Tommy Mokkelbost

Depolarised gas anodes for aluminium electrowinning

Consumable carbon anodes are used in the electrowinning of aluminium by the Hall-Heroult process. Emissions of CO2 may be eliminated by introducing an inert oxygen evolving anode, which however will require a higher anode potential. An alternative approach is to use a natural gas or hydrogen gas anode to reduce the CO2 emissions and lower the anode potential. Preliminary laboratory experiments were carried out in an alternative molten salt electrolyte consisting of CaCl2-CaO-NaCl at 680 °C. Porous anodes of platinum and tin oxide were tested during electrolysis at constant current. The behaviour of inert anode candidate materials such as tin oxide and nickel ferrite were also studied.

SnO2-based gas (hydrogen) anodes for aluminum electrolysis

Abstract A novel SnO 2 -based gas anode was developed for aluminum electrolysis in molten cryolite at 850 °C to reduce energy consumption and decrease CO 2 emissions. Hydrogen was introduced into the anode, participating in the anode reaction. Carbon and aluminum were used as the cathode and reference electrodes, respectively. Cyclic voltammetry was applied in the cell to investigate the electrochemical behavior of oxygen ion on platinum and SnO 2 -based materials. The potential for oxygen evolution on these electrode materials was determined. Then, galvanostatic electrolysis was performed on the gas anode, showing a significant depolarization effect (a decrease of ∼0.8 V of the anode potential) after the introduction of hydrogen, compared with no gas introduction or the introduction of argon. The results indicate the involvement of hydrogen in the anode reaction (three-phase-boundary reaction including gas, electrolyte and electrode) and give the possibility for the utilization of reducing gas anodes for aluminum electrolysis.

Depolarized Gas Anodes for Electrowinning of Aluminium From Cryolite-Alumina Melts in a Laboratory Cell

Consumable carbon anodes are used in the electrowinning of aluminium by the Hall-Heroult process and in other proposed processes for electrowinning in molten salts. Emissions of CO2 may be eliminated by introducing an inert oxygen evolving anode, which however will require a higher anode potential. By introducing natural gas or hydrogen to the anode the CO2 emissions can be reduced and the anode potential can be lowered. Laboratory experiments were carried out in a modified Hall-Heroult electrolyte with excess AlF3 at 850 °C. Anodes of platinum, tin oxide and graphite were tested during electrolysis at constant current, with the supply of argon, methane and hydrogen through or at the anodes.

Gas Anodes Made of Porous Graphite for Aluminium Electrowinning

One of the major downsides of the current aluminium production process is the high CO2 emission. One alternative is to replace the consumable carbon anodes with inert anodes so that oxygen evolves instead of CO2. Also PFC emissions will be eliminated by using inert anodes. However, so far a sufficiently inert anode has not been found. Another option is to utilize natural gas through porous anodes in order to change the anode process. This will decrease CO2 emission remarkably and also eliminate PFC emissions and anode effect. The porous anode could be made of carbon or it can be inert. However, the as-mentioned problem still exists regarding porous inert anodes. Therefore, at the moment porous carbon anodes seem to be the best practical option. In this study, porous anodes made of different grades of graphite were used for electrolysis experiments in a laboratory cell. Also, off-gas analysis was performed to get an insight of the ongoing reactions. Our results show that for some types of graphite anodes, methane participates effectively in the anodic reaction.