The European Union (EU) is trying to reach its goal of climate neutrality (net-zero greenhouse gas emissions) by 2050. It is worth noting that, about 60 % of the crude steel is still produced through the blast furnace-basic oxygen furnace (BF–BOF) route in Europe. Although the BF-BOF route shows high production efficiency, the CO2 emissions per ton of crude steel are higher than other alternative steelmaking routes. One promising alternative steelmaking route is using a hydrogen-based fluidized bed to produce hydrogen direct reduced iron (HDRI), followed by an electric arc furnace (EAF) process. To improve the fluidization and reduction behaviors of magnetite iron ore fines in the fluidized bed, some pretreatments of the material are investigated.
This thesis tests some pre-treatments, i.e., pre-oxidation and MgO addition, of the magnetite ore fines to maintain the fluidization state. The influence of pre-oxidation temperature, oxidation degree and the addition amount of MgO on the fluidization and reduction behaviors of the magnetite iron ore fines are analyzed. The result shows that the raw magnetite iron ore fines are de-fluidized when reduction degree reaches only around 20%. At reducing temperature 600 °C, the de-fluidization can be avoided by a prior oxidation treatment. At higher reduction temperatures, 650-800 °C, the fluidization behavior can be further enhanced by an addition of 0.5 wt.-% MgO. The magnetite sample with higher oxidation temperature (1000 °C) shows better fluidization behavior. While lower oxidation temperature (800 °C) is more beneficial for the reduction rate, especially in the later reduction stage. The influence of pre-oxidation degree of the magnetite sample on its fluidization and reduction behaviors can be ignored. The primary and secondary influencing factors are oxidation temperature, pre-oxidation degree, MgO addition amount, and gas velocity. The optimum condition is that the magnetite iron ore is deeply oxidized at 800 °C, mixed with 1.5 wt.% of MgO powder, and reduced in the fluidized bed at a gas velocity of 0.45 m/s.
As for the oxidation of magnetite samples, the oxidation rate peaks appear at around 330 °C and 550 °C, which indicates the appearance of γ-Fe2O3 and α-Fe2O3. Theoretically, the surface energies of the hematite (104) and (110) crystal surfaces are larger than that of the (113) crystal surface. Hence, the hematite crystals may preferentially orient themselves along the (104) or (110) crystal surfaces. Therefore, the hematite phase shows a particular growth habit. The oxidation first occurs at the surface forming the grid-like hematite structures and then extends to the inside resulting in hematite needles. The specific surface area and pore volume decreases significantly during the oxidation due to the sintering effect. Based on in situ high-temperature X-ray diffraction (HT-XRD) analysis, the lattice constants of Fe3O4 and α-Fe2O3 increase with an increase in temperature because of the thermal expansion and can be successfully fitted with temperature by second-order polynomials. With Fe3O4 being oxidized into α-Fe2O3, the α-Fe2O3 crystallite grows.