Hydrogen reduction of low-grade banded iron ore

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 degrees 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 degrees 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 degrees C restricted the reduction to magnetite, and the optimal conditions; 600 degrees C, 60 min, 0.5 LPM H2, and 3.3 x 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.