WO2002070786A1

WO2002070786A1 - Metal-based anodes for aluminium production cells

Info

Publication number: WO2002070786A1
Application number: PCT/IB2002/000667
Authority: WO
Inventors: Thinh T. Nguyen; Vittorio De Nora
Original assignee: Moltech Invent S.A.
Priority date: 2001-03-07
Filing date: 2002-03-04
Publication date: 2002-09-12
Also published as: EP1381716B1; ATE296367T1; DE60204307T2; NZ527307A; EP1381716A1; ES2239709T3; AU2002236142B2; CA2437671A1; DE60204307D1

Abstract

A metal-based anode substrate of a cell for the electrowinning of aluminium comprises a nickel-alloy based core, a layer of silver on the core and a layer comprising nickel and iron covering the silver layer and serving as an anchorage layer for anchoring an electrochemically active surface coating on top of the anode substrate. The electrochemically active coating may be a cerium-based coating. The silver layer inhibits diffusion of fluoride species into the core and prevents interdiffusion of constituents of the core and constituents of the anchorage layer.

Description

METAL-BASED ANODES FOR ALUMINIUM PRODUCTION CELLS

Field of the Invention

This invention relates to metal-based anodes for aluminium production cells, aluminium production cells operating with such anodes as well as operation of such cells 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 6,077,415 (Duruz/de Nora) discloses a metal-based anode comprising a metal-based core covered with an oxygen barrier layer and an electrochemically active outer layer, the barrier layer and the outer layer being separated by an intermediate layer to prevent dissolution of the oxygen barrier layer. US Patents 4,614,569 (Duruz/Derivaz/Debely/Adorian) , 4,966,674 (Bannochie/Sheriff) , 4,683,037 and 4,680,094 (both in the name of Duruz) describe 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 by the addition of small amounts of cerium to the molten cryolite.

Along the same lines, EP Patent application 0 306 100 and US Patents 5,069,771, 4,960,494 and 4,956,068 (all in the name of Nyguen/Lazouni/Doan) disclose aluminium production anodes having an alloy substrate protected with an oxygen barrier layer that is covered with a copper- nickel layer for anchoring a cerium oxyfluoride operative surface coating.

Although the above mentioned prior art metal-based anodes showed a significantly improved lifetime over known oxide and cermet anodes, they have not as yet found commercial acceptance. Objects 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. An important object of the invention is to reduce the solubility of the surface of an aluminium electrowinning anode, thereby maintaining the anode dimensionally stable without excessively contaminating the product aluminium.

Another object of the invention is to provide a cell for the electrowinning of aluminium utilising metal-based anodes, and a method to produce aluminium in such a cell and preferably maintain the metal-based anodes dimensionally stable.

A main object of the invention is to provide a metal- based anode for the production of aluminium which is resistant to fluoride attack.

A subsidiary object of the invention is to prevent diffusion of chromium in a metal-based anode that comprises chromium as an oxygen barrier layer. Summary of the Invention

It has been observed that prior art aluminium production metal-based anodes are attacked during use by fluorides. Also when aluminium production cells are operated with an electrolyte at reduced temperature, i.e. below 960°C, fluoride attack increases, as the fluoride content is higher.

Without being bound to any theory, it is believed that metal oxides present at the surface of metal-based anodes, like oxides of iron, nickel, copper, chromium etc., combine during use with fluorides of the electrolyte to produce soluble oxyfluorides .

The invention is based on the observation that silver can be used as a barrier layer to fluoride attack. At high temperature, i.e. above 450°C, silver does not form an oxide and remains as a metal . It follows from the above theory that during use fluorides cannot form oxyfluorides by exposure to the silver layer which is devoid of oxide, and the fluorides cannot corrode the silver layer. Therefore, the invention relates to a metal-based anode substrate of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The substrate comprises a nickel-alloy based core, a layer of silver on the core and a layer comprising nickel and iron covering the silver layer and serving as an anchorage layer for anchoring an electrochemically active surface coating on top of the anode substrate. The silver layer inhibits diffusion of fluoride species into the core and prevents interdiffusion of constituents of the core and constituents of the anchorage layer.

The silver layer may have an average thickness in the range of 5 to 100 micron.

As silver is not an efficient barrier to oxygen, when the core of the anode substrate is not by itself sufficiently resistant to oxygen attack, it is preferable to protect it with an oxygen barrier layer, such as a chromium barrier layer. Therefore, in one embodiment, the anode substrate comprises a further layer of silver and a layer of chromium, the chromium layer being located between the core and the anchorage layer and separated therefrom by the silver layers. The chromium layer may have an average thickness in the range of 10 to 100 micron.

Thus, the oxygen barrier layer is separated from the core by a first layer of silver and from the anchorage layer by a second layer of silver. The silver layers prevent interdiffusion of chromium from the barrier layer with constituents of the core and with constituents of the anchorage layer.

As silver is insoluble in chromium and vice-versa, the silver layers confine the chromium within the barrier layer and do not mix with the chromium, thereby securing a long-lasting integrity of the chromium barrier layer.

By having an oxygen barrier layer of chromium separated by silver layers from miscible anode constituents, this embodiment of the anode according to the present invention is efficiently protected against oxidation for a longer period of time than prior art anodes .

Conversely, in prior art anodes, e.g. as disclosed in US Patents 5,069,771, 4,960,494 and 4,956,068 mentioned above, the chromium barrier layer contacts miscible metals such as nickel and/or copper. During use, a slow interdiffusion of chromium with nickel and/or copper takes place, and the contaminated chromium barrier layer becomes pervious to oxygen permitting oxidation of the anode material underneath.

The anchorage layer and/or the core may comprise one or more additives selected from yttrium, tantalum and niobium in a total amount of 0.1 to 5 weight% . Preferably, the anchorage layer and/or the core comprise yttrium, for instance in an amount of less than 1 weight% .

It has been observed that when a nickel-iron alloy is used as anode material, the iron of the alloy slowly diffuses to the surface, becomes oxidised by anodically evolved oxygen and dissolves in the electrolyte during use. The addition of yttrium to the nickel-iron alloy greatly inhibits diffusion of iron from inside the alloy to its surface. Indeed, in such an alloy, yttrium is mainly located at the joints between the grains forming the nickel-iron alloy and constitutes a mechanical obstacle to diffusion of the grains within the alloy.

The anchorage layer may have an average thickness in the range of 30 to 300 micron. The anchorage layer can be made of a bottom layer of nickel and iron and a top layer of copper. In this case, the nickel-iron bottom layer may have an average thickness in the range of 30 to 300 micron and the copper top layer an average thickness in the range of 5 to 50 micron. Upon heat treatment, the copper top layer is usually partly interdiffused with the nickel-iron layer adjacent to it.

Usually, upon exposure to an oxidising atmosphere and/or during use, one or more of the layers on the anode core are at least partly oxidised. The invention also relates to a metal-based anode that comprises an anode substrate as described above which is coated with an electrochemically active surface coating made of one or more cerium compounds, in particular cerium oxyfluoride. The electrochemically active surface coating may comprise at least one additive selected from yttrium, tantalum and niobium.

The electrochemically active surface coating can be an electrolytically deposited coating or applied before use, for instance from a cerium-based slurry.

Another aspect of the invention is a cell , for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte. The cell according to the invention comprises at least one of the above described metal-based anodes. The electrolyte of the cell preferably comprises cerium species to maintain the electrochemically active surface coating dimensionally stable.

The cell of the invention may be operated with or without a crust and/or a sideledge of frozen electrolyte.

Advantageously, the cell has an electrolyte at reduced temperature, i.e. below 960°C, for instance in the range from 860° to 930°C.

A further aspect of the invention is a method of producing aluminium in the above described cell. The method of the invention comprises dissolving alumina in the electrolyte and passing an electrolysis current between the or each anode and a facing cathode whereby oxygen is anodically evolved on the electrochemically active surface coating and aluminium is cathodically reduced.

The invention will be further described in the following Examples:

Example 1 Ar^de__subs_trate_:_

An anode substrate made of a nickel-iron core covered with a silver barrier layer and a nickel-iron anchorage layer according to the invention was prepared as follows :

A he i-spherical nickel-containing anode core having a diameter of 20 mm and a length of 30 mm was machined from a nickel-iron alloy rod made of 80 weight% nickel and 20 weight% iron. The surface of the anode core was sandblasted, degreased and rinsed carefully with deionised water . The anode core was then immersed in an AgCN/KCN bath at room temperature and polarised in order to electrolytically deposit silver thereon from a silver counter electrode . A cathodic current with a current density of about 50 mA/cm² was passed at the surface of anode core. During the electrolytic deposition, the AgCN/KCN bath was moderately agitated. After 10 minutes electrodeposition was interrupted.

The anode core was removed from the AgCN/KCN bath and carefully rinsed with deionised water. An electrodeposited silver layer having an average thickness of about 25 to 30 micron had been formed on the anode core .

The silver plated anode core was then immersed and polarised in a FeS0₄-NiS0₄-NiCl₂/Boric - Salicylic acid bath at a temperature of 55°C. A nickel-iron alloy was deposited onto the silver plated anode core from an alloy counter electrode made of a 50 weight% nickel and 50 weight% iron. An electrolysis current was passed between the plated anode core and the counter-electrode at a current density of about 60 mA/cm² at the surface of the plated anode core. As before, the bath was moderately agitated during the electrolytic deposition.

After 30 minutes electrodeposition was interrupted. The plated anode core was removed from the bath and rinsed carefully with deionised water. A nickel iron anchorage layer made of 54 weight% nickel and 46 weight% iron with an average thickness of about 30 to 35 micron had been deposited onto the silver plated anode core which thus constituted an anode substrate according to the invention.

The anode substrate was oxidised in air at a temperature of about 1100°C for 1 hour. An iron oxide based black adherent layer consisting of 95-97 weight% iron oxide and 3-5% nickel oxide was formed on the anode substrate.

Ele.ctrolyBJLS_Testing. ι

The oxidised anode substrate was then immersed and anodically polarised in ^' a laboratory aluminium electrowinning cell operating with a cryolite-based electrolyte consisting of about 21 weight% AlF₃ , 4 weight%

A1₂0₃, 3 weight% CeF₃ and 72 weight% Na₃AlF₆ at a temperature of about 920°C The cell used an aluminium pool as a cathode.

At the beginning of electrolysis,^' to permit formation of an electrochemically active cerium oxyfluoride coating on the anode substrate, a reduced electrolysis current was passed between the anode substrate and the aluminium cathodic pool at an anodic current density of about 0.5

A/cm². After 5 hours the current density was increased to about 0.8 A/cm². To compensate depletion of CeF₃ and Al₂0₃ during electrolysis, the cell was periodically supplied with a powder feed of Al₂0₃ containing 1 weight% CeF₃. The feeding rate corresponded to 30% of the cathodic current efficiency. After 24 hours the anode was removed from the molten bath and cooled down to room temperature.

Examination,_After_T_.estinsI

Visual examination of the anode showed that a blue and uniform cerium oxyfluoride coating had been deposited on the part of the anode substrate that had been immersed in the cryolite-based electrolyte.

The anode was cut perpendicular to a cerium oxyfluoride coated surface and the section was examined under a SEM microscope .

It was observed that the cerium-based coating had a thickness of about 500 to 700 micron. Underneath the cerium-based coating, the nickel-iron anchorage layer had been completely oxidised and transformed into a black and adherent matrix of iron-nickel mixed oxyfluorides . The electroplated silver layer had remained un-oxidised. Underneath the silver layer, the anode core showed no sign of corrosion or exposure to fluorides. However, a surface layer containing a uniform distribution of iron oxide inclusions and having a thickness of about 200 micron had been formed on the core. Example 2 toode__subs_trate_:_

Another anode substrate made of a nickel-iron core covered with a silver barrier layer, a chromium barrier layer, a further silver layer and a nickel-iron anchorage layer according to the invention was prepared as follows: An anode core was plated with a layer of silver as in Example 1.

An oxygen barrier layer of chromium was then formed on the plated anode core by immersing and polarising it in a Cr0₃/H₂S0₄ bath at a temperature of 35°C. A dimensionally stable counter electrode was used. An electrolysis current was passed between the plated anode core and the counter- electrode at a current density of about 300 mA/cm² on the plated anode core in order to deposit chromium from the bath onto the anode core. The bath was moderately agitated during the electrolytic deposition.

After 30 minutes electrodeposition was interrupted.

The plated anode core was removed from the bath and rinsed carefully with deionised - water. A mat chromium electrodeposited layer of about 15 micron had been deposited onto the silver layer.

Then, the chromium layer was activated in a NiCl₂/HCl bath by anodic polarisation at a current density of about 30 mA/cm² for 3 minutes followed by a cathodic polarisation at the same current density for 6 minutes. A layer of nickel having a thickness of about 1 micron was deposited onto the chromium coating.

After activation, the plated anode core was removed from the activation bath, rinsed carefully with deionised water and immediately plated with a further layer of silver following the above-described silver plating procedure and then with a nickel-iron anchorage layer air oxidised as described in Example 1.

J_lectrol _3_is_Testins

The anode substrate was coated with a cerium oxyfluoride electrochemically active layer to form an anode according to the invention and then used for 24 hours in a cell as described in Example 1.

Exa inati n,_Aft_er_ Testing_

The anode was cut ^" perpendicular to a cerium oxyfluoride coated surface and the section was examined under a SEM microscope .

It was observed that the cerium-based coating had a thickness of about 500 to J00 micron. Underneath the cerium-based coating, the nickel-iron anchorage layer had been completely oxidised and transformed into a black and adherent matrix of iron-nickel mixed oxyfluorides . The electroplated silver layers had remained un-oxidised. The chromium oxygen barrier layer was oxidised to a depth of about 2 to 5 micron.

No nickel or iron from the core or the anchorage layer was found in the chromium layer which demonstrated that silver acts as an efficient barrier preventing interdiffusion of constituents of the core and constituents of the anchorage layer.

Underneath the silver layers and the chromium layer, the anode core showed no sign of corrosion or exposure to fluorides . No oxide was found in the anode core demonstrating the efficiency of the chromium oxygen barrier layer.

Claims

1. A metal-based anode substrate of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprising a nickel-alloy based core, a layer of silver on the core and a layer comprising nickel and iron covering the silver layer and serving as an anchorage layer for anchoring an electrochemically active surface coating on top of the anode substrate, the silver layer inhibiting diffusion of fluoride species into the core and preventing interdiffusion of constituents of the core and constituents of the anchorage layer.

2. The anode substrate of claim 1, wherein the silver layer has an average thickness in the range of 5 to 100 micron.

3. The anode substrate of claim 1 or 2 , which comprises a further layer of silver and a layer of chromium, the chromium layer being located between the core and the anchorage layer and separated therefrom by the silver layers .

4. The anode substrate of claim 3 , wherein the chromium layer has an average thickness in the range of 10 to 100 micron.

5. The anode substrate of any preceding claim, wherein the anchorage layer and/or the core comprise one or more additives selected from yttrium, tantalum and niobium in a total amount of 0.1 to 5 weight% .

6. The anode substrate of claim 5, wherein the anchorage layer and/or the core comprise yttrium in an amount of less than 1 weight%.

7. The anode substrate of any preceding claim, wherein the anchorage layer has an average thickness in the range of 30 to 300 micron.

8. The anode substrate of any _.one of claims 1 to 6, wherein the anchorage layer comprises a bottom layer of nickel and iron and a top layer of copper.

9. The anode substrate of claim 8, wherein the nickel- iron bottom layer has an average thickness in the range of 30 to 300 micron and the copper top layer has an average thickness in the range of 5 to 50 micron.

10. The anode substrate of claim 8, wherein the copper top layer is partly interdiffused with the nickel-iron layer adjacent to it.

11. The anode substrate of any preceding claim, wherein one or more of the said layers are at least partly oxidised.

12. A metal-based anode comprising an anode substrate according to any preceding claim, wherein the anchorage layer is coated with an electrochemically active surface coating made of one or more cerium compounds .

13. The anode of claim 12, wherein the electrochemically active surface coating comprises cerium oxyfluoride.

14. The anode of claim 12 or 13, wherein the electrochemically active surface coating comprises at least one additive selected from yttrium, tantalum and niobium.

15. The anode of claim 12, 13 or 14, wherein the electrochemically active surface coating is an electrolytically deposited coating.

16. .The anode of claim 12, 13 or 14, wherein the electrochemically active surface coating is a slurry- applied coating.

17. A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte, comprising at least one metal-based anode according to any one of claims 12 to 16.

18. The cell of claim 17, wherein the electrolyte comprises cerium species to maintain the electrochemically active surface coating dimensionally stable.

19. The cell of claim 17 or 18, which comprises a crust and/or a sideledge of frozen electrolyte.

20. The cell of claim 17, 18 or 19, wherein the electrolyte is at a temperature in the range from 860° to 930°C.

21. A method of producing aluminium in a cell as defined in any one of claims 17 to 20, comprising dissolving alumina in the electrolyte and passing an electrolysis current between the or each anode and a facing cathode whereby oxygen is anodically evolved and aluminium is cathodically reduced.