Developing a Low-Temperature, Carbon-Lean Hybrid Valorisation Process for Bauxite Residue (Red Mud) Towards Metallic Fe and Al Recovery

摘要

The present study deals with the recovery of metallic Fe and an Al-ion-rich liquid from bauxite residue (BR), with an Naion-rich liquid phase that could potentially be recycled or recovered as NaOH. First, BR was mixed with sodium hydroxide in mass ratios of 80/20 and 74/26, respectively. Then, each mixture was roasted under pure -H2 for 2 h at 500 degrees C, 550 degrees C and 600 degrees C. The thermal products were analyzed by powder X-ray diffraction, which revealed the formation of metallic iron, sodium aluminum silicon oxide and perovskite, among other phases, along with an amorphous phase. Subsequent water leaching of the milled products resulted in an Al- and Na-ion-rich liquid and a metal-containing Fe-rich insoluble product. Chemical analysis of the liquid phase of the sample with the mass ratio of 74/26 after roasting at 600 degrees C showed that Al recovery was as high as 77%, while the average Fe content of the solid fraction reached approximately 38.5 wt%. A similar Fe content was also observed at lower temperatures, but the Al recovery was lower. The composition of the remaining solid phase consisted mainly of Ca, Si, Ti and some undissolved Na and Al, which accounted for less than 6 wt%. The findings of this study suggest that hydrogen reduction of BR is not only as efficient as carbothermic reduction, perhaps even more so, but also that it has the added advantage of producing significantly less -CO2 emissions, especially when green hydrogen is used in the process.