Variation of the redox conditions and the resultant phase assemblages during iron ore sintering

Abstract

Highlights• The oxygen potential in a sinter bed is measured by a zirconia oxygen probe to be ~ 0.01 atm.• This oxygen potential in the sinter is appreciably more oxidizing than previous estimates.• The contents of a quenched sinter pot show that at the flame front, only magnetite and slag are present.• SFCA phases only form to a limited extent at the top of the quenched bed.• Thermodynamic modeling at a pO 2 of 0.01 atm is in qualitative agreement with the phase analysis of the sinter pot. AbstractThe oxygen potential prevailing during iron ore sintering was measured with a zirconia sensor in a series of sinter pot experiments. This was done to get a better indication of the redox conditions during commercial sintering. It was found that the pO 2 is appreciably more oxidizing than previously assumed, with a minimum value of ~0.01 atm. It is concluded that this value represents the oxygen potential of the gas phase and it is therefore a mixture of combustion gas and downdraft air. The contents of a quenched sinter pot where the reactions were interrupted with the flame front situated midway through the sinter bed were investigated. X-ray diffraction analysis, using an internal standard to quantify the amorphous slag phase, revealed that at the flame front only magnetite and slag was present. SFCA phases only formed at the top of the bed after the flame front had passed. Thermodynamic modeling of the phases at equilibrium agree qualitatively with the phase analysis and explained the extensive presence of magnetite and melt, as well as the formation of calcium ferrite phases during cooling below 1100°C.

Measurement of Process Conditions Present in Pilot Scale Iron Ore Sintering

“…However, these stages do little to define the process conditions that are present during the sinter process and their potential impact on sinter microstructure, which ultimately determine the properties of the sinter product. It has been shown in the present and previous studies [4,5] that temperature and oxygen partial pressure are key process variables. Analysis of the results obtained in the present study show that both equilibrium and non-equilibrium phases can be formed depending on these conditions and the thermal history.…”

“…Multiple reducing peaks were observed, both before and after the peak bed temperature, while only a single peak was previously observed either before [4] or at the peak bed temperature [5]. The p O2 peaks were more reducing than that previously observed [4,5], with a variation in the peaks observed between sinter pot tests.…”

“…The p O2 measured during sintering, and the corresponding temperature, differed significantly from that previously measured [4,5]. Multiple reducing peaks were observed, both before and after the peak bed temperature, while only a single peak was previously observed either before [4] or at the peak bed temperature [5].…”

“…; on cooling, the measured oxygen partial pressure was close to 0.21 atm. Research by van den Berg et al [4] indicated that the most reducing conditions, a pO2 of ~10 −2 atm, occurred prior to the peak bed temperature. To perform the pO2 and temperature measurements, an oxygen probe and a thermocouple were inserted from the top of the sinter bed, into the centre of the sinter pot.…”

“…After achievement of the peak bed temperature, the sinter is cooled by the air drawn down through the sinter bed. Despite the widespread industrial use of this process, relatively few measurements of the oxygen partial pressure (pO2) within the sinter bed during sintering have been carried out [4,5]. The pO2 measurements by Sugiwara et al [5] indicated that the lowest oxygen pressure, pO2 of ~10 −8 atm, occurred at the peak bed temperature of approximately 1673 K. The position and method of introducing the probe into the sinter bed was not described in the paper.…”