Study on the Inter-electrode Process of Aluminum Electrolysis
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TRODUCTION
IN the Hall–Heroult process, it is generally accepted that the major loss in current efficiency (CE) is due to the ‘‘back reaction’’ of product aluminum and CO2. A fraction of the product aluminum dissolves in the electrolyte and becomes re-oxidized by the dissolved anode gas/anode bubbles.[1] Main reaction : 1=2Al2 O3 þ 3=4C ¼ Al þ 3=4CO2 ½1
Back reaction : Al þ 3=2CO2 ¼ 1=2Al2 O3 þ 3=2CO ½2 To obtain the optimum energy efficiency during industrial production, the ACD is a compromise between high CE and low electrolyte voltage drop. With low ACD, a sharp decrease in CE occurred due to both high mass transfer coefficient (caused by the faster gas flow rate) and increased amplitude of the metal waves. Once the dissolved aluminum contacts with CO2, the back reaction occurs leading to the loss of CE. The rate of back reaction during electrolysis raises a question about the thickness, properties, and stability of boundary layers existing in Hall–Heroult cell, such as the metal/bath boundary layer, the anode/gas boundary layer for understanding the potential of low ACD for CE improvement.
YOUJIAN YANG, Ph.D. Candidate, BINGLIANG GAO, ZHAOWEN WANG, and ZHONGNING SHI, Professors, and XIANWEI HU, Associate Professor, are with the School of Materials and Metallurgy, Northeastern University, Mail Box 117, Shenyang 11004, P.R. China. Contact e-mail:[email protected] Manuscript submitted on November 22, 2014. Article published online November 12, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
During the electrowinning of aluminum, it is observed that the so-called aluminum fog forms near the cathode.[2,3] The formation of the colored solution is due to the nucleation of liquid aluminum from electrolyte oversaturated with metal. Since the electrolyte is well mixed, this visual phenomenon is also explained as the concentration gradients with respect to the dissolved metal which only exists in the cathode boundary layer.[4] For CE considerations, the concentration gradients are important since the loss of CE can be related to the transfer of dissolved metal or electrons through that boundary layer.[1] However, the metal flows with an average velocity of 0.1 m s1 in an industrial aluminum cell,[5] and the heights of gas-induced waves at the bath–metal interface in industrial cells are in the magnitude of centimeters.[6] Rolseth has shown that the amplitude of the metal waves increased as the ACD was reduced.[7] The position of the cathode boundary layer as well as the position of the gas bubbles in the inter-electrode space would sooner or later coincide at low ACD, each contact of the two gives a certain amount of metal loss with Reaction [2]. The CE loss is then proportional to the probability of this coincidence. Another interesting phenomenon at the cathode boundary layer was proposed by Solheim referring to the crystallization of cryolite and alumina. Since the electrolyte facing the cathode during electrolysis contains less aluminum fluoride than the bulk due to the diffusion processes in the aluminum cathode bou
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