Anne Støre

The Effect of Current Density on Cathode Expansion During Start-Up

During start-up of aluminium reduction cells, sodium penetration causes expansion in the carbon cathode, which may influence the lifetime of the cathode lining. Traditionally, the sodium expansion has been measured at cathode current densities up to 0.75 A/cm2. However, it is well known that the current distribution in the cathode is non-uniform, and high local current densities may be experienced close to the sideledge, which commonly is associated with the W wear pattern. Hence, the sodium expansion may cause both local stresses in the cathode blocks, as well as in the total cell lining. The aim of this study is to determine the sodium expansion over a wider range of current densities. Typically, it is found that the sodium expansion starts to increase again above 0.7 A/cm2, after the plateau reached at 0.2 A/cm2. Apparently, this second increase continues outside the range of 1.5 A/cm2 applied in this work.

INVESTIGATION OF THE CATHODE WEAR MECHANISM IN A LABORATORY TEST CELL

Cathode wear has become one of the major challenges for the life time of high amperage aluminum reduction cells due to the use of graphitized cathodes. The fundamentals of the cathode wear are still a matter of debate, and a laboratory procedure for testing of cathode materials is desired. Here, we present a laboratory electrolysis cell, which has been designed for cathode wear tests of industrial cathode materials. The formation and transport of aluminum carbide have been considered to be an important factor for cathode wear, and the laboratory test cell was designed in such a way that the cathode is exposed directly to the electrolyte. Aluminum carbide formed at the cathode may dissolve directly into the electrolyte. Here we present the study of the cathode wear of a commercial high density graphitic material, where the influence of the cathode surface morphology, diffusion and hydrodynamics in the electrolyte, have been in focus. The cathode wear and the penetration of electrolyte into the cathode were investigated by optical and electron microscopy. The influence of current density, hydrodynamics and transport of carbide in the electrode are discussed in relation to the experimental results.

Critical Reflections on Laboratory Wear Tests for Ranking Commercial Cathode Materials in Aluminium Cells

The lifetime of high amperage cells with graphitized carbon cathodes is mainly determined by cathode wear. Several attempts have been made to investigate cathode wear in laboratory test cells, but the underlying mechanism is still a matter of discussion. This is reflected in the fact that test methods enabling the ranking of different commercial cathode materials are still to be developed. In the present paper we report on a laboratory test cell where the cathode is directly exposed to the electrolyte, which accelerates the wear rate by an order of magnitude relative to the wear rate in industrial cells. In this study three different commercial carbon cathode materials have been tested; graphitized carbon, high density graphitized carbon, and anthracitic carbon. No significant differences in wear rate could be detected under the test conditions used. Possible reasons for this unexpected result are discussed, and suggestions for modifications of the test cell are provided.

PFC evolution characteristics during aluminium and rare earth electrolysis

In addition to aluminium electrolysis, the electrolysis of rare earth (RE) metals from fluoride melts is a significant source of perfluorocarbon (PFC) emissions to the atmosphere. These processes have many similarities, they are both based on molten fluoride salt electrolysis at temperatures around 1000 °C, and are utilizing carbon materials as the anode. Although PFC emissions from aluminium industry and rare earth electrolysis have similar overall reactions, they are often reported to have different characteristics. In order to get a better understanding of these differences and similarities, different laboratory experiments focusing on anode reactions and gas compositions in Al2O3 saturated cryolite and REF3-LiF melts during aluminium and rare earth metal electrolysis were studied. The results obtained, combined with thermodynamic data analysis allowed to better understand onset, evolution and termination behaviour of PFC evolution in molten fluoride systems of different chemistries.

Electrochemical Processing of Rare Earth Alloys

The light rare earth metals Nd, Pr, La, Ce as well as some alloys with Fe, are today produced in China by electrolysis in molten fluorides using oxide raw materials. A major challenge is to obtain a good cell operation without de-composing the electrolyte leading to emissions of perfluorinated carbon (PFC) green-house gases to the atmosphere. This work is focused on understanding the fundamental requirements to run the electrolysis cells for DyFe alloy production in an efficient and environmental friendly way. Electrolysis experiments was carried out in DyF3-LiF melts at 1050 °C. A Fe rod was used as consumable cathode and the (consumable) anode was made of graphite. To establish at which anode potential PFC occurred and thus enabling optimisation of the oxide batch feed rate, analysis of the anode gases was performed with Fourier Transform Infrared Spectrometer (FTIR). The produced DyFe alloy was characterised by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS).

Perfluorocarbon Formation During Rare Earth Electrolysis

A challenge during rare earth (RE) electrolysis is to avoid emissions of perfluorocarbon (PFC) green-house gases. The objective of this work was to study how to operate the RE electrolysis process with neither PFC formation nor anode effect. Linear Sweep Voltammetry was carried out at 1050 and 1100 °C, and electrolysis was performed in REF3-LiF melts at ca. 1050 °C during on-line off-gas analysis. To avoid anode effect, the current density values must be strictly less than 0.43 and 0.68 A cm−2 at working temperatures of 1050 and 1100 °C, respectively. The optimal REO batch feed rate for avoiding PFC formation could be established by correlating the onset of PFC with the values and the changes that occurred in the anode potential.

Optimization of the Anode-Stub Contact: Effect of Casting Temperature, Contact Stress and Temperature and Surface Roughness

In electrolysis of aluminum, the contact between the anode and the anode stub is normally facilitated by cast iron. Important parameters are the effect on the anode stub contact resistance from: Casting temperature of the iron, Temperature of the stub-anode coupling Uniaxial stress between carbon and cast iron Surface roughness