Electrochemical reduction and dissolution of liquid aluminium in thin layers of molten halides

Abstract

New methods of liquid metals refining and separation operated with low energy consumption and environmental impact are highly desirable nowadays. This work presents an approach of Al refining and extraction from scrap in a thin layer of the multiple-pore molten salt electrochemical system, which appears promising. The new single-capillary cell design with a quasi-reference electrode on the TiB2 substrate was used to study the kinetics of aluminium reduction and dissolution in a narrow 1 mm-diameter channel filled with KF-AlF3 (1.1<[KF]/[AlF3]<1.5 mole/mole) or equimolar NaCl-KCl with N wt.% of AlF3 (3<N<20) at 700–850 °C. It was found that the chloride-based melts are more preferable than the fluoride mixtures due to the much bigger electrode potential window between Al and Alkali metal (1.3 vs. 0.4 V), higher apparent limiting currents (0.9 vs. 0.3 A cm−2) and lower overvoltage (4–16 vs. 66–247 mV at 0.1 A cm−2) resulted from the diffusion kinetics dominance. However, anodic dissolution of aluminium in NaCl-KCl-AlF3 may be chemical reaction-controlled with the exchange current density of 105 mA cm−2, and the reaction order of 0.34 observed in the experiment. The temperature drastically affected the whole kinetics picture with the activation energy for alkali metal reduction found to be 94.762 kJ mole−1. The preferable temperature for liquid metal refining was found to be 800 °C. The cathodic process in the KF-AlF3 melt has mixed kinetics. It possesses surprisingly low apparent diffusion limiting current (0.3 A cm−2 at 800 °C and [KF]/[AlF3]=1.1 mole/mole) and chemical reaction control at higher [KF]/[AlF3] (with exchange current density of 50 mA cm−2 and the reaction order of 0.08 at [KF]/[AlF3]=1.5 mole/mole). It may happen due to the co-deposition of Al and K with the evolution of solid cryolite. The composition of the melt plays a crucial role in refining performance. The optimal [KF]/[AlF3] for Al refining is 1.2 mole/mole. It allows operating at a current density not higher than 0.4 A cm−2 with a thickness of a thin-layer system less than 60 μm. The preferable AlF3 concentration in the NaCl-KCl-AlF3 composition is 10%. It allows operating at 1.4 A cm−2 with a thickness of 22 μm or lower.