Kinetics of reduction of ferric iron in Fe2O3-CaO-SiO2-Al2O3 slags under argon, CO-CO2, or H2-H2O

The rates of reduction of ferric iron in Fe₂O₃-CaO-SiO₂-Al₂O₃ slags containing 3 to 21 wt pct Fe₂O₃ under impinging argon, CO-CO₂, or H₂-H₂O have been studied at 1370 °C under conditions of enhanced mass transfer in the slag using a rotating alumina disc just in contact with the slag surface. For a 6 wt pct Fe slag at a stirring speed of 900 rpm the observed reduction rates by 50 pct H₂-H₂O were a factor of 2 to 3 times higher than those by 50 pct CO-CO₂ and more than one order of magnitude higher than those under pure argon. The observed rates were analyzed to determine the rate-controlling mechanisms for the present conditions. Analysis of the rate data suggests that the rates under 50 pct H₂-H₂O are predominantly controlled by the slag mass transfer. The derived values of the mass-transfer coefficient followed a square-root dependence on the stirring speed for a given slag and, at a given stirring speed, a linear function of the total iron content of the slags. The rates of oxygen evolution during reduction under pure argon were shown to be consistent with a rate-controlling mechanism involving a fast chemical reaction at the interface and relatively slow mass transfer in the gaseous and the slag phases. The rates of reduction by CO-CO₂ (pCO=0.02 to 0.82 atm) were found to be likely of a mixed control by the slag mass transfer and the interfacial reaction. A significant contribution of oxygen evolution to the overall rates was observed for more-oxidized slags and for experiments with relatively low values of pCO. Assuming a parallel reaction mechanism, the estimated net reduction rates due to CO were found to be of the first order in pCO, with the first-order rate constants being approximately a linear function of the ferric concentration. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS. The original symposium appeared in the October 2000 Vol. 31B issue.