Mechanism of puffing and the role of puffing inhibitors in the graphitization of electrodes from needle cokes

Abstract

A mechanism of puffing that occurs during graphitization of needle coke in the electrode is proposed based on the experimental results in the literature. Gases may evolve from within needle coke grains via porosity, causing pore walls to expand irreversibly when they are softened during graphitization. Hence, extents of puffing are influenced by rate and temperature range of gas evolution, the nature of evolved gases, location of the evolved heteroatoms such as sulfur and nitrogen in the coke, and the structure (especially grain size), porosity (pores and cracks), and preferred orientation of the coke, because these factors are related to the pressure induced by the evolved gas, timing for the softening of the coke and gas evolution and the resistivity of the wall against the pressure. Porosity within a coke grain can be modified by the coke derived from binder and impregnation pitches that can diffuse into the porosity of filler coke in the electrode. The graphitization also leads to shrinkage of the carbon. Hence, the size of the electrode is influenced by both puffing and shrinkage, these having opposing effects. The extent of puffing can be described semi-quantitatively in terms of influential factors described above. Thus, it is understandable that the puffing increased the porosity in a certain range (0.1–1 μm) and that puffing is suppressed by the increased porosity of 1–100μm. Based on the mechanism, procedures for puffing inhibition are suggested including: 1. 1. moderate acceleration of graphitization 2. 2. capture of heteroatoms to be liberated 3. 3. controled gasification to increase porosity of the filler cokes 4. 4. removal of heteroatoms from the needle coke or its starting feedstock The roles of puffing inhibitors are discussed based on the mechanism of puffing.