Study Of Al-Sc Alloy Prepared By Molten Salt Electrolysis Method

The research works on the solubility of Sc2O3 in the electrolyte of Al electrolysis, some important physical-chemical properties of the Sc containing electrolyte and thermodynamics and the electro-chemical process of direct electrolysis of madding Al-Sc alloy is carried out to evaluate the feasibility of industrial application of direct electrolysis for Al-Sc alloy.The relationships of the solubility of Sc2O3 in cryolite-aluminium oxide system electrolyte with CR, temperature and addition contents of Al2O3 and LiF during Al-Sc alloys direct electrolysis has been revealed by the research work. The results show that reducing the addition of Al2O3 and rising temperature 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 liquids temperature, electrical conductivity, density and thereto-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 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 reduce environment pollution under the normal superheat (10K～40K) in the electrolysis process, and at the same time Sc2O3 won't be lost through volatilization. The properties such as liquids 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 decomposition voltage 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, it is a reaction controlled by diffuseness and polarization, 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 valet Sc ions and secondary reactions as well.Has carried out an analog computation on the activity coefficient of the scandium in Al-Sc alloy with the Miedema model, union with a little fundamental thermodynamics relational expression, we could get the coefficient expression of any double-phase system. So it's critical important to maintain the content of scandium oxide at a high level and the aluminium oxide at a low level in the electrolyte. Under the electrolysis conditions, the balance activity coefficient could be large as 2.33% when Sc and Al separating together. And it makes the coefficient of Sc in Al far smaller than 1 because scandium and aluminium generate a stable high-melting-point compound, it's possible to manufacture Al-Sc alloy (the content of Sc is about 2%) directly in the electrolyzing cell.The direct electrolysis operation process of Al-Sc alloys is studied on the basis of thermodynamics analysis. The results show that the content of Sc in the alloys is going up with Sc2O3 content rising and Al2O3 content reducing in electrolyte, and increases with the higher electrolysis temperature, longer pole distance and stronger current density which, however, have small influence on Sc content in alloys. The increase of Sc2O3 content in the electrolyte raises current efficiency, but Al2O3 has the contrary effect.Metallographic and SEM analysis of the Al-Sc alloy samples from direct electrolysis has been carried out. The results show that the alloys containing 0.5 wt% Sc can exhibit superior performances owing to the fine, dispersed Al3Sc particles in the alloys.When Al3Sc content in alloys reaches 2%, the particle sizes in the samples from direct electrolysis almost keep unchanged, and those from thermal reduction become very big. It can gain better metallographic structured production and higher efficiency can be got as well by direct electrolysis.