Composition-Structure Relationship and Routes of Formation of Blocklike Ferrospheres Produced by Pulverized Combustion of Two Coal Types.

The composition-structure relationship of blocklike ferrospheres (FSs) isolated from fly ash produced during the combustion of two different types of coal was studied systematically by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. Monoblock globules were shown to consist of large sintered crystallites of Mg, Mn ferrospinel, which are formed from excluded siderite particles containing isomorphic impurities of magnesium and manganese carbonates. The common groups of globules for which the gross composition of polished sections corresponds to the general equations for the relationship of the concentrations SiO = (AlO) and CaO = (SiO) were highlighted from FSs of two series. These globules are formed during the thermochemical transformation of associates of siderite, quartz, calcite, and anorthite, which have a silicate modulus of SiO/AlO equal to 1.18, which corresponds to the coefficients in the general equations of the relationship SiO = (AlO). SEM analysis of polished cross-sections of the globules of selected FS groups demonstrates that the crystallite size of ferrospinel decreases, while the content of the glass phase increases with the declining FeO concentration in individual globules. The crystallite size and shape are found to depend on the size of the local melt area where the concentration of spinel-forming oxides is >85 wt %. The observed increase in the glass-phase content is attributed to the broadening of the liquation zone in the FeO-FeO-SiO system as the oxidative potential increases and to the higher content of [FeO] and [Fe O] ferrite complexes in calcium-rich melts.