Isothermal Hydrogen Reduction of a Lime-Added Bauxite Residue Agglomerate at Elevated Temperatures for Iron and Alumina Recovery

Abstract

The hydrogen reduction of bauxite residue lime pellets at elevated temperatures was carried out to recover iron and alumina from the bauxite residue in a new process route. Prior to the H2 reduction, oxide pellets were initially prepared via the mixing of an industrial bauxite residue with fine calcite powder followed by calcination and high-temperature sintering. The chemical, compositional, and microstructural properties of both oxide and reduced pellets were studied by advanced characterization techniques. It was found that iron in the oxide pellets is mainly in the form of brownmillerite, and calcium–iron–titanate phases, while upon reduction they are converted to wüstite and shulamitite intermediate phases and further to metallic iron. Moreover, it was found that the reduction at lower temperature of 1000 °C is faster than that at higher temperatures of 1100 °C and 1200 °C. The slower rate and extent of reduction at the higher temperatures is attributed to the porosity loss and reduction mechanism change to a diffusion-controlled process step. In addition, it was found that Al-containing phases in the raw materials are converted mainly to gehlenite in sintered pellets and further to the leachable mayenite phase. The alkaline leaching of selected reduced pellets by a sodium carbonate solution yielded up to 87% Al recovery into the solution, while the metallic iron was not affected.

Water and Alkali Leaching for Simultaneous Iron and Alumina Separation from Hydrogen-Reduced Bauxite Residue-Calcite Pellets

“…During the Na 2 CO 3 leaching process, Na + and CO 3 −2 from the bulk solution moved to the matrix interface, where the matrix is mostly composed of metallic iron and mayenite. Here, the assumption is that, in the reduced pellets along with metallic iron and mayenite, other phases such as perovskite and larnite are present; however, those phases are stable during alkali leaching [10]. The reaction of Na + with mayenite occurs on the surface of mayenite and NaAlO 2 is leached out from the pellets to the bulk solution, leaving behind CaCO 3 on the reduced pellets.…”

“…Iron and alumina recovery has been studied by Skibelid et al (2022), and they studied the hydrogen reduction in BR-calcite sintered pellets at various temperatures followed by alkali leaching for alumina recovery. They found that at a reduction temperature of 1000 • C, all iron oxides were converted to metallic iron and the alumina recovery went above 87% during the alkali leaching [10]. An amount of alumina is lost in the form of complex phases such as gehlenite, which forms during the sintering and reduction of the pellets.…”

“…The low enrichment of iron in the magnetic fraction is due to the encapsulated matrix of iron and alumina, which results in both the presence of iron and alumina in the magnetic fraction. In another applied method, alumina was recovered from the reduced pellets after alkali leaching, followed by the magnetic separation of iron from the leaching residue, which was mostly composed of iron and calcite [10]. In this study, we tried to recover iron and alumina from the hydrogen-reduced BR-calcite pellets by employing simultaneous leaching and magnetic separation.…”

High-Temperature Hydrogen Reduction of Bauxite Residue for Iron Recovery

“…The hydrogen reduciton of the lime fluxed BR pellets can be done in isothermal and non-isothermal regimes. Results from isothermal reduction regime is presented in reference [9] and this study presents results from non-isothermal reduction regime.…”

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

“…Following the leaching process, the residue became enriched with calcite and rare earth elements. This enriched calcite residue was subsequently recycled to produce bauxite residue calcite pellets, completing a sustainable materials loop within the process [12,13].…”