Aluminium production cells with iron-based metal alloy anodes

a production cell and metal alloy technology, applied in the direction of electrical-based machining apparatus, machining electrodes, electrodes, etc., to achieve the effect of low contamination of aluminum products

Inactive Publication Date: 2005-01-06
NGUYEN THINH T +2
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

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.

Method used

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Examples

Experimental program
Comparison scheme
Effect test

example 1

[0075] An anode rod of diameter 20 mm and total length 200 mm was prepared by casting the composition of Sample D of Table I, using a sand mould.

[0076] Electrolysis was carried out in a laboratory scale cell equipped with this anode immersed to a depth of 50 mm in a fluoride-containing molten electrolyte at 880°C. The electrolyte contained cryolite with 24 weight % excess of AlF3 and further containing 4 weight % of CaF2.

[0077] The current density was about 0.7 A / cm2 and the concentration of dissolved alumina in the electrolyte was 5 weight %. This concentration of alumina was maintained during the entire electrolysis by periodically feeding fresh alumina into the cell.

[0078] After 22 hours electrolysis was interrupted and the anode extracted. Upon cooling the anode was examined externally and in cross-section. The anode was covered by an external scale of Fe2O3 about 10-15 micron thick on top of a layer of Fe aluminate (probably Fe(FeAl)2O4 spinel phase) about 150-200 micron thi...

example 2

[0081] This Example illustrates the wear rate of the iron-based metal anode of Example 1.

[0082] An estimation of the wear rate is based on the following parameters and assumptions:

[0083] With a current density of 0.7 A / cm2 and a current efficiency of 90% an aluminium electrowinning cell produces daily approximately 50 kg aluminium per square meter of active cathode surface.

[0084] Assuming a contamination of the produced aluminium by 800 ppm of iron, which corresponds to the experimentally measured quantities in typical tests, the wear rate of an iron sample corresponds to approximately 5 micron / day.

example 3

[0085] An anode rod of diameter 20 mm and total length 20 mm was prepared by casting the composition of Sample F of Table I, using a sand mould.

[0086] The anode was subjected to testing as in Example 1 but with electrolysis during 72 hours with the cell voltage maintained stable at 3.8 to 4.0 volts. After 72 hours, the electrolysis was stopped, the anode was extracted and, upon cooling, was examined externally and in cross-section. The anode was covered by a coherent and dense external scale of Fe2O3 20 to 30 micron thick, over a cermet zone about 50 micron thick composed of a Ni—Al and Ni—Fe network with inclusions of NiXFe2-xO3 and Al2O3, or a mixture thereof.

[0087] Comparing with the results described in Example 1, the decrease of the Al content from 10 to 6 weight % and he presence of nickel increased the oxidation resistance of the alloy core by formation of protective scales of NiXFe2-XO3 and Al2O3.

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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...

Claims

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Application Information

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Patent Type & Authority Applications(United States)
IPC IPC(8): C25C3/06C25C3/12
CPCC25C3/12C25C3/06
Inventor NGUYEN, THINH T.DURUZ, JEAN-JACQUESDE NORA, VITTORIO
Owner NGUYEN THINH T
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