Arne Petter Ratvik

Diffusion of alkali metals in the first stage graphite intercalation compounds by vdW-DFT calculations

Diffusion of alkali metal cations in the first stage graphite intercalation compounds (GIC) LiC6, NaC6, NaC8 and KC8 has been investigated with density functional theory (DFT) calculations using the optPBE-vdW van der Waals functional. The formation energies of alkali vacancies, interstitials and Frenkel defects were calculated and vacancies were found to be the dominating point defects. The diffusion coefficients of the alkali metals in GIC were evaluated by a hopping model of point defects where the energy barriers for vacancy diffusion were derived from transition state theory. For LiC6, NaC6, NaC8 and KC8, respectively, the diffusion coefficients were found to be 1.5 × 10−15, 2.8 × 10−12, 7.8 × 10−13 and 2.0 × 10−10 m2 s−1 at room temperature, which is within the range of available experimental data. For LiC6 and NaC6 a curved vacancy migration path is the most energetically favourable, while a straight pathway was inferred for NaC8 and KC8. The diffusion coefficients for alkali metal vacancy diffusion in first stage GICs scales with the graphene interlayer spacing: LiC6 ≪ NaC8 < NaC6 ≪ KC8.

Depolarised gas anodes for aluminium electrowinning

Consumable carbon anodes are used in the electrowinning of aluminium by the Hall-Heroult process. Emissions of CO2 may be eliminated by introducing an inert oxygen evolving anode, which however will require a higher anode potential. An alternative approach is to use a natural gas or hydrogen gas anode to reduce the CO2 emissions and lower the anode potential. Preliminary laboratory experiments were carried out in an alternative molten salt electrolyte consisting of CaCl2-CaO-NaCl at 680 °C. Porous anodes of platinum and tin oxide were tested during electrolysis at constant current. The behaviour of inert anode candidate materials such as tin oxide and nickel ferrite were also studied.

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.

Particulate Emissions from Electrolysis Cells

In the dry cleaning of the exhaust gas from the aluminium cells impurities are accumulated in the finer fractions of secondary alumina from the dry scrubbers. The present work describes new methods for the determination of dust composition, aiming at increasing the understanding of the effect of cell operation on the amount and the composition of dust in the fume. New and advanced analysis methods are used to characterize a broad specter of emissions. An Electrical Low Pressure Impactor is used to sample and analyze the dust from the cells. The equipment enables real-time particle size distribution analysis of 12 particle classes in the range 30 nm — 10 μm. The size classified samples are analyzed by means of SEM/EDS and XRD to determine the characteristic chemical composition of the different fractions. Understanding the evolution, evaporation, and condensation of particulates in the cell emissions under different operational conditions may facilitate new standards for environmental friendly and energy efficient high amperage electrolysis cells.

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.

SnO2-based gas (hydrogen) anodes for aluminum electrolysis

Abstract A novel SnO 2 -based gas anode was developed for aluminum electrolysis in molten cryolite at 850 °C to reduce energy consumption and decrease CO 2 emissions. Hydrogen was introduced into the anode, participating in the anode reaction. Carbon and aluminum were used as the cathode and reference electrodes, respectively. Cyclic voltammetry was applied in the cell to investigate the electrochemical behavior of oxygen ion on platinum and SnO 2 -based materials. The potential for oxygen evolution on these electrode materials was determined. Then, galvanostatic electrolysis was performed on the gas anode, showing a significant depolarization effect (a decrease of ∼0.8 V of the anode potential) after the introduction of hydrogen, compared with no gas introduction or the introduction of argon. The results indicate the involvement of hydrogen in the anode reaction (three-phase-boundary reaction including gas, electrolyte and electrode) and give the possibility for the utilization of reducing gas anodes for aluminum electrolysis.

Interaction of Sodium Vapor and Graphite Studied by Thermogravimetric Analysis

Intercalation of sodium in carbon materials is of paramount importance for the Hall-Heroult process. The interaction of sodium and graphite has been investigated for decades, but despite considerable efforts, the transport and nature of sodium in carbon materials are still poorly understood. Here we report on a study of the interaction between graphite and sodium vapor by thermogravimetric analysis. A graphitized carbon material was exposed to sodium vapor, and the equilibration of sodium uptake in the carbon material was monitored. The kinetics of the sodium uptake is discussed with respect to surface adsorption, bulk diffusion and the solid solubility of sodium in graphite. The kinetics of the reaction was analyzed with support from finite element method simulations. Finally, recent density functional theory simulations of sodium intercalation compounds are presented, demonstrating the low thermodynamic stability of such sodium intercalation compounds reflecting the low reactivity of sodium with carbon.

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.

Depolarized Gas Anodes for Electrowinning of Aluminium From Cryolite-Alumina Melts in a Laboratory Cell

Consumable carbon anodes are used in the electrowinning of aluminium by the Hall-Heroult process and in other proposed processes for electrowinning in molten salts. Emissions of CO2 may be eliminated by introducing an inert oxygen evolving anode, which however will require a higher anode potential. By introducing natural gas or hydrogen to the anode the CO2 emissions can be reduced and the anode potential can be lowered. Laboratory experiments were carried out in a modified Hall-Heroult electrolyte with excess AlF3 at 850 °C. Anodes of platinum, tin oxide and graphite were tested during electrolysis at constant current, with the supply of argon, methane and hydrogen through or at the anodes.

Correlation between Moisture and HF Formation in the Aluminium Process

Hydrogen fluoride (HF) emission to the working atmosphere is still a problem in the aluminium industry. Moisture in secondary alumina fed to the cell and humidity in the ambient air reacts with fluorides in the bath and fluoride vapours to form hydrogen fluoride. The relation between the various sources of water and the resulting HF emission is still not well understood. In this work, industrial measurements have been done to determine where HF escapes from the bath. The quantities of HF and moisture at the specific sites have also been determined. Measurements were done in the duct during normal operation as well as during anode change, above the feeder hole and above an open hole in the crust. A strong correlation between feed cycle and HF levels was measured. Increased HF emissions were also recorded during anode change.