Aluminium production cells with iron-based metal alloy anodes

Abstract

An iron-based metal anode for the electrowinning of aluminium by the electrolysis of alumina in a molten fluoride electrolyte has an electrochemically active integral outside oxide layer on an iron-based alloy that consists of 75 to 90 weight % iron; 0.5 to 5 weight % in total of at least one rare earth metal, in particular yttrium; 1 to 10 weight % aluminium; 0 to 10 weight % copper; 0 to 10 weight % nickel; and 0.5 to 5 weight % of other elements. The total amount of aluminium, copper and nickel is in the range from 5 to 20 weight %; and the total amount of rare earth metal(s), aluminium and copper is also in the range from 5 to 20 weight %. The electrochemically active surface layer is predominantly of iron oxide that slowly dissolves into the electrolyte during operation and is maintained by progressive slow oxidation of iron at the interface of the bulk metal of the alloy with the oxide layer. This progressive slow oxidation of iron corresponds to the dissolution of iron into the electrolyte which remains at or below saturation level at the operating temperature, the operating temperature being maintained sufficiently low to limit the contamination of the product aluminium to an acceptable level, and the electrolyte being circulated to maintain a sufficient concentration of alumina in the anode cathode gap.

Description

FIELD OF THE INVENTION [0001] This invention relates to iron-based metal anodes for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte and to a cell and method for the electrowinning of aluminium using such iron-based metal anodes. BACKGROUND ART [0002] Using non-carbon anodes in aluminium electrowinning cells should drastically improve the aluminium production process by reducing pollution and the cost of aluminium production. Many attempts have been made to use oxide anodes, cermet anodes and metal-based anodes for aluminium production, however they were never adopted by the aluminium industry. [0003] U.S. Pat. No. 6,248,227 (de Nora / Duruz) discloses a non-carbon, metal-based slow-consumable anode of a cell for the electrowinning of aluminium that self-forms during normal electrolysis an electrochemically-active oxide-based surface layer. The rate of formation of this layer is maintained substantially equal to its r...

AI Technical Summary

Benefits of technology

[0009] An object of the invention is to provide an iron-based metal anode with an iron-rich alloy having an integral outside oxide layer which can be progressively formed during use at a rate corresponding to a controlled dissolution into the electrolyte at the operating temperature, or which can even be stabilised by maintaining an amount of iron species in the electrolyte, leading to an acceptably low contamination of the product aluminum.

Problems solved by technology

Many attempts have been made to use oxide anodes, cermet anodes and metal-based anodes for aluminium production, however they were never adopted by the aluminium industry.