Effect of SiO₂ on the Softening-Melting and Dropping Behavior of Magnesia Flux Pellets

High-proportion pellet smelting is the direction of optimization of blast furnace burden structures in the future. In this paper, the effect of SiO2 content on the softening and dropping properties of magnesia flux pellets was studied. And the influence of SiO2 content on the mineralogical composition and microstructure of magnesia flux pellets was analyzed, which clarified the reason for the air permeability of the middle and lower columns of magnesia flux pellets becoming poor during smelting. The results show that with increasing SiO2 content, the T-10 (softening start temperature) and TS (melting start temperature) of the column gradually decrease, and the melting range increases; in particular, T-10 and Ts decrease dramatically when the SiO2 content exceeds 6%. The maximum pressure difference increases from 15.0 kPa to 20.7 kPa, and the characteristic value increases from 644.8 kPa center dot degrees C to 1 334.8 kPa center dot degrees C when the SiO2 content of magnesia flux pellets increases from 6% to 8%. The mineralogical composition difference between low-silica magnesia flux pellets (SiO2= 3.5%) and high-silica magnesia flux pellets (SiO2= 7.0%) at different temperatures is caused by a large amount of unreacted SiO2 in high-silica magnesia flux pellets. SiO2 and silicate produce a large amount of liquid phase at low temperature, which reduces TS. In addition, the dropping temperature of high- silica magnesia flux pellets is higher than that of low-silica magnesia flux pellets. It's due to the existence of particulate SiO2 in the primary slag, which causes the slag to become sticky and makes it difficult to separate the slag from iron.