Development of an electrochemical sensor for alumina concentration measurements and dissolution characteristics of alumina in cryolite melt
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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 potential values after stabilization for some of the experiments carried out under the first and second experiemental systems. The initial decrease of the potential after the adding alumina to the electrolyte, the establishing of the plateau as well as the agreement between calculated and measured potential supported the fundamental working principle of the sensor. However, for the experiments carried out under the experimental system 3, the measured potential values were in less agreement with the calculated values. The difference arises most likely from the differences in the experimental conditions such as gas composition above the melt, initial chemical composition of the melt with the addition of AlF3, CaF2, and LiF or from the assumptions made in calculating the potentials. The dissolution times obtained based on the sensor showed significant faster dissolution process for the secondary alumina than the primary alumina, which is consistent with previous results reported in the literature, and suggested that the sensor was capable for estimating alumina dissolution time. The results from the video recordings were also in fair accordance with the emf measurements, revealing a considerably shorter time needed for the secondary alumina to dissolve. The video recordings also revealed that the dissolution of the primary alumina was characterized by the formation of a crust, followed by the detachment, and sinking of alumina-bath flakes. The dissolution mechanism for the secondary alumina was characterized by rapid dispersion of all the particles throughout the melt leading to a faster dissolution. An increase in the dissolution time with increasing initial alumina concentration in the melt during the experiments was generally observed in the emf measurements and video recordings. The video recordings revealed that for low alumina concentration the dissolution mainly proceeds through the rapid dispersion and dissolution of a white cloud consisting of alumina particles followed by the formation and rapid dissolution of a thin crust. On the contrary, for high alumina concentration, a thicker crust is formed, and it mainly dissolves by detachment and sinking of alumina-bath flakes. The dissolution process of alumina fines and bulk alumina and possible correlations with its other physicochemical properties were also studied. Video recordings showed significant differences in the characteristics and time of the dissolution between secondary bulk alumina and secondary alumina fines. The dissolution mechanism of bulk alumina was characterized by a rapid dispersion and dissolution of the alumina particles immediately after the alumina get in contact with the bath. Further, a crust is formed on the top of the bath, followed by detaching and sinking of alumina-bath flakes from it. The last phase of the dissolution is characterized by the decrease in the thickness of the crust until it completely disappears when the dissolution is completed. On the contrary, alumina fines showed less dispersion of the particles in the bath immediately after feeding and caused a considerably thicker crust than the bulk alumina. The dissolution process of the alumina fines takes place only at the alumina layer-bath surface interface. Towards the end of the dissolution process of alumina fines the alumina layer appears to have smaller pores and be more compact than the crust of the bulk alumina. Alumina fines were characterized by longer dissolution times compared to bulk alumina. The dissolution times of the alumina fines and bulk alumina were found to have a negative correlation with the particle size distribution (PSD), meaning that particles with small surface area tend to promote longer dissolution times. The small size of the particles and the lack of dispersion of alumina fines upon addition leads to the formation of a crust with less contact area between the alumina and the melt. On the other hand, the better dispersion of bulk alumina during addition leads to the formation of a less compact crust. Then the melt can penetrate the crust resulting in more contact between the melt and the alumina leading to the detachment and sinking of alumina flakes and shorter dissolution times.
Has partsBracamonte, Luis Carlos Izaguirre; Nilsen, Karoline Aasen; Rosenkilde, Christian; Sandnes, Espen. Alumina Concentration Measurements in Cryolite Melts. The Minerals, Metals & Materials Series 2020 s. 600-607 https://doi.org/10.1007/978-3-030-36408-3_82
Bracamonte, Luis Carlos Izaguirre; Aulie, Vegard; Rosenkilde, Christian; Einarsrud, Kristian Etienne; Sandnes, Espen. Dissolution Characteristics and Concentration Measurements of Alumina in Cryolite Melts. The Minerals, Metals & Materials Series 2021 s. 495-503 https://doi.org/10.1007/978-3-030-65396-5_70
Bracamonte, Luis Carlos Izaguirre; Einarsrud, Kristian Etienne; Rosenkilde, Christian; Sandnes, Espen. Measurements and Simultaneous Optical Observations During Dissolution of Alumina in Cryolite Melt. In: Eskin, D. (eds) Light Metals 2022. The Minerals, Metals & Materials Series. Springer, Cham. https://doi.org/10.1007/978-3-030-92529-1_52
Bracamonte, Luis Carlos Izaguirre; Gylver, Sindre Engzelius; Einarsrud, Kristian Etienne; Sandnes, Espen. Influence of Secondary Alumina Properties on Alumina Dissolution in Cryolite Melt.