Porous Carbon Anodes for the Supply of Methane during Electrowinning of Aluminium

Currently consumable carbon anodes are utilized in the aluminium production process. The major drawback of this type of anodes is the high amount of CO₂ emissions. Furthermore, due to the consumption of the anodes during the electrolysis it is necessary to adjust the anode – cathode distance continuously and finally replace the anode. Introduction of hydrogen or natural gas through the anode can decrease the CO₂ emissions significantly and reduce the consumption of the anode. The use of inert porous anodes is interesting since the energy demand will be reduced due to natural gas/hydrogen participating in the anode reaction. However, due to unavailability of a sufficiently inert anode for aluminium electrolysis until now, carbon anodes seem to be more attractive. When natural gas/hydrogen is purged through the porous carbon anode, there will be competing anodic reactions depending on whether carbon from the anode or the CH₄/H₂ reacts with the oxygen ions from the bath. Previous studies using porous carbon anodes showed 0.3 V depolarization as well as 20% decrease in carbon anode consumption using methane compared to passing argon through the anode¹. On the other hand, using CH₄/H₂ can lead to the formation and evolution of additional hydrogen fluoride. This was studied before and some ideas on how to deal with this extra HF evolution were presented². In this study, further laboratory experiments were performed and the effect of carbon anodes with different porosities and designs were studied. Besides, off-gas analysis was performed and HF evolution will be discussed.

References:

1. S. Xiao, T. Mokkelbost, O. Paulsen, A.P. Ratvik, and G.M. Haarberg, Metallurgical and Materials Transactions, 44B, (2013) 1311.
2. T. Mokkelbost, O. Kjos, O. Paulsen, B. Øye, H. Gudbrandsen, A.P. Ratvik, and G.M. Haarberg, Light metals (2014), 756.