Structure–Composition Relationship of Skeletal and Dendritic Ferrospheres Isolated from Calcium-Rich Power Plant Ash

Abstract

This paper presents a systematic SEM–EDS study of polished sections of individual skeletal and dendritic ferrospheres in the–0.04 + 0.032 mm size fraction, isolated from fly ash from the combustion of brown coal from the Berezovskoe field. The ferrospheres are characterized by a wide range of variations in the macrocomponent composition of local areas. 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: SiO₂ = f(FeO), SiO₂ = f(Al₂O₃), and CaO = f(SiO₂). Such equations make it possible to identify the nature of the mineral precursors involved in the formation of the globules. FeO-rich skeletal ferrospheres with low CaO concentration originate from the thermochemical transformation of pyrite and illite associates. Skeletal and dendritic ferrospheres with monotonically increasing CaO and SiO₂ concentrations are formed from pyrite and montmorillonite associates, with the participation of a melt containing quartz and decomposition products of Ca-humates of the initial coal. Skeletal and dendritic spinel ferrite crystallization is due to a magnesium aluminate spinel “seed,” resulting from the thermal transformation of illite and montmorillonite from the parent coal. The observed increase in glass phase concentration and the change from the skeletal type of crystallization to a dendritic in the ferrospheres containing ≤64 wt % FeO and ≥6.5 wt % CaO are due to the low concentration of the spinel-forming cations Fe²⁺ and Fe³⁺ in the melt and the increase in the percentage of [Fe³⁺O₂]⁻ and [Fe₂³⁺ O₅]⁴⁻ ferrite complexes with an increase in the degree of oxidation of the melt.