Metallurgical grade alumina is produced worldwide through the well-known Bayer process, which unavoidably generates bauxite residue (BR, also known as red mud) in almost equal amounts to alumina. This study aims the valorization of BR through a smelting-reduction process to obtain calcium aluminate slags that can be a proper feed for alumina recovery via the Pedersen process. It investigates the thermodynamics and characteristics of the slags and pig iron produced from mixtures of BR, a bauxite beneficiation byproduct, and lime. In this context, the evolution of the different phases in the slags is studied with advanced analytical techniques and thermodynamic calculations. According to the results, a CaO/Al₂O₃ mass ratio within 1.3 to 1.4 in the slags can yield more Al₂O₃-containing leachable phases, such as CaO·Al₂O₃ and 12CaO·7Al₂O₃. The cooling dictates the amount and the characteristics of these phases, and the slower cooling rate yields improved slag characteristics. The distribution of the elements between the slag and metal phases shows that iron is separated, and the majority of the P, Cr, Ni, and V are distributed in the produced pig iron, while S, Ti, and Si are mostly concentrated in the slags.

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.

In this study, the isothermal reduction of bauxite residue-calcite sintered pellets by hydrogen at elevated temperatures and different gas flow rates was investigated. A thermogravimetric technique was applied to study the kinetics of the direct reduction by H₂ at 500–1000 °C. It was observed that iron in sintered oxide pellets mainly exists in the form of brownmillerite, srebrodolskite and fayalite. The reduction of brownmillerite, the dominant Fe-containing phase, with hydrogen produces mayenite, calcite and metallic iron. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence (XRF), BET surface area, pycnometer and mercury intrusion porosimeter analyses were adopted on reduced pellets to interpret the experimental results. The order of the reduction process changes from first-order reaction kinetics to second-order with an increasing reduction temperature. The change in reaction order may be due to sintering at higher reduction temperatures and corresponding physical and microstructural changes in pellets. The activation energy of reduction was calculated as 55.1–96.6 kJ/mol based on the experimental conditions and using different kinetic model equations. From the experimental observations, it was found that 1000 °C with 60 min is the most suitable condition for bauxite residue-CaO sintered pellets’ reduction with hydrogen.

Hydrogen will presumably become an important substitute for carbon as a reductant in the metallurgical industry for processes such as steel production. However, the challenge to supply enough CO₂-free hydrogen for metallurgical processes has not been resolved yet. This paper reviews different production technologies for hydrogen and their advantages and drawbacks. Additionally, it will highlight the development of plasma technology to produce hydrogen and carbon black which has been taking place at SINTEF during the last 30 years.