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Fog Formation during Electrodeposition of Aluminium
ABSTRACT
Laboratory experiments were performed to study the behaviour of so-called metal fog formed at the cathode during deposition of aluminium from cryolite-alumina melts and molten cryolite-sodium chloride. Double potential step chronoamperometry was used to detect fog formed during electrodeposition of aluminium. The effects of dissolved iron species, curent density and the presence of aluminium and CO2 on the fog formation were studied.
1 INTRODUCTION
The fact that aluminium is soluble in molten cryolite-based electrolytes is important for the loss in current efficiency during industrial electrowinning of aluminium, which can reach 95-96% in modern cells. Both dissolved aluminium and dissolved sodium are formed in molten cryolite in contact with aluminium [1]. Dissolved Al is most likely present as a subvalent species (AlF2- ), which will not contribute to electronic conductivity [2]. The presence of dissolved Na has been demonstrated to give rise to a significant electronic conductivity [3]. Migration of dissolved metal species such as AlF2- may take place, which was demonstrated by Thonstad and Oblakowski [4] who observed migration of dissolved metal toward the anode in cryolite-alumina melts. Metal solubility data [5,6] show that the solubility of aluminium decreases with increasing AlF3 content in the molten NaF-AlF3 system. Values in the range from 0.03 - 0.06 wt% are reported for melts of industrial relevance at temperatures around 1000oC.
So-called metal fog is a visual phenomenon which is often observed in the electrolyte near the cathode during deposition of liquid metals from molten salts. Results from visual observations have been reported [7, 8, 9]. Metal solubility in molten salts leads to the formation of coloured solutions, suggesting that in some cases at least part of the fog is due to dissolved metal. Visual observations and studies of nucleation of magnesium deposition from chloride melts indicated that the fog consisted of tiny Mg droplets formed by homogeneous nucleation from a supersaturated solution of dissolved Mg [10]. Fog formation was facilitated at cathodes with poor wetting properties. A similar conclusion was drawn by Grjotheim et al. [11] from experimental studies of Al deposition from fluoride melts.
2 EXPERIMENTAL
Studies of metal fog were performed by visual observations and electrochemical measurements. Visual observations were performed from above an open furnace. A graphite crucible with sintered alumina lining served as the anode, while rods of steel, tungsten or carbon (graphite or glassy carbon) were used as cathodes. Electrolysis was run galvanostatically in molten Na3AlF6-Al2O3 at 1000oC.
Electrochemical studies were carried out in a closed furnace with argon atmosphere. Cyclic voltammetry and potential step measurements were applied by using a three-electrode system; glassy carbon or tungsten as working electrode, graphite as counter electrode and aluminium placed inside a tube of sintered alumina or boron nitride as reference electrode. The experiments were conducted in the molten systems Na3AlF6-Al2O3 at 1010oC and Na3AlF6-AlF3(10 wt%)-Al2O3 at 990oC. The influence of dissolved iron species was studied by additons of FeF2 and by employing an Fe working electrode. The influence of CO2 was also examined.
Experiments were also carried out in the model system molten Na3AlF6(30wt%)-NaCl at 775oC. This melt is less corrosive than cryolite due to the lower temperature and the content of NaCl. This allowed for using quartz as crucible material, and visual observations could be made in a closed cell with argon atmosphere.
3 RESULTS AND DISCUSSION
Visual Observations
The pure cryolite-alumina melt was clear and transparent. When adding aluminium fog was observed after the metal was molten. The production of fog continued for several minutes. Fog was formed again upon stirring the melt or the liquid metal. The melt gradually turned opaque due to the fog formation.
The fog formed during electrolysis was found to depend on the cathodic current density (cd). Above 0.5 A/cm2 fog was observed immediately or after a few seconds after start of electrolysis. The fog was apparently formed near the cathode and spread rapidly in the electrolyte. No fog was observed at cds lower than 0.1 A/cm2. By increasing the cd to 0.25 A/cm2 fog was formed for a limited time (minutes), but was observed again when increasing the cd to 0.3 A/cm2. The fog formation, which was more pronounced at higher cds, caused the melt to turn non-transparent and in some cases opaque. During long time electrolysis at high cd (above 1 A/cm2) the bulk of the electrolyte was clear after some time (minutes to an hour) in spite of continuous fog formation at the cathode. Gas or disssolved gas from the anode apparently reacted with the fog. After terminating electrolysis the melt gradually turned clear and was usually transparent after 10 minutes. The cathode substrates steel, tungsten and graphite were found to have no specific influence on the fog formation.
Additions of metals other than aluminium caused fog formation in the cases where the melting point of the metal was lower than the actual temperature of the melt.
These observations indicate that the behaviour and movement of fog is controlled by convection and diffusion rather than density differences or possible charge.
4 ELECTROCHEMICAL MEASUREMENTS
Cyclic voltammetry was used to identify possible electrode reactions. Glassy carbon was found to be best suited as indicator electrode. The potential window is limited by underpotential deposition of sodium starting a few hundred millivolts positive of Al deposition and gas evolution (CO/CO2) starting at ~1.8 V. The reported potentials are referred to the Al reference electrode, which was found to maintain a stable potential throughout the measurements.
Formation of metal fog was studied by double potential step measurements; cathodic polarization for a certain time (10-60 seconds) followed by anodic polarization at 0.9 V until steady state. The cathodic potential was varied. Figure 1 shows current versus time behaviour during a double step experiment. Aluminium formed during cathodic polarization, will oxidize at 0.9 V. The anodic current was found to reach a steady value after a few minutes. Cathodic polarization at potentials more negative than -150 mV gave rise to abrupt fluctuations of the measured current during the anodic polarization. This behaviour was attributed to oxidation of particles in the melt. By integrating the current with time for these current peaks, it was possible to determine the size of the particles assuming they consist of Al. The radius of the droplets is in the order of 10 - 50 um, indicating that the dispersion may have some stability. Droplets of the same size were found in a similar study during magnesium deposition from chloride melts [12]. However, the first report on oxidation of small metal particles is due to Rolseth [13], who reported strong current oscillations at a Pt anode polarized at 1.4 V in molten cryolite-alumina in contact with liquid aluminium.
The amount of fog produced was essentially independent of the applied cathode potential at potentials more negative than ~-400 mV versus the Al reference electrode. The apparent current efficiency with respect to fog formation was found to be about 8% in these experiments.
Addition of aluminium to the melt did not give significant changes of the anodic current time decay curves. However, the amount of Al formed during cathodic polarization was found to decrease. The steady state oxidation current at 0.9 V was found to increase by a factor of 10 – 100 in the presence of excess Al. This is due to oxidation of dissolved Al; AlF2- species are anodically oxidixed to Al(III). Contributions from electronic conduction due to dissolved Na were believed to be negligible during these experiments.
Effects of dissolved iron species were studied by additions of FeF2. The fog production was found to decrease drastically in the presence of dissolved Fe(II) species, which is due to reaction between dissolved Al and Fe(II). The oxidation current was also observed to decrease. More Al was apparently formed during cathodic polarization when iron was present. Addition of FeF2 to a melt containing excess Al caused decreased oxidation current and reduced formation of fog. The reaction between Fe(II) and dissolved Al was apparently slow.
As expected bubbling by CO2 was found to reduce both the oxidation current and the fog production.
Figure 1. Potentiostatic current transients in molten Na3AlF6-AlF3(10 wt%)-Al2O3 at 990oC at a glassy carbon electrode. Potential steps: -0.8 V, 60 seconds followed by +0.9 V, 200 seconds.
5 ACKNOWLEDGEMENTS
This work was supported by the Norwegian aluminium industry, The Norwegian Research Council and by the Scientific Grand Agency of the Ministry of Education and the Slovak Academy of Sciences under contract No. 2/5117/98.
6 REFERENCES
[1]. K. Grjotheim, C. Krohn, M. Malinovsky, K. Matiasovsky and J. Thonstad, "Aluminium Electrolysis", 2nd ed., Aluminium Verlag, Düsseldorf, 1982.
[2]. R. deg rd, Electrochim. Acta, 33, 527 (1988).
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[4]. J. Thonstad and R. Oblakowski, Electrochim. Acta, 25, 223 (1980).
[5]. R. deg rd, . Sterten and J. Thonstad, "The Solubility of Aluminium in Cryolitic Melts", Light Metals 1987, p. 389.
[6]. X. Wang, R. D. Peterson and N. E. Richards, " Dissolved Metals in Cryolitic Melts", Light Metals 1991, p. 323.
[7]. Qui Zhuxian, Fan Liman, K. Grjotheim and H. Kvande, J. App. Electrochem., 17, 707 (1987).
[8]. W.E. Haupin and W.C. McCrew, Aluminium, 51, 273 (1975).
[9]. W.E. Haupin, R.S. Danchik and J.F. Luffy, Light Metals, 1, 159 (1976).
[10]. G.M. Haarberg, S.R. Johansen, J. Melaas and R. Tunold, Proc. Seventh Int. Symp. Molten Salts, Volume 90-17, p. 449, 1990.
[11]. K. Grjotheim, H. Kvande, Qui Zhuxian and Fan Liman, Aluminium, 65, 157, (1989).
[12]. S. Rolseth, "Tilbakereaksjonen i aluminiumelektrolysen", Dr. ing. thesis, University of Trondheim, July 1980.
[13]. B. B rresen, "Electrodeposition of Magnesium from Halide Melts", Dr.ing. thesis, Department of Electrochemistry, NTH, Trondheim, January 1995.
Author's Names: Geir M. Haarberg, Trond Store, Jomar Thonstad, Stanislaw Pietrzyk & Alexander Silny
Firm: SINTEF Materials Technology, Hydro Aluminium, Technology Centre, Dep. of Materials Technology and Electrochemistry, Faculty of Non-Ferrous Metals, Institute of Inorganic Chemistry
Address: N-7465 Trondheim, N-5870 vreordal, NTNU, N-7491 Trondheim, University of Mining and Metallurgy, 30-059, Krakow, Slovak Academy of Sciences, 84236 Bratislava
Country: Norway, Poland & Slovakia
http://www.aluminium.sk/aluminium/prednasky/3.htm