On the cathodic overvoltage on aluminium in cryolite-alumina melts—I

doi:10.1016/0013-4686(78)85050-6

Electrochimica Acta

Volume 23, Issue 3, March 1978, Pages 223-231

https://doi.org/10.1016/0013-4686(78)85050-6 Get rights and content

Abstract

When a constant cathodic current is applied to an aluminium electrode in a Na₃AlF₆Al₂O₃ melt at 1010°C, the potential decreases gradually and linearly with the square root of time to more negative values. Pronounced potential oscillations occurred at cds above 2A/cm² and the evolution of sodium gas was observed at very high cds. Steady state measurements yielded straight η vs log i plots with a slope of −0.23V/decade, the overvoltage being −0.19V at 1A/cm². The overvoltage decreased markedly when the melt was stirred. Potential decay measurements yielded linear log time plots with slopes of 0.01–0.5V/decade. The charge-transfer resistance was determined by double pulse measurements to be 0.0031 ohm cm². Ac impedance measurements gave a similar result. The charge transfer overvoltage accounts for only about two percent of the total overvoltage, the rest is apparently diffusion controlled. The cathodic overvoltage in industrial aluminium cells is of similar magnitude as found in the laboratory investigations.

References (38)

K. Grjotheim et al.
Aluminium Electrolysis. Aluminium Verlag
(1977)
C.E. Ransley et al.
J. Inst. Metals
(1950)
E.W. Dewing
Met. Trans.
(1972)
E. Grünert
Z. Elektrochem.
(1942)
M.M. Vetyukov et al.
Tsvet. Met
(1971)
H. Ginsberg et al.
Metall
(1973)
M.M. Makansi et al.
J. phys. Chem.
(1955)
K. Yoshida et al.
Met. Trans.
(1972)
J. Thonstad
Can. J. Chem.
(1965)
M.M. Vetyukov et al.

L.N. Antipin

Z. Fiz. Khim.

(1955)

C. Kubik et al.

Electrochim. Acta

(1964)

C. Kubik et al.

Chem. Zvesty

(1967)

C. Kubik et al.

Chem. Zvesty.

(1968)

A. Silny et al.

Chem. Zvesty

(1969)

R. Piontelli et al.

Electrochim. Met.

(1966)

P. Drossbach

Z. Elektrochem.

(1936)

R. Piontelli et al.

Alluminio

(1953)

R. Piontelli et al.

Alluminio

(1956)

R. Piontelli

Metallurgia Ital.

(1960)

Cited by (39)

Advancing the prediction of bath penetration and electrochemical degradation in Hall-Héroult cell cathodes: Insights into ionic species transport in a porous electrode model
2024, MethodsX
This study presents a groundbreaking approach to modeling the Hall-Héroult cathode used in aluminum production. Our innovative model is grounded in a sophisticated porous electrode methodology coupled with state-of-the-art numerical simulations. This enables us to capture the intricate physicochemical processes within the system precisely, encompassing the migration, diffusion, and convection of ionic species. A key feature of our model is the integration of detailed electrochemical reaction kinetics at the microscale, providing a nuanced understanding of the internal dynamics of the cathode. Furthermore, we have incorporated a unique penalization method that rigorously enforces the principles of electroneutrality and ionic thermodynamic equilibrium, ensuring the model's fidelity to real-world phenomena. Computational simulation using the finite element method (FEM) serves as the backbone of our model, offering unparalleled accuracy and robustness. This has been confirmed through validation against empirical data, underlining the model's potential to significantly enhance both the efficiency and sustainability of aluminum production processes.
- •
  Development of an advanced porous electrode model for the Hall-Héroult process, utilizing numerical simulations42 to unravel complex physicochemical dynamics.
- •
  Incorporation of a novel penalization method for ensuring electroneutrality and thermodynamic equilibrium, enhancing 44 model accuracy.
- •
  Validation of the model against empirical data using FEM, demonstrating potential improvements in aluminum 46 production efficiency and sustainability.
Electrochemical description of the interfacial tension between the liquid metal pad and cryolitic melts in industrial electrolysis cells
2023, Journal of Molecular Liquids
An accurate knowledge of the interfacial tension between liquid aluminium and cryolitic melts is critical to better understand the process parameters influencing the current efficiency in electrolysis cells. In principle, the interfacial tension can be predicted from the surface tension of liquid aluminium, cryolitic melts and their adhesion energy. In industrial electrolysis cells, liquid aluminium contains several impurities (Na, Si, Fe, V, O, …) for which the concentration is modulated by the current density and the thermochemical and electrochemical reactions between aluminium and cryolitic melts, strongly impacting the surface tension of the liquid metal and therefore the interfacial tension. The aim of this paper is to present a predictive model describing the interfacial tension between the liquid aluminium pad and cryolitic melts taking into account the thermochemical and electrochemical reactions within industrial electrolysis cells, depending on the current density. The impurity concentrations within the metal pad are calculated from the cryolitic melt compositions and the current density. The presence of Na and O, even if just a few ppm, may drastically decrease the surface tension of the liquid pad. As no experimental data is available for Al-Na-O system, it has been predicted based on a theoretical approach. Overall, the predicted interfacial tension with the proposed model is in good agreement with the different experimental datasets reported in the literature. A mapping of interfacial tension as a function of electrolyte composition, temperature and current density is proposed. Lastly, based on the proposed interfacial tension model, the probability of precipitation of undissolved alumina particles at the bottom of the cells and its impact upon the current efficiency is discussed.
The content of sodium in aluminium during electrolysis of the molten systems Na<inf>3</inf>AlF<inf>6</inf>-NaCl-Al<inf>2</inf>O<inf>3</inf> and NaF-NaCl
2004, Electrochimica Acta
The content of sodium in molten aluminium during electrolysis of the molten system Na₃AlF₆–NaCl–Al₂O₃ was investigated at the temperatures 750, 890 and 1000 °C. Simultaneously, the cathodic overvoltage was measured. It was found that the experimental data on the cathodic overvoltage agree well with the values predicted on the basis of the concentrations of sodium in molten aluminium. Saturation of molten aluminium with sodium was not achieved by electrolysis of a cryolite-based electrolyte.
To achieve saturation of sodium in liquid aluminium the eutectic mixture NaF–NaCl was used as electrolyte. The experiments were carried out at the temperatures 710, 750, 800, 900 and 960 °C, i.e. below and above the boiling temperature of sodium at atmospheric pressure. In this case, saturation of the molten aluminium with sodium was reached. The obtained data are treated by a thermodynamic model.
Cathodic overvoltage and the contents of sodium and lithium in molten aluminium during electrolysis of cryolite-based melts
2001, Electrochimica Acta
The cathodic concentration overvoltage in Na₃AlF₆–AlF₃–Al₂O₃(sat.) melts was investigated at the temperature of 1000°C. The melts contained 10 and 20 wt.% of AlF₃. Experimental data on the cathodic overvoltage agreed well with values calculated from the content of sodium in aluminium. The dependence of the cathodic overvoltage on temperature was studied as well. It was shown that the addition of 5 wt.% CaF₂ or 5 wt.% MgF₂ does not influence the overvoltage significantly, while the addition of 2 wt.% LiF decreases the overvoltage by 20 mV at a cathodic current density of 0.25 A cm⁻² and by 50 mV at 0.75 A cm⁻². These results are consistent with the influence of these additions on the content of sodium in aluminium.
Cathodic processes at TiB<inf>2</inf>/C-composite electrodes during aluminium electrolysis
1993, Electrochimica Acta
The results show that the cathodic overpotential for aluminium deposition decreases gradually as the TiB₂ content of the TiB₂/C composite electrodes is increased. Increasing the TiB₂ content of the composite electrodes makes it easier for them to sustain a thin and continuous film of molten aluminium metal on their surface and hence less energy is required to achieve bulk deposition of aluminium.
The overvoltage data can be plotted as straight lines on Tafel plots in the current density range of 0.1–0.7 A cm⁻² for those composites which have TiB₂ contents greater than about 60 wt.%. For those electrodes whose TiB₂ content is less than about 60 wt.% and which are not able to sustain a thin, uniform film of metallic aluminium on their substrates, the Tafel plots exhibited two distinct regions corresponding to Na and Al codeposition, respectively. The results show that the cathodic overpotential on TiB₂/C composite cathodes is of the order of 140–200 mV at 1 A cm⁻².
Electrochemical studies of the effect of TiO<inf>2</inf> and B<inf>2</inf>O<inf>3</inf> additions on the aluminium deposition reaction in the molten cryolite bath
1992, Electrochimica Acta
Potential sweep studies have shown that the wetting properties of graphite electrodes can be enhanced by the addition of small quantities of TiO₂ or B₂O₃ into the cryolite-based melts. The oxides enhance the wettability by initiating the formation of AlTi and AlB alloys on the surface of the electrodes. The potential sweep studies and the examination of appropriate phase diagrams have been used to identify the concentration limits of the additives that enhance the wetting properties and of the alloying elements in the cathodically deposited aluminium. This work has also been used to study and explain the reactions and mechanisms for the deposition and reoxidation of titanium and boron.
The results produced by the scanning electron microscope (SEM) studies are in agreement with those obtained by the potential sweep studies.

View all citing articles on Scopus

View full text

On the cathodic overvoltage on aluminium in cryolite-alumina melts—I

Abstract

Aluminium Electrolysis. Aluminium Verlag

J. Inst. Metals

Met. Trans.

Z. Elektrochem.

Tsvet. Met

Metall

J. phys. Chem.

Met. Trans.

Can. J. Chem.

Z. Fiz. Khim.

Electrochim. Acta

Chem. Zvesty

Chem. Zvesty.

Chem. Zvesty

Electrochim. Met.

Z. Elektrochem.

Alluminio

Alluminio

Metallurgia Ital.