Study of Aluminum Carbide Formation in Hall-Heroult Electrolytic Cells.

Transcription

1 Study of Aluminum Carbide Formation in Hall-Heroult Electrolytic Cells. Abdelhalim Zoukel 1, Patrice Chartrand 2, Gervais Soucy 1. 1 Département de génie chimique (REGAL), Université de Sherbrooke, 2500 Boul. université, Sherbrooke, Qc, J1K2R1, Canada. 2 Département de génie chimique (REGAL), École Polytechnique de Montréal, C.P. 6070, Succursale Centre-ville, Montréal, Qc, H3C3A7, Canada. Keywords: Aluminum Electrolysis, Aluminum Carbide Formation, Phase Distribution. Abstract The trend in the aluminum reduction industry today is to operate cells with graphitized carbon cathode blocks, increased current density and acidic bath chemistry. The resulting problem is an accelerated wear of the graphitized cathode blocks, thought to be caused by the formation and subsequent dissolution of aluminum carbide at the cathode surface. This phenomenon is now recognized as one of the important factors limiting the cell lifetime, and will be discussed further in the literature review. A special laboratory test method has also been developed to elucidate the mechanism of aluminum carbide formation. The aluminum carbide formation has also been studied using X-ray diffraction, as well as optical and scanning electron microscopy. Preliminary analysis of the results will be presented in this paper. Introduction Several chemical and electrochemical reactions of formation of aluminum carbide in Hall-Heroult cells have been proposed in the literature. The direct chemical reaction between the reduced liquid aluminum and the carbon at the cathode surface has been evidenced [1]. Al (l) + C (s) = Al 4 C 3 (s, diss) (1) Reaction (1) is thermodynamically strongly favored at cell operating temperatures. According to Worrell, at 1000 C [2] the standard Gibbs energy change for this reaction is 170 kj. The presence of an alumina film on the carbon cathode surface can inhibit the formation of aluminum carbide at lower temperatures [3]. Sørlie and Øye [4] suggested that the layer of aluminum carbide formed on the surface can act as diffusion barrier and protect the carbon surface from further reaction with liquid aluminum. Molten cryolite tends to increase the rate of Reaction (1) [5,6] which is probably due to the dissolution of the alumina layer by the melt, and possibly some wetting effects. It is well known that both sodium from the liquid metal and the bath penetrate the carbon cathode during aluminum electrolysis. The chemical reaction between the molten electrolyte and intercalated sodium within the carbon cathode is considered as another source of aluminum carbide formation (Reaction (2) [1]); also alumina dissolved in the bath can react with sodium and carbon to produce the aluminum carbide as proposed in Reaction (3) [1]: 4 Na 3 AlF 6 (l) + 12 Na (in C) + 3 C (s) = Al 4 C 3 (s) + 24 NaF (l) (2) 8 Al 2 O 3 (s) +12 Na (in C) + 3 C (s) = Al 4 C 3 (s) + 12 NaAlO 2 (s) (3) Based on wetting phenomena, non-wetting fluids do not penetrate the pores of a material without the application of an external pressure. Thus, it has been shown that the presence of aluminum in the electrolyte, and sodium in the carbon, improves the wettability of the carbon cathode by the electrolyte. Therefore the pores can be rapidly filled with molten electrolyte. Metallic sodium in the cathode acts as a reactive in the formation of Al 4 C 3 according to reactions (2) and (3). It has been reported that that the electrolyte penetration front in the cathode block follows the Na penetration front [1, 5, 6]. Reaction (2) is not stable in the presence of oxygen (in the form of CO and/or CO 2 ) or nitrogen, Al 4 C 3 can only be formed if oxygen and nitrogen are fully reacted. Electrochemical formation of aluminum carbide is also possible and is generally accepted to be a function of current density: 3 C + 4 AlF 3 (diss) + 12 e = Al 4 C 3 (s, diss) + 12 F (diss) (4) 4 Al C +12 e = Al 4 C 3 (s, diss) (5) The effect of current density on Al 4 C 3 formation has been proposed in the first place by Keller et al. [7] (Reaction (4)). Liao et al. [8] suggested that the aluminum carbide formation is electrochemical in nature, and might arise through Reaction (5). The aluminum ion is supposed to come from the partial anodic dissolution of liquid aluminum at the metal/electrolyte interface. The driving force for the anodic dissolution of Al 3+, and its transport to the cathode block, will be increased with the increasing current density within the system, this leads to enhancement of Al 4 C 3 formation [7, 9]. It is well known that aluminum carbide is less soluble in liquid aluminum than in cryolitic melts. The cycle for the formation and dissolution of aluminum carbide is recognized as one of the modes of deterioration or wear of the carbon blocks in industrial cells. Intensive investigations of the aluminum carbide solubility in cryolite melts and in molten aluminum have been carried out by Dewing et al. [10] and Ødegard et al. [11, 12]. For these tests, molten aluminum and bath components were placed in a carbon crucible for 4 to 5 hours. Samples were quenched and the content of Al 4 C 3 was measured via the release of CH 4 from the decomposition of carbide in an acid. Dewing et al. [10] suggested the dissolution of Al 4 C 3 in cryolitic melts in the form of Al 2 CF 2, according to the following Reaction: Al 4 C AlF 3 = 3 Al 2 CF 2 (6) The results of Ødegard et al. [11, 12] show that Al 4 C 3 is dissolved in the electrolyte as the ionic species, Al 3 CF 8 3-, while it is soluble 1123

2 in molten aluminum as elemental carbon. For low bath ratios 4- Al 4 CF 12 is not excluded. The dissolution of Al 4 C 3 in cryolite melts was postulated from the following equilibrium: 3- Al 4 C 3 (s) + 5 AlF 3 (diss) + 9 NaF (l) = Al 3 CF Na + (7) Ødegard et al. [11, 12] reported a value of the equilibrium constant K(C) of at 1020 C. Their results were confirmed by Gudbranson et al. [13] during solubility tests of Al 4 C 3 under electrolysis conditions. Reaction (7) is generally accepted with respect to the dissolution of Al 4 C 3 in cryolitic baths. Note that the carbide solubility in cryolitic melts (2.1% at CR=1.8 and 1020 o C [14]) is much higher than that in molten aluminum (0.03% [15]). The rate of dissolution of aluminum carbide in a cryolitic bath layer over molten aluminum is limited by the low solubility into the metal. Dispersed fine carbon particles released into the electrolyte have been observed both in laboratory and in industrial cells. This carbon dust might originate through the oxidation of dissolved aluminum carbide by the anodic gas, or, directly deposited at the anode through anodic oxidation (Gudbranson et al. [13], Sørlie et Øye. [1], Ødegard et al. [11, 12]). Dissolved carbide present in the melt may be transported to the anode, via the molten aluminum layer by the diffusion controlled process, into the electrolyte, where it is oxidized. The first oxidation mechanism proposed in the literature was the oxidation of dissolved Al 4 C 3 by anodic gases. Dissolved aluminum carbide may come into contact with entrained CO 2 bubbles or dissolved CO 2 present in the electrolyte, and get oxidized to alumina and CO (g) according to the following oxidation reactions: bath can react with these dispersed carbon particles to form dissolved Al 4 C 3, which causes a loss in current efficiency with respect to the aluminum formation. Some theoretical aspects of the transfer of carbon, from the cathode to the anode, have been discussed previously [13, 16, 17]. At low cell voltages, it was mentioned that Reaction (4) is the main cathodic reaction (cathodic dissolution of carbon), and Reaction (11) is the main anodic reaction (carbon deposition at the anode); which means that the main overall cell reaction, will be a transfer of carbon from the cathode at the anode (i.e. C cathode = C anode ). This paper provides an investigation of aluminum carbide formation in both cryolitic baths and on carbon cathode surfaces during electrolysis. The goal of this study is to identify the mechanisms by which aluminum carbide is formed, and to study its behavior in the cell. Experimental and Results The experimental electrolytic cell was designed in order to simulate the real operating conditions of modern alumina reduction cell. For this reason electrolysis was conducted according to the conventional arrangement (i.e. the anode is suspended by a rod above the cathode) using an alumina saturated bath. The whole assembly used to perform electrolysis is shown in Figure 1. Al 4 C 3 (diss) + 9 CO 2 (g) = 2 Al 2 O 3 (diss) + 12 CO (g) (8) Al 4 C 3 (diss) + 6 CO 2 (g) = 2 Al 2 O 3 (diss) + 3 C (s) + 6 CO (g) (9) Reactions (8) and (9) result in the consumption of dissolved aluminum carbide in the bath, thus preventing the latter from reaching saturation. The second oxidation mechanism, also proposed in the literature, is the anodic oxidation of dissolved Al 4 C 3 resulting in the electrodeposition of carbon at the anode. It has also been suggested that dissolved Al 4 C 3 can be oxidized electrochemically [13] according to Reactions (10) and (11): Na 3 Al 3 CF 8 (l) + NaF (l) = C (s) + 3 AlF 3 (l) + 4 Na e (10) Al 3 CF 8-3 (diss) + F - (diss) = C (s) + 3 AlF 3 (diss) + 4 e (11) For low current densities it was demonstrated that Reaction (11) may take place on a graphite substrate [11, 12]. This indicates that the reverse reaction will also be favored in the cathodic region (i.e. the cathodic dissolution of carbon in cryolite melts as carbide); more precisely, in the cracks and in the large open porosity of cathode blocks, caused by ohmic voltage drop inside the cathode. Reactions (10) and (11) lead to solid carbon being deposited at the anode, resulting in the formation of the so-called anode spikes [11, 12]. Dispersed carbon particles formed by Reaction (11) can eventually be oxidized by CO 2. Thus, dissolved aluminum in the Figure 1: Experimental set-up. The cathode crucibles used in the cell were machined directly from graphitized commercial grade carbon blocks. The cathodesamples were cylindrical cores, with a height of 260 mm and an external diameter of 125 mm. In order to contain the anode and the electrolyte, a hole was drilled in the cathode crucibles. An alumina tube was inserted in each crucible to cover the internal sidewalls in order to obtain more uniform and vertical current distribution. The electrolyte composition used is 95.3 wt. % Na 3 AlF 6 and 4.7 wt. % CaF 2 (CR=3.0) with no alumina. For the two experiments (6 and 12 hours), the initial amount of bath was kept at 704 g. The alumina cylinder covering the internal wall of the cathode crucible is in direct contact with the bath, and should provide minimal alumina content to the bath. No aluminum metal was added initially. 1124

3 Cylindrical anodes were machined from graphite blocks with a diameter of 25.4 mm and a length of mm. A small cone was machined at the bottom of each anode to prevent the accumulation of gas bubbles on the anodic surface. The vertical position of the anode was adjusted at 2 cm above the bottom of cathode; the anode was centered along the vertical axis of the cell. The crucible was installed in a gas-tight inconel furnace purged with a nitrogen flow. Electrolysis was performed at a current density of 0.8 A/cm 2 at 980 C. The electrolysis time for the 2 tests was 6 and 12 hours respectively. At the end of the electrolysis period, the bath was allowed to solidified (the cooling rate of the bath was between 2 and 3 C/min in the first hours after electrolysis). Samples from the bath were taken at 3 different positions (see Figure 2). Figure 3 shows a post-mortem picture of the crucible containing the solidified bath and metal for the 12- hour test. Note that the metal produced during electrolysis formed nodules. Figure 3: Post-Mortem photograph after electrolysis. Figure 4 shows pieces of solidified bath in contact with the molten aluminum nodule after the electrolysis test lasting 12 hours. The yellow layers shown in Figure 4 were identified by XRD and SEM as aluminum carbide. These layers were found in the position A below the anode, between the surface of the electrolyte and the surface of the aluminum produced during the electrolysis. Figure 2: Illustration showing the three sampling positions. The cathode crucibles were also cut in the following regions: Interface aluminum-carbon (i.e. cathodic surface, under the anode where the bulk of the metal produced was formed). Interface cryolite bath-carbon. (i.e. cathodic surface, far from the anode). Aluminum Carbide Formation in Cryolitic Baths Aluminum carbide is very reactive in the presence of oxygen and humidity. Al 4 C 3 can eventually be hydrolyzed by H 2 O to form alumina or aluminum hydroxide, and is also rapidly oxidized by O 2 to alumina, according to the following reactions: Figure 4: Aluminum carbide layers at the interface between the bath and the molten aluminum. No solid aluminum carbide layer at the metal/bath interface was observed for the electrolysis test lasting 6 hours. The Al 4 C 3 film reached a thickness of up to 85 μm as can be seen in Figure 5. Al 4 C H 2 O = 4 Al (OH) CH 4 (12) Al 4 C H 2 O = 2 Al 2 O CH 4 (13) Al 4 C 3 + O 2 = Al 2 O 3 + CO 2 (14) In order to avoid hydrolysis and oxidation of Al 4 C 3, the samples were stored under argon atmosphere. The sample transfer to the analysis instruments was completed over a short time to minimize the reaction with moisture. Figure 5: Thickness of aluminum carbide layer (optical photograph) at the metal/bath interface. 1125

4 Figure 6 shows the x-ray digital mapping of the metal-bath interface. Carbon signal can be observed. Main phases observed in solidified bath (6 hours of electrolysis) Despite the fact that aluminum carbide can be oxidized quickly by Reactions (12), (13) or (14) it is possible to observe a small amount of carbon dispersed in the Al 4 C 3 layer between the surface of the aluminum-bath interface. The elemental maps also show the presence of oxygen in this layer; this can be explained by the formation of alumina through the Reaction (14), when the aluminum carbide is exposed to oxygen from air or can come from the formation of solid alumina from the bath during cooling. Rrelative intensity Na3AlF6 Na5A3lF14 Na2Ca3Al2F14 Al2O3 Al AlF3 A B C Main phases observed in solidified bath (12 hours of elctrolysis) Relative intensity Na3AlF6 Na5A3lF14 Na2Ca3Al2F14 AlF3 Al4C3 Al CaF2 Figure 6: SEM and x-ray digital mapping of the Al 4 C 3 layer at the interface aluminum-bath (CR =2.56, 12 hours of electrolysis). The Al 4 C 3 layer was also observed by SEM as shown in Figure 7. This picture presents the solidified bath/carbide interface. Analysis of the solidified bath after electrolysis was performed by X-ray Diffraction (XRD) and Scanning Electron Microscopy. XRD measurements were made using Phillip Xpert Pro diffractometer, equipped with a Cu kα anode. The scanned area was cm 2. The scans were performed from 10 to 90, with a step of and a 0.5 s counting time. XRD patterns were performed by utilizing JADE software for phase identifications. The results from XRD measurements are shown in Figure 8. A B C Figure 8: Main phases observed in the solidified bath (under nitrogen atmosphere) at positions A, B and C. Our results for the 6- and 12-hour tests are: 1) The solid phases detected by XRD in the solidified bath in the 3 zones A, B and C are cryolite (Na 3 AlF 6 ), chiolite (Na 5 Al 3 F 14 ), Ca-cryolite (Na 2 Ca 3 Al 2 F 14 ), aluminum fluoride (AlF 3 ), fluorite (CaF 2 ), corundum (Al 2 O 3 ), aluminum (Al) and aluminum carbide (Al 4 C 3 ). Other possible phases were not detected because of: XRD patterns of the solidified bath are very complicated. This makes it difficult to interpret the small diffraction peaks usually overlapped by larger peaks of major component. Phases were not crystalline. 2) In the electrolysis experiment lasting 12 hours, the peaks for Al 2 O 3 were note observed indicating that alumina in the bath was completely consumed during the electrolysis period, this being confirmed by a rapid increase in voltage over a short time (anode effect) at the end of the electrolysis period. 3) The time of electrolysis is also an important parameter in the formation of new phases such as Al 4 C 3. The presence of this phase in the bath is observed just for the 12 hours test not in the 6 hours test. Al 4 C 3 was only found just below the anode (the position A where electrolysis is performed). Figure 7: SEM image showing the Al 4 C 3 layer on the electrolyte surface. Other analyses were performed on samples from the cathode surface by optical microscope as shown in Figure

5 contact with the bath, only if the bath is saturated with aluminum carbide. Otherwise this solid aluminum carbide at the metal bath interface could have formed from a bath unsaturated in aluminum carbide during the cooling process after electrolysis. This is the first possible mechanism of aluminum carbide formation. At this point, further investigations are necessary. During electrolysis, carbon has been transferred to the metal-bath interface. It is thus necessary to consider the various mass transfers in order to explain how aluminum carbide is transferred from the cathodic surface towards the metal-bath interface. Literature data shows that carbon particles found in the bath during electrolysis can come from two different processes; 1) via the formation of aluminum carbide (Reactions (4) and (5)) at the bath-carbon interface, followed by the dissolution (Reaction(7)) and subsequent oxidation of aluminum carbide (see Reaction (9)) which then leads to the formation of carbon dust within the bath; 2) carbon particles can also come from the carbon dusting of the anodes (mechanical degradation). Figure 9: Optical microscope images showing the cathodic surface after electrolysis (12 hours): A) Region between the carbon surface and the metal. B) Region between the carbon surface and the electrolytic bath. Finally, carbon spikes were observed at the surface of the anode for the 12 hour test, as shown in Figure 10. Solid carbon particles in suspension in the bath could react with the liquid aluminum nodules to form solid aluminum carbide at the metal-bath interface if the supply of these carbon particles to the metal is more intense than the dissolution rate of the formed aluminum carbide in the bath. This is the second possible mechanism of aluminum carbide formation. In our experiments, the carbon dusting of the anode must have been negligible since the anodes were machined from electro-graphite material. In both experiments, the carbon consumption was lower than 100%. So the first process is most probably the dominating one, if the carbide was formed from solid carbon particles. Our observations show that 1) the bath-carbon interface is much larger than the metal-carbon interface (Figure 3); 2) the carbide formed at the bath-carbon interface is all dissolved in the bath (Figure 9.B); 3) the carbide formed at the metal-carbon interface is not dissolved by the bath (Figure 9.A) - Note that the solubility of the aluminum carbide in the cryolitic melt (about 2 wt % for a CR=1.8) is higher than in molten aluminum (around 100 ppm) [11]. 4) Carbon spikes are formed on the anode surface (Figure 10). All these observations indicate that the bath contained of high level of carbide. Reactions (8-11) are reducing the carbide level in the bath, which is probably under saturated. Figure 10: Spikes formed at the surface of the anode after electrolysis (12 hours). Discussion Conventional carbon cathodes are not wetted by liquid aluminum. In the present tests, the cathode surface was not completely covered by liquid aluminum even with an electrolysis time of up 12 hours. This led to an agglomeration of the aluminum in the form of a liquid nodule, with a high-contact-angle between the aluminum and the cathode (Figure 3). The contact area between the bath and the carbon is much larger than the contact area between aluminum and carbon. It is well known that Al 4 C 3 is formed in aluminum electrolysis cells whenever carbon and aluminum are in mutual contact. The results show that the layer of carbide formed at the metal-bath interface is far from the cathode surface (Figure 4). Solid aluminum carbide can form at 980 o C in In order to compare and quantify the concentrations (calculated from a simple mass transfer model) of the carbon particles dispersed in the electrolyte or metal, more recent studies, conducted by Skybakmoen et al. [18] have shown that the concentration of carbon particles "as aluminum carbide is much greater than the carbon particles as carbon in the electrolyte or in the metal phases. This should be in agreement with the high dissolution of Al 4 C 3 at the bath-carbon interface showed in Figure 9, and with the literature data, as discussed above. This indicates that the majority of the amount of carbon that forms the carbide layer at the metal-bath interface comes directly from the carbide formed at the bath-carbon interface. Skybakmoen et al. [18] proposed that the concentration of carbon "as aluminum carbide or as elemental carbon, at the metal-bath interface (metal side) is always less than the concentration of carbon at the metal-bath interface (bath side). The results in the 1127

6 present work (Figures 4 and 7) are in agreement with these observations. Due to the continual aluminum carbide oxidation in the bath, most probably more by anodic oxidation (Reaction (10)) than by CO 2 oxidation (Reactions (8) and (9)), the saturation of the bath with aluminum carbide does not occur in the whole volume of the bath. The formation of anode spikes (Figure 10) is a proof that aluminum carbide has been consumed. So the total carbon formed by the second oxidation mechanism (Reactions 10 and 11) will not participate in the formation of aluminum carbide at the aluminum-bath interface. Following up on the above discussion and analysis, it can be concluded that the major mechanism which explains the presence of carbon (as carbide) at the metal-bath interface is the transfer of dissolved Al 4 C 3 in the bath from the bath-carbon interface to the bath-metal interface. Before reaching the bath-metal interface, some of this aluminum carbide might be oxidized by CO 2 bubbles producing carbon particles which can then reach the aluminum bath interface to form solid aluminum carbide. Based on the results obtained by DRX, Al 4 C 3 was found only in the position under anode A. Conclusions Experiments were conducted in order to produce aluminum carbide in a laboratory electrolysis cell containing cryolite. Preliminary results described above present a good picture of the formation of aluminum carbide, especially at the aluminum-bath interface. 1) The distribution of phases was analyzed at three different positions in the solidified bath. The phase distribution in the B and C positions was quite similar but otherwise different from the A position. Solid aluminum carbide was only observed in the A position for the 12 hour test. 2) This solid Al 4 C 3 layer was formed at the aluminum-bath interface. 3) Mechanisms were proposed to explain the formation of Al 4 C 3 at the aluminum-bath interface. 4) Carbon spikes were observed on the anode surface confirming the electro decomposition of aluminum carbide according the Reactions (10) and (11). Future work is needed to evaluate carbide formation when aluminum metal layer which cover the full cathode surface is added before electrolysis. Acknowledgements Financial support from the Fond Québécois de la Recherche en Nature et Technologie (FQRNT) is gratefully acknowledged. 2. W. L. Worrell, "Carbothermic Reduction of Alumina. A Thermodynamic Analysis," Canadian Metallurgical Quarterly,4 (1965), H. A. Øye and B. J. Welch, "Cathode Performance: The Influence of Design, Operations, and Operating Conditions," JOM,50 (1998), M. Sørlie and H. A. Øye, "Deterioration of Carbon Linings in Aluminum Reduction Cells. Part II - Chemical and Physical Characterization of Cathode Carbons," Metall,38 (1984), P. Brilloit, L. P. Lossius, and H. A. Øye, "Melt Penetration and Chemical Reactions in Carbon Cathodes During Aluminium Electrolysis. Laboratory Experiments", Light Metals, (1993), M. Sørlie and H. A. Øye, "Survey on Deterioration of Carbon Linings in Aluminium Reduction Cells," Metall,36 (1982), R. Keller, J. W. Burgman, and P. J. Sides, "Electrochemical Reactions in the Hall-Heroult Cathode", Light Metals, (1988), X. Liao and H. A. Øye, "Carbon Cathode Corrosion by Aluminum Carbide Formation in Cryolitic Melts", Light Metals, (1999), P. Rafiei, F. Hiltmann, M. Hyland, B. James, and B. Welch, "Electrolytic Degradation Within Cathode Materials", Light Metals, (2001), E. W. Dewing, "Solubility of Aluminum Carbide in Cryolite Melts," Transaction of the Metallurgical Society of AIME,245 (1969), R. Ødegard, "On the Solubility of Aluminum Carbide in Cryolitic Melts," Metallurgical and Materials Transactions B,19 (1988), R. Ødegard, A. Sterten, and J. Thonstad, "On the Solubility of Aluminum Carbide and Electrodeposition of Carbon in Cryolitic Melts," Journal of the Electrochemical Society,134 (1987), H. Gudbrandsen, A. Sterten, and R. Ødegard, "Cathodic Dissolution of Carbon in Cryolitic Melts", Light Metals, (1991), R. Ødegard and S. H. Midtlyng, "Electrochemical and Chemical Reactivity of Carbon Electrodeposited From Cryolitic Melts Containing Aluminum Carbide," Journal of the Electrochemical Society,138 (1991), R. C. Dorward, "Aluminium Carbide Contamination of Molten Aluminium," Aluminium,49 (1973), X. Liao and H. A. Øye, "Physical and Chemical Wear of Carbon Cathode Materials", Light Metals, (1998), X. Liao and H. A. Øye, "Carbon Cathode Wear in Aluminum Electrolysis," Tribologia,17 (1998), E. Skvbakmoen, A. P. Ratvik, A. Solheim, S. Rolseth, and H. Gudbrandsen, "Laboratory Test Methods for Determining the Cathode Wear Mechanism in Aluminium Cells", Light Metals, (2007), References 1. M. Sørlie and H. A. Øye, Cathodes in Aluminium ElectrolysisDüsseldorf Aluminium-Verlag ), 408 p. 1128