WO2003102273A1

WO2003102273A1 - Anode system for use in metal reduction processes and method for the same

Info

Publication number: WO2003102273A1
Application number: PCT/NO2003/000173
Authority: WO
Inventors: Jilai Xue; Arne Petter Ratvik
Original assignee: Sinvent As
Priority date: 2002-05-31
Filing date: 2003-05-28
Publication date: 2003-12-11
Also published as: AU2003232694A1

Abstract

The present invention is to provide a method and an apparatus for producing metal, primarily aluminium, through a gas anode system using reducing gases. The gas anode system has been created not only to carry electrical current, but also to convey and distribute reducing gas, for instance, natural gas to the anode - electrolyte interface. The anode is made of electrically conductive materials, preferentially, graphite or carbon based materials with connecting pores or channels for gas distribution in high temperature applications. However, the carbon anode itself is not normally involved in the anodic reaction. The manner of the distribution of the reducing gas enables protection of the carbon anode from the anodic oxidation. The invention renders possible a more effective, environment-friendly aluminium reduction process with reduced CO2 emission and lower consumption of the anode materials. The invention can also extend to other metals reduction processes where said reducing gas is involved.

Description

ANODE

INTRODUCTION

The present invention relates to an anode system for use in metal reduction processes, particularly for an aluminium reduction process, and a method or process for the same.

BACKGROUND OF THE INVENTION

Today, primary aluminium is commercially produced by the Hall-Heroult (H-H) cell process. The main reaction is

Al₂0₃ + 3/2 C = 2 Al(l) + 3/2 C0₂ (g) (1 )

The H-H cell operates in cryolite (Na₃AIFβ) based molten salts (as electrolyte) at about 960 °C. The electrolyte may have varying cryolite radio (Na₃/AIF₃) and con- tain other additives such as CaF₂, LiF, MgF₂, etc. The electrolyte serves as the solvent for dissolving alumina (Al₂0₃). Electrical current passes via the anode and the cathode through the cryolitic electrolyte, thus electrochemically reducing the dissolved alumina into aluminium metal (liquid) at the cathode and C0₂ at the anode. The Al metal produced rests on a carbon cathode while C0 leaves the cell as gas bubbles.

The modern H-H cells use prebaked carbon anodes. The existing prebaked anode system usually consists of a solid carbon anode body, solid steel stub and aluminium-steel rod. The stub holds the anode body and is connected to the anode rod. The solid stub and anode rod also carry the electrical current to the carbon anode that again contacts directly to the cryolitic electrolyte. The anodic reaction occurs at the anode - electrolyte interface. The present invention can also apply to the older, but still currently used process, involving a Søderberg anode.

The theoretical decomposition voltage is about 1.2 V for Equation (1 ), while the voltage in real life is about 1.7 V. This is mainly due to over-potentials related to chemical and electrochemical reactions of CO and C0₂ at the carbon anode surface. Additional voltage is needed to maintain a heat balance for the cell operation. The heat to keep the cell at operating temperature of about 960 °C is mainly generated by l²R effect in the electrolyte. The cell voltage of the commercial H-H cell today is about 4.3 to 4.5 V with a current efficiency of about 93 to 95%.

It is well known in the industry that there are many problems associated with the use of the existing carbon anode in H-H cells for aluminium production.

The carbon anode is consumed during the aluminium reduction process according to Reaction (1). The theoretical anode carbon consumption is 334 kg per ton of Al produced, but the actual value ranges from 400 to 500 kg per ton of Al. The prebaked carbon anodes are expensive and represent a significant part (about 14 to 20 %) of the aluminium production cost.

The consumption of the anode requires a periodic adjustment of the height of the anodes in order to maintain a certain anode-to-cathode distance. The periodic change of the anodes leads to disturbance of the cell operation and makes cell automation more difficult. The operation of changing anodes is a significant part of aluminium production cost related to the equipment, operation, maintenance and labour.

Furthermore, the varied anode consumption results in an uneven anode surface that is originally made with flat geometry. The uneven anode surface forces the H- H cells to maintain a significant anode - cathode distance during operation in order to avoid any possible shorting between the anode and the cathode. This practice leads to extra voltage drop through the electrolyte.

The exit gas from the reduction cells contains more than 90 % C0₂. It is a greenhouse gas with increasing environmental objection today. The C0₂ formation is directly associated with the current aluminium production process. In addition, other emissions inherent to the use of the current H-H cells are CO, HF, perfluoro- carbon volatiles, etc. The solutions associated to reduce these emissions represent a significant cost and problem for the aluminium producers. For a foreseeable future, the environmental-related costs are expected to continue to increase if no radical change is made to the current H-H cell technology. In the H-H cells, anode effect occurs (with good current practice 0.2 to 0.5 times per week) when the local Al₂0₃ concentration in the cryolitic electrolyte becomes too low. In this case, C0₂ evolution at the carbon anode is interrupted and CF₄ and C₂F₆ are released that are environmentally harmful. The anode effect also restricts the maximum current allowed at the anode, that is, the rate of aluminium production in a H-H cell.

To overcome the shortcomings of the existing anode system mentioned above, various types of so-called non-consumable anodes (NCA) have been tried over the last decades. With the NCA, the basic chemical reaction for aluminium electrolysis becomes

Al₂0₃ = 2 Al(l) + 3/2 0₂(g) (2)

However, the decomposition voltage for Equation (2) is about 2.2 V, compared to 1.2 V with carbon anode. This offsets, partly, its advantage to the carbon anode. The most promising materials for the inert anodes are ceramic oxides or ceramic - metal mixture. Unfortunately, these materials often have a certain solubility in the cryolite electrolyte at about 960 °C. The dissolution of these materials into the cryolitic electrolyte causes contamination of the Al produced.

US Patent 6 039 862 (March 21 , 2000) presented a method to produce aluminium through modification/adaptation of H-H cells with a non-consumable anode of the type used for solid oxide fuel cells. The anode is in a hollow form and is made of zirconia-based solid electrolyte. This anode material allows oxygen-ions from the cryolitic electrolyte to migrate through the wall of the anode to the hollow inner space where the oxygen-ions react with a fuel. It should be noted that no gas bubbles appear at the anode - electrolyte interface and the cryolitic electrolyte is sep- arated from the fuel by the anode wall.

The above process (US 6 039 862) could be implemented at a significantly lower temperature. However, at normal H-H cell operating temperature of about 960 °C, the solubility of zirconia in the cryolitic electrolyte is quite high (0.4 to 8 wt%). Also, the resistance to thermal shock and chemical attack by metallic Al is usually poor. Failure or corrosion of the zirconia-based materials would certainly result in contamination of the aluminium produced. In addition, the mechanical connection between the anode body and the metal stub would be difficult to survive in a harsh environment of thermal shock and chemical corrosion encountered in the H-H cells. In the absence of bubbles formation at the anode - electrolyte interface, the convection and diffusion in the electrolyte melts may become inadequate to transport the dissolved alumina to the anode region. It is also a question if this system at lower temperature is capable of transporting sufficient current to maintain a high productivity, as a high migration rate of the oxygen-ions requires high temperature.

So far, the mentioned non-consumable anodes have not been commercialised. It is still a great challenge, both technically and economically, to implement the concept of the non-consumable anode in retrofitting the current H-H cells.

DE 3721311 A1 presents a method for feeding alumina to the electrolyte cell. A hole is drilled in the anode and a tube for pneumatic transport of feed materials is inserted. The gas used for pneumatic transportation of the solid materials to the electrolyte under the anode may be a reducing gas. As the purpose of the tube is to add alumina, a tube of significant dimensions is needed, probably above 5 cm diameter to avoid clogging. Using reducing gas for the pneumatic transportation of alumina may facilitate a reduced consumption of the carbon anode, however, the distribution over the anode is not controlled.

SUMMARY OF THE INVENTION

The present invention is conceived to solve or at least alleviate the problems with the prior art as outlined above.

In order to reach the objective a gas anode system has been created not only to carry electrical current, but also to convey and distribute reducing gas, for instance, natural gas to the anode - electrolyte interface. The reducing gas is distributed to the anode - electrolyte interface through connecting pores or channels in the anode body, facilitating an anodic reaction involving the reducing gas at the anode-electrolyte interface. In a preferred embodiment of the invention the anode system comprises a combined gas supply and current conductor supplying both current and the reducing gas to the anode body.

For high temperature applications, such as aluminium electrolysis using cryolitic electrolyte, the anode is preferentially made of carbon or graphite based materials with connecting pores or channels for gas distribution. The connecting pores, which are required for gas distribution, may be created during baking of the anode by deliberately varying the particle size in combination with proper binder composi- tions. The gas distribution system may also be inserted in the green composite material and baked with the material to solid anodes. This is especially feasible using synthetic binders, which requires lower baking temperatures than the pitch- based binders, although a steel distribution system is feasible with the present pitch-based binders.

The carbon anode itself may not be involved in the anodic reaction during aluminium electrolysis. The manner of the distribution of the reducing gas according to this invention enables protection of the carbon anode from the anodic oxidation. Other materials, e.g. ceramics and carbon - metal oxide composites can also be used in the anode in accordance with an embodiment of the present invention, although a sufficient current conductivity must be maintained.

The gas anode system and method according to the invention renders possible an environment-friendly aluminium reduction process with material saving of merit. A relative dimension-stable anode due to the reduced material consumption permits to lower the anode-cathode distance for further energy savings.

The method and gas anode system in accordance with the present invention do not require lower operating temperature for use in H-H cells. No major changes in operation condition are needed in adapting this method and the proposed gas anode system in the H-H cells. This would offer a lower cost and more feasible solution for aluminium producer to retrofit the existing H-H cells. The method and the gas anode system also allow various new designs of the electrolysis cells for the aluminium production.

In general, the reducing gas may not be limited only to natural gas. The invention can also extend to other metals reduction processes where said reducing gas is involved. In broadest terms, the present invention as a whole or any part of it (such as holes or channels of the anode) is not limited to size, geometry and thickness, or to the overall scale of the gas anode system and of any application of the method disclosed here. The anode is also applicable to other electrolyte systems where oxygen takes part in the anode reaction. In such systems other conducting materials than carbon may be more feasible, e.g. due to catalytic effects. The invention is defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be explained by referring to the drawings, where:

FIGURE 1 is a schematic drawing of an aluminium electrolysis cell using the method and the apparatus of a gas anode system in accordance with an embodiment of the invention;

FIGURE 2 is a schematic showing of an alternative embodiment of the anode system in accordance with the present invention;

FIGURE 3 is a schematic illustration of a multiple anode system as an embodi- ment of the method and the design concept in accordance with the present invention;

FIGURE 4 is a schematic showing of a multiple bipolar anode system using the method and design concept in accordance with the present invention; and

FIGURE 5 is a schematic view of an aluminium reduction cell with drained cathode and inclined anodes constructed according to the method and design concept of the present invention. DETAILED DISCRETION OF THE INVENTION

The invention is based on the recognition of the following reaction that can occur if reducing gas, for instance, natural gas or reformed natural gas, is used to replace the carbon as reductant. For a general metal production process this is expressed in the following equation:

Me_xO_y + y/4 CH₄ (g) = x Me (l,s) + y/4 C0₂ (g) + y/2 H₂0 (aq.g) (3)

or in the case of aluminium:

Al₂0₃ + 3/4 CH₄ (g) = 2 Al (I) + 3/4 C0₂ (g) + 3/2 H₂0 (g) (3)

FIGURE: 1 schematically illustrates a modified Hall-Heroult cell using a gas anode in accordance with an embodiment of the present invention. The cell includes a gas anode system introducing the reducing gas 1 via a hollow tube. In the embodiment shown in FIGURE 1, the hollow tube carries both electrical current and reducing gas to the anode - electrolyte interface. This gas/current conductor 2 is connected to an anode 6 containing graphite or carbon materials with connecting pores or channels. The anode 6 is immersed in an electrolyte 7 that is contained in a vessel, in this case, of carbon 4 with a fused-alumina lining 5. The gas products from the anode reaction escape through an exit tube 3. A metal pad 8 rests on the bottom of the crucible that serves as a cathode 9.

The apparatus of the gas anode system in accordance with the present invention of using reducing gas to produce aluminium, as shown in FIGURE 1 , comprises

1 ) a carbon - or graphite - containing anode with connecting pores and channels,

2) a hollow tube with a function of connecting to the anode body and 3) reducing gas and current. The configuration may, however, not be restricted to FIGURE 1 , as its basic function of the invention is to carry both electrical current and reducing gas to the anode - electrolyte interface.

The gas/current conductor in FIGURE 1 may be made of stainless steel or other materials like Al, Cu, Ni, etc. As the current conductor also is used to carry the gas to the anode, the upstream opening of the current conductor is connected to a gas supply facility, and its downstream opening is inserted into the anode body. The insertion of the downstream opening is made in a manner that permits the reducing gas to be transported to the anode-electrolyte interface area that is immersed in the liquid electrolyte, rather than to the area that is above the electrolyte level.

The same reference numbers denotes the same parts throughout the different embodiments in the following description.

The current conductor and the gas supply may be two separate systems, i.e., a standard current conductor according to present practice and a separate gas sup- ply system for the anode. Different embodiments of such anode systems having separate current and gas supply systems are shown in FIGURES 2, 4 and 5.

FIGURE 2 shows an alternative embodiment of the anode system in accordance with the present invention. The major difference to the anode system shown in FIGURE 1 is that the electricity is supplied through an anode rod 10 and the reducing gas through separate pipes 11. The reducing gas is distributed through pores or multiple-channels 12, and then reaches the interface between the anode 6 and the electrolyte 7.

FIGURE 3 discloses a multiple-anode system in electrolysis cells according to an embodiment of the present invention using a combined gas/current conductor. The anodes 13 and the cathodes 14 are placed vertically parallel to each other in the cell. The reducing gas runs through a distributing pipe 15 into each anode having pores or channels supplying the gas to the anode - electrode interface. The cat- hode 14 can be made of conventional carbon / graphite materials, or preferably materials wettable to the metal produced e.g. TiB₂ -based materials. 8. The electrical current goes through the anodes and cathodes via the anode distributing pipe 15 and the cathode collector bar 16, both made of conductive metals or materials. The anodes and the cathodes are electrically connected in parallel. The side- wall 17 and the bottom of the cell are fabricated in any material with good resistance to high temperature and corrosion. The cell linings 18 can be made of any material resistant to the corrosion of the electrolyte 7. A cover 19 is used to prevent emission from the cell. In FIGURE 4, a multiple bipolar electrode system according to another embodiment of the present invention is schematically disclosed. The bipolar electrode consists of an anode part 20 and a cathode part 21. All bipolar electrodes are vertically placed parallel to each other in the cell. The reducing gas runs through a distributing pipe 22 into each anode part having pores or channels distributing the reducing gas to the anode - liquid electrolyte interface. The electricity is distributed via an anode conductor 23 and a cathode conductor 24. All electrodes are electrically connected in series and the current goes from the end of a single anode 13 to the end of a single cathode 14 through all the bipolar electrodes immersed in the electrolyte 7.

FIGURE 5 schematically illustrates a multiple electrode system as another alternative embodiment according to the present invention. The electricity through anode rods 10 and the reducing gas through pipes 11 are separately supplied to the ano- des 6. The reducing gas runs through a collector pipe 22 into each anode with pores or channels supplying the gas to the anode - liquid electrolyte interface. The anode surface and cathode surface may be adjusted to a certain angle of inclination (0 < angle < 90°). The inclined cathode is made of the materials wettable to the metal produced, for instance, TiB₂ -based materials. The metal 8 is collected in a bottom pool 25. The inclined arrangement facilitates the draining of the metal produced and the escape of the gas generated.

The reaction rate for the aluminium reduction process in equation (3) using the method in accordance with the present invention is controlled by a combination of regulating the gas-flowing rate and the current density on the anode system.

As used herein, the term of "reducing gas" may not be limited to the natural gas or CH₄, as other type of reducing gases, for instance, H₂, C₂H₆, etc. could function well using the method and in the gas anode system in accordance with the present invention.

Unlike the method making the reaction of the fuel gas with the migrated oxygen- ions to take place into the inner hollow of an anode body as proposed in US 6,039,862, the method and the gas anode system in accordance with the pre- sent invention carry the reducing gas to the anode - electrolyte interface. The reducing gas, for instance, CH₄, participates in the anodic reaction at the anode - electrolyte interface, rather than in an inner hollow space of the anode.

In this case, the reducing gas has direct contact with the electrolyte and the anode surface. The apparatus and the method of the gas anode system enable formation of a continuous, fine bobble layer of the reducing gas covering the anode surface. The reducing gas layer reduces or prevents the carbon oxidation reaction of Equation (1) and makes the reaction of Equation (3) prevailing.

The gas anode system according to the invention renders possible an environment-friendly aluminium reduction process. By comparing Equation (1 ) and Equation (3), it indicates that a 50% reduction in C0₂ emission for producing the same amount aluminium could be realised using the CH₄ gas. The carbon anode is not involved in the anodic reaction according to the Equation (3), thus, at least theoretically, being not consumable during aluminium electrolysis. Application of the invention will make the anode dimension-stable due to the nature of non-involvement in the anodic reaction, compared with the current use of the carbon anode in H-H process.

This will provide all the advantages of a dimension-stable anode: constant anode- to-cathode distance, easy and inexpensive construction of the anode system, lower operation and maintenance costs. This will significantly reduce the aluminium production cost and improve the cell operation efficiency.

The occurrence frequency of the anode effect is also expected to be reduced, as no surface oxygen-carbon complex compounds (a direct cause to the anode effect) are generated from the anodic oxidation of the carbon anode as they are in the existing H-H cells. This would allow a higher aluminium production rate with no disturbances of sudden cell voltage increase caused by the anode effect.

The decomposition voltage of the aluminium reduction is about 1.1 V at 960 °C due to the depolarisation effect of the reacting gas. This means that the anode system that has been described above can not only offer the advantages as dimension-stable anodes, but also have a desirable lower decomposition voltage, in contrast to the inert anode.

Moreover, because the decomposition voltage (1.1 V) is similar to the current value (1.2 V), the heat balance (controlled by the voltage components) of the H-H cell can still be maintained almost the same. This will greatly reduce the work and cost in design of new cells and retrofit of the existing cells by implementing this invention.

In the embodiments of the present invention, there is no requirement of lower operating temperature for use in H-H cells. No major changes in operation condition are needed in adapting this method and the gas anode system in the H-H cells. This would offer a lower cost and higher feasibility solution for aluminium producer to retrofit the existing H-H cells.

In case that any failures in gas supply occur or the channels (or pores) become blocked, the anode in this invention can still work as a traditional carbon anode (just become consumable again). No contamination from the failures or corrosion of the anode could happen - an advantage in contrast to the well known ceramic inert anodes.

Example

The electrolysis cell was arranged as shown in FIGURE1. A graphite anode with fine connecting pores and channels was used to distribute CH gas to the anode surface. The electrolyte had a composition of NaF/AIF₃ radio = 2.1 , 5% CaF₂ and 5% Al₂0₃. The cell was operated at 960 °C.

The decomposition voltage (thermodynamic decomposition voltage + polarising over-potentials) was 1.60 V for the anode without the reducing gas, and 1.45 V for the anode with the gas. Under identical anode material and testing conditions, the anode without the reducing gas shows a significant consumption, while the anode with the reducing gas through the method and the apparatus in accordance with the present invention shows only minor consumption.

The cell structure, anode material and structures, operating conditions, etc., as mentioned above, is intended to demonstrate some examples of embodiments of the present invention, and should not be viewed as any limitations for the scope and the extension of the embodiment of the invention.

Other examples of embodiments are vertically or tilted electrode arrangements, which may be used as single electrode cells or in bipolar cells. Such embodiments are shown in Figures 2-5. In bipolar cells the anode side of the electrode is manufactured with pores or channels for gas distribution while the cathode side is manufactured without pores or channels. The anode and cathode side may be of similar material composition or the cathode side may be a totally different material from the anode side, e.g. steel, cermets or other materials with cathodic resistance to the produced metal or applied electrolyte.

Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.

Claims

C L A I M S

1. Anode system for use in an electrolysis cell using a reducing gas in the anodic reduction process at an anode - electrolyte interface, the anode system com- prising an anode body (6) for immersion into the electrolyte (7), a reducing gas supply (2, 11 , 15) through the anode body to the anode - electrolyte interface, a current conductor (2, 10, 15, 23) for supplying current to the anode body (6) immersed in the electrolyte, c h a r a c t e r i z e d i n t h a t the anode body (6) having pores or channels for distributing the reducing gas to the anode-electrolyte interface.

2. Anode according to claim 1 , characterized in a combined gas supply and current conductor (2, 15), the anode gas/current conductor (2, 15) supplying both current and the reducing gas to the anode body (6).

3. Anode according to claim 2, characterised in that the anode gas/current conductor (2, 15) is made of a solid material and having a downstream opening inserted into the anode body in a manner only permitting transport of the reducing gas to the anode-electrolyte interface area.

4. Anode according to claims 2 or 3, characterized in that the anode gas/current conductor (2) is a hollow tube.

5. Anode according to claim 1 , characterized in that the anode body (6) com- prises carbon or graphite.

6. Anode according to claims 1 or 5, characterised in that the anode body (6) comprises composite carbon materials, materials containing carbon/graphite, and/or oxide materials.

7. Anode according to claim 1 , 4 or 5, characterised in that the anode body (6) comprises composite carbon materials, material containing carbon/graphite, and/or oxide materials providing the connecting pores or channels for gas transportation.

8. Anode according to claim 1 , comprising an exit tube (3) for the gas products from the reduction reaction at the anode.

9. Anode according to one of claims 1 , wherein the electrolyte is a cryolitic electrolyte containing alumina and the produced metal is aluminium.

10. Anode according to claim 1 , characterized in that the electrolysis cell is a modified Hall-Herouit cell.

11. Anode according to claim 9, characterized in that the cryolitic electrolyte is contained in a carbon vessel (4) with an alumina lining (5).

12. Anode according to claim 10, characterized in an aluminium metal pad (9) on the bottom of the vessel serving as a cathode.

13. Anode according to claim 1 , characterized in that the electrolysis cell is a bipolar cell or multiple electrodes cell having vertical, horizontal or inclined anode arrangements and with gas distributing system and cell structure parts of various refractory or ceramic materials.

14. Method for production of a metal by a reduction process at an anode - electrolyte interface in an electrolysis cell, the method comprising supplying a reducing gas to an anode-electrolyte interface by using an anode having connecting pores or channels.

15. Method according to claim 14, comprising supplying both current and reducing gas to the anode - electrolyte interface through the anode.

16. Method according to claim 14, comprising using natural gas as the reducing gas.

17. Method according to claim 14, comprising controlling the reaction rate in the reduction process by regulating the reducing gas flow rate and the current density in combination.

18. Method according to claim 13, comprising using various designs of modified H-H cells, bipolar cell, multiple electrodes cell, and vertical, horizontal, or inclined anode arrangements with gas distributing system and cell structure parts of various refractory or ceramic materials.