Anode gas bubble behavior and anode effect on graphite and industrial carbon rod-shaped anode in a cryolite melt have been studied using a see-through furnace. The different carbon materials have different properties which can affect bubble behavior and electrochemical properties. Industrial carbon is more inhomogeneous with respect to structure, pore, aggregates and impurities in comparison to the graphite. More bubbles were nucleated on the industrial carbon than on the graphite for the same current density. The time related to the coalescence process for both anodes was found to be in interval 16 to 24 ms and independent of the current densities. Bubbles detached from the horizontal surface of the anode have similar average diameter value for both anodes for current densities < 1.0 A cm⁻², while for current densities > 1.0 A cm⁻², the average diameter is lower for the industrial carbon anode. The onset of the anode effect occurred faster on the graphite than on the industrial anode. The PFC-containing gas layer appeared to be thicker and more stable on the graphite anode than on the industrial carbon anode.

This work aims to study the CO-CO₂ gas composition at low potentials and low current densities in cryolite melt with relatively low alumina content (≤2 wt%). There is a scarcity of data in the literature regarding the low current density region and also for bath low in alumina. The experimental setup was constructed to minimize the back reaction as well as the Boudouard reaction. For potentials up to 1.55 V and corresponding current densities up to 0.07 A cm⁻², it was found that CO is the dominant product. Between 1.55 and 1.65 V (corresponding current density region 0.07 to 0.2 A cm⁻²), CO₂ becomes the dominant gas product. These potential values are probably slightly large due to suspected Boudouard reaction between CO₂ and carbon particles in the melt formed by disintegration of the graphite anode. The results are discussed in relation to the literature data and thermodynamic calculations.

Identifying optimum anode baking level and mixing temperature are important when producing high quality anodes. The effect of varying mixing temperature and baking level were investigated in terms of the resulting apparent anode density, specific electrical resistivity (SER), air permeability, coefficient of thermal expansion (CTE), air reactivity, and CO₂ reactivity. Six pilot-scale anodes were prepared at Hydro Aluminium using a single source petroleum coke and <2 mm coke fractions. A coal tar pitch was used with Mettler softening point of 119.1 °C. The aggregate was mixed at 150 °C or 210 °C and baked to a low, medium, or high baking level. A 2² full-factorial design analysis was performed to determine the response of the analyzed properties to the applied mixing and baking temperature. Apparent density, SER, and air permeability were found to be highly dependent on mixing temperature. Apparent density and SER were also slightly affected by baking level. CTE was found to be independent of both baking level and mixing temperature. Air reactivity was found to be mainly dependent on baking level, while CO₂ reactivity was dependent on both mixing temperature and baking level.

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.