Bubble Behavior and Anode Effect on Different Carbon Anode Designs in Cryolite Melt Applying a See-through Furnace

Abstract

Anodic CO2 gas evolution is one of the main processes in the Hall-Héroult electrolysis cell for primary aluminium production. This work is a laboratory scale study of the effect of anode geometry, anode orientation and carbon anode material on bubble behavior and bubble properties in cryolite melt. Increased knowledge of the bubble behavior is important for improved design of industrial cells, but also for laboratory‐scale experiments. The anodes studied in this work are frequently applied for investigation of reaction kinetics and mass transport, anode effect, current efficiency, anode quality properties, etc. It is therefore of interest to study the bubble dynamics of these anodes in more detail because bubble evolution is relevant for all the above-mentioned phenomena.The anode potential has been shown to be highly dependent on anode geometry and orientation. For this study four different anode designs were made: horizontal (downward-facing), inverted horizontal (upward-facing), vertical, rod (with both vertical and horizontal surface). For most of the designs, characteristics such as bubble noise, bubble size, and wetting were obtained as a function of current density or anode potential/cell voltage. The bubble noise normally went through a maximum, while the bubble size decreased in the whole current density range. The wetting contact angle showed little variation with current density. It was assumed that the current density had a stronger effect on the contact angle through its effect on the bubble size than the polarization and associated surface roughening.Also, phenomena like nucleation, growth, coalescence, bubble detachment and anode effect were studied for both graphite and industrial carbon as anode material. The nucleation was more pronounced at the industrial carbon due to more active sites confirmed by CT analysis. The coalescence process was found to be independent of current density/potential and carbon anode material and was in the range 16-24 msec. On increased currents anode effect occurred on both anodes, the current being lower for the graphite anode at onset. Video recordings and electrochemical measurements showed more abrupt initiation of the anode effect on the graphite and a strong de-wetting of the anode. With the existence of a (C – F) surface compound on the anode, as long as the anode effect gas could escape the electrode surface, the anode seems to be electrochemically active towards both CO/CO2 and PFC formation during anode effect.In addition to the bubble characteristics and phenomena described, the impact of the results for better anode design for laboratory scale studies is discussed.