Characterization Of Porous Structure And Its Correlation To Sodium Penetration-Expansion-Creep Of Carbon Cathode Materials

The Chinese technology in the design of super-high ampage cell for aluminum reduction is of first-class in the world. However, the service life of the cell is still behind the advanced technology. The carbon cathodes in aluminum electrolytic cell directly affect the service life of cell, energy consumption level and the emissions of solid waste. Previous studies consider that generated metallic sodium penetrates into carbon cathodes making cathode expansion and creep, which can result in sodium-carbon intercalation and expansion of cathode materials. This is a major cause for generating internal stresses and cracks, shortening the service life of carbon cathodes. Past research focuses on the performance of carbon cathode material entities, and there is little research on the influence of the pore structure due to lack of quantitative characterization methods. Therefore, based on digital image analysis technology, the characterization method was established suitable for the pore structure of carbon cathode materials. Porous structures and their evolution of graphitic cathode materials at various baking temperatures were investigated using this image analysis method. The relationship between the porous structures and sodium penetration-expansion-creep were also studied. The information obtained could be useful for building a new method for materials properties evaluation and improving the quality and performance of the graphitic cathodes for use in aluminum reduction cells.According to the characters of cathode materials and its pores size, the image analysis method has been improved to be suitable for the porous structure of the carbon cathode materials. The porosity, average pore diameter, pore specific surface area, aspect ratio, orientation factor, connectivity, coordination number, and fractal characteristics are defined as pore structure parameters and associated key characteristic parameters with the carbon cathode penetration, expansion and creep properties. This method can be quantitative, accurate characterization of the internal porous structure, micro porous structure, and pore evolution process correlated with performances.The porous structures and their evolution of graphitic cathode materials at various baking temperatures, pressures and graphitization degree were investigated using the image analysis method. It is found that the fractal dimension increased from1.25to1.50after1200℃baked. The change of porous structures of graphitic cathodes is found following the law of fractal behaviors and hence an evolution model has been proposed, so that the image analysis parameters and the fractal dimension can apply for characterizing the degree of porous structures evolution when subjected to a given baking temperature. When the pressure increases, the aspect ratio and connectivity of the pores become higher while the porosity first decreases and then increases. The porosity of three typical industrial products (semi graphite, graphite, graphitized) cathode increase with the degree of graphitization ascension (23%,25%and30%, respectively), but the pore specific surface area, connectivity and orientation factor decrease which means pore tended to be isotropic and more uniform distribution.The composition and porous structure of carbon cathode materials were analyzed before and after electrolysis by SEM-EDS and image analysis. NaF is the major phase penetration into cathode after aluminum electrolysis. The aspect ratio, connectivity and coordination number of the cathode materials calculated by image analysis is closely related to the penetration deep and rate. The long and narrow pores which have high aspect ratio, connectivity and coordination number are effect on penetration process significantly. The law of the molten salt electrolyte penetration into cathode of the same degree graphitization can be simulated by improved Carman-Kozeny model and two-dimensional image pore parameters.The alienation phenomenon of pores evolution in adjacent regions of the cathode graphite materials was found in situ expansion research of high temperature electrolysis. The porosity of pores in the center region decrease, while the porosity of pores in the side region increase. The corresponding evolution between porous structure and interior stresses can be explained according to the analysis of the interior stresses. It means that the changes in the pore spaces make the interior stress and strain equilibrated through pores self-organization behavior without external constraint stress.At the same time, it is found that the porosity of cathode materials decreased in the initial creep, but increased both in central and side area after a longer time to creep in situ creep research of high temperature electrolysis. Under the condition of laboratory, creep behavior eventually lead to porosity increased50%. With external constraint, when the expansion behavior as the main role, internal pores can still release the non-unifonn stress through self-organization behavior. When the external stress cause creep behavior as the main role, the pores collapse and degrade with new pores and micro crack increasing. The size of carbon cathode is larger in industrial aluminum reduction cell, so the cathode block is under non-equilibrium state due to the internal stress uneven distribution. The pores self-organization will release the internal stress of cathode materials, reducing the material damage caused by creep behavior.