Microstructural evolution of high temperature treated anthracites of different rank

The present work is focused on the microstructural evolution of major components of carbonaceous samples during high temperature treatment, i.e. vitrinite in raw anthracites and dense particles in the heated samples. Three anthracites from distinct geological settings, with an Rr of 2.62%, 5.23%, and 6.25% (PBEB, ACB, and DB, respectively) were chosen for this research. The raw samples were previously carbonized at 1000 °C, and then subjected to high temperature treatment (HTT) at 1500, 2000, and 2500 °C, in a graphite furnace. From the results obtained by optical microscopy (RIS parameters), micro-Raman spectroscopy, and X-ray diffraction, it was possible to evaluate the subsequent structural and microtextural evolution of the heat treated materials. It was found that, during the carbonization process up to 1500 °C, the transformations are essentially of a chemical nature and are expressed by chemical and optical parameters (RIS main axes and Rev parameter). The magnitude of these changes is higher for the lower rank heat treated anthracite (PBEBC series). The structural order parameters obtained by X-ray diffraction show an increase in disorder at 1000 °C and a slight improvement after heat treatment at 1500 °C. The G band FWHM (cm− 1) exhibits the same tendency at 1000 °C. However, due to the small size of the crystallites, this Raman parameter continues to increase at 1500 °C. After 2000 °C, there is an increase in the structural order in the materials of the heat treated samples. A three-dimensional ordering was observed in the materials of the DBC carbonized anthracite at 2500 °C, as exhibited by the X-ray diffraction parameters and Raman spectral features. The PBEBC and ACBC carbonized anthracites still retain their turbostratic structure after heat treatment at 2500 °C.