3 K2MnO4 + 4 CO2 + 2 H2O → 4 KHCO3 + MnO2 + 2 KMnO4
This is an oxidation-reduction (redox) reaction:
2 MnVI - 2 e- → 2 MnVII (oxidation)
MnVI + 2 e- → MnIV (reduction)
K2MnO4 is both an oxidizing and a reducing agent (disproportionation (dismutation)).