Chemical composition and structure of the shell of fly ash non-perforated cenospheres produced from the combustion of the Kuznetsk coal (Russia)

The relationship between the composition and structure of the shell of low-density non-perforated cenospheres of fly ashes from the combustion of coal from the Kuznetsk Basin (Russia) at temperatures of 1500 and 1650 °C has been investigated. Narrow fractions of cenospheres of constant composition have been separated from concentrates with the use of the technological scheme including stages of hydrodynamic gravitational separation, grain-size classification, and magnetic separation. It has been established that the concentrations of the major components of the chemical composition of the products obtained are related by linear regression equations. In particular, the aluminosilicate composition of the non-perforated cenospheres with a low Fe2O3 concentration (2.5–3.5 wt.%) is described by two general regression equations [SiO2] = 80 − 0.7·[Al2O3] and SiO2/Al2O3 = 5.54 − 0.12·[Al2O3] with correlation coefficients of −0.98 and −0.99, respectively. In this case, narrow fractions of products with different Fe2O3 contents are characterized by nearly constant SiO2/Al2O3 ratios. The shell structure has been investigated using scanning electron microscopy. It has been revealed that the outer and inner surfaces of globules are covered by a film with a thickness of 30–50 nm. The surface of magnetic cenospheres contains heterogeneous regions with extended linear ferrospinel aggregates of crystallites with sizes ranging from 50 to 1000 nm. It has been shown that the size of globules and the thickness and porosity of the shell in each series depend on the composition of the melt from which they are formed and on its viscosity. Nonmagnetic cenospheres are formed from the high-silica K–Al–Si melt with the SiO2/Al2O3 ratio varying over a wide range (1.6–3.6). Narrow fractions of magnetic cenospheres formed at low temperatures (1500 °C) are characterized by nearly constant SiO2/Al2O3 ratios. Globules of this type are formed from two immiscible high-silica K–Al–Si and Fe–Al–Si melts.