Structural components in needle-cokes as studied by etching with chromic acid
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Structures within commercial calcined needle-cokes are characterized by optical microscopy to reveal optical texture and by scanning electron microscopy before and after etching with chromic acid solution at 400 to 423 K. The needle-cokes exhibit an optical texture of medium and coarse-grained mosaics, acicular flow domains and flow domains. The etching studies indicate that the acicular flow domains are made up of rolled and convoluted lamellae which etch to form laths (needle-like) ∼ 3 μm across. The mosaics appear when the rolled structures are viewed microscopically sectioned perpendicular to the lamellae. The acicular flow domains appear in longitudinal section. The etching of the flow domains shows that they have a sedimentary structure, 1 to 2 μm layer thickness, suggesting that mesophase coalesces within a layer but not between layers. A model is proposed to explain the origin and separation of structures in needle-cokes as created in the delayed coker.
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