Investigation on the structure of carbonized pitch and calcined coke-carbonized pitch interface in carbon anodes by etch
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atrick Coulombe Aluminerie Alouette Inc., Sept-Îles, Québec G4R 5M9, Canada (Received 4 July 2016; accepted 5 October 2016)
A novel, simple, quick, and economic method has been developed to etch samples for characterizing the structural aspects of carbonized pitch alone and in baked anodes. Hot air is used to etch the polished carbonized pitch surface for creating its topography; followed by the characterization of the structure using scanning electron microscope. Hot air preferentially etches the carbonized pitch, which make the differentiation of carbonized pitch from the calcined coke particles possible in baked anode. After etching, lamellar parallel cracks are created and fine granular mosaics are observed on the surfaces of carbonized pitch. The structural composition in baked anode differs visibly from the pure carbonized pitch baked under the same conditions. This may be due to the effect of fine coke particles in anode on the formation of structure during baking. The etching technique permits the determination of the internal structure of carbonized pitch and its interface with coke in anode.
I. INSTRUCTION
Coal tar pitch is an anode raw material that binds the coke particles together during the production of carbon anodes used in aluminum industry. During the baking process, pitch undergoes pyrolysis. The thermal decomposition of pitch results in the release of volatiles and the formation of pitch-coke, namely, carbonized pitch. Carbonized pitch exhibits different structure than that of coke due to its properties, mixing, and compacting conditions used as well as the heat treatment that it is subjected to during anode fabrication. The structure of carbonized pitch and its interface with coke particles have a significate influence on the strength and reactivity of baked anodes.1,2 Polarized light microscopy is a well-known method that is used for the examination of polished surfaces of metallurgical cokes.3–10 Optical textural units of varying size and color can be seen on the polished coke surface due to the interaction between the structure and the incident light.4 Two types of optical structure of carbonized pitch (isotropic and anisotropic) can be identified by optical microscopy depending on pitch composition and baking conditions. The granular texture in carbonized pitch under polarized light exhibits stronger physical bonding with coke due to stronger possibility of forming chemical bond with coke particles compared to that of Contributing Editor: Paolo Colombo a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.396
carbonized anisotropic pitch. Therefore, carbonized pitch with relatively granular structure is preferred for anode production due to more favorable anode strength.11,12 Moreover, disordered carbonized coke has less open porosity that protects the underlying coke from air/CO2 oxidation better than a more ordered anisotropic carbonized coke in addition to obtaining anode with better strength with this type of pitch.1,2 Because of the similar hard
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