Development of Optical Anisotropy in the Coal Grain during Cocarbonization Process

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Abstract
In order to utilize non-fusible coals as a source of blast-furnace cokes, it is necessary to modify their carbonization properties, and for implementation, the development of optical anisotropy is one of the most important means. For its development, the fused state at the final stage of carbonization around 500°C that can be achieved by addition of a suitable additive is one of the most decisive factors. In the present paper, anisotropic development in the coal grain was studied to reveal the role of mass transfer during cocarbonization, using four kinds of English coals (a semianthracite, a strong, a weak and a non-coking coal) and hydrogenated Ashland A200 pitch (H-A200). A coking coal and a non-coking coal, which are in contact, were carbonized rather independently of each other. Moreover, they exhibited their original isotropic and anisotropic areas (Fig. 1). In contrast, H-A200, a powerful modifier for carbonization, can effect optical textural development in the coke, the extent of the development being subject to the coal properties. The optical texture of the coal grain of Bettshanger East, a semi-anthracite, was modified only around its periphery to give F (fibrous), Mc (coarse mosaic) and Mm (medium mosaic) textures; however, the texture at the central area of the grain remained the same as the original D (Domain) texture (Fig. 2a). Grain coal, when cocarbonized, lost its margin in the coke derived from the pitch to provide Mc and F structures as observed in the cocarbonization of powdered coal, however differing from the Mf texture produced by carbonization singly (Fig. 2b). In the case of Markham Deep Hard, a weakly caking coal, which produced an isotropic coke when carbonized singly, though slightly fusible, the Mm structure was formed around the coal grain, but the isotropic texture still remained in the central area of large pieces (Fig. 3a, b). In addition, some inert pieces from the coal were observable in the coke from the pitch (Fig. 3c), indicating that dissolution and migration of part of the coal grain into the pitch. Lea Hall Shallow Seam (non-coking coal) showed similar cocarbonization behavior, however, the area of anisotropic development was more restricted to its periphery (Fig. 4). Effects of H-A200 on cocarbonization with coals may be ascribed to the dissolution of coal and/or to solvolytic reaction by hydrogen transfer from the hydrogenated part to the coal during heat treatment. The liquid phase thus derived in the cocarbonization process can induce development of optical anisotropy because of its appropriate fluidity and rate of carbonization as in the case of partial hydrogenation of condensed aromatic rings indicated in previous papers16). The dissolution of coal and/or the solvolytic reaction is, on the other hand, governed by mutual mass-transfer or diffusion of the reacting substances. The carbonizing substances from a fusible coal as a liquid mixture can contact efficiently with the additive added. In contrast, slightly or non-fusible coals can only be liquified by the additive through the surface reaction between solid coal and liquid additive. Therefore, the extent of liquefaction is strongly influenced by the rate of increase in the reactive region between coal and additive matrix that provides constantly a fresh surface of the coal to the additive. Such a mechanism indicates a lesser probability of appropriate cocarbonization at the inner region of the grain and a lesser extent of anisotropic development, maintaining intrinsic optical texture of the coal-coke being produced at the core of the grain.

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