Reduction characteristics of magnetite-limonite mixed pellets in gaseous environments of CO and H2

Abstract

Recently, the steel industry has been pursuing the reduction of greenhouse gas emissions and carbon neutrality due to the global warming. The usage of pellets is increasing in the ironmaking process, and the depletion of high-grade iron ore is accelerating, and the development of pellet manufacturing technology using low-grade limonite, which is abundant in reserves, is expected to become important. Magnetite-limonite mixed pellets were manufactured, and the physical and chemical properties of the mixed pellets were observed according to the limonite mixing ratio. Most of the Fe-oxides(Fe3O4, FeO(OH)) contained in the magnetite and limonite iron ores were changed to the Fe2O3 phase during the pellet manufacturing process. As the limonite content increased, the volume shrinkage and mass reduction tended to increase due to the evaporation of crystal water inside the limonite. Up to 10-30% limonite content, the compressive strength of the mixed pellets increased (~5kN/pellet) and the porosity decreased. And when it exceeded 30%, the compressive strength decreased and the porosity increased. When reducing the mixed pellets in the CO environment, the reduction degree decreased as the limonite content increased up to 30% and it increased when it exceeded 30%. The higher the limonite content, the larger the cracks in the pellets, which is thought to have a significant impact on the reduction degree. The reduction degree of the mixed pellets in the H2 environment was 1.6 to 1.8 times higher than that in the CO environment, and the reduction rate was also faster. It is because the size of H2 molecules are small, so they can penetrate into the pellet more quickly during the reduction and reduce Fe-oxide more quickly. When reducing the mixed pellets in the H2 environment, larger cracks appeared on the surface of the mixed pellets, which is presumed to be caused by H2O molecules generated during the reduction. According to the results of this study, the pellets mixed with limonite up to 30% can be used in the blast furnace, and if the limonite content exceeds 30%, it is considered difficult to use because the differentiation rate is expected to increase due to increase the porosity of mixed pellets and cracks that occurred during the reduction.