Low temperature operating cell for the electrowinning of aluminium

A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, using anodes that are based on alloys of iron and at least one of nickel and cobalt to produce aluminium of low contamination and of commercial high-grade purity, the cell comprising in combination:

• (a) a plurality of anodes that:
• are based on alloys of iron and at least one of nickel and cobalt immersed in the molten electrolyte, each anode having an oxygen-evolving electrochemically active surface spaced by an inter-electrode gap from an aluminium-wettable drained cathode surface;
• (b) means for inducing, by upward lift of oxygen released from the anodes, circulation of the electrolyte towards the molten electrolyte surface and down to the inter-electrode gap;
• (c) cell side walls contacted by the molten electrolyte, the cell side walls being made of material resistant to the molten electrolyte;
• (d) a thermal insulation, including a sidewall insulation and an insulating cover above the molten electrolyte surface, for preventing the formation of any crust of solidified electrolyte or ledge of solidified electrolyte on the cell side walls, the cover being arranged to allow the removal and insertion of anodes from/into the molten electrolyte;
• (e) means for supplying powdered alumina between the insulating cover and the molten electrolyte surface, arranged to distribute the supplied powdered alumina over the molten electrolyte surface, from where the alumina dissolves as it enters the electrolyte to continuously maintain it substantially saturated with alumina;
• (f) the aluminium-wettable drained cathode surface on which aluminium is produced and from which the produced aluminium is drained comprising, or being associated with, inclined drained surfaces adjacent to the side walls, said inclined drained surfaces being inclined down towards the centre of the cell to keep the produced aluminium out of contact from the side walls;
• and
• (g) a central aluminium collection reservoir for collecting molten aluminium draining from the drained cathode surfaces and/or from said inclined drained surfaces from where the produced aluminium can be evacuated from the cell;
• and wherein
• (h) the molten electrolyte is substantially saturated with alumina, particularly on the electrochemically active anode surf ace, and contains dissolved species of at least one major metal present at the surface of the anodes, which inhibits dissolution of the anodes, and results in a concentration of the metal species in the produced molten aluminium within commercially acceptable limits, said dissolved species being selected from species of iron, nickel and cobalt and being present in the molten electrolyte at or nearly at a saturation concentration so as to inhibit dissolution of the electrochemically active anode surface.