FT-IR and micro-Raman spectroscopic characterization of minerals in high-calcium coal ashes

Two high-calcium coal ashes were prepared in a muffle furnace at 815 °C. The mineral matter in both coal ashes was characterized by Fourier transform infrared (FT-IR) spectroscopy, micro-Raman spectroscopy, and X-ray diffraction (XRD). The overlapping bands of original FT-IR spectra were resolved into individual ones by using second derivative method. The presence of two most intense absorption bands (1154 and 1120 cm−1) in the original spectra indicate that both coal ashes contain high levels of anhydrite, consistent with the XRD result. The presence of amorphous silica and metakaolinite was found from the second derivative spectra. Calcite and anhydrite in both ashes show marked Raman bands at 1086 and 1017 cm−1, respectively. In addition, the Raman intensity of anatase in both ashes is very strong, due to the high polarizability of TiOTi. FT-IR and micro-Raman spectroscopy are complementary for the identification of anhydrite in coal ashes. Moreover, a combination of both spectroscopic techniques can provide more information on mineral composition and structure as compared to XRD, since XRD fails to identify the presence of amorphous silica, metakaolinite, and anatase.