ALUMINIUM ELECTROWINNING WITH METAL-BASED ANODES

Abstract: A process for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte in a cell operating at reduced temperature, typically below 870 DEG C, utilising nickel-alloy based anodes, in particular nickel-iron alloy anodes. The electrolyte contains AlF3 in such a high concentration, usually above 20 weight%, in addition to cryolite, that fluorine-containing ions rather than oxygen ions are oxidised on the anodes. However, only oxygen is evolved, the evolved oxygen being derived from the dissolved alumina present near the anodes. The anodes may be porous at the surface so as to provide a high active surface area for operation at low current density.

Description: ALUMINIUM ELECTROWINNING WITH METAL-BASED ANODES Field of the Invention This invention relates to a process and cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte using non-carbon, metal-based anodes. Background Art The production of aluminium since Hall and Heroult has been carried out by dissolving the feed material consisting of pure alumina obtained from bauxite in a cryolite-based electrolyte at about 950°C. This process has not evolved for more than one hundred years as many other electrochemical processes . Different types of carbon have been used as anode, cathode and sidewall material. All attempts to utilise other materials have failed with the exception of silicon carbide for sidewalls and more recently TiB 2 protective coatings on carbon cathodes instead of or in addition to a thick pool of aluminium protecting the cathodes against cryolite attack. The carbonaceous anodes must be replaced every few weeks. During electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting C0 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 1/3 higher than the theoretical amount of 333 Kg/Ton. Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production. US Patent 4,374,050 (Ray) discloses inert anodes made of specific multiple metal compounds which are produced by mixing powders of the metals or their compounds in given ratios followed by pressing and sintering, or alternatively by plasma spraying the powders onto an anode substrate. The possibility of obtaining the specific metal compounds from an alloy containing the metals is mentioned. US Patent 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes 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 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, US Patents 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium production anodes with an oxidised copper-nickel surface on an alloy substrate with a protective oxygen barrier layer. However, full protection of the alloy substrate was difficult to achieve . US Patent 4,681,671 (Duruz) discloses aluminium production from alumina dissolved in an electrolyte between 680° and 690°C in a cell utilising metal anodes that have an electrochemically active surface whose area is increased at least 5 times compared to conventional anodes. The anodes are arranged for the discharge of oxide ions preferentially to fluorine ions using a low current density at the anode. Use of such a process with a multimonopolar arrangement of non-consumable electrodes that are vertical or at a slope, is described in US Patent 5,725,744 (Duruz /de Nora) . In Belyaev & Studentsov: Electrolysis of Alumina in Fused Cryoli te wi th Oxide Anodes, Legkie Metali 6 No . 3, 1937, pp. 17-24 and Belyaev: Electrolysis of Al umina wi th Ferri te Anodes, Legkie Metali 7 No. 1, 1938, pp. 7-20, it has been established in tests using anodes made of precious metals such as platinum, and bulk ceramic oxides such as ferrites that the primary anodic product resulting from the electrolysis of cryolite-alumina melts is oxygen. Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. Many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry because they had a short life and contaminated the aluminium produced. All efforts made to utilise non-carbon anodes and avoid pollution by C0 2 and organic fluorides have not succeeded because all non-noble metal oxides, which are the only materials commercially acceptable and resistant to oxygen, are more or less soluble in cryolite which was chosen and is still used as the electrolyte because it is a good solvent of oxides such as alumina. Objects of the Invention An object of the invention is to provide a process and cell for aluminium electrowinning using long-lasting non-carbon anodes so as to eliminate carbon-generated pollution . Another object of the invention is to provide a process and cell for aluminium electrowinning using metal- based anodes, in which the conditions are such as to inhibit corrosion or oxidation of the anodes. A further object of the invention is to provide an aluminium electrowinning process and cell with anodes having a high electrochemical activity and a low or no solubility in the electrolyte. Another object of the invention is to provide an aluminium electrowinning process and cell utilising improved metal-based anodes made of readily available material (s ) . A major object of the invention is to provide an aluminium electrowinning process and cell using metal anodes and operating under such conditions that the contamination of the product aluminium is limited. Summary of the Invention The present invention concerns an aluminium electrowinning process in a cell containing alumina dissolved in a fluoride-based molten electrolyte and utilising specific metal alloy-based anodes which do not require to be made of oxides in order to be electrochemically active and resistant to the attack of the molten electrolyte and of oxygen gas. Several models of anodic reactions can be considered to explain the production of oxygen gas during the electrowinning process of the invention, namely: [1] 20 2 - - 4e = 0 2 [2] 2A10 3 " - 6e = Al 2 0 3 + 3/20 2 [3] 2AlO ~ - 2e = Al 2 0 3 + l/20 2 [4] 2F- - 2e = F 2 ; and 2A1 2 0 3 + 6F 2 = 4A1F 3 + 0 2 [5] F- - e = F; and A1 2 0 3 + 6F = 2A1F 3 + 0; and 0 + 0 = 0 2 [6] 2AlF g " + A1 2 0 3 - 6e = 2Al 2 F 6 + 3/20 2 [7] 2A1F 3 + Al 2 0 3 = Al 2 F 6 0 2 - + Al0 2 + Al 3+ or 2A1F 3 + A10 2 = A1 2 F 6 0 2~ + 0 2" + Al 3+ or 2A1F 3 + 0 2 - = A1 2 F 5 0 2 -; and A1 2 F 6 0 2 - - 2e = A1 2 F 6 0; and A1 2 F 6 0 = Al 2 F 6 + l/20 2 Whereas mechanisms [1] to [7] have been defined in terms of stoichiometric compounds, it is possible that corresponding mechanisms involving non-stoichiometric compounds may occur during electrolysis. The present invention is based on the observation that under specific cell operating conditions, i.e. reduced electrolysis temperature and high fluoride content in the electrolyte, the electrochemical oxidation reaction of oxygen ions or fluorine-free ionic oxides to form oxygen gas, i.e. reactions [1] to [3], can be minimised or even suppressed. Hence, the oxidation of fluorine ions or ionic fluorine-containing compounds, i.e. reactions [4], [5], [6] and [7], in particular the reaction involving the oxidation of F ~ to nascent fluorine F and/or of aluminium oxyfluoride ions [7], become the main or only electrochemical reactions occurring on the electrochemically active anode surface. This inhibits direct contact of reactive oxygen species, in particular nascent monoatomic oxygen, with the electrochemically active surface, which greatly reduces the risk of oxidation and corrosion of the anode by these oxygen species . Furthermore, it has been observed that nickel alloys, in particular nickel-iron metal alloys, are electrochemically active with a low overvoltage for the oxidation of fluorine ions or ionic fluorine-containing compounds such as aluminium oxyfluoride ions and, surprisingly, are stable and substantially do not react with the product of the anodic electrolysis even after several hundred hours of electrolysis under specific cell operating conditions. The anodes used in this invention consist essentially of a nickel alloy, in particular of a nickel- iron based alloy, and can be used as such for efficient and successful operation in a melt having a high concentration of aluminium fluoride and operated at reduced temperature. Cermet anodes which have been described in the past in relation to aluminium production have an oxide content which forms the major phase of the anode. Conversely, the anode according to the invention is made predominantly of metal, possibly covered with a thin oxide layer. For the first time, this invention permits utilisation of a non-noble metal anode which is resistant to a fluoride-based molten electrolyte, electrochemically active and has a very long life the limit of which has not been determined yet. The invention relates to a process for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte in a cell operating at reduced temperature and utilising metal-based anodes. The anodes comprise an alloy of nickel and an alloying metal, in particular iron, having an outer part consisting predominantly of nickel which forms an electrochemically active surface for the oxidation of ions. In this process the electrolyte contains AlF 3 in such a high concentration that fluorine-containing ions, such as aluminium oxyfluoride ions, predominantly rather than oxygen ions are oxidised on the electrochemically active surfaces. However, only oxygen is evolved, the evolved oxygen being derived from the dissolved alumina present near the electrochemically active anode surfaces. As in the fluorine oxidation reactions [4], [5], [6] and [7] listed above, the oxidation of fluorine- containing ions covers oxidation of ions of fluorine as such as well as ions contained in a fluorine compound such as AlF^ " or Al 2 F 6 0 2" . As explained below, the outer part of the nickel alloy advantageously has an open porosity defining a high surface area electrochemically active surface. The total amount of electrolysis current passed between the anode and facing cathode which corresponds to about to 0.5 to 1.5 A/cm 2 at the cathode surface of an industrial cell corresponds to a lower current density on the high surface area electrochemically active surface. The actual current density on the surface of the pores of the anode is typically 5 to 50 times smaller than the corresponding density on the cathode. To prevent anode effects and corrosion of the anode by fluorine-containing ions oxidised on the electrochemically active anode surface, a sufficient concentration of dissolved alumina is permanently present in the molten electrolyte near the electrochemically active anode surfaces so that fluorine-containing ions react before or after their oxidation with oxygen ions from the dissolved alumina to evolve oxygen gas instead of fluorine . The cell is preferably operated with a crustless and ledgeless electrolyte, as described in co-pending application PCT/IB99/01739 (de Nora/Duruz) . To ensure sufficient dissolution of alumina in the electrolyte at reduced temperature, the cell is preferably fitted with an alumina spraying device to spray and distribute alumina over substantially the entire surface of the molten electrolyte, as disclosed in PCT/IB99/00697 (de Nora/Berclaz) . To promote circulation of molten electrolyte rich in dissolved alumina to the electrochemically active anode surface, the electrodes may be designed as disclosed in W099/41429 (de Nora/Duruz) and in PCT/IB99/01740 (de Nora) . Preferably, the anodes have a foraminate electrochemically active structure to permit circulation of the molten electrolyte therethrough, as disclosed in PCT/IB99/00018 (de Nora) , which is advantageously fitted with a funnel-like arrangement to guide the molten electrolyte from and to the electrochemically active anode surfaces as disclosed in PCT/IB99/00017 (de Nora) . Normally, the molten electrolyte contains cryolite and, in addition to cryolite, an excess of A1F 3 in an amount of at least 20 weight% of the electrolyte typically 23 weight% or more, preferably between 25 and 35 weight%, in particular between 27 to 30 weight%, for example about 28 weight% of the electrolyte. The electrolyte may further contain CaF 2 and/or MgF 2 . The reduced temperature of the molten electrolyte should be at 900°C or 910°C at the most, typically below 880°C and preferably below 870°C, and above the melting point of aluminium, but usually above 730°C. As stated above, the cell may advantageously be fitted with means to circulate electrolyte containing dissolved aluminium to constantly maintain a sufficient concentration of dissolved alumina near the electrochemically active anode surfaces. comprise an alloy of nickel and an alloying metal, in particular iron, having an outer part consisting predominantly of nickel which forms an electrochemically active surface for the oxidation of ions. The electrolyte contains A1F 3 in such a high concentration that fluorine- containing ions, such as aluminium oxyfluoride ions, predominantly rather than oxygen ions are oxidised on the electrochemically active surfaces, but only oxygen is evolved, the evolved oxygen being derived from the dissolved alumina present near the electrochemically active anode surfaces. Preferably, aluminium is produced on an aluminium- wettable cathode, in particular on a drained cathode, for instance as disclosed in US Patent 5,683,559 (de Nora) or in PCT application WO99/02764 (de Nora/Duruz) . In one embodiment of the cell, each anode is a nickel-iron alloy based anode. The anode before use has an electrochemically active surface with an oxide film. When it is polarised in a molten electrolyte of a cell, it becomes electrochemically active for the oxidation of fluorine ions rather than oxygen ions. However, only oxygen is evolved which is derived from the dissolved alumina present near the electrochemically active anode surfaces . Before use, the alloy of which the anode is made may have a Ni/Fe, or more generally nickel/alloying metal, atomic ratio below 1. Alternatively, the Ni/Fe atomic ratio may be at least 1, in particular from 1 to 4. As described below, when the outer part of the anode is made porous by oxidation and removal of the alloying metal, a higher content of alloying metal leads to a greater porosity whereas a lower content of alloying metal leads to a smaller removal and formation of a reduced porosity. The alloy can further contain one or more additives. Before use, the alloy may contain nickel and the alloying metal, in particular iron, in a total amount of at least 85 weight%, in particular at least 95 weight%, and the balance additive (s) . For example, one or more additives can be selected from chromium, copper, cobalt, silicon, titanium, tantalum, tungsten, vanadium, yttrium, of at least 85 weight%, in particular at least 95 weight%, and the balance additive (s). For example, one or more additives can be selected from chromium, copper, cobalt, silicon, titanium, tantalum, tungsten, vanadium, yttrium, molybdenum, manganese, aluminium and niobium in a total amount of up to 5 or even 10 weight% of the alloy before use. One or more additives may be catalytically active for the desired reaction (s) and selected from iridium, palladium, platinum, rhenium, rhodium, ruthenium, tin or zinc metals, Mischmetals and their oxides and metals of the Lanthanide series and their oxides as well as mixtures and compounds thereof in a total amount of up to 5 weight% of the alloy before use. The outer part of the anodes may comprise more than 75 weight% nickel, preferably between 85 and 95 weight% nickel . The nickel metal rich outer part typically has a porosity defining a high surface area electrochemically active surface and which can be obtained by oxidation in an oxidising atmosphere before use. Usually, the porosity contains cavities which are partly or completely filled before use with nickel and/or iron oxides or more generally oxides of nickel and/or the alloying metal and during use with one or more fluorine-containing compounds of at least one metal selected from nickel, iron or other alloying metal, and aluminium. The porosity defining a high surface area electrochemically active surface can alternatively be obtained or can be completed by dissolving part of the iron or other alloying metal into the electrolyte of the aluminium electrowinning cell, or of another electrolytic cell and then transferred into the aluminium electrowinning cell, this dissolution taking place usually soon after electrolysis start-up. During use, the porosity usually contains cavities which are partly or completely filled with fluorides of at least one metal selected from nickel, iron or other alloying neta L and aluminium. In one embodiment the nickel alloy underlying the electrochemically active surface has a decreasing concentration of iron or other alloying metal (s) towards the electrochemically active surface layer. The nickel metal rich outer part can comprise nickel metal and iron or other alloying metal in a Ni/Fe or more generally nickel/alloying metal atomic ratio of more than 3 where it reaches the electrochemically active surface . A suitable nickel-iron alloy based anode for such a cell can be produced as follows. A nickel alloy substrate, in particular a nickel-iron alloy substrate, is heat treated in an oxidising atmosphere to form a nickel alloy based anode having an integral thin oxide film and anodically polarised in a molten electrolyte contained in a cell as described above, whereby fluorine-containing ions predominantly rather than oxygen ions are oxidised on the electrochemically active surface of the nickel-iron anode . When the alloy is covered with a thin oxide film obtainable by oxidation before use, during use the oxides of nickel and iron or other alloying metal present on and possibly in the alloy substrate originating from the oxidation treatment in the oxidising atmosphere may be dissolved in the molten electrolyte without being replaced, or may be substituted with one or more fluorine- containing compounds of aluminium from the electrolyte and of iron and nickel from the anode. The nickel-iron or other nickel alloy substrate can be heat treated in an oxidising atmosphere for 20 minutes to 5 hours or even 6 hours, preferably 30 to 240 minutes, for example about 120 minutes during use, at a temperature of 900 to 1200°C. It can be heat treated in an oxidising atmosphere containing 10 to 100 molar% 0 2 and the balance one or more inert gases. The nickel-iron or other nickel alloy substrate can also be heat treated in air. After formation of the integral oxide film, the nickel-iron or other nickel alloy substrate may further be heat treated in an inert atmosphere. As nickel and cobalt behave very similarly under the above described cell conditions, in a modification of the above aspects of the invention, the nickel of the metal-based anodes, in particular of their outer part, is wholly or predominantly substituted by cobalt. For example, the anode is made from a nickel-cobalt-iron alloy or a cobalt-iron alloy, in which case its outer part is rich in nickel and cobalt metal, or rich in cobalt metal only, respectively. The invention also relates to the use of a nickel alloy, in particular a nickel-iron alloy, which comprises a surface electrochemically active for the oxidation of fluorine ions as an anode of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte. The electrochemically active surface of the anode is a surface of the nickel alloy as such or oxidised before or during electrolysis. Detailed Description The invention will be further described in the following Examples: Example 1 Anode Prep_a . ratj.on_: An anode suitable for producing aluminium according to the invention was made by pre-oxidising in air at 1100°C for 30 minutes a substrate of a nickel-iron alloy consisting of 50 weight% nickel and 50 weight% iron, to form a very thin oxide surface film on the alloy. The surface oxidised anode was cut perpendicularly to the anode operative surface and the resulting section of the anode was subjected to microscopic examination. Before use, the anode had an external oxide surface layer having a thickness of up to 20-25 micron. This layer in the given example of a nickel-iron alloy consisted of an iron-rich nickel-iron oxide and, underneath, an iron-depleted nickel-iron alloy outer part containing generally round columnar pores filled with iron-rich nickel-iron oxide. The pores had a diameter of about 2 to 5 micron. The nickel-iron alloy of the outer part contained about 80-85 weight% nickel. Underneath this outer part, the nickel-iron alloy had remained substantially unchanged. Example 2 Electro1ys_is_Testing_ ι An anode prepared as in Example 1 was tested in an aluminium electrowinning cell containing a molten electrolyte at 850°C consisting essentially of NaF and A1F 3 in a weight ratio NaF/AlF 3 of about 0.7 to 0.8, i.e. an excess of AlF 3 in addition to cryolite of about 26 to 30 weight% of the electrolyte, and approximately 3 weight% alumina. The alumina concentration was maintained at a substantially constant level throughout the test by adding alumina at a rate adjusted to compensate the cathodic aluminium reduction. The test was carried out at an apparent current density of about 0.6 A/cm 2 which generally corresponds to a current density of less than about 0.06 A/cm 2 on the surface of the pores. The electrical potential of the anode remained substantially constant at 4.2 volts throughout the test. During electrolysis aluminium was cathodically produced while fluorine and/or fluorine-containing ions, such as aluminium oxyfluoride ions, rather than oxygen ions were oxidised on the nickel-iron anodes. However, only oxygen was evolved which was derived from the dissolved alumina present near the anodes. After 72 hours, electrolysis was interrupted and the anode was extracted from the cell. The external dimensions of the anode had remained unchanged during the test and the anode showed no signs of damage. The anode was cut perpendicularly to the anode operative surface and the resulting section of the anode was subjected to microscopic examination, as in Example 1. It was observed that the anode had an electrochemically active surface covered with a discontinuous, macroporous, non adherent iron oxide layer of the order of between 500 to 1000 micron thick, hereinafter called the "excess iron oxide layer". The excess iron oxide layer was pervious to and contained molten electrolyte, indicating that it had been formed during electrolysis. The excess iron oxide layer resulted from the excess of iron contained in the part of the nickel-iron alloy underlying the electrochemically active surface and which diffuses therethrough. In other words, the excess oxide layer resulted from an iron migration from inside to outside the anode during the electrolysis. Such an iron oxide layer has no or little electrochemical activity. It slowly diffuses and dissolves into the electrolyte until the part of the anode underlying the electrochemically active surface reaches an iron content of about 15-20 weight% corresponding to an equilibrium under the operating conditions at which iron ceases to diffuse, and thereafter the layer continues to dissolve into the electrolyte. The anode's aforesaid outer part had been transformed during electrolysis. Its thickness had grown from 20-25 micron to about 500 to 1000 micron and the cavities had also grown in size to vermicular form but were only partly filled with nickel and iron compounds . The cavities had a length of about 10 to 20 micron and a diameter of about 2 to 5 micron. The nickel and iron oxides filling the cavities had been fluorised to form fluoride-containing nickel and iron ceramic compounds. The presence of the fluoride-containing nickel and iron ceramic compounds attests the anodic fluorine reaction, i.e. mechanisms [4], [5], [6] and/or [7]. The cavities also contained aluminium fluoride but no electrolyte was detected and no sign of corrosive damage appeared throughout the anode . Underneath the outer part, the nickel-iron alloy had remained unchanged. The shape and external dimensions of the anode remained unchanged after electrolysis which demonstrated stability of this anode structure under the operating conditions in the molten electrolyte. In another test a similar anode was operated under the same conditions for several hundred hours at a substantially constant current and cell voltage which demonstrated the long anode life compared to known non- carbon anodes . Example 3 Anode_jorep_arat_i n_ Another anode suitable for producing aluminium according to the invention was prepared by coating a nickel-rich nickel-iron alloy substrate with a layer of nickel-iron alloy richer in iron, and heat treating this coated substrate. The alloy substrate consisted of 80 weight% nickel and 20 weight% iron. The alloy layer consisted of about 50 weight% nickel and 50 weight% iron. The alloy layer was electrodeposited onto the alloy substrate using an appropriate electroplating bath prepared by dissolving the following constituents in deionised water at a temperature of about 50 °C: a. Nickel sulfate hydrate (NiS0 4 .7 H 2 0) : 130 g/1 b. Nickel chloride hydrate (NiCl 2 . 6 H 2 0) : 90 g/1 c. Ferrous sulfate hydrate (FeS0 4 .78 H 2 0) : 52 g/1 d. Boric acid H 3 B0 3 : 49 g/1 e. 5-Sulfo-salicylic acid hydrate (C 7 H 6 0 6 S.2 H 2 0) : 5 g/1 f . o-Benzoic acid sulfimide Sodium salt hydrate (C 7 H 4 Na0 3 S.aq) : 3.5 g/1 g. 1-Undecanesulfonic acid Sodium salt (C 11 H 23 Na0 3 S) : 3.5 g/1 To assist dissolution, the constituents were stirred in the deionised water. - 15 - with an average composition of 47.5 weight% nickel and 52.5 weight% iron. After deposition, the coated alloy substrate was surface oxidised at 1100°C in air for 1 hour and cooled to room temperature. The surface-oxidised anode was then cut perpendicularly to the anode operative surface and the resulting section of the anode was subjected to microscopic examination as in Example 1. It was observed that the external anode surface was covered with iron-rich nickel-iron oxides over a thickness of about 20 to 25 micron. The alloy layer had an iron-depleted nickel-iron alloy outer part with a thickness of about 50 micron, this outer part containing vermicular iron-rich nickel-iron oxide inclusions in a nickel-iron alloy containing about 70 to 75 weight% nickel metal. Underneath this outer part, the composition of the alloy layer had remained substantially unchanged. Some minor interdiffusion of iron was also observed at the interface between the alloy layer and the alloy substrate enhancing the adherence of the layer on the substrate. Example 4 lectr_o1ys_is _Testing_ ι An anode prepared as in Example 3 was tested in an aluminium electrowinning cell as in Example 2 except that the electrolyte contained approximately 4 weight% alumina and that the anode was tested during 75 hours. During electrolysis aluminium was produced and oxygen evolved. The anode when inspected showed no signs of having been subjected to the usual type of oxidation/passivation mechanisms observed with prior art processes. This lead to the conclusion that predominantly fluorine and/or fluorine-containing ions, such as aluminium oxyfluoride ions, rather than oxygen ions were oxidised on the nickel-iron anodes. However, only oxygen of having been subjected to the usual type of oxidation/passivation mechanisms observed with prior art process . This lead to the conclusion that predominantly fluorine and/or fluorine-containing ions, such as aluminium oxyfluoride ions, rather than oxygen ions were oxidised on the nickel-iron anodes. However, only oxygen was evolved which was derived from the dissolved alumina present near the anodes. After electrolysis the anode was extracted from the cell and examined. The external surfaces of the anode were crust free and its external dimensions were practically unchanged. No sign of damage was visible. The anode was cut perpendicularly to the operative surface and the resulting section of the anode was subjected to the microscopic examination as in Example 1. It was observed that the anode surface was covered with an iron rich oxide over a thickness of less than 25 to 50 micron. The thinness of this oxide layer attested the fact that the anode had not, or only marginally, been exposed to nascent monoatomic oxygen, hence that the oxidation process of fluorine-containing ions was predominant over the process of oxygen ions . The anode's outer part (depleted in iron metal) had grown from 50 to about 250 micron containing mainly empty pores . The pores were vermicular with a length limited to the thickness of the overall alloy layer and a diameter of about 10 micron. The outer part was further depleted in iron metal and had a composition of about 75 weight% nickel and 25 weight% iron. The structure and composition of the alloy substrate had remained substantially unchanged, with the exception of empty pores of random shape having a size of about 5 to 10 micron that were located at the substrate/layer interface and up to a depth of 100 to 150 micron. The empty pores resulted from the internal oxidation and diffusion towards the anode's surface of iron during electrolysis. Example 5 Anode_ preparat_L_qn_ A metallic anode consisting of an alloy of 70 weight% nickel and 30 weight% iron was conditioned to be suitable for electrolysis according to the invention by anodic polarisation in an electrolytic cell. The electrolytic cell contained a molten electrolyte at 850°C consisting essentially of NaF and AlF 3 in a weight ratio NaF/AlF 3 of about 0.7 to 0.8, i.e. an excess of AlF 3 in addition to cryolite of about 26 to 30 weight% of the electrolyte. The electrolyte contained no alumina other than that included in impurities of the added AlF 3 making about 2 weight% of the electrolyte. Before immersion into the electrolyte, the anode was pre-heated for 0.5 hour over the cell to a temperature of about 750°C. After immersion into the conditioning electrolyte, the anode was polarised at an initial current density of about 0.06-0.1 A/cm 2 which decreased over time to less than about 0.01 A/cm 2 . The cell voltage was about 2.2 volt and the anode potential was below 2 volt. Thus, substantially no oxygen could be evolved during polarisation. The current passed during polarisation was essentially due to selective anodic dissolution of iron present at and close to the surface of the anode. After 24 hours, polarisation was interrupted and the anode was extracted from the cell. The external dimensions of the anode had remained unchanged and was covered with black oxide. This conditioned anode was ready to be used for the production of aluminium according to the invention. The anode's composition was ascertained by cutting it perpendicular to the operative surface and the resulting section of the anode was subjected to the microscopic examination, as in Example 1. It was observed that the anode surface was covered with a very thin film of iron -rich oxide having a thickness of less than 1 micron. Underneath, the anode had an outer iron-depleted nickel-iron alloy part which had an average thickness of 100 to 150 micron. This outer alloy part had vermicular pores with a diameter of 10 to 30 micron that were empty except for small oxide inclusions. The average metal composition of the outer alloy part was about 80 weight% nickel and 20 weight% iron. Below the outer alloy part, the initial nickel-iron alloy composition had remained substantially unchanged. In a variation of this Example, the composition of the anode can be changed. For instance, the starting alloy contains 30 weight% nickel and 70 weight% iron or 80 weight nickel and 20 weight% iron. A coated substrate as described in Example 3 can also be conditioned to form an anode suitable for the production of aluminium according to the invention by dissolving part of the iron of the anode as described in Example 5. All or part of the nickel content of the anodes of Examples 1, 3 and 5 can be replaced by cobalt. Examp1e 6 Electr_ 1ys_i_s _Te_sting_:_ An anode as prepared in Example 5 was used in an aluminium electrowinning cell containing a molten electrolyte as described in Example 4. As in Example 4, during electrolysis aluminium was produced and oxygen evolved. The anode inspection also led to the conclusion that fluorine-containing ions predominantly rather than oxygen ions were oxidised on the anode surface. After 75 hours, electrolysis was interrupted and the anode was extracted from the cell. The external surfaces of the anode were crust free and its external dimensions were practically unchanged. No sign of damage was visible. The anode was cut perpendicularly to the operative surface and the resulting section of the anode was subjected to the microscopic examination as in Example 1. It was observed that the anode surface was covered with a iron rich oxide over a thickness of less than 25 to 50 micron. The anode surface was covered by a very thin film of iron-rich oxide having a thickness of less than 100 micron, which indicated that the iron depletion during electrolysis was less than for a pre-oxidised anode as in Example 2. The anode outer part had grown from 150 micron to about 500 to 750 micron and contained pores that were substantially empty in their majority. Below this outer part, the alloy composition had remained unchanged. Example 7 A_nc_de__Cons_truc_t_iqn__and__E1ect_rqly_s_is_ Tes ting_ An anode having an active structure of 210 mm diameter was made of three concentric rings spaced from one another by gaps of 6 mm. The rings had a generally triangular cross-section with a base of about 19 mm and were connected to one another and to a central vertical current supply rod by six members extending radially from the vertical rod and equally spaced apart from one another around the vertical rod. The gaps were covered with chimneys for guiding the escape of anodically evolved gas to promote the circulation of electrolyte and enhance the dissolution of alumina in the electrolyte as disclosed in PCT publication WO00/40781 (de Nora) . The anode and the chimneys were made from cast nickel-iron alloy containing 50 weight% nickel and 50 weight% iron that was heat treated as in Example 1. The anode was then tested in a laboratory scale cell containing an electrolyte as described in Example 2 except that it contained approximately 4 weight% alumina. During the test, a current of approximately 280 A was passed through the anode at an apparent current density of about 0.8 A/cm 2 on the apparent surface of the anode which generally corresponds to a current density of less than about 0.08 A/cm 2 on the surface of the columnar pores of the anode. The electrical potential of the anode remained substantially constant at approximately 4.2 volts throughout the test. The electrolyte was periodically replenished with alumina to maintain the alumina content in the electrolyte close to saturation. Every 100 seconds an amount of about 5 g of fine alumina powder was fed to the electrolyte. The alumina feed was periodically adjusted to the alumina consumption based on the cathode efficiency, which was about 67%. As in Examples 4 and 6, during electrolysis aluminium was produced and oxygen evolved. The anode inspection also led to the conclusion that fluorine- containing ions predominantly rather than oxygen ions were oxidised on the anode surface. After more than 1000 hours, i.e. 42 days, electrolysis was interrupted and the anode was extracted from the cell and allowed to cool. The external dimensions of the anode had not been substantially modified during the test but the anode was covered with iron-rich oxide and bath. The anode showed no sign of damage. The anode was cut perpendicularly to the anode operative surface and the resulting section of a ring of the active structure was subjected to microscopic examination, as in Example 1. It was observed that the porous outer alloy part had grown inside the anode ring to a depth of about 7 mm leaving only an inner part of about 5 mm diameter unchanged, i.e. consisting of a non-porous alloy of 50 weight% nickel and 50 weight% iron. The outer porous alloy part of the anode had a concentration of nickel varying from 85 to 90 weight% at the anode surface to 70 to 75 weight% nickel close to the non-porous inner part, the balance being iron. The iron depletion in the porous alloy outer part corresponded about to the accumulation of iron present as oxide on the surface of the anode, which indicated that the iron oxide had not substantially dissolved into the electrolyte during the test. Summary of Examples In summary, the analysis of the anodes tested in all the above Examples showed that, at equal anode current, the oxidation rate of nickel-alloy anodes was between about 20 and 100 times smaller than the oxidation rate under conventional conditions in which the oxidation of oxygen ions is the sole or the predominant mechanism occurring at the surface of the anode, so in the above described Examples the nickel-alloy anodes should last several thousand hours, whereas in a normal cryolite electrolyte the anodes last less than 50 hours. It is believed that the greatly reduced oxidation of iron at the anode surface under the present electrolysis conditions can have two causes. The first possible cause of oxidation is exposure to nascent oxygen produced by the oxidation of oxygen ions at the anode surface which may marginally occur in parallel to the oxidation of fluorine-containing ions and which might represent less than 1% of the overall oxidation mechanism at the anode surface. The second cause of oxidation is exposure to dissolved molecular oxygen which is marginally present in the electrolyte at a theoretical pressure of about 10 -10 atm under the test conditions. If the surface of nickel-iron alloy anodes described above were exposed to significant oxygen concentration in the electrolyte, the nickel of the anode would be rapidly oxidised into NiO which would passivate the anode and prevent electrolysis. The absence of such oxidation/passivation confirms that no or substantially no oxygen ions are oxidised at the surface of the nickel- alloy anodes. In addition, the presence of sodium-free fluorides, such as nickel, iron and aluminium fluorides and oxyfluorides , was observed in the pores of the tested anodes. This indicates that not electrolyte but fluorine or fluorides from the active anode surface penetrated into these pores, and confirms that the mechanism of oxidation of fluorine-containing ions took place at the surface of the anodes .

Claims: CLAIMS 1. A process for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte in a cell operating at reduced temperature and utilising metal-based anodes comprising an alloy of nickel and an alloying metal having an outer part consisting predominantly of nickel which forms an electrochemically active surface for the oxidation of ions, in which the electrolyte contains AlF 3 in such a high concentration that fluorine-containing ions predominantly rather than oxygen ions are oxidised on the electrochemically active surfaces, however, only oxygen is evolved, the evolved oxygen being derived from the dissolved alumina present near the electrochemically active anode surfaces. 2. The process of claim 1, wherein dissolved alumina predominantly combines with oxidised fluorine ions to produce aluminium fluoride and oxygen. 3. The process of claim 2, wherein dissolved alumina combines with monoatomic nascent fluorine formed by oxidation of fluorine ions to produce oxygen gas and partly dissociated aluminium fluoride. 4. The process of claim 1, wherein aluminium oxyfluoride ions predominantly rather than oxygen ions are oxidised. 5. The process of claim 4, wherein aluminium oxyfluoride ions resulting from the combination of aluminium fluoride and alumina rather than oxygen ions are oxidised on the electrochemically active surfaces into transient aluminium oxyfluoride which decomposes into oxygen and aluminium fluoride. 6. The process of any preceding claim, wherein the operating temperature of the electrolyte is below 900°C, preferably below 880°C, and even more preferably below 870°C. 7. The process of any preceding claim, wherein the electrolyte contains cryolite and, in addition to cryolite, an excess of A1F 3 in an amount of at least 20 weight% of the electrolyte, preferably between 25 and 35 weight% of the electrolyte. 8. The process of any preceding claim, wherein the electrolyte further contains CaF 2 and/or MgF 2 . 9. The process of any preceding claim, wherein said alloying metal of the nickel alloy is iron. 10. The process of any preceding claim, wherein the outer part of the anode comprises more than 75 weight% nickel, preferably between 85 and 95 weight% nickel. 11. The process of any preceding claim, wherein the outer part has an open porosity defining a high surface area electrochemically active surface, current being passed at a low current density on the high surface area electrochemically active surface. 12. The process of claim 11, wherein part of said alloying metal of the nickel alloy dissolves into the electrolyte to form said open porosity. 13. The process of any preceding claim, comprising circulating electrolyte containing dissolved aluminium to constantly maintain dissolved alumina near the electrochemically active anode surfaces. 14. The process of any preceding claim, modified in that the nickel of the metal-based anodes, in particular of their outer part, is wholly or predominantly substituted by cobalt. 15. A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte operating at reduced temperature and utilising metal-based anodes comprising an alloy of nickel and an alloying metal having an outer part consisting predominantly of nickel which forms an electrochemically active surface for the oxidation of ions, in which the electrolyte contains AlF 3 in such a high concentration that fluorine-containing ions predominantly rather than oxygen ions are oxidised on the electrochemically active surfaces, however, only oxygen is evolved, the evolved oxygen being derived from the dissolved alumina present near the electrochemically active anode surfaces. 16. The cell of claim 15, wherein the temperature of the electrolyte is below 900°C, preferably below 880°C, even more preferably below 870°C. 17. The cell of claim 13 or 14, wherein the electrolyte contains cryolite and, in addition to cryolite, an excess of AlF 3 in an amount of at least 20 weight% of the electrolyte, preferably between 25 and 35 weight% of the electrolyte . 18. The cell of claim 15, 16 or 17, wherein the electrolyte further contains CaF 2 and/or MgF 2 . 19. The cell of any one of claims 15 to 18, wherein said alloying metal of the nickel alloy is iron. 20. The cell of any one of claims 15 to 19, wherein the outer part of the anodes comprises more than 75 weight% nickel, preferably between 85 and 95 weight% nickel. 21. The cell of any one of claims 15 to 20, wherein the nickel alloy has a decreasing concentration of said alloying metal towards the electrochemically active surface layer. 22. The cell of claim 21, wherein the nickel alloy has a nickel metal rich outer part with a porosity defining a high surface area electrochemically active surface, said porosity containing cavities which are partly or completely filled during use with fluorides of at least one metal selected from nickel, said alloying metal and aluminium. 23. The cell of claim 21 or 22, wherein the nickel metal rich outer part comprises nickel metal and said alloying metal in a nickel/alloying metal atomic ratio of more than 3 where it reaches the electrochemically active surface. 24. The cell of any one of claims 15 to 23, wherein the alloy of nickel with said alloying metal has before use a nickel/alloying metal ratio below 1. 25. The cell of any one of claims 15 to 23, wherein the alloy of nickel with said alloying metal has before use a nickel/alloying metal ratio of at least 1, in particular from 1 to 4. 26. The cell of any one of claims 15 to 25, wherein the alloy of nickel with said alloying metal contains one or more additives, the alloy before use containing nickel with said alloying metal in a total amount of at least 85 weight%, preferably at least 95 weight%, and the balance said additive(s). 27. The cell of claim 26, wherein one or more additives are selected from chromium, copper, cobalt, silicon, titanium, tantalum, tungsten, vanadium, yttrium, molybdenum, manganese, aluminium and niobium in a total amount of up to 10 weight%, in particular up to 5 weight%, of the alloy before use. 28. The cell of claim 26 or 27, wherein one or more additives are catalytically active and selected from iridium, palladium, platinum, rhenium, rhodium, ruthenium, tin or zinc metals, Mischmetals and their oxides and metals of the Lanthanide series and their oxides as well as mixtures and compounds thereof in a total amount of up to 5 weight% of the alloy before use. 29. The cell of any one of claims 15 to 28, wherein before anodic polarisation the nickel alloy is covered with an integral oxide film obtainable by oxidising the alloy in an oxidising atmosphere. 30. The cell of any one of claims 15 to 29, wherein each anode is a nickel iron alloy-based anode. 31. The cell of any one of claims 15 to 30, modified in that the nickel of the metal-based anodes, in particular of their outer part, is wholly or predominantly substituted by cobalt.

Inventors: DE NORA VITTORIO, DURUZ JEAN-JACQUES, DE NORA, VITTORIO, DURUZ, JEAN-JACQUES
Applicants: DURUZ JEAN JACQUES, MOLTECH INVENT SA, NORA VITTORIO DE, DE NORA, VITTORIO, DURUZ, JEAN-JACQUES, MOLTECH INVENT S.A.
Application year: 2000
Main classification: C25C3
IPC codes: C25C3