Iron and alumina recovery from bauxite residue through hydrogen reduction

Abstract

The study primarily centers on agglomerating bauxite residue with calcite pellets and reducing in the presence of hydrogen. This process aims to reduce iron oxide complexes while simultaneously forming leachable calcium aluminate phases under hydrogen flow at elevated temperatures.

Two types of pellets were produced: sintered oxide pellets produced at high temperatures, and self-hardened carbonate pellets. Several physical separation methods have been utilized, with wet magnetic separation emerging as the most efficient for separating iron from the oxide matrix.

Smelting was utilized to recover iron and separate calcium aluminate slag, which was then subjected to an alkaline leaching to extract alumina. Thermodynamic calculations and characterization techniques supported the research.

Reduction of sintered bauxite residue calcite pellets starts at lower temperatures, reaching full reduction with proper rate at 1000 °C due to the formation of brownmillerite.

In self-hardened pellets, the iron initially present as hematite, undergoes faster reduction compared to sintered pellets. This is due to the transformation into brownmillerite during the heating process in sintered pellets, which impacts reduction rates.

Reduction rate of different iron pellets depends on grain size, pellet porosity, and reduction temperature. Heat treatment of reduced pellets increases metallic iron particle size, while smelting at 1500 °C effectively separates iron from the oxide matrix that forms a molten slag. Alumina leaching achieves a maximum of around 75%, impeded by calcite layer formation during leaching and gehlenite formation during reduction and sintering.