Carbothermic Reduction Kinetics of Phosphorous Vaporization from Tri-calcium Phosphate (TCP) Under Microwave Rapid Heating With/Without the Presence of Fe₃O₄

The kinetics of vapor phase dephosphorization from tri-calcium phosphate (TCP) by carbothermic reduction was studied with and without the presence of Fe₃O₄. Microwave heating was utilized to obtain large variations in the heating rate (HR). In the reduction of TCP alone, the phosphorous removal fraction (RF; equal to ∆P₂O₅/P₂O₅ ⁰, where ∆P₂O₅ is the weight change and P₂O₅ ⁰ is the P₂O₅ weight before heating) decreased as the HR increased. In other words, a shorter residence time at a high temperature resulted in a smaller reduction fraction of TCP. An apparently third-order reaction was postulated to account for the kinetics using a fitting simulation based on the additive law of the reaction progress. On the other hand, the phosphorous removal (dephosphorization) rate (RR; equal to ∆P₂O₃/t_MW, where t_MW is the microwave heating time period) increased as the HR increased above 1200 °C. The reduction ratio of Fe₃O₄ above 1100 °C is higher than 97 pct regardless of the heating rate. The reduction of TCP in the presence of Fe₃O₄ showed that RF increased slightly with increasing HR despite a shorter residence time at a high temperature. The RR also increased with the HR even though RF decreased to half of the values observed in the cases without Fe₃O₄ for temperatures above 1200 °C. The practicality and optimal operation conditions of phosphorus vapor removal were discussed.