Performance Evaluation of Low-Temperature KF-NaF-AlF3 Electrolytes for Aluminum Electrolysis Using Vertical Inert Cu–Ni–Fe Alloy Anodes

Abstract

In this study, the performance of low-temperature electrolytes in a 40 A laboratory oxygen-evolving aluminum electrolysis cell with vertical inert electrodes for two compositions of NaF-KF-AlF3 based electrolyte is investigated. Two compositions of Cu–Ni–Fe alloy, single-phase homogenized, 20Cu-42Ni-38Fe wt% and 5Cu-48Ni-47Fe wt%, are evaluated as anodes. Titanium diboride (TiB2) is used as a wetted cathode. The characteristics of a protective oxide layer, pre-formed on the anode through high-temperature air oxidation, are examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that a multilayered oxide morphology, revealing Fe2O3 and NiFe2O4 as major constituents, is dense and adherent to the base alloy. The effects of electrolyte compositions, anode alloys, and electrolysis conditions on cell voltage evolution, current efficiency, contamination levels in the electrolyte and produced aluminum, and anode wear rate are systematically studied. A protective oxide layer, formed through oxidation treatment on the 20Cu-42Ni-38Fe wt% alloy, not only remains compact post-electrolysis but also performs significantly better in a K-rich electrolyte in terms of wear rate and the purity of the aluminum obtained. Influences of electrolyte composition on anode degradation behavior are discussed.