GRAPHITIZATION OF HIGH RANK COALS - THE ROLE OF SHEAR STRAIN - EXPERIMENTAL CONSIDERATIONS

The graphitization of anthracite coal has been investigated following experimental deformation under constant high confining pressure (500 MPa), deviatoric stress, variable temperature (300-600-degrees-C) and strain (to 33%). Under the experimental conditions anthracite is characterized by a power law dependency of stress to strain rate with the stress exponent of 3.3 and activation energy of 68.6 kJ mol-1. Such an activation energy is considered to be associated with the rate controlling processes involved with the loss of CH4, H+ and O2- by diffusion. Transmission electron microscopy indicates that in highly strained specimens the basic structural units (BSUs) or stacks of aromatic layers have become more ordered during deformation and have been progressively aligned in the plane of flattening. The structural re-arrangement of the BSUs is not homogeneous throughout the deformed specimens. Such improvement of the organization during the experiments of this study occurs at a much lower activation energy than reported from thermal experiments at ambient pressure or even high confining pressure (68.6 kJ mol-1 as compared to up to about 1000 kJ mol-1) and is attributed to shear strain(s) which has led to straightening of the aromatic layers closer to a more uniform packing. Change in optical properties between the deformed and undeformed anthracite can be explained in terms of the progressive alignment of BSUs. High temperature and strain have faciliated reorientation of the optical indicating surface such that the maximum reflectance is parallel, or close to parallel, to the plane of flattening during deformation. Increase in anisotropy (bireflectance) and an increase in maximum reflectance accompanies deformation. Orientation of the optical indicating surface of some of the deformed samples suggests the maximum reflectance is a composite product of the original orientation of BSUs and orientations produced during deformation.