AN ELECTROANALYTICAL STUDY OF ELECTRODE REACTIONS ON CARBON ANODES DURING ELECTROLYTIC PRODUCTION OF ALUMINUM

In connection with the electrolytic production of aluminum the anodic reactions have been studied on carbon microelectrodes by voltammetry and chronoamperometry. Anode gases have been analyzed by gas chromatography on-line during controlledpotential electrolysis in a laboratory-scale aluminum reduction cell. When the voltage exceeds a critical value (about 3 V vs Al/Al), the cell current drops precipitously. We attribute this to the formation of a highly resistive film on the surface of the anode. The existence of this putative film was shown to be strictly potential dependent the film could be formed and removed at will by regulation of applied potential. The rate of PFC generation was found to vary with the magnitude of the anodic overpotential. A PFC reduction strategy that takes into account the design of the power supply is proposed. Introduction In the primary production of aluminum by the Hall-Héroult process, a cell malfunction known as the anode effect results in the generation of the perfluorocarbons (PFCs) CF4 and C2F6 [1]. Owing to the high global warming potentials of these gases [2] and the fact that, in the U.S., aluminum smelting is the number one point source of PFC emissions, the Environmental Protection Agency and the primary aluminum producers have established the Voluntary Aluminum Industrial Partnership (VAIP) with the goal of substantially reducing PFC emissions [3]. To gain a better understanding of the mechanism of PFC generation, VAIP has undertaken two projects: (1) measurements of PFC emissions from industrial smelters [4], and (2) basic research into the attendant electrochemistry. The latter is the substance of the present article. Previous work in this laboratory had shown that, in conformity with industrial data, PFCs are generated only when the cell goes on anode effect [5]. Furthermore, time-resolved data taken by Kimmerle et al. on industrial cells [6] and by Nissen and Sadoway on laboratory-scale cells [5] confirm that by far the highest PFC levels are attained during the first several minutes after the cell goes on anode effect. Clearly, any plan to reduce emissions needs to address this fact. Accordingly, the present investigation was directed at the underlying kinetics of PFC generation, more specifically the nature of the reactions at the electrode. The fundamental electrochemistry was studied by cyclic voltammetry and chronoamperometry. Subsequently, controlled-potential electrolysis with analysis of anode gas was performed. This article reports the results of these experiments and speculates on the mechanistic features of the reactions occurring on the anode at various potentials. In addition, the role of the power supply in sustaining undesirable PFC generation is addressed. Experimental The cell design was largely identical to that used previously in this laboratory [5,7]. Electrochemical measurements were made with a potentiostat (Solartron Electrochemical Interface, model 1287, Allentown, PA) controlled by a personal computer running Corrware (Scribner Associates, Southern Pines, NC). The working electrode was made of either a graphite or a glassy carbon rod, about 2 to 3 mm in diameter, shrouded by BN, so that about 10 mm in length was exposed to the melt and electrochemically active. The molybdenum crucible served as the counter electrode. The reference electrode was Al/AlF3 based upon the design patented by Sadoway [8,9]. For the analysis of anode gas during the electrolysis, a tubular anode was used [7]. The outer vertical surface is insulated with BN; hence, only the inside walls (8 mm in dia., 10 mm height) contact the electrolyte. Anode gas was drawn through an alumina tube from the chamber formed above the melt “inside” the electrode. Gas analysis was performed by gas chromatograph (Model M200 Dual Gas Analyzer, MTI Analytical Instruments, Fremont, CA). The electrolyte was prepared from reagent-grade chemicals: AlF3 (98%), CaF2 (99%) and Na3AlF6 (97%) dried at 500C under Ar flow for about 12 hours and pre-melted before being introduced into the reduction cell. Prior to cell testing, melts were subjected to pre-electrolysis to rid them of impurities. All experiments were conducted at a temperature of 975oC. Results and Discussion Cyclic Voltammetry Cyclic voltammetry was performed on melts doped with alumina. Because the technique is conducted with a microelectrode and relies upon depletion of the electroactive species on a time scale comparable to that of the voltage sweep, it is necessary to fix the alumina concentration at a value much lower than is typically encountered in industrial operations. In this set of experiments, alumina levels ranged from 0.01 wt % to 0.60 wt %. The solvent was 84 wt % Na3AlF6 11 wt % AlF3 5 wt % CaF2. Figure 1 shows the behavior of a melt containing 0.01 wt % Al2O3 on a graphite electrode at a sweep rate of 100 mV s. On the forward (anodic) sweep, peaks are evident at 1.8 V, 3.4 V, and 4.3 V. Repeatedly, on the reverse scan nothing measurable was found in the way of a cathodic peak, indicating that the electrode process is highly irreversible. Actually, the voltammogram of the reverse scan is rather similar to that of forward scan. We attribute the rise in current at 1.8 V to the discharge of oxide species, since the potential is not extreme enough to support the discharge of fluoride: the lowest potential for PFC evolution is 2.5 V for CF4. We suspect that the peaks at 3.4 V and 4.3 V are related to the discharge of fluoride.