Diffusion and Flow of CO2 in Carbon Anode for Aluminium Smelting

• Published in: Metallurgical and materials transactions. B, Process metallurgy and materials processing science Process metallurgy and materials processing science, 2019-04, Vol.50 (2), p.846-856
• Main Authors: Putri, Epma, Brooks, Geoffrey, Snook, Graeme A., Eick, Ingo, Lossius, Lorentz Petter
• Format: Article
• Language: English
• Subjects: Aluminum; Anodes; Carbon; Carbon dioxide; Characterization and Evaluation of Materials; Chemistry and Materials Science; Convective flow; Correlation analysis; Curve fitting; Diffusion coefficient; Diffusion effects; Electric potential; Gaseous diffusion; High temperature; Intrusion; Materials Science; Metallic Materials; Nanotechnology; Permeability; Pore size; Porosity; Smelting; Structural Materials; Surfaces and Interfaces; Thin Films; Voltage drop
• ISSN: 1073-5615; 1543-1916
• DOI: 10.1007/s11663-018-1497-z

Using a well-made carbon anode with the right porosity characteristics is essential to the successful operation of a Hall–Héroult cell. Bubble generation at this carbon anode and its contribution to the overall voltage drop in aluminum production holds significant potential for reducing overpotential and improving energy savings. This voltage drop is believed to be greatly influenced by the gas diffusion in the anode carbon, for which there are a limited number of measurements to correlate with the carbon anode properties. In the present study, the CO 2 gas diffusion characteristics were first investigated via measuring anode porosity using mercury intrusion porosimetry (MIP) for different samples with different permeability. Second, diffusion experiments were conducted by flowing CO 2 gas into the anode sample at elevated temperatures (up to 960 °C). The impact of temperature, average pore size, and permeability on the diffusion coefficient was investigated. The diffusion coefficient was consequently calculated by curve fitting using diffusion theories. The value obtained varied from 1.38 × 10 −6 to 7.89 × 10 −6 m 2 /s. A high diffusion coefficient was measured in the carbon sample with larger average pore size and higher permeability. It was found that anomalous diffusion behavior in the temperature range 600 °C to 960 °C was caused by convective flow effects, different diffusion mechanisms, and the Boudouard reaction for these carbon anode samples. Furthermore, by plotting the measured diffusion coefficient against average pore size and permeability, an increasing trend of diffusivity with an increase in pore size and permeability was observed.