A Multi-scale Mathematical Model of Growth and Coalescence of Bubbles Beneath the Anode in an Aluminum Reduction Cell
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NTRODUCTION
IN the modern aluminum reduction cell (as shown in Figure 1), the direct current passes through the anode, molten electrolyte (bath) and metal pad to the nearest cathode collector bar. In this process, the electrochemical reaction occurs as the alumina is fed to and dissolved in the molten electrolyte layer at a high temperature of approximately 1223 K. Tiny bubbles are generated under the anode and then grow because of mass transport and coalescence. After that, the larger bubbles leave the nucleation site in a slow and creeping motion. Finally, they move toward the anode edge and release from the anode bottom. The bubbles have both positive and negative effects, such as:
MEIJIA SUN is with the School of Metallurgy, Northeastern University, Shenyang, 110819, Liaoning, China and also with the State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming, 650093, China. BAOKUAN LI is with the School of Metallurgy, Northeastern University. Contact e-mail: [email protected] LINMIN LI is with the College of Energy and Electricity, Hohai University, Nanjing, 210098, Jiangsu, China. Manuscript submitted November 18, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
(a) Improving the mixing and dissolution rates of the alumina in the bath. (b) Promoting the heat transfer between the bath and sidewall. (c) Reducing the contact area between the bath and anode, reducing the electrochemical reaction rate. (d) Increasing the electrical resistance in the anode-cathode distance (ACD) resulting in the increase of voltage drop. (e) Increasing of the bath perturbation due to bubble detachment from the anode bottom, resulting in MHD instability. To take full advantage of the bubble benefits and minimize its negative effects, a better understanding of bubble dynamics in the bath is necessary. The bubble motion is difficult to measure by ordinary means because of the harsh operating conditions with high temperatures in the aluminum reduction cell. Many physical experiments[1–9] have been done to investigate the bubble dynamic in the air-water system. Fortin et al.[1] used a full-scale water model of the aluminum reduction cell to study the bubble behaviors. The results indicated that an increase in current density increases the bubble size, bubble thickness and bubble velocity. A higher electrolyte velocity decreases the bubble size and the gas coverage, while the bubble velocity and release
frequency increase. Vekony et al.[2] developed a real-size air-water electrolysis cell model with two inclined anodes to study the bubble layer under the anode. It was found that the maximum height of bubble is up to 2 cm because of the presence of Fortin bubbles. Simonsen and Einarsrud[3] studied the bubble coalescence and movement under an inclined surface in a water model of a laboratory-scale electrolysis. The surface tension is increased by adding NaCl. The bubble size in the NaCl solutions is larger than that in the water. Moreover, the NaCl solution appears to be close to a self-organized state. Alam et a
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