INVESTIGATION OF THE ALUMINUM ELECTRODEPOSITION PROCESS IN CRYOLITE-BASED MELTS USING A ROTATING RING-DISC ELECTRODE - EVIDENCE FOR THE EXISTENCE OF A SUBVALENT INTERMEDIATE SPECIES

The rotating ring-disc electrode technique has been used to investigat e the reaction mechanism of the aluminium electrodeposition process in cryolite-based electrolytes. Laboratory studies using high temperatur e gold-molybdenum and platinum-molybdenum rotating ring-disc electrode s have provided evidence for the existence of a subvalent intermediate species (A1(I)). In a cryolite-alumina electrolyte (bath ratio: 1.5), two well separated convective-diffusion controlled oxidation processe s were observed at both a gold and a platinum ring during aluminium el ectrodeposition at the disc. On the basis of the data presented, a rea ction scheme involving reduction of A1(III) to A1(0) via A1(I), follow ed by chemical dissolution of A1(0) into the bulk electrolyte was prop osed. The loss of current efficiency in aluminium smelting was primari ly attributed to the chemical dissolution of A1(0), rather than to the formation of a subvalent intermediate species.