AN ANODE SUBSTRATE COATED WITH RARE EARTH OXYCOMPOUNDS

Abstract: An anode for electrowinning a metal from a molten salt electrolyte comprises an electroconductive oxycompound substrate which in use carries a surface coating comprising at least one rare earth oxycompound. The substrate comprises a sintered body composed of a nickel-copper-lithium oxide solid solution, wherein the solid solution preferably contains 70-90 mol % nickel oxide, 5-29 mol % copper oxide and 1-10 mol % lithium oxide.

Description: AN ANODE SUBSTRATE COATED WITH RARE EARTH OXYCOMPOUNDS The invention relates to anodes for electrowinning metals such as aluminum from molten salt electrolytes, of the type comprising a substrate made of an electroconductive oxycompound which in use is coated with a surface coating comprising at least one rare earth oxycompound, typically including cerium oxyfluoride. The invention also relates to electrowinning processes using such anodes. BACKGROUND ART Materials used as non-consumable anodes in molten electrolytes must have a good stability in an oxidising atmosphere, good mechanical properties, good electrical conductivity and be able to operate for prolonged periods of time under polarising conditions. It is well known that ceramic materials have better resistance to chemical corrosion. However, their low electrical conductivity and difficulties of making mechanical and electrical contact as well as difficulties in shaping and machining these materials seriously limit their use. US Patent 4 614 569 describes a method of electrowinning metals by electrolysis of a melt containing a dissolved species of the metal to be won using an anode immersed in the melt wherein the anode has a metal, alloy or cermet substrate and an operative anode surface which is a protective surface coating containing a compound of a metal less noble than the metal to be electrowon, the protective coating being preserved by maintaining in the melt a suitable concentration of a species of this less noble metal. Usually the protective anode coating comprises a fluorine-containing oxycompound of cerium (referred to as "cerium oxyfluoride") alone or in combination with additives such as compounds of tantalum, niobium, yttrium, lanthanum, praesodymium and other rare earth elements, this coating being maintained by the addition of cerium and possibly other elements to the electrolyte. The electrolyte can be molten cryolite containing dissolved alumina, i.e. for the production of aluminum. This electrowinning method potentially has very significant advantages. To date, however, there remain problems with the anode substrate. When the substrate is a metal, alloy or cermet, it may be subject to oxidation leading to a reduced life of the anode, despite the excellent protective effect of the cerium oxyfluoride coating which protects the substrate from direct attack by corrosive electrolyte. When the substrate is a ceramic oxycompound, conductivity and corrosion are major problems, A promising solution to these problems has been the use of a ceramic/metal composite material of at least one ceramic phase and at least one metallic phase, comprising mixed oxides of cerium with aluminum, nickel, iron and/or copper in the form of a skeleton interwoven with a continuous metallic network of an alloy or intermetallic compound of cerium with aluminum, nickel, iron and/or copper, as described in EP-A-0 257 708. When used as electrode substrates, these materials have promise, particularly those based on cerium and aluminum because even if they corrode, this does not lead to corrosion products that contaminate the electrowon aluminum. Nevertheless corrosion of the substrate remains a problem. US Patent 4 374 050 discloses inert electrodes for aluminum production fabricated from at least two metals or metal compounds to provide a combination metal compound. For example, an alloy of two or more metals can be surface oxidised to form an oxycompound of the metals at the surface on an unoxidised alloy substrate. US Patent 4 374 761 discloses similar compositions further comprising a dispersed metal powder in compositions which may be applied as a preformed oxide composition on a metal substrate by cladding or plasma spraying. Such application techniques, however, are known to involve many drawbacks and the adhesion is particularly poor. US Patent 4 620 905 describes an oxidised alloy electrode based on tin or copper with nickel, iron silver, zinc, magnesium, aluminum and yttrium, either as a cermet or partially oxidised at its surface. Such partially oxidised alloys suffer serious disadvantages in that the oxide layers formed are far too porous to oxygen, and not sufficiently stable in corrosive environments. In addition, at high temperatures the partially oxidised structures continue to oxidize and this uncontrolled oxidation causes subsequent segregation of the metal and/or oxide layer. Adherence at the ceramic-metal interfaces is particularly difficult to achieve and this very problem has hampered use of such simple composites. Finally, none of these materials has proven satisfactory as substrate for cerium oxyfluoride coatings of the type discussed. Improved metal-based substrates are described in European Patent Applications 88201957.3, 88201851.8, 88201852.6, 88201853.4 and 88201854.2 all as yet unpublished. These typically include a substrate made of an alloy of chromium with nickel, cobalt and/or iron. On the surface of the substrate is a chromium oxide film on top of which is a layer of copper oxide in solid solution with nickel or manganese, obtained by oxidising a layer of nickel/copper or manganese/copper which is applied eg by electroplating. It was also mentioned that the nickel oxide in the surface layer may have its electrical conductivity improved by doping with lithium. Such composite layers nevertheless remain difficult to prepare and although they have demonstrated superior performance over previous anodes, considerable development is still required to optimize their lifetime and reduce the production cost. The aforementioned European Patent Application 88201854.2, mentions further embodiments of ceramic intermediate layers which in use serve as anchorage for the in-situ maintained protective coating of cerium oxyfluoride to the metal substrate, these intermediate layers including: nickel ferrite; copper oxide and nickel ferrite; doped, non-stoichiometric and partially substituted ceramic oxide spinels containing combinations of divalent nickel, cobalt, magnesium, manganese, copper and zinc with divalent/trivalent nickel, cobalt, manganese and/or iron, and optionally dopants selected from 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 2+ (see US patent No. 4 552 630); as well as coatings based on rare earth oxides and oxyfluorides, in particular pre-applied cerium oxyfluoride alone or in combination with other components. To date, very little progress has been made with anode substrates made of ceramic electroconductive oxycompounds. The most widely tested materials in this category on account of their acceptable conductivity have been based on tin dioxide. However, it has not yet been ■ possible to make an adequate electrode substrate based on tin dioxide despite expedients devised to reduce the amount of substrate material dissolved in the electrolyte. See for example EP-A-0 257 709 (E00208) which proposed doping the oxyfluoride coating with tantalum to render it more impervious and thereby reduce contamination of the electrolyte and the electrowon aluminum with tin from the substrate. SUMMARY OF THE INVENTION An object of the invention is to provide electrode substrates based on electroconductive oxycompounds which can be produced easily, have excellent conductivity and perform well as anode substrates when coated with an oxyfluoride-type coating. The invention is based on the realization that sintered copper-nickel oxide suitably doped with lithium oxide to enhance conductivity fulfills the sought-after requirements of a material for the anode substrate. According to the invention, an anode for electrowinning a metal from a molten salt electrolyte, comprising an electroconductive oxycompound substrate which in use carries a surface coating comprising at least one rare earth oxycompound, is characterized in that the substrate comprises a sintered body composed of a nickel-copper-lithium oxide solid solution. Preferably, the nickel oxide is present in the solid solution in an amount of at least 70 mol%, the copper oxide is present in an amount of at most 29 mol% and the lithium oxide is present in an amount of at most 10mol%. Preferably still, the solid solution contains 70-90 mol% nickel oxide, 5-29 mol% copper oxide and 1-10 mol% lithium oxide. The concentration the lithium dopant preferably ranges from 1 to 10 atom % with an optimum value at about 5 atom %, this usually in combination with about 70-80 mol% nickel oxide and about 20-25 mol% copper oxide. It has been shown that a concentration of 1 to 5 atom% lithium increases the conductivity of the (Ni-Cu)O solid solution by two orders of magnitude, up to about 200(ohm cm) " at 1000°C. This makes the material an attractive substrate material for cerium oxyfluoride coatings for aluminum electrowinning. A method of preparing an anode substrate according to the invention comprises mixing powders of nickel oxide, copper oxide and a compound of lithium, firing at 900-1100°C, cooling, grinding, cold pressing and sintering at 1000-1300°C for 30-40 hours. Shapes of lithium doped (Ni-Cu)O solid solution can thus conveniently be prepared by mixing powders of Li(NO_), Li 2 C0 3 or LiOH; CuO; and NiO in the right proportions and firing for example at 900-1050°C in air for about 24 hours at a heating rate of about 100°C/hour. After cooling, the material is ground, 2 cold pressed (eg at 10 tons/cm ) and sintered at 1000-1300°C e.g. 1100-1150°C for 30-40 hours. The resulting sintered material shows a density of at least 70% theoretical density, typically 80%, and an electrical conductivity of about 150 (ohm cm) at 980°C compared to 1 (ohm cm)- for the undoped (Ni-Cu)O. A sintered specimen of such composition was tested as an anode substrate in a neutral cryolite containing 1.5% CeF- and 1.5 Ta_0 5 . A very dense tantalum-doped cerium oxyfluoride coating was obtained. No noticeable change in the substrate composition near the interface was observed. The process may be optimized to improve densification by hot pressing and/or the addition of sintering aids. The anode substrate according to the invention can be used as a massive body supporting the rare earth oxycompound coating. But it can, if desired, incorporate a metal or other current collector to assist the supply of electric current and facilitate connection to the power supply. It has been observed that using the anode substrate according to the invention in a cryolite melt containing dissolved alumina and cerium species produces very dense, adherent and homogeneous cerium-oxyfluoride coatings. This is believed to be related to the presence of copper oxide in the substrate surface and to the surface porosity of the sintered material. The rare earth oxide coating may be cerium oxyfluoride alone or preferably may be cerium oxyfluoride together with at least one compound of tatalum, niobium, yttrium, lanthanum, praesodymium and other rare earth elements. The invention also provides a method of eletrowinning aluminum from molten cryolite containing alumina wherein an anode is immersed, the anode having an eletroconductive oxycompound substrate carying a surface coating comprising at least one rare earth oxycompound, the surface coating being maintained by the presence of cerium species in the molten cryolite, the method comprising passing electrolysis current between the anode and a cathode to evolve oxygen and to maintain the surface coating at the anode and to produce aluminum at the cathode. The invention will be further illustrated by the following Example. Exam le A Li. 5 Ni._ 0 Cu. 2 -.O sample was prepared using powder metallurgy techniques: Li(NO.,), CuO and NiO powders were mixed in the right proportions and fired at 1000°C in air for 24 hours. The heating rate was 100°C/hour. After cooling the specimen was powdered, cold 2 pressed at 10 tons/cm and sintered for 35 hours at 1150°C. The microstructure of the resulting sample showed a porosity of nearly 20% and CuO precipitates at the grain boundaries due to the slow cooling rate. A typical SEM-EDX 2 analysis over a window of about 0.25 mm gave nickel 71.0 atom% and copper 28.6 atom%, whereas for individual grains the composition was: nickel 76.6 atom% and copper 23.2 atom%. This analytical method is not -suitable for detecting the lithium. 2 An ingot with a surface area of 7.5 cm was prepared from this sample and exposed for 5 hours in a neutral cryolitic bath of 900g containing 1.5% of Ta 2 0_. and 6g (i.e. about 0.7%) CeF„ . Using a current density of 200 mA/cm , a dense tantalum-doped cerium oxyfluoride coating was formed on the substrate at a rate 2 ranging from 0.15 to 0.16 g/cm per hour. EDX analysis revealed that the concentration of nickel and copper did not significantly change during the cerium oxyfluoride deposition: 70 and 30 atom% for nickel and copper respectively for a window analysis; 76 atom% nickel and 23 atom% copper for a grain analysis. This stability of the composition of the sample is an indication of the protective role of the deposit for the cryolite in the previously mentioned conditions. The relative stability of the potential during the deposition may be related to the conductivity of the substrate which is strongly dependant on the lithium concentration.

Claims: CLAIMS : 1. An anode for electrowinning a metal from a molten salt electrolyte, comprising an electroconductive oxycompound substrate which in use carries a surface coating comprising at least one rare earth oxycompound, characterized in that the substrate comprises a sintered body composed of a nickel-copper-lithium oxide solid solution. 2. The anode of claim 1, wherein the nickel oxide is present in the solid solution in an amount of at least 70mol%, the copper oxide is present in an amount of at most 29mol% and the lithium oxide is present in an amount of at most 10mol%. 3. The anode of claim 2, wherein the solid solution contains 70-90 mol% nickel oxide, 5-29 mol% copper oxide and 1-10 mol% lithium oxide. 4. The coating of claim 3, wherein the solid solution contains about 70-80 mol% nickel oxide, about 20-25 mol% copper oxide and about 5 mol% lithium oxide. 5. The anode of any preceding claim, wherein the substrate is coated with a surface coating comprising cerium oxyfluoride. 6. The anode of claim 5, wherein the surface coating further comprises at least one compound of tantalum, niobium, yttrium, lanthanum, praesodymium and other rare earth elements. 7. A method of preparing the anode substrate of an anode according to any preceding claim, comprising mixing powders of nickel oxide, copper oxide and a compound of lithium, firing at 900-1100°C, cooling, grinding, cold pressing, and sintering at 1000-1300°C for 30-40 hours. 8. A method of electrowinning aluminum from molten cryolite containing alumina, characterized by using an anode as claimed in any one of claims 1-7. 9. The method of claim 8, wherein a surface coating comprising cerium oxyfluoride is maintained on the anode by the presence of cerium species in the molten cryolite. 10. A method of eletrowinning aluminum from molten cryolite containing alumina wherein an anode is immersed, the anode having an eletroconductive oxycompound substrate carying a surface coating comprising at least one rare earth oxycompound, the surface coating being maintained by the presence of cerium species in the molten cryolite, the method comprising passing electrolysis current between the anode and a cathode to evolve oxygen and to maintain the surface coating at the anode and to produce aluminum at the cathode.

Inventors: JORDA JEAN-LOUIS, JORDA, JEAN-LOUIS
Applicants: MOLTECH INVENT SA, MOLTECH INVENT S.A.
Application year: 1990
Main classification: C25C3
IPC codes: C25C3, C25C7