Studies Of The Pore Structure And The Cracking Mechanism During Forming Of Carbon Anode Usded In The Aluminum Industry

Carbon anode is the key part of the large-scale aluminum reduction cell, is directly related to the carbon anode consumption, current efficiency, metal aluminum purity and others important technical indicators. In recent years, carbon anode raw materials (especially petroleum coke) quality deteriorates increasingly. On the other hand, the application of the large electrolysis cell with high current density needs the carbon anode with higher quality. In this paper, basing on the systematic summary of previous research works on the control of anode material quality and the process of mixing, forming and baking, the connection beween the carbon materials and anode pore structure was studied. The relationship between the stress distribution and crack of green anode in the vibration forming process and the evolution of pores in baking process were studied. These works provide the scientific basis and technical support for the advanced manufacture of high quality and performance carbon anode, and the low carbon development of aluminum industry.First of all, the influences of the coke calcinations levels on pore structure of carbon anode were studied. Basing on the selective oxidation theory of carbon anode and the methods of the image analysis, mercury porosimetry and confocal microscopy, We find that the decrease of the coke calcinations level can increses the coke shrinkage rate in baking, and thereby redeuces the volume of medium pores, which closed surround the large coke particles and formed by the different shrinkage rates of coke and pitch coke in baking. But the increase of the coke shrinkage rate will reduce the mutual connection between cokes, and lead to generate the large elongated pore between cokes, which easy to develop cracking. When the calcination temperature was 900 ℃, the really density and Lc value of cokes were 1.967-1.985g/cm3 and 1.9-2.0 nm respectively, the conductive, porosity, permeability, air/CO2 reactivity and compressive/shear strength had the optimal values.Secondly, The coupling relationship between the different parameters of mixing process and the pore structure of carbon anode was studied.The studies show that the conductive, compressive/shear strength, density, porosity, air permeability and reactivity of anode have the optimum values when the mixing temperature and time are 170℃ and 35min respectively. Under the optimum mixing temperature and time, the porosity and permeability of the samples decreased with the mixing temperature and time increasing for the better mixing effects. When the mixing temperatures exceeded 170℃, the volumes of pores with diameters of 20-80μm increased for the releasing of the volatiles of coal tar pitch. When mixing time exceed 35 min, the volumes of pores with diameters of 70-150μm increased. These may be attributed to the crush of cokes in mixing. The coupling relationship provides a scientific basis on the optimization of the baking process and the pore structure.Then, the stress distribution of the industrial anode in the vibration forming process was modeled based on the finite element method for the first time. The study shows the stress in green anode reduces continuously from the green anode surface to inward under external loads during the forming process. The area of the minimum stress in the green anode is easy to develop cracking for the weak connection. The cracking probability of green anode decreases with the stress value (center vertices) increasing. The stress value increases with the anode L/W ratio increasing. The relationship between the stress value (center vertices) and cracking probability of green anode was obtained to predict the probability of forming crack in the green anode with different shapes.In addition, the effects of coke and pitch properties, coke size composition, pitch ratio and mixing temperature on the expansion of green anode after forming were studied. The results show the expansion volume of green anode after forming increased with the ratio of pitch in a paste and the mixing temperature increasing. Compared with the medium temperature pitch, the modified pitch provided more restriction on the expansion effect of green anode. When the modified pitch content was large enough, the green anode appeared volume contraction. The expansion was not directly related with the green density, had inversely ratio with the close porosity of coke, and had direct ratio with the real density of coke.Finally, the formation and evolution characteristics of the pores were studied by testing the changes of anode volume and mass in baking with different heating rates. The elongated pores were formed due to the release of stress in the 100-210 ℃ range. The appropriate fast heating rate is conducive to improve the anode permeability, increase the release rate of volatile, and reduce the volume expansion of the anode. When the temperature exceeded 210℃, the anode began to lose weight for the release of the pitch volatiles, and the round pores were formed in the anode; The weight loss of anode reached the maximum value in 410 ℃, and then decreased gradually to zero at 498℃. The proportion of the round pores in the anode reached the maximum value at 498℃. When temperature exceeds the 498℃, the redius and volume of pores formed in the previous process began to reduce for the shringe of pith coke, and the new elonged pores was formed by the different shringe rate of coke and pitch coke. The ratio of elongated pores increased with the baking temperature and the shrinkge rate of pith coke increasing.