Phase Transformations During Softening of Iron Ore Sinter of Varying Basicity in the CaO-SiO₂-FeO System

The cohesion zone of a blast furnace is instrumental in determining the gas-dynamic regime and the efficiency of reducing gas utilization. The extent of this phenomenon is contingent upon the initial and final temperatures at which iron ore undergoes softening, which, in turn, are determined by the chemical and phase composition, as well as the degree of reduction of the charge. The present study investigated sinter with a basicity (CaO/SiO₂) ranging from 1.2 to 3.0 using a combination of methods. The experimental program involved the use of X-ray diffraction (XRD) with refinement using the Rietveld method, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and load-dependent softening tests. It was established that as the basicity increased, the content of the calcium-aluminum silicoferrite (SFCA) binder phase increased from 6.2 to 17.5 wt.%, whilst the amount of hematite decreased from 12.6 to 2.3 wt.%. The softening onset temperature increases from 1185 to 1260 °C, the softening end temperature from 1345 to 1415 °C, and the softening interval narrows from 160 to 155 °C. The evolution of the phase composition of sinter during controlled reduction (0-95%) has been investigated for the first time. It has been demonstrated that the maximum accumulation of wustite (FeO) is attained at a reduction degree of 40-60%, irrespective of the basicity of the substance. It is precisely in this range that the minimum softening start (1040-1065 °C) and end (1170-1210 °C) temperatures are observed, which is associated with the formation of low-melting eutectics. The sinter belongs to the CaO-SiO₂-FeO-Al₂O₃-MgO system, and the softening behavior is governed by the FeO-CaO-SiO₂ system where low-melting eutectics form. When the reduction rate exceeds 60%, the metallic phase becomes dominant, leading to an increase in softening temperatures and a narrowing of the cohesion zone. It is evident from the data obtained that the optimal basicity range of the sinter is 2.0-2.5. Furthermore, it is recommended that a reduction degree of at least 60% is implemented in order to improve gas dynamics and increase blast furnace productivity. The findings can be utilized to enhance the efficiency of charge materials and refine mathematical models of the blast furnace process.