LiF对Na3AlF6-Al2O3熔盐电解阴极过程的影响

摘要

电解铝工业中铝在阴极上析出，铝析出反应的机理对电解铝生产具有理论指导意义。在运用循环伏安法研究的基础上，通过理论计算，对Na3AlF6-Al2O3和Na3AlF6-Al2O3-LiF体系中金属铝在钨电极上的电化学沉积行为以及铝钨金属间化合物的形成机理进行了研究。结果表明：两体系中铝钨金属间化合物在50 mV·s-1≤ν≤150 mV·s-1扫描速率下的形成过程是受扩散控制的准可逆过程。在化合物形成的过程中，两体系中Al3+的扩散系数从4.54×10-9 cm2·s-1增长到5.71×10-9 cm2·s-1，Al3+反应的活化能分别为11.14 kJ·mol-1和10.47 kJ·mol-1。在Na3AlF6-Al2O3-LiF体系的还原过程中，Li并没有还原析出，而在氧化过程中Al在金属间化合物中的氧化电流增大；在恒电流电解时，Al-W金属间化合物并不溶于熔盐中，会附着在工作电极表面，LiF的加入会使电极表面的WAl4量变小，取而代之的是Al2O3的增加，说明LiF的加入使电解更加稳定，抑制了电极表面WAl4的生长。%Electrochemical reactions of Al3+and formation mechanism of Al-W intermetallic compound on tungsten electrode were investigated by means of cyclic voltammetry and theoretical calculation analysis. Al3+was reduced to Al metal in a single step and the deposition potential of sodium was more negative than aluminum in molten cryolite system. In both Na3AlF6-Al2O3 system and Na3AlF6-Al2O3-LiF system, the deposition reaction of Al3+ as Al-W intermetallic compounds on tungsten electrode was quasi-reversible, involving diffusion process from the sweep rate range of 50 mV·s-1 to 150 mV·s-1. During the forming process of Al-W intermetallic compound, Al3+ diffusion coefficient increased from 4.54×10-9 cm2·s-1 to 5.71×10-9 cm2·s-1, and reaction activation energy decreased from 11.14 kJ·mol-1 to 10.47 kJ·mol-1. Al-W intermetallic compound was insoluble in both Na3AlF6-Al2O3 system and Na3AlF6-Al2O3-LiF system. In Na3AlF6-Al2O3-LiF system, Li did not deposit out on tungsten electrode during the reduction process and oxidation current was larger than that in Na3AlF6-Al2O3 system during the formation process of Al-W intermetallic compound. In the constant current electrolysis experiment, XRD, SEM, EDS showed that Al-W intermetallic compound was insoluble in molten salt, but adhered to the surface of the working electrode. Adding LiF decreased Al-W intermetallic compound (WAl4) quantity but increased Al2O3 quantity. With the formation of Al-W intermetallic compound, Li-Al intermetallic compound also formed at the same time and it was soluble in Na3AlF6-Al2O3-LiF system. It confirmed that the working electrode was more stable during the electrolysis process after adding LiF. LiF inhibited the growth of WAl4 and increased the activity of Al3+in Na3AlF6-Al2O3-LiF system. As the cathode of aluminum electrolysis, alloying effect was obvious on tungsten electrode during the electrolysis process. In order to guarantee normal electrolysis, tungsten wire electrode should be pretreated to form a stable alloy.