Catalytic graphitization of residual carbon from gasification fine slag with ferric chloride as catalyst

Abstract

Coal-based graphite obtained from catalytic graphitization has raised great interests in many industrial areas including adsorbent, electrodes, hydrogen storage and many more. In this paper, residual carbon (RC), acquired from gasification fine slag (GFS) after flotation and acid leaching, was used as carbon precursor in graphitization process due to its high carbon content and graphitization tendency. Ferric chloride (FeCl3) was added as catalyst to facilitate graphitization process. The microstructure of the graphite samples obtained with and without FeCl3 was determined by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscope - energy dispersive spectroscopy (SEM-EDS) and Raman spectroscopy, so that the structural differences can be studied and catalytic mechanism can be analyzed. The results revealed that with the increase of temperature and presence of FeCl3, the layer spacing (d002) gradually decreased. Accordingly, the graphitization degree (g), crystallite size (La) and stacking height (Lc) presented an increase trend, which illustrated that the graphite domain gradually became larger. And the g value of graphite sample derived from RC at 2800 °C with FeCl3 was even higher than that of sample obtained with anthracite at 3000 °C. Besides, the better-defined XRD diffractions peaks and Raman spectra also indicated better ordered graphite structures. The possible catalytic mechanism could involve the formation of an intermediate eutectic carbide (FexCy), which appeared as spherical droplets at 2400 °C and could significantly facilitate the graphite process. The result was confirmed by the kinetic analysis results, as the reaction activation energy of the graphitization process with FeCl3 in the temperature range of 2000–2400 °C was significantly lower than that without FeCl3 or in other temperature ranges. With the analysis of the microstructures, one possible catalytic mechanism appropriate for the graphitization of residual carbon with FeCl3 was proposed.