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.

Carbon anodes for aluminium production are produced from calcined petroleum coke (CPC), recycled anode butts and coal tar pitch (CTP). The CO 2 produced during anode consumption contributes to a substantial amount of the CO 2 footprint of this industrial process. Charcoal from wood has been suggested to partly replace coke in anodes but high porosity, low electrical resistivity and high ash content contributes negatively to final anode properties.In this work, charcoal from Siberian larch and spruce was produced by heat treatment to 800 °C, 1200 °C and 1400 °C and acid-washed with H 2 SO 4 . Acid-washing resulted in reduced metal impurity and the porosity decreased with increasing heat treatment. Pilot anodes were made from CTP, CPC with some additions of spruce and larch charcoal. Another set of pilot anodes were produced using a green binder. Compared to reference anodes, the CO 2 reactivity of anodes containing larch was less affected compared to anodes containing spruce. The green binder was found to be highly detrimental for the anodes' CO 2 reactivity properties. Electrochemical consumption increased for anodes containing both green binder, larch and spruce compared to the reference anode.

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

No abstract

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.