Metal-based anodes for aluminium electrowinning cells

Abstract

An anode of a cell for the electrowinning of aluminium comprises a nickel-iron alloy substrate having an openly porous nickel metal rich outer portion whose surface is electrochemically active. The outer portion is optionally covered with an external integral nickel-iron oxide containing surface layer which adheres to the nickel metal rich outer portion of the nickel-iron alloy and which in use is pervious to molten electrolyte. During use, the nickel metal rich outer portion contains cavities some or all of which are partly or completely filled with iron and nickel compounds, in particular oxides, fluorides and oxyfluorides.

Description

FIELD OF THE INVENTION [0001] This invention relates to non-carbon, metal-based, anodes for use in cells for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, methods for their fabrication, and electrowinning cells containing such anodes and their use to produce aluminium. BACKGROUND ART [0002] The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950° C. is more than one hundred years old. This process, conceived almost simultaneously by Hall and Héroult, has not evolved as many other electrochemical processes. [0003] The anodes are still made of carbonaceous material and must be replaced every few weeks. During electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting CO2 and small amounts of CO and fluorine-containing dangerous gases. The actual consumption of the anode is as much as 450 Kg / Ton of alumini...

AI Technical Summary

Benefits of technology

[0011] A major object of the invention is to provide an anode for aluminium electrowinning which has no carbon so as to eliminate carbon-generated pollution and has a long life.

[0020] In addition, the open porosity of the nickel-metal rich outer part provides an electrochemically active surface of high surface area. Hence, the anode can be operated with an apparent high electrolysis current while having a low effective current density at the anode's electrochemically active surface which makes it suitable for use in an electrolyte at reduced temperature containing a limited concentration of dissolved alumina.

[0027] Such an external integral oxide layer offers the advantage of limiting the width of possible pores and / or cracks present in the surface layer to a small size, usually below about a tenth of the thickness of the surface layer. When a small pore and / or crack is filled with molten electrolyte, the electrochemical potential difference in the molten electrolyte across the pore and / or crack is below the reduction-oxidation potential of any metal oxide of the surface layer present in the molten electrolyte contained in the pore and / or crack. Therefore, such a surface layer cannot be dissolved by electrolysis of its constituents within the pores and / or cracks.

[0029] As mentioned above, the thinness of the external integral oxide layer permits circulation of electrolyte to the openly porous outer portion. When monoatomic oxygen evolved during electrolysis or resulting from dissolution in the electrolyte of biatomic molecular oxygen possibly reaches nickel metal instead of iron metal of the nickel metal rich outer portion, the nickel metal is oxidised to passive nickel oxide on the surface of the nickel metal rich outer portion. However, the presence of oxygen near the metal of the openly porous nickel-metal rich outer portion can be minimised by oxidising fluoride-containing ions instead of oxygen ions at the electrochemically active surface, as discussed in greater detail in the Examples and in PCT / IB99 / 01976 (Duruz / de Nora).

[0049] Advantageously, the method includes substantially saturating the molten electrolyte with alumina and species of at least one major metal, usually iron and / or nickel, present in the nickel-rich openly porous outer portion of the anode(s) to inhibit dissolution of the anode(s). The molten electrolyte may be operated at a temperature sufficiently low to limit the solubility of the major metal species thereby limiting the contamination of the product aluminium to an acceptable level.

Problems solved by technology

Such anodes have an overall electrical conductivity which is higher than that of solid ceramic anodes but insufficient for industrial commercial production.

However, in case such a thick oxide layer is damaged, molten electrolyte may penetrate into cracks between the metallic inner part and the oxide layer.

The surfaces of the crack would then form a dipole between the metallic inner anode part and the oxide layer, causing electrolytic dissolution of the metallic inner part into the electrolyte contained in the crack and corrosion of the metallic anode part underneath the thick oxide layer.