Thermomagnetic properties of the goethite transformation during high-temperature treatment

Magnetizing roasting-magnetic separation is a promising technique to beneficiate low-grade iron ore. This technique is based on phase transformations of weakly magnetic minerals into strongly magnetic minerals that can be extracted by magnetic separation. In this paper, transformations of synthetic goethite and brown goethite ore (Kerch iron ore basin) under oxidative (in air) and reducing (mixed with 4% of starch) conditions were examined using XRD, SEM, TGA, and magnetic studies. When heated under oxidative conditions, synthetic goethite and natural brown goethite exhibited two significant changes in weight, which meant the removal of absorbed water and OH groups. The transformation of goethite into hematite occurred at about 270 °C for the synthetic sample and at 310 °C for the natural brown sample. The weight changes were accompanied by slight changes in the magnetic susceptibility of both samples. The reduction of the samples with starch led to the magnetite formation at 270 °C for synthetic goethite and at 500 °C for brown goethite. An increase in the saturation magnetization from <1 Am2/kg to 70 Am2/kg was observed as a result of synthetic goethite reduction. The saturation magnetization of reduced natural goethite rose to 25 Am2/kg. The reduction of goethite ore with charcoal occurred at the temperature that is 200 °C higher than that of starch. The optimum conditions for magnetizing roasting of brown goethite ore with starch may include: temperature - 750 °C, time - 5 min, and starch content - at least 4%. Therefore, biomass-based roasting technique can be used to beneficiate brown goethite ore by magnetizing roasting-magnetic separation.