Investigation of carbothermic microwave reduction followed by acid leaching for recovery of iron and aluminum values from Indian red mud

Abstract

The red mud is an abundantly generated by-product of the aluminum industry rich in iron values mainly in the form of hematite and goethite phase. The favorable dielectric properties of iron oxide enable efficient microwave heating of red mud, and ~750 °C is achieved in 10 min with 10.7% C at 800 W. The heating rate in the mixture depends on the charcoal dosage and phase composition. The theoretical thermal decomposition kinetics suggest that the hematite to magnetite conversion in the red mud is chemically controlled and requires activation energy of 20.95 kJ/mol. The carbon gasification findings suggest that the CO formation is initiated below the bulk temperature of 700 °C during microwave heating, possibly because of accelerated microwave-induced heating in carbon having a significant effect on the reduction reaction mechanism. Volumetric heating enables efficient reduction and iron enrichment in magnetic concentrate with iron grade 52% and ~98% recovery constituting the magnetite phase majorly in 10 min, consuming 478.8 kJ energy. The iron metallization is dependent on the charcoal dosage, and ~15% metallic iron is formed in 30 min with 1435.4 kJ energy expenditure. The non-magnetic fraction is leached in 0.5 M HCl to recover alumina values in the mullite phase with an Al purity of 48% suitable for refractory applications. The non-magnetic residue containing iron values can be mixed with magnetic fraction to increase the Fe-product yield to ~84%.