Determination of the crystallinity of calcined and graphitic cokes by X-ray diffraction†

Abstract

Although the Scherrer equation has been the basis of the XRD method for the determination of the crystallinity of calcined coke, the most accurate interpretation of coke crystallinity involves profile analysis. In this paper, the general background related to coke crystallinity (L_c) determination is described, Alcan and ASTM standard methods are compared and line breadth as a function of crystallite size is characterized. At present, it is generally understood that the coke graphitization process occurs during calcination of coke and is related not only to the calcination temperature, but also to other parameters (composition, process conditions). As coke graphitization affects electrolytic cell performance, it should be monitored and quantified. The methods used for the determination of the degree of graphitization (DOG) and described in the literature are useful for cases involving calcination temperatures above 1800 °C. These methods are critically examined. Because known methods do not apply to the low-temperature graphitization processes (1200–1500 °C), a new approach was conceived. It is based on profile line analysis and an estimate of the graphitic contribution in coke specimen can be made. The new approach involves a commercial X-ray diffractometer and a corresponding software package and relies on profile fitting of the (002) coke peak. A calcined coke specimen or the presence of a graphitic portion in calcined coke can then be clearly recognized. During the profile fitting operation it is possible to resolve the graphitic portion from the calcined peak profile mathematically, and then to express it quantitatively. Among several profile functions that were tested, the split Pearson VII was found to give the best fit to data corresponding to calcined coke. A novel equation by which the DOG can be estimated in calcined coke is given. Selected examples of the DOG determination are described.