X-ray scattering from coals

Abstract

This paper describes the results of X-ray scattering experiments on a series of vitrain-coals of varying rank. The scattering at high angles is interpreted in terms of condensed aromatic layers; the diameters of the layers are determined and are found to increase with increasing rank; the percentage carbon in the form of layers is also estimated. Fourier transforms calculated for the scattering at lower angles show that the layers occur partly singly, and partly in groups of two, three or more stacked parallel to each other. The degree of local parallel stacking increases with increasing rank. There is a preferred orientation of the layers parallel to the bedding plane, which becomes more marked the higher the rank of the coal. The diffraction peak at ~20 Å, found for coals with 85 to 94% C, is considered to be a consequence of ‘liquid-type’ packing of the most frequently occurring groups containing two to three layers. The diameters of the layers determined from the transforms (~8 Å) are in good agreement with the results of intensity calculations, and the values obtained from the scattering at high angles. The scattering at very small angles is measured with a two-crystal spectrometer for spacings up to ~5000 Å. The scattering is related to porosity; the scattering curves do not support the existence of a close-packed arrangement of ‘micelles’ of fairly constant diameter less than ~5000 Å, but suggest that there are anisotropic cracks and pores of a wide range of sizes, some of which must exceed ~5000 Å. The total scattering indicates the existence of disk cracks preferentially orientated parallel to the bedding plane. In terms of a proposed structural model, coalification is a process of condensation, ordering of the layers, and flattening of the structure. A ‘liquid-type’ structure is formed, which is most perfect at ~89% C and accounts for the minimum of porosity. Anthracitization is probably accompanied by a clustering of the layers, which results in a rapid increase of layer diameter, in irregular packing, and in an increase of porosity. The properties of the proposed structure model of coal are discussed and compared with the known physical and chemical properties of coal, e. g. density, optical data, porosity and mechanical properties, and some of the problems still outstanding are indicated.