Upgrading Low-Grade Iron Ore through Gangue Removal by a Combined Alkali Roasting and Hydrothermal Treatment

In this study, a combination of alkali roasting and hydrothermal treatment is used as a method of gangue (Si, Al, and P) removal from iron ores as a means to upgrade low-grade iron ore (limonite) into a high-grade iron ore with low gangue content, low porosity, and high Fe and Fe₂O₃ content to enhance the sustainable development of iron and steel industries. The effects of the combined treatments (NaOH hydrothermal treatment and H₂O/NaOH hydrothermal treatment of the alkali roasted sample), the iron ore type, their physical properties, and their calcination/roasting temperatures on the removal extent of gangue are investigated. The extent of Si, Al, and P removal by subjecting iron ores to a 5 M NaOH hydrothermal treatment at 300 °C reached 10-91%, 39-70%, and 38-76%, respectively. When the iron ores are roasted with NaOH at 350 °C, α-FeOOH in limonite transfers to NaFeO₂. On the other hand, for alkali roasted iron ores that inherently contain Fe₂O₃, Fe₂O₃ and Na₂CO₃ are also observed after the roasting treatment. Higher Al and P removal extents are observed for H₂O leaching at room temperature in the prepared roasted samples (Roasting/H₂O_RT) as compared to NaOH hydrothermal treatment, whereas that of Si is low for all samples, except the iron ore with the highest Fe content. After the H₂O leaching process, the Fe form is found to be in the amorphous form for all samples, except for the iron ore sample of the highest Fe content. The reason for this is thought to be due to the large amount of unreacted Fe₂O₃ with NaOH during the roasting process. The specific surface area significantly increases after the Roasting/H₂O_RT treatment in all samples due to the dehydration of goethite (α-FeOOH → Fe₂O₃ + H₂O) during the roasting treatment and gangue removal during H₂O leaching. When the roasted samples are supplied for hydrothermal treatment by H₂O at 300 °C (Roasting/H₂O_SC), the removal rate of Si and P increases as compared with the Roasting/H₂O_RT treatment. The influence of temperatures of calcination and the roasting treatment on the extent of gangue removal in 5 M NaOH hydrothermal, Roasting/H₂O_RT, and Roasting/H₂O_SC treatments is small. When NaOH hydrothermal treatment is carried out on the samples that have undergone the Roasting/H₂O_RT treatment, a gangue removal extent of above 70-97% was achieved, except for the iron ore with the lowest P content, which had the largest loss of ignition and the lowest Fe content. In addition, it is revealed that low-grade iron ore with a high pore properties, α-FeOOH content, and gangue content can be upgraded to a high-grade iron ore with a low pore property (low specific surface area and pore volume), high Fe₂O₃ content, and low gangue content using the above method. Therefore, this method is promising as a method for upgrading low-grade iron ore.