Carbonization in the tube bomb leading to needle coke: III. Carbonization properties of several coal-tar pitches

Carbonization properties of several coal-tar pitches of different origins were studied, by using tube bomb, to correlate their analytical characteristics with their carbonization properties and to find their respective optimum carbonization conditions for the production of excellent needle coke. The pitch rich in naphthenic structure produced an excellent needle coke of low CTE and uni-axially arranged flow texture under a wider range of carbonization conditions, whereas the pitches of high oxygen and alkyl contents or of very high aromaticity gave cokes of larger CTE under the standard carbonization conditions of 500 °C, 8 kg/cm², because of more mosaic texture or poor uni-axial orientation in the resultant cokes. However, the two kinds of poor pitches were found to produce better cokes under their respective appropriate conditions. The former one did at a slightly lower carbonization temperature of 480 °C and the latter one did under a lower carbonization pressure of 4 kg/cm². A lower temperature allowed the development of bulk mesophase through the moderation of carbonization reactions for the former reactive pitch. The lower pressure improved uni-axial arrangement of mesophase molecules at the solidification stage through the sufficient gas evolution of good timing for the latter highly aromatic pitch. The higher pressure of carbonization delayed the solidification to be off-timing to the gas evolution. The mechanism of needle coke formation was discussed to explain the respective optimum conditions for the pitches of different analytical characteristics.