The Research On Direct Electrolytic Al-Sc Alloys In Molten Salt

The basic properties, extracting methods of Scandium and its superior application performances are introduced briefly in this article. The latest research status on Al-Sc alloys is reviewed. The main obstacles and problems for application of Al-Sc alloys on a large scale are analyzed. A technology development project and its scheme are proposed firstly in this paper that the Sc oxide with not high purity is extracted from bauxite or red mud in alumina production, then the direct molten salt electrolysis process is applied to produce Al-Sc alloys by using the Sc oxide as electrolysis raw material. After the technology is developed and applied , the Sc containing resources will be utilized effectively, the Al-Sc alloy production process will be simplified, the production cost will be greatly reduced and a large-scale application of Al-Sc alloy will be promoted.The research work on solubility of Sc2O3 in the electrolyte for Al electrolysis, some important physical-chemical properties of the Sc containing electrolyte and thermodynamics and the electro-chemical process of direct electrolysis of Al-Sc alloy is carried out to evaluate the feasibility of industrial application of direct electrolysis for Al-Sc alloys.The relationship of solubility of Sc2O3 in Na3AlF6-3%MgF2-3%CaF2 electrolyte with CR, temperature and addition of A12O3 during Al-Sc alloys direct electrolysis has been revealed by the research work. The results show that reducing the addition of Al2O3 and temperature rising can increase the solubility. CR has little influence on the solubility. Under normal operation conditions of the modern Al electrolysis cells with point feeding the solubility of Sc2O3 in the electrolyte can reach up to 3-5%, which meets the need for direct electrolysis of Al-Sc alloy.The conclusions can be obtained from the research results on physical chemistry properties (such as liquidus temperature, electrical conductivity, density and thermo-lost weight rate etc.) that the electrolysis temperature can be reduced to save energy by addition of Sc2O3; existing and normal addition range of Sc2O3 in Al electrolysis has a little influence on the density and electrical conductivity of electrolyte and consequently on the stability of the cell as well; addition of Sc2O3 into the electrolyte can decrease the volatile loss from the electrolyte and reduced environment pollution under the normal superheat (20K-40K) in the electrolysis process, and at the same time Sc2O3 won't be lostthrough volatilization. The properties such as liquidus temperature, electrical conductivity, density and thermo-lost weight rate of the Sc containing electrolyte can satisfy the need of direct electrolysis process for Al-Sc alloys so that Al-Sc alloys direct electrolysis is feasible.The electro-chemical reduction process of Sc3+ on Al pole and decompositionvoltage of Sc2O3 on graphite pole in cryolite melt have been investigated by using cycling volt-current analysis, steady-state polarized method and linear volt-current analysis. The results show that the electrolysis reduction process of Sc3+ in melts is a simple electron transferring process by obtaining 3 electrons in one step, as a result of which the direct electrolysis process to produce Al-Sc alloys will reach a high current efficiency because of no creation of low valent Sc ions and secondary reactions as well. The decomposition voltage of Sc2O3 is 1.2V which is close to that of Al2O3 under the same temperature (1.15V, CO2=70%) so that the SciOs will not be accumulated in melts during electrolysis which keeps the process stable.The equilibrium constant K of the reaction of A1-SC2O3 under the electrolysis conditions is calculated. It is found by thermodynamics analysis that there is no possibility to form Al3Sc in liquid alloys under normal electrolysis conditions. The activity coefficient of Sc in liquid Al is approximately calculated and found to be far smaller than 1 if Sc content is very low in the alloys by use of the Miedema model. It is shown from the experiments results and thei...