Optical properties of carbonized vitrinites

The optical properties have been examined of three vitrinite-rich coals (carbon, daf: 82.5, 88.0 and 93.1%), carbonized at temperature intervals of 25 °C over the greater part of the range 20–950 °C using a heating rate of 2.45 °C/min. The small temperature interval employed has allowed a satisfactory correlation between alterations in the physical state of the carbonized products and fluctuations in the rates of change of their optical properties during carbonization. In general, as temperature rises the variation of all the optical properties reflects the growth and development of organized polycondensed aromatic systems, but in detail reflectivity does not prove as sensitive an indicator of structural changes as do other optical parameters, particularly refractive index. The behaviour of the refractive-index curves of the carbonized vitrinites is governed by changes in the heights of the crystallites forming the aromatic systems. The shape of these curves closely follows the shape of curves produced in independent X-ray diffraction studies giving the heights of crystallites. While all the refractive-index and all the absorptiveindex curves are similar in shape and trend to one another, each of these curves for carbonized coking-coal vitrinite is displaced from the equivalent curves for low-rank bituminous and anthracitic vitrinites. The displacement can be related to the formation of the highly-plastic stage during carbonization of the coking-coal vitrinite, which ultimately yields weakly cross-linked structures with good crystallite orientation, in contrast to the strongly cross-linked structures with lower ordering developed in carbonization of vitrinites of both lower and higher rank.