This invention relates to inhibiting dissolution of an oxygen-evolving anode of a cell for the production of aluminium from alumina dissolved in an sodium ion-containing molten electrolyte.
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950xc2x0 C. is more than one hundred years old. This process, conceived almost simultaneously by Hall and Hxc3xa9roult, 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 CO2 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), U.S. Pat. No. 4,680,094 (Duruz), U.S. Pat. No. 4,683,037 (Duruz) and U.S. Pat. No. 4,966,674 (Bannochie/Sherriff) describe non-carbon 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 a cerium compound to the molten cryolite electrolyte. This made it possible to have a protection of the surface from the electrolyte attack.
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.
WO00/06802 (Duruz/de Nora/Crottaz) discloses a method of keeping an anode with a transition metal oxide layer dimensionally stable during operation in an aluminium electrowinning cell by maintaining in the electrolyte a sufficient concentration of transition metal species and dissolved alumina.
U.S. Pat. No. 6,248,227 (de Nora/Duruz) discloses an aluminium electrowinning anode having a metallic anode body which can be made of various alloys. During use, the surface of the anode body is oxidised by anodically evolved oxygen to form an integral electrochemically active oxide-based surface layer, the oxidation rate of the anode body being equal to the rate of dissolution of the surface layer into the electrolyte. This oxidation rate is controlled by the thickness and permeability of the surface layer which limits the diffusion of anodically evolved oxygen therethrough to the anode body.
WO00/06803 (Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz), WO01/42534 (de Nora/Duruz), WO01/42536 (Duruz/Nguyen/de Nora) disclose further developments of metal-based aluminium production anodes.
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 for commercial aluminium production because their lifetime is limited.
An object of the invention is to provide a method of increasing the lifetime of transition metal-containing alloy anodes during operation in an aluminium electrowinning cell, in particular anodes made of a homogeneous metal alloy, such as a cast alloy or possibly an electroformed alloy.
The invention relates to a method of inhibiting dissolution of an oxygen-evolving anode of a cell for the production of aluminium from alumina dissolved in an sodium ion-containing molten electrolyte comprising a cathodic material that is predominately active for the reduction of sodium ions rather than aluminium ions. The oxygen-evolving anode comprises a transition metal-containing alloy having an integral oxide layer containing predominantly one or more transition metal oxides which slowly dissolve in the electrolyte and are compensated by oxidation of the alloy at the alloy/oxide layer interface.
According to the invention, the method comprises providing a sodium-inert layer on the sodium-active cathodic material and electrolysing the dissolved alumina whereby oxygen is anodically evolved and aluminium ions rather than sodium ions are cathodically reduced on the sodium-inert layer to inhibit the presence in the molten electrolyte of soluble cathodically-produced sodium metal that constitutes an agent for chemically reducing the transition metal oxides and evolved oxygen, in particular molecular oxygen. The sodium-inert layer is used as a dissolution inhibitor of the anode by its effect in inhibiting reduction of the transition metal oxides by sodium metal and in maintaining the evolved oxygen at the anode at a concentration such as to produce at the alloy/oxide layer interface stable and coherent transition metal oxides having a high level of oxidation.
The present invention is based on two different observations about the operation of a cell utilising transition metal-alloy anodes.
The first observation relates to the quality of the anode""s integral oxide layer which slowly dissolves in the electrolyte and is compensated by oxidation of the alloy at the alloy/oxide layer interface.
A high concentration of oxygen, in particular molecular oxygen, at the anode surface permits the formation of transition metal oxides having a high level of oxidation. It has been observed that such metal oxides have a greater stability in the electrolyte and thus a lower dissolution rate than metal oxides of lower oxidation level. In addition, metal oxides having a high level of oxidation have a greater coherence and form integral anode oxide layers with a greater imperviousness to electrolyte and oxygen diffusion which also reduces the oxidation rate of the alloy and inhibits corrosion.
Thus a high concentration of oxygen, in particular molecular oxygen, at the surface of a transition metal-alloy anode with an integral oxide layer surprisingly maintains the anode whereas a low concentration of oxygen leads to faster oxidation and corrosion of the anode.
The second observation relates to the wear-rate of a transition metal alloy-based anode operated in an aluminium production cell which has surprisingly been found to be significantly higher when the cell is operated with a cathodically polarised carbon material which is directly exposed to the molten electrolyte than when the carbon material is shielded from the electrolyte by a sodium-inert layer, such as molten aluminium, a boride coating or a fused alumina layer.
As opposed to sodium-inert materials, a sodium-active material leads to the reduction of sodium ions rather than aluminium ions. Usually such sodium-active materials, e.g. carbon, chemically combine with sodium during cathodic reduction which lowers the required sodium reduction energy in comparison to the energy of sodium reduction on an inert or neutral surface, such as molten aluminium, to an extent that sodium ions rather than aluminium ions are cathodically reduced.
Furthermore, sodium metal produced by cathodic reduction of sodium ions is very soluble in the molten electrolyte and thus can easily migrate to the anode.
It follows that sodium metal near the anode will chemically reduce the oxygen evolved on the anode leading to depletion of oxygen at the anode. As mentioned above, a lower concentration of oxygen at the anode leads to faster oxidation and corrosion of the anode.
Furthermore, sodium metal dissolved in the electrolyte at the anode may chemically reduce oxides of the anode""s surface which causes corrosion of the anode or the sodium metal may be oxidised by the anodic current which reduces the cell""s current efficiency. Therefore, the sodium-inert layer also inhibits reduction of the anode""s transition metal oxides by sodium metal and increases the current efficiency.
Thus, hiding or shielding cathodically polarised sodium-active material, e.g. carbon, from the electrolyte surprisingly reduces the wear rate of transition metal alloy anodes in the electrolyte.
The inhibition of dissolution of the alloy anodes can be achieved by shielding the sodium-active cathodic material from the electrolyte using various materials all chemically inert to sodium. Such shielding materials include molten aluminium and refractory hard material-based layers, in particular layers disclosed in WO01/42168 (de Nora/Duruz) and WO01/42531 (Nguyen/Duruz/de Nora). Examples of aluminium production cells with such coatings have been disclosed in U.S. Pat. No. 5,683,559 (de Nora), U.S. Pat. No. 6,258,246 (Duruz/de Nora), WO98/53120 (Berclaz/de Nora), WO99/02764, WO99/41429 (both de Nora/Duruz), WO00/63463 (de Nora), WO01/31086 (de Nora/Duruz) and WO01/31088 (de Nora).
These references all disclose applying a protective coating of a refractory material such as titanium diboride to a carbon component of an aluminium electrowinning cell, by applying thereto a slurry of particulate refractory material and/or precursors thereof in a colloid and/or inorganic polymer. Coatings with preformed refractory material have shown outstanding performance compared to previous attempts to apply refractory coatings to cathodes of aluminium electrowinning cells. These aluminium-wettable refractory boride coated bodies can be used in conventional cells with a deep aluminium pool and also permit the elimination of the thick aluminium pool required to partially protect the carbon cathode, enabling the cell to operate with a drained cathode.
The following attributes of these refractory boride coatings have been disclosed: excellent wettability by molten aluminium, inertness to attack by molten aluminium and cryolite, low cost, environmentally safe, ability to absorb thermal and mechanical shocks, durability in the environment of an aluminium production cell, and ease of production and processing. The boride coating also acts as a barrier to sodium penetration into the cathode, which is particularly detrimental when the cathode is made of carbon material.
However, such protective coatings and other sodium-inert cathodic materials, in particular molten aluminium and aluminium-wettable components placed on a cathodic bottom as for instance disclosed in U.S. Pat. No. 4,824,531 (Duruz/Derivaz) and U.S. Pat. No. 4,650,552 (de Nora/Gauger/Fresnel/Adorian/Duruz), have never been disclosed for their ability to inhibit dissolution of anodes having a transition metal-containing alloy with an integral oxide layer.
In fact, the effect produced at the anode by shielding from the electrolyte a cathode made of carbon or another sodium-active material has never been examined and thus never led to any technical measure and commercial utilisation.
The layer of sodium-inert material covering the sodium-active cathodic material may be electrically conductive over its entire surface or over only part thereof. For example, a conductive cell trough can be covered with a sodium-inert layer that is electrically conductive as described above where it faces the anodes and electrically non-conductive, e.g. fused alumina, where no aluminium is produced, e.g. on the sidewalls of the conductive cell trough.
The sodium-active cathodic material may comprise carbon in the form of petroleum coke, metallurgical coke, anthracite, graphite, amorphous carbon, fullerene, low density carbon or mixtures thereof.
The sodium-inert material, in particular in the form of a powder-sintered or slurry-applied or plasma-sprayed coating or possibly tiles or other preformed components, may comprises one or more refractory hard materials, for example as disclosed in the above references, in particular borides, such as borides of titanium, chromium, vanadium, zirconium, hafnium, niobium, tantalum, molybdenum, cerium, nickel and iron. The sodium-inert material, when produced from a slurry, may comprises consolidated boride particles, in particular in a dried inorganic polymeric and/or colloidal binder, for example alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminium phosphate or cerium acetate or combinations thereof, all in the form of colloids and/or inorganic polymers. Furthermore, the sodium-inert material may comprise a conductive element or compound, in particular a metal such as Cu, Al, Fe or Ni for enhancing the electrical conductivity of the layer and its adherence to the cathode.
Advantageously, the sodium-inert material comprises an aluminium-wetting agent selected from at least one metal oxide and/or at least one partly oxidised metal, such as iron, copper, cobalt, nickel, zinc and manganese in the form of oxides and partly oxidised metals and combinations thereof. Such metal oxide and/or partly oxidised metal particles are reactable with molten aluminium when exposed thereto to form an alumina matrix containing metal of these particles and aluminium. Further details of such a material are disclosed in the abovementioned WO01/42168 (de Nora/Duruz). Such wetting-agents are particularly suited for use in combination with aluminium-resistant refractory compound, in particular selected from borides, silicides, nitrides, carbides, phosphides, oxides and aluminides, such as alumina, silicon nitride, silicon carbide or boron nitride or combinations thereof.
The aluminium-resistant refractory compound can be in the form of a coating, a reticulated structure or another preformed component, such as a tile, placed against the sodium-active material.
The alloy of the oxygen-evolving anode can comprise at least one transition metal selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo, Ru, Rh, Pd, Ir, Pt, Au, Ce and Yb and combinations thereof. For example, the alloy contains at least one of iron, nickel and cobalt, in particular iron alloys such as alloys with nickel and/or cobalt. In addition to transition metal(s), the alloy may contain at least one further metal selected from Li, Na, K, Ca, Y, La, Ac, Al, Zn, Ga, Zr, Ag, Cd and In. The alloy may also contain non-metals or compound thereof, in particular one or more constituent selected from elemental and compounds of H, B, C, O, F, Si, P, As, Se and Te.
Suitable anodes comprising a transition metal-alloy with an integral oxide layer containing predominantly one or more transition metal oxides have been disclosed in the prior art, in particular in the above references, as well as in, WO00/40783 (de Nora/Duruz) and U.S. Pat. No. 6,077,415 (Duruz/de Nora). Suitable designs for metal-based anodes are disclosed in WO00/40781 and WO00/40782 (both de Nora).
As mentioned above, the anode has a transition metal-containing alloy that self-forms during normal electrolysis an integral electrochemically-active oxide-based surface layer containing predominantly one or more transition metal oxides which slowly dissolve in the electrolyte.
The rate of formation of this oxide layer is substantially equal to its rate of dissolution at the surface layer/electrolyte interface thereby maintaining its thickness substantially constant and forming a limited barrier controlling the oxidation rate.
Such an anode wear mechanism is disclosed in greater details in WO00/06805 and U.S. Pat. No. 6,248,227 (both de Nora/Duruz). By using the cell environment and operating conditions of the present invention the anode wear and corrosion can be significantly reduced.
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 anodes 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.
Advantageously, the anode body comprises an iron alloy which when oxidised will form an oxide-based surface layer containing iron oxide, such as hematite or a mixed ferrite-hematite, 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 as disclosed in WO00/06805 (de Nora/Duruz) and WO00/40783 (de Nora/Duruz).
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 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.
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 Ti4+, Zr4+, Sn4+, Fe4+, Hf4+, Mn4+, Fe3+, Ni3+, Co3+, Mn3+, Al3+, Cr3+, Fe2+, Ni2+, Co2+, Mg2+, Mn2+, Cu2+, Zn2+ 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 surface of the anode may be in-situ or ex-situ 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.
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.
In one embodiment, the anode has a highly conductive metallic structure with an active anode surface on which, during electrolysis, oxygen is anodically evolved, and which is suspended in the electrolyte substantially parallel to a facing cathode. Such metallic structure comprises a series of parallel horizontal anode members, each having an electrochemically active surface on which during electrolysis oxygen is anodically evolved, the electrochemically active surfaces being in a generally coplanar arrangement to form said active anode surface. The anode members are spaced laterally to form longitudinal flow-through openings for the circulation of electrolyte, in particular for the up-flow of alumina-depleted electrolyte driven by the upward fast escape of anodically evolved oxygen, and for the down-flow of alumina-rich electrolyte to an electrolysis zone spacing the anode(s) and the cathode.
Depending on the cell configuration some or all of the flow-through openings may serve for the flow of alumina-rich electrolyte to an electrolysis zone between the anode(s) and the cathode and/or for the flow of alumina-depleted electrolyte away from the electrolysis zone. When the anode surface is horizontal or inclined these flows are ascending and descending. Part of the electrolyte circulation may also take place around the metallic anode structure.
A substantially uniform current distribution can be provided from a current feeder through conductive transverse metallic connectors to the anode members and their active surfaces.
As opposed to known oxygen-evolving anode designs for aluminium electrowinning cells, in such an anode, the coplanar arrangement of the anode members provides an electrochemically active surface extending over an expanse which is much greater than the thickness of the anode members, thereby limiting the material cost of the anode.
The active anode surface may be substantially horizontal, vertical or inclined to the horizontal.
In special cases, the electrochemically active anode surface may be vertical or substantially vertical, the horizontal anode members being spaced apart one above the other, and arranged so the circulation of electrolyte takes place through the flow-through openings. For example, the anode members may be arranged like venetian blinds next to a vertical or substantially vertical cathode.
In one embodiment, two substantially vertical (or downwardly converging at a slight angle to the vertical) spaced apart adjacent anodes are arranged between a pair of substantially vertical cathodes, each anode and facing cathode being spaced apart by an inter-electrode gap. The adjacent anodes are spaced apart by an electrolyte down-flow gap in which alumina-rich electrolyte flows downwards until it circulates via the adjacent anodes"" flow-through openings into the inter-electrode gaps. The alumina-rich electrolyte is electrolysed in the inter-electrode gaps thereby producing anodically evolved oxygen which drives alumina-depleted electrolyte up towards the surface of the electrolyte where the electrolyte is enriched with alumina, and induces the downward flow of alumina-rich electrolyte.
The anode members may be spaced-apart blades, bars, rods or wires. The bars, rods or wires may have a generally rectangular or circular cross-section, or have in cross-section an upper generally semi-circular part and a flat bottom. Alternatively, the bars, rods or wires may have a generally bell-shape or pear-shape cross-section.
Each blade, bar, rod or wire may be generally rectilinear or, alternatively, in a generally concentric arrangement, each blade, bar, rod or wire forming a loop to minimise edge effects of the current during use. For instance, each blade, bar, rod or wire can be generally circular, oval or polygonal, in particular rectangular or square, preferably with rounded corners.
Each anode member may be an assembly comprising an electrically conductive first or support member supporting or carrying at least one electrochemically active second member, the surface of the second member forming the electrochemical active surface. To avoid unnecessary mechanical stress in the assembly due to a different thermal expansion between the first and second members, the first member may support a plurality of spaced apart xe2x80x9cshortxe2x80x9d second members.
The electrochemically active second member may be electrically and mechanically connected to the first support member by an intermediate connecting member such as a flange. Usually, the first member is directly or indirectly in contact with the electrochemically active second member along its whole length which minimises during cell operation the current path through the electrochemically active member. Such a design is particularly well suited for a second member made of an electrochemically active material which does not have a high electrical conductivity.
The parallel anode members are transversally connected by at least one transverse connecting member. Possibly the anode members are connected by a plurality of transverse connecting members which are in turn connected together by one or more cross members.
For concentric looped configurations, the transverse connecting members may be radial. In this case the radial connecting members extend radially from the middle of the parallel anode member arrangement and optionally are secured to or integral with an outer ring at the periphery of this arrangement.
Advantageously, the transverse connecting members are of variable section to ensure a substantially equal current density in the connecting members before and after each connection to an anode member. This also applies to the cross member when present.
Alternatively, the parallel anode members can be connected to one another for instance in a grid-like, net-like or mesh-like configuration of the anode members. To avoid edge effects of the current, the extremities of the anode members can be connected together, for example they can be arranged extending across a generally rectangular peripheral anode frame from one side to an opposite side of the frame.
In other designs, each anode comprises a vertical current feeder arranged to be connected to a positive bus bar which is mechanically and electrically connected to at least one transverse connecting member or to one or more cross members connecting a plurality of transverse connecting members, for carrying electric current to the anode members through the transverse connecting member(s) and, where present, through the cross member. Where no transverse connecting member is present the vertical current feeder is directly connected to the anode structure which can be a grid, net, mesh or a perforated plate.
The vertical current feeder, anode members, transverse connecting members and where present the cross members may be secured together for example by being cast as a unit. Assembly by welding or other mechanical connection means is also possible.
For all these anode designs, the anode""s active layer obtained by surface oxidation of a metallic anode substrate is made of metal oxide such as iron oxide, and a sufficient amount of anode constituents may be maintained in the electrolyte to keep the anode(s) substantially dimensionally stable by reducing dissolution thereof into the electrolyte.
The cell may comprise at least one aluminium-wettable cathode. The aluminium-wettable cathode may be in a drained configuration. Examples of drained cathode cells are described in U.S. Pat. No. 5,683,130 (de Nora), WO99/02764 and WO99/41429 (both in the name of de Nora/Duruz).
The cell may also comprise means to facilitate dissolution of alumina fed into the electrolyte, for instance by using electrolyte guiding members above the anode members as described in PCT/IB99/00017 (de Nora), the content of which is disclosed in WO00/40781, inducing an up-flow and/or a down-flow of electrolyte through and possibly around the anode structure.
The electrolyte guide members may be secured together by being cast as a unit, welding or using other mechanical connecting means to form an assembly. This assembly can be connected to the vertical current feeder or secured to or placed on the foraminate anode structure.
The cell may also comprise means to thermally insulate the surface of the electrolyte to prevent the formation of an electrolyte crust on the electrolyte surface, such as an insulating cover above the electrolyte, as described in co-pending application WO98/02763 (de Nora/Sekhar).
The electrolyte of the aluminium production cell usually comprises sodium fluoride and aluminium fluoride, in particular cryolite, possibly with at least one further fluoride selected from fluorides of calcium, lithium and magnesium. The electrolyte can be at temperature in the range from 660xc2x0 to 1000xc2x0 C., in particular from 720xc2x0 to 960xc2x0 C., preferably from 850xc2x0 to 940xc2x0 C. Examples of electrolyte compositions are given in U.S. Pat. No. 4,681,671 (Duruz), U.S. Pat. No. 5,725,744 (de Nora/Duruz) and in the abovementioned WO00/06802.
The invention also relates to a method of electrowinning aluminium in a cell for the production of aluminium from alumina dissolved in a sodium ion-containing molten electrolyte. Such a cell comprises a cathodic material that is predominately active for the reduction of sodium ions rather than aluminium ions and an oxygen-evolving anode that comprises a transition metal-containing alloy having an integral oxide layer containing predominantly one or more transition metal oxides which are slowly dissolved in the electrolyte and compensated by oxidation of the alloy at the alloy/oxide layer interface. This method comprises using a sodium-inert layer on the cathodic material to inhibit dissolution of the anode, as described above and cathodically producing aluminium.
Anodes of the present invention may be covered with an iron oxide-based material, in particular hematite-based, obtained by oxidising the surface of an anode substrate which contains iron. Suitable anode materials are described in PCT/IB99/00015 (de Nora/Duruz) and PCT/IB99/00016 (Duruz/de Nora) the contents of which are published in WO00/40783 and WO00/06803 respectively. These two patent applications disclose the use for aluminium electrowinning of a metal iron-alloy anode having an integral electrochemically active oxide layer which during operation is progressively further formed by surface oxidation of the anode""s iron-alloy by controlled oxygen diffusion through the electrochemically active oxide layer, and is progressively dissolved into the electrolyte at the electrolyte/anode interface.
Furthermore, the invention generally concerns cells for the production of aluminium from alumina dissolved in an sodium ion-containing molten electrolyte. The cells comprise a cathodic material, in particular carbon, that is predominately active for the reduction of sodium ions rather than aluminium ions and an oxygen-evolving anode that comprises a transition metal-containing alloy having an integral oxide layer containing predominantly one or more transition metal oxides which slowly dissolve in the electrolyte and are compensated by oxidation of the alloy at the alloy/oxide layer interface.
More particularly, the invention relates to the use in such a cell of a sodium-inert layer on the sodium-active cathodic material as a dissolution inhibitor of the anode. This sodium-inert layer is active for the cathodic reduction of aluminium ions rather than sodium ions and inhibits the presence in the molten electrolyte of soluble cathodically-produced sodium metal that constitutes an agent for chemically reducing the anode""s transition metal oxides and the anodically-evolved oxygen, in particular molecular oxygen, thereby inhibiting reduction of the anode""s transition metal oxides by sodium metal and maintaining the evolved oxygen at the anode at a concentration such as to produce at the alloy/oxide layer interface stable and coherent transition metal oxides having a high level of oxidation.
A further aspect of the invention relates to a cell for the production of aluminium from alumina dissolved in a molten electrolyte comprising ions of at least one metal selected from sodium, lithium and potassium. The cell comprises a cathodic material that is predominately active for the reduction of such electrolyte metal ions rather than aluminium ions and an oxygen-evolving anode that comprises a transition metal-containing alloy having an integral oxide layer containing predominantly one or more transition metal oxides which slowly dissolve in the electrolyte and are compensated by oxidation of the alloy at the alloy/oxide layer interface.
More specifically the invention relates to a use in such a cell of a layer that is inert to these electrolyte metal ions on such a cathodic material as a dissolution inhibitor of the anode. This inert layer is active for the cathodic reduction of aluminium ions rather than the electrolyte metal ions to inhibit the presence in the molten electrolyte of soluble cathodically-reduced electrolyte metal ions that act as agents for chemically reducing the anode""s transition metal oxides and the evolved oxygen, in particular molecular oxygen, thereby inhibiting reduction of the anode""s transition metal oxides by said cathodically-reduced electrolyte metal ions and maintaining the evolved oxygen at the anode at a concentration such as to produce at the alloy/oxide layer interface stable and coherent transition metal oxides having a high level of oxidation.
Yet another aspect of the invention relates to a method of inhibiting dissolution of an oxygen-evolving anode of a cell for the production of aluminium from alumina dissolved in an molten electrolyte comprising ions of at least one metal selected from sodium, lithium and potassium. This cell comprises a cathodic material that is predominately active for the reduction of such electrolyte metal ions rather than aluminium ions. The oxygen-evolving anode comprises a transition metal-containing alloy having an integral oxide layer containing predominantly one or more transition metal oxides which slowly dissolve in the electrolyte and are compensated by oxidation of the alloy at the alloy/oxide layer interface.
The method of the invention comprises providing a layer that is inert to these electrolyte metal ions on such a cathodic material and electrolysing the dissolved alumina whereby oxygen is anodically evolved and aluminium ions rather than these electrolyte metal ions are cathodically reduced on this inert layer to inhibit the presence in the molten electrolyte of soluble cathodically-reduced electrolyte metal ions that constitute agents for chemically reducing the anode""s transition metal oxides and evolved oxygen, in particular molecular oxygen. The inert layer is used as a dissolution inhibitor of the anode by its effect in inhibiting reduction of the anode""s transition metal oxides by said cathodically-reduced electrolyte metal ions and in maintaining the evolved oxygen at the anode at a concentration such as to produce at the alloy/oxide layer interface stable and coherent transition metal oxides having a high level of oxidation.
Yet a further aspect of the invention relates to a cell for the production of aluminium from alumina dissolved in a molten electrolyte. The cell comprises a carbon-based material that is reactable with oxygen, in particular molecular oxygen, and/or carbon dioxide, to form carbon monoxide, or that produces carbon dust, and an oxygen-evolving anode that comprises a transition metal-containing alloy having an integral oxide layer containing predominantly one or more transition metal oxides which slowly dissolve in the electrolyte and are compensated by oxidation of the alloy at the alloy/oxide layer interface,
More particularly the invention relates to the use in such a cell of an oxygen-stable layer on the carbon-based material as a dissolution inhibitor of the anode. The oxygen-stable layer inhibits the presence in the molten electrolyte of carbon dust or carbon monoxide that constitutes an agent for chemically reducing the anode""s transition metal oxides and the evolved oxygen, in particular molecular oxygen to form carbon dioxide, thereby inhibiting reduction of the anode""s transition metal oxides by the carbon dust or carbon monoxide and maintaining the evolved oxygen at the anode at a concentration such as to produce at the alloy/oxide layer interface stable and coherent transition metal oxides having a high level of oxidation.
Furthermore, the invention relates to a method of inhibiting dissolution of an oxygen-evolving anode of a cell for the production of aluminium from alumina dissolved in an molten electrolyte. The cell comprises carbon-based material that is reactable with oxygen, in particular molecular oxygen, and/or carbon dioxide, or that produces carbon dust. The oxygen-evolving anode comprises a transition metal-containing alloy having an integral oxide layer containing predominantly one or more transition metal oxides which slowly dissolve in the electrolyte and are compensated by oxidation of the alloy at the alloy/oxide layer interface.
According to the invention, the method comprises providing an oxygen-stable layer on the carbon-based material and electrolysing the dissolved alumina whereby oxygen is anodically evolved and aluminium ions are cathodically reduced. The oxygen-stable layer inhibiting the presence in the molten electrolyte of the carbon dust or carbon monoxide that constitutes an agent for chemically reducing the anode""s transition metal oxide and the evolved oxygen, in particular molecular oxygen, to form carbon dioxide. The oxygen stable layer is used as a dissolution inhibitor of the anode by its effect in inhibiting reduction of the anode""s transition metal oxides by the carbon dust or carbon monoxide and in maintaining the evolved oxygen at the anode at a concentration such as to produce at the alloy/oxide layer interface stable and coherent transition metal oxides having a high level of oxidation.
This oxygen-stable layer can comprise nitrides and/or carbides, such as silicon nitride, silicon carbide and/or boron nitride, or a stable oxide such as fused alumina. The oxygen-stable layer may comprise an aluminium-wetted coating, the aluminium retained in the coating forming a barrier to oxygen.
For example, the cell comprises sidewalls made of a carbon-based material which produces carbon dust that is reactable with oxygen.
Furthermore, the carbon dust, carbon monoxide, sodium, lithium or potassium may be oxidised by the anodic current which reduces the cell""s current efficiency. Therefore, the sodium-inert layer may also inhibit reduction of the anode""s transition metal oxides by sodium metal.
Additionally, the abovementioned carbon dust, carbon monoxide, sodium, lithium or potassium metal in the electrolyte at the anode may chemically reduce oxides of the anode""s surface which causes corrosion of the anode. Sodium, lithium or potassium metal may also be oxidised in the electrolyte by the anodic current which reduces the cell""s current efficiency. Therefore, the sodium-inert layer also inhibits reduction of the anode""s transition metal oxides and increases the current efficiency.