Influence of Al 2O3/SiO2 Ratio on the Viscosity and Structure of Aluminate Ironmaking Slag Systems in the HIsmelt Process

The HIsmelt process, as a potential technology for smelting high-alumina iron ores, encounters practical challenges such as high slag viscosity and free running temperature. This paper investigated the flowability and structure of CaO–SiO2–MgO–Al2O3–FeO slag with varying Al2O3/SiO2 ratios. The research revealed that the viscosity initially decreased and then increased with increasing Al2O3/SiO2 ratio from 0.53 to 1.45, troughing at ratios near 0.88, while the free running temperature exhibited the opposite trend. Elevating the Al2O3 level to 26–29 wt pct enhanced the slag’s flowability compared to the baseline slag with 17 wt pct Al2O3, thereby meeting the smelting requirements of high-alumina iron ores. Molecular dynamics simulations confirmed that Si4+ and Al3+ in the slag were predominantly tetrahedrally coordinated. Bond strength weakening triggered by an increased Al–O bond number dominated the viscosity decrease. Conversely, the increase in polymerization resulting from the higher tetrahedral number and bridging oxygen concentration accounted for the subsequent viscosity increase.