Abstract
Gas bubble behavior on a carbon anode in a cryolite melt has been studied using a see-through cell. The phenomena studied have been growth, coalescence, detachment, and wetting during electrolysis. The surface orientation affects bubble behavior. Therefore, two different anode designs were tested, an anode with a horizontal downward-facing surface and an anode with a vertical surface. At the horizontal anode, it was found that one large bubble was formed by the growth and coalescence of smaller bubbles, and finally, the large bubble detached periodically. For the vertical anode surface, the detaching bubbles were smaller, and most of them had been going through a coalescence process prior to detachment. The bubbles detached randomly. The coalescence process from the initiation to the final bubble shape at the vertical surface took about 0.016–0.024 s. The current density did not affect the duration of the coalescence. The bubble diameter was decreasing with increasing current density for both anodes. The values were in the range 7.2 to 5.7 mm for the horizontal anode in the current density interval 0.2–1.0 A cm−2 and in the range 3.7 mm to 1.5 mm for the vertical anode in the current density interval 0.1–2.0 A cm−2. The wetting contact angle for the vertical anode stayed more or less constant with an increase in current density, which likely can be attributed to the decreasing bubble size rather than an increase in polarization. In addition to the bubble phenomena described and bubble properties found, the impact of the results for better design of laboratory-scale studies is discussed.
Highlights
Industrial production of aluminum is performed by the Hall–Héroult process in which alumina is dissolved in a molten bath consisting mainly of cryolite, which is predominantly used because of its capacity as a solvent for alumina and fairly suitable ionic conductivity.Aluminum-containing bath species are reduced at the aluminum cathode, forming molten aluminum, while CO2 is formed in the anode reaction with oxygen-containing bath species and the anode carbon as reactants [1,2,3]
More details on wetting and contact angles are included in the down to 105° at 1.0 A cm
The anode is shielded with boron nitride and according to Åsheim et al [22], Boron nitride (BN) is better wetted by the cryolite in compari carbon
Summary
Industrial production of aluminum is performed by the Hall–Héroult process in which alumina is dissolved in a molten bath consisting mainly of cryolite, which is predominantly used because of its capacity as a solvent for alumina and fairly suitable ionic conductivity.Aluminum-containing bath species are reduced at the aluminum cathode, forming molten aluminum, while CO2 is formed in the anode reaction with oxygen-containing bath species and the anode carbon as reactants [1,2,3]. Industrial production of aluminum is performed by the Hall–Héroult process in which alumina is dissolved in a molten bath consisting mainly of cryolite, which is predominantly used because of its capacity as a solvent for alumina and fairly suitable ionic conductivity. Gas present at the anode surface contributes to an increased cell voltage. The extra voltage drop in an industrial cell due to the presence of bubbles is about 0.15–0.35 V out of a typical total cell voltage of ~4.5 V [4]. The presence of gas bubbles under the anode contributes to the overall bath resistance because the gas is non-conducting. Models have been developed for calculating the resistance effect based on the average layer thickness and fractional surface coverage of the surface
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