The evolution of coal, examining the transitions from anthracite to natural graphite: a spectroscopy and optical microscopy evaluation

Coal-derived natural graphite (CDNG) has multiple industrial applications. Here, ten metamorphic coals from anthracite to CDNG were obtained from Lutang and Xinhua in the Hunan Province and Panshi in the Jilin Province. Bulk characterization (proximate and ultimate analyses, X-Ray powder diffraction (XRD), and powder Raman spectroscopy), along with optical microscopy, scanning electron microscope (SEM) and micro-Raman spectroscopy were utilized to examine the transitions from anthracite to semi-graphite to CDNG. The XRD and Raman spectroscopy data indicate that from anthracite to highly ordered graphite the average crystal diameter (L_a) and height (L_c) increased from 6.1 and 4.6 nm to 34.8 and 27.5 nm, respectively. The crystalline parameters of the CDNG samples from Panshi and Lutang varied slightly when closer to the intrusive body. Optical microscopy and SEM indicated that in the anthracite samples there were thermoplastic vitrinite, devolatilized vitrinite, and some “normal” macerals. In the meta-anthracite, pyrolytic carbon, mosaic structure, and crystalline tar were present. In the CDNG there were flake graphite, crystalline aggregates, and matrix graphite. The crystalline aggregates show the highest structural ordering degree as determined from Raman spectral parameters (full-width at half maxima (G-FWHM) −20 cm⁻¹, D1/(D1 + D2 + G) area ratio (R2) value < 0.5). The flake graphite is less ordered with G-FWHM ∼28 cm⁻¹ and 0.5 < R2 < 1, but a larger grain size (up to 50 µm). The mosaic structures were likely the precursors of the matrix graphite through in situ solidstate transformation. The pyrolytic carbon and crystalline tars are the transient phase of gas-state and liquid-state transformations. This study is beneficial to realize the rational utilization of CDNG.