Bubble Behavior on Horizontal and Vertical Carbon Anode Surfaces in Cryolite Melt Applying a See-Through Cell

Abstract

Gas bubble behavior on a carbon anode in a cryolite melt have been studied by direct observation using a see-through cell. The bubble phenomena studied have been growth, coalescence and detachment during electrolysis. The anode geometry and surface orientation affect bubbles behavior. Therefore, two different anodes were tested, an anode with a horizontal facing-downwards surface and an anode with a vertical surface. Galvanostatic and potentiostatic measurements were performed for different current densities and different potentials with simultaneous video recording. At the horizontal anode for a constant current density/potential it was found that one large bubble was formed by growth and coalescence of smaller bubbles and finally the large bubble detached periodically. The frequency of the bubble release events observed from the video recordings was in agreement with the dominant frequency from the Fast Fourier Transform (FFT) analysis. For the vertical anode surface smaller bubbles were formed and detached either due to being pushed by the formation of other bubbles or by coalescence obtaining enough buoyancy. FFT analysis gave no dominant frequency. The diameter of detached bubbles from the horizontal surface and vertical surface was measured. The value was in a range 5.7 mm to 7.2 mm for the horizontal surface and in a range 1.5 mm to 3.7 mm for the vertical surface, strongly depending on the applied current density. The bubble diameter was decreasing with increasing current density for both surfaces. The smaller bubble diameter might be explained by a larger bubble induced convection and increased wetting.