6 CuFeS2 + 34 KNO3 + 20 H2SO4 → 6 CuSO4 + 34 NO + 3 Fe2(SO4)3 + 17 K2SO4 + 20 H2O
This is an oxidation-reduction (redox) reaction:
12 S-II - 96 e- → 12 SVI (oxidation)
34 NV + 102 e- → 34 NII (reduction)
6 FeII - 6 e- → 6 FeIII (oxidation)
CuFeS2 is a reducing agent, KNO3 is an oxidizing agent, CuFeS2 is a reducing agent.