Fe→FeS→H2S→K2S→H2S→S→SO2.
Fe + S = FeS
FeS + 2HCl = H₂S + FeCl₂
H₂S + 2KOH = K₂S + 2H₂O
K₂S + 2HNO₃ = 2KNO₃ + H₂S
2H₂S + O₂ = 2S + 2H₂O
S + O₂ = SO₂