Abstract:
A non-carbon, metal-based slow-consumable anode of a cell for the electrowinning of aluminium self-forms during normal electrolysis an electrochemically-active oxide-based surface layer ( 20 ). The rate of formation ( 35 ) of the layer ( 20 ) is substantially equal to its rate of dissolution ( 30 ) at the surface layer/electrolyte interface ( 25 ) thereby maintaining its thickness substantially constant, forming a limited barrier controlling the oxidation rate ( 35 ). The anode ( 10 ) usually comprises an alloy of iron with at least one of nickel, copper, cobalt or zinc which during use forms an oxide surface layer ( 20 ) mainly containing ferrite.

Description:
This application is a continuation of co-pending international application designating the USA, PCT/IB99/01358, filed on Jul. 30, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to non-carbon, metal-based, slow consumable anodes for use in cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte, and to methods for their fabrication and reconditioning, 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 Héroult, 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 CO 2  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 ⅓ 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. 
     U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes 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 cerium to the molten cryolite electrolyte. This made it possible to have a protection of the surface only from the electrolyte attack and to a certain extent from the gaseous oxygen but not from the 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, U.S. Pat. Nos. 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 oxygen 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. As mentioned hereabove, many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide a non-carbon, metal-based anode for the electrowinning of aluminium so as to eliminate carbon-generated pollution and reduce the frequency of anode replacement, such an anode having an outside layer well resistant to chemical electrolyte attack whose surface is electrochemically active for the oxidation of oxygen ions contained in the electrolyte and for the formation of gaseous oxygen. 
     A further object of the invention is to provide a metal-based anode capable of generating during normal electrolysis at its surface an electrochemically active oxide layer which slowly and progressively dissolves into the electrolyte. 
     A major object of the invention is to provide an anode for the electrowinning of aluminium which has no carbon so as to eliminate carbon-generated pollution and reduce the high cell voltage. 
     SUMMARY OF THE INVENTION 
     The invention relates to a non-carbon, metal-based slow-consumable anode of a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-based electrolyte. The anode self-forms during normal electrolysis an electrochemically-active oxide-based surface layer, the rate of formation of said layer being substantially equal to its rate of dissolution at the surface layer/electrolyte interface thereby maintaining its thickness substantially constant forming a limited barrier controlling the oxidation rate. 
     In this context, metal-based anode means that the anode contains at least one metal as such or as an alloy, intermetallic and/or cermet. 
     During normal operation, the anode thus comprises a metallic (un-oxidised) anode body (or layer) on which and from which the oxide-based surface layer is formed. 
     The electrochemically active oxide-based surface layer may contain an oxide as such, or in a multi-compound mixed oxide and/or in a solid solution of oxides. The oxide may be in the form of a simple, double and/or multiple oxide, and/or in the form of a stoichiometric or non-stoichiometric oxide. 
     The oxide-based surface layer has several functions. Besides protecting in some measure the metallic anode body against chemical attack in the cell environment and its electrochemical function for the conversion of oxygen ions to molecular oxygen, the oxide-based surface layer controls the diffusion of oxygen which oxidises the anode body to further form the surface layer. 
     When the oxide-based surface layer is too thin, in particular at the start-up of electrolysis, the diffusion of oxygen towards the metallic body is such as to oxidise the metallic anode body at the surface layer/anode body interface with formation of the oxide-based surface layer at a faster rate than the dissolution rate of the surface layer into the electrolyte, allowing the thickness of the oxide-based surface layer to increase. The thicker the oxide-based surface layer becomes, the more difficult it becomes for oxygen to reach the metallic anode body for its oxidation and therefore the rate of formation of the oxide-based surface layer decreases with the increasing thickness of the surface layer. Once the rate of formation of the oxide-based surface layer has met its rate of dissolution into the electrolyte an equilibrium is reached at which the thickness of the surface layer remains substantially constant and during which the metallic anode body is oxidised at a rate which substantially corresponds to the rate of dissolution of the oxide-based surface layer into the electrolyte. 
     In contrast to carbon anodes, in particular pre-baked carbon anodes, the consumption of the non-carbon, metal-based anodes according to the invention is at a very slow rate. Therefore, these slow consumable anodes in drained cell configurations do not need to be regularly repositioned in respect of their facing cathodes since the anode-cathode gap does not substantially change. 
     To practically realise the invention, the anode body can comprise an iron alloy which when oxidised will form an oxide-based surface layer containing iron oxide, such as hematite or a mixed ferrite-hematite, some of which adheres to the iron alloy, providing a good electrical conductivity and electrochemical activity, and a low dissolution rate in the electrolyte. 
     Optionally, the anode body may also comprise one or more additives selected from beryllium, magnesium, yttrium, titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhodium, silver, aluminium, silicon, tin, hafnium, lithium, cerium and other Lanthanides. 
     Suitable kinds of anode materials which may be used for forming the oxide-based surface layer comprise high-strength low-alloy (HSLA) steels. 
     It has been observed that low-carbon HSLA steels such as Cor-Ten™, 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 known 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 environment promoting dissolution or delamination of the surface layer, in particular in an aluminium electrowinning cell, 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. 
     High-strength low-alloy (HSLA) steels are a group of low-carbon steels (typically up to 0.5 weight % 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 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 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. Alternatively, the oxidation treatment may take place in air or in oxygen for 5 to 25 hours at 750 to 1150° C. 
     In order to prevent thermal shocks causing mechanical stresses, a high-strength low-alloy steel body may be tempered or annealed after pre-oxidation. Alternatively, the high-strength low-alloy steel body 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 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. 
     Advantageously, the anode comprises cerium which is oxidised to ceria in the formation of the oxide-based surface layer to provide on the surface of the oxide-based surface layer a nucleating agent for in-situ formation of an electrolyte-generated protective layer. Such electrolyte-generated protective layer usually comprises cerium oxyfluoride when cerium ions are contained in the electrolyte and may be obtained by following the teachings of U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) which describes a protective anode coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, and maintained by the addition of small amounts of cerium to the molten electrolyte. 
     The oxide-based surface layer may alternatively comprise ceramic oxides containing combinations of divalent nickel, cobalt, magnesium, manganese, copper and zinc with divalent/trivalent nickel, cobalt, manganese and/or iron. The ceramic oxides can be in the form of perovskites or non-stoichiometric and/or partially substituted or doped spinels, the doped spinels further comprising dopants selected from the group consisting of Ti 4+ , Zr 4+ , Sn 4+ , Fe 4+ , Hf 4+ , Mn 4+ , Fe 3+ , Ni 3+ , Co 3+ , Mn 3+ , Al 3+ , Cr 3+ , Fe 2+ , Ni 2+ , Co 2+ , Mg 2+ , Mn 2+ , Cu 2+ , Zn 2+  and Li + . 
     The anode can also comprise a metallic anode body or layer which progressively forms the oxide-based surface layer on an inert, inner core made of a different electronically conductive material, such as metals, alloys, intermetallics, cermets and conductive ceramics. 
     In particular, the inner core may comprise at least one metal selected from copper, chromium, nickel, cobalt, iron, aluminium, hafnium, molybdenum, niobium, silicon, tantalum, tungsten, vanadium, yttrium and zirconium, and combinations and compounds thereof. For instance, the core may consist of an alloy comprising 10 to 30 weight % of chromium, 55 to 90 weight % of at least one of nickel, cobalt and/or iron and up to 15 weight % of at least one of aluminium, hafnium, molybdenum, niobium, silicon, tantalum, tungsten, vanadium, yttrium and zirconium. 
     Resistance to oxygen may be at least partly achieved by forming an oxygen barrier layer on the surface of the inner core by surface oxidation or application of a precursor layer and heat treatment. Known barriers to oxygen are chromium oxide, niobium oxide and nickel oxide. 
     Advantageously, the inner core is covered with an oxygen barrier layer which is in turn covered with at least one protective layer consisting of copper, or copper and at least one of nickel and cobalt, and/or oxide(s) thereof to protect the oxygen barrier layer by inhibiting its dissolution into the electrolyte. 
     The invention also relates to a method of producing such anodes. The method comprises immersing an anode with an oxide-free or pre-oxidised surface into a molten fluoride-containing electrolyte and self-forming or growing an electrochemically active oxide-based surface layer as described hereabove. 
     An anode according to the invention can be restored when the metallic anode body or layer is worn and/or damaged. The method for restoring the anode comprises clearing and cleaning at least the worn and/or damaged parts of the anode; reconstituting the anode and optionally pre-oxidising the surface of the anode; immersing it into a molten fluoride-containing electrolyte; and self-forming or growing an electrochemically active oxide-based surface layer as described above. 
     A further aspect of the invention is a cell and a method for the electrowinning of aluminium comprising at least one anode which during normal electrolysis is oxidised, self-forming the electrochemically active oxide-based surface layer as described above. 
     Preferably, the cell comprises an aluminium-wettable cathode. Even more preferably, the cell is in a drained configuration by having a drained cathode on which aluminium is produced and from which aluminium continuously drains, as described in U.S. Pat. No. 5,651,874 (de Nora/Sekhar) and U.S. Pat. No. 5,683,559 (de Nora). 
     The cell may be of monopolar, multi-monopolar or bipolar configuration. A bipolar cell may comprise the anodes as described above as a terminal anode or as the anode part of a bipolar electrode. 
     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 the electrolyte. Such means can for instance be provided by the geometry of the cell as described in co-pending application PCT/IB99/00222 (de Nora/Duruz) or by periodically moving the anodes as described in co-pending application PCT/IB99/00223 (Duruz/Bellò). 
     The cell may be operated with the electrolyte at conventional temperatures, such as 950 to 970° C., or at reduced temperatures as low as 700° C. 
     The invention also relates to a method of producing aluminium in a cell for the electrowinning of aluminium. The method comprises immersing a metallic anode having an oxide-free or a pre-oxidised surface into a molten fluoride-containing electrolyte, self-forming an electrochemically active oxide-based surface layer as described hereabove, and then electrolysing the dissolved alumina to produce aluminium in the same or a different fluoride-based electrolyte. 
     The surface of the anode may be in-situ or exsitu pre-oxidised, for instance in air or in another oxidising atmosphere or media, or it may be oxidised in a first electrolytic cell and then transferred into an aluminium production cell. 
     Another aspect of the invention is an anode comprising an oxide-free or a pre-oxidised surface which when (further) oxidised during cell operation as described above gives origin to the above described self-formed, electrochemically active oxide-based surface layer. 
     When the anode has a pre-oxidised surface layer which is thicker than its thickness during steady operation, the rate of formation of the oxide-based surface layer is initially less than its rate of dissolution but increases to reach it. Conversely, when the anode has an oxide-free surface or a pre-oxidised surface forming an oxide-based layer which is thinner than its thickness during steady operation, the rate of formation of the oxide-based surface layer is initially greater than its rate of dissolution but decreases to reach it. 
     The pre-oxidised surface layer may be of such a thickness that after immersion into the electrolyte and during electrolysis the thick oxide-based surface layer prevents the penetration of nascent monoatomic oxygen beyond the oxide-based surface layer. Therefore the mechanism for forming new oxide by further oxidation of the anode is delayed until the existing pre-oxidised surface layer has been sufficiently dissolved into the electrolyte at the surface layer/electrolyte interface, no longer forming a barrier to nascent oxygen. 
     Anodes made according to the invention when worn can be replaced during normal use of a cell with new anodes or restored anodes. 
     A further aspect of the invention is a method for preparing an anode and using it for producing aluminium in a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte, the method comprising preparing an anode as described above, and then utilising the anode to electrolyse dissolved alumina in a molten electrolyte contained in an aluminium electrowinning cell to produce aluminium by passing an ionic current between the anode and a facing cathode of the cell. 
     The anode may be pre-oxidised in-situ, or in a different electrolytic cell and then transferred to an aluminium production cell. Alternatively, the anode may be pre-oxidised in an oxygen containing atmosphere, such as air. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is made to the drawings wherein: 
     FIGS.  1 ( a ),  1 ( b ) and  1 ( c ) are schematic representations of the evolution in time of an anode according to the invention with a self-formed oxide-based surface layer; 
     FIGS.  2 ( a ) and  2 ( b ) are schematic representations of the evolution in time of an anode similar to the anode shown in FIGS.  1 ( a ),  1 ( b ) and  1 ( c ) which further comprises an inner metal core. 
    
    
     DETAILED DESCRIPTION 
     FIGS.  1 ( a ),  1 ( b ) and  1 ( c ) show an anode comprising a metallic (un-oxidised) anode body  10  which is slowly consumed as a self-formed electrochemically active oxide-based surface layer  20  progresses according to the invention when the anode is anodically polarised in an electrolytic bath  40 , such as a fluoride-based electrolyte  40  at about 950° C. containing 1 to 10% dissolved alumina in a cell for the electrowinning of aluminium. The anode for example comprises an alloy of iron with nickel, copper and/or cobalt which forms an oxide-based surface layer  20  containing ferrites. 
     FIG.  1 ( a ) shows part of a pre-oxidised anode according to the invention shortly after its immersion into the electrolyte  40 . In FIG.  1 ( a ) the anode is in a transitional period during which the pre-oxidised surface layer  20 ′ is grown from the metallic anode body  10  at the surface layer/anode body interface  15  at a faster rate than its dissolution  30  into the electrolyte  40  at the surface layer/electrolyte interface  25 , thereby progressively increasing its thickness The dashed line  25 ′ shows the initial position of the surface layer/electrolyte interface  25  at or shortly after immersion of the anode into the electrolyte  40 . 
     FIGS.  1 ( b ) and  1 ( c ) illustrate the situation where the anode has reached its steady state of operation. The oxide-based surface layer  20  has grown from its original thickness shown in FIG.  1 ( a ) to its equilibrium thickness as shown in FIGS.  1 ( b ) and  1 ( c ). The rate of dissolution  30  of the surface layer  20  into the electrolyte  40  at the surface layer/electrolyte interface  25  is substantially equal to its rate of formation  35  at the surface layer/anode body interface  15 , consuming the metallic anode body  10  at an equivalent rate. Furthermore, the surface layer/electrolyte interface  25  slowly withdraws from its initial position  25 ′ while the oxide-based surface layer  20  is dissolved into the electrolyte  40 . 
     FIGS.  2 ( a ) and  2 ( b ) show an anode comprising an electronically conductive and oxidation resistant inner core  5 , for instance nickel-based, supporting a metallic anode layer  10 ′ having an electrochemically active oxide-based surface layer  20  as described previously. 
     FIG.  2 ( a ) illustrates the oxide-based surface layer  20  grown from the metallic anode layer  10 ′ at the surface layer/anode layer interface  15 . The formation rate  35  of the surface layer is equal to its dissolution rate  30  into the electrolyte  40  as illustrated in FIGS.  1 ( b ) and  1 ( c ). 
     In FIG.  2 ( b ), the oxide-based surface layer  20  has progressed until the metallic anode layer  10 ′ covering the inner core  5  has been nearly completely consumed. Since the inner core  5  is resistant to oxidation, further dissolution  30  of the oxide-based surface layer is not replaced by oxidation of the inner core once the metallic anode layer  10 ′ has worn away. The remaining surface layer  20  will slowly dissolve into the electrolyte  40  at the surface layer/electrolyte interface  25  and its thickness slowly decreases. 
     An anode having an oxidisable metallic anode layer  10 ′ covering an inner core  5  may still remain in the electrolyte  40  after its metallic anode layer  10 ′ is completely consumed, provided the inner core  5  is not fully passivated when exposed to oxygen, until the oxide-based surface layer  20  is too thin to allow the conversion of ionic oxygen to molecular oxygen. When this conversion is no longer possible the anode needs to be extracted and replaced or restored. However, the anode can be removed earlier if desired. 
     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-Ten™ type low-carbon high-strength (HSLA) steel doped with niobium, titanium, chromium and copper in a total amount of less than 4 weight %, which is commercially available from US-Steel. The anode was pre-oxidised in air at about 1050° C. for 15 hours to form a dense hematite-based outer layer constituting an oxide-based surface layer on an un-oxidised anode body. 
     The anode was then tested in a fluoride-containing molten electrolyte at 850° C. containing cryolite and 15 weight % excess of AlF 3  and approximately 3 weight % alumina at a current density of about 0.7 A/cm 2 . 
     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 to slow down the 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. 
     As variations, other HSLA steel may be used as anodes, such as a HSLA steel doped with manganese 0.4 weight %, niobium 0.02 weight %, molybdenum 0.02 weight %, copper 0.3 weight %, nickel 0.45 weight % and chromium 0.8 weight %, or a HSLA steel doped with nickel, copper and silicon in a total amount of less than 1.5 weight %. 
     EXAMPLE 2 
     A non-carbon metal-based anode according to the invention was obtained from a 15×15×80 mm sample of a nickel-iron based alloy. The sample was made of cast alloy consisting of 79 weight % nickel, 10 weight % iron and 11 weight % copper. 
     The sample was pre-oxidised in air at about 1100° C. for 5 hours in a furnace to form the anode with a pre-oxidised surface layer. 
     After pre-oxidation, the anode was immersed in molten cryolite contained in a laboratory scale cell. The molten cryolite contained approximately 6 weight % of dissolved alumina. Current was passed through the anode sample at a current density of 0.5 A/cm 2 . After 100 hours, the anode was extracted from the cell for analysis. 
     The anode was crack-free and its dimensions remained substantially unchanged. On the surface of the anode a well adherent oxide surface layer of a thickness of about 0.6 mm had grown providing an adequate protection. 
     EXAMPLE 3 
     This Example illustrates the wear rate of the nickel-iron containing anode of Example 2 and is based upon observations made on dissolution of nickel-based samples in a fluoride-based electrolyte. 
     An estimation of the wear rate is based on the following parameters and assumptions: 
     With a current density of 0.7 A/cm 2  and a current efficiency of 94% an aluminium electrowinning cell produces daily 53.7 kg aluminium per square meter of active cathode surface. 
     Assuming a contamination of the produced aluminium by 200 ppm of nickel, which corresponds to the experimentally measured quantities in typical tests, the wear rate of a nickel-iron sample corresponds to approximately 1.2 micron/day. Therefore, it will theoretically take about 80 to 85 days to wear 0.1 mm of the anode.