High-temperature heat treatment of hydrogen-reduced bauxite residue-calcite pellets for iron particles growth and separationTraitement thermique à haute température de boulettes de résidus de bauxite et calcite réduites à l’hydrogène pour la croissance et la séparation des particules de fer

Abstract

ABSTRACT Bauxite residue characterised as a polymetallic source containing iron, alumina, titanium, calcium, silicon and rare earth elements offers an opportunity for reduced waste volume and the recovery of valuable metals. This study focuses on the agglomeration of bauxite residue with calcite, followed by hydrogen reduction at 1000°C. The resulting iron particles in the reduced pellets were initially very small distributed in a porous oxide matrix, posing challenges for physical separation methods. To address this issue, further heat treatments were conducted on the reduced pellets in an argon atmosphere, ranging from 1100°C to 1550°C. Phase analysis and microstructural examinations were performed using X-ray powder diffraction and scanning electron microscopy, respectively. Experimental findings indicate that the size of iron particles in the reduced pellets increases with higher heat treatment temperatures. Up to 1200°C, the oxide matrix remains a solid state, transitioning to a semi-molten and molten state above this heat treatment temperature and complete separation of iron from the oxide matrix above 1500°C. An alkali-leachable phase is present in the reduced pellets, initially in the form of calcium aluminate (mayenite), while above 1400°C, this phase dissociates to form monocalcium aluminate, and complete conversion at 1500°C. As observed in the microstructural analysis, there is an increase in the particle size of iron in both the solid and semi-molten states with an increasing in temperature from 1100°C up to 1300°C. This comprehensive study sheds light on the transformative effects of varying heat treatment temperatures on the composition and microstructure of bauxite residue-based materials.