Structural study of graphite materials prepared by HTT of unburned carbon concentrates from coal combustion fly ashes

Unburned carbon concentrates with different mineral matter contents were obtained from coal combustion fly ashes by an oil agglomeration procedure. The concentrates were then heated in the temperature interval 1800-2700 degrees C for the purpose of exploring their ability to graphitize. The influence of the treatment temperature and mineral matter of the unburned carbon on the structural characteristics of the materials prepared was studied. The interlayer spacing, d(002), and crystallite sizes along the c-axis, L-c, and the a-axis, L-a, calculated from X-ray, diffractometry (XRD) as well as the relative intensity of the Raman D-band, I-D/I-t, were used to assess the degree of structural order of the materials. Graphite materials with structural characteristics comparable to those of other oil-derived synthetic graphites were prepared from the unburned carbon concentrates at temperatures >= 2400 degrees C. It was also observed that more-ordered materials were obtained from the unburned carbon concentrates with higher mineral matter content. The influence of the mineral matter on the graphitization of the unburned carbon concentrates is the result of two countereffects, thus limiting its extent. On the one hand, the lateral coalescence of the crystallites is preferentially promoted. Reasonably good linear correlations were attained between the mineral matter of the unburned carbon concentrate and the XRD parameter L-a of the materials. However, on the other, this coalescence also facilitates the flattening of the pores, thus decreasing the temperature at which their breakage occurs. As a consequence, from this point on, the structural evolution of the materials with increasing mineral matter is only noticeable by the slow vegetative growth of the crystallites along the a-axis.