Characterization of mineral matters and carbonaceous structure of high-ash coals by vibrational spectroscopy

Mineral matters and carbonaceous structure of both raw and acid-washed Guangxi Heshan (GX) and Pingdingshan (PD) coals were characterized by Fourier Transform Infrared Spectroscopy (FT-IR), Raman spectroscopy and X-ray diffraction (XRD). The FT-IR spectra demonstrate that the two raw coals are most abundant in kaolinite, followed by quartz and calcite. Some new mineral matters such as muscovite, serpentine, gypsum and alkali-feldspar are clearly observed from the second derivative FT-IR spectra. In addition, three OH stretching vibration peaks are shown in the FT-IR spectra at 3 695, 3 651 and 3 619 cm^-1, indicating that the kaolinite in coals is not well crystallized. For demineralized coals, aromatic C=C peak (1 600 cm^-1) and (002) diffraction peak of microcrystalline carbon are obviously shown in FT-IR and XRD spectra, respectively. In the case of raw coals, both FT-IR and XRD spectra show that the carbonaceous structure is almost completely inhibited by mineral matters. However, the defect carbon (D peak) and graphite carbon (G peak) are clearly found in the Raman spectra for both raw and acid-washed coals, since the mineral matters are completely inhibited by carbon due to more intensive signal. The crystalline carbon is found to be slightly less ordered for demineralized coals than for raw coals, and therefore the carbonaceous structure is slightly affected by acid treatment.