Preparation Of Aluminum-silicon Alloy By Molten Salt Electrolysi

Al-Si alloy has the advantages of light weight,corrosion resistance,high specific strength,easy recovery and extrusion molding,high thermal conductivity and electrical conductivity,so it is widely used in aviation,transportation,building materials,electronics,communications and other fields.The existing preparation method of Al-Si alloy has the disadvantages of long process,high energy consumption and many impurities in the product,which are difficult to control.Therefore,it is of great significance to find a production method of Al-Si alloy with lower cost,shorter process and higher alloy quality.In this paper,Al-Si alloy is prepared by molten salt electrolysis based on the molten salt system of Na₃AlF₆-AlF₃-Al₂O₃-SiO₂.This method has the characteristics of short process,low energy consumption and high product purity,and has unique environmental protection and economic advantages.In this paper,the liquidus temperature,SiO₂solubility and volatilization loss of cryolite-aluminum fluoride-SiO₂ molten salt system were measured,the electrochemical behavior of SiO₂ was studied,the influence of process parameters on Si content in Al-Si alloy prepared by electrolysis was studied,and the alloy morphology was characterized.The results are as follows:（1）The liquidus temperature of molten salt system was studied by step cooling curve.It was found that the addition of SiO₂（0wt%-8wt%）significantly increased the liquidus temperature of Na₃AlF₆-AlF₃-SiO₂ system with CR=2.2 and Na₃AlF6-AlF₃-Al₂O₃-SiO₂-Ca F₂-MgF₂-LiF system with CR=2.2 by 9℃and 7℃respectively.The addition of SiO₂ can obviously reduce the liquidus temperature of Na₃AlF₆-AlF₃-SiO₂-Ca F₂-MgF₂-LiF,and the reduction range can reach 5-11℃,but it has no obvious effect on the liquidous temperature of other systems.After adding SiO₂ into the current industrial aluminum electrolyte,the electrolytic temperature can be controlled at about956℃.（2）The solubility of SiO₂ in cryolite molten salt was studied by rotating quarts tablet method.The results show that the solubility of SiO₂ in Na₃AlF₆-AlF₃ was9.12wt%-12.52wt%when the superheat is 10℃.The addition of Al₂O₃will obviously improve the solubility of SiO₂.At 975℃,after adding Ca F₂,MgF₂and LiF,the solubility of SiO₂ decreased to about 6wt%.（3）When the temperature is higher than 800℃,AlF₃ and SiO₂will react to form gaseous Si F₄,which will lead to the increase of volatilization loss of molten salt.When the superheat is 10℃,the volatilization loss of Na₃AlF₆-AlF₃-SiO₂ molten salt is2.3wt%-7.5wt%,and the volatilization loss of molten salt increases with the increase of SiO₂ addition;The addition of Al₂O₃ can obviously reduce the volatilization loss of molten salt and SiO₂;The addition of Ca F₂,MgF₂and LiF has little effect on volatilization loss.The composition of electrolyte with less volatilization loss was found through experiments:Na₃AlF₆-AlF₃-2wt%Al₂O₃-3wt%SiO₂-5wt%Ca F₂-1wt%MgF₂-2wt%LiF,The volatilization loss is between 2.3wt%and 2.6wt%,and the r emaining SiO₂ in the electrolyte accounts for more than 94.9%of the added amount of SiO ₂.（4）The cyclic voltammetry curve and steady polarization curve of molten salt show that Al₂O₃ is reduced to Al in one step,while the electrolysis of SiO₂→Si is completed in two steps.The voltage at which Al₂O₃ and SiO₂ begin to decompose is similar,but the decomposition voltage of SiO₂ is lower.（5）Al-Si alloy with Si content of 8wt%-20wt%was prepared by molten salt electrolysis,and the Si content was much higher than that of Al-Si alloy produced by industrial electrolytic aluminum reduction cells at present;The optimum current density for preparing alloy by electrolysis is 0.7A/cm²-0.8A/cm²,and the appropriate amount of SiO₂ is 2wt%-3wt%,at this time,the recovery rate of SiO₂ is high and the evaporation loss of electrolyte is small;Si in Al-Si alloy obtained by electrolysis exists in the form of needle-like structure.