INVESTIGATION OF THE COAL MINERAL PRECURSORS OF FLY ASH GLASSES USING CLUSTER ANALYSIS OF PARTICLE SEM DATA

Particle SEM or automated SEM uses a computer-controlled stage and associated software to perform SEM-EDS analysis of individual particles in a sample of a material such as fly ash. This has been applied to analyze the glassy component of some typical commercial fly ashes. The glass particles are automatically identified by characteristic size range, i.e.2.0- 25μm and circularity < 1.3. For each particle meeting these criteria, concentrations of 16 elements are measured. Up to 10,000 glass particles are analyzed in a given fly ash sample. In order to visualize patterns in this very large set of data, the particle compositions have been plotted in the glass science triaxial diagram which has axes of SiO₂ + Al₂O₃ + Fe₂O₃, MgO + CaO and Na₂O + K₂O. This reveals that the compositions are not random, but instead appear to fall into a limited set of clusters which can be related to the compositions of the mineral precursors in the coal source. Plotting the compositions of typical coal mineral classes such as clays or micas in the diagram indicates that the fly ash glass clusters are usually deficient in the alkalis Na₂O + K₂O. This implies that the glass formation process involves the volatilization of these constituents during melting. However, some very calcium-rich glasses do not plot along the dealkalization reaction paths, which suggests that instead of having a single mineral precursor, they are formed by reactions involving more complex particles of biological origin such as diatoms or phytoliths. Finally, there is a region in the diagram in the calcium-poor zone next to the SiO₂ + Al₂O₃ + Fe₂O₃ axis where no glass clusters are found. The compositions in this region are characteristic of kaolinite which tends to crystallize into mullite1 rather than forming glasses.