Aluminum Electrolysis in an Inert Anode Cell

Oxides that can form films on oxygen-evolving inert anodes have a certain solubility in fluoride melts. The solubility of most oxides increases as the alumina concentration in the electrolyte decreases, and consequently, the anode is corroded to a higher degree [9]. So it is expected that the corrosion of metal anodes can be reduced by decreasing the operation temperature and increasing the alumina concentration in the electrolyte relative to conventional practice. 

Molten Salts Chemistry and Technology, First Edition. Edited by Marcelle Gaune-Escard and Geir Martin Haarberg. 2014 John Wiley Sons, Ltd. Published 2014 by John Wiley Sons, Ltd. 

All electrolysis tests were carried out in a 20 A cell fitted with an aluminum bronze anode and wetted cathodes at an anode current density of 0.45 A/cm in a temperature range between 700 and 800 °C. The anode-cathode distance was 2 cm. The duration of experiments varied from 24 to 56 h. The KF-AIF3 system with CR =1.3 was the main electrolyte composition. NaF, in the amount of 2 or 4wt%, was added into the potassium-aluminum fluoride system. The CR in potassium-sodium electrolyte was considered as (N aP+Nkf)/Naif3, mol/mol. 

Alumina needed to be added throughout the process to replace the alumina consumed in the primary reaction. To achieve this, an automatic feeding system was designed to deliver regular amounts of alumina at defined intervals. 

Before, during, and after electrolysis, samples of electrolyte were collected by means of electrolyte freezing on an alumina rod or by pulling it up into an alumina tube. Concentrations of aluminum, potassium, and sodium, as well as concentrations of possible impurities (Cu, Fe, and others), in the electrolyte were 

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Low-temperature aluminum electrolysis in a 20 A cell fitted with an inert aluminum bronze anode was successfully performed in KF-AIF3, KF-AlFg-NaF (2wt%), and KF-AIFg-NaF (4wt%) electrolytes with CR=1.3. 

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