(Co,Ni)O Coated Anodes for CO2-Free Al Production

Abstract

Canadian aluminum production is an important source of greenhouse gases (GHGs) with 6 Mt of CO2 eq emitted in 2017, which is equivalent to the amount of GHG generated annually by about 2 million cars. The current technology consumes carbon anodes during the electrolysis of aluminum to form CO2 according to the overall reaction: Al2O3 + 3/2 C = 2 Al + 3/2 CO2. The most effective solution would be to replace the consumable carbon anodes with so-called inert anodes that emit O2 rather than CO2 and that are based on the following overall reaction: Al2O3 = 2 Al + 3/2 O2. This would reduce GHGs by 75 to 100% depending on the type of emissions (CO2, CFx, NOx, SOx, etc.). However, the design of inert anodes is a major challenge because of the severe conditions during aluminum electrolysis that require materials with excellent corrosion and thermal shock resistance while having the same adequate electrochemical properties [1].Among inert anodes studied so far, Cu-Ni-Fe-based alloys appear to be the most promising owing to their ability to form a layer of nickel ferrite (NiFe2O4) on the surface of the anode upon Al electrolysis [2-4]. This nickel ferrite has low solubility in a cryolithic medium. However, the formation of the nickel ferrite protective layer is slow and, depending on the experimental conditions, might not be formed fast enough to provide effective protection of the underlying substrate Cu-Ni-Fe alloy. One strategy to help in the formation of this layer is to coat the Cu-Ni-Fe alloy with a sacrificial layer that would be stable for a period long enough to allow the formation of NiFe2O4 on the surface of the anode.In this context, the use of (Co,Ni)O-based protective coatings for metallic anode appears promising [5]. However, it is challenging to produce coherent and crack-free oxide layer as required for industrial Al production. A potentially relevant method to produce (Co,Ni)O coated inert anodes is by direct deposition of (Co,Ni)O oxide compounds by thermal spray techniques such as suspension plasma spray (SPS) and high velocity oxygen fuel (HVOF). They are well-established technologies for producing protective oxide coatings for various industrial applications (e.g. gas turbines). Additionally, thermal spray could be used on Al production site to restore protective (Co,Ni)O coating on end-of-life inert anodes.As a first step toward this goal, single phase (Co,Ni)O powders with various Co/Ni ratios that could be used as raw materials for the thermal spraying of protective coatings onto Cu-Ni-Fe inert anodes have been produced [6]. The crystalline structure, thermal stability, electrical conductivity and solubility in cryolite media of the produced (Co,Ni)O powders are characterized depending on their Co/Ni ratio. Finally, selected (Co,Ni)O powders have been used as raw materials for the thermal spraying (HVOF and SPS) of protective coatings onto Cu-Ni-Fe inert anodes and the electrochemical behaviour of the coated inert anodes under Al electrolysis conditions is presented.