| Due to the many unavoidable drawbacks in the traditional Hall-Heroul process,it is gradually unable to meet the strict requirements for environmental protection and energy saving.Therefore,it is significantly necessary to find new aluminum smelting methods and technologies.In this situation,the research of low temperature aluminum electrolysis has become one of the most popular research directions,and it is expected to optimize and improve the current aluminum electrolysis process.Among them,Na-cryolite low temperature melts are considered to have good prospects.When the mole ratio of NaF to AlF3(CR)is in the range of 1.15～1.45,the electrolyte have many advantages.For example,low liquidus temperature,and moderate variation of the liquidus temperature with the variation in the electrolyte composition.Different from the difficult-to-dissolve crust of Na3AlF6 formed by the current cell,the main component of the crust formed at the cathode is Na5Al3F14,which is relatively easy to melt.Moreover,the electrolyte density is also reduced,which is conducive to the separation of electrolyte and aluminum.Finally,the interfacial tension between aluminum and the electrolyte increases,and the secondary reaction of aluminum is diminished.At the same time,due to the large increase in the AlF3 content in the electrolyte and the significant decrease in liquidus temperature,the alumina solubility and conductivity are very low.And the alumina raw materials used in many domestic aluminum electrolysis cell contain Li2O and K2O.Therefore,some KF was added to improve the alumina solubility,and some LiF was added to improve the conductivity.Therefore,this paper has studied aluminum electrolysis in a quaternary NaF-LiF-KF-AlF3(CR=1.3)low temperature melts with KF content x[KF]=0～20%and LiF content x[LiF]=0～20%.The main research results are as follows:First,a series of electrochemical methods were used to study the cathode process of Al(Ⅲ)ions reduction in tungsten and graphite in a binary NaF-AlF3(CR=1.3)low temperature melts.The results show that the deposition potential of metal Al on tungsten and the graphite is basically the same,and both are accompanied by the deposition of metal Na.Unlike the one-step reduction process of Al(Ⅲ)ions on graphite,the reduction on tungsten is also accompanied by a two-electron transfer reaction(AlF4-→AlF2-).Metal Al will form an intercalation compound Na-C before being deposited on graphite electrode,and that on tungsten is Al4W alloy.The deposition of Al4W alloy and metal A1 on the surface of tungsten electrode are quasi-reversible reactions controlled by the diffusion process.The diffusion coefficient of Al(Ⅲ)ions is 1.77×10-8 cm2·s-1,and the critical cathode current for metal Al deposition is about 150 mA.Secondly,Alumina solubility of the quaternary NaF-LiF-KF-AlF3(CR=1.3)low temperature melts at various KF and LiF contents and different temperatures were studied using the mass loss method of the rotating corundum disk.The alumina solubility formula and equivalent alumina solubility diagram under the conditions were given,and a simple analysis of the phase and morphology of the crust formed when the alumina is excessive.The results show that in the range of 830～890℃,x[LiF]=0～20%,x[KF]=0～20%,alumina solubility meets the following empirical regression equation:S=-47.963+0.0102x[KF]0.0642x[LiF]+0.0999t+1.34×10-3x[KF]2+1.63×10-4x[LiF]2-4.83×10-5t2,t is the temperature.KF has a promoting effect on alumina solubility,and LiF has a suppressing effect on alumina solubility.According to the equivalent alumina solubility diagram,it is suggested that further research should focus on the electrolyte components of x[KF]=4%～20%,x[LiF]=0～6%,and the temperature is 860～890℃.When the alumina is added into the melts,it will settle at the bottom to form a crust.The crust phase will change with the changes in the content of LiF and KF,and the morphology of alumina will change from a close-packed lamellar structure to a chaotic,random-packed,fragmentary structure.Finally,the influencing factors and feasibility of electrolysis aluminum in NaF-LiF-KF-AlF3(CR=1.3)low temperature melts were studied,and some problems and challenges that need to be faced by the industrial application research of this low temperature melts were briefly analyzed and summarized.The results show that when the current is constant,the larger the electrode distance is,the larger the cell voltage is,the more stable the voltage is,and the voltage fluctuation is smaller when the d=4 cm,which basically stabilizes.Considering the factors such as stability of the electrolyte,the fluctuation of the ceii voltage during the electrolysis process,and the energy consumption,the research of NaF-LiF-KF-AlF3(CR=1.3)low temperature melts should choose a temperature of about 860℃,the electrolyte composition is x[KF]=4%～20%,x[LiF]=0～6%,and the content of K and Li should not be too high,d should be controlled at about 4 cm.In addition,there are some outstanding issues that need to be paid attention to in the industrial application research of this melts,such as more research on the basic physical and chemical properties of electrolyte,optimization and improvement of the production process,design and development of new low temperature aluminum electrolysis cell. |
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