Characterization of Mineral Matter in Coal Ashes with Infrared and Raman Spectroscopy

The analysis of mineral matter in coal ash is based on the analysis and characterization of mineral composition. Mineral matter in two high-silicon and high-aluminum coal ashes were well characterized and identified with Fourier Transform Infrared Spectroscopy (FTIR), Raman spectroscopy, and X-ray diffraction (XRD). The results from combined use of FTIR and Raman spectroscopy were then compared with those of XRD. Results of FUR show the presence of the strongest band in the range of 1100 similar to 1000 cm(-1), such as the bands for quartz (1089 cm(-1)) and metakaolinite (1042 cm(-1)), which are both assigned to Si-O stretching vibration. In contrast to original infrared spectra, the second derivative infrared spectra show the positions of overlapping adsorption bands, which are masked in the original infrared spectra. The analysis of the overlapping adsorption bands are useful for the determination of mineral composition and thus provides more detailed information on mineral matter. For the anhydrite in coal ashes, the three Raman bands (1157, 1126, and 674 cm(-1)) are obviously similar to the three corresponding FTIR bands (1151, 1120, and 678 cm(-1)), for they show the identical vibration mode and close peak position. Moreover, the anhydrite in coal ashes shows other different bands in its FTIR and Raman spectra. Therefore, FTIR and Raman spectroscopy techniques are complementary for the identification of mineral phases in coal ashes. Although the anatase content of both coal ashes is very low, the Raman band of anatase (144 cm(-1)) is far more intense than the band of quartz (461 cm(-1)) because of the significantly high polarizability of Ti-O. The results of XRD show that the mineral components in both ashes are primarily quartz, muscovite, hematite, anhydrite, and unknown amorphous mineral phase. In addition to these minerals, combined use of FTIR and Raman spectroscopy indicates the presence of metakaolinite, amorphous silica, feldspar, calcite, anatase, etc. The combination of FTIR and Raman spectroscopy can therefore provide more detailed mineral composition than XRD for qualitative analysis of mineral matter in coal ashes.