6 Fe3O4 + K2Cr2O7 + 31 H2SO4 → Cr2(SO4)3 + 9 Fe2(SO4)3 + K2SO4 + 31 H2O
This is an acid-base reaction (neutralization): Fe3O4 is a base, H2SO4 is an acid.
This is an oxidation-reduction (redox) reaction:
18 Fe8/3 - 6 e- → 18 FeIII (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
Fe3O4 is a reducing agent, K2Cr2O7 is an oxidizing agent.