Abstract
The organic and mineral components in two coals and resulting high-temperature ashes with high silicon content were characterized by second-derivative infrared spectroscopy, Raman spectroscopy, and X-ray diffraction (XRD). The infrared spectra of raw coals show weak organic functional groups bands but strong kaolinite bands because of the relatively high silicates content. In contrast, the Raman spectra of raw coals show strong disordered carbon bands but no mineral bands since Raman spectroscopy is highly sensitive to carbonaceous phases. The overlapping bands of mineral components (e.g., calcite, feldspar, and muscovite) were successfully resolved by the method of second-derivative infrared spectroscopy. The results of infrared spectra indicate the presence of metakaolinite in coal ashes, suggesting the thermal transformation of kaolinite during ashing. Intense quartz bands were shown in both infrared and Raman spectra of coal ashes. In addition, Raman spectra of coal ashes show a very strong characteristic band of anatase (149 cm–1), although the titanium oxides content is very low. Combined use of second-derivative infrared spectroscopy and Raman spectroscopy provides valuable insight into the analyses of mineralogical composition. The XRD results generally agree with those of FTIR and Raman spectroscopic analyses.
Highlights
The characterization of coals is complicated because of mineralogical and chemical heterogeneity of the raw materials and compositional and structural changes resulting from heat treatment
Mineral phases with low crystallinity or low content in coal ashes were successfully identified by combined use of Fourier transform infrared (FTIR) and Raman spectroscopy techniques
FTIR, Raman spectroscopy, and X-ray diffraction (XRD) are complimentary for the identification of minerals in coals and coal ashes
Summary
The characterization of coals is complicated because of mineralogical and chemical heterogeneity of the raw materials and compositional and structural changes resulting from heat treatment. Fourier transform infrared (FTIR) spectroscopy is one of the most important techniques available for the analysis of both organic and inorganic components in a coal. The identification of the mineralogical composition of raw coals and resulting coal ashes by second-derivative infrared spectroscopy has not yet been systematically studied. Raman spectral changes from raw coals to resulting coal ashes have not yet been reported The purpose of this investigation is to identify the organic and mineral components of coals and resulting high-temperature ashes by second-derivative infrared spectroscopy and Raman spectroscopy. The second-derivative infrared spectroscopy was conducted to thoroughly analyze the mineralogical composition of the coals and coal ashes based on the interpretation of original and overlapping bands. Mineral phases with low crystallinity or low content in coal ashes were successfully identified by combined use of FTIR and Raman spectroscopy techniques
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