FeS2 + 24 KOH + 9 Cl2 → K2FeO4 + 18 KCl + 2 K2SO4 + 12 H2O
This is an oxidation-reduction (redox) reaction:
2 S-I - 14 e- → 2 SVI (oxidation)
18 Cl0 + 18 e- → 18 Cl-I (reduction)
FeII - 4 e- → FeVI (oxidation)
FeS2 is a reducing agent, Cl2 is an oxidizing agent, FeS2 is a reducing agent.