A Laboratory Study of Partial Anode Effects during Aluminium Electrowinning

Perfluorocarbons (PFCs) are potent greenhouse gases (GHG) with long lifetimes and high global warming potentials, giving them a high capacity to inflict harm on the environment.  The aluminium industry is one of the bigger emitters of these gases and consequently a lot of effort has been put into understanding the mechanism behind the emissions.  

Aluminium is produced electrochemically by consuming alumina (dissolved in molten cryolite) and carbon. In addition to aluminium this also produces CO₂ with a fraction of CO. The reversible potential for the reaction is about 1.2 V, but due to overpotentials related to the electrodes and IR drops in different parts of the cell, especially from the electrolyte, a regular industrial cell needs to operate at a much higher voltage (3.8 – 4.5 V). If the alumina concentration in the electrolyte gets too low the polarisation increases which opens the possibility for other anode reactions, like the production of the PFC gases CF₄ and C₂F₆. The phenomenon is termed anode effect (AE) and traditionally when this occurred the voltage could raise to levels of 20 V or higher, and the current distribution over the anodes would become erratic.

The existence of PFCs not related to the traditional anode effect in industrial aluminium production cells has been documented for a wide range of technologies [1-4]. Industrial measurements have shown that PFC production can begin at one anode during normal cell potentials and propagate to the full cell given the right conditions [5]. To experience the phenomenon in the laboratory, a system with more than one anode was designed.

The setup consists of two separate chambers made out of a single cylindrical block of graphite, which acts as a shared cathode for the system. Each chamber has its own electrolyte solution, reference electrode and anode. A thermocouple is placed within each anode to get instant feedback on the heat production of the reactions occurring (Figure 1). The anodes are connected in parallel to a power supply unit that can operate under both constant current (CC) and constant voltage (CV). Individual anode currents are measured over a resistance connected in series with each parallel anode, and voltages are measured against the respective reference.

By varying factors such as alumina concentration it is possible to emulate the local variances in feeding that can develop over the vastness of an industrial cell. By measuring individual anode voltages, currents and temperatures valuable information regarding partial and low voltage anode effects can be gained.

The results indicate that it is possible for at least one anode to go into, and out of what is believed to be partial production of PFC. It was also discovered that this mode could be sustained for a half hour before a traditional anode effect with full blocking of the surface was experienced.

Keywords: GHG, PFC, NAE-PFC, Anode effect, Aluminium Electrolysis, Low voltage AE, LCV-AE

References

[1]   W. Li, Q. Zhao, S. Qiu, S. Zhang, and X. Chen, “PFC Survey in Some Smelters of China,” Light Metals 2011, pp. 357-360: John Wiley & Sons, Inc., 2011.

[2]   L. Wangxing, C. Xiping, Y. Jianhong, H. Changping, L. Yonggang, L. Defeng, and G. Huifang, “Latest Results from PFC Investigation in China,” Light Metals 2012, pp. 617-622: John Wiley & Sons, Inc., 2012.

[3]   J. Marks, and C. Bayliss, "GHG Measurement and Inventory for Aluminum Production,” Light Metals 2012, pp. 803-808: John Wiley & Sons, Inc., 2012.

[4]   H. Åsheim, T. A. Aarhaug, A. Ferber, O. S. Kjos and G. M. Haarberg, “Monitoring of Continuous PFC Formation in Small to Moderate Size Aluminium Electrolysis Cells,” Light Metals 2014, pp. 535-539: John Wiley and Sons, Inc., 2014.

[5]   S. Kolås, P. McIntosh and A. Solheim, “High Frequency Measurements of Current through Individual Anodes: Some Results from Measurement Campaigns at Hydro,” Light Metals 2015, pp. 729-734: John Wiley and Sons, Inc., 2015.