Espen Sandnes

Anode Processes on Carbon in Chloride-Oxide Melts

Alkali and alkaline earth metals such as lithium, sodium, magnesium and calcium are produced by electrowinning in chloride melts. Direct cathodic reduction of metal oxides in chloride melts has been proposed for the FFC process. Fundamental data for the anodic process on carbon in mixed chloride/oxide melts are lacking. Anodes of graphite and vitreous carbon (VC) were studied in NaCl-NaO, and in binary melts of the type NaCl-MCl2-MO with 0-7 mol% oxide, (M = Ca, Sr and Ba), using linear polarization, CV and EIS. The gaseous products were Cl2, CO and CO2 and the exit concentration of carbon oxides were analysed by gas chromatography. In this paper the emphasis is on the NaCl-CaCl2-CaO system.

Anodic Voltage Oscillations in Hall-Heroult Cells

Experiments on lab- and industrial scale cells have been conducted in order to study the behaviour of anodic gas bubbles under various operating conditions. Traditional voltage measurements have been supplied with high-speed video recordings of the bath surface showing a good correspondence between voltage fluctuations and escaping gas bubbles. On average, 0.5 and 2 bubbles were observed per second in each respective case. Average frequencies obtained by a FFT of the voltage signal however show significantly lower values, approximately half of that observed. It is shown that this discrepancy can be due to large variations in the bubble release times. Observed bubble events can be related to FFT frequencies by means of a frequency based on statistically significant periods. For industrial anodes, the possibility of overlapping bubbles is investigated as alternative effect resulting in the mismatch between observed and calculated frequencies.

Anode Effect Initiation during Aluminium Electrolysis in a Two-Compartment Laboratory Cell

Most laboratory cells used in the investigation of the alumina reduction process use a single anode. When investigating the initiation of the anode effect an approach with more than one anode might give better results, as the probability of obtaining partial anode effect is higher. Additionally, the design is closer to the industrial, where several anodes are connected in parallel. The system constructed consisted of two anodes in separate electrolyte compartments connected in parallel with a single combined cathode. The results indicate that an anode can go in and out of partial anode effect with little influence on the current, although, kept untreated a full anode effect is likely imminent. The results also show that under certain current and alumina conditions, with only two anodes in parallel, an anode can handle approximately the whole load of a fully passivated anode for a certain time.

Partial Anode Effect in a Two-Compartment Laboratory Alumina Reduction Cell

Most laboratory systems investigating the aluminium production process utilize a single anode set-up. When approaching alumina depletion under constant current conditions in such a system, the potential will increase to high levels (>10 V) and initiate an anode effect and perfluorocarbon generation. However, it has been discovered by industrial measurements that perfluorocarbon generation may also occur at normal cell voltages. With the use of a two-anode setup in parallel with an electronic load this phenomena was investigated in the laboratory. The results indicate that as long as the rest of the cell can acquire the extra load, partial passivation of one or more anodes is possible and can be accompanied by small amounts of PFC evolution (0–3 ppm mol CF4). Individual anode potentials can be highly elevated, albeit the changes get buried in the total cell voltage. Only when the total load becomes too large the voltage rises abruptly and substantial amounts of PFC can be produced (≫1000 ppm mol CF4).