Structure, Morphology and Magnetic Properties of Hematite and Maghemite Nanopowders Produced from Rolling Mill Scale

The work is devoted to developing a cost-efficient method for the processing of metallurgical wastes such as oiled mill scale formed upon the mechanical cleaning of a hot-rolled steel strip in scalebreakers. The most significant parameters of a chemical-metallurgical process for producing expensive and highly marketed products, such as α-Fe₂O₃ and γ-Fe₂O₃ nanopowders, are experimentally determined. The properties of initial materials and nanodispersed products have been studied by X-ray diffractometry, energy dispersive spectroscopy, scanning and transmission electron microscopy, and Mössbauer spectrometry. The temperature and field dependences for the powder magnetization have been plotted according to the measurements performed with the use of a vibration magnetometer. The mill scale under investigation consists of three main phases: wustite, magnetite and hematite at a weight ratio of 6 : 8 : 7, respectively. The initial scale was activated in a magnetic mill in a hydrogen flow and dissolved in a mixture of hydrochloric and nitric acids. The resulting solutions have been used to obtain α-Fe₂O₃ nanocrystalline hematite by a chemical-metallurgical method, the main stages of which consist in hydroxide precipitation with the use of alkali at constant pH, washing, drying, and dehydration. Maghemite γ-Fe₂O₃ has been obtained from hematite in two stages. At the first stage, hydrogen reduction has been performed, whereas at the second stage, the obtained magnetite has been oxidized in air. The particles of synthesized nanodispersed oxide powders are in the aggregated condition. The particles of α-Fe₂O₃ are spherical, whereas the particles of γ-Fe₂O₃ are rod-shaped. According to Mössbauer spectroscopy, the lattices of both oxides contain magnesium, aluminum, silicon, chromium, and manganese that originate from the initial scale. These elements determine magnetic properties of α-Fe₂O₃ and γ-Fe₂O₃ nanopowders. The set of properties inherent in nanodispersed hematite and maghemite powders obtained from metallurgical wastes (mill scale) is recommended for the application in catalytic processes, in the systems of industrial wastewater purification from heavy metal ions, as well as in the manufacturing of blood analysis markers.