Anodic Bubble Behavior and Voltage Drop in a Laboratory Transparent Aluminum Electrolytic Cell
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THE Hall-He´roult process is the only way for primary aluminum production at the industrial scale, since its invention in 1886.[1] In such a process, alumina powders are fed to and dissolved in molten cryolite at approximate 1223 K (950 C) in aluminum electrolytic cells, in which a direct current up to 600 kA flows through submerged anodes via bath layer and metal layer to the underlying cathodes. The current plays two necessary roles in this process: (a) aluminum is electrically reduced from alumina and (b) Joule heat is generated to maintain the cell temperature. The anodic bubbles are generated at the anode bottom from anodic chemical reaction, released out of anode edges and rose in side channel in a cyclic pattern. The bubble motion is a main driver for the circulation of electrolyte, by which the alumina is transported from the
ZHIBIN ZHAO, Ph.D. Candidate, is with the School of Materials and Metallurgy, Northeastern University, Mail Box 117, Shenyang 110819, China, and also with the CSIRO Mineral Resources Flagship, Clayton, VIC, 3168, Australia. ZHAOWEN WANG, BINGLIANG GAO, and ZHONGNING SHI, Professors, and XIANWEI HU, Associate Professor, are with the School of Materials and Metallurgy, Northeastern University Contact E-mail: blgao@ mail.neu.edu.cn YUQING FENG, Senior Research Scientist, is with the CSIRO Mineral Resources Flagship. Manuscript submitted July 12, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
feeding holes to anode bottom for chemical reaction and the joule heat is carried to the vessel wall to maintain the delicate heat balance. The anode bottom surface is always partly covered by gas bubbles. It was reported that the bubble coverage ranged from 24 to 90 pct[2] and the thickness of bubble layer is about 5 mm[3,4] resulting an extra voltage drop of about 0.15 to 0.35 V[3] due to the high electrical resistance of these bubbles. In addition, the release of bubbles causes some wave-like variations on metal-bath interface that may reduce the current efficiency and cell stability. While the presence of bubbles is an inherent phenomenon in aluminum electrolytic process, a detailed understanding of the bubble dynamics is necessary for quantitatively assess its relevance on cell performance. The most direct way of assessing bubble dynamics is industrial measurement (IM). Haupin et al.[3] positioned a voltage probe under the anode and moved it upward slowly from the metal layer to the anode bottom. Based on the change of voltage drop between the probe and anode rod, the thickness of bubble layer and extra voltage drop can be obtained. Richards et al.[5,6] estimated the anodic gas coverage by calculating the ratio of nominal current density to the real current density in commercial cells. The effects of cell design (e.g., a slot on anode) and cell operation (e.g., alumina concentration) on gas coverage were discussed. However, restricted by the high temperature, heavily corrosive environment and the opaque nature of the molten salt, industrial
measurements particularly for detailed bubble dyn
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