Relationship between composition and structure of globules in narrow fractions of ferrospheres

A systematic study has been carried out on the relationship between the composition, morphology, and structural characteristics of iron-containing phases of 25 narrow fractions of ferrospheres with the Fe2O3 content in the range 30-92 wt.%, which are separated from four known (S, FS, CS, and FCS) types of fly ashes. It has been established that the compositions of the major components of all the studied narrow fractions of ferrospheres and magnetic concentrates obtained from 17 power plants in Russia, Ukraine, and Kazakhstan are described by two general linear regression equations [SiO2] = 65.71-0.71 center dot[Fe2O3] and [Al2O3] = 24.92-0.26 center dot[Fe2O3] with the correlation coefficients of -0.99 and -0.97, respectively. In the general dependence of the ferrosphere composition, there are two regions differing in the properties of the melt from which the ferrospheres are formed. In a wide range of the Fe2O3 content variations from 30 to 79 wt.%, the behavior of the melt is determined by the properties of the FeO-SiO2-Al2O3 system. This system is characterized by the formation of an aluminum-magnesium-ferrite spinel. The content of the spinel increases with increasing iron concentration, and the unit cell parameter of the spinel also increases in the range from 8.3440 to 8.3897 angstrom. The unit cell parameters of the alpha-Fe2O3 phase are substantially smaller than those of stoichiometric hematite. For a higher Fe2O3 content (> 83 wt.%) and a low Al2O3 content (< 2 wt.%), the behavior of the melt is determined by the properties of the FeO-CaO system. This system is characterized both by the formation of the CaO-promoted ferrospinel, which has the unit cell parameter exceeding the corresponding parameter of stoichiometric magnetite, and by the formation of the hematite phase with the unit cell parameters close to those of stoichiometric alpha-Fe2O3. An increase in the iron concentration and the modulus of basicity leads to a decrease in the viscosity of the melt from which the ferrospheres are formed. In this case, the main morphological type of ferrospheres changes in the following sequence: porous (foam-like), glass-like, fine-grained (dendritic and point), skeletal-dendritic and coarse-grained (block-like). (C) 2013 Elsevier Ltd. All rights reserved.