| China’s capacity of aluminum production leads the world, and the aluminum electrolysis process is energy intensive. Therefore, the technology for saving energy is the important direction for current research. The anode reaction generates gases which can react with the aluminum dissolved in cryolite melts, and reduce the current efficiency and rise the cell voltage. To reduce the cell voltage, researchers try to use the slotted anode, sloped anode and loading the ultrasound on the anode. The core concept of above-mentioned methods is to accelerate the bubbles escaping from the anode, and short the time stayed underneath the anode. In previous work, there were little research about the effects of the frequency and intensity of ultrasound, change of anode-cathode distance, distribution of bubbles on anode and their effect on current efficiency. For the above-mentioned problems, a systematic research should take place.Firstly, to aim at applying of ultrasound in electrolytic process, the effect of ultrasound frequency was studied. The results show that, lower frequency (20kHz) ultrasound can make better effect.Secondly, it is difficult to observe the bubble at high temperature about930-960℃, the distribution of bubbles’ diameter, area percentage and coverage were studied at room temperature. The results showed that the maximum diameter of bubble and coverage increased with the current density increasing when the current density was lower than0.2A/cm2. The maximum diameter of bubble and coverage had no remarkable change with the current density increasing when the current density was higher than0.2A/cm2. At0.05A/cm2, the maximum diameter of bubble and coverage increased slightly with decreasing the anode-cathode distance (ACD). At0.2A/cm2, the maximum diameter of bubble and coverage had no pronounced rule with ACD. It showed that there was little relationship between the maximum diameter or coverage of bubbles and ACD when the current density was higher.Furthermore, due to the predominant effect of ultrasound on anodic bubbles, for the optimal input power of ultrasound and ACD, the bubbles’ behavior and cell voltage was tested at room temperature. The results showed that, the regional coverage of bubbles on anode decreased and the reduced value of cell voltage became higher with increasing ultrasound intensity. The bubbles were eliminated completely when the ultrasound intensity was hither than0.027W/cm2, continued to increase the ultrasound intensity, the effect of reducing the cell voltage was weaken. At the ultrasound intensity0.022W/cm2, the region coverage increased with the ACD decreasing, indicated that the static pressure hindered the bubble escaping from the anode. The bubbles distribution on the anode and coverage not only has an important significance on saving energy and improving current efficiency, and also play an important role of simulating the magnetic field and flow field.The systematic research of the ultrasound effect on cell voltage was made in aluminum electrolytic process. The results showed that the reduced values of cell voltage increased but amplification was lower with the ultrasound intensity increasing. In giving ultrasound intensity, the reduced value of cell voltage decreased with shorting ACD. The effective resistivity of cell decreased with the current density increasing, and The effective resistivity of cell can be lower by introducing ultrasound. On the one hand, it can have no influence on heat balance to ensure the cell stable operation, on the other hand, provide a technical method and theoretical basis for increasing capacity in the same equipment.Based on above-mentioned results, the effects of ultrasound and ACD on current efficiency were studied. The results showed that at0.75A/cm2, the current efficiency increased by introducing ultrasound. The improved effect of current efficiency was about2%by ultrasound and had little relation with changing ACD. Furthermore, the coupled mass transfer model for CO2bubbles growth at the aluminum electrolytic anode and floating of the bubbles which escaped from the bottom of the anode was constructed. CO2mass transfer rate was bigger when the diameter of bubble consisted a certain gas was smaller, and caused more loss of current efficiency. Mass transfer rate in bubbles’growing process is more than floating process. Current density has pronounced influence on the mass transfer rate in growing process, and little influence on the mass transfer rate in floating process. When mass transfer rate of gases is close to loss rate of aluminum, current efficiency (CE) doesn’t change remarkably with the anode and cathode distance (ACD) descending, but in experimental data current density decreased obviously with the ACD decreasing. At ACD35-40mm, the calculated values is close to the measured values. When the ACD is lower than35mm, calculated values is smaller than the measured values, the other factors related to the CO2mass transfer must be considered to predict the current efficiency exactly. |
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