| Summary: | Mössbauer spectroscopy and X-ray diffraction have been used to study samples of fly ashes formed after combustion of coal from the Ekibastuz basin at the thermal power plants TPP-2 and TPP-3 in Almaty (Kazakhstan). It has been established that the fractions of fly ashes contain iron in the form of magnetite Fe<sub>3</sub>O<sub>4</sub> and hematite α-Fe<sub>2</sub>O<sub>3</sub>. The mixed valence of iron Fe<sup>3+</sup> ↔ Fe<sup>2+</sup> in the octahedral sublattice of magnetite is destroyed by isostructural substitution impurities. Maghemite γ-Fe<sub>2</sub>O<sub>3</sub> is additionally present in the fly ash of TPP-3 as a product of magnetite slow oxidation. It was shown that at T ≥ 1400 °C the proportion of magnetite in fly ashes increases due to decomposition of hematite, maghemite, hercynite and the drop of iron content in mullite. It was concluded that the amount of iron in magnetite is a temperature indicator of fly ashes formation. The parameters of hyperfine interactions have been determined in the iron-containing minerals of fly ashes. It was identified that formation of the fly ashes structure occurs in oxidizing atmosphere, since no traces were revealed of reducing environment effect on the phase composition.
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