Straw-induced hematite-to-magnetite conversion in alkaline media: Experimental study and potential in Bayer process

High-iron bauxite is a vital iron-aluminum symbiotic mineral, primarily processed through the Bayer method for alumina extraction. This method generates a substantial quantity of highly alkaline red mud, which possesses significant application potential due to its elevated iron content. However, the iron-bearing minerals in red mud predominantly exist as weakly magnetic hematite, making it challenging to achieve an optimal recovery rate through straightforward physical separation techniques. Consequently, additional treatment is required for effective iron recovery. This paper presents an innovative approach that employs agricultural waste straw as a reductant, facilitating the directional transformation of hematite into magnetite via a one-step dissolution process integrated within the Bayer method framework. This addresses the issue of low recovery rates associated with the weak magnetism of iron-bearing phases in red mud. The transformation process from hematite to magnetite induced by alkali thermal dissolution of straw was examined using X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). During the simulation of the Bayer process for the alkali thermal reduction of hematite, it was noted that increases in temperature and straw quantity significantly enhanced the reduction and conversion of hematite. Specifically, when the caustic concentration of the sodium aluminate solution reached 240 g/L, with a molecular ratio of 3.1, a reaction temperature of 280 °C, a liquid-to-solid ratio of 4:1, and a mass ratio of straw to hematite of 1:4, the reduction rate of iron oxide achieved an impressive 94.21 % after 60 min of dissolution. Analysis using a vibrating sample magnetometer (VSM) and laser particle size analysis (LPSA) demonstrated that the magnetic saturation intensity and average particle size of the transformed magnetite increased by over 80 times and 10 times, respectively, reaching 86.84 emu/g and 20.73 µm, thus enhancing its susceptibility to separation by magnetic force in subsequent roughing processes. Scanning surface zeta potential analysis (SSZPA) revealed that the absolute values of the zeta potential of the desilication product (DSP), hematite, and transformed magnetite under strong alkaline conditions followed the order: magnetite > DSP > hematite, with magnetite exhibiting stronger repulsive forces. Further scanning tunneling microscopy (STM) analysis indicated that the solvation tendency of magnetite particles was magnetite < DSP < hematite, making them more prone to aggregation and growth during the alkaline thermal dissolution process. By further incorporating straw into the Bayer process dissolution of high-iron bauxite, the recovery rates of alumina and the reduction rates of hematite reached 90.81 % and 85.31 %, respectively. XRD and SEM-EDS analyses confirmed the conversion of hematite to magnetite induced by straw under alkaline thermal conditions, facilitating the dissociation of iron minerals from other minerals. This study proposes a novel process for converting weakly magnetic hematite to strongly magnetic magnetite during alumina production, offering new insights for reducing red mud stockpiling and promoting sustainable development in the alumina industry.