasting means

Roasting is heating an ore in excess air or oxygen to convert sulfides to oxides or sulfates and to release sulfur dioxide. It is a key step before reduction or leaching in zinc metallurgy.

Balanced equations for ZnS (sphalerite)

Main oxidizing roast: 2 ZnS(s) + 3 O₂(g) → 2 ZnO(s) + 2 SO₂(g) (exothermic)
Possible over-roast to sulfate (controlled conditions):
ZnS(s) + 2 O₂(g) → ZnSO₄(s)
Sulfation roast (used for special leaching routes):
ZnO(s) + SO₂(g) + 1/2 O₂(g) → ZnSO₄(s)

Process control

• Temperature: Fluidized-bed roasters run ~900–1000 °C for efficient conversion.
• Gas handling: Produced SO₂ is concentrated and sent to a sulfuric acid plant to prevent pollution and to generate value.
• Target: High ZnS → ZnO conversion with minimal unburned sulfide and controlled sulfate content depending on downstream flowsheet.

Why roasting is used in zinc extraction

Roasting turns insoluble ZnS into ZnO, which can be reduced to zinc metal or dissolved in acid for electrolytic zinc. It also separates sulfur as SO₂, which is easier to capture and convert than solid residues.