| dc.description.abstract | This doctoral work is an experimental laboratory scale study of the effect of anode geometry, anode orientation and carbon anode material on bubble behavior and bubble properties in cryolite melt. The increased knowledge of the bubbles is useful for further studies, especially in laboratory‐scale studies which typically applies similar anode designs. The anodes used in this work are frequently used to study reaction kinetics and mass transport, anode effect, current efficiency, anode quality properties, etc. It is therefore interesting to study bubble dynamics of these anodes in more detail because bubbles are 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). All anodes were through experimental work redesigned, though retaining its original orientation, in order to improve its performance. From polarization curves it was found that the vertical anode and the inverted horizontal anode operated at lowest potentials. Above 1 A cm−2 the vertical anode showed the lowest potential. As the current increases the transition towards smaller noise is pronounced for the horizontal anode and to some degree for the vertical anode and inverted horizontal anode. Only random bubble noise was found for the vertical and the inverted horizontal anode and is probably due to more effective and random bubbles detachment from these surfaces in comparison with the rod and the horizontal 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 in 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 dominant product. Between 1.55 V and 1.65 V (corresponding current density region 0.07 A cm−2 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 the relation with the literature data and thermodynamic calculations.
Gas bubble behavior on a carbon anode in a cryolite melt have been studied using a see-through cell. The phenomena studied have been growth, coalescence, detachment and wetting during electrolysis. The surface orientation affects the bubble behavior. Therefore, two different anode designs were tested, an anode with a horizontal facing-downwards surface and an anode with a vertical surface. At the horizontal anode it was found that one large bubble was formed by 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 16-24 ms. 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 mm 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.
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, pores, 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 the interval 16-24 ms and independent of the current density. 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.
To investigate the effect of different anode size regarding bubble behavior, anodes with 10 mm and 20 mm diameter with a downward-facing horizontal surface were made. It was found that the bubble diameter was decreasing with increasing current density for both anodes. Bubbles detached from the 20 mm anode are larger in size than bubbles detached from the 10 mm anode for the same current density. The thickness of the bubble just before it starts to slide towards edge to be detached from the anode surface, was found to be around 4.6 mm for the 10 mm anode while for the 20 mm anode it was 3.9 mm. At the moment of the anode effect initiation the bubble formation and bubble detachment stopped and the gas layer, a mixture of CO, CO2 and PFC gases, was formed at the surface completely covering the surface thus preventing contact between surface and electrolyte.
Bubble behavior and dynamics on an upward-facing horizontal carbon anode was studied in order to get more detailed understanding of the regular electrolysis and anode effect in the Hall-Héroult process. The presence of weak saw-tooth features for this anode at low current densities was explained by bubble retention time long enough for coalescence into a larger bubble to take place. Different stages of bubble growth were observed: hemispherical spreading, cylindrical spreading, cylindrical growth, and necking. Coalescence was observed only in the hemispherical spreading stage. The evolution of the anode effect depends on the possibility of the gas bubbles and gas layer to be detached from the anode surface. If the gas is trapped the gas layer is covering the anode surface, disabling contact between anode and electrolyte and further gas production is strongly hindered. The anode surface of the inverted horizontal anode after anode effect was found to be still electrochemically active. The gas was not trapped and the gas evolution could continue. Results from this doctoral work supported and also gave new information to the anode effect mechanism regarding formation of surface compounds and/or gas layer insulating the anode surface. It was found that the PFC-containing gas layer only can cover the whole anode surface if the gas is not able to leave the anode. As long as produced gas can escape there is no gas layer completely covering the anode surface. With the existence of a (C – F) surface compound on the anode, the anode seems to be electrochemically active towards both CO/CO2 and PFC formation. | en_US |
