AU622000B2 - An anode substrate coated with rare earth oxycompounds - Google Patents

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

OPI DATE 09/10/90 APPLN. I D 51803 t P r.AOJP DATE 15/11/90 PCT NUMBER PCT/EP90/0O364 INTERNATIONAL APLCTOj;Lh A TflPAAET COOPERATION TREATY'(PCT)

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(51) International Patent Classification 5 :I(1Itra~a ~llcainNme:W 0 03 C25C3/2, /02Al (43) Internatioaal Publication Date: 20 September 1990 (20.09.90) International Application Number; PCT/EP90/00364 (74) Agent: CRONIN, Brian, Moltech 9, route de Troiflex, CH-1227 Carouge (22) International Filing Date: 6 March 1990'(06.03.90) (81) Designated States: AT (European patent), AU, BE (Euro- Pri~rity data: pean patent), BR, CA, CH (European patent), DE (Eu- <49810175.3 7 March 1989 (07,03 19) EP ropean patent), DK (European patent), ES (European (34) Countries for which, the regional patent), FR (European patent',, GB (European patent), or international application IT (European patent), JP, LU (European patent), NL was filed: AT et al. (European patent), NO, SE (European patent), SU, UC.

(71) Applicant (for all designated States aeept MOLTECH Published ,.IN VENT S.A. [LU/LU]; Boulevard de la Petrusse 68-70, With international search report.

*Box 0~61 10 13 Luxembourg Before the expiration of the time lim it for amending the and claims and to be republished in the event of the receipt of 72) Inventor; adamendments, (75) Irnventor/Applieant (for US only). JORDA, Jean-Louis [FR/ FR] Chaumont; F-74270 Frangy (FR).

(54)Tille: ANANODE SUBSTRATE COATED WITH R :RE EARTH OXYCOMPOUNDS (57) Abstract An at~de for electrowinning a metal from a molten salt electrolyte comprises an electrocond ctive oxycompound substrate -Which in use carries a surrace coating comprising at, least one iare earth oxycompodnd, The substri te comprises a sintered body composed of a nickel-copper-lithium oxide solid ksolutioll, wherein the solid soitution preferably contains 70-90 mol nickel oxide, 5-29 mol copper oxide an'&I10 mol li~hium oxide, f 42'- <2 14 WO 90/10735 PCT/EP90/00364 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. 7Th invention also relates to electrowinning processes using such anodes.

SACKGROUND 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 condition It is well known that ceramic materials have better resistance to chemical corrositon. However, their low electrical conductivity and difficulties of making mechanical and electrical contact Sas wel as difficulties in shapinguand mchiAi7ng these mat eals seriously limit thei r-ue.

US Patent 4 614 569 describes a method of i lectrowinning metals by electrolysis of a melt cot ining S a dissolved ,species of the metal t6 be woNusing a ode immersed in the melt wherein the anode has a metal, aloy or cermet substrate and an operative anode surface whh ch WO 90/10735 PCT/EP90/00364 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 cf cerium (referred to as "cerium oxyfluoride") alone or in combination with additives such as compounds of tantalum, niobium, yttrium, lanthanum, praesodynmium 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 I containing dissolved lumina, 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 Smetal, 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 ti1e 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' ~4ast one ceramic phase and at least one metallic phase', eomprising Smixed oxides of cerium with aluminum, nickel, iron and/or copper in the form of a skeleton interwoven with a continuous me llic network of an alloy ot intermetallic compound of cerium with aluminum, nickel, iron and/or copper, as described in EP-A-0 257 708. When used as Selectrode substrates, these materials have prormise, Sparticularly those based on cerium and aluminum because (k e 4 -3even 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 J 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, S 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 15 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 Publications 0 308 014 Al, 0 306 099 Al, 0 306 100 Al, 0 306 101 Al and 0 306 102 Al.

r;i WO 90/10735 PCI/EP90/00364 4 npublished These typically include a SAstrate 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 Sperformance over previous anodes, considerable development I is still required to optimize their lifetime and reduce the production cost.

1.0 C> 1I AOZ F1 SThe aforementioned European Patent ppliasa tie 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 Ssubstitute ceramic oxide spinels containing combinations of'-ivalent nickel, cobalt, magnesium, manganese, copper and zinc with divalent/trivalent nickel, cobalt, manganese and/or iron, and optionally dopants selected from Ti 4 4 4+ 4+ 4+ 4+ F3+ N 3 Zr Sn, Fe Hf, Me Fe Ni 3+ 3 Mn3+ 3 3 2+ 2+ 2+ Co Al Cr Ni Co 2+ 2+ 2+ 2+ V 2+ ,Mg Mn Cu Zn a'd 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 j oxyfluoride alone or in combination with other components.

o Todate, very littleoprogess has been made with o> date 411 I I WO90/10735 PCT/EP90/00364 anode substrates made of ceramic electroconductive oxycompou-nds-. 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 Fbstrate 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 subs rate comprises a sintered body composed of a nickel-copper-lithium oxide solid solution.

o f WO 90/10735 PC/EP90/00364 6 Preferably, the nickel oxide is presen;- in the solid ,iution in an amount of at least 70 mol%, the copper &xide is present in an amount of at most 29 mol% and the lithium oxide is present in an amount of at most 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 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 atom% lithium increases the conductivity of the (Ni-Cu)O solid solution by two orders of magnitude, up to about -i~ 200(ohm cm)- at 1000 0 C. This makes the material an attractive substrate material for cerium oxyfluoride coatings for aluminum electrowinning.

A ethod 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 thas conveniently e prepared by mixing powders of Li(NO 3 i 2

CO

3 or LiOH; CuO; and NiO in the right roportions and firing for exampl at 900-1050°C in air for about 24 hours at a heating rate of about 100°C/hour. Aftercogoling, the material is ground, 2 cold pressed (egat.l0 tons/cm and sintered at 1000-1300 0 C e.g. 1100-1150°C for 30-40 hours. The resulting sintered material shows a density of at least theoretical density, typically 80%, and an electrical conductivity of about 150' (o6 cm) 1 at 980 C compared to (om for the ndoped (Ni-C)O. A sinte to 1 (ohm cm' for the undoped (Ni-Cu)O. A sintereid 0 11 1 1 1 1 11 1 1 1 1 1 1 I k S-7specimen of such composition was tested as an anode substrate in a neutral cryolite containing 1.5% CeF 3 and 1.5 Ta 2 O5. 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 tantalum, niobium, ytt tum, lanthanum, praseodymium and other rare earth elements.

The invention also provides a method of electrowinning aluminum from milten cryolite containing alumina wherein an anode is immersed, the anode having an electroconductive oxycompound substrate carrying a surface coating S comprising at least one rare earth oxycompound, the surface coating being S maintained by the presence of cerium species in the molten cryolite, the method comprising passing electrolysis current between the anode according to the invention, or whose substrate is prepared by the method according to the invention, and a cathode to evolve oxygen and o maintain the surface coating at the anode and to produce aluminum at the cathode.

The invention will be further illustrated by the following Example.

0 1; o^VL A i WO 90/1 735 PCT/EP90/00364 .8 Example A Li. 05 Ni70Cu 250 sample was prepared using powder metallurgy techniques, Li(NO3), 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 pressed ,at 10 tons/cm 2 and sintered for 35 hours at 1150 0 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 Ta205 and 6g about CeF 3 Using a current density of 200 mA/cm a dense tantalum-doped cerium Soxy luoride coating was formed on the substrate at a rate ranging from 0.5S 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 respect ively for a window analysis; 76 atom% nickel and 23 atom% c pperfo a grain analysis. This stability of the S 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 conductiviy of the substrate which is strongly dependant on the lithium concentration-.

Claims (2)

19. -9- CLAIMS: 1. fAn anode for electrowinning a metal from a molten sal/t 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 'ickel-copper-lithium oxide solid solution. 2. The anode of claim 1, wherein the nickel oxide is Spresent in the solid solution in an amount of at least the copper oxide is present in an amount of at most 29mol% and the lithium oxide is present in an amount of at most 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. S-^oAe 4. The coingof 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 anod4 pof 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 comoound of tantalum, niobium, yttr lanthanum, praes dymium and other rare earth elements. 4 I i _ii. 7. An anode for electrowinning a metal from a molten salt electrolyte, substantially as herein described with reference to the Example. 8. A method of preparing the anode substrate of an anode according to any one of rlaims 1 to 7, 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. 9. A method of electrowinning aluminum from molten cryolite containing alumina, characterized by using an anode as claimed in any one of claims 1-7. The method of claim 9, wherein a surface coating comprising cerium oxyfluoride is maintained on the anode by the presence of cerium species in the molten oryolite, 11. A method of preparing the anode substrate of an anode according to any S one of claims 1 to 7 which method is substantially as herein described with reference to the Example. *00 12. A method of electrowinning aluminum from molten cryolite containing 0*0 alumina wherein an anode Is immersed, the anode having an electroconductive oxycompound substrate carrying a surface coating comprising at least one rare earth oxycompound, the surface coating being maintained by the presence of S. cerium species in the molten cryolite, the method complising passing electrolysis current between the anode according to any one of claims 1 to 7, or whose substrate is prepared by the method of claim 8, and a cathode to evolve oxygen and to maintain the surface coating at the anode and to produce aluminum at the 0he* cathode. DATED this 10th day of January 1992. SMOLTECH INVENT SA. By their Patent Attorneys: CALLINAN LAWRIE i .c 1 I I t<°~i.g 1) .4 I INTERNATIONAL SEARCH AEPORT International Application No POT/EP 90/00364 1. CLASSIFICA.TIPN OF SUIJECT MAt' 'ER (if sevoral classification symbols apply. indicate all) According to, itentional Patent Classification! 1PC)I or to both National Classiication and IPC C' 25 3/12, C 25 C 7/02 11. FiEIl,'.A SEARCHED L Minimum Documentation SearchedI Ctasification System, i Classification Symbols IPC C 25"C 3, C 25 C 7 Documentation Searched other thlan Minimum Documentation to the Extent that such Documents are tncluded In the Fields SearchedI III, DOCUMENTS CONSIDERED TO BE RELEVANT' Category *I Citation of ocument, 11 with Indication, where appropriate, of the relevant passages$2 Relevant to Claim No,.1 3 A EP, A, 0114085 (ELTECH SYSTEMS CORP.) July 1984 see pages 11-12; claims (cited in the application) EP, A, 0306102 (ELTECH SYSTEMS CORP.) 8 March 1989 see page, 5, lines 7-32 1,5,6,8,9 1,5,6,8,9, A C214 a00rl6 of C,di~djoctjmontsl If "Al docijnient 6ari,'ing the 'genoral slate of the art which (I not Considered to be of pfrlcular rliivanta "E/alrdocumornitby Published on or atear the Irtiernational filing dot#e document which may throw doubte on Priority claim(s) or which is cited to establisi~ the publication daie of another Citation~ Or Other special reii!yf sl 0 documori referring to an re 4dl"scosure, use exhibition or oth*0 means IP-0 dciurilont, Published oy or to the International fl 3 gdate but ,at*r than the priority<dale claimed later locumint published aft the International filing data or priority dale end not In cornict with the application but Ci1010 to Understand the principle or theory un terlying the Invention document of particular relevance; the claimed Invention cannot be considered novel or cannot be considered to lnqvo or% Inventive step document of Particular relevance;' the cialmod invention Cannot be considarert to Involve an Inventive step wheon the document is combined with. one or more other such docU. ments, such combination being obvious Ico a person skilled In the Art, 0AI document member of the same patent for 1 rilio 2 (N Inter I Setal rching Authority- EUI EWPATENT OFFICE form !oCT/lSk, ,f4s*c~nd sheetf) (Jantuary 405) I I I, ANNEX'TO TfHE INTERNATIONAL SEARCH REPORT ON INTEIZNATIONAL PATENT APPLICATION NO. EP 9000364 SA 34889 This annex lists the patent family members relating to the patent documents cited in the above-mentioned international search report. The members are as contained in the European Patent Office EDP ile on 03107/90 The European Patent Office in no way liable for these particulars which are merely given for tCie purpofe of information. IPatent document Pblication Patent family Publication citedin search report IT Pdate member(s)* date EP-A- 0114085 25-07-84 AU-B- AU-A- CA-A- DE-A- WO-A- JP -T- US-A- 578598 2415684 1257559 3467777 8402724 60500218 4614569 03-11-88 02-08-84 18-07-89 07-01-88 19-07-84 2 1-02-85

30-09-86 EA-0306102 08-03-89 AU-A,- 2320088 31-03-89 AU-A- 2327688 31-03-89 AU-A- 2424388 31-03-89 AU-A- 2428988 31-03-89 WO-A- 89,01991 09-03-89 WO-A- 8901992 09-03-89 WO-A- 8901993 09-03-89 WO-A- 8901994 09-03-89 EP-A- 0306099 08-03-89 EP-A- 0306100 08-03-89 EP-A- 0306101 08-08-89 /1 For more details: aout this annex, ee Official Journal of 1 Uropelarvatt"Pt Office, No. 1,219