Study on the mechanism of vacuum carbothermal reduction of Ca3(PO4)2 enhanced by SiO2

Vacuum carbothermal reduction mechanism of Ca3(PO4)2 with addition SiO2 was studied by combining FactSage 8.1 thermodynamic theoretical calculations with experiments. The theoretical calculations showed that adding SiO2 effectively reduced the initial reaction temperature. The experimental results showed that the addition of SiO2 significantly improved weight loss and volatilization rates of phosphorus. Ca2SiO4 and CaSiO3 were generated during the reaction. When the SiO2/CaO ratio was 1.25 at 1300 degrees C, the reduction effect of Ca3(PO4)2 and the volatilization rate of phosphorus was optimal, with a volatilization rate of 91.98 %. Excessive SiO2 led to insufficient contact between the reactants and reducing agent C, thereby hindering the reaction. Simultaneously, a large amount of liquid phase was produced, which deteriorated the kinetic conditions of gas diffusion and affected the reaction. In the reduction process, Ca3(PO4)2 reacted with reducing agent C to generate CaO, then reacted with external SiO2 to generate low melting point CaSiO3, which wrapped the unreacted core. With an increase in the low melting point CaSiO3, the mass transfer in the reaction changed from diffusion mass transfer to flow mass transfer, which promoted the inward diffusion of reducing agent C to contact the unreacted core until the reduction reaction was completed.