Characterization of the mineralogy and microchemistry of fly ash

Abstract

The microstructures of coal fly ash particles were studied by transmission electron microscopy. Some particles are glassy and microscopically homogeneous; however, most particles examined contained one or more crystalline phases. Frequently, particles contain crystallites of a spinel compound in a glass matrix. It is believed that these crystallites nucleate homogeneously during cooling. Particles containing a greater fraction of crystalline material often have a dendritic structure. In principle, it should be possible to calculate solidification rates based on crystallite size or dendrite arm spacings. The distribution of trace elements was studied by thermogravimetric analysis and by studying their partitioning between fly ash and bottom ash. Trace element partitioning was then related to the distribution of particle types observed optically. This analysis indicated that only a few elements are present as volatile species. Many important trace elements appear to be associated with a particular particle type, namely, the ferrospheres. These ash particles have been shown to be the oxidized remains of framboidal pyrite in the feed coal.