Microwave Reduction of Red Mud for Recovery of Iron Values

Abstract

India’s steel vision 2030 necessitates the exploitation of low-grade and alternative iron sources as there will be a scarcity of high-grade iron ores. In this context, red mud—a waste product of the aluminum industry—is attempted for recovery of iron values. Red mud is an abundant byproduct rich in iron values generated by the Bayer process. In this study, microwave-assisted reduction is carried out to recover iron-enriched concentrate. It is found that red mud is susceptible to microwave exposure and increases its magnetic property considerably. The red mud is microwave reduced in the temperature range of 700–1000 °C for 10–50 min and reductant dosage of 8–12%. A regression model is developed from response-surface method statistical design for iron grade and iron recovery. It can be concluded that exposure time is the most influencing factor for iron grade and reduction temperature in iron recovery. An iron-rich concentrate with 48.5 wt% iron content and 95% iron recovery rate can be achieved at 1000 °C in only 10 min. The aluminum, sodium, and silicon present in the system interact with each other to form nepheline which gets chemically bonded with the reduced ferrous phase and gets trapped in the magnetic concentrate. At higher temperature, the formation of hercynite reduces the iron grade in the magnetic concentrate, and the hercynite formation indicates that nepheline (Na₂O·Al₂O₃·2SiO₂) interacts with Bayer’s iron phase. With prolonged exposure time (50 min), the formation of pseudobrookite (Fe₂O₃·TiO₂) is observed. The magnetic concentrate is also investigated using VSM analysis to identify the ferromagnetic and paramagnetic phases. Out of all iron phases, hematite, magnetite, hercynite, and Ilmenite, magnetite is the only ferromagnetic phase. Based on SEM-EDS and XRD analyses, titanium present in the red mud preferentially reacts with iron to form ilmenite, and the remaining iron interacts with aluminum to form spinel phase, whereas silicon, sodium, and aluminum form nepheline phase.