3 SiO3 + CaSO4 + 13 C → CaSi3 + S + 13 CO
This is an oxidation-reduction (redox) reaction:
SVI + 6 e- → S0 (reduction)
13 C0 - 26 e- → 13 CII (oxidation)
3 SiVI + 20 e- → 3 Si-2/3 (reduction)
CaSO4 is an oxidizing agent, C is a reducing agent, SiO3 is an oxidizing agent.