Issues of technology and conditions of smelting of low-silicon cast iron in blast furnaces

The issues of technology are considered and the conditions for smelting cast iron with a low silicon content (0.2‒0.3%) in blast furnaces are discussed. The relation between the regime parameters of the blast furnace and the silicon content in the cast iron is analyzed using actual data. One of the conditions for obtaining the cast iron with a low silicon content is a reduced heating of smelting products, i.e. the implementation of the technology of cold blast furnace smelting. It is shown that low-sulfur low-silicon cast iron is possible with a decrease in the fluctuation of the chemical composition of iron ore raw materials and an improvement in the quality of coke, in the production of raw materials in which silica is bound into hard-to-recover silicates, while controlling the chemical composition of all raw materials in the stream. To reduce the concentration of silicon in cast iron, it is necessary to increase the basicity of the slag (CaO/SiO2) and increase the concentration of MgO in the slag with a ratio in the slag Al2O3/MgO = 0.85‒1.10. In this case, calcium silicates CaO•SiO2 (or even bicalcium silicate 2CaO•SiO2) are formed in the slag system), from which silicon reduction is more “hard” than from free silica. The transition of silicon from the gas phase to cast iron depends on the reaction surface area of the metal-gas contact. Therefore, by reducing the thickness of the cohesion zone and the residence time of the melts in this zone, it is possible to reduce the saturation of cast iron with silicon. It is shown that in the production of low-silicon cast iron it is necessary to work with the opened center ‒ to carry out the axial gas flow. The contact time of cast iron and gas in the cohesion zone is also determined by the properties of coke ‒ primarily by its hot strength. With low hot strength, coke significantly impairs the properties of the coke oven nozzle and increases the metal filtration time in the lower part of the furnace ‒ in the coke oven nozzle zone, thereby contributing to the saturation of cast iron with silicon. The silicon reduction process is accompanied by an increase in the volume of the gas phase. Therefore, increasing the pressure in the blast furnace makes it difficult to recover the silicon. In the production of low-silicon cast iron, smelting must be carried out at high blowing pressure. As the blast pressure increases, it is necessary to increase the excess pressure of the top gas, establishing the necessary difference in gas pressure along the height of the charge layer. Increasing the specific productivity of the blast furnace is an effective factor in reducing the concentration of silicon in cast iron. This is associated with an increase in the flow rate of liquid melting products, a reduction in their residence time in the cohesion zone and, consequently, to a decrease in the silicon content in cast iron.