Effect of MgO on Vacuum Carbothermal Reduction Mechanism of Ca3(PO4)2 in SiO2-C-Ca3(PO4)2-MgO-Based System

Phosphate rocks mainly contain Ca-3(PO4)(2), SiO2, and MgO. The vacuum carbothermal reduction of Ca-3(PO4)(2) with different MgO contents was investigated to determine the maximum volatilization rate of P-2 and the transformation of phases in a SiO2-C-Ca-3(PO4)(2)-MgO-based system. The application of reduced slag in the dephosphorization of a converter was discussed. The carbon excess coefficient (CEC), reduction temperature, and equilibrium phase were calculated using FactSage 8.1. The quinoline phosphomolybdate weight method, scanning electron microscopy-energy dispersive X-ray spectroscopy, and X-ray diffraction were used to characterize the volatilization rate of P-2, phases, and micromorphology of the reduced slag. The volatilization rate of P-2 first increased and then decreased, with the highest volatilization rate of 93.77% obtained at n(MgO/CaO) = 0.50. The reduced slag phase changed from CaSiO3 to 2CaO center dot SiO2 center dot MgO center dot SiO2, CaO center dot SiO2 center dot MgO center dot SiO2, and 3CaO center dot P2O5 center dot 2CaO center dot SiO2 solid solutions with increasing MgO content. The diffusion of graphite was hindered by an excessive MgO solid phase when n(MgO/CaO) > 0.50, resulting in the reaction of unreacted Ca-3(PO4)(2) with 2CaO center dot SiO2 center dot MgO center dot SiO2 to form 2CaO center dot SiO2 center dot 3CaO center dot P2O5 (C2S-C3P2) during the holding process, thus reducing the reduction rate of Ca-3(PO4)(2). Reduced slag with a small amount of phosphorus can be used in converter smelting to achieve low-temperature dephosphorization. [GRAPHICS] .