Elucidating the Crystallization Behavior of CaO–SiO2–MgO–Al2O3 Slags Towards the Smooth Operation of Blast Furnace

This manuscript expounds the crystallization behavior of CaO–SiO₂–MgO–Al₂O₃ (CSMA) slags towards the smooth operation of the blast furnace, where a structure-oriented elucidation for viscous flow was highlighted for deeper understanding of the crystallization behavior. The CCT curves collected from confocal laser scanning microscopy showed that the initial crystallization temperatures decreased with lower CaO/SiO₂ mass ratios, and the primary crystal phases transformed from a merwinite phase (Ca₃MgSi₂O₈) with a stronger crystallization ability to a melilite solid solution (Ca₂Al₂SiO₇–Ca₂MgSi₂O₇) with a comparatively weaker crystallization ability, which correlates well with the variation of Mg/Al mole ratios in melilite and verifies the formation of Ca₂MgSi₂O₇ as one of the main reasons for the poor fluidity of the slag. Furthermore, a higher viscosity was observed for modified slags with CaO/SiO₂ mass ratios from 1.34 to 1.05, accompanied by the activation energy increased from 173.0 to 182.7 kJ/mol, which is consistent with the decreased NBO/Si value and the increased Q³/Q² ratio detected by Raman spectra. The crystal phase with melting point higher than the discharged temperature and the enhanced degree of polymerization of the slag structure will lead to the poor fluidity of the slag, which should be emphasized in the actual modification of the blast furnace slag.