CA2360094C - High-strength low-alloy steel anodes for aluminium electrowinning cells - Google Patents

Abstract

An anode of a cell for the electrowinning of aluminium comprises a low-carbo n high-strength low-alloy (HSLA) steel body or layer whose surface is oxidised to form a coherent and adherent outer iron oxide- based layer the surface of which is electrochemically active for the evolution of oxygen. The iron oxide-based layer has a low solubility in the molten electrolyte. During use, the thickness of the iron oxide-based layer is such as to reduce or prevent diffusion of oxygen from the electrochemically active surface into the steel body or layer. During cell operation, the anode may be maintained dimensionally stab le by saturating the electrolyte with anode constituents.

Description

HIGH-STRENGTH LOW-ALLOY STEEL ANODES FOR
ALUMINIUM ELECTROWINNING CELLS
Field of the Invention 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 such as cryolite, and to methods for their fabrication, as well as to electrowinning cells containing such anodes and their use to produce aluminium.
Background Art 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 Heroult, has not evolved as many other electrochemical processes.
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 C02 and small amounts of CO and fluorine-containing dangerous gases. The actual consumption of the anode is as much as 450 Kg/Ton of aluminium produced which is more than 1/3 higher than the theoretical amount of 333 Kg/Ton.
Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production.
US Patent 4,999,097 (Sadoway) describes anodes for conventional aluminium electrowinning cells provided with an oxide coating containing at least one oxide of zirconium, hafnium, thorium and uranium. To prevent consumption of the anode, the bath is saturated with the materials that form the coating. However, these coatings are poorly conductive and have not been used.
US Patent 4,504,369 (Keller) discloses a method of producing aluminium in a conventional cell using anodes whose dissolution into the electrolytic bath is reduced by adding anode constituent materials into the electrolyte, allowing slow dissolution of the anode. However, this method is impractical because it would lead to a contamination of the product aluminium by the anode constituent materials which is considerably above the acceptable level in industrial production.
US Patent 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes metal anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained during electrolysis by the addition of small amounts of a cerium compound to the molten cryolite electrolyte. This made it possible to have a protection of the surface from the electrolyte attack and to a certain extent from gaseous oxygen but not from nascent monoatomic oxygen.
EP Patent application 0 306 100 (Nyguen/Lazouni/
Doan) describes anodes composed of a chromium, nickel, cobalt and/or iron based substrate covered with an oxygen barrier layer and a ceramic coating of nickel, copper and/or manganese oxide which may be further covered with an in-situ formed protective cerium oxyfluoride layer.
Likewise, US Patents 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium production anodes with an oxidised copper-nickel surface on an alloy substrate with a protective barrier layer. However, full protection of the alloy substrate was difficult to achieve.
Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. Many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry because they had a short life and contaminated the aluminium produced.

Obiects of the Invention 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 increase the anode life.
A further object of the invention is to provide an aluminium electrowinning anode material with a surface having a high electrochemical activity and a low solubility in the electrolyte.
Another object of the invention is to provide an anode for the electrowinning of aluminium which is covered with an electrochemically active layer with limited ionic conductivity for oxygen ions.
Yet another object of the invention is to provide an anode for the electrowinning of aluminium which is made of readily available material(s).
An important object of the invention is to substantially reduce the solubility of the surface layer of an aluminium electrowinning anode, thereby maintaining the anode dimensionally stable.
Yet another object of the invention is to provide operating conditions for an aluminium electrowinning cell under which the contamination of the product aluminium is limited.
Summarv of the Invention This invention is based on the observation that low-carbon high-strength low-alloy (HSLA) steels such as Cor-TenTM even at high temperature form under oxidising conditions an iron oxide-based surface layer which is dense, electrically conductive, electrochemically active for oxygen evolution and, as opposed to oxide layers formed on standard steels or other iron alloys, is highly adherent and less exposed to delamination and limits diffusion of ionic, monoatomic and molecular oxygen.
HSLA steels are used for their strength and resistance to atmospheric corrosion especially at lower temperatures (below 0°C) in different areas of technology such as civil engineering (bridges, dock walls, sea walls, piping), architecture (buildings, frames) and mechanical engineering (welded/bolted/riveted structures, car and railway industry, high pressure vessels). However, these HSLA steels have never been proposed for applications at high temperature, especially under oxidising or corrosive conditions, in particular in cells for the electrowinning of aluminium.
It has been found that the iron oxide-based surface layer formed on the surface of a HSLA steel under oxidising conditions limits also at elevated temperatures the diffusion of oxygen oxidising the surface of the HSLA
steel. Thus, diffusion of oxygen through the surface layer decreases with an increasing thickness thereof.
If the HSLA steel is exposed to an oxidising environment which maintains or preserves the surface layer, the iron oxide-based surface layer grows until its thickness constitutes a sufficient barrier to oxygen and then remains dimensionally stable. If the HSLA steel is exposed to an environment promoting dissolution or delamination of the surface layer, the rate of formation of the iron oxide-based surface layer (by oxidation of the surface of the HSLA steel) reaches the rate of dissolution or delamination of the surface layer after a transitional period during which the surface layer grows or decreases to reach an equilibrium thickness in the specific environment.
Anodes and Manufacture The invention relates in particular to an anode of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte.
This anode comprises a low-carbon high-strength low-alloy (HSLA) steel body or layer whose surface is oxidised to form a coherent and adherent outer iron oxide-based layer the surface of which is electrochemically active for the evolution of oxygen. The iron oxide-based layer has a low solubility in the molten electrolyte. The thickness of the iron oxide-based layer is such as to reduce or prevent diffusion of oxygen from the electrochemically active surface into the steel body or layer during use.
During steady operation the reduced rate of diffusion through the oxide-based layer can be such that oxygen only diffuses into the steel body or layer in a controlled manner without significant increase of the thickness of the oxide-based layer.
High-strength low-alloy (HSLA) steel designates a group of low-carbon steels (typically up to 0.5 weighto carbon of the total) that contain small amounts of alloying elements. These steels have better mechanical properties and sometimes better corrosion resistance than carbon steels.
The surface of the high-strength low-alloy steel body or layer may be oxidised in an electrolytic cell or in an oxidising atmosphere, in particular a relatively pure oxygen atmosphere. For instance the surface of the high-strength low-alloy steel body or layer may be oxidised in a first electrolytic cell and then transferred to an aluminium production cell. In an electrolytic cell oxidation would typically last 5 to 15 hours at 800 to 1000°C. Oxidation may also take place in air or in oxygen for 5 to 25 hours at 750 to 1150°C before electrolysis.
In order to prevent thermal shocks causing mechanical stresses, a high-strength low-alloy steel body or layer may be tempered or annealed after pre-oxidation.
Alternatively, the high-strength low-alloy steel body or layer may be maintained at elevated temperature after pre oxidation until immersion into the molten electrolyte of an aluminium production cell.
The high-strength low-alloy steel body or layer may comprise 94 to 98 weight% iron and carbon, the remaining constituents being one or more further metals selected from chromium, copper, nickel, silicon, titanium, tantalum, tungsten, vanadium, zirconium, aluminium, molybdenum, manganese and niobium, and possibly small amounts of at least one additive selected from boron, sulfur, phosphorus and nitrogen.

In one embodiment, the anode comprises a layer of high-strength low-alloy steel on an oxidation resistant metallic core. The layer of high-strength low-alloy steel may be applied on the metallic core before or after formation of the outer iron oxide-based layer.
The metallic core is preferably electrically highly conductive and may be made of copper or a copper alloy. To enhance the mechanical properties, the metallic core may contain minor amounts of at least one oxide, such as alumina, hafnia, yttria and/or zirconia. Furthermore, to enhance oxidation resistance, the metallic core may be coated with at least one metal selected from nickel, chromium, cobalt, iron, aluminium, hafnium, manganese, molybdenum, niobium, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium, and alloys, intermetallic compounds and combinations thereof.
The metallic core may be coated with an oxygen barrier layer of chromium and/or niobium.
The layer of high-strength low-alloy steel may be plasma sprayed, arc sprayed, slurry-applied or electrodeposi''~Eed onto the metallic core. Alternatively, to enhance adhesion, the high-strength low-alloy steel layer may be bonded to the metallic core through at least one intermediate layer, in particular a film of silver, typically 0.1 to 10 micron thick, which is in intimate and continuous contact with the metallic core and with the steel layer, and/or at least one layer of nickel and/or copper.
The invention also relates to a bipolar electrode of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing electrolyte, comprising on its anodic side an anode as described above.
The high strength low allow (HSLA) steel body can also be bonded or connected to an electrically conductive anode structure of special design as disclosed in WO00/40781 and WO00/40782 (both in the name of de Nora ) .

One aspect of the invention is an anode precursor comprising a low-carbon high-strength low-alloy (HSLA) steel body or layer and which can be converted into a fully manufactured anode as described above by oxidising the surface of the steel body or layer to form the coherent and adherent outer iron oxide-based layer.
Another aspect of the invention is a method of manufacturing an anode as described above comprising:
- providing a low-carbon high-strength low-alloy (HSLA) steel body or layer; and - oxidising the surface of the high-strength low-alloy steel body or layer to form the coherent and adherent outer iron oxide-based layer the surface of which is electrochemically active for the evolution of oxygen.
Cells and Aluminium Production A further aspect of the invention is a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte comprising at least one anode having a low-carbon high-strength low-alloy (HSLA) steel body or layer and an electrochemically active outer iron oxide-based layer whose surface is electrochemically active, as described above.
During normal operation the electrochemically active layer of the or each anode may be progressively further formed by surface oxidation of the steel body or layer by controlled oxygen diffusion through the electrochemically active layer, and progressively dissolved into the electrolyte at the electrolyte/anode interface, the rate of formation of the outer iron oxide-based layer being substantially equal to its rate of dissolution into the electrolyte.
However, it has been observed that this type of anode may be maintained dimensionally stable under specific cell operating conditions.
In known processes, even the least soluble anode material releases excessive amounts of constituents into _ g _ the bath, which leads to an excessive contamination of the product aluminium. For example, the concentration of nickel (a frequent component of proposed metal-based anodes) found in aluminium produced in small scale tests at conventional cell operating temperatures is typically comprised between 800 and 2000 ppm, i.e. 4 to 10 times the maximum acceptable level which is 200 ppm.
Iron oxides and in particular hematite (Fe203) have a higher solubility than nickel in molten electrolyte.
However, in industrial production the contamination tolerance of the product aluminium by iron is also much higher (up to 2000 ppm) than for other metal impurities.
Solubility is an intrinsic property of anode materials and cannot be changed otherwise than by modifying the electrolyte composition and/or the operating temperature of a cell.
Small scale tests utilising a NiFe204/Cu cermet anode and operating under steady conditions were carried out to establish the concentration of iron in molten electrolyte and in the product aluminium under different operating conditions.
In the case of iron oxide, it has been found that lowering the temperature of the electrolyte deacreases considerably the solubility of iron species. This effect can surprisingly be exploited to produce a major impact on cell operation by limiting the contamination of the product aluminium by iron.
Thus, it has been found that when the operating temperature of the cell is reduced below the temperature of conventional cells (950-970°C) an anode covered with an outer layer of iron oxide can be made dimensionally stable by maintaining a concentration of iron species and alumina in the molten electrolyte sufficient to reduce or suppress the dissolution of the iron-oxide layer, the concentration of iron species being low enough not to exceed the commercial acceptable level of iron in the product aluminium.

The presence of dissolved alumina in the electrolyte at the anode surface has a limiting effect on the dissolution of iron from the anode into the electrolyte, which reduces the concentration of iron species necessary to substantially stop dissolution of iron from the anode.
Therefore, anodes according to the invention may be kept dimensionally stable by maintaining a sufficient amount of dissolved alumina and iron species in the electrolyte to reduce or prevent dissolution of the outer oxide layer.
The cell should be operated at a sufficiently low temperature to limit the solubility of iron species in the electrolyte, thereby limiting contamination of the product aluminium by constituents of the outer iron oxide-based layer of the anodes) to a commercially acceptable level.
When the cell is operated with a fluoride-based melt the operating temperature of the electrolyte should be below 910°C, usually from 730 to 870°C.
The amount of iron species and alumina dissolved in the electrolyte preventing dissolution of the iron oxide-based outside surface layer of the or each anode should be such that the product aluminium is contaminated by no more than 2000 ppm iron, preferably by no more than 1000 ppm iron, and even more preferably by no more than 500 ppm iron.
Usually the iron species are intermittently fed into the electrolyte, for instance together with alumina, to maintain the amount of iron species in the electrolyte constant which, at the operating temperature, prevents the dissolution of the iron oxide-based outside surface layer of the anodes. However, the iron species can also be continuously fed, for instance by dissolving a sacrificial electrode which continuously feeds the iron species into the electrolyte.
The iron species may be fed in the form of iron metal and/or an iron compound, in particular iron oxide, iron fluoride, iron oxyfluoride and/or an iron-aluminium alloy.

WO 00/d0?83 PCT/IB00100028 Advantageously, the cell may comprise an aluminium-wettable cathode which can be a drained cathode on which aluminium is produced and from which it continuously drains, as described in US Patent 5,651,874 (de Nora/Sekhar) and 5,683,559 (de Nora).
Usually, the cell is in a monopolar, mufti-monopolar or in a bipolar configuration. The bipolar cell comprises a terminal cathode facing a terminal anode and thereinbetween at least one bipolar electrode, the anodes) described above forming the anodic side of the or each bipolar electrode and/or of the terminal anode.
In such a bipolar cell an electric current is passed from the surface of the terminal cathode to the surface of the terminal anode as ionic current in the electrolyte and as electronic current through the bipolar electrodes, thereby producing aluminium on each cathode surface and oxygen on each anode surface.
Preferably, the cell comprises means to improve the circulation of the electrolyte between the anodes and facing cathodes and/or means to facilitate dissolution of alumina in tile electrolyte. Such means can for instance be provided by the geometry of the cell as described in co-pending application W099/41429 (de Nora/Duruz) or by periodically moving the anodes as described in co-pending application W099/41430 (Duruz%Bellb).
Yet another aspect of the invention is a method of producing aluminium in a cell as described above. The method comprises dissolving alumina in the electrolyte and passing an ionic electric current between the electrochemically active surface of the anodes) and the surface of the cathode(s), thereby producing aluminium on the cathode surfaces) and oxygen on the anode surface(s).
Yet a further aspect of the invention is a method of manufacturing an anode and producing aluminium in an electrolytic cell comprising inserting an anode precursor as described above into the electrolyte of an electrolytic cell and forming the iron oxide-based layer to produce a fully manufactured anode, and producing oxygen on the surface of the iron oxide-based layer and aluminium on a facing cathode in the same (or nearly the same) or in a different electrolyte.
The thus-produced anode may then be transferred from the electrolytic cell in which it was produced to an aluminium electrowinning cell. Alternatively the composition of the electrolyte in which the anode was produced can be suitably modified, for instance by dissolving alumina and optionally iron species, and electrolysis continued in the same cell to produce aluminium.
Detailed Description The invention will be further described in the following Examples:
Example 1 Electrolysis was carried out in a laboratory scale cell equipped with an anode according to the invention.
The anode was made with a Cor-TenTM type low-carbon high-strength (HSLA) steel containing niobium, titanium, chromium and copper in a total amount of less than 4 weighto. The anode was pre-oxidised in air at about 1050°C
for 15 hours for the formation of a dense hematite-based outer layer.
The anode was then tested in a fluoride-containing molten electrolyte at 850°C and at a current density of about 0.7 A/cm2. The electrolyte contained cryolite and 15 weighto excess of AlF3, approximately 3 weighto alumina and approximately 200 ppm iron species obtained from the dissolution of iron oxide thereby surely saturating the electrolyte with iron species and inhibiting dissolution of the hematite-based anode surface layer.
To maintain the concentration of dissolved alumina in the electrolyte, fresh alumina was periodically fed into the cell. The alumina feed contained sufficient iron oxide so as to replace the iron which had been deposited into the product aluminium, thereby maintaining the concentration of iron in the electrolyte at the limit of solubility and preventing dissolution of the hematite-based anode surface layer.
After 140 hours electrolysis was interrupted and the anode extracted. Upon cooling the anode was examined externally and in cross-section. No corrosion was observed at or near the surface of the anode.
The produced aluminium was also analysed and showed an iron contamination of about 700 ppm which is below the tolerated iron contamination in commercial aluminium production.
Example 2 As in Example 1, aluminium was produced in a laboratory scale cell equipped with an anode according to the invention.
The anode was made with a low-carbon high-strength (HSLA) steel containing manganese 0.4 weighto, niobium 0.02 weighto, molybdenum 0.02 weight%, copper 0.3 weighto, nickel 0.45 weighto and chromium 0.8 weighto. The anode was pre-oxidised in air at about 850°C for 12 hours to form a dense hematite-based outer layer.
The anode was then tested under similar conditions as in Example 1 and the test results were similar.
Example 3 As in Example 1, aluminium was produced in a laboratory scale cell equipped with an anode according to the invention.
The anode was made with a low-carbon high-strength (HSLA) steel containing nickel, copper and silicon in a total amount of less than 1 . 5 weight o . The anode was pre-oxidised in air at about 850°C for 12 hours to form a dense hematite-based outer layer.
The anode was then tested under similar conditions as in Example 1 and the test results were similar.

Claims (41)

1. ~An anode of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, said anode comprising a low-carbon high-strength low-alloy (HSLA) steel body or layer whose surface is oxidised to form a coherent and adherent outer iron oxide-based layer the surface of which is electrochemically active for the evolution of oxygen, said iron oxide-based layer having a low solubility in the molten electrolyte, the thickness of said iron oxide-based layer being such as to reduce or prevent diffusion of oxygen from the electrochemically active surface into the steel body or layer.

2. The anode of claim 1, wherein the high-strength low-alloy steel body or layer comprises 94 to 98 weight% iron and carbon, the remaining constituents being one or more further metals selected from chromium, copper, nickel, silicon, titanium, tantalum, tungsten, vanadium, zirconium, aluminium, molybdenum, manganese and niobium, and possibly small amounts of at least one additive selected from boron, sulfur, phosphorus and nitrogen.

3. The anode of claim 1, comprising a layer of high-strength low-alloy steel on an oxidation resistant metallic core.

4. The anode of claim 3, wherein the metallic core is made of copper or a copper alloy.

5. The anode of claim 4 wherein the metallic core contains minor amounts of at least one oxide reinforcing the mechanical properties of the metallic core.

6. The anode of claim 5, wherein said at least one reinforcing oxide is selected from alumina, hafnia, yttria and zirconia.

7. ~The anode of claim 3, wherein the metallic core is coated with at least one metal selected from nickel, chromium, cobalt, iron, aluminium, hafnium, manganese, molybdenum, niobium, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium, and alloys, intermetallic compounds and combinations thereof.

8. ~The anode of claim ?, wherein the metallic core is coated with an oxygen barrier layer of chromium and/or niobium.

9. ~The anode of claim 3, wherein the high-strength low-alloy steel layer is bonded to the metallic core through at least one intermediate layer.

10. ~The anode of claim 9, wherein the high-strength low-alloy steel layer is bonded to the metallic core through a film of silver, and/or at least one layer of nickel and/or copper.

11. A bipolar electrode of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing electrolyte, comprising on its anodic side an anode as defined in claim 1.

12. A method of manufacturing an anode as define d in claim 1 comprising:
- providing a low-carbon high-strength low-alloy (HSLA) steel body or layer; and - oxidising the surface of the high-strength low-alloy steel body or layer to form the coherent and adherent outer iron oxide-based layer the surface of~
which is electrochemically active for the evolution of oxygen.

13. The method of claim 12, comprising applying a layer of high-strength low-alloy steel on an oxidation resistant metallic core before or after formation of said outer iron oxide-based layer.

14. The method of claim 13, comprising plasma spraying, arc spraying or electrodepositing the high-strength low-alloy steel layer on the metallic core.

15. The method of claim 13, comprising bonding the high -strength low-alloy steel layer to the metallic core through at least one intermediate bonding layer.

16. The method of claim 12, comprising oxidising the surface of the high-strength low-alloy steel body or layer in a molten electrolyte at 800 to 1000°C for 5 to 15 hours.

17. The method of claim 16, comprising oxidising the surface of the high-strength low-alloy steel body or layer at 750 to 1150°C for 5 to 25 hours in an oxidising atmosphere such as air or oxygen before electrolysis.

18. A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte comprising at least one anode having a low-carbon high-strength low-alloy (HSLA) steel body or layer and an electrochemically active outer iron oxide-based layer as defined in claim 1.

19. The cell of claim 18, wherein during normal operation the electrochemically active layer of the or each anode is progressively further formed by surface oxidation of the steel body or layer by controlled oxygen diffusion through the electrochemically active layer, and progressively dissolved into the electrolyte at the electrolyte/anode interface, the rate of formation of the outer iron oxide-based layer being substantially equal to its rate of dissolution into the electrolyte.

20. The cell of claim 18, wherein the or each anode is kept dimensionally stable by maintaining a sufficient amount of dissolved alumina and iron species in the electrolyte to prevent dissolution of the outer oxide layer of the or each anode.

21. The cell of claim 20, which is operated at a sufficiently low temperature to limit the solubility of the outer iron oxide-based layer of the anode(s), thereby limiting the contamination of the product aluminium by constituents of the outer iron oxide-based layer of the anode(s).

22. The cell of claim 18, which is in a bipolar configuration, comprising a terminal cathode facing a terminal anode and thereinbetween at least one bipolar electrode, and wherein said anode(s) form(s) the anodic side of the or each bipolar electrode and/or the terminal anode.

23. A method of producing aluminium in a cell as defined in claim 18, the method comprising dissolving alumina in the electrolyte and passing an ionic electric current between the electrochemically active surface of the anode(s) and the surface of the cathode(s), thereby producing aluminium on the cathode surface(s) and oxygen on the anode surface(s).

24. The method of claim 23, wherein the electrochemically active layer of the or each anode is progressively further formed by surface oxidation of the steel body or layer by controlled oxygen diffusion through the electrochemically layer, and progressively dissolved into the electrolyte at the electrolyte/anode interface, the rate of formation of the outer iron oxide-based layer being substantially equal to its rate of dissolution into the electrolyte.

25. The method of claim 23, comprising keeping the or each anode dimensionally stable by maintaining a sufficient amount of dissolved alumina and iron species in the electrolyte to prevent dissolution of the outer oxide layer of the or each anode.

26. The method of claim 23, comprising operating the cell at a sufficiently low temperature to limit the solubility of the outer iron oxide-based layer of the anode(s), thereby limiting contamination of the product aluminium by constituents of the outer iron oxide-based layer of the anode(s).

27. The method of claim 24, wherein the cell is operated with an operative temperature of the electrolyte below 910°C.

28. The method of claim 25, wherein the cell is operated at an electrolyte temperature from 730 to 870°C.

29. The method of claim 26, wherein the amount of iron species and alumina dissolved in the electrolyte which prevents dissolution of the iron oxide-based outside surface layer of the anode, is such that the product aluminium is contaminated by no more than 2000 ppm iron.

30. The method of claim 29, wherein the product aluminium is contaminated by no more than 1000 ppm iron.

31. The method of claim 29, wherein the product aluminium is contaminated by no more than 500 ppm iron.

32. The method of claim 25, wherein iron species are intermittently or continuously fed into the electrolyte to maintain the amount of iron species in the electrolyte which prevents at the operating temperature the dissolution of the iron oxide-based outside surface layer of the or each anode.

33. The method of claim 32, wherein the iron species are fed in the form of iron metal and/or an iron compound.

34. The method of claim 33, wherein the iron species are fed into the electrolyte in the form of iron oxide, iron fluoride, iron oxyfluoride and/or an iron-aluminium alloy.

35. The method of claim 32, wherein the iron species are periodically fed into the electrolyte together with alumina.

36. The method of claim 32, wherein a sacrificial electrode continuously feeds the iron species into the electrolyte.

37. The method of claim 23, for producing aluminium on an aluminium-wettable cathode.

38. The method of claim 37, wherein the produced aluminium continuously drains from said cathode.

39. The method of claim 23, comprising circulating the electrolyte between the anodes and facing cathodes thereby improving dissolution of alumina into the electrolyte and/or improving the supply of dissolved alumina under the active surfaces of the anodes.

40. Use of a low-carbon high-strength low-alloy (HSLA) steel body or layer as an anode precursor which can be converted into an aluminium electrowinning anode as defined in claim 1 by oxidising the surface of the steel body or layer to form the coherent and adherent outer iron oxide-based layer.

41. A method of manufacturing an anode and producing aluminium in an electrolytic cell comprising inserting a low -carbon high-strength low-alloy (HSLA) steel body or layer as an anode precursor into a fluoride-containing molten electrolyte of an electrolytic cell, oxidising in-situ the surface of the anode precursor to produce an electrochemically active iron oxide-based layer, thereby converting the anode precursor into an anode as defined in claim 1, and producing oxygen on the surface of the iron oxide-based layer and aluminium on a facing cathode in the same or in a different electrolyte.