Development of an electrochemical sensor for alumina concentration measurements and dissolution characteristics of alumina in cryolite melt

The work studied the development and behaviour of an in-situ electromotive force (emf) based electrochemical sensor composed of a graphite probe during the addition and dissolution of alumina in cryolite-based melts. The principle of the alumina sensor is based on the measured potential of the graphite electrode immersed in the bath (chamber I) against a reference electrode (chamber II). Thus, the potential between the graphite probe and the reference electrode is due to the difference in activity of alumina in the two chambers. Then, the emf measurements can be indirectly related to a change in alumina concentration when properly calibrated. This method allows to determine the absolute concentration of alumina after addition only if the initial condition or concentration is known. In the present work, alumina concentration measurements are always referring to the measured potentials. The half-cell reactions for the graphite electrode and the reference electrode were derived together with the corresponding Nernst equation. Further, the potentials were calculated for comparison with the experimental results. Two reference electrodes were tested during the experiments. An aluminium reference electrode and an alumina-saturated graphite quasi-reference electrode. Dissolution of alumina were investigated using emf-measurements at three different experimental systems: 1) Dissolution of alumina in a bath composed of 99 wt% synthetic cryolite and 1 wt% alumina in a sealed furnace under N2 gas. 2) Dissolution of alumina in a partially open see-through furnace under N2 gas with the same bath composition as in 1). Video recordings were performed separately with a small variation in the chemical composition of the bath. 3) Dissolution of alumina in an open see-through furnace under nitrogen gas/ambient air. The initial chemical composition of the bath was 75 wt% cryolite, 15 wt% AlF3, 6.5 wt% CaF2 and 3.5 wt% LiF and 1 wt% alumina. Video recordings were performed simultaneously as the sensor measurements. The emf measurements were generally characterized by a gradual decrease in the potential after the addition of the alumina, followed by a stabilization and establishment of a plateau when the potential reached a constant value. The gradual decrease in potential was interpreted as an increase in the alumina concentration and the creation of the plateau as the finalization of the dissolution. There was a satisfactory agreement between the measured and calculated potent