Investigation of an electrolytic process for the production of aluminium using hydrogen as the reducing agent

Abstract

One of the disadvantages of the Hall-Heroult process that uses pre-bake technology for producing aluminium is the need for periodic carbon anode replacement. This results in voltage instability, varying cell cavity geometry, and heat imbalance. Furthermore, greenhouse gases are formed as a by-product with the use of carbon anodes. Due to these shortcomings, alternative technologies such as inert anodes have been investigated and tested, but with little success to date. The use of hydrogen as the anode in aluminium electrowinning has merits, as aluminium can be produced at a reversible potential similar to that using a carbon anode, while also overcoming the disadvantages as outlined above. Laboratory scale electrolysis experiments were conducted in order to investigate the production of aluminium using hydrogen diffusion anodes. An electrolyte based on the KF-AlF3-AhO3 system was used at 700°C and 750 °C. The anode materials used were porous carbon and porous nickel alloy. Prior to the electrolysis trials, characterisation of the anode was carried out by conducting permeability experiments, thermodynamic analysis and contact angle measurements. The parameter specification for the anode pore structure and the electrolysis trials were based on the characterisation experiments of the anode materials With the carbon anode, there was a decrease in voltage when hydrogen was supplied to the anode. However, there was an increase in anode porosity after the electrolysis and also carbon oxides were detected in the electrolysis off-gas analysis indicating that carbon anode was consumed. With nickel alloy anode, there was a measurable depolarisation of the anode voltage when hydrogen was supplied to the anode. The water vapour formation at the anode was also further confirmed by the water condensate found at the electrolysis exit gas pipe. The current efficiency based on water vapour evolution was approximately 40 % and the hydrogen fluoride evolved was close to 5.5 g / kg of theoretical aluminium produced. The presence of metallic aluminium was confirmed by the material analysis. The experiments conducted in the galvanodynamic mode suggested that the rate limiter for hydrogen oxidation was the availability of surface hydrogen at the electrode / electrolyte interface. The performance of the anode deteriorated with time due to the reduction in the active anode surface. The bath also penetrated into the anode material to a distance of 3 mm from the electrolyte/ anode interface. The concept of using a hydrogen anode in aluminium electrowinning is technically achievable on a laboratory scale. However, its application may be constrained by the material limitation of the porous anode material ( as in the case of inert anodes). Hence future work should focus on developing a hydrogen anode material which can maintain a stable pore structure for sustained operating periods.