Relationships between pitch composition and optical texture of cokes

The optical texture of cokes obtained at 500 °C (60 minutes soak time) from a range of pitches, coal-tar and petroleum pitches and from the extrography fractions of three of the pitches, was related to the chemical composition of the parent material. Small differences observed in the amount of polyaromatic hydrocarbons and basic nitrogen polyaromatic compounds present in vacuum-distilled and thermally treated binder pitches do not have a significant influence on the optical textures of the resultant cokes. However, when pitches of very different origins (binder pitches, impregnating pitches, petroleum pitches and low-temperature tar pitches) are compared, different distributions of classes of compounds in terms of molecular size and functional groups (as determined by extrography) have a definite effect on the optical texture of the coke, a wide range of structures being observed. The carbonization of pitch fractions obtained by extrography gave valuable information on the effect of specific types of compounds on the optical texture of cokes. Fractions rich in basic PANCs provided cokes with the best developed optical texture, whereas fractions constituted by PAHs were not completely converted into mesophase under the same pyrolysis conditions. It was also observed that pitch material not eluted during the extrographic fractionation, which is highly polar, adversely affects the development of the optical texture of the cokes.