Hydrogen reduction of low-grade banded iron ore

Abstract

The current study investigates hydrogen reduction of low-grade iron ore (∼37 % Fe) containing dispersed bands of hematite and jasper/quartzite phases. Gaseous hydrogen reduction is carried out at isothermal conditions in a custom-made tubular furnace, followed by magnetic separation. The reduction temperature (300–600 °C), time (30–90 min), particle size (0.5–3.3 mm), and hydrogen flow rate (0.5–1.5 LPM) were optimized by statistical design of experiments to maximize the saturation magnetization, degree of reduction and metallization. Complete reduction of hematite was achieved in 2 h at 600 °C; however, the separation of quartz from reduced ferrite was inefficient evidenced by the lower saturation magnetization. The association of impurities with hematite does not affect the reduction rate; however, the trapped quartz particles in the reduced phases deteriorate the magnetic separation efficiency. Temperature and time synergize the reduction rate, improving reduction and metallization degree; meanwhile, flow rate and particle size have minimal effect. Reduction below 450 °C restricted the reduction to magnetite, and the optimal conditions; 600 °C, 60 min, 0.5 LPM H2, and 3.3 × 2 mm particle size yielded ferrite concentrate possessing saturation magnetization of 134.6 emu/g (68 % FeM) with 97.9 % DOR, 88.8 % DOM and 47.3 % yield. The reduction of hematite results in generation of micro cracks and porous morphology of the ferrite. Iron oxide reduction follows the Fe2O3 → Fe3O4 → Fe, evidenced by the absence of wustite (FeO) or fayalite (FeO.SiO2) phases in the product. The ferrite particles form a fused and porous layer trapping the silica particles.