In order to investigate the influence of SiC on the composition and structure of mixed slag (blast-furnace slag: steel slag = 1:9), the chemical composition, equilibrium-phase composition, and microscopic morphological characteristics and elemental distribution in the microscopic region of the SiC-reagent-tempered slag samples were analyzed by X-ray diffractometer (XRD), FactSage7.1 thermodynamic analysis software, scanning electron microscope, and energy spectrum analyzer. It was found that the main physical phases of the tempered slag samples were magnesia–silica–calcite (Ca3Mg(SiO4)2, C3MS2), calcium–aluminum yellow feldspar (Ca2Al2SiO7, C2AS), C2S, and iron alloy. Theoretical calculations suggest that the experimental temperature should be higher than 1500 °C to facilitate the combination of P5+ with Fe and Mn in the liquid phase to form an alloy, reduce the P5+ content in the tempered slag, and create conditions for the self-powdering of the conditioned slag. The doping of the SiC reagent can increase the liquid phase line temperature and reduce the binary basicity in the liquid phase; the liquid phase line temperatures were 1150 °C, 1200 °C, and 1300 °C and the basicities in the liquid phase were 4.68, 4.13, and 3.10 for the doping amounts of 3%, 4%, and 5% of the SiC reagent, respectively. The mixed slag doped with 4% SiC reagent achieves self-powdering and reduction of ferroalloys during the air-cooling and cooling processes, realizing the purpose of “resource utilization” of blast-furnace slag and steel slag.
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