Influence of Basicity and Calcium-Containing Substances on the Consolidation Mechanism of Fluxed Iron Ore Pellets

Abstract

The application of fluxed pellets in iron making industry has attracted considerable attention because of the better metallurgical properties than acid pellets and environmental friendliness compared to sinters. However, fluxed pellets with different binary basicity (CaO/SiO2) exhibited significant differences in phase composition, microstructure and consolidation mechanism. These differences mainly stemmed from the influence of calcium-containing substances in fluxed pellets. Herein, the theoretical investigation discovered the calcium-containing substances from fluxed pellets, including calcium iron silicate, calcium silicate and complex calcium ferrite (SFCA), which determined the properties of fluxed pellets. Microstructure analysis revealed that the calcium-containing substances filled between hematite particles were used as a binding phase to assist in pellets’ consolidation. Furthermore, the calcium-containing binding phase formed in the low-basicity (0.4–1.0) pellets was mainly composed of the calcium iron silicate glassy phase, while the binding phase of the high-basicity (1.0–1.2) pellets was dominated by SFCA belonging to SiO2-Fe2O3-CaO-Al2O3 multivariate system. In comparison, SFCA exhibited better crystallinity and reducibility than calcium iron silicate. Within the roasting temperature range of 1200–1250 °C, the increase of basicity contributed to the fluxed pellets obtaining better strength. To sum up, fluxed pellets with SFCA as the main calcium-containing binding phase can be obtained by increasing the basicity above 1.0–1.2, which was imperative for further improving the physical and metallurgical properties of fluxed pellets.