Avaliação do limite de detecção do método de difracção de raios-x na determinação do trifluoreto de alumínio em amostras de uma solução electrolítica

Abstract

The main objective of this work is to find the minimum amounts of excess aluminum trifluoride present in the electrolytic solution, which can be determined by the X-ray diffraction method under production conditions. This is done by comparing the results obtained by X-ray diffraction with those obtained by a reference volumetric method. The study was developed with a view to its application in the control of electrolytic cells, that is, in maximizing their efficiency for the production of aluminum. One way to improve current efficiency is to lower the cell temperature. This is achieved by adding additives such as aluminum trifluoride. The reason for using additives is the loss of less energy to the outside and less resistance of the electrolytic solution to the passage of current, hence the concentrations of all additives must be monitored at regular intervals. For this purpose, the X-ray diffraction method is used, compared with a reference volumetric method. The volumetric method is used as a reference method since during the initial start-up phase of the electrolytic cells the electrolytic solution contains low levels of aluminum trifluoride which lead to the coexistence of several crystalline phases. Low aluminum trifluoride contents lead to the appearance of three crystalline phases simultaneously namely fluorite, cryolite and calcium cryolite. The sample containing these three phases can be analyzed by X-ray diffraction, but there is an overlap between the various peaks of these substances, which leads to a result with low precision. Considering the results obtained, the detection limit of the X-ray diffraction method, for the determination of aluminum trifluoride, was estimated for a statistical safety of 2o (95%) as corresponding to 1.50% of chiolite in the electrolytic solution. This means that only from concentrations above 1.50% of kiolite can the X-ray diffraction method be safely applied in the determination of aluminum trifluoride in the electrolytic solution, under the conditions of production. (The main objective of this work is to find the minimum amounts of excess aluminum trifluoride present in the electrolytic solution, which can be determined by the X-ray diffraction method under production conditions. This is done by comparing the results obtained by X-ray diffraction with those obtained by a reference volumetric method.

The study was developed with a view to its application in the control of electrolytic cells, that is, in maximizing their efficiency for the production of aluminum. One way to improve current efficiency is to lower the cell temperature. This is achieved by adding additives such as aluminum trifluoride. The reason for using additives is the loss of less energy to the outside and less resistance of the electrolytic solution to the passage of current, hence the concentrations of all additives must be monitored at regular intervals. For this purpose, the X-ray diffraction method is used, compared with a reference volumetric method. The volumetric method is used as a reference method since during the initial start-up phase of the electrolytic cells the electrolytic solution contains low levels of aluminum trifluoride which lead to the coexistence of several crystalline phases. Low aluminum trifluoride contents lead to the appearance of three crystalline phases simultaneously namely fluorite, cryolite and calcium cryolite. The sample containing these three phases can be analyzed by X-ray diffraction, but there is an overlap between the various peaks of these substances, which leads to a result with low precision. Considering the results obtained, the detection limit of the X-ray diffraction method, for the determination of aluminum trifluoride, was estimated for a statistical safety of 2o (95%) as corresponding to 1.50% of chiolite in the electrolytic solution. This means that only from concentrations above 1.50% of kiolite can the X-ray diffraction method be safely applied in the determination of aluminum trifluoride in the electrolytic solution, under the conditions of production. (TRADUÇÃO NOSSA)