Bubble Evolution on Different Carbon Anode Designs in Cryolite Melt
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RODUCTION
THE anode reaction in the Hall–He´roult process is complicated because it involves discharge of oxide present in oxyfluoride complex ions, among others 2Al2OF26 and Al2O2F4 , presumably adsorption of oxygen at the carbon anode and a following thermal or electrochemical desorption.[1,2] The electrolysis takes place in cryolite melt (3NaF-AlF3) with dissolved alumina but discharge of fluoride ions does not take place during normal electrolysis since this potential is higher than the discharge potential of the oxyfluoride complexes. The overall reaction can be one or both of the following:
2Al2 O3 ðlÞ þ 3C ðsÞ ! 4AlðlÞ þ 3CO2 ðgÞ E0 ¼ 1:16 V
½1
Al2 O3 ðlÞ þ 3C ! 2AlðlÞ þ 3COðgÞ E0 ¼ 1:02 V ½2 The cathode product is liquid aluminum, and the anode product is a CO2/CO gas mixture. As a result, carbon anodes are consumed. The main primary anode product is CO2 (g), but some CO can be formed at low current densities 0.05 - 0.1 A cm2.[3] Several mechanisms are proposed for the anode reaction. Picard et al.[4] studied reaction [1] using electrochemical impedance and proposed the following three-step mechanism at low current densities and low overpotentials: 1. Diffusion of the oxyfluoroaluminate species from the bulk of the melt to the surface of the graphite anode: 1x AlOF1x x ðmeltÞ ! AlOFx ðelectrodeÞ
NIKOLINA STANIC, IVANA JEVREMOVIC, and ESPEN SANDNES are with the Norwegian University of Science and Technology, 7491 Trondheim, Norway. Contact e-mail: [email protected] ANA MARIA MARTINEZ is with the SINTEF Industry, 7034 Trondheim, Norway. Manuscript submitted June 20, 2019. Article published online April 14, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS B
2. Dissociation of the oxyfluoroaluminate species and adsorption of the oxide ions, followed by their discharge and the formation of the adsorbed carbon–oxygen species: þ C ! COads þ AlF3x þ 2e AlOF1x x x
VOLUME 51B, JUNE 2020—1243
3. Dissociation of the oxyfluoroaluminate species and the adsorption of the oxide ions, their discharge in the presence of the COads and the formation of the gaseous CO2: þ COads ! CO2 ðgÞ þ AlF3x þ 2e AlFO1x x x A similar mechanism with the electrochemical desorption step was also proposed by Kisza et al.[5] Thonstad[3] also proposed an electrochemical adsorption followed by a thermal desorption step involving a combination of two COads species. The most important reaction responsible for the loss in current efficiency in the aluminum electrolysis is the back reaction between dissolved CO2 and dissolved Al and Na in the melt. The sodium is created through the reaction between aluminum and sodium fluoride at cathode/electrolyte interface. The back reaction can be written[6,7]: 2Al ðdiss:Þ + 3CO2 ðdiss:Þ ! Al2 O3 ðdiss:Þ + 3CO (g) ½3 The CO content in the anode off-gas can arise from the electrochemical reaction directly, from the back reaction and also from the Boudouard reaction: CO2 þ C ! 2CO
½4
The carbon can come from the anode itself or carbon dust in the melt. At 950 to 1000 C, the equilibrium is displace
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