Formation of CO2 and CO on Carbon Anodes in Molten Salts

• Published in: Molten Salts Chemistry and Technology pp. 1 - 8
• Main Authors: Thonstad, J, Sandnes, E
• Format: Book Chapter
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 23.05.2014
• Subjects: Chloride melts; CO/CO2 formation; Fluoride melts
• ISBN: 9781118448731; 1118448731
• DOI: 10.1002/9781118448847.ch1a

Aluminium electrolysis is conducted in NaF‐AlF3‐Al2O3 melts at around 960 °C, using carbon anodes. Several carefully conducted laboratory investigations have shown that the primary anode product is CO2, in spite of the fact that CO formation is thermodynamically favoured. However, the off‐gas from the electrolysis cells contains about 20 % CO, due to secondary reactions between CO2 and the cathode product (Al) and carbon particles suspended in the electrolyte. In 1936 Drossbach [1] postulated that the primary anode gas composition is determined by the chemical equilibrium, CO + 1/2 O2 = CO2. Taking into account the very high anodic polarisation, which as long as equilibrium conditions are established, raises the partial pressure of oxygen and shifts the reaction to the right, it was concluded with support from experimental gas analysis that the primary anode product is CO2. However, the same reasoning can apparently not be applied to electrolysis in NaCl‐Na2O, NaCl‐CaCl2‐CaO and NaCl‐SrCl2‐SrO melts. A recent work in these melts by Sandnes et al. [2] revealed that the primary product contained more CO than CO2 for oxide concentrations comparable to those applied in industrial aluminium electrolysis.