Metal Anode Performance in Low-Temperature Electrolytes for Aluminum Production

Abstract

An investigation has been undertaken into the performance of metal alloy anodes used to produce aluminum via an electrochemical method. Alumina was electrolyzed in NaF/AlF 3 and KF/AlF 3 electrolytes and mixtures thereof with copper-nickel-iron (Cu:Ni:Fe) alloy anodes and titanium diboride (TiB 2 ) cathodes. The operating temperatures of the electrochemical cells ranged from 973 K to 1123 K (700°C to 850°C), with an anode current density of 5000 A/m 2 . Cells ranged in current capacity from 10 to 300 amperes, with oxygen gas formed at the anode and molten aluminum collected from the cathode. Posttest anodes were sectioned, and elemental maps were performed to characterize the distribution of the chemical phases, including the metal electrodes, bath phases, and aluminum metal production, which were used to determine the reaction mechanisms of the cell. The metal alloy slowly corroded and formed an adherent, electronically conducting nickel ferrite plus copper scale during the operation of the cell. The proposed mechanisms of the anode performance are described herein.

Layer growth mechanisms on metallic electrodes under anodic polarization in cryolite-alumina melt

“…This diffusion leads to the creation of pores and voids in the wüstite layer, especially at the metal/wüstite interface, and at the grain boundaries where the diffusion rate is higher. Unlike the results of Gallino [18], Khramov [25] and Beck [29], no FeF x phase is observed at the metal/oxide interface or in the infiltrated salt. This is attributed to the short polarization time (maximum 1000 s compared to at least 72 h in the other studies), not long enough to allow iron fluoride formation.…”

Studies on the oxidation rate of metallic inert anodes by measuring the oxygen evolved in low-temperature aluminium electrolysis

“…Low-melting systems based on KF instead of a conventional electrolyte decrease the working temperature by 100-150°C. These systems could lead to substantially prolonged lifetimes of the anode because of the significant reduction of the oxidation rate [1][2][3]. Towards this end, we report here our investigation of the oxidation rate of metallic anodes in the course of low-temperature aluminium electrolysis.…”

Synthesis and thermal stability of (Co,Ni)O solid solutions

“…Among many metallic materials studied so far, Ni-Fe-Cu-based alloys appear to be promising in their ability to form a layer of nickel ferrite (NiFe 2 O 4 ) on the surface of the anode that is recognized for its low solubility in the cryolithic bath. [3][4][5] However, these alloys are sensitive to the fluoridation reaction with the cryolithic medium, forming non-conductive nickel or iron fluoride at the metal/ oxide interface, which increases the cell voltage and reduces energy efficiency of the electrolysis process. 6 It is therefore essential to remedy this problem by protecting Ni-Fe-Cu anodes with an appropriate fluoridation barrier.…”

“…In this context, the use of CoO-based protective coatings for metallic anode appears promising as shown by Nguyen and de Nora. 7 However, CoO is likely to be converted into Co 3 is an n-type semiconductor that displays a significantly higher O 2 evolution overpotential than p-type semiconducting CoO. 7 It has been found that CoO can be stabilized under Al electrolysis conditions by adding about 15 wt% of Ni to form (Co,Ni)O solid solution.…”