Hydrogen Reduction of Bauxite Residue Pellets and Effect of Calcite Addition

The utilization of bauxite residue (BR) was experimentally studied through hydrogen reduction, focusing on iron oxide reduction and the formation of leachable alumina phases. Three different types of pellets were made: one from bauxite residue, and the other two via calcite addition with varying calcite amounts. All pellets were isothermally reduced by H2 gas in a thermogravimetry furnace at constant temperature under fixed hydrogen flow rate and reduction times. X-ray diffraction (XRD) and Scanning electron microscopy (SEM) coupled with Energy dispersive spectroscopy (EDS), were used for the phase and microstructural analysis. The hydrogen reduction rate is influenced by iron-bearing oxides formed during heating. Bauxite residue pellets exhibit a higher initial reduction rate than bauxite residue–calcite pellets as brownmillerite formation occurs in the latter one, which hinders iron oxide phase reducibility. The reduction kinetics are affected by the reduction temperature and calcite quantity added, while dominant phase formation during hydrogen reduction depends on their combined effect. There is no mayenite (Ca12Al14O33) phase formation in reduced BR pellets, while with the addition of calcite, small amount of mayenite phase starts to appear during reduction, which increases with more calcite addition.