The Current Efficiency for Aluminium Deposition from Molten Fluoride Electrolytes with Dissolved Alumina

Abstract

Controlled laboratory experiments were carried out to determine the current efficiency for aluminium deposition from fluoride electrolytes containing dissolved alumina. Electrolysis was performed at constant current density, and the amount of deposited aluminium was determined. Effects of temperature, cathodic current density, electrolyte composition and the presence of dissolved impurities were studied. The loss in current efficiency is strongly linked to the solubility of dissolved aluminium. The rate of the so called back reaction between dissolved aluminium and CO2 depends on the diffusion of dissolved aluminium near the cathode. Both dissolved aluminium and dissolved alkali metals must be considered. Impurities with several oxidation states, such as phosphorus, cause a loss in current efficiency by undergoing cyclic red/ox reactions at the electrodes. The results are of interest for the industrial process of producing aluminium.