Structure–Composition Relationship of Platelike Ferrospheres in Calcium-Rich Power Plant Ash

Abstract

The structure–composition relationship for polished sections of individual platelike ferrospheres in the–0.04 + 0.032 mm size fraction isolated from fly ash from the combustion of brown coal has been studied systematically by scanning electron microscopy and energy dispersive X-ray spectroscopy. We have identified groups of globules whose overall composition, as well as the composition of local areas on their polished sections, can be represented by general equations for component concentrations: CaO = f(FeO) and SiO₂ = f(FeO). Analysis of the structure–composition relationship for the globules leads us to conclude that their structure is determined by transformations that occur in the CaO–Fe_xO_y system in response to an increase in its oxidation potential. It has been shown that the platelike globules containing 68–73 wt % FeO are made up of Ca₂Fe₂O₅ and CaFe₂O₄ crystallites resulting from the oxidative transformation of Fe, Ca, and Mg complex humates in the parent brown coal. The ferrospheres containing 79–90 wt % FeO have a fragmentary core–shell structure, where the platelike shell consists of Ca₂Fe₂O₅ and CaFe₂O₄ crystallites, and the core consists of Fe₂O₃ and a Ca-, Mg-, and Al-promoted magnetite. Precursors for the formation of this type of globule are pyrite associates with complex humates. It has also been demonstrated that the low concentration of aluminum and silicon oxides in the composition of the globules and the viscosity of their melt have no effect on the structure of the platelike ferrospheres.