LiF对Na_3AlF_6-Al_2O_3熔盐电解阴极过程的影响

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≤150mV·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。

Cathodic electrochemical behavior in Na_3AlF_6-Al_2O_3-LiF-based melts at tungsten electrode with various cryolite ratios

The cathodic behavior at tungsten electrode in Na3AlF6-Al2O3-LiF-based melt with various cryolite ratios was investigated by means of potentiodynamic cathodic polarization, potentiostatic electrolysis, chronopotentiometry, and open-circuit chronopotentiometry. The results show that the formation process of Al-W intermetallic compound is controlled by both diffusion and charge transfer when the cryolite ratio is below 2.5, and is completely controlled by diffusion when cryolite ratio is above2.5. The deposition process of metal aluminum is completely charge-transfer controlled. Sodium vapor releases along with the deposition of metal aluminum as crylite ratio increases, which leads to a great influence on current efficiency. When the cryolite ratio is lower than 2.0, the critical cathodic current density of deposited aluminum at tungsten electrode is about 150 mA·cm^（-2）,but the current density is above 200 mA·cm^（-2） under other experimental conditions. A higher cryolite ratio can cause a higher cathodic overvoltage. The relative content of Al layer is higher with the decrease of cryolite ratio, and Al layer easily strips into the molten salt when the cryolite ratio is higher than 2.5.

低分子比NaF-KF-LiF-AlF_(3)体系电解质在石墨阴极中的渗透行为

研究分子比为1.3的NaF−KF−LiF−AlF_(3)体系电解质在石墨阴极中的渗透行为,通过XRD分析对电解前后阴极中的电解质进行系统研究,并通过MUAD的半定量计算对结果进行分析。结果表明,KF可以促进电解质的渗透,KF含量越高,渗透性越强。在电解质渗透过程中,K_(2)NaAlF_(6)和含有KF的固溶体在含有KF的体系中发挥重要作用。LiF有效地抑制电解质渗透,Na_(3)Li_(3)Al_(2)F_(12)相在高LiF含量的体系中发挥了重要作用。