Thermodynamic Analysis of Mineral Phase Composition of Steel Slag System

Abstract

In order to transform the crystalline form of Ca2SiO4 (C2S) in phosphorus-containing slag from monoclinic β-polycrystalline to square γ-polycrystalline, a volume expansion of about 11% was generated, which caused the phosphorus-containing slag to undergo self-powdering. The CaO-SiO2-Al2O3-MgO-MnO-P2O5-FeO slag system was analyzed using FactSage7.1 thermodynamic software, and the effects of different P2O5, FeO and basicity on the mineral phase composition of slag system were analyzed in the range of 1300~1700 °C. It was shown that P2O5, FeO and basicity all have an effect on the composition of the mineral phases. When the mass fraction of P2O5 in the slag was lower than 0.25%, it had less effect on the transformation of C2S crystalline structure. When the P2O5 content was higher than 0.25%, it was favorable to the generation of low-melting-point substances, but the P2O5 in the slag reacted with C2S in the silicate phase, making P5+ solidly soluble in C2S, inhibiting the transformation of β-C2S to γ-C2S and hindering the self-powdering of the slag. The FeO content in the slag system ranged from 20% to 28%, and as the FeO content increased, the C2S content in the silicate phase decreased from 33.3% to 25.9%, while the temperature at which the silicate was completely dissolved into the liquid phase decreased from 1600 °C to 1500 °C and the complete melting temperature of the slag decreased. The low FeO content facilitates the self-powdering of slag. In the high-phosphorus slag, at temperatures below 1450 °C, with the increase of basicity, the proportion of C2S in the silicate phase first increased and then decreased. With basicity at 1.8; the highest content of silicate phase, accounting for 33.7%; and the temperature exceeding 1450 °C, the silicate phase dissolved into the liquid phase, which is conducive to the removal of phosphorus from the slag, achieving the self-powdering of high-phosphorus slag.