Comparative Study of Hydrogen Reduction of Bauxite Residue-Calcium Sintered and Self-Hardened Pellets Followed by Magnetic Separation for Iron Recovery

Abstract

To minimize the carbon footprint in the industrial valourization of bauxite residue, hydrogen was used as a reducing agent. The current study experimentally investigated hydrogen reduction of bauxite residue-CaO sintered and self-hardened pellets at 1000 °C, along with magnetic separation of these reduced pellets for iron recovery. Calcium was introduced to bauxite residue to form leachable calcium aluminate phases with the existing alumina in bauxite residue. This involved the addition of either CaCO3 or a mixture of CaO and CaCO3 while maintaining the fixed Ca content during pelletization. The former underwent sintering at 1150 °C, while the latter was self-hardened through the cementing effect of CaO in exposure to moisture and air. Both types of pellets were reduced in a thermogravimetry furnace at an elevated temperature under similar conditions. The pellets were characterized by the X-ray diffraction (XRD) method and scanning electron microscope (SEM) coupled with energy dispersive spectroscopy (EDS), and their physical and mechanical properties were measured via standard techniques. During hydrogen reduction, a negligible amount of gehlenite (Ca2Al2SiO7) was formed in the self-hardened pellets, while this phase dominated in the sintered pellets. Alumina in the bauxite residue converted to mayenite phases during reduction in both the pellet types; however, reduced self-hardened pellets had a higher amount of alumina containing mayenite leachable phase. The two pellets showed similar reduction behaviour, while different chemical, physical, and mechanical properties were observed. The magnetic properties of milled reduced pellets were examined through a Davis Tube magnetic separator in a wet environment under a constant magnetic field of 800 G. Higher iron recovery was observed for the self-hardened reduced (41%) pellets than for the sintered pellets (27%).

Comparative Kinetics Study of Different Iron-Containing Pellets with Hydrogen at Isothermal Conditions

“…However, the half of reduction completion is different for pellets reduced at 700 • C. The halftime of reduction for sintered BR-CaO pellets was longer compared to other three pellets. As from the previous work, the iron present in the sintered pellets is in the form of brownmillerite [15]. The reduction of brownmillerite with hydrogen is thermodynamically feasible near 1000 • C as per thermodynamics calculation, and its reducibility is obviously lower at lower temperatures regarding the curves in Fig.…”

“…The sintered BR-CaO pellets were produced by drying BR-CaCO 3 green pellets overnight in an oven and then sintered at 1150 ± 10 • C for 120 min in a muffle furnace [14]. Self-hardened pellets were made by mixing the BR with CaO and CaCO 3 with appropriate composition [15]. The green pellets were aged in air (room temperature) for three days to increase the pellet strength.…”

“…The green pellets were aged in air (room temperature) for three days to increase the pellet strength. The mechanism of self-hardening of pellets was presented in our previous study [15]. The commercial iron ore pellets were from the ferrous industry.…”

“…The reduction experiments were carried out in a TG furnace, which comprised a vertical tube furnace, stainless steel hollow cylindrical crucible, a gas flow controller, an electronic balance, and a temperature controller. The schematic view of the furnace and the working principle were described in our previous paper [15]. These pellets were heated in the presence of argon with a flow of 1NL/min up to the targeted temperature 1000 ± 10 • C. The heating rate was maintained at 10 • C/min to homogenize temperature inside the pellet bed.…”

Kinetics Study on the Hydrogen Reduction of Bauxite Residue-Calcite Sintered Pellets at Elevated Temperature

“…In the sintered pellet, the majority of iron oxide was present in the brownmillerite phase (Ca 2 Fe 1.632 Al 0.364 O 5 ); however, during reduction, the brownmillerite phase is reduced to metallic iron, mayenite, CaO and water vapor [23]. XRD analysis (Figure 10) showed that the intensity of the iron, CaO and mayenite peaks was increased with the increasing reduction temperature; meanwhile, the brownmillerite peaks' intensities were decreasing and completely disappeared at the highest temperature.…”