Study On Carbon Anode Consumption And The Influence Of Ultrasound During Aluminium Electrolysis

Recently, more and more high and super-high ampagere aluminum reduction cells with current of400～600kA are in production, and hence the demand of their low-carbon running attracts increasing attention. Reduction of carbon anodes consumption is of great importance for aluminum electrolysis cell to save energy and promote low-carbon aluminum metallurgical industry. Studies showed that selective oxidation and Bucherer reaction directly affects the carbon anode consumption. However, most of previous studies focused on the improvements in calcining of petroleum coke and kilning process of anode, modification of additives, controlling of electrolysis operating parameters, etc. The mechanism of anode current density affecting the anode porosity and carbon consumption are still clear. There is little knowledge on quantitative characterization of the relationship among current density, pore and carbon consumption. Therefore, this work is aimed to study the impact of the anode current density on pore structure changes of the carbon anode and its correlation with carbon consumption and carbon dusting formation using multiple approaches including digital image analysis. The influence of calcining of petroleum coke and the porous structures on the anode consumption and the selective oxidation will be investigated, while a new method using ultrasound to reduce carbon anode consumption will be tried to reduce the anode carbon consumption.Firstly, the carbon consumptions of typical industrially used carbon block anode with different anodic current density are tested. By image analysis method, the variation of characteristic parameters for surface and internal porosity after electrolysis is quantitatively studied. The relationship between anodic current density, pore structure and carbon consumption is obtained. The results show that the anode consumption and the amount of carbon dusting can be precisely characterized by the obtained parameters from by the image analysis. It is concluded carbon consumption can be reduced by lowering of formation factor, porosity rate, average pore size, connectivity. Uneven distribution of anode current density has a big impact on the formation of carbon dusting. Large pores with size of800～1500μm on the anode are formed due to the formation of carbon dusting mainly by the aggregate particle shedding. The critical pore size by the formation of carbon dusting may be600μm. Amount of carbon dusting in anode edge is greater than that in the central region, which is particularly evident for current density of0.75and1.00A/cm2.Secondly, petroleum cokes are calcined at different temperatures to study the effect of the oxidation activity of anode coke on the carbon consumption behavior. The pore structure, physical and chemical properties and carbon consumption of anodes which are calcined at different levels are analyzed. The results show that when the grain size of calcined coke is in range of17-27A, low calcined anode coke porosity, pore connectivity and specific surface area of porosity decrease gradually with decreasing of the calcination degree of petroleum coke, and the corresponding air/CO2reaction is mainly determined by the pore structure of the anode and does not increase with increasing of oxidation activity of calcined coke. The amount of carbon dusting decreases significantly for low calcined coke in the anodic electrolysis process, the amount of carbon gasification increases and the total consumption reduces. This is mainly due to the changes of consumption mechanism of calcined anode coke for low calcinations. With oxidation activity of calcined coke increasing, the selective oxidation of the anode-electrolyte reaction interface is suppressed, while Bucherer reaction is strengthened in the internal pores.Then, the distribution of pressure field under ultrasound in the electrolyte cell and the formation, development characteristics of electrolyte-anode gas two-phase flow in the initial stage of ultrasonic action are investigated by computational fluid dynamics. The measurement of sound pressure is also carried out. It is concluded simulation and experimental results are in consistence. The results show that the pressure field and velocity field exhibits large gradient in the area near the center of the bottom surface of the carbon anode. It is the main area of the ultrasonic vibration. Flow velocity increases from the center to the edge region along the bottom surface of the anode. The maximum value is1.30m/s, and the minimum value is0.03m/s. There is a substantial attenuation in the vertical direction. It does not cause the disturbance in the cathode area, or increase carbon consumption for one ton of aluminum due to secondary aluminum loss. The results provide a theoretical basis for further study of two-phase flow and distribution in the aluminium reduction cell, and reference data for further revealing the ultrasound-assistance mechanism to reduce carbon anode consumption.Besides, in the ultrasound-assisted aluminum electrolysis process, it is firstly found ultrasound can reduce carbon anode consumption. The effects of ultrasound on the carbon anode consumption behavior are investigated on the premise of not changing the shape and properties of carbon anode. The results show that ultrasound can reduce the carbon anode consumption of5.6%to14.2%with current densities of0.75-1.00A/cm2, and the carbon dusting of52%to86%. Carbon anode porosity, apparent pore surface area, aspect ratio, connectivity are increasing; significantly reduce the uneven consumption between anode underside center and edge. The ultrasound can reduce the consumption of carbon anode through getting rid of the bubbles on the bottom surface, weakening Bucherer reaction (reduction of CO ratio in anodic gas); and on the other hand, anode current density distribution tends to be uniform while bubbles on the bottom surface are removed. The anode-electrolyte interface reaction is more uniform, and thus the amount of carbon dusting can decrease. Besides, it is found that ultrasound works mainly along the axial direction of the anode, which is mainly decided by intrinsic characteristics and loading type of ultrasonic.