Physical modeling studies of electrolyte flow due to gas evolution and some aspects of bubble behavior in advanced Hall
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INTRODUCTION
T H E need for reducing energy consumption led to the concept of advanced Hall-Heroult cells that could be operated at a lower anode-to-cathode distance (ACD) (presently in the range o f 4 to 5 cm). However, Dorward [1] showed that reduction in the anode to cathode gap (ACG) did not give the anticipated energy savings. Dorward attributed this to the increase in the void fraction of bubbles as the ACD was reduced. This increased the effective resistivity of the electrolyte and canceled part of the energy savings obtained by bringing the electrodes closer, a phenomena referred to as "bubble effect." Consequently, studies were carried out to enhance the removal of the electrolytically generated gas bubbles from the ACG by modifying the existing design of Hall cells. A water model study by Dorward and PayneI2] showed that inclining the electrodes at an angle of 3 deg led to a drastic reduction in interpolar resistance. A slight reduction in cell resistance was also demonstrated in cells equipped with near-vertical electrodes.ISl Shekhar and Evans[4,51 carried out extensive velocity and interpolar resistance measurements in a water model of an advanced Hall cell in three electrode configurations, namely, horizontal, near-horizontal, and near-vertical. Here, the electrolytically generated gas bubbles were simulated by sparging air through a porous graphite plate which simulated the anode. The top surface of a plastic box represented the cathode. In addition to the conventional "flat" anode used in existing Hall cells, they also employed a novel "grooved" anode, details of which can be found in
R. SHEKHAR, Assistant Professor, is with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208 016, India. J.W. EVANS, Professor, is with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted July I8, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
Reference 6. The grooves serve as a means to remove the anode gas bubbles from the ACG. Their study explained the observations of Dorward and Payne.t2.3I Of significance is their conclusion that the existing cell configuration may not be the optimum configuration for low-energy advanced Hall cells. In fact, their investigations suggest that lower energy consumption would be obtained in cells operated with near-horizontal electrodes equipped with grooved anodesISI which were completely submerged in the electrolyte. The rationale for a submerged anode, a concept foreign to existing Hall cells because of the corrosive nature of cryolite, has been dealt with in Reference 5. Also, the changes suggested by Shekhar and Evans were such that they could be implemented in existing cells, an important consideration because Hall cells are capital intensive. Evans and Shekhart6J also observed that a large number of bubbles did not enter the grooves readily but traveled a considerable distance along the extended face of the anode. The avoidance of this phenomeno
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