Evolution Of And Coupling Mechanisms Between Electrical Conductivity And Na-Bath Penetration For Carbon Cathode Materials In Aluminum Electrolysis

Electrical conductivity and the resistance to Na-bath penetration are very important for carbon cathode materials used for aluminum reduction cells. In the literature, both can be related to microstructures of the carbon cathodes, and the deformation mechanism of the carbon cathodes attacked by the penetration has been established. However, the evolution of and coupling mechanism between electrical conductivity and the penetration in correlation with the microstructures of the carbon cathodes is concerned scarcely. In this thesis, the porous and the crystalline structures of the carbon cathodes before and after electrolysis were characterized using XRD, SEM-EDS analysis and image analysis method, in order to illustrate the coupling effects between the microstructures evolution and the penetration in carbon cathode materials. Furthermore, the influencing mechanism of this coupling on the resistivity of the carbon cathodes was also studied and discussed. The aim of this work is to well understand the physical and chemical factors resulting in the variation of the resistivity during the electrolysis, to enrich the theoretical foundation of optimizing the structures and performances of the carbon cathodes, and to provide key parameters for designing long-life cells with lower energy consumption. The main results of this thesis are summarized as follows:(1) The relationship of ambient electrical resistivity (AER) depending on porosity, crystal structure and graphite content is put forward. The results indicate that the AER (p) is very sensitive to the specific resistivity (po) and the porosity (ε). A modified model based on percolation theory, ρ=ρ0-(1-ε)", is proposed to describe their relationship. The po can reflect the intrinsic conductivity of the carbon cathodes without the effects of pores, which is dependent on the crystalline structure of the carbon cathodes. The exponent n is dependent on the aggregate materials in the carbon cathodes, but can work well with an averaged value of-4.65for all the carbon cathodes under investigation. Moreover, the ρ0can be related to graphite content by the simple rule of mixture in the graphitic cathodes with mixed carbon aggregates of anthracite and artificial graphite.(2) The coupling effects between Na-bath penetration and the microstructures evolution in the carbon cathodes is revealed. In the characteristic parameters of porous structure, the connectivity and coordination numbers of pores have significant influence on the electrolyte penetration, and the mean diameter and the size distribution of pores have secondary influence, while the porosity and the numbers of pores are irrelevant factors. The electrolyte penetrated along the porous structure provides a facile approach and material source for sodium penetration, which excites the sodium penetrating into the carbon cathodes more deeply. Adjusting the porous structure to decrease the connectivity and coordination numbers of pores and make the mean diameter and the size distribution of pores reasonable, can restrict the penetration. Meanwhile, a self-organization behavior of crystalline structure in the carbon cathodes is discovered, which increases the degree of graphitization of the non-penetrated zone inside the carbon cathodes and depresses the sodium penetration.(3) The influencing mechanism of sodium/bath penetration on the resistivity is illustrated for carbon cathodes, combined with the evolution of microstructural and the variation of micro area compositions by the penetration. During the electrolysis, the change of the conductivity of the carbon cathodes is a competing result between the two factors:The penetrated sodium and electrolyte are benefit for improving the conductivity, while the chemical reactions between the penetrated substances and carbon aggregates deteriorate it. The conductive mechanism of carbon cathode materials is conformed to both electron conduction and band theory. And a transformation temperature of conductive mechanism is existed in the carbon cathodes. At low-temperature, the conductivity of carbon cathodes is dependent on the concentration of charge carriers, but it changes to depend on the scattering mechanism of charge carriers at high-temperature. For semi-and full-graphitic cathodes, the penetration increases the concentration of charge carriers and simultaneously weakens the scattering, which improves the conductivity of the cathodes at high-temperature. For graphitized cathode, however, the penetration increases the lattice vibration scattering from the crystal structure, which deteriorates the high-temperature conductivity of the cathodes.(4) The graphitized cathodes have good resistance to sodium/bath penetration but with bad conductivity variation during the electrolysis. In order to solve this problem, external pressure is applied to excite the self-organization behavior of the microstructure in carbon cathodes before the penetration. The best effects are acquired when the external pressure is conducted at ambient temperature, and it is strengthened during the heating process. During the electrolysis, the external pressure associating with the effects of thermal field and the penetration results in an enhanced catalytic graphitization process for the carbon cathodes, which improves both the resistance to the penetration and the high temperature conductivity. The mechanism of the improvements is described that the strengthened catalytic graphitization process increases the concentration of charge carriers, at the same time the external pressure decreases the electrolyte penetration resulting in inhibiting the chemical reactions between the penetrated substances and the carbon aggregates.(5) In order to improve both the resistance to the penetration and the high temperature conductivity of semi-and full-graphitic cathodes, TiB2instead of partial powder aggregates to fill in the binder burnt is proposed. When10wt-%TiB2with the particle size of-200mesh replaces the same amount of carbon aggregates with same size, associating with the external pressure applied before the electrolysis, an improved effect is obtained. Meanwhile, the mechanism of TiB2increasing the resistance to the penetration for the cathodes is worked at three aspects:Firstly, TiB2facilitates the graphitization process by catalysis during the baking stage of the cathodes; Secondly, the penetrated electrolyte is changed from NaF without TiB2to NaF·xAlF3when TiB2is added, in which the latter wetting and spreading harder on the surface of C/TiB2cathodes results in the electrolyte difficult penetration; Finally, liquid Al wetting and spreading very well on the surface of the cathodes results in Al penetrating prior and then filling the porous structure of the cathodes, which interdicts the quick access for sodium penetration and decreases the sodium penetration.