Composition of ferrospinel phase and magnetic properties of microspheres and cenospheres from fly ashes

The methods of Mössbauer spectroscopy, magnetic measurements, chemical analysis, and thermodynamic calculation of the phase composition were used in systematic studies of oxide iron–aluminosilicate (SiO2–Al2O3–Fe2O3) system in a wide range of its macrocomponent composition: 0.6–65.6 wt.% SiO2; 0.05–20.5 wt.% Al2O3; 4.6–92.5 wt.% Fe2O3. These correspond to the composition of magnetic microspheres and cenospheres from fly ashes originated from powdered-coal combustion of five varieties of energy coals. It was established that the main phase governing the magnetic properties is magnetite with a defective structure. The microspheres saturation magnetization increases with the iron content range of 4.6–80.0 wt.% Fe2O3 is caused by the increase of the spinel phase content. The magnetization reduction in ferruginous microspheres with the high iron content (83.0–92.5 wt.% Fe2O3) is the result of the hematite–magnetite displacement.