Antonenko T. The peculiarities of transformation of structure and properties of iron oxides and hydroxides under the influence of external factors

The investigation of changes of magnetic properties and structure of magnetically ordered iron oxides and hydroxides and methods of their production are in the focus of attention during last years. The aim of the study was to investigate the processes of transformation of iron oxides and hydroxides and changing of their magnetic characteristics that occur in aqueous medium. The processes of transformation of iron oxides and hydroxides were studied for next samples: synthesized iron oxides and hydroxides (synthetic goethite, hematite, magnetite, lepidocrocite) and natural iron oxides and hydroxides (goethite ore of the Kerch basin, goethite ores of the Kryvyi Rih basin, hematite ores of the Kryvyi Rih basin). Phase composition of synthetic and natural samples was determined by X-ray diffraction (XRD). Magnetic characteristics (saturation magnetization) of initial and obtained samples were determined using magnetometry method. Curie temperature of obtained samples was determined by thermomagnetic analysis. The synthesis of iron oxides and hydroxides by chemical co-precipitation of two- and trivalent iron in alkaline medium were investigated. Samples with different Fe3+/Fe2+ ratio were synthesized. It was shown that the size of synthesized particles was in the range of 9-30 nm and saturation magnetization varies from 19 to 65 A•m2/kg. Synthesized samples consist mainly of magnetite. It was shown that amount of nanomagnetite almost do not increase with Fe3+/Fe2+ > 2. The transformation of synthetic and natural goethite and hematite to magnetite was performed by treatment up to 98 °С in aqueous Fe2+-containing solution under convectional and microwave heating. Saturation magnetization for initial samples was less than 1 А∙m2/кg. Saturation magnetization of samples, obtained using convectional heating was within the range 5-14 A•m2/kg. By the data of thermomagnetic analysis, it was shown that Curie temperature of obtained samples is close to Curie temperature of pure magnetite. Saturation magnetization of samples, obtained using microwave heating was within the range 8-32 A•m2/kg. It was shown by XRD method that phase of magnetite appears in all obtained samples after transformation. Microwave heating enhances the processes of structure transformation in the aqueous medium. Regularities of natural hematite to magnetite transformation under heating in the presence of different amounts of starch have been studied. The processes of hematite reduction to magnetite with starch were investigated in the temperature range of 420-650 °С. It was established that saturation magnetization of natural hematite ore in the process of magnetization increased almost 20 times and reached the value of 20 А•m2/kg. Also, the transformation of natural goethite and hematite in water solution of starch under the influence of microwave heating was conducted. Saturation magnetization of samples after such treatment was within the range 30-67 A•m2/kg. The phenomenon of Fe (II) ions uptake was investigated in the process of transformation of natural goethite and hematite samples. A method of spectrophotometry was used to study the phenomenon of Fe (II) ions uptake by the initial samples of iron ores. Fe+2-containing aqueous solution and alkali were added step-by-step to the initial crushed raw material, followed by heating at a temperature of 98 °C for 30 minutes. The monitoring of the Fe+2 ions concentration was performed after each stage of the procedure. It was established that the highest sorption ability corresponds to a sample of hematite. The behavior of natural goethite and hematite samples, as the value of pH changes, in the aqueous medium has been investigated. The behavior of natural goethite and hematite samples in the Fe+2-containing solution and Fe+3-containing solution is significantly different, namely, in the Fe+2-containing solution rapid increase of pH begins with the addition of 2 ml of ammonium hydroxide, and in Fe+3-containing solution – after addition of 16 ml of ammonium hydroxide. Possible mechanisms of transformation of goethite and hematite to magnetite in aqueous medium are described. According to these mechanisms, magnetite phase is formed by the addition of Fe2+ to weakly magnetic phase. Fe2+ ions adsorb at the surface of the goethite or hematite, after that the electron exchange between Fe2+ and Fe3+ occurs. The electrons migrate to the inner ions with further conversion of Fe3+ ions of the bulk crystal into Fe2+ ions. The results could be used to solve the fundamental problems, associated with the mechanisms of transformation of iron oxides and hydroxides in an aqueous medium and applied problems associated with the recycling of oxidized iron ores of the Kryvyi Rih basin and solving the environmental problem of Kryvyi Rih.