Interfacial Mechanisms and Inert Anodes Materials for Aluminium Electrolysis – MIMINELA

The proposal is concerned with the development of ecofriendly molten salt electrolysis process for aluminum production where consumable carbon anodes are replaced by inert anodes.
The so called HH process consists in electrochemical reduction of aluminum, using alumina extracted from bauxite as the feeding material. Although the process has been challenged many times for 50 years, it is still the only one used by all the aluminum companies such as ALCOA (Aluminum Company of America ), RIO TINTO ALCAN (RTA, Aluminum Pechiney is a French law company belonging to RTA), RUSAL (a major Russian company), CHINALCO ( a major Chinese company), ..
The process developed in 1886 is based on the dissolution of alumina in cryolithe (Na3AlF6) rich electrolytic bath between 950°C and 1000°C and by current flow reduce alumina to aluminum at the cathode and oxidize the carbon to CO2 gas at the anode (1,5 t CO2/t Al).

In a smelter, the pots operate with high current (from 200 kA up to 400 kA, depending on the technology), and typically 300 of them are connected in série ???
Carbon is the only material able to resist to cryolithe under voltage. CO2 emission results from the process. Carbon oxidation takes place with energy yield by 80 %, that leads to electric consumption by around 3 kWh/t Al, that to say 20% of the total specific energy consumption, but goes with following drawbacks :
Lack of alumina in the vicinity of the anode leads to anode effect with CF4 and C2F6 greenhouse gas emission. The environmental impact of CF4 and C2F6 gas are much more severe as compared to CO2 : 7400 higher than CO2 in term of weighted impact -6500 for CF4and 9200 for C2F6, the mean effect being estimated to 2 t CO2 eq./ t Al.
Carbon of high purity is requested as raw material for anode (i.e. sulfur content to limit SO2 emission ) and is getting rare (to be imported, crude oil derivatives ).
Carbon anode consumption leads to anode change every 25 to 30 days, with pot opening and fluor gas emission.
The mass production of carbon anodes takes place in a large workshop and use carbon binder which curing might generate toxic substances.

The project is aimed at understanding the corrosion mechanisms of multi-material inert anodes. Such anodes are made of Ceramic Metal Composite –CMC- with increasing Ni ferrite content from the core (possibly metallic) to the electrochemical active surface.
Any material used as inert anode should resist to severe corrosion environment (high temperature, aggressive bath composition) ; the degradation of the material is mainly resulting from diffusion phenomenon, combined with thermo-mechanical weakening. Any development of multi-material anode requests multi-disciplinary and multi-scale models.

The studies carried out up to now by the partners have led to building up a extended corpus of fundamental and experimental knowledge and to identify priority axis: the proposed studies are focused on characterization of the interfacial phenomena taking place at operating temperature at internal and external interfaces –i.e. alloy/CMC, CMC/external active layer and external active layer/bath, respectively. The approach is completed by bath speciation study.
Diffusion calculations in the materials allow to fine tune our understanding of the anode material degradation.
The multi-scale modeling of the species transport (?) involved in the corrosion phenomena (such as iron) enable to develop a material optimization strategy in conventional bath and to consider new bath composition and temperature.