Ball-milled Cu-Ni-Fe-O materials as inert anodes for aluminum electrolysis in low-temperature KF-AlF3 electrolyte

Abstract

A series of compounds with the general formula (Cu65Ni20Fe15)97.7O2.3 were prepared by high energy ball milling and evaluated as inert anodes for aluminum electrolysis at 700°C. All compounds had the same nominal composition but O2 was introduced at different moments during the milling operation. All compounds show the presence of an fcc phase (γ-phase). Upon heat-treatment at 1000°C during the subsequent powder consolidation, the added O reacts with Fe to form more or less well dispersed Fe2O3 precipitates. Dry oxidation tests at 700°C showed that the material oxidation behavior strongly depended on the moment when O2 is added. The best results were obtained when O2 is added after the γ-phase is formed. The use of this anode for Al electrolysis in low-temperature (700°C) KF-AlF3 electrolyte for 20 h showed that the cell voltage is stable at ca. 4.2 V and the Cu contamination of the produced Al is 0.2 wt.%. In this case, the size, dispersion and concentration of Fe2O3 precipitates in the consolidated powder are optimized to promote the formation of a protective NiFe2O4 layer on the anode surface.