Effect of SiO2 content in magnesia flux pellets on softening-melting and dripping behavior of comprehensive burden structure

Abstract

High-proportion pellet smelting is the current development direction of blast furnace burden structures in China. And that is an inevitable trend for the future steel industry to achieve pollution reduction and carbon reduction. This study focuses on the mixed burden of magnesia flux pellets with different SiO₂ contents, sinter, and lump ore. The influence of SiO₂ content on the softening-melting behavior of comprehensive burden and the high-temperature interaction between magnesia flux pellets and sinter were studied through droplet experiments and visual experiments. The results show that with increasing SiO₂ content, the T₁₀ of magnesia flux pellets gradually decreases, while the T₁₀ of the comprehensive burden shows no significant change. The T_S of both shows a gradually decreasing trend with increasing SiO₂ content. However, due to the good matching of the melting range between sinter and magnesia flux pellets in the comprehensive burden, the trend of T_S change in the comprehensive burden is relatively slow. The air permeability of the comprehensive burden has significantly improved compared with the single magnesia flux pellets; The interaction between magnesia flux pellets and sinter occurs through the liquid phase. The fayalite phase in the pellets reacts with the main high melting point substance Ca₂SiO₄ in the sinter to generate a new low melting point kirschsteinite. With the increase of SiO₂ content, the content of kirschsteinite in the comprehensive burden increases. That is also the reason for the decrease in T_S of the high silicon comprehensive burden; With the increase of SiO₂ content, the maximum pressure difference and characteristic values of magnesia flux pellets and comprehensive burden gradually increase. When the SiO₂ content exceeds 6%, the maximum pressure difference and characteristic value of a single pellet sharply increase, while the trend of the maximum pressure difference and characteristic value change of comprehensive burden is relatively gentle. Its characteristic values are below 980 kPa·°C. At this time, the air permeability of the comprehensive burden is significantly improved compared to the single magnesia flux pellets. In the case of SiO₂ content exceeding 6%, the addition of a sinter can effectively address the soft melting performance of high-silica magnesia flux pellets and enhance column air permeability. In addition, the high drop temperature of the high-silica comprehensive burden is due to the presence of a large amount of MgO in the magnesia wustite during the later stage of reduction, which increases the melting point. And the MgO content in the slag is relatively low. That causes a sharp increase in slag viscosity and makes it difficult to separate the slag from iron.