Diffusion of alkali metal cations in the first stage graphite intercalation compounds (GIC) LiC
6
, NaC
6
, NaC
8
and KC
8
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 LiC
6
, NaC
6
, NaC
8
and KC
8
, 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
m
2
s
−1
at room temperature, which is within the range of available experimental data. For LiC
6
and NaC
6
a curved vacancy migration path is the most energetically favourable, while a straight pathway was inferred for NaC
8
and KC
8
. The diffusion coefficients for alkali metal vacancy diffusion in first stage GICs scales with the graphene interlayer spacing: LiC
6
< NaC
8
< NaC
6
< KC
8
.
The diffusion constant of alkali metals in graphite intercalation compounds is proportional to the graphene interlayer distance.
Full text
Available for:
IJS, KILJ, NUK, UL, UM, UPUK
The raw gas composition from primary aluminium production depends mostly on the process technology applied and the composition of the raw materials. At steady state, a stationary condition is ...established among the material sources, gas production and sinks: the gas treatment centre and escaping gases. Only a few papers discuss the off-gas composition itself; most papers deal with the gas composition from laboratory-scale experiments performed under inert conditions. In this article, an overview of the literature describing gas production from aluminium electrolysis is given. Effects of temperature and chemical equilibrium on the stationary condition are also discussed. The typical chemical composition of the raw materials is presented to evaluate their input into the gas composition, especially with respect to their impurity levels.
Autopsies of six spent potlinings with different carbon cathode block grades, amperage regimes and cell designs were conducted at three separate smelters to reveal possible mechanisms causing cathode ...wear. The microstructure of the cathode samples from the autopsies was investigated by optical and electron microscopy and x-ray computed tomography, while the composition of the solid bath at the surface and in the interior pores was investigated by x-ray 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 play a major role in 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 partial solidification of the bath and spatial variation of the current density.
This article gives a brief overview of the history of modern aluminium production, which started with the invention of electrowinning of dissolved aluminium oxide in molten cryolite (Na
3
AlF
6
...)-based electrolytes in 1886. All primary aluminium production is still based on the same principles, although numerous improvements and adoptions have been developed over the years. A historical development of the process is followed by brief fundamental and thermodynamic aspects of the reactions. Some of the most important developments are described, with a focus on the carbon anode production process. Carbon anodes, which are responsible for the majority of direct greenhouse gas (GHG) emissions by the industry, are discussed thoroughly, and recent developments in inert anodes and utilization of biomass in anode formulation are presented. Finally, the environmental footprint of the process and the trends and objectives of the industry towards improving energy efficiency and mitigating environmental emissions are elaborated.
Full text
Available for:
EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
•The deactivation was due to coating loss or combined with passivation of the titanium substrate.•The coating loss was occurring as coating dissolution, coating spalling and coating peeling off.•No ...critical value of the amount of the residual iridium was found in this work to predict the eventual deactivation before forming the passive oxide film.•Deactivation of the anodes was found to be more dependent on the calcination temperature than other manufacturing parameters.
In this work, series of IrO2-Ta2O5 anodes were investigated. The catalytic activity towards oxygen evolution reaction (OER) of these anodes are determined by calcination temperature, coating loading (coating thickness), pretreatment of titanium substrate and coating method. The difference in OER performance among the anodes are ascribed to the crystallinity of the IrO2 phase and the phase composition of the coatings. The durability of the anodes were also studied by conducting an accelerated lifetime test (ALT) in acidic 0.9 mol L−1 Na2SO4 solution at a current density of 5 kA m−2. An anode prepared at a moderate temperature exhibits an excellent lifetime of almost one year although its catalytic activity is not the best. Nevertheless, using the electrostatic spraying method to replace the hand-brush method in the coating preparation can prolong the service life even further and with less amount of coating loading. Moreover, it reveals that the coating loss or combined with titanium substrate passivation results in the eventual deactivation of the anodes during ALT. No critical value of the amount of the residual iridium was found in this work to predict the eventual deactivation before forming the passive oxide film. In addition, the deactivation of the anodes strongly depends on the calcination temperature.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
During production of aluminum in Hall-Héroult cells, carbon anodes electrochemically oxidize to form mainly CO2. The CO2 bubbles block the anode working surface during growth and coalescence, leading ...to cell voltage loss and voltage oscillations. Lower grade isotropic petroleum cokes are currently being introduced to anodes worldwide, but little is known about effects of coke quality on the unavoidable voltage losses linked to bubble formation. To investigate this, a pilot carbon anode series was made with various blended ratios of isotropic to anisotropic coke. The anodes were characterized with respect to voltage oscillations related to bubble formation and release, wettability toward electrolyte and surface roughness before and after electrolysis. Results showed that voltage noise caused by gas bubbles was reduced for anodes containing isotropic coke. For blended anodes of isotropic and anisotropic coke, the potential oscillation amplitude was reduced by ∼0.19 V compared to a 100% anisotropic coke anode. The percentage of the anode surface screened by gas bubbles was also reduced as isotropic coke was introduced. Increased wettability between electrolyte and two anodes containing isotropic coke was observed after anode polarization. The reduced bubble screening on the anode surface was attributed to better wetting between anode and electrolyte.
Aluminosilicate refractory lining constitutes a major part of anode baking kilns, and during anode baking the lining is exposed to harsh conditions which limits the lifetime. Here, autopsies of ...refractory linings from both an open and closed top furnace are reported aiming to determine the microstructural and mineralogical changes in the lining during operation. Significant variations in density and porosity were observed, both across the brick cross sections, but also for the vertical position in the lining. The chemical and mineralogical composition of the bricks were investigated by electron microscopy and X‐ray diffraction. Only minor changes in the mineralogical composition of the materials were observed, and sodium originating from green anodes was only observed to a minor degree. Evidence of silicon oxide transport from the lower to the upper part of the linings was observed, reflecting the changes in reducing‐oxidizing conditions during an anode baking cycle. The main degradation mechanisms in these linings were due to densification of the material over time, particularly the uneven densification across a lining wall. The present findings are discussed and related to the main differences in the two furnace designs.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
Bio-pitch is considered as a promising candidate to replace fossil coal-tar-pitch used in carbon anodes. The electrochemical performance of the carbon anode is affected by the properties of its ...constituents and replacement of coal-tar-pitch with bio-pitch requires that the electrochemical performance is not significantly reduced. In this work the impact of the bio-pitch on the electrochemical behavior of carbon anodes was investigated using different electrochemical methods including, electrolysis under constant current, cyclic voltammetry and electrochemical impedance spectroscopy in a laboratory scale electrolysis cell. The electrochemical behavior of the bio-pitch anodes was compared to that of the reference anodes made of coal-tar-pitch. The bio-pitch anodes demonstrated slightly improved electrochemical performance, compared to the reference anodes. The reaction overpotential decreased while an increase in the double layer capacitance, which referrers to the interfacial contact between the anode and the molten electrolyte, was noticed upon using the bio-pitch as a binder. The finding of this work proved the potential of using the bio-pitch, as a binder, which can even be beneficial for the electrochemical performance of carbon anodes.
Full text
Available for:
EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
One significant contribution to the anodic potential during aluminum electrolysis is the formation of CO2 bubbles that screen the anode surface. This effect creates an additional ohmic resistance as ...well as an increased reaction overpotential, hyperpolarization, as the effective surface area decreases. This work aims to improve the understanding of how anode properties - including isotropy at the optical domain level, wettability (toward electrolyte), surface roughness and porosity - affect bubble evolution. Pilot anodes, made with single source coke types varying in isotropy, were used to study bubble evolution by electrochemical methods. In order to retain bubbles during experiments, anodes were designed to have only horizontal surface area. Bubble formation and release were monitored at different current densities, and were tracked by measuring the oscillations in anode potential and series resistance. Anodes made from different cokes were found to have different bubble evolution properties, possibly due to variation in the density of nucleation sites at the surface of each anode and varying anode-electrolyte wettability.
PDF
