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Metal sulphide like HgS reacts with O₂ to form mercuric oxide and sulphur dioxide. Mercuric oxide is unstable changes to Hg(l) which can be refined by distillation.

2HgS₄ + 3O₂ → 2HgO + 2SO₂

HgS + 2HgO → 3Hg + SO₂

Sulphide ore like Zn, on roasting (in presence of O₂) gives metal oxide which on reduction with metal sulphide (ZnS) or suitable reducing agent gives metal which is refined by electrolyte refining.

2ZnS + 3O₂ → 2ZnO + 2SO₂

2ZnO + ZnS → 3Zn + SO₂

Alternative method of reduction :

ZnO + C → Zn + CO

Pure zinc is obtained by electrolytic refining