Analysis of Dephosphorization Thermodynamics Based on the Melt Structure of CaO–SiO2–FeO–MgO System

To reduce the phosphorus content in steel, the CaO–SiO2–FexO–MgO-based dephosphorization slag was designed to clarify the microscopic reaction behavior of phosphorus. In the present study, Raman spectra of final slag were measured and deconvoluted. The spectral analysis showed that P5+ ions removed from liquid iron existed in the form of Q0(P) and Q1(P) groups. According to the result, phosphate capacity and phosphorus distribution ratio (LP*) were redefined. The results showed that lgLP* was proportional to molar ratio of Q0(P)/Q1(P), indicating that Q0(P) exerted a significant influence on LP* compared with Q1(P). As the increases of CaO/SiO2 from 0.15 to 1.25 and FeO from 7.2 to 21.9 mass% in the final slag, lgLP* gradually increased, whereas lgLP* showed a downward trend when FeO increased to 29.2 mass%. This was because the increase of O2− ions generated by the dissociations of CaO, MgO and FeO continuously destroyed the network structure and formed more Q0(P) units, which were compensated by the increasing Ca2+, Mg2+, and Fe2+ cations to form stable groups. Meanwhile, the Q0(Si) units formed by slag depolymerization further played a role in fixing Q0(P). However, due to the stronger polarization of Fe2+ than Ca2+ and Mg2+, Q0(P) and Q1(P) units were easily deformed and decomposed by Fe2+. The excessive Fe2+ diluted the proportion of Ca2+ and Mg2+, and made Q0(P) and Q1(P) lose stability. The P5+ ions in the Q0(P) and Q1(P) units were reduced to liquid iron, and the rephosphorization phenomenon occurred, resulting in a decrease of lgLP*.