Low-temperature biomass pyrolytic reduction and recovery of iron oxides from red mud

Abstract

In this study, the pyrolytic reduction of iron oxides contained in red mud using biomass-derived reductive gases was explored. Wheat straw and rice husk were evaluated together with activated carbon and graphite for their effectiveness in converting iron oxides to magnetite. Thermodynamic analyses using thermogravimetric-mass spectrometry (TG-MS) demonstrated that CO, H₂, CH₄, and biochar could effectively reduce iron oxides, with CO being particularly effective at lower temperatures. Optimal parameters for iron recovery were identified through an orthogonal experimental design, highlighting wheat straw as the superior biomass feedstock due to its high yield of reducing gases and fast reduction kinetics. Single-factor optimization emphasized the importance of temperature and red mud-to-biomass ratio. Optimization of pyrolytic reduction conditions revealed that wheat straw achieved the highest iron recovery rate of 75 % and a concentrate grade of 42 % at 550 °C, with a red mud-to-wheat straw ratio of 1:2, a heating rate of 12 °C/min, and a reduction time of 40 min. Mechanistic studies using X-ray diffraction, X-ray fluorescence, and scanning electron microscopy showed hematite in red mud transforming into magnetite, with some aluminum atoms substituting for iron to form iron-aluminum spinel, impacting the concentrate grade. This is a promising method for recycling iron from red mud, mitigating environmental impact, and conserving resources.