Experimental and thermodynamic analysis of MgO saturation in the CaO–SiO2–Al2O3–MgO slag system melted in a laboratory resistive furnace

Abstract

Slags from the CaO–SiO2–Al2O3–MgO system (CSAM) are commonly employed for the secondary refining treatment of special steels. Those slags must be designed to work in the refining reactions, presenting properties that maximize their capability to capture impurities from the liquid steel. With this in context it becomes relevant to study the phases of those slags in the operating temperatures. In the present study, four slags from the CSAM system, with increments of MgO (5.98–23.43wt.%) and binary basicity (1.96–2.48), were melted in a high temperature resistive electric furnace. The objective was to analyze the phases present in the slags in secondary refining temperatures (1500°C (1773K) to 1600°C (1873K)), depending on their chemical composition. Fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscopy (SEM) with dispersive energy spectrometer (EDS) were used for the slag analyses. The experimental results were compared with thermodynamic calculations obtained by using the FactSage™ 7.1. As the MgO contents increased in the slag (>14.96wt.%), the precipitation of MgO solids were predicted by thermodynamic calculations, indicating the saturation of the slag in this compound. In the SEM/EDS analysis, this saturation was confirmed by the presence of dark and dendritic phases rich in MgO in the slag microstructure. However, for MgO contents up to 7.93wt.%, the microstructures obtained were mainly composed of C2S (dicalcium silicate) phases dispersed in the liquid matrix, identifying the unsaturated condition for the MgO compound.