Diffusion Measurements of CO₂ Within Carbon Anodes for Aluminium Smelting

A study of the CO/CO₂ diffusion coefficients within the carbon anode and cryolite was carried out. Gas diffusion experiments were conducted to measure the diffusion coefficient of CO₂ in conjunction with anode characterization using Mercury Intrusion Porosimetry (MIP). The diffusion coefficient value obtained varied from 1.38 × 10⁻⁶ m²/s to 7.89 × 10⁻⁶ m²/s in the temperature range from 25 to 960 °C. In addition, the diffusion coefficient of CO₂ in cryolite at 960 °C was predicted to be 2.74 × 10⁻⁹ m²/s. Two verification studies were attempted to measure the diffusion coefficient in cryolite, but no reliable results were obtained. The results from this study suggest the gas transport for bubble growth is controlled by gas flow in the anode due to slower diffusion coefficient value in cryolite which is three orders of magnitude lower when gas diffusion coefficient for CO₂ through the anode samples.