One significant contribution to the anodic potential during aluminium electrolysis is the formation of CO 2 bubbles that screen the anode surface. This effect creates an additional ohmic resistance as well as an increased reaction overpotential, hyperpolarisation, as the effective surface area decreases. This work aims to improve the understanding of how anode properties -including isotropy at the optical domain level, wettability (towards electrolyte), surface roughness and porosity -affect bubble 2 evolution. Pilot anodes, made with single source coke types varying in isotropy, were used to study bubble evolution by electrochemical methods. In order to retain bubbles during experiments, anodes were designed to have only horizontal surface area.Bubble formation and release were monitored at different current densities, and were tracked by measuring the oscillations in anode potential and series resistance. Anodes made from different cokes were found to have different bubble evolution properties, possibly due to variation in the density of nucleation sites at the surface of each anode and varying anode-electrolyte wettability.

20Although the anode process during aluminium electrolysis has a substantial 21 overpotential that increases the energy demand and production cost of aluminium, 22properties of the coke that can influence the electrochemical reactivity in the 23 industrial anode itself have not been well documented.In this work the 24 electrochemical performance of anodes fabricated from single source (anisotropic and 25 2 isotropic) cokes, including an ultrapure graphite as reference material, was 26 determined, and compared to the material properties of the cokes and baked anodes. 27Cokes and anodes were characterised with respect to air and CO 2 reactivity, optical 28 texture, presence of oxygen surface groups, as well as to microstructure (fractions of 29 basal, edge and defect sites on the surface and pore volume below 16 nm). Results 30show that anodes made from more isotropic cokes (increasing optical texture 31 fineness) had a slight improvement in the electrochemical performance compared to 32 those made from more anisotropic cokes. For all anodes, electrochemical reactivity 33 correlated well with the electrochemically-wetted surface area, as determined by the 34 double layer capacitance. This appears to be related to microstructure and the volume 35 of pores with width below 16 nm, and possibly also to differences in surface 36 chemistry, rather than differences in surface roughness and porosity as determined by 37 optical techniques (i.e. on a μm-scale). 38 39 40

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Availability of anode raw materials in the growing aluminum industry results in a wider range of petroleum cokes being used to produce carbon anodes. The boundary between anode grade cokes and what previously was considered non-anode grades are no longer as distinct as before, leading to introduction of cokes with higher sulfur and higher trace metal impurity content in anode manufacturing. In this work, the chemical nature of sulfur in five industrial cokes, ranging from 1.42 to 5.54 wt pct S, was investigated with K-edge XANES, while the reactivity of the cokes towards CO2 was measured by a standard mass loss test. XANES identified most of the sulfur as organic sulfur compounds. In addition, a significant amount is identified (16 to 53 pct) as S-S bound sulfur. A strong inverse correlation is observed between CO2-reactivity and S-S bound sulfur in the cokes, indicating that the reduction in reactivity is more dependent on the amount of this type of sulfur compound rather than the total amount of sulfur or the amount of organic sulfur.

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To investigate cathode wear, an autopsy of a shutdown aluminium electrolysis cell was conducted. The original lining consisted of a fully impregnated and graphitized carbon block and the cell was shut down after 2461 days operation. The cell was cleaned down to the surface of the carbon cathode, revealing the profile of the cathode wear. Generally, the cathode wear was uneven across the cell with typical potholes. At a finer length scale, the wear was characterized by small "pitholes" resembling wide shallow pitting corrosion. Samples of the cell lining were obtained by drilling cylindrical samples at different locations in the cell. These samples were analysed with respect to phase composition and microstructure by a combination of X-ray computed tomography, optical and electron microscopy. The findings are discussed in relation to the current understanding of the underlying mechanism(s) for cathode wear.

A hard carbon build-up layer often forms on the flue wall surface in anode baking furnaces. The layer accumulates over thermal circles and needs to be mechanically removed regularly to ensure sufficient space for the anodes between flue walls. The underlying mechanisms are still unknown and the extent of the carbon build-up varies from plant to plant. The build-up on the flue wall, taken from an autopsy of an open top furnace, has been examined. Microstructure and phase compositions of the carbon build-up, especially towards the refractory interface, were studied by optical microscopy, X-ray computed tomography (CT), SEM/EDS, and XRD. Pyrolytic carbon was found to be the main part of the carbon build-up layer in addition to packing coke particles. The transport of silicon from the refractory material, condensating on the flue wall surface, is found as nucleation sites for the formation of carbon build-up. Formation mechanisms of the carbon build-up are proposed with reaction schemes supported by thermodynamic calculations.

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