Mineralogy, Chemical Composition, and Microstructure of Ferrospheres in Fly Ashes from Coal Combustion

Fourteen samples of coal and ferrospheres, which were recovered by dry magnetic separation from fly ashes, were collected from five power plants in China. The mineralogy, chemical composition, and microstructure of ferrospheres in fly ashes have been studied by optical microscopy, X-ray diffraction (XRD), Mossbauer spectroscopy, and field emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (FSEM-EDX). Iron in ferrospheres mainly occurs as Fe3O4, α-Fe2O3, γ-Fe2O3, MgFe2O4, and Fe3+−glass, ferrian spinel, and so on. On the basis of iron content, the ferrospheres in fly ashes are classified into four groups, namely ferrooxides (Fe ≥ 75%), aluminosilicate-bearing ferrooxides (75% > Fe ≥ 50%), high-ferriferous aluminosilicates (50% > Fe ≥ 25%), and ferroaluminosilicates (Fe < 25%). Ferrooxides are derived from the oxidation of iron-bearing minerals, whereas aluminosilicate-bearing ferrooxides, high-ferriferous aluminosilicates, and ferroaluminosilicates are formed by the fu...