Correlation between Coke Type, Microstructure and Anodic Reaction Overpotential in Aluminium Electrolysis

Abstract

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

Bubble Evolution on Different Carbon Anode Designs in Cryolite Melt

“…Thorne et al reported similar trends. [14,15] The inverted horizontal anode has similar behavior to the vertical anode at current densities > 0.5 A cm À2 , i.e., random potential oscillation, and at lower current densities (0.1 to 0.3 A cm À2 for vertical and 0.1 to 0.5 A cm À2 for inverted horizontal anode) saw-tooth features are present. However, inverted horizontal anode has larger noise as discussed below.…”

“…The height for the vertical anode was only 5 mm but still considered to be comparable to the other anodes as the bubbles on the vertical surface have been reported to be relatively small and this design was earlier successfully used. [13,15] Experiments were performed under inert N 2 atmosphere in a cryolite melt. The cryolite was supplied from Sigma Aldrich (purity ‡ 97 pct, cryolite ratio = 3).…”

Anode Overvoltages on the Industrial Carbon Blocks

“…The works of many researchers [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] are devoted to the anodic overvoltage measurements. For a given current density, the overvoltage values differ greatly for differently graphitized materials and for different classes of structural ordering.…”

“…According to the different researchers [10,11,23], there is a significant difference in the magnitude of overvoltage even for one class of carbon materials; the difference in overvoltages can reach more than 300 mV when measured on samples made from different industrial anode blocks. It can be assumed that in different experiments carbon materials of different degree of graphitization, as well as with different content of impurities, were used, which, apparently, affected the structure of surface layers and their reactivity [26][27][28][29][30][31][32].…”

Comparing Anodic Bubble Behavior of Anodes from Anisotropic and Isotropic Cokes in Laboratory Scale Aluminum Electrolysis Cells

“…The exact reason for these observed differences is not known and are beyond the scope of this study. A more detailed investigation of the material properties is provided in a previous work(53), and this showed that the anisotropic coke had a higher ratio of basal surface planes as well as lower amount of surface oxides (0.02 g/m 2 for the anisotropic coke and 0.035 g/m 2 for the isotropic coke). These material property differences observed in the coke may provide an explanation to the observed differences in electrochemical behavior and wetting properties of the anodes.The results obtained in this work indicate that there might be benefits from introducing isotropic cokes in industrial scale cells with respect to a reduction of the cell voltage/voltage oscillation amplitude.…”