Isothermal reduction behavior and kinetics of Russian high-chromium vanadium-titanium magnetite pellets under gas atmospheres of CO–CO2–N2 and CO–N2 at 873 K–1173 K

Abstract

ABSTRACT In this paper, detailed investigations on the reduction characteristics and kinetics mechanism of Russian high-chromium vanadium–titanium magnetite pellets under gas atmospheres of CO–CO2–N2 and CO–N2 at 873 K–1173 K were made, and comparisons of the atmospheres were researched. It was found that the reduction temperature and gas atmosphere have great effects on pellet reduction at this temperature range, and the reverse reduction appeared uniquely for some time after 2 h reduction in the CO2-bearing gas atmosphere. The reduction was further researched with crushing strength tests, X-ray diffraction (XRD) patterns, and SEM microscopic examination, and results showed that the cause of the change in crushing strength, the phase transformation patterns, and microstructures were consistent across both atmospheres. For the CO–CO2–N2 gas atmosphere, the controlling step was predominantly the interfacial chemical reaction with the reaction activation value of 38.01 kJ/mol at the initial stage. This gradually became mixed control of the interfacial chemical reaction and gas internal diffusion through the solid product layer, with the reduction proceeding with 23.55 kJ/mol before 120 min. However, the controlling step became irregular after 120 min. For the CO–N2 gas atmosphere, the rate controlling step was predominantly the interfacial chemical reaction with 31.41–38.27 kJ/mol at the initial stage, after which the rate controlling mechanism was the mixed control with 17.97–19.33 kJ/mol until 180 min mark.