Electrochemical behaviour of carbon anodes in Na3AlF6-K3AlF6-based low-melting electrolytes for aluminium electrolysis

Abstract

The anode processes of carbon electrodes in 49 wt% Na3AlF6 - 21 wt% K3AlF6 - 30 wt% AlF3-Al2O3 (saturated or free) melts were studied in a wide range of potentials by cyclic voltammetry. Two electrolysis processes were observed on the cyclic voltammogram traces. It is considered that the first process corresponds to the discharge of oxide ions (or residual oxide ions), and the discharge of fluoride ions occur after the discharge of oxide ions, leading to the anode effect. As the potential increases, the second process occurs with the emission of Perfluorocarbon (PFC) gases. Compared to the cyclic voltammogram trace obtained from the traditional electrolyte for aluminium industry 84 wt% Na3AlF6 - 11 wt% AlF3 - 5 wt% CaF2 - Al2O3 (saturated), it can be seen, as the addition of a large amount of AlF3 and the decrease of electrolysis temperature, the current density of the second process increases obviously. The performance of low temperature aluminium electrolysis increases the risk of a larger emission rate of PFCs.