Formation of thin mesophase layers at the interface between filler and binder in prebaked anodes. Effect of mixing on mesophase

Abstract

Prebaked anodes used for aluminum electrolysis are prepared by mixing petroleum coke as a filler with coal-tar pitch as a binder. Pitches which contain too large amounts of mesophase spheres are not suitable for preparing high quality anodes since mesophase microstructure is destroyed by mixing. Both the well extended parallel preferred orientation and the spherical shape disappear. It is observed that disorganized mesophase forms a shell around the coke particles. Such shells reduce the wettability of the coke by the pitch. This transformation was reproduced experimentally by strong grinding. When a disorganized mesophase is heat-treated at 700°C, it recovers a local molecular orientation, the extension of which widely varies from one area to the next one. The smaller the orientation extent, the lower the graphitizability. A mixture of non graphitizing microporous carbon and of partially graphitizing macroporous carbon is thus obtained. Such porous carbons are also found in anodes after baking as shells around the filler.