Effect of limestone and dolomite flux on the quality of pellets using high LOI iron ore

Abstract

Owing to the depletion of high-grade iron ore quality, many steel plants over the world are now using pellets after beneficiation of low grade ores as blast furnace feed. For the effective utilization of low-grade iron ore resources (59–62% Fe) with high loss of ignition (LOI), mineralogical characteristics play the vital role to improve the process efficiency. The present work illustrates the effect of limestone and dolomite as flux material in pelletization of an Indian goethetic-hematite iron ore with 59.75% Fe, 4.52% SiO2, 3.84% Al2O3, and 4.85% LOI. As per mineralogical analysis, this ore contains 30.11% goethite and 9.71% kaolinite, which contribute major LOI of the sample. The self-binding properties of the ore ensured the complete elimination of an external binder due to the presence of kaolinite. LOI of the ore indeed affected the quality of indurated pellets by cracking, which might be the reason for the degradation of pellet quality. Concurrently, the same LOI enhanced the pellet quality when suitable fluxes were added. Limestone, dolomite, and their combinations were used as fluxing agent. The effects of CaO and MgO for the improvement of inherent pellet properties were evaluated through the physical, mineralogical, thermal, chemical, and metallurgical properties of the fluxed pellet using different analytical techniques, i.e., Thermogravimetric Analysis (TGA), X-ray diffraction, Stereomicroscopy, Optical Microscopy, Field Emission Scanning Electron Microscopy, and Wavelength Dispersive Microscopy with Electron Microprobe. TGA results indicated the thermal decomposition of goethite, kaolinite, and fluxes at different temperatures. With the addition of dolomite and limestone in the pellet feed material, CCS (Cold Compressive Strength/kg.pellet−1), RI (Reducibility Index/%), RDI (Reduction Degradation Index/%), SI (Swelling Index/%) and porosity could be improved. Due to the loss of mass in flux materials, by increasing the MgO content, it is possible to reduce swelling index and increase the pellet porosity. Simultaneously, the strength of the pellet increases because of slag formation in the form of calcium/magnesium aluminum silicate. The physical and metallurgical properties, i.e., CCS, porosity, SI, RDI, and RI of the fluxed pellets were optimized. At particular ratio of limestone and dolomite, synergistic effect was observed with respect to pellet characteristics.