PHYSICAL MODELING STUDIES OF ELECTROLYTE FLOW DUE TO GAS EVOLUTION AND SOME ASPECTS OF BUBBLE BEHAVIOR IN ADVANCED HALL CELLS .1. FLOW IN CELLS WITH A FLAT ANODE

The need for energy reduction in the electrolytic production of alumin um led to the concept of advanced Hall cells that can be operated at l ower interelectrode gaps compared to existing cells. However, gas bubb les generated by the anodic reaction increase the resistivity of elect rolyte and cancel out part of the reduction in interelectrode resistan ce expected from bringing the electrodes closer together. Therefore, t he primary objective of this work was to determine a cell design in wh ich flow can be managed to promote the removal of anode gas bubbles fr om the interelectrode gap. In particular, this article focuses on adva nced Hall cells equipped with ''flat'' anodes, similar to those used i n existing cells. The principal experimental tool has been a ''water'' model consisting of a large tank in which simulated anodes can be sus pended in either the horizontal or near-horizontal configurations. Gas was generated by forcing compressed air through porous graphite, and the fine bubbles characteristic of inert anodes used in advanced Hall cells were produced by adding butanol to water. Velocities were measur ed using a laser-Doppler velocimeter (LDV). This study indicates that the existing cell configuration might not be the optimum configuration for advanced Hall cells. The results also show that operation of an a dvanced Hall cell with a fully submerged anode should give rise to hig her electrolyte velocities and thus rapid removal of bubbles. The bubb le effect should be further lowered in a near-horizontal configuration ; however, the flow pattern could have an adverse effect on current ef ficiency and alumina distribution in the cell. It has also been shown that the bubble size, and, therefore, the physical properties of the e lectrolyte, can have a significant effect on the electrolyte flow patt ern in the interelectrode gap.