Samuel Senanu

Characterization of the off-gases from a 500 A inert electrode aluminum electrolysis cell operating at a temperature of ca. 800 °C has been conducted using characterization units comprising gas chromatography, tuneable diode laser spectroscopy, and Fourier-transform infrared spectroscopy. The off-gases detected from the inert electrode aluminum electrolysis cell have been compared to the off-gases released from the traditional Hall–Héroult aluminum electrolysis cell. All the measurements from the inert electrode cell confirmed oxygen as the main process gas during the electrolysis process. Hydrogen fluoride, HF, which is assumed to result from reactions between moisture in the alumina raw material or the surrounding air and the fluoride melt, was also observed together with gases assumed to come from air introduced, such as nitrogen and CO ₂ . There were no indications that these gases came from the production process. A very small amount of SiF ₄ was also detected. Graphical Abstract

Biochar from seaweed, specifically sugar kelp, cultivated on the Norwegian coastline has been investigated as a possible biocarbon source for the metal industry to reduce the dependency on terrestrial biomass. Pre-processing of the biomass prior to pyrolysis is needed to reduce the water and ash content and was performed by water and acid washing followed by drying. The three types of biochar were obtained after pyrolysis at a temperature of 550 °C. Characterization of the three batches of biochars showed that pre-processing of the seaweed as was done during the water and acid washing, plays an important role on the removal of ash content. Due to the enormous amount of woody biomass needed for example in the ferroalloy industry to replace fossil coal, replacing only parts of the woody biomass with kelp biochar could have a significant impact. Water washing combined with acid washing had the best results considering the ash and fixed carbon contents. Microstructural analysis of the seaweed biochars showed a very porous material with the crystal structure resembling that of charcoal, albeit a lower degree of crystallinity. Graphical Abstract

Reliable monitoring of oxide concentrations in electrowinning of metals in fluoride-based electrolytes is very important for avoiding unwanted reactions, notably perfluorocarbons emissions which are very potent greenhouse gases. A probe, consisting of a working electrode and a counter electrode, has been developed for online monitoring of metal oxide concentrations in molten fluoride electrolytes. During measurements, the immersed probe in the electrolyte is subject to fast anodic polarisation sweeps while the electric current response is recorded. During anodic polarisation, oxide species in the electrolyte are rapidly depleted at the probe, resulting in a response called anode effect on the probe tip, triggering a rapid voltage increase and current drop. The rate of oxide depletion and the consequent anode effect is proportional to the dissolved oxide concentration. This allows to correlate voltage-current–time curves to the oxide concentration. Measurements in cryolite melts with different oxide concentrations showed different voltages at which anode effect occurs.

Electrolysis experiments to produce Al-Sc alloys were carried out in galvanostatic mode using a cryolitic melt with a NaF/AlF₃ molar ratio of 2.2 at 980 °C, using both synthetic and waste feeds. After elucidation of the cryolite electrolyte bath chemistry when adding Sc₂O₃, small-laboratory scale trials allowed for the demonstration of the process and the study and for the optimisation of the electrolysis parameters. Experiments in large-scale electrolysis cells allowed us to run long-term trials in continuous operation, while the on-line monitoring of the cell off-gases ensured the environmentally benign performance of the process. The aluminium product obtained contained 0.6–2.6 wt% Sc, depending on the current density applied. The material is suited to prepare Al-Sc master alloys for 3D printing powders.