3 K2MnVIO4 + 4 CO2 + 2 H2O 4 KHCO3 + MnIVO2 + 2 KMnVIIO4

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).