Electrical Conductivity of Low-Temperature KF-Naf-AlF3 Melts with CaF2 and Al2O3 Addition

Abstract

Replacing consumable carbon anodes with oxygen evolving inert anodes, in aluminium reduction cells, has long been a goal for the aluminium industry. Using inert anodes would have several advantages, one of them being the elimination of greenhouse gas emission during the electrolysis process. Metallic alloys have proven to be a promising category of material candidates for inert anodes. The industrial electrolyte is a cryolite based fluoride, and the process temperature is around 960°C. This is a very corrosive environment for the anodes, which have failed to be inert. Lowering the electrolysis temperature is key to improving anode performance. The liquidus can be lowered by reducing the NaF ratio, often termed Cryolite Ratio (CR), of the NaF-AlF3 melt, but that is not feasible as it drastically reduces the alumina solubility. If some, or all, of the NaF in the melt is replaced by KF, acceptable alumina solubility is maintained even at the low CR necessary for melting point down to 700°C. Replacing NaF with KF is known to reduce the electrical conductivity, and therefore it is of interest to determine the electrical conductivity of KF-NaF-AlF3-Al2O3 melts with different parameters. In the present study, we determine the electrical conductivity of KF-NaF-AlF3 melt at different parameters such as temperature, cryolite ratio (CR = xNaF+xKF/xAlF3), potassium ratio (xKF/xKF+xNaF) and composition of CaF2 and Al2O3 additives. Continuously varying cell constant (CVCC) technique is used to determine the electrical conductivity of the melt, which is governed by equation (1). (1)Here κ is the electrical conductivity of the melt in S cm−1, A is the cross-sectional area of the conductivity cell in cm2, l is the length of the cell in cm, and Rm is the melt resistance in Ω. The experiments are performed at various cell geometries, as shown in figure 1a.Electrochemical impedance spectroscopy (EIS) is used to determine the resistance (R) of the conductivity cell. AC-technique with a small amplitude of 10 mV and high frequencies ranging from 100 Hz to 100 kHz is used. R = Rm + Rp + Rc (2) Rm is the melt resistance, Rp is the polarisation resistance, and Rc is the resistance related to electrical connections. When EIS is performed at different l values, Rp and Rc remain constant while Rm changes. Thus, the Rp+Rc value can be estimated by extrapolating the line plotted between R and cell constant to 0 (cell constant value) (see figure 1b). Figure 1a. Conductivity cell: 1: Stainless steel rod, 2: Alumina tube, 3: BN tube, 4: Molten electrolyte, 5: Counter electrode electrical connector, 6: Graphite working electrodes, 7: Graphite crucible/counter electrode, 8: k-type thermocouple. 1b. An example graph plotted between the cell resistance and cell constant to estimate the melt resistance. Figure 1