Hydrogen fluidised-bed reduction of Australian hematite-goethite iron ores

Fluidised-bed reactors are emerging as promising technologies for hydrogen reduction of low-grade iron ores. This study reports the fluidised-bed reduction behaviour of 4 low-grade ores with hematite-goethite mineralogy (57–61 wt% Fe) sourced from the Pilbara region of Western Australia, following initial calcination (500 °C) in air. Calcined ore batches were fluidised and reduced in hydrogen at temperatures ranging from 500 °C to 900 °C. Stable fluidisation was obtained during the reduction of all investigated ores between 500 °C and 800 °C, with only minor differences in the observed reduction rates. However, attempts to reduce the calcined ores at 900 °C were less successful, resulting in particle sticking, gas channelling and bed defluidisation for 3 out of 4 of the tested ores. This indicates that there is a maximum practicable operating temperature for hydrogen reduction of hematite-goethite ores in fluidised-bed reactors. Reduction degrees above 93% were obtained within 12 and 18 min of reduction at 700 °C and 800 °C, respectively. At these temperatures, the final reduction degree was impacted by the formation of fayalite (Fe₂SiO₄), due to side-reactions with silicon-bearing impurities in the ores. At high temperatures, the reduction of wüstite is rate-limiting. This reaction proceeds throughout the entire particle by growth of micro-scale regions of iron metal which surround shrinking zones of wüstite. Off-gas measurements indicate that the rate of wüstite reduction at 800 °C was close to the theoretical maximum rate, indicating excellent potential for achieving high reaction intensities and hydrogen utilisation efficiencies in an industrial-scale process.