Digital image analysis of needle cokes and other solid carbons and their thermal expansion behavior.

摘要

Needle coke is a special type of petroleum coke with extremely high graphitizability. It is used as a filler material in manufacturing graphite electrodes for electric-arc furnaces to produce iron and steel. Properties of needle cokes, in particular the coefficient of thermal expansion (CTE), control the performance of graphite electrodes. The microstructure, (optical texture, and porosity) of needle cokes is considered to determine their thermal expansion behavior. Conventional texture characterization methods rely on subjective point counting procedures and assignment of texture indices to different anisotropic structures. The principal objective of this study is to develop effective methods to characterize the optical texture and porosity of needle cokes and other solid carbons by using computer-aided image analysis coupled with polarized-light microscopy. Studying the relationships between optical texture, porosity, of coke and carbon samples and their thermal expansion behavior is of particular interest.; Three image analysis methods developed in this study reflect different structural characteristics of calcined cokes and semi-cokes. The texture indices derived from these three methods can have different applications in relating texture, characteristics to coke properties. Only calcined needle cokes were analyzed for characterization of macroporosity that can be resolved by optical microscopy using digital image analysis. These macropores of needle cokes are formed predominantly in the calcination stage. The shape and orientation of the macropores, especially those with comparable sizes to texture components, were found to correlate with the thermal expansion behavior of calcined needle cokes. Porosity analysis of calcined needle cokes can, therefore, offer a simpler means of structural characterization as an alternative to texture analysis.; The texture and porosity of green, baked, and graphitized test bars, and commercial graphite electrode samples were analyzed to investigate the structural changes as a function of thermal treatment relevant to graphite electrode manufacture. The filler coke particles and the associated macropores near the edge of test bars were found to be better oriented than those at the center of the bars. Increasing the heat treatment temperature from green to graphitized test bars produced a wider pore size distribution, increasing the average pore size. The baked bars had the largest porosity, however, consistent with devolatilization of the binder during baking.