CA2498622C - Aluminium electrowinning cells with metal-based anodes - Google Patents
Aluminium electrowinning cells with metal-based anodes Download PDFInfo
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- CA2498622C CA2498622C CA2498622A CA2498622A CA2498622C CA 2498622 C CA2498622 C CA 2498622C CA 2498622 A CA2498622 A CA 2498622A CA 2498622 A CA2498622 A CA 2498622A CA 2498622 C CA2498622 C CA 2498622C
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- aluminium
- fluoride
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- 229910052782 aluminium Inorganic materials 0.000 title claims abstract description 73
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 title claims abstract description 73
- 239000004411 aluminium Substances 0.000 title claims abstract description 72
- 229910052751 metal Inorganic materials 0.000 title claims abstract description 26
- 239000002184 metal Substances 0.000 title claims abstract description 26
- 238000005363 electrowinning Methods 0.000 title claims abstract description 21
- 239000003792 electrolyte Substances 0.000 claims abstract description 98
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims abstract description 71
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims abstract description 61
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims abstract description 40
- 238000000576 coating method Methods 0.000 claims abstract description 37
- 239000011248 coating agent Substances 0.000 claims abstract description 34
- 229910052759 nickel Inorganic materials 0.000 claims abstract description 34
- NROKBHXJSPEDAR-UHFFFAOYSA-M potassium fluoride Chemical compound [F-].[K+] NROKBHXJSPEDAR-UHFFFAOYSA-M 0.000 claims abstract description 34
- KLZUFWVZNOTSEM-UHFFFAOYSA-K Aluminium flouride Chemical compound F[Al](F)F KLZUFWVZNOTSEM-UHFFFAOYSA-K 0.000 claims abstract description 32
- PUZPDOWCWNUUKD-UHFFFAOYSA-M sodium fluoride Chemical compound [F-].[Na+] PUZPDOWCWNUUKD-UHFFFAOYSA-M 0.000 claims abstract description 22
- 229910045601 alloy Inorganic materials 0.000 claims abstract description 20
- 239000000956 alloy Substances 0.000 claims abstract description 20
- 239000011698 potassium fluoride Substances 0.000 claims abstract description 20
- 239000010941 cobalt Substances 0.000 claims abstract description 19
- 229910017052 cobalt Inorganic materials 0.000 claims abstract description 19
- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt atom Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 claims abstract description 19
- 229910052742 iron Inorganic materials 0.000 claims abstract description 19
- 239000000470 constituent Substances 0.000 claims abstract description 17
- 235000003270 potassium fluoride Nutrition 0.000 claims abstract description 17
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 claims abstract description 11
- 239000010949 copper Substances 0.000 claims abstract description 11
- 235000013024 sodium fluoride Nutrition 0.000 claims abstract description 11
- 239000011775 sodium fluoride Substances 0.000 claims abstract description 11
- KRHYYFGTRYWZRS-UHFFFAOYSA-M Fluoride anion Chemical compound [F-] KRHYYFGTRYWZRS-UHFFFAOYSA-M 0.000 claims abstract description 10
- XVVDIUTUQBXOGG-UHFFFAOYSA-N [Ce].FOF Chemical compound [Ce].FOF XVVDIUTUQBXOGG-UHFFFAOYSA-N 0.000 claims abstract description 10
- 229910052802 copper Inorganic materials 0.000 claims abstract description 10
- WUKWITHWXAAZEY-UHFFFAOYSA-L calcium difluoride Chemical compound [F-].[F-].[Ca+2] WUKWITHWXAAZEY-UHFFFAOYSA-L 0.000 claims abstract description 9
- 229910001634 calcium fluoride Inorganic materials 0.000 claims abstract description 8
- 229910052595 hematite Inorganic materials 0.000 claims abstract description 8
- 239000011019 hematite Substances 0.000 claims abstract description 8
- LIKBJVNGSGBSGK-UHFFFAOYSA-N iron(3+);oxygen(2-) Chemical compound [O-2].[O-2].[O-2].[Fe+3].[Fe+3] LIKBJVNGSGBSGK-UHFFFAOYSA-N 0.000 claims abstract description 8
- 239000010955 niobium Substances 0.000 claims abstract description 6
- 229910052758 niobium Inorganic materials 0.000 claims abstract description 5
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 claims abstract description 5
- UQSXHKLRYXJYBZ-UHFFFAOYSA-N Iron oxide Chemical compound [Fe]=O UQSXHKLRYXJYBZ-UHFFFAOYSA-N 0.000 claims description 23
- GWEVSGVZZGPLCZ-UHFFFAOYSA-N Titan oxide Chemical compound O=[Ti]=O GWEVSGVZZGPLCZ-UHFFFAOYSA-N 0.000 claims description 11
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 10
- 229910052760 oxygen Inorganic materials 0.000 claims description 10
- 239000001301 oxygen Substances 0.000 claims description 10
- 239000000463 material Substances 0.000 claims description 8
- OGIDPMRJRNCKJF-UHFFFAOYSA-N titanium oxide Inorganic materials [Ti]=O OGIDPMRJRNCKJF-UHFFFAOYSA-N 0.000 claims description 8
- PQXKHYXIUOZZFA-UHFFFAOYSA-M lithium fluoride Chemical compound [Li+].[F-] PQXKHYXIUOZZFA-UHFFFAOYSA-M 0.000 claims description 6
- 229910000640 Fe alloy Inorganic materials 0.000 claims description 4
- XJHCXCQVJFPJIK-UHFFFAOYSA-M caesium fluoride Chemical compound [F-].[Cs+] XJHCXCQVJFPJIK-UHFFFAOYSA-M 0.000 claims description 4
- 239000010406 cathode material Substances 0.000 claims description 4
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 claims description 4
- 238000000034 method Methods 0.000 claims description 4
- 229910000480 nickel oxide Inorganic materials 0.000 claims description 4
- AHLATJUETSFVIM-UHFFFAOYSA-M rubidium fluoride Chemical compound [F-].[Rb+] AHLATJUETSFVIM-UHFFFAOYSA-M 0.000 claims description 4
- 238000009736 wetting Methods 0.000 claims description 4
- 229910001635 magnesium fluoride Inorganic materials 0.000 claims description 3
- GNRSAWUEBMWBQH-UHFFFAOYSA-N oxonickel Chemical compound [Ni]=O GNRSAWUEBMWBQH-UHFFFAOYSA-N 0.000 claims description 3
- KEAYESYHFKHZAL-UHFFFAOYSA-N Sodium Chemical compound [Na] KEAYESYHFKHZAL-UHFFFAOYSA-N 0.000 claims description 2
- OYLGJCQECKOTOL-UHFFFAOYSA-L barium fluoride Chemical compound [F-].[F-].[Ba+2] OYLGJCQECKOTOL-UHFFFAOYSA-L 0.000 claims description 2
- 229910001632 barium fluoride Inorganic materials 0.000 claims description 2
- QCCDYNYSHILRDG-UHFFFAOYSA-K cerium(3+);trifluoride Chemical compound [F-].[F-].[F-].[Ce+3] QCCDYNYSHILRDG-UHFFFAOYSA-K 0.000 claims description 2
- 239000011195 cermet Substances 0.000 claims description 2
- 239000003795 chemical substances by application Substances 0.000 claims description 2
- 229910000428 cobalt oxide Inorganic materials 0.000 claims description 2
- IVMYJDGYRUAWML-UHFFFAOYSA-N cobalt(ii) oxide Chemical compound [Co]=O IVMYJDGYRUAWML-UHFFFAOYSA-N 0.000 claims description 2
- 239000002019 doping agent Substances 0.000 claims description 2
- ORUIBWPALBXDOA-UHFFFAOYSA-L magnesium fluoride Chemical compound [F-].[F-].[Mg+2] ORUIBWPALBXDOA-UHFFFAOYSA-L 0.000 claims description 2
- 150000001247 metal acetylides Chemical class 0.000 claims description 2
- 150000004767 nitrides Chemical class 0.000 claims description 2
- FVRNDBHWWSPNOM-UHFFFAOYSA-L strontium fluoride Chemical compound [F-].[F-].[Sr+2] FVRNDBHWWSPNOM-UHFFFAOYSA-L 0.000 claims description 2
- 229910001637 strontium fluoride Inorganic materials 0.000 claims description 2
- 229910052715 tantalum Inorganic materials 0.000 claims description 2
- GUVRBAGPIYLISA-UHFFFAOYSA-N tantalum atom Chemical compound [Ta] GUVRBAGPIYLISA-UHFFFAOYSA-N 0.000 claims description 2
- 229910052799 carbon Inorganic materials 0.000 abstract description 14
- 239000000203 mixture Substances 0.000 description 25
- 239000010410 layer Substances 0.000 description 23
- 239000002245 particle Substances 0.000 description 13
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 11
- 238000005868 electrolysis reaction Methods 0.000 description 11
- 235000013980 iron oxide Nutrition 0.000 description 10
- 238000004519 manufacturing process Methods 0.000 description 10
- 230000000052 comparative effect Effects 0.000 description 7
- 229910001030 Iron–nickel alloy Inorganic materials 0.000 description 6
- -1 aluminium oxyfluoride ions Chemical class 0.000 description 5
- 238000011109 contamination Methods 0.000 description 5
- 238000004090 dissolution Methods 0.000 description 5
- DBJLJFTWODWSOF-UHFFFAOYSA-L nickel(ii) fluoride Chemical compound F[Ni]F DBJLJFTWODWSOF-UHFFFAOYSA-L 0.000 description 5
- 239000011148 porous material Substances 0.000 description 5
- ZLMJMSJWJFRBEC-UHFFFAOYSA-N Potassium Chemical compound [K] ZLMJMSJWJFRBEC-UHFFFAOYSA-N 0.000 description 4
- 150000001785 cerium compounds Chemical class 0.000 description 4
- 238000001816 cooling Methods 0.000 description 4
- 229910052700 potassium Inorganic materials 0.000 description 4
- 239000011591 potassium Substances 0.000 description 4
- 239000011253 protective coating Substances 0.000 description 4
- 230000001681 protective effect Effects 0.000 description 4
- YCKRFDGAMUMZLT-UHFFFAOYSA-N Fluorine atom Chemical compound [F] YCKRFDGAMUMZLT-UHFFFAOYSA-N 0.000 description 3
- 229910001610 cryolite Inorganic materials 0.000 description 3
- 239000011737 fluorine Substances 0.000 description 3
- 229910052731 fluorine Inorganic materials 0.000 description 3
- 239000007789 gas Substances 0.000 description 3
- 150000002500 ions Chemical class 0.000 description 3
- VBMVTYDPPZVILR-UHFFFAOYSA-N iron(2+);oxygen(2-) Chemical class [O-2].[Fe+2] VBMVTYDPPZVILR-UHFFFAOYSA-N 0.000 description 3
- 150000002739 metals Chemical class 0.000 description 3
- 230000003647 oxidation Effects 0.000 description 3
- 238000007254 oxidation reaction Methods 0.000 description 3
- 238000010079 rubber tapping Methods 0.000 description 3
- 239000000758 substrate Substances 0.000 description 3
- 239000002344 surface layer Substances 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- 229910052582 BN Inorganic materials 0.000 description 2
- PZNSFCLAULLKQX-UHFFFAOYSA-N Boron nitride Chemical compound N#B PZNSFCLAULLKQX-UHFFFAOYSA-N 0.000 description 2
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 2
- 239000004372 Polyvinyl alcohol Substances 0.000 description 2
- 239000002253 acid Substances 0.000 description 2
- 239000000654 additive Substances 0.000 description 2
- 230000004888 barrier function Effects 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 239000003575 carbonaceous material Substances 0.000 description 2
- 229910052804 chromium Inorganic materials 0.000 description 2
- 239000011651 chromium Substances 0.000 description 2
- GOECOOJIPSGIIV-UHFFFAOYSA-N copper iron nickel Chemical compound [Fe].[Ni].[Cu] GOECOOJIPSGIIV-UHFFFAOYSA-N 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 230000007797 corrosion Effects 0.000 description 2
- 238000009792 diffusion process Methods 0.000 description 2
- 229910002804 graphite Inorganic materials 0.000 description 2
- 239000010439 graphite Substances 0.000 description 2
- AMWRITDGCCNYAT-UHFFFAOYSA-L hydroxy(oxo)manganese;manganese Chemical compound [Mn].O[Mn]=O.O[Mn]=O AMWRITDGCCNYAT-UHFFFAOYSA-L 0.000 description 2
- 238000011065 in-situ storage Methods 0.000 description 2
- 239000011810 insulating material Substances 0.000 description 2
- UGKDIUIOSMUOAW-UHFFFAOYSA-N iron nickel Chemical compound [Fe].[Ni] UGKDIUIOSMUOAW-UHFFFAOYSA-N 0.000 description 2
- 238000002161 passivation Methods 0.000 description 2
- 229920002451 polyvinyl alcohol Polymers 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- 239000002002 slurry Substances 0.000 description 2
- 125000006850 spacer group Chemical group 0.000 description 2
- 229910052723 transition metal Inorganic materials 0.000 description 2
- 150000003624 transition metals Chemical class 0.000 description 2
- 229910052684 Cerium Inorganic materials 0.000 description 1
- 229910001313 Cobalt-iron alloy Inorganic materials 0.000 description 1
- 229910000881 Cu alloy Inorganic materials 0.000 description 1
- 229910000570 Cupronickel Inorganic materials 0.000 description 1
- DGAQECJNVWCQMB-PUAWFVPOSA-M Ilexoside XXIX Chemical compound C[C@@H]1CC[C@@]2(CC[C@@]3(C(=CC[C@H]4[C@]3(CC[C@@H]5[C@@]4(CC[C@@H](C5(C)C)OS(=O)(=O)[O-])C)C)[C@@H]2[C@]1(C)O)C)C(=O)O[C@H]6[C@@H]([C@H]([C@@H]([C@H](O6)CO)O)O)O.[Na+] DGAQECJNVWCQMB-PUAWFVPOSA-M 0.000 description 1
- 229910017709 Ni Co Inorganic materials 0.000 description 1
- KGWWEXORQXHJJQ-UHFFFAOYSA-N [Fe].[Co].[Ni] Chemical compound [Fe].[Co].[Ni] KGWWEXORQXHJJQ-UHFFFAOYSA-N 0.000 description 1
- 230000001464 adherent effect Effects 0.000 description 1
- 238000005054 agglomeration Methods 0.000 description 1
- 230000002776 aggregation Effects 0.000 description 1
- RHZUVFJBSILHOK-UHFFFAOYSA-N anthracen-1-ylmethanolate Chemical compound C1=CC=C2C=C3C(C[O-])=CC=CC3=CC2=C1 RHZUVFJBSILHOK-UHFFFAOYSA-N 0.000 description 1
- 239000003830 anthracite Substances 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- 239000011449 brick Substances 0.000 description 1
- 150000001721 carbon Chemical class 0.000 description 1
- 238000005524 ceramic coating Methods 0.000 description 1
- GWXLDORMOJMVQZ-UHFFFAOYSA-N cerium Chemical compound [Ce] GWXLDORMOJMVQZ-UHFFFAOYSA-N 0.000 description 1
- 239000008199 coating composition Substances 0.000 description 1
- 230000001427 coherent effect Effects 0.000 description 1
- 238000007596 consolidation process Methods 0.000 description 1
- 230000008602 contraction Effects 0.000 description 1
- YOCUPQPZWBBYIX-UHFFFAOYSA-N copper nickel Chemical compound [Ni].[Cu] YOCUPQPZWBBYIX-UHFFFAOYSA-N 0.000 description 1
- 229910000431 copper oxide Inorganic materials 0.000 description 1
- 238000007598 dipping method Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 230000003628 erosive effect Effects 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- 238000003682 fluorination reaction Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000007654 immersion Methods 0.000 description 1
- 239000012535 impurity Substances 0.000 description 1
- 229910001026 inconel Inorganic materials 0.000 description 1
- 238000009413 insulation Methods 0.000 description 1
- 238000009830 intercalation Methods 0.000 description 1
- 230000002687 intercalation Effects 0.000 description 1
- SZVJSHCCFOBDDC-UHFFFAOYSA-N iron(II,III) oxide Inorganic materials O=[Fe]O[Fe]O[Fe]=O SZVJSHCCFOBDDC-UHFFFAOYSA-N 0.000 description 1
- WPBNNNQJVZRUHP-UHFFFAOYSA-L manganese(2+);methyl n-[[2-(methoxycarbonylcarbamothioylamino)phenyl]carbamothioyl]carbamate;n-[2-(sulfidocarbothioylamino)ethyl]carbamodithioate Chemical compound [Mn+2].[S-]C(=S)NCCNC([S-])=S.COC(=O)NC(=S)NC1=CC=CC=C1NC(=S)NC(=O)OC WPBNNNQJVZRUHP-UHFFFAOYSA-L 0.000 description 1
- 239000011159 matrix material Substances 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 229910001092 metal group alloy Inorganic materials 0.000 description 1
- 230000005012 migration Effects 0.000 description 1
- 238000013508 migration Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- NQNBVCBUOCNRFZ-UHFFFAOYSA-N nickel ferrite Chemical compound [Ni]=O.O=[Fe]O[Fe]=O NQNBVCBUOCNRFZ-UHFFFAOYSA-N 0.000 description 1
- 230000002093 peripheral effect Effects 0.000 description 1
- 230000035699 permeability Effects 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 239000011241 protective layer Substances 0.000 description 1
- 239000011819 refractory material Substances 0.000 description 1
- 230000002787 reinforcement Effects 0.000 description 1
- 229910052710 silicon Inorganic materials 0.000 description 1
- 239000010703 silicon Substances 0.000 description 1
- 238000005245 sintering Methods 0.000 description 1
- 229910052708 sodium Inorganic materials 0.000 description 1
- 239000011734 sodium Substances 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 239000007921 spray Substances 0.000 description 1
- 238000003466 welding Methods 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- VWQVUPCCIRVNHF-UHFFFAOYSA-N yttrium atom Chemical compound [Y] VWQVUPCCIRVNHF-UHFFFAOYSA-N 0.000 description 1
- 229910000859 α-Fe Inorganic materials 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
- C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
- C25C7/02—Electrodes; Connections thereof
- C25C7/025—Electrodes; Connections thereof used in cells for the electrolysis of melts
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
- C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
- C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
- C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
- C25C3/12—Anodes
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
- C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
- C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
- C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
- C25C3/12—Anodes
- C25C3/125—Anodes based on carbon
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Electrolytic Production Of Metals (AREA)
Abstract
A cell for the electrowinning of aluminium comprises a metal-based anode (10) containing at least one of nickel, cobalt and iron, for example an anode made from an alloy consisting of 50 to 60 weight% in total of nickel and/or cobalt; 25 to 40 weight% iron; 6 to 12 weight% copper; 0.5 to 2 weight% aluminium and/or niobium; and 0.5 to 1.5 weight% in total of further constituents. The anode (10) may have an applied hematite-based coating and optionally a cerium oxyfluoride-based outermost coating. The cell contains a fluoride-containing molten electrolyte (5) at a temperature below 940~C, in which the anode is immersed and which consists of: 5 to 14 weight% dissolved alumina; 35 to 45 weight% aluminium fluoride; 30 to 45 weight% sodium fluoride; 5 to 20 weight% potassium fluoride; 0 to 5 weight% calcium fluoride; and 0 to 5 weight% in total of one or more further constituents. A nickel-containing anode stem (14b) can be used to suspend the anode (10) in the electrolyte facing a cathode (21,21A,25) that has an aluminium-wettable surface (20), in particular a drained horizontal or inclined surface.
Description
WO 2004/035871 _ 1 _ PCT/IB2003/004649 ALUMINIUM ELECTROWINNING CELLS WITH METAL-BASED ANODES
Field of the Invention This invention relates to aluminium electrowinning cells having metal-based anodes which contain at least one of nickel, iron and copper and which du~'ing use are inhibited from passivating and dissolving and from causing unacceptable contamination of the product aluminium.
Background Art The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950°C is more than one hundred years old and still uses carbon anodes and cathodes.
Using metal anodes in commercial aluminium electrowinning cells would be new and drastically improve the aluminium process by reducing pollution and the cost of aluminium production.
US Patents 4,614,569 (Duruz/Derivaz/Debely/
Adorian), 4,680,094 (Duruz), 4,683,037 (Duruz) and 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 anode 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 (Nguyen/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 Nguyen/Lazouni/Doan)
Field of the Invention This invention relates to aluminium electrowinning cells having metal-based anodes which contain at least one of nickel, iron and copper and which du~'ing use are inhibited from passivating and dissolving and from causing unacceptable contamination of the product aluminium.
Background Art The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950°C is more than one hundred years old and still uses carbon anodes and cathodes.
Using metal anodes in commercial aluminium electrowinning cells would be new and drastically improve the aluminium process by reducing pollution and the cost of aluminium production.
US Patents 4,614,569 (Duruz/Derivaz/Debely/
Adorian), 4,680,094 (Duruz), 4,683,037 (Duruz) and 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 anode 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 (Nguyen/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 Nguyen/Lazouni/Doan)
- 2 _ PCT/IB2003/004649 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 6,248,227 (de Nora/Duruz) discloses an aluminium electrowinning anode having a metallic anode body which can be made of various alloys, for example a nickel-iron-copper alloy. 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 is 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.
US Patent 6,372,099 (Duruz/de Nora) discloses the use of transition metal species in an electrolyte below 910°C of an aluminium electrowinning cells to inhibit dissolution of metal-based anodes of the cell.
W000/06803 (Duruz/de Nora/Crottaz) and WO00/06804 (Crottaz/Duruz) both disclose an anode produced from a nickel-iron alloy which is surface oxidised to form a coherent and adherent outer iron oxide-based layer whose surface is electrochemically active. WO00/06804 also mentions that the anode may be used in an electrolyte at a temperature of 820° to 870°C containing 23 to 26.5 weight o AlF3, 3 to 5 weight o A1203, 1 to 2 weight a ZiF
and 1 to 2 weighty MgF2.
US Patents 5,006,209 and 5,284,562 (both Beck/Brooks), 6,258,247 and 6,379,512 (both Brown/
Brooks/Frizzle/Juric), 6,419,813 (Brown/Brooks/Frizzle) and 6,436,272 (Brown/Frizzle) all disclose the use of nickel-copper-iron anodes in an aluminium production electrolyte at 660°-800°C containing 6-26 weighto NaF, 7-33 weighto KF, 1-6 weighto ZiF and 60-65 weighto A1F3.
The electrolyte may contain A1203 in an amount of up to 30 weighto, in particular 5 to 10 or 15 weight%, most of which is in the form of suspended particles and some of which is dissolved in the electrolyte, i.e. typically 1 to 4 weighto dissolved A1203. In US Patents 6,258,247, 6,379,512, 6,419,813 and 6,436,272 such an electrolyte WO 2004/035871 _ 3 _ PCT/IB2003/004649 is said to be useable at temperatures up to 900°C. In US
Patents 6,258,247 and 6,379,512 the electrolyte further contains 0.004 to 0.2 weighto transition metal additives to facilitate alumina dissolution and improve cathodic operation.
US Patent 5,725,744 (de Nora/Duruz) discloses an aluminium production cell having anodes made of nickel, iron and/or copper in a electrolyte at a temperature from 680 ° to 880 °C containing 42-63 weight o A1 F3, up to 48 weight o NaF, up to 48 weight o LiF and 1 to 5 weight o A1203. MgF~, KF and CaF2 are also mentioned as possible bath constituents.
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 was too short and needs to be increased.
Summarv of the Invention One object of the invention is to provide an aluminium electrowinning cell incorporating metal-based anodes which remain substantially insoluble at the cell operating temperature and which can be operated without passivation or excessive contamination of the produced aluminium.
Another object of the invention is to provide an aluminium electrowinning cell operating with a crustless and ledgeless electrolyte, which can achieve high productivity, low contamination of the product aluminium, and whose components resist corrosion and wear.
The invention relates to a cell for electrowinning aluminium from alumina. The cell comprises: a metal-based anode having an outer part that contains at least one of nickel, cobalt and iron and that has an electrochemically active oxide-based surfaced and a fluoride-containing molten electrolyte at a temperature below 940°C, in particular in the range from 880° to 920°C, in which the active anode surface is immersed.
The electrolyte consists of : 5 to 14 weight o overall of WO 2004/035871 _ 4 _ PCT/IB2003/004649 dissolved alumina; 35 to 45 weighto aluminium fluoride;
30 to 45 weighto sodium fluoride; 5 to 20 weighto potassium fluoride; 0 to 5 weighto calcium fluoride; and 0 to 5 weighto in total of one or more further constituents.
For instance, the electrolyte consists of: 7 to 10 weighto dissolved alumina; 38 to 42 weight% aluminium fluoride; 34 to 43 weighto sodium fluoride; 8 to 15 weighto potassium fluoride; 2 to 4 weighto calcium fluoride; and 0 to 3 weighto in total of one or more further constituents.
Such an electrolyte composition is well adapted for aluminium electrowinning at reduced temperature, i.e. at a temperature below the conventional aluminium electrowinning temperature of about 950°C, using a metal-based anode containing at least one of nickel, cobalt and iron, usually in metallic and/or oxide form.
The electrolyte is particularly adapted for anodes containing at least one of metallic nickel, metallic cobalt and oxides of iron. Oxides of iron include ferrous oxide, hematite, magnetite and ferrites (e. g.
nickel ferrite), in stoichiometric and non-stoichiometric form. For example, the anode has a metallic alloy body that contains one or more of these metals - nickel, cobalt and iron - and that is covered with an integral active oxide layer or film.
The presence in the electrolyte of potassium fluoride in the given amount has two effects. On the one hand, it leads to a reduction of the operating temperature by up to several tens of degrees without increase of the electrolyte's aluminium fluoride content or even a reduction thereof compared to standard electrolytes operating at about 950°C with an aluminium fluoride content of about 45 weighto. On the other hand, it maintains a high solubility of alumina, i.e. up to above about 14 weighto, in the electrolyte even though the temperature of the electrolyte is reduced by a few tens of degrees compared to conventional temperature.
Hence, in contrast to prior art low temperature electrolytes which carry large amounts of undissolved alumina in particulate form, according to the present invention a large amount of alumina in the electrolyte is in a dissolved form.
s~:n ~ without being bound to any theory, it is believed that combining a high concentration of dissolved alumina in the electrolyte and a limited concentration of aluminium fluoride leads predominantly to the formation of (basic) fluorine-poor aluminium vxyfluoride ions ( [A,1202F41 Z-) instead of (acid) fluorine-rich aluminium vxyfluoride ions ( [A120F6]2-) near the anode. As opposed to acid fluorine-rich aluminium oxyfluoride ions, basic fluorine-poor aluminium ~oxyfluoride ions do not significantly paaeivate the anode's nickel and cobalt, or dissolve the anode's iron. In particular, basic fluorine-poor aluminium oxyfluoride ions dv not significantly passivate metallic nickel and cobalt, or dissolve iron oxides. The weight ratio of dissolved alumina/alurninium fluoride in the electrolyte should be above 1/7, and often above 1/6.5 or even above 1/6, to vbta3~n a favourable ratio of the fluorine-poor aluminium oxyfluoride ions and the fluorine-rich aluminium oxyfluoride ions.
It follows that the use of the above described electrolyte with metal-based anodes containing at Isast one of nickel, cobalt and iron inhibits paaaivation and corrosion thereof.
In order to maintain the alumina concentration abvv~ the given threshold during normal electrolysis, the cell is preferably fitted with means tv monitor and adjust the electrolyte's alumina content.
The abovernentioned one yr more further constituents of the electrolyte may comprise at least one fluoride selected from magnesium fluoride, lithium fluoride, cesium fluoride, rubidium fluoride, strontium fluoride, barium fluoride and cerium fluoride_ Advantageously, the cell is sufficiently insulated tv be operated with a substantially crustless and/or ledgeless electrvlytp_ Suitable cell insulation is disclosed in U5 Patent 6,402,928 (de Nora/Sekhar), W002/070784 and US Publicativzl 2003/0102228 (both de Nara/Herclaz).
The cell can have a cathode that has an aluminium-4o wettabla surface, in particular a drained horiaontal or inclined surface. Suitable cathode designs are for examples disclosed in US Patents 5,683,559, 5,888,360, 6,093,304 (all de Nora), 6,258,246 (Duruz/de Nora), 6,358,393 (Berclaz/de Nora) and 6,436,273 (de Nora/Duruz), and in PCT publications W099/02764 (de Nora/Duruz), WO00/63463 (de Nora), W001/31086 (de Nora/Duruz), W001/31088 (de Nora), W002/070785 (de Nora), W002/097168 (de Nora), W002/097168 (de Nora), W003/023091 (de Nora) and W003/023092 (de Nora).
The cathode can have an aluminium-wettable coating that comprises a refractory boride and/or an aluminium-wetting oxide. Suitable aluminium-wettable materials are disclosed in W001/42168 (de Nora/Duruz), W001/42531 (Nguyen/Duruz/de Nora), W002/070783 (de Nora), W002/096831 (Nguyen/de Nora) and W002/096830 (Duruz/
Nguyen/de Nora).
The anode can have a metallic or cermet body and an oxide layer integral with or applied on the anode body.
Usually, the anode body is made from an iron alloy, in particular an alloy of iron with nickel and/or cobalt. Suitable alloys are disclosed in US Patents 6,248,227 (de Nora/Duruz), 6,521,115 (Duruz/de Nora/Crottaz), 6,562,224 (Crottaz/Duruz), and in PCT
publications W000/40783 (de Nora/Duruz), W001/42534 (de Nora/Duruz), W001/42536 (Duruz/Nguyen/de Nora), W002/083991 (Nguyen/de Nora), W003/014420 (Nguyen/Duruz/de Nora) and W003/078695 (Nguyen/de Nora).
For example, the anode body is made from an alloy consisting of:
- 40 to 80o nickel and/or cobalt, in particular 50 to 60 weighto;
- 9 to 55 weighto iron, in particular 25 to 40 weight%;
- 5 to 15 weight o copper, in particular 6 to 12 weight o ;
- 0 to 4 weighto in total of at least one of aluminium, niobium and tantalum, in particular 0.5 to 2 weighto; and - 0 to 2 weighto in total of further constituents, in particular 0.5 to 1 weighto.
Typically such an alloy is oxidised prior to or during use. This can lead to diffusion of metals in the anode, especially at the alloy's surface, which locally changes the alloy's composition.
WO 2004/035871 _ ~ _ PCT/IB2003/004649 The anode body can be covered with an integral iron oxide-based layer containing less than about 35 weighto nickel oxide and/or cobalt oxide, in particular from 5 to 10 weighto nickel oxide. Such integral layers are usually obtained by preoxidation of the body before and/or during use in the cell.
The anode may also comprise an applied iron oxide-based coating. Suitable iron oxide-based coatings are disclosed in US Patents 6,361,681 (de Nora/Duruz), 6,365,018 (de Nora), 6,379,526 (de Nora/Duruz) and 6,413,406 (de Nora), and in PCT applications PCT/IB03/01479, PCT/IB03/03654 and PCT/IB03/03978 (all Nguyen/de Nora). For example, the anode coating contains Fe~03 and optionally: at least one dopant selected from TiO~, Zn0 and Cu0 and/or at least one inert material selected from nitrides and carbides.
Especially when used in the upper part of the abovementioned operating temperature range (e.g. 910°-940°C), the anode can comprise an applied cerium oxyfluoride-based outermost coating, for example as disclosed in the abovementioned US Patents 4,614,569, 4,680,094, 4,683,037 and 4,966,674 or PCT Applications W002/070786 (Nguyen/de Nora) and W002/083990 (de Nora/Nguyen). Such a coating may be applied before or during use and maintained during use by the presence of cerium species in the electrolyte.
A nickel-containing stem can be used to suspend the anode in the electrolyte, in particular a stem having a nickel-containing core covered with an applied oxide coating, such as a coating containing aluminium oxide and titanium oxide. The core of the stem can comprise a copper inner part and a nickel-based outer part. Further details of anode stems are disclosed in PCT/IB03/02702 (Crottaz/Duruz).
Suitable anode designs are for example disclosed in W099/02764 (de Nora/Duruz), W000/40781, W000/40782, W003/023091, W003/023092 and W003/006716 (all de Nora).
Usually, the cell comprises at least one component, e.g. the cathode, that contains a sodium-active cathodic material, such as elemental carbon. This sodium-active cathodic material is preferably shielded from the electrolyte by a sodium-inert layer to inhibit the WO 2004/035871 _ 8 _ PCT/IB2003/004649 presence in the molten electrolyte of soluble cathodically-produced sodium metal that constitutes an agent for dissolving the active oxide-based anode surface. This mechanism is explained in greater detail in US Application 2003/0075454 and W003/083176 (both de Nora/Duruz).
The invention also relates to a cell that comprises:
- a metal-based anode having an outer part that has an electrochemically active oxide-based surface and that is made from an alloy consisting of : 50 to 60 weight o in total of nickel and/or cobalt; 25 to 40 weight%
iron; ~ to 12 weighto copper; 0.5 to 2 weighty aluminium and/or niobium; and 0.5 to 1.5 weighto in total of further constituents, the anode comprising an applied hematite-based coating and optionally a cerium oxyfluoride-based outermost coating;
- a nickel-containing anode stem for suspending the anode in the electrolyte, the stem being covered with a coating of aluminium oxide and titanium oxide;
- a fluoride-containing molten electrolyte at a temperature in the range from 880° to 920 or 930°C, in which the active anode surface is immersed and which consists of : 7 to 10 weight o dissolved alumina; 38 to 42 weighto aluminium fluoride; 34 to 43 weighto sodium fluoride; 8 to 15 weighto potassium fluoride; 2 to 4 weighto calcium fluoride; and 0 to 3 weighto in total of one or more further constituents; and - a cathode having an aluminium-wettable surface, in particular a drained horizontal or inclined surface, formed by an aluminium-wettable coating of refractory hard material and/or aluminium-wetting oxide.
A further aspect of the invention relates to a method of electrowinning aluminium in a cell as described above. The method comprises electrolysing the dissolved alumina to produce oxygen on the anode and aluminium cathodically, and supplying alumina to the electrolyte to maintain therein a concentration of dissolved alumina of 5 to 14 weight%, in particular 7 to 10 weighto.
Brief Description of Drawings The invention will be further described with reference to the accompanying drawings, in which:
WO 2004/035871 - 9 _ PCT/IB2003/004649 - Figures 1a and 1b schematically show respectively a side elevation and a plan view of an anode for use in a cell according to the invention;
- Figures 2a and 2b show a schematic cross-sectional view and a plan view, respectively, of an aluminium production cell for equipment with a potassium fluoride-containing electrolyte and a metal-based anode according to the invention; and - Figure 3 shows a schematic cross-sectional view of another aluminium production cell for equipment with a potassium fluoride-containing electrolyte and a metal-based anode according to the invention.
Detailed Description Figures 1a and 1b schematically show an anode 10 which can be used in a cell for the electrowinning of aluminium according to the invention.
The anode 10 comprises a series of elongated straight anode members 15 connected to a cast or profiled support 14 for connection to a positive bus bar.
The cast or profiled support 14 comprises a lower horizontally extending foot 14a for electrically and mechanically connecting the anode members 15, a stem 14b for connecting the anode 10 to a positive bus bar and a pair of lateral reinforcement flanges 14c between the foot 14a and stem 14b.
The anode members 15 may be secured by force-fitting or welding the foot 14a on flats 15c of the anode members 15. As an alternative, the connection between the anode members 15 and the corresponding receiving slots in the foot 14a may be shaped, for instance like dovetail joints, to allow only longitudinal movements of the anode members.
The anode members 15 have a bottom part 15a which has a substantially rectangular cross-section with a constant width over its height and which is extended upwardly by a tapered top part 15b with a generally triangular cross-section. Each anode member 15 has a flat lower oxide surface 16 that is electrochemically - zo -active for the anodic evolution of oxygen during operation of the cell. Also, the anode may be covered with a coating of iron oxide-based material, for example applied from a composition as set out in Table III
below, and/or a coating of one or more cerium compounds in particular cerium oxyfluoride.
The anode members 15, in particular their bottom parts 15a, are made of an iron alloy comprising nickel and/or cobalt as disclosed in Table II below. The lifetime of the anode may be increased by a protective coating made of cerium compounds, in particular cerium oxyfluoride as discussed above.
The anode members 15 are in the form of parallel rods in a coplanar arrangement, laterally spaced apart from one another by inter-member gaps 17. The inter-member gaps 17 constitute flow-through openings for the circulation of electrolyte and the escape of anodically-evolved gas released at the electrochemically active surfaces 16.
Figure 2a and 2b show an aluminium electrowinning cell having a series of metal-based anodes 10 in a fluoride-containing cryolite-based molten electrolyte 5 containing dissolved alumina according to the invention.
The electrolyte 5 has a composition that is selected from Table I below. The metal-based anodes 10 have a composition selected from Table II below, optionally with a protective coating made of cerium compounds, in particular cerium oxyfluoride as discussed above.
The anodes 10 are similar to the anode shown in Figs. 1a and 1b. Suitable alternative anode designs are disclosed in W000/40781, W000/40782 and W003/006716 (all de Nora ) .
The drained cathode surface 20 is formed by tiles 21A which have their upper face coated with an aluminium-wettable layer. Each anode 10 faces a corresponding tile 21A. Suitable tiles are disclosed in greater detail in W002/096830 (Duruz/Nguyen/de Nora).
Tiles 21A are placed on upper aluminium-wettable faces 22 of a series of carbon cathode blocks 25 extending in pairs arranged end-to-end across the cell.
As shown in Figures 2a and 2b, pairs of tiles 21A are spaced apart to form aluminium collection channels 36 that communicate with a central aluminium collection groove 30.
The central aluminium collection groove 30 is located in or between pairs of cathode blocks 25 arranged end-to-end across the cell. The tiles 21A
preferably cover a part of the groove 30 to maximise the surface area of the aluminium-wettable cathode surface 20.
As explained hereafter, the cell is thermally sufficiently insulated to enable ledgeless and crustless operation.
The cell comprises sidewalk 40 made of an outer layer of insulating refractory bricks and an inner layer of carbonaceous material exposed to molten electrolyte 5 and to the environment thereabove. These sidewalls 40 are protected against the molten electrolyte 5 and the environment thereabove with tiles 21B of the same type as tiles 21A. The cathode blocks 25 are connected to the sidewalls 40 by a peripheral wedge 41 which is resistant to the molten electrolyte 5.
Furthermore, the cell is fitted with an insulating cover 45 above the electrolyte 5. This cover inhibits heat loss and maintains the surface of the electrolyte in a molten state. Further details of suitable covers are disclosed in the abovementioned references.
In operation of the cell illustrated in Figs. 2a and 2b, alumina dissolved in the molten electrolyte 5 at a temperature of 880° to 940°C is electrolysed between the anodes 10 and the cathode surface 20 to produce gas on the operative anodes surfaces 16 and molten aluminium on the aluminium-wettable drained cathode tiles 21A.
The cathodically-produced molten aluminium flows on the drained cathode surface 20 into the aluminium collection channels 36 and then into the central aluminium collection groove 30 for subsequent tapping, WO 2004/035871 _ 12 - PCT/IB2003/004649 The cell shown in Figure 3 comprises a plurality of metal-based anodes 10 dipping in a molten electrolyte 5 according to the invention.
The anodes 10 are similar to the anode shown in Figs. la and lb. Suitable alternative anode designs are disclosed in W000/40781, W000/40782, W003/006716 and W003/023092 (all de Nora).
The cell bottom comprises a series of pairs of spaced apart carbon cathode blocks 25 placed across the cell and having an aluminium-wettable upper surface 22 formed by an aluminium-wettable layer. The upper surfaces 22 are covered with aluminium-wettable openly porous plates 21 which are filled with molten aluminium to form an aluminium-wetted drained active cathode surface 20 above the upper surfaces 22 of the carbon cathode blocks 25. Further details of such a cathode bottom are disclosed in W002/097168 and W002/097169 (both de Nora).
The cathode blocks 25 are made of graphite and have a reduced height, e.g. 30 cm, and are coated with an aluminium-wettable layer which forms the upper surface 22 and which protects the graphite from erosion and wear. Suitable aluminium-wettable layers are disclosed in US Patent 5,651,874, W098/17842, W001/42168 and W001/42531. The aluminium-wettable openly porous plates 21 covering the coated cathode blocks 25 can be made of the material disclosed in W002/070783 (de Nora).
The cell bottom further comprises a centrally-located recess 35 which extends at a level below the upper surfaces 22 of the carbon cathode blocks 25 and which during use collects molten aluminium 60 drained from the aluminium-wettable drained active cathode surface 20.
The aluminium collection recess 35 is formed in a reservoir body 30 which is placed between the blocks 25 of each pair of cathode blocks and spaces them apart across the cell. As shown in Figure 3, the recess 35 formed in the reservoir body 30 is generally U-shaped with rounded lower corners and an outwardly curved upper part.
WO 2004/035871 _ 13 _ PCT/IB2003/004649 The reservoir body 30 is made of two generally Z-shaped sections 31 assembled across the cell. The reservoir sections 31 are made of anthracite-based material. The aluminium-wettable layer forming the upper surfaces 22 extends in the recess 35 to protect the reservoir body 30 during use against wear and sodium or potassium intercalation.
As shown in Figure 3, the reservoir body 30 extends below the cathode blocks 25 into the refractory and insulating material 26 of the cell bottom permitting maximisation of the capacity of the aluminium collection recess 35.
Furthermore, the reservoir body 30 has a solid base 32 which extends from above to below the bottom face of the cathode blocks 25 and provides sufficient mechanical resistance to keep the blocks 25 properly spaced apart across the cell when exposed to thermal expansion during start-up of the cell and normal operation. As shown in dotted lines in the upper part of the reservoir body 30, longitudinally spaced apart spacer bars 33 placed across the reservoir body 30 may provide additional mechanical strength to the reservoir body 30. Such spacer bars 33 can be made of carbon material coated with an aluminium-wettable protective layer.
The openly porous plates 21 placed on the upper surfaces 22 of the carbon cathode blocks 25 and located in the central region of the cell bottom extend over part of the aluminium collection recess 35 so that during use the protruding part of the aluminium-wetted drained active cathode surface 20 is located over the recess 35.
The openly porous plates 21 are spaced apart over the aluminium collection recess 35 to leave an access for the tapping of molten aluminium through a conventional tapping tube. The spacing between the openly porous plates 21 over the aluminium collection recess can be much smaller along the remaining parts of the recess 35, thereby maximising the surface area of the active cathode surface 20.
The cell shown in Figure 3 comprises a series of corner pieces 41 made of the same openly porous material as plates 21 and filled with aluminium and placed at the periphery of the cell bottom against sidewalls 40. The sidewalls 40 and the surface of the electrolyte 5 are covered with a ledge and a small crust of frozen electrolyte 6. The cell is fitted with an insulating cover 45 above the electrolyte crust 6. Further details of suitable covers are disclosed in the abovementioned references.
The cell is also provided with exhaust pipes (not shown) that extend through the cover 45 for the removal of gases produced during electrolysis.
The cell comprises alumina feeders 50 with feeding tubes 51 that extend through the insulating cover 45 between the anodes 10. The alumina feeders 50 are associated with a crust breaker (not shown) for breaking the crust 6 underlying the feeding tube 51 prior to feeding.
In a variation, the insulating material of the sidewalls 40 and cover 45 may be sufficient to prevent formation of any ledge and crust of frozen electrolyte.
In such a case, the sidewalls 40 are preferably completely shielded from the molten electrolyte 5 like in the cell of Figs. 2a and 2b or by a lining of the aforesaid openly porous material filled with aluminium.
Enhanced alumina dissolution may be achieved by utilising an alumina feed device which sprays and distributes alumina particles over a large area of the surface of the molten electrolyte 5. Suitable alumina feed devices are disclosed in US Patent 6,572,757 (de Nora/Berclaz) and in W003/006717 (Berclaz/Duruz).
Furthermore, the cell may comprise means (not shown) to promote circulation of the electrolyte 5 from and to the anode-cathode gap to enhance alumina dissolution in the electrolyte 5 and to maintain in permanence a high concentration of dissolved alumina close to the active surfaces of anodes 10, for example as disclosed in 60000/40701 (de Nora) .
During operation of the cell shown in Figure 3, alumina dissolved in the electrolyte 5 is electrolysed to produce oxygen on the anodes 10 and aluminium 60 on the drained cathode surfaces 20. The product aluminium 60 drains from the cathode surfaces 20 over the openly porous plates 21 that extend over part of the reservoir 30 into the reservoir 30 from where it can be tapped.
Hence, aluminium is produced on the drained active cathode surface 20 which covers not only the cathode blocks 25 but also part of the reservoir 30, thereby maximising the useful aluminium production area (i.e.
the drained cathode surface 22) of the cell.
Figs. 2a, 2b and 3 show specific aluminium electrowinning cells by way of example. It is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art.
For instance, the cell may have a sloping cathode bottom, as disclosed in W099/02764 (de Nora/Duruz), and optionally one or more aluminium collection reservoirs across the cell, each intersecting the collection groove to divide the drained cathode surface into four quadrants as described in W000/63463 (de Nora).
Examples of electrolyte compositions according to the invention are given in Table 1, which shows the weight percentages of the indicated constituents for each specimen electrolyte A1-Il at a given temperature.
TABLE l AlF3 NaF KF CaF~ A1~03 TC
A1 40.4 42.6 6 3 8 935 B1 40.6 41.4 7 3 8 930 C1 40.4 39.6 9 3 8 915 Dl 40.2 37.8 11.5 2.5 8 900 E1 43.5 40 6.5 2 g 8g5 F1 40 3~ 13 3 g 8g0 H1 36 36.5 16 3.5 8 880 Examples of alloy compositions of suitable metal-based anode are given in Table 2, which shows the weight percentages of the indicated metals for each specimen alloy A2-K2.
Ni Co Fe Cu Al Nb Ta other A2 57 - 30 1fl 2 - - 1 D2 25 43 25 7 - - - _ E2 - 42 50 6 0.5 - 1 0.5 F2 - 45 45 9 - - _ 1 G2 25 25 38 10 - 2 - _ H2 45 - 40 11 - - 2.5 1.5 The "other" elements refer to minor additives such as manganese, silicon and yttrium which may be present in individual amounts of 0.2 to 1.5 weighto. Usual impurities, such.as carbon, have not been listed in Table 2.
Usually, these alloys will be surface oxidised before use and further oxidised during use, as described in the Examples below.
Examples of starting compositions of particle mixtures for producing hematite-based protective anode coatings are given in Table 3, which shows the weight percentages of the indicated constituents for each specimen starting composition of the coating A3-L3.
Fe203 BN A1N ZrC Ti02 Zr02 Zn0 Ta~05 Cu0 A3 78 10 __ __ 10 __ __ __ 2 B3 78 10 __ __ __ __ 10 _- 2 D3 78 10 __ __ __ 10 __ __ 2 F3 78 10 __ __ __ __ __ 10 2 G3 78 -- 10 -- 10 __ __ __ 2 H3 78 -- 12 __ __ __ 5 3 2 I3 70 10 4 3 -- 2 5.5 3 2.5 J3 75 14 __ __ 5 5 __ __ 1 K3 85 5 4 __ __ __ 6 __ __ L3 75 __ __ 12 5 __ __ 5 3 Comparative Example A metal-based anode was tested in a potassium fluoride-free electrolyte at 900°C.
The anode was manufactured from a rod of diameter mm and total length 20 mm made from a cast nickel-iron alloy having the composition of sample A2 of Table 15 2. The anode rod was supported by a stem made of an alloy containing nickel, chromium and iron, such as Inconel, protected with an alumina sleeve. The anode was suspended for 16 hours over the molten fluoride-based WO 2004/035871 _ 18 _ PCT/IB2003/004649 electrolyte whereby its surface was oxidised prior to immersion into the electrolyte.
Electrolysis was carried out by fully immersing the anode rod in the molten electrolyte. The potassium fluoride-free electrolyte contained 49 weighto aluminium fluoride (A1F3), 43 weighto aluminium fluoride (NaF), 4 weight% calcium fluoride (CaF2) and 4 weighto alumina (A1~03). The saturation concentration of alumina in such an electrolyte, unattainable in practice, is at 5 weighto.
The current density was about 0.8 A/cm~ and the cell voltage was at 3.6-3.8 volt for 24 hours. The concentration of dissolved alumina in the electrolyte was maintained during the entire electrolysis by periodically feeding fresh alumina into the cell.
After 32 hours the cell voltage increased to 10 volt and electrolysis was interrupted. The anode was extracted. Upon cooling the anode was examined externally and in cross-section.
The anode's outer dimensions had remained substantially unchanged. The anode's oxide outer part had grown from an initial thickness of about 70 micron to a thickness after use of about up to 1000 micron. A
yellow-green layer of nickel fluoride (NiF~) was observed between the oxide outer part and the metallic inner part of the anode. Such a nickel fluoride layer is substantially non-conductive and passivates the anode, which caused the voltage increase.
Furthermore, a vermicular structure was observed in the metallic inner part immediately underneath the nickel fluoride layer over a depth of about 2 to 3 mm.
The vermicular structure had mainly empty pores that had an average diameter of about 20 to 30 micron.
Example 1 A test was carried out with a cell according to the invention comprising: a molten potassium fluoride-containing electrolyte at 900°C having the composition of sample D1 of Table I, i.e. rich in dissolved alumina, WO 2004/035871 _ 19 _ PCT/IB2003/004649 and an anode made from a nickel-iron alloy having the composition of sample A2 of Table 2.
The anode was manufactured like in the Comparative Example and suspended for 16 hours over the molten electrolyte.
Electrolysis was carried out in the same potassium fluoride-containing electrolyte: The current density was about 0.8 A/cm2 and the cell voltage was stable at 3.8 volt during the entire test. The dissolved alumina-content was maintained around 8 weighto by periodically feeding fresh alumina into the cell.
After 50 hours electrolysis was interrupted and the anode extracted. Upon cooling the anode was examined externally and in cross-section.
The anode's outer dimensions had remained substantially unchanged. The anode's oxide outer part had grown from an initial thickness of about 70 micron to a thickness after use of about up to 500 micron, instead of the 1000 micron observed in the Comparative Example. Also, no passivating yellow-green layer of nickel fluoride (NiF~) was observed.
Immediately underneath the oxide outer part, a vermicular structure was observed in the metallic inner part over a depth of about 0.5 to 1 mm, instead of the 2 to 3 mm of the Comparative Example. The vermicular structure had pores which were partly filled with oxides, in particular iron oxides, and which had an average diameter of about 2 to 5 micron.
Example 2 Example 1 was repeated with an anode made form the nickel-cobalt-iron alloy composition of sample D2 of Table 2 which was prepared, like in Example 1, over a potassium fluoride-containing electrolyte having the composition of sample D1 of Table l, i.e. rich in dissolved alumina. The anode was then tested in the electrolyte like in Example 1 and showed similar results.
WO 2004/035871 _ 2 p - PCT/IB2003/004649 Example 3 Example 1 was repeated with an anode made from the nickel-iron alloy composition of sample H2 of Table 2 prepared, like in Example 1, over a potassium fluoride-s containing electrolyte having the composition of sample D1 of Table 1, i.e. rich in dissolved alumina. The anode was then tested in the electrolyte like in Example 1.
After 50 hours electrolysis was interrupted and the anode extracted. Upon cooling the anode was examined externally and in cross-section.
The anode's outer dimensions had remained substantially unchanged. The anode's oxide outer part had grown from an initial thickness of about 70 micron to a thickness after use of about up to 1000 micron like in the Comparative Example. However, no passivating yellow-green layer of nickel fluoride (NiF2) was observed.
A vermicular structure was observed in the metallic inner part immediately underneath the oxide outer part over a depth of about 1.5 to 2 mm, instead of the 2 to 3 mm of the Comparative Example. The vermicular structure had pores which were partly filled with oxides, in particular iron oxides, and which had an average diameter of about 2 to 5 micron.
Example 4 Example 1 was repeated with an anode made from the nickel-iron alloy composition of sample A2 of Table 2 which was prepared, like in Example 1, over a potassium fluoride-containing electrolyte having the composition of sample A1 of Table l, i.e. rich in dissolved alumina.
The anode was then tested in the electrolyte like in Example 1 and showed similar results.
Example 5 Examples 1 to 4 can be repeated using different combinations of electrolyte compositions (A1-I1) selected from Table 1 and anode alloy compositions (A2-K2) selected from Table 2.
WO 2004/035871 _ 21 _ PCT/IB2003/004649 Example 6 Another aluminium electrowinning anode was prepared as follows:
A slurry for coating an anode was prepared by suspending in 32.5 g of an aqueous solution containing 5 weighto polyvinyl alcohol (PVA) 67.5 g of a particle mixture made of hematite Fe203 particles, boron nitride particles, Ti02 particles and Cu0 particles (with.
particle size of -325 mesh, i.e. smaller than 44 micron) in a weight ratio corresponding to sample A3 of Table 3.
An anode made of the nickel-iron alloy of sample A2 of Table 2 was covered with ten layers of this slurry that were applied with a brush. The applied layers were dried for 10 hours at 140°C in air and then consolidated at 950°C for 16 hours to form a protective hematite based coating which had a thickness of 0.4 to 0.45 mm.
During consolidation, the Fe~03 particles were sintered together into a microporous matrix with a volume contraction. The TiO~ particles and Cu0 particles were dissolved in the sintered Fe203. The boron nitride particles remained substantially inert during the sintering but prevented migration and agglomeration of the micropores into cracks.
Underneath the coating, an integral oxide scale mainly of iron oxide had grown from the anode's alloy during the heat treatment and combined with iron oxide and titanium oxide from the coating to firmly anchor the coating to the oxidised alloy. The integral oxide scale contained titanium oxide in an amount of about 10 metal weighto. Minor amounts of copper, aluminium and nickel were also found in the oxide scale (less that 5 metal weighto in total).
Electrolysis was carried out in a potassium fluoride-containing electrolyte at 900°C having the composition of sample D1 of Table 1, i.e. rich in dissolved alumina. The current density was about 0.8 A/cm2 and the cell voltage was stable at 3.6 volt during the entire test, instead of the 3.8 volt observed in Examples 1 to 4. The dissolved alumina-content was i CA 02498622 2005-03-10 - as -~ maintained around 8 weight% by periodically feeding fresh alumina into the ce7.l_ After 50 hours electrolysis was interrupted and the anode extracted. Upon cooling the anode was examined externally and in cross-section.
The anode's outer dimensions as well as the anode's coating had remained substantially unchanged. However, Ti02 had selectively been dissolved in the electroiyte from the coating. The anode's structure underneath the coating was similar to the structure observed in Examples 1 to Samples of the used electrolyte and the" product aluminium were also analysed. It Was found that the electrolyte contained lees that 7o ppm nickel and the produced aluminium contained lees than 30D ppm nickel which is significantly lower than with an uncoated anode that can cause a typical nickel contamination of 1000 ppm in the product aluminium.
Example 7 Example 6 can be repeated using d~.fferent combinations of electrolyte compositions (A1-I1) selected from Table 1, anode alley eompvsitions (A2-KZj selected from Table 2 and coating compositions (A3-L3) selected from Table 3.
Further details on the application of such anode coatings and .suitable compositions are disclosed in W~03/087435, W02004/016731 and 6102004/024994 (aIl Nguyen/de Norm) .
In summazy, as can he seen by carnparing Example 1-5 to the Comparative Example, using the potassium-fluoride electrolyte pf the invention containing about s weight%
dissolved alumina instead of a potassium-fluoride free electrolyte containing only 4 weight% dissolved alumina, inhibits fluorination and passivation of the nickel and/or cobalt of the anode and reduces wear (oxidation and dis3olution v~ the anode's iron).
WO 2004/035871 _ ~3 _ PCT/IB2003/004649 Furthermore, as can be observed from Examples 6-7, use of a crack-free nickel-free hematite-based protective coating on a nickel-iron anode alloy reduces the cell voltage and significantly inhibits contamination of the product aluminium by nickel from the anode, compared to an uncoated nickel-iron anode operated in the same type of electrolyte.
US Patent 6,248,227 (de Nora/Duruz) discloses an aluminium electrowinning anode having a metallic anode body which can be made of various alloys, for example a nickel-iron-copper alloy. 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 is 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.
US Patent 6,372,099 (Duruz/de Nora) discloses the use of transition metal species in an electrolyte below 910°C of an aluminium electrowinning cells to inhibit dissolution of metal-based anodes of the cell.
W000/06803 (Duruz/de Nora/Crottaz) and WO00/06804 (Crottaz/Duruz) both disclose an anode produced from a nickel-iron alloy which is surface oxidised to form a coherent and adherent outer iron oxide-based layer whose surface is electrochemically active. WO00/06804 also mentions that the anode may be used in an electrolyte at a temperature of 820° to 870°C containing 23 to 26.5 weight o AlF3, 3 to 5 weight o A1203, 1 to 2 weight a ZiF
and 1 to 2 weighty MgF2.
US Patents 5,006,209 and 5,284,562 (both Beck/Brooks), 6,258,247 and 6,379,512 (both Brown/
Brooks/Frizzle/Juric), 6,419,813 (Brown/Brooks/Frizzle) and 6,436,272 (Brown/Frizzle) all disclose the use of nickel-copper-iron anodes in an aluminium production electrolyte at 660°-800°C containing 6-26 weighto NaF, 7-33 weighto KF, 1-6 weighto ZiF and 60-65 weighto A1F3.
The electrolyte may contain A1203 in an amount of up to 30 weighto, in particular 5 to 10 or 15 weight%, most of which is in the form of suspended particles and some of which is dissolved in the electrolyte, i.e. typically 1 to 4 weighto dissolved A1203. In US Patents 6,258,247, 6,379,512, 6,419,813 and 6,436,272 such an electrolyte WO 2004/035871 _ 3 _ PCT/IB2003/004649 is said to be useable at temperatures up to 900°C. In US
Patents 6,258,247 and 6,379,512 the electrolyte further contains 0.004 to 0.2 weighto transition metal additives to facilitate alumina dissolution and improve cathodic operation.
US Patent 5,725,744 (de Nora/Duruz) discloses an aluminium production cell having anodes made of nickel, iron and/or copper in a electrolyte at a temperature from 680 ° to 880 °C containing 42-63 weight o A1 F3, up to 48 weight o NaF, up to 48 weight o LiF and 1 to 5 weight o A1203. MgF~, KF and CaF2 are also mentioned as possible bath constituents.
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 was too short and needs to be increased.
Summarv of the Invention One object of the invention is to provide an aluminium electrowinning cell incorporating metal-based anodes which remain substantially insoluble at the cell operating temperature and which can be operated without passivation or excessive contamination of the produced aluminium.
Another object of the invention is to provide an aluminium electrowinning cell operating with a crustless and ledgeless electrolyte, which can achieve high productivity, low contamination of the product aluminium, and whose components resist corrosion and wear.
The invention relates to a cell for electrowinning aluminium from alumina. The cell comprises: a metal-based anode having an outer part that contains at least one of nickel, cobalt and iron and that has an electrochemically active oxide-based surfaced and a fluoride-containing molten electrolyte at a temperature below 940°C, in particular in the range from 880° to 920°C, in which the active anode surface is immersed.
The electrolyte consists of : 5 to 14 weight o overall of WO 2004/035871 _ 4 _ PCT/IB2003/004649 dissolved alumina; 35 to 45 weighto aluminium fluoride;
30 to 45 weighto sodium fluoride; 5 to 20 weighto potassium fluoride; 0 to 5 weighto calcium fluoride; and 0 to 5 weighto in total of one or more further constituents.
For instance, the electrolyte consists of: 7 to 10 weighto dissolved alumina; 38 to 42 weight% aluminium fluoride; 34 to 43 weighto sodium fluoride; 8 to 15 weighto potassium fluoride; 2 to 4 weighto calcium fluoride; and 0 to 3 weighto in total of one or more further constituents.
Such an electrolyte composition is well adapted for aluminium electrowinning at reduced temperature, i.e. at a temperature below the conventional aluminium electrowinning temperature of about 950°C, using a metal-based anode containing at least one of nickel, cobalt and iron, usually in metallic and/or oxide form.
The electrolyte is particularly adapted for anodes containing at least one of metallic nickel, metallic cobalt and oxides of iron. Oxides of iron include ferrous oxide, hematite, magnetite and ferrites (e. g.
nickel ferrite), in stoichiometric and non-stoichiometric form. For example, the anode has a metallic alloy body that contains one or more of these metals - nickel, cobalt and iron - and that is covered with an integral active oxide layer or film.
The presence in the electrolyte of potassium fluoride in the given amount has two effects. On the one hand, it leads to a reduction of the operating temperature by up to several tens of degrees without increase of the electrolyte's aluminium fluoride content or even a reduction thereof compared to standard electrolytes operating at about 950°C with an aluminium fluoride content of about 45 weighto. On the other hand, it maintains a high solubility of alumina, i.e. up to above about 14 weighto, in the electrolyte even though the temperature of the electrolyte is reduced by a few tens of degrees compared to conventional temperature.
Hence, in contrast to prior art low temperature electrolytes which carry large amounts of undissolved alumina in particulate form, according to the present invention a large amount of alumina in the electrolyte is in a dissolved form.
s~:n ~ without being bound to any theory, it is believed that combining a high concentration of dissolved alumina in the electrolyte and a limited concentration of aluminium fluoride leads predominantly to the formation of (basic) fluorine-poor aluminium vxyfluoride ions ( [A,1202F41 Z-) instead of (acid) fluorine-rich aluminium vxyfluoride ions ( [A120F6]2-) near the anode. As opposed to acid fluorine-rich aluminium oxyfluoride ions, basic fluorine-poor aluminium ~oxyfluoride ions do not significantly paaeivate the anode's nickel and cobalt, or dissolve the anode's iron. In particular, basic fluorine-poor aluminium oxyfluoride ions dv not significantly passivate metallic nickel and cobalt, or dissolve iron oxides. The weight ratio of dissolved alumina/alurninium fluoride in the electrolyte should be above 1/7, and often above 1/6.5 or even above 1/6, to vbta3~n a favourable ratio of the fluorine-poor aluminium oxyfluoride ions and the fluorine-rich aluminium oxyfluoride ions.
It follows that the use of the above described electrolyte with metal-based anodes containing at Isast one of nickel, cobalt and iron inhibits paaaivation and corrosion thereof.
In order to maintain the alumina concentration abvv~ the given threshold during normal electrolysis, the cell is preferably fitted with means tv monitor and adjust the electrolyte's alumina content.
The abovernentioned one yr more further constituents of the electrolyte may comprise at least one fluoride selected from magnesium fluoride, lithium fluoride, cesium fluoride, rubidium fluoride, strontium fluoride, barium fluoride and cerium fluoride_ Advantageously, the cell is sufficiently insulated tv be operated with a substantially crustless and/or ledgeless electrvlytp_ Suitable cell insulation is disclosed in U5 Patent 6,402,928 (de Nora/Sekhar), W002/070784 and US Publicativzl 2003/0102228 (both de Nara/Herclaz).
The cell can have a cathode that has an aluminium-4o wettabla surface, in particular a drained horiaontal or inclined surface. Suitable cathode designs are for examples disclosed in US Patents 5,683,559, 5,888,360, 6,093,304 (all de Nora), 6,258,246 (Duruz/de Nora), 6,358,393 (Berclaz/de Nora) and 6,436,273 (de Nora/Duruz), and in PCT publications W099/02764 (de Nora/Duruz), WO00/63463 (de Nora), W001/31086 (de Nora/Duruz), W001/31088 (de Nora), W002/070785 (de Nora), W002/097168 (de Nora), W002/097168 (de Nora), W003/023091 (de Nora) and W003/023092 (de Nora).
The cathode can have an aluminium-wettable coating that comprises a refractory boride and/or an aluminium-wetting oxide. Suitable aluminium-wettable materials are disclosed in W001/42168 (de Nora/Duruz), W001/42531 (Nguyen/Duruz/de Nora), W002/070783 (de Nora), W002/096831 (Nguyen/de Nora) and W002/096830 (Duruz/
Nguyen/de Nora).
The anode can have a metallic or cermet body and an oxide layer integral with or applied on the anode body.
Usually, the anode body is made from an iron alloy, in particular an alloy of iron with nickel and/or cobalt. Suitable alloys are disclosed in US Patents 6,248,227 (de Nora/Duruz), 6,521,115 (Duruz/de Nora/Crottaz), 6,562,224 (Crottaz/Duruz), and in PCT
publications W000/40783 (de Nora/Duruz), W001/42534 (de Nora/Duruz), W001/42536 (Duruz/Nguyen/de Nora), W002/083991 (Nguyen/de Nora), W003/014420 (Nguyen/Duruz/de Nora) and W003/078695 (Nguyen/de Nora).
For example, the anode body is made from an alloy consisting of:
- 40 to 80o nickel and/or cobalt, in particular 50 to 60 weighto;
- 9 to 55 weighto iron, in particular 25 to 40 weight%;
- 5 to 15 weight o copper, in particular 6 to 12 weight o ;
- 0 to 4 weighto in total of at least one of aluminium, niobium and tantalum, in particular 0.5 to 2 weighto; and - 0 to 2 weighto in total of further constituents, in particular 0.5 to 1 weighto.
Typically such an alloy is oxidised prior to or during use. This can lead to diffusion of metals in the anode, especially at the alloy's surface, which locally changes the alloy's composition.
WO 2004/035871 _ ~ _ PCT/IB2003/004649 The anode body can be covered with an integral iron oxide-based layer containing less than about 35 weighto nickel oxide and/or cobalt oxide, in particular from 5 to 10 weighto nickel oxide. Such integral layers are usually obtained by preoxidation of the body before and/or during use in the cell.
The anode may also comprise an applied iron oxide-based coating. Suitable iron oxide-based coatings are disclosed in US Patents 6,361,681 (de Nora/Duruz), 6,365,018 (de Nora), 6,379,526 (de Nora/Duruz) and 6,413,406 (de Nora), and in PCT applications PCT/IB03/01479, PCT/IB03/03654 and PCT/IB03/03978 (all Nguyen/de Nora). For example, the anode coating contains Fe~03 and optionally: at least one dopant selected from TiO~, Zn0 and Cu0 and/or at least one inert material selected from nitrides and carbides.
Especially when used in the upper part of the abovementioned operating temperature range (e.g. 910°-940°C), the anode can comprise an applied cerium oxyfluoride-based outermost coating, for example as disclosed in the abovementioned US Patents 4,614,569, 4,680,094, 4,683,037 and 4,966,674 or PCT Applications W002/070786 (Nguyen/de Nora) and W002/083990 (de Nora/Nguyen). Such a coating may be applied before or during use and maintained during use by the presence of cerium species in the electrolyte.
A nickel-containing stem can be used to suspend the anode in the electrolyte, in particular a stem having a nickel-containing core covered with an applied oxide coating, such as a coating containing aluminium oxide and titanium oxide. The core of the stem can comprise a copper inner part and a nickel-based outer part. Further details of anode stems are disclosed in PCT/IB03/02702 (Crottaz/Duruz).
Suitable anode designs are for example disclosed in W099/02764 (de Nora/Duruz), W000/40781, W000/40782, W003/023091, W003/023092 and W003/006716 (all de Nora).
Usually, the cell comprises at least one component, e.g. the cathode, that contains a sodium-active cathodic material, such as elemental carbon. This sodium-active cathodic material is preferably shielded from the electrolyte by a sodium-inert layer to inhibit the WO 2004/035871 _ 8 _ PCT/IB2003/004649 presence in the molten electrolyte of soluble cathodically-produced sodium metal that constitutes an agent for dissolving the active oxide-based anode surface. This mechanism is explained in greater detail in US Application 2003/0075454 and W003/083176 (both de Nora/Duruz).
The invention also relates to a cell that comprises:
- a metal-based anode having an outer part that has an electrochemically active oxide-based surface and that is made from an alloy consisting of : 50 to 60 weight o in total of nickel and/or cobalt; 25 to 40 weight%
iron; ~ to 12 weighto copper; 0.5 to 2 weighty aluminium and/or niobium; and 0.5 to 1.5 weighto in total of further constituents, the anode comprising an applied hematite-based coating and optionally a cerium oxyfluoride-based outermost coating;
- a nickel-containing anode stem for suspending the anode in the electrolyte, the stem being covered with a coating of aluminium oxide and titanium oxide;
- a fluoride-containing molten electrolyte at a temperature in the range from 880° to 920 or 930°C, in which the active anode surface is immersed and which consists of : 7 to 10 weight o dissolved alumina; 38 to 42 weighto aluminium fluoride; 34 to 43 weighto sodium fluoride; 8 to 15 weighto potassium fluoride; 2 to 4 weighto calcium fluoride; and 0 to 3 weighto in total of one or more further constituents; and - a cathode having an aluminium-wettable surface, in particular a drained horizontal or inclined surface, formed by an aluminium-wettable coating of refractory hard material and/or aluminium-wetting oxide.
A further aspect of the invention relates to a method of electrowinning aluminium in a cell as described above. The method comprises electrolysing the dissolved alumina to produce oxygen on the anode and aluminium cathodically, and supplying alumina to the electrolyte to maintain therein a concentration of dissolved alumina of 5 to 14 weight%, in particular 7 to 10 weighto.
Brief Description of Drawings The invention will be further described with reference to the accompanying drawings, in which:
WO 2004/035871 - 9 _ PCT/IB2003/004649 - Figures 1a and 1b schematically show respectively a side elevation and a plan view of an anode for use in a cell according to the invention;
- Figures 2a and 2b show a schematic cross-sectional view and a plan view, respectively, of an aluminium production cell for equipment with a potassium fluoride-containing electrolyte and a metal-based anode according to the invention; and - Figure 3 shows a schematic cross-sectional view of another aluminium production cell for equipment with a potassium fluoride-containing electrolyte and a metal-based anode according to the invention.
Detailed Description Figures 1a and 1b schematically show an anode 10 which can be used in a cell for the electrowinning of aluminium according to the invention.
The anode 10 comprises a series of elongated straight anode members 15 connected to a cast or profiled support 14 for connection to a positive bus bar.
The cast or profiled support 14 comprises a lower horizontally extending foot 14a for electrically and mechanically connecting the anode members 15, a stem 14b for connecting the anode 10 to a positive bus bar and a pair of lateral reinforcement flanges 14c between the foot 14a and stem 14b.
The anode members 15 may be secured by force-fitting or welding the foot 14a on flats 15c of the anode members 15. As an alternative, the connection between the anode members 15 and the corresponding receiving slots in the foot 14a may be shaped, for instance like dovetail joints, to allow only longitudinal movements of the anode members.
The anode members 15 have a bottom part 15a which has a substantially rectangular cross-section with a constant width over its height and which is extended upwardly by a tapered top part 15b with a generally triangular cross-section. Each anode member 15 has a flat lower oxide surface 16 that is electrochemically - zo -active for the anodic evolution of oxygen during operation of the cell. Also, the anode may be covered with a coating of iron oxide-based material, for example applied from a composition as set out in Table III
below, and/or a coating of one or more cerium compounds in particular cerium oxyfluoride.
The anode members 15, in particular their bottom parts 15a, are made of an iron alloy comprising nickel and/or cobalt as disclosed in Table II below. The lifetime of the anode may be increased by a protective coating made of cerium compounds, in particular cerium oxyfluoride as discussed above.
The anode members 15 are in the form of parallel rods in a coplanar arrangement, laterally spaced apart from one another by inter-member gaps 17. The inter-member gaps 17 constitute flow-through openings for the circulation of electrolyte and the escape of anodically-evolved gas released at the electrochemically active surfaces 16.
Figure 2a and 2b show an aluminium electrowinning cell having a series of metal-based anodes 10 in a fluoride-containing cryolite-based molten electrolyte 5 containing dissolved alumina according to the invention.
The electrolyte 5 has a composition that is selected from Table I below. The metal-based anodes 10 have a composition selected from Table II below, optionally with a protective coating made of cerium compounds, in particular cerium oxyfluoride as discussed above.
The anodes 10 are similar to the anode shown in Figs. 1a and 1b. Suitable alternative anode designs are disclosed in W000/40781, W000/40782 and W003/006716 (all de Nora ) .
The drained cathode surface 20 is formed by tiles 21A which have their upper face coated with an aluminium-wettable layer. Each anode 10 faces a corresponding tile 21A. Suitable tiles are disclosed in greater detail in W002/096830 (Duruz/Nguyen/de Nora).
Tiles 21A are placed on upper aluminium-wettable faces 22 of a series of carbon cathode blocks 25 extending in pairs arranged end-to-end across the cell.
As shown in Figures 2a and 2b, pairs of tiles 21A are spaced apart to form aluminium collection channels 36 that communicate with a central aluminium collection groove 30.
The central aluminium collection groove 30 is located in or between pairs of cathode blocks 25 arranged end-to-end across the cell. The tiles 21A
preferably cover a part of the groove 30 to maximise the surface area of the aluminium-wettable cathode surface 20.
As explained hereafter, the cell is thermally sufficiently insulated to enable ledgeless and crustless operation.
The cell comprises sidewalk 40 made of an outer layer of insulating refractory bricks and an inner layer of carbonaceous material exposed to molten electrolyte 5 and to the environment thereabove. These sidewalls 40 are protected against the molten electrolyte 5 and the environment thereabove with tiles 21B of the same type as tiles 21A. The cathode blocks 25 are connected to the sidewalls 40 by a peripheral wedge 41 which is resistant to the molten electrolyte 5.
Furthermore, the cell is fitted with an insulating cover 45 above the electrolyte 5. This cover inhibits heat loss and maintains the surface of the electrolyte in a molten state. Further details of suitable covers are disclosed in the abovementioned references.
In operation of the cell illustrated in Figs. 2a and 2b, alumina dissolved in the molten electrolyte 5 at a temperature of 880° to 940°C is electrolysed between the anodes 10 and the cathode surface 20 to produce gas on the operative anodes surfaces 16 and molten aluminium on the aluminium-wettable drained cathode tiles 21A.
The cathodically-produced molten aluminium flows on the drained cathode surface 20 into the aluminium collection channels 36 and then into the central aluminium collection groove 30 for subsequent tapping, WO 2004/035871 _ 12 - PCT/IB2003/004649 The cell shown in Figure 3 comprises a plurality of metal-based anodes 10 dipping in a molten electrolyte 5 according to the invention.
The anodes 10 are similar to the anode shown in Figs. la and lb. Suitable alternative anode designs are disclosed in W000/40781, W000/40782, W003/006716 and W003/023092 (all de Nora).
The cell bottom comprises a series of pairs of spaced apart carbon cathode blocks 25 placed across the cell and having an aluminium-wettable upper surface 22 formed by an aluminium-wettable layer. The upper surfaces 22 are covered with aluminium-wettable openly porous plates 21 which are filled with molten aluminium to form an aluminium-wetted drained active cathode surface 20 above the upper surfaces 22 of the carbon cathode blocks 25. Further details of such a cathode bottom are disclosed in W002/097168 and W002/097169 (both de Nora).
The cathode blocks 25 are made of graphite and have a reduced height, e.g. 30 cm, and are coated with an aluminium-wettable layer which forms the upper surface 22 and which protects the graphite from erosion and wear. Suitable aluminium-wettable layers are disclosed in US Patent 5,651,874, W098/17842, W001/42168 and W001/42531. The aluminium-wettable openly porous plates 21 covering the coated cathode blocks 25 can be made of the material disclosed in W002/070783 (de Nora).
The cell bottom further comprises a centrally-located recess 35 which extends at a level below the upper surfaces 22 of the carbon cathode blocks 25 and which during use collects molten aluminium 60 drained from the aluminium-wettable drained active cathode surface 20.
The aluminium collection recess 35 is formed in a reservoir body 30 which is placed between the blocks 25 of each pair of cathode blocks and spaces them apart across the cell. As shown in Figure 3, the recess 35 formed in the reservoir body 30 is generally U-shaped with rounded lower corners and an outwardly curved upper part.
WO 2004/035871 _ 13 _ PCT/IB2003/004649 The reservoir body 30 is made of two generally Z-shaped sections 31 assembled across the cell. The reservoir sections 31 are made of anthracite-based material. The aluminium-wettable layer forming the upper surfaces 22 extends in the recess 35 to protect the reservoir body 30 during use against wear and sodium or potassium intercalation.
As shown in Figure 3, the reservoir body 30 extends below the cathode blocks 25 into the refractory and insulating material 26 of the cell bottom permitting maximisation of the capacity of the aluminium collection recess 35.
Furthermore, the reservoir body 30 has a solid base 32 which extends from above to below the bottom face of the cathode blocks 25 and provides sufficient mechanical resistance to keep the blocks 25 properly spaced apart across the cell when exposed to thermal expansion during start-up of the cell and normal operation. As shown in dotted lines in the upper part of the reservoir body 30, longitudinally spaced apart spacer bars 33 placed across the reservoir body 30 may provide additional mechanical strength to the reservoir body 30. Such spacer bars 33 can be made of carbon material coated with an aluminium-wettable protective layer.
The openly porous plates 21 placed on the upper surfaces 22 of the carbon cathode blocks 25 and located in the central region of the cell bottom extend over part of the aluminium collection recess 35 so that during use the protruding part of the aluminium-wetted drained active cathode surface 20 is located over the recess 35.
The openly porous plates 21 are spaced apart over the aluminium collection recess 35 to leave an access for the tapping of molten aluminium through a conventional tapping tube. The spacing between the openly porous plates 21 over the aluminium collection recess can be much smaller along the remaining parts of the recess 35, thereby maximising the surface area of the active cathode surface 20.
The cell shown in Figure 3 comprises a series of corner pieces 41 made of the same openly porous material as plates 21 and filled with aluminium and placed at the periphery of the cell bottom against sidewalls 40. The sidewalls 40 and the surface of the electrolyte 5 are covered with a ledge and a small crust of frozen electrolyte 6. The cell is fitted with an insulating cover 45 above the electrolyte crust 6. Further details of suitable covers are disclosed in the abovementioned references.
The cell is also provided with exhaust pipes (not shown) that extend through the cover 45 for the removal of gases produced during electrolysis.
The cell comprises alumina feeders 50 with feeding tubes 51 that extend through the insulating cover 45 between the anodes 10. The alumina feeders 50 are associated with a crust breaker (not shown) for breaking the crust 6 underlying the feeding tube 51 prior to feeding.
In a variation, the insulating material of the sidewalls 40 and cover 45 may be sufficient to prevent formation of any ledge and crust of frozen electrolyte.
In such a case, the sidewalls 40 are preferably completely shielded from the molten electrolyte 5 like in the cell of Figs. 2a and 2b or by a lining of the aforesaid openly porous material filled with aluminium.
Enhanced alumina dissolution may be achieved by utilising an alumina feed device which sprays and distributes alumina particles over a large area of the surface of the molten electrolyte 5. Suitable alumina feed devices are disclosed in US Patent 6,572,757 (de Nora/Berclaz) and in W003/006717 (Berclaz/Duruz).
Furthermore, the cell may comprise means (not shown) to promote circulation of the electrolyte 5 from and to the anode-cathode gap to enhance alumina dissolution in the electrolyte 5 and to maintain in permanence a high concentration of dissolved alumina close to the active surfaces of anodes 10, for example as disclosed in 60000/40701 (de Nora) .
During operation of the cell shown in Figure 3, alumina dissolved in the electrolyte 5 is electrolysed to produce oxygen on the anodes 10 and aluminium 60 on the drained cathode surfaces 20. The product aluminium 60 drains from the cathode surfaces 20 over the openly porous plates 21 that extend over part of the reservoir 30 into the reservoir 30 from where it can be tapped.
Hence, aluminium is produced on the drained active cathode surface 20 which covers not only the cathode blocks 25 but also part of the reservoir 30, thereby maximising the useful aluminium production area (i.e.
the drained cathode surface 22) of the cell.
Figs. 2a, 2b and 3 show specific aluminium electrowinning cells by way of example. It is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art.
For instance, the cell may have a sloping cathode bottom, as disclosed in W099/02764 (de Nora/Duruz), and optionally one or more aluminium collection reservoirs across the cell, each intersecting the collection groove to divide the drained cathode surface into four quadrants as described in W000/63463 (de Nora).
Examples of electrolyte compositions according to the invention are given in Table 1, which shows the weight percentages of the indicated constituents for each specimen electrolyte A1-Il at a given temperature.
TABLE l AlF3 NaF KF CaF~ A1~03 TC
A1 40.4 42.6 6 3 8 935 B1 40.6 41.4 7 3 8 930 C1 40.4 39.6 9 3 8 915 Dl 40.2 37.8 11.5 2.5 8 900 E1 43.5 40 6.5 2 g 8g5 F1 40 3~ 13 3 g 8g0 H1 36 36.5 16 3.5 8 880 Examples of alloy compositions of suitable metal-based anode are given in Table 2, which shows the weight percentages of the indicated metals for each specimen alloy A2-K2.
Ni Co Fe Cu Al Nb Ta other A2 57 - 30 1fl 2 - - 1 D2 25 43 25 7 - - - _ E2 - 42 50 6 0.5 - 1 0.5 F2 - 45 45 9 - - _ 1 G2 25 25 38 10 - 2 - _ H2 45 - 40 11 - - 2.5 1.5 The "other" elements refer to minor additives such as manganese, silicon and yttrium which may be present in individual amounts of 0.2 to 1.5 weighto. Usual impurities, such.as carbon, have not been listed in Table 2.
Usually, these alloys will be surface oxidised before use and further oxidised during use, as described in the Examples below.
Examples of starting compositions of particle mixtures for producing hematite-based protective anode coatings are given in Table 3, which shows the weight percentages of the indicated constituents for each specimen starting composition of the coating A3-L3.
Fe203 BN A1N ZrC Ti02 Zr02 Zn0 Ta~05 Cu0 A3 78 10 __ __ 10 __ __ __ 2 B3 78 10 __ __ __ __ 10 _- 2 D3 78 10 __ __ __ 10 __ __ 2 F3 78 10 __ __ __ __ __ 10 2 G3 78 -- 10 -- 10 __ __ __ 2 H3 78 -- 12 __ __ __ 5 3 2 I3 70 10 4 3 -- 2 5.5 3 2.5 J3 75 14 __ __ 5 5 __ __ 1 K3 85 5 4 __ __ __ 6 __ __ L3 75 __ __ 12 5 __ __ 5 3 Comparative Example A metal-based anode was tested in a potassium fluoride-free electrolyte at 900°C.
The anode was manufactured from a rod of diameter mm and total length 20 mm made from a cast nickel-iron alloy having the composition of sample A2 of Table 15 2. The anode rod was supported by a stem made of an alloy containing nickel, chromium and iron, such as Inconel, protected with an alumina sleeve. The anode was suspended for 16 hours over the molten fluoride-based WO 2004/035871 _ 18 _ PCT/IB2003/004649 electrolyte whereby its surface was oxidised prior to immersion into the electrolyte.
Electrolysis was carried out by fully immersing the anode rod in the molten electrolyte. The potassium fluoride-free electrolyte contained 49 weighto aluminium fluoride (A1F3), 43 weighto aluminium fluoride (NaF), 4 weight% calcium fluoride (CaF2) and 4 weighto alumina (A1~03). The saturation concentration of alumina in such an electrolyte, unattainable in practice, is at 5 weighto.
The current density was about 0.8 A/cm~ and the cell voltage was at 3.6-3.8 volt for 24 hours. The concentration of dissolved alumina in the electrolyte was maintained during the entire electrolysis by periodically feeding fresh alumina into the cell.
After 32 hours the cell voltage increased to 10 volt and electrolysis was interrupted. The anode was extracted. Upon cooling the anode was examined externally and in cross-section.
The anode's outer dimensions had remained substantially unchanged. The anode's oxide outer part had grown from an initial thickness of about 70 micron to a thickness after use of about up to 1000 micron. A
yellow-green layer of nickel fluoride (NiF~) was observed between the oxide outer part and the metallic inner part of the anode. Such a nickel fluoride layer is substantially non-conductive and passivates the anode, which caused the voltage increase.
Furthermore, a vermicular structure was observed in the metallic inner part immediately underneath the nickel fluoride layer over a depth of about 2 to 3 mm.
The vermicular structure had mainly empty pores that had an average diameter of about 20 to 30 micron.
Example 1 A test was carried out with a cell according to the invention comprising: a molten potassium fluoride-containing electrolyte at 900°C having the composition of sample D1 of Table I, i.e. rich in dissolved alumina, WO 2004/035871 _ 19 _ PCT/IB2003/004649 and an anode made from a nickel-iron alloy having the composition of sample A2 of Table 2.
The anode was manufactured like in the Comparative Example and suspended for 16 hours over the molten electrolyte.
Electrolysis was carried out in the same potassium fluoride-containing electrolyte: The current density was about 0.8 A/cm2 and the cell voltage was stable at 3.8 volt during the entire test. The dissolved alumina-content was maintained around 8 weighto by periodically feeding fresh alumina into the cell.
After 50 hours electrolysis was interrupted and the anode extracted. Upon cooling the anode was examined externally and in cross-section.
The anode's outer dimensions had remained substantially unchanged. The anode's oxide outer part had grown from an initial thickness of about 70 micron to a thickness after use of about up to 500 micron, instead of the 1000 micron observed in the Comparative Example. Also, no passivating yellow-green layer of nickel fluoride (NiF~) was observed.
Immediately underneath the oxide outer part, a vermicular structure was observed in the metallic inner part over a depth of about 0.5 to 1 mm, instead of the 2 to 3 mm of the Comparative Example. The vermicular structure had pores which were partly filled with oxides, in particular iron oxides, and which had an average diameter of about 2 to 5 micron.
Example 2 Example 1 was repeated with an anode made form the nickel-cobalt-iron alloy composition of sample D2 of Table 2 which was prepared, like in Example 1, over a potassium fluoride-containing electrolyte having the composition of sample D1 of Table l, i.e. rich in dissolved alumina. The anode was then tested in the electrolyte like in Example 1 and showed similar results.
WO 2004/035871 _ 2 p - PCT/IB2003/004649 Example 3 Example 1 was repeated with an anode made from the nickel-iron alloy composition of sample H2 of Table 2 prepared, like in Example 1, over a potassium fluoride-s containing electrolyte having the composition of sample D1 of Table 1, i.e. rich in dissolved alumina. The anode was then tested in the electrolyte like in Example 1.
After 50 hours electrolysis was interrupted and the anode extracted. Upon cooling the anode was examined externally and in cross-section.
The anode's outer dimensions had remained substantially unchanged. The anode's oxide outer part had grown from an initial thickness of about 70 micron to a thickness after use of about up to 1000 micron like in the Comparative Example. However, no passivating yellow-green layer of nickel fluoride (NiF2) was observed.
A vermicular structure was observed in the metallic inner part immediately underneath the oxide outer part over a depth of about 1.5 to 2 mm, instead of the 2 to 3 mm of the Comparative Example. The vermicular structure had pores which were partly filled with oxides, in particular iron oxides, and which had an average diameter of about 2 to 5 micron.
Example 4 Example 1 was repeated with an anode made from the nickel-iron alloy composition of sample A2 of Table 2 which was prepared, like in Example 1, over a potassium fluoride-containing electrolyte having the composition of sample A1 of Table l, i.e. rich in dissolved alumina.
The anode was then tested in the electrolyte like in Example 1 and showed similar results.
Example 5 Examples 1 to 4 can be repeated using different combinations of electrolyte compositions (A1-I1) selected from Table 1 and anode alloy compositions (A2-K2) selected from Table 2.
WO 2004/035871 _ 21 _ PCT/IB2003/004649 Example 6 Another aluminium electrowinning anode was prepared as follows:
A slurry for coating an anode was prepared by suspending in 32.5 g of an aqueous solution containing 5 weighto polyvinyl alcohol (PVA) 67.5 g of a particle mixture made of hematite Fe203 particles, boron nitride particles, Ti02 particles and Cu0 particles (with.
particle size of -325 mesh, i.e. smaller than 44 micron) in a weight ratio corresponding to sample A3 of Table 3.
An anode made of the nickel-iron alloy of sample A2 of Table 2 was covered with ten layers of this slurry that were applied with a brush. The applied layers were dried for 10 hours at 140°C in air and then consolidated at 950°C for 16 hours to form a protective hematite based coating which had a thickness of 0.4 to 0.45 mm.
During consolidation, the Fe~03 particles were sintered together into a microporous matrix with a volume contraction. The TiO~ particles and Cu0 particles were dissolved in the sintered Fe203. The boron nitride particles remained substantially inert during the sintering but prevented migration and agglomeration of the micropores into cracks.
Underneath the coating, an integral oxide scale mainly of iron oxide had grown from the anode's alloy during the heat treatment and combined with iron oxide and titanium oxide from the coating to firmly anchor the coating to the oxidised alloy. The integral oxide scale contained titanium oxide in an amount of about 10 metal weighto. Minor amounts of copper, aluminium and nickel were also found in the oxide scale (less that 5 metal weighto in total).
Electrolysis was carried out in a potassium fluoride-containing electrolyte at 900°C having the composition of sample D1 of Table 1, i.e. rich in dissolved alumina. The current density was about 0.8 A/cm2 and the cell voltage was stable at 3.6 volt during the entire test, instead of the 3.8 volt observed in Examples 1 to 4. The dissolved alumina-content was i CA 02498622 2005-03-10 - as -~ maintained around 8 weight% by periodically feeding fresh alumina into the ce7.l_ After 50 hours electrolysis was interrupted and the anode extracted. Upon cooling the anode was examined externally and in cross-section.
The anode's outer dimensions as well as the anode's coating had remained substantially unchanged. However, Ti02 had selectively been dissolved in the electroiyte from the coating. The anode's structure underneath the coating was similar to the structure observed in Examples 1 to Samples of the used electrolyte and the" product aluminium were also analysed. It Was found that the electrolyte contained lees that 7o ppm nickel and the produced aluminium contained lees than 30D ppm nickel which is significantly lower than with an uncoated anode that can cause a typical nickel contamination of 1000 ppm in the product aluminium.
Example 7 Example 6 can be repeated using d~.fferent combinations of electrolyte compositions (A1-I1) selected from Table 1, anode alley eompvsitions (A2-KZj selected from Table 2 and coating compositions (A3-L3) selected from Table 3.
Further details on the application of such anode coatings and .suitable compositions are disclosed in W~03/087435, W02004/016731 and 6102004/024994 (aIl Nguyen/de Norm) .
In summazy, as can he seen by carnparing Example 1-5 to the Comparative Example, using the potassium-fluoride electrolyte pf the invention containing about s weight%
dissolved alumina instead of a potassium-fluoride free electrolyte containing only 4 weight% dissolved alumina, inhibits fluorination and passivation of the nickel and/or cobalt of the anode and reduces wear (oxidation and dis3olution v~ the anode's iron).
WO 2004/035871 _ ~3 _ PCT/IB2003/004649 Furthermore, as can be observed from Examples 6-7, use of a crack-free nickel-free hematite-based protective coating on a nickel-iron anode alloy reduces the cell voltage and significantly inhibits contamination of the product aluminium by nickel from the anode, compared to an uncoated nickel-iron anode operated in the same type of electrolyte.
Claims (23)
1. A cell for electrowinning aluminium from alumina, comprising:
- a metal-based anode having an outer part that has an electrochemically active oxide-based surface and that contains at least one of nickel, cobalt and iron;
a fluoride-containing molten electrolyte in which, during cell operation to electrowin aluminum, the active anode surface is immersed and which is at a temperature in the range from 8800 to 940°C, and which consists of:
- 5 to 14 weight% dissolved alumina;
- 35 to 45 weight% aluminium fluoride;
- 30 to 45 weight% sodium fluoride;
- 5 to 20 weight% potassium fluoride;
- 0 to 5 weight% calcium fluoride; and - 0 to 5 weight% in total of one or more further constituents.
- a metal-based anode having an outer part that has an electrochemically active oxide-based surface and that contains at least one of nickel, cobalt and iron;
a fluoride-containing molten electrolyte in which, during cell operation to electrowin aluminum, the active anode surface is immersed and which is at a temperature in the range from 8800 to 940°C, and which consists of:
- 5 to 14 weight% dissolved alumina;
- 35 to 45 weight% aluminium fluoride;
- 30 to 45 weight% sodium fluoride;
- 5 to 20 weight% potassium fluoride;
- 0 to 5 weight% calcium fluoride; and - 0 to 5 weight% in total of one or more further constituents.
2. The cell of claim 1, wherein the electrolyte contains 7 to 10 weight%
alumina.
alumina.
3. The cell of claim 1 or 2, wherein the electrolyte contains 38 to 42 weight%
aluminium fluoride.
aluminium fluoride.
4. The cell of any one of claims 1 to 3, wherein the electrolyte contains 34 to 43 weight% sodium fluoride.
The cell of any one of claims 1 to 4, wherein the electrolyte contains 8 to 15 weight% potassium fluoride.
6. The cell of any one of claims 1 to 5, wherein the electrolyte contains 2 to weight% calcium fluoride.
7. The cell of any one of claims 1 to 6, wherein the electrolyte contains 0 to weight% of said one or more further constituents.
8. The cell of any one of claims 1 to 7, wherein said one or more further constituents comprise at least one fluoride selected from magnesium fluoride, lithium fluoride, cesium fluoride, rubidium fluoride, strontium fluoride, barium fluoride and cerium fluoride.
9. The cell of any one of claims 1 to 8, comprising a cathode that has an aluminium-wettable surface.
10. The cell of claim 9, wherein the cathode has an aluminium-wettable coating that comprises a refractory boride and/or an aluminium-wetting oxide.
H. The cell of any one of claims 1 to 10, wherein the anode has a metallic or cermet body and an oxide layer on the anode body.
12. The cell of any one of claims 1 to 11, wherein the anode body is made from an iron alloy containing nickel and/or cobalt.
13. The cell of claim 12, wherein the anode body is made from an alloy consisting of:
40 to 80% nickel and/or cobalt;
- 9 to 55 weight% iron;
- 5 to 15 weight% copper;
- 0 to 4 weight% in total of at least one of aluminium, niobium and tantalum;
and - 0 to 2 weight% in total of further constituents.
40 to 80% nickel and/or cobalt;
- 9 to 55 weight% iron;
- 5 to 15 weight% copper;
- 0 to 4 weight% in total of at least one of aluminium, niobium and tantalum;
and - 0 to 2 weight% in total of further constituents.
14. The cell of claim 12 or 13, wherein the anode body is covered with an integral iron oxide-based layer containing up to 35 weight% nickel oxide and/or cobalt oxide.
15. The cell of any one of claims 1 to 14, wherein the anode comprises an applied iron oxide-based coating.
16. The cell of claim 15, wherein the anode coating contains Fe2O3 and optionally: at least one dopant selected from TiO2, ZnO and CuO and/or at least one inert material selected from nitrides and carbides.
17. The cell of any one of claims 1 to 16, wherein the anode comprises a cerium oxyfluoride-based outermost coating.
18. The cell of any one of claims 1 to 17, wherein the anode is suspended in the electrolyte by a nickel-containing stem.
19. The cell of claim 18, wherein the nickel containing stem is covered with an applied coating containing aluminium oxide and titanium oxide.
20. The cell of claim 18 or 19, wherein the core of the stem comprises a copper inner part and a nickel-based outer part.
21. The cell of any one of claims 1 to 20, comprising at least one component that contains a sodium-active cathodic material, said sodium-active cathodic material being shielded from the electrolyte by a sodium-inert layer to inhibit the presence in the molten electrolyte of soluble cathodically-produced sodium metal that constitutes an agent for dissolving the active oxide-based anode surface.
22. A cell according to claim 1, comprising:
- a metal-based anode having an outer part that has an electrochemically active oxide-based surface and that is made from an alloy consisting of:
- 50 to 60 weight% in total of nickel and/or cobalt;
- 25 to 40 weight% iron;
- 6 to 12 weight% copper;
- 0 5 to 2 weight% aluminium and/or niobium; and - 0 5 to 1.5 weight% in total of further constituents, the anode comprising an applied hematite-based coating and optionally a cerium oxyfluoride-based outermost coating;
- a nickel-containing anode stem for suspending the anode in the electrolyte, the stem being covered with a coating of aluminium oxide and titanium oxide;
a fluoride-containing molten electrolyte in which the active anode surface is immersed and which is at a temperature in the range from 880° to 930°C and which consists of:
- 7 to 10 weight% dissolved alumina;
- 38 to 42 weight% aluminium fluoride;
- 34 to 43 weight% sodium fluoride;
- 8 to 15 weight% potassium fluoride;
- 2 to 4 weight% calcium fluoride; and - 0 to 3 weight% in total of one or more further constituents;
and - a cathode having a drained horizontal or inclined aluminium-wettable surface, formed by an aluminium-wettable coating of refractory hard material and/or aluminium-wetting oxide.
- a metal-based anode having an outer part that has an electrochemically active oxide-based surface and that is made from an alloy consisting of:
- 50 to 60 weight% in total of nickel and/or cobalt;
- 25 to 40 weight% iron;
- 6 to 12 weight% copper;
- 0 5 to 2 weight% aluminium and/or niobium; and - 0 5 to 1.5 weight% in total of further constituents, the anode comprising an applied hematite-based coating and optionally a cerium oxyfluoride-based outermost coating;
- a nickel-containing anode stem for suspending the anode in the electrolyte, the stem being covered with a coating of aluminium oxide and titanium oxide;
a fluoride-containing molten electrolyte in which the active anode surface is immersed and which is at a temperature in the range from 880° to 930°C and which consists of:
- 7 to 10 weight% dissolved alumina;
- 38 to 42 weight% aluminium fluoride;
- 34 to 43 weight% sodium fluoride;
- 8 to 15 weight% potassium fluoride;
- 2 to 4 weight% calcium fluoride; and - 0 to 3 weight% in total of one or more further constituents;
and - a cathode having a drained horizontal or inclined aluminium-wettable surface, formed by an aluminium-wettable coating of refractory hard material and/or aluminium-wetting oxide.
23. A method of electrowinning aluminium in a cell as defined in any one of claims 1 to 22, comprising electrolysing the dissolved alumina to produce oxygen on the anode and aluminium cathodically, and supplying alumina to the electrolyte to maintain therein a concentration of dissolved alumina of 5 to 14 weight%.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IBPCT/IB02/04059 | 2002-10-18 | ||
| IB0204059 | 2002-10-18 | ||
| PCT/IB2003/004649 WO2004035871A1 (en) | 2002-10-18 | 2003-10-17 | Aluminium electrowinning cells with metal-based anodes |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2498622A1 CA2498622A1 (en) | 2004-04-29 |
| CA2498622C true CA2498622C (en) | 2011-09-20 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA2498622A Expired - Fee Related CA2498622C (en) | 2002-10-18 | 2003-10-17 | Aluminium electrowinning cells with metal-based anodes |
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|---|---|
| US (1) | US20110031129A1 (en) |
| EP (1) | EP1554416B1 (en) |
| CN (1) | CN1735717B (en) |
| AT (1) | ATE543927T1 (en) |
| AU (1) | AU2003269385B2 (en) |
| CA (1) | CA2498622C (en) |
| ES (1) | ES2381927T3 (en) |
| NO (1) | NO20052377L (en) |
| NZ (1) | NZ538777A (en) |
| RU (1) | RU2318924C2 (en) |
| SI (1) | SI1554416T1 (en) |
| WO (1) | WO2004035871A1 (en) |
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| CA2505285C (en) * | 2003-02-20 | 2012-07-17 | Moltech Invent S.A. | Aluminium electrowinning cells with metal-based anodes |
| US7740745B2 (en) * | 2004-03-18 | 2010-06-22 | Moltech Invent S.A. | Non-carbon anodes with active coatings |
| CN100465350C (en) * | 2005-06-24 | 2009-03-04 | 曹大力 | Method of preparing aluminium-iron base alloy in electrolytic tank using iron and its alloy as anode |
| ATE546567T1 (en) * | 2008-09-08 | 2012-03-15 | Rio Tinto Alcan Int Ltd | HIGH CURRENT DENSITY METALLIC OXYGEN EVOLVING ANODE FOR ALUMINUM REDUCTION CELLS |
| CN101586250B (en) * | 2009-06-10 | 2010-12-29 | 中南大学 | Composite coating, preparation method and application thereof |
| RU2457286C1 (en) * | 2011-03-02 | 2012-07-27 | Федеральное государственное бюджетное учреждение науки Институт высокотемпературной электрохимии Уральского отделения Российской Академии наук | Electrolysis method of molten salts with oxygen-containing additives using inert anode |
| CN103484891B (en) * | 2012-06-11 | 2016-06-15 | 内蒙古联合工业有限公司 | A kind of electrolgtic aluminium electrolyzer and use the electrolysis process of this electrolyzer |
| AU2013275997B2 (en) * | 2012-06-11 | 2016-06-16 | Inner Mongolia United Industrial Co., Ltd. | Electrolyte used for aluminum electrolysis and electrolysis process using the electrolyte |
| CN103014769A (en) * | 2012-11-26 | 2013-04-03 | 中国铝业股份有限公司 | Alloy inert anode for aluminium electrolysis and preparation method thereof |
| US10975484B2 (en) * | 2013-07-09 | 2021-04-13 | United Company RUSAL Engineering and Technology Centre LLC | Electrolyte for obtaining melts using an aluminum electrolyzer |
| CN105452538B (en) * | 2013-08-19 | 2018-02-02 | 俄铝工程技术中心有限责任公司 | For obtaining the iron-based anode of aluminium by being electrolysed melt |
| CN105132952B (en) * | 2015-08-26 | 2017-09-29 | 贵州理工学院 | It is a kind of to reduce the electrolyte system of perfluocarbon discharge capacity |
| KR102562722B1 (en) * | 2016-02-01 | 2023-08-03 | 재단법인 포항산업과학연구원 | Anode for electrolysis, electrolytic cell comprising the same, and electrolysis process using the electrolytic cell |
| RU2698162C2 (en) * | 2017-03-01 | 2019-08-22 | Общество с ограниченной ответственностью "Объединенная Компания РУСАЛ Инженерно-технологический центр" | Perforated metal inert anode for aluminium production by molten electrolysis |
| RU2686408C1 (en) * | 2018-06-20 | 2019-04-25 | Федеральное государственное бюджетное учреждение науки Институт высокотемпературной электрохимии Уральского отделения Российской Академии наук | Electrolytic production method of aluminum |
| CN114717610B (en) * | 2022-05-16 | 2023-08-08 | 中国铝业股份有限公司 | Method for reducing potassium content in aluminum electrolysis fluorine-carrying aluminum oxide |
| WO2024030044A1 (en) * | 2022-08-02 | 2024-02-08 | Владислав Владимирович ФУРСЕНКО | Method for producing aluminium by electrolysis of a solution of alumina in cryolite |
Family Cites Families (11)
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|---|---|---|---|---|
| US400766A (en) * | 1889-04-02 | Process of reducing aluminium by electrolysis | ||
| US400664A (en) * | 1886-07-09 | 1889-04-02 | M Hall Charles | Process of reducing aluminium from its fluoride salts by electrolysis |
| SU554318A1 (en) * | 1974-03-19 | 1977-04-15 | Институт общей и неорганической химии АН Украинской ССР | Electrolyte to obtain aluminum-silicon alloys |
| US5006209A (en) * | 1990-02-13 | 1991-04-09 | Electrochemical Technology Corp. | Electrolytic reduction of alumina |
| US5725744A (en) * | 1992-03-24 | 1998-03-10 | Moltech Invent S.A. | Cell for the electrolysis of alumina at low temperatures |
| US5284562A (en) * | 1992-04-17 | 1994-02-08 | Electrochemical Technology Corp. | Non-consumable anode and lining for aluminum electrolytic reduction cell |
| US6258247B1 (en) * | 1998-02-11 | 2001-07-10 | Northwest Aluminum Technology | Bath for electrolytic reduction of alumina and method therefor |
| US6497807B1 (en) * | 1998-02-11 | 2002-12-24 | Northwest Aluminum Technologies | Electrolyte treatment for aluminum reduction |
| US6692631B2 (en) * | 2002-02-15 | 2004-02-17 | Northwest Aluminum | Carbon containing Cu-Ni-Fe anodes for electrolysis of alumina |
| US6800191B2 (en) * | 2002-03-15 | 2004-10-05 | Northwest Aluminum Technologies | Electrolytic cell for producing aluminum employing planar anodes |
| US7740745B2 (en) * | 2004-03-18 | 2010-06-22 | Moltech Invent S.A. | Non-carbon anodes with active coatings |
-
2003
- 2003-10-17 WO PCT/IB2003/004649 patent/WO2004035871A1/en active Search and Examination
- 2003-10-17 AT AT03751166T patent/ATE543927T1/en active
- 2003-10-17 EP EP03751166A patent/EP1554416B1/en not_active Expired - Lifetime
- 2003-10-17 CA CA2498622A patent/CA2498622C/en not_active Expired - Fee Related
- 2003-10-17 CN CN2003801070764A patent/CN1735717B/en not_active Expired - Lifetime
- 2003-10-17 US US10/530,884 patent/US20110031129A1/en not_active Abandoned
- 2003-10-17 RU RU2005115103/02A patent/RU2318924C2/en not_active IP Right Cessation
- 2003-10-17 NZ NZ538777A patent/NZ538777A/en not_active IP Right Cessation
- 2003-10-17 ES ES03751166T patent/ES2381927T3/en not_active Expired - Lifetime
- 2003-10-17 AU AU2003269385A patent/AU2003269385B2/en not_active Ceased
- 2003-10-17 SI SI200332133T patent/SI1554416T1/en unknown
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Also Published As
| Publication number | Publication date |
|---|---|
| WO2004035871A1 (en) | 2004-04-29 |
| RU2005115103A (en) | 2005-10-27 |
| NO20052377L (en) | 2005-05-13 |
| CA2498622A1 (en) | 2004-04-29 |
| CN1735717B (en) | 2011-12-28 |
| EP1554416B1 (en) | 2012-02-01 |
| AU2003269385A1 (en) | 2004-05-04 |
| NZ538777A (en) | 2007-02-23 |
| ES2381927T3 (en) | 2012-06-01 |
| SI1554416T1 (en) | 2012-05-31 |
| CN1735717A (en) | 2006-02-15 |
| US20110031129A1 (en) | 2011-02-10 |
| RU2318924C2 (en) | 2008-03-10 |
| AU2003269385B2 (en) | 2009-06-04 |
| ATE543927T1 (en) | 2012-02-15 |
| EP1554416A1 (en) | 2005-07-20 |
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