Research on influence of grinding blast furnace return ore on reduction swelling index of low silicon fluxed pellets and production practice

Pellet quality is one of the key technologies restricting the smelting of high proportion pellets in the blast furnace, especially the reduction swelling index of low silicon fluxed pellets. In this paper, the inhibition effect of grinding blast furnace return ore on the reduction swelling index of low silicon fluxed pellets was studied and application has been conducted in Shougang Jingtang Company. The experimental results show that the addition of 5–15% fine grinding blast furnace ore into pellet production significantly reduced the reduction swelling index of pellets from 23.5% to 14.3%. Microstructure and energy spectrum analysis shows that calcium ferrite, magnesium ferrite and liquid phase in the pellets with fine grinding blast furnace return ore increased obviously with a more uniform distributed liquid phase between hematite particles than in the basic group pellets and tended to be “homogenized”. It is conducive to inhibiting the generation of ferric whiskers in the reduction process of the hematite phase, and thus contributes to the reduction of the swelling index. Since its implementation at Shougang Jingtang in 2020, this technology has yielded positive results. The fluxed pellets exhibit good quality with an alkalinity of 1.1, a SiO2 content of 2.1%, and a reduction in the swelling index from 18.1% to 16.5%. Besides, the pellet ratio into blast furnace has reached 55%, the cumulative average utilization coefficient of three 5500 m3 super-large blast furnaces is 2.43t/(m3. d), and the monthly average for a single furnace reaches 2.62t/(m3. d). Meanwhile, consumption reduction of calcium-containing flux slaked lime in fluxed pellets by 0.9% is greatly beneficial to reducing the cost of pellets and expanding the resources of pellet powder.