| This paper was supported on the "863" program of national funding projects (2008AA030503)-Research on the new technology of inert electrode electrolytic at low temperature. In this paper the third sub-subject of this project was studied-Research on the physical and chemical properties of new aluminum electrolyte. Parts of physical and chemical properties of Na3AlF6-K3AlF6-AlF3system were studied, the testing methodologies of liquidus temperature and aluminum solubility in this system were explored. Based on the research of composition, temperature and additives on the influence of the physical and chemical properties for Na3AlF6-K3AlF6-AlF3system, some electrolytes meeting the requirements of the low-temperature electrolysis were effectively chosed. Compared with the experiment results of Aluminum Corporation of China, and low-temperature electrolyte of Northeastern University, the differences of physical and chemical characters of aluminum electrolyte of these systems were analyzed. The conclusions of this paper are presented as follows:(1)Differential thermal analysis and step cooling differential were used to measure liquidus temperatures of Na3AlF6-K3AlF6-AlF3system. The results were similar in measuring liquidus temperature of NaCl by using the two methods, the error was below0.5%. However, for the system of Na3AlF6-K3AlF6-AlF3, the results varied significantly. There were no significant mutations of temperature by using DTA, so it was difficult to determine the liquidus temperature of the system. However, the largest peak determined liquidus temperature can be found by using the step cooling differential method. By comparing the results with other authors, step cooling differential method was credible. Therefore, the step cooling differential method was selected in this paper.(2)The density of Na3AlF6-K3AlF6-AlF3system was studied by the hydrostatic weighing method. The influences of LiF and Al2O3additives on density of molten salt were studied. To choose good electrolyte system, the density of some systems which liquidus temperature was less than850"Cwere measured. The results showed that the density of the system decreased with the temperature gradually increased after adding LiF and Al2O3.With the contents of LiF increased, the density of the same system gradually reduced. With the contents of Al2O3increased, the density of corresponding system gradually reduced. The low temperature electrolyte system which contains potassium cryolite, the liquidus temperature is less than850℃, its density is less than2.0g/cm3, which has great density difference with molten aluminum, in these conditions, the better separation could be achieved.(3)The solubility of aluminum electrolyte in Na3AlF6-K3AlF6-AlF3system at low temperature were studied by mass difference methods. The influences of time, depth of molten salt, composition, temperature, additives on the solubility of aluminum in this system were studied. The results showed that the soluble mass of aluminum increased with the time, but the dissolved salt saturated after3h; evaporation and loss of aluminum reduced with the depth of molten salt increased; the solubility of aluminum increased with the temperature increased; With the increasing of the contents of AlF3and potassium cryolite, the solubility of aluminum reduced; the addition of LiF and A12O3could result in the increasing of solubility of aluminum.(4)Based on the physical and chemical properties studies on the Na3AlF6-K3AlF6-AlF3low temperature electrolytic system, and also the characters of low electrical conductivity and high solubility of alumina in potassium cryolite system. Through filtering different contents of potassium cryolite and compositions of fluoride aluminum, LiF and NaCl were added in this system. At last the better overall performance was optimized for this low-temperature aluminum electrolytic system which met the following requirements: liquidus temperature≤850℃, the conductivity≥1.6Ω-1·cm-1, the solubility of alumina≥4.5wt.%, the density difference between aluminum fluid and electrolyte≥0.3g/cm3. This study provided a technical basis for low-temperature aluminum electrolyte.(5) Compared some physical and chemical characters of the optimized low-temperature aluminum electrolyte, the experimental low-temperature electrolytes of Aluminum Corporation of China, and the low-temperature electrolytes of Northestern University. The results showed that the second was superior in the electrolyte conductivity, but liquidus temperature was above900℃. The third has low liquidus temperature, lower conductivity and the lower solubility of alumina. The optimized system Na3AlF6-K3AlF6-AlF3-LiF-NaCl has some advantages such as the lower liquidus temperature, the higher solubility of alumina at low temperature, and meet the basic requirements of aluminum electrolyte, and could improve current efficiency and save energy, and has a good foreground. |
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