An Invitation on Characterization of H2-Reduced Bauxite Residue and Recovering Iron through Wet Magnetic Separation Processes

Abstract

Recovering iron from the bauxite residue (BR) is one of the long-standing challenges in the mining industry. However, there is a substantial lack of information in the literature regarding sample properties and iron extraction by reducing hydrogen. The present study aims at reducing a Greek BR using hydrogen, its characterization, and separating iron by magnetic separation processes. To this end, the reduced sample was characterized using X-ray diffractometry analysis (XRD), X-ray fluorescence spectrometer analysis (XRF), thermomagnetic analysis (TMA), automated mineralogy (AM), and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The effect of particle size (−200 + 100 µm, −100 + 75 µm, and <75 µm) was investigated through a medium-intensity magnetic separation (MIMS, Davis Tube) at 1000–2500 Gauss and a Slon® magnetic separator (1000 G). The effects of solid content (3% and 10% w/w) in a wet low-intensity magnetic separation (WLIMS, 350 G) and a two-stage MIMS followed by WLIMS were investigated. It was revealed that through reduction at 500 °C and 2 h with 20 wt% NaOH under 5 vol.% H2 + 95 vol.% N2, iron oxides and ferric oxyhydroxide (Fe2O3 and FeOOH) were converted into magnetite (Fe3O4), whereas aluminum (oxy)hydroxides (Al(OOH), Al(OH)3) were reacted with Na+ towards sodium aluminates (NaAlO2). The AM observations indicated that only 3% of iron was in the phase of liberated magnetite, and the remaining was associated with Na, Al, and Ti phases with different intensities. The dissemination of iron throughout the matrix of the sample was recognized as the principal challenge in the physical separation processes. It was found that increasing magnetic intensity from 1000 G to 2500 G resulted in improved recovery for all studied particle size fractions in Davis Tube tests. The particle range of −106 + 74 µm was chosen as the most appropriate size to achieve the maximum Fe content of 41%. The results of WLIMS (350 G) showed the maximum Fe grade but revealed less recovery of 52% and 27% at 10% and 3% solid contents, respectively, compared to the Davis Tube trials.

Optimization of Fe, Al, and Na Recovery from H2-Reduced Bauxite Residue (“Red Mud”) Using Response Surface Methodology (RSM)

“…As demonstrated in Figure 5c, the highest recovery of Na increased with increas NaOH addition and reduction time. However, both NaOH addition and reduction ti were not as significant parameters as the temperature, according to the ANOVA analy The highest recovery of Na was predominantly observed when the experiments involv elevated temperatures, higher NaOH additions, and extended reduction time [12]. In Figure 5, the relationship between Na recovery and different input variables is depicted using three-dimensional response surface plots.…”

“…The as-received BR from the Mytilineos Aluminum Greece plant was dried at 110 °C for 12 h in a drying oven (Memmert, Schwabach, Germany), followed by sieving (<500 µm). The characterization is reported elsewhere [3,8,9,12]. For the pelletization, the BR (100 g) was mixed with NaOH solution (addition of 10-25 wt% based on dry weight, The as-received BR from the Mytilineos Aluminum Greece plant was dried at 110 • C for 12 h in a drying oven (Memmert, Schwabach, Germany), followed by sieving (<500 µm).…”

“…For the pelletization, the BR (100 g) was mixed with NaOH solution (addition of 10-25 wt% based on dry weight, The as-received BR from the Mytilineos Aluminum Greece plant was dried at 110 • C for 12 h in a drying oven (Memmert, Schwabach, Germany), followed by sieving (<500 µm). The characterization is reported elsewhere [3,8,9,12]. For the pelletization, the BR (100 g) was mixed with NaOH solution (addition of 10-25 wt% based on dry weight, Sigma Aldrich-99.9% purity, Zedelgem, Belgium), followed by mixing/pelletization using an Eirich mixer (Type: EL1).…”

“…However, both NaOH addition and reduction time were not as significant parameters as the temperature, according to the ANOVA analysis. The highest recovery of Na was predominantly observed when the experiments involved elevated temperatures, higher NaOH additions, and extended reduction time [12].…”

“…In the Bayer process, about 1.5-2.5 tons of bauxite residue (BR) is generated as waste per ton of alumina, depending on the type of bauxite ore and process efficiency [1,2]. The global inventory of BR is estimated to be around 5.5 billion tons by 2022 [3,4], most of it being disposed in landfills [1]. BR's high alkalinity and fine particle size poses a potential risk to land, ecosystems, and groundwater.…”