Effect of basicity on the reduction swelling behavior and mechanism of limestone fluxed iron ore pellets

Abstract

Limestone fluxed iron ore pellets are reduced in CO-rich atmosphere generally accompanied by swelling behavior. If the reduction swelling index (RSI) exceeds the normal range (≥20%), it will seriously hinder the stable and anterograde production of iron making. To better guide the industrial production, the effect of binary basicity (CaO/SiO2) on the reduction swelling behavior and mechanism of limestone fluxed iron ore pellets was investigated. Meanwhile, X-ray diffraction technique was used to analyze the phase transformation in pellets with different basicity during the reduction process, and the microstructures of different substances were examined by SEM-EDS. The above researches confirmed that the crystal volume expansion caused by crystal structure transformation during hematite (α-Fe2O3) reduced to maghemite (γ-Fe2O3) was the fundamental reason for the swelling of limestone fluxed pellets. Low basicity (0.4–0.8) amplified the volume expansion produced in this process, while high basicity (1.0–1.2) reversed it. This inconsistency stemmed from the difference in the composition of binding phase. Compared with calcium iron silicate in low basicity pellets, complex calcium ferrite (SFCA) as the binding phase of high basicity pellets had excellent reducibility, which promoted uniform reduction rate of high basicity pellets. The increased of SFCA content i reduced the quantities of hematite and effectively inhibited swelling of pellets. From this investigation, increasing the basicity above 1.0 to promote the formation of SFCA was an effective way for decreasing the reduction swelling index of limestone fluxed pellets.