Concentrations of dissolved rare earth metal oxides, Nd2O3, and Pr2O3 or their mixtures in different fluoride electrolytes composed of NdF3, PrF3, and LiF at ca. 1040 °C were monitored using a graphite probe inserted into the electrolyte during the dissolution process. Fast voltage sweeps of 100 V/s were applied to the graphite probe, and the current response was measured. As the oxide concentration in the diffusion layer towards the electrode depletes, a passive layer is, at a certain point, formed on the probe, resulting in a current drop. The magnitude of the peak current attained before the formation of the passive layer reflects the concentration of the dissolved oxide and, thus, is applied to determine the oxide concentration. The oxide concentration in the electrolyte samples determined using the inert gas fusion technique showed a good correlation to the peak current determined by the probe.

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.

Autopsies of six spent potlinings with different carbon cathode block grades, amperage regimes and cell design were conducted at three separate smelters to reveal possible mechanisms causing cathode wear. Microstructure of the cathode samples from the autopsies was investigated by optical and electron microscopy and X-ray computed tomography, while the composition of solid bath at the surface and in the interior pores were investigated by Xray diffraction and electron microscopy. The present findings revealed that the cathode surface was characterized by a wear pattern resembling pitting corrosion, and it is discussed that the observed variations in the bath chemistry plays a major role for the wear mechanism and the pitting of the surface. A hypothesis involving initiation and termination of the main reaction causing the cathode wear is proposed based on the effect of the consumption of aluminium fluoride in the molten bath layer between the carbon and the molten aluminium pad, resulting in partly solidification of the bath and spatial variation of the current density.

Reduced fossil carbon footprint is currently a hot topic in the metalproducing industries. Replacement of fossil carbon with bio based equivalents is thus an important topic also for the primary aluminium industry. Several challenges related to biocarbon have been discussed in the literature as the quality and performance of carbon materials made from bio based sources tend to be inferior to the fossil based materials, unless they undergo expensive treatments for adaptation to the aluminium industry. The most likely successful scenario appears to be a gradual introduction of biocarbon into the fossil based carbon materials already being used. Strategies being considered include replacement of the fossil binder material with a bio based binder as well as introducing small amounts of bio-coke in the production of carbon anodes and ramming paste. The current paper reviews research works on candidate bio based carbon materials in the aluminium industry. Sustainability of biocarbon with respect to availability is also discussed.

Autopsies of six spent potlinings with different carbon cathode block grades, amperage regimes and cell design were conducted at three separate smelters to reveal possible mechanisms causing cathode wear. Microstructure of the cathode samples from the autopsies was investigated by optical and electron microscopy and X-ray computed tomography, while the composition of solid bath at the surface and in the interior pores were investigated by Xray diffraction and electron microscopy. The present findings revealed that the cathode surface was characterized by a wear pattern resembling pitting corrosion, and it is discussed that the observed variations in the bath chemistry plays a major role for the wear mechanism and the pitting of the surface. A hypothesis involving initiation and termination of the main reaction causing the cathode wear is proposed based on the effect of the consumption of aluminium fluoride in the molten bath layer between the carbon and the molten aluminium pad, resulting in partly solidification of the bath and spatial variation of the current density.