Metal-based anodes for aluminium electrowinning cells

Abstract

An anode of a cell for the electrowinning of aluminium comprises a nickel-iron alloy substrate having an openly porous nickel metal rich outer portion whose surface is electrochemically active. The outer portion is optionally covered with an external integral nickel-iron oxide containing surface layer which adheres to the nickel metal rich outer portion of the nickel-iron alloy and which in use is pervious to molten electrolyte. During use, the nickel metal rich outer portion contains cavities some or all of which are partly or completely filled with iron and nickel compounds, in particular oxides, fluorides and oxyfluorides.

1 - 53 . (canceled)  
     
     
         54 . An anode of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, said anode comprising a cobalt-iron alloy having an openly porous cobalt rich outer portion which consists predominantly of cobalt metal and whose surface constitutes an electrochemically-active anode surface of high active surface area, the openly porous cobalt-rich outer portion having a vermicular porosity obtainable by removal of at least part of the iron from the cobalt-iron alloy.  
     
     
         55 . The anode of  claim 54 , wherein the cobalt-iron alloy is further alloyed with nickel, cobalt being predominant over nickel.  
     
     
         56 . The anode of  claim 54 , wherein the cobalt rich openly porous outer portion contains pores which are partly or completely filled with iron and cobalt compounds.  
     
     
         57 . The anode of  claim 55 , wherein the pores have an average diameter of up to 5 micron and an average length of up to 30 micron.  
     
     
         58 . The anode of  claim 55 , wherein the openly porous cobalt-rich outer portion has a thin integral oxide film which underlies the electrochemically active anode surface.  
     
     
         59 . The anode of  claim 58 , wherein said oxide film has a thickness of less than 1 micron.  
     
     
         60 . The anode of  claim 54 , which is covered with a thick external integral cobalt-iron containing oxide layer which adheres to the openly porous outer portion and which is pervious to molten electrolyte.  
     
     
         61 . The anode of  claim 60 , wherein the external integral oxide layer has a thickness of less than 50 micron, in particular from 5 to 30 micron.  
     
     
         62 . The anode of  claim 60 , wherein said external integral oxide layer comprises iron-rich cobalt-iron oxide.  
     
     
         63 . The anode of  claim 62 , wherein said external integral oxide layer comprises cobalt-ferrite.  
     
     
         64 . The anode of  claim 63 , wherein the cobalt-ferrite of said external integral oxide surface layer contains non-stoichiometric cobalt-ferrite having an excess of iron or cobalt, and/or an oxygen deficiency.  
     
     
         65 . The anode of  claim 54 , wherein the cobalt-iron alloy comprises a non-porous inner portion.  
     
     
         66 . The anode of  claim 65 , wherein the non-porous inner portion has a Ni/Fe atomic ratio below 1 before use.  
     
     
         67 . The anode of  claim 54 , wherein the cobalt-rich openly porous outer portion has a Ni/Fe atomic ratio of at least 1, in particular from 1 to 4, before use.  
     
     
         68 . The anode of  claim 54 , wherein the cobalt rich openly porous outer portion has a decreasing concentration of iron metal towards its outermost part.  
     
     
         69 . The anode of  claim 68 , wherein the outermost part of the openly porous cobalt rich outer portion comprises cobalt metal and iron metal in an Co/Fe atomic ratio of more than 3.  
     
     
         70 . The anode of  claim 54 , wherein the cobalt-iron alloy comprises cobalt metal and iron metal in a total amount of at least 65 weight %, in particular at least 80 weight %, preferably at least 90 weight % of the alloy.  
     
     
         71 . The anode of  claim 70 , wherein the cobalt-iron alloy comprises at least one further metal selected from chromium, copper, nickel, silicon, titanium, tantalum, tungsten, vanadium, zirconium, yttrium, molybdenum, manganese and niobium in a total amount of up to 10 weight % of the alloy.  
     
     
         72 . The anode of  claim 70 , wherein the cobalt-iron alloy comprises at least one catalyst selected from iridium, palladium, platinum, rhodium, ruthenium, tin or zinc metals, Mischmetals and their oxides and metals of the Lanthanide series and their oxides as well as mixtures and compounds thereof, in a total amount of up to 5 weight % of the alloy.  
     
     
         73 . The anode of  claim 70 , wherein the cobalt-iron alloy comprises aluminium in an amount less than 20 weight %, in particular less than 10 weight %, preferably from 1 to 5 or even 6 weight % of the alloy.  
     
     
         74 . The anode of  claim 54 , comprising a core made of an electronically conductive material, such as metals, alloys, intermetallics, cermets and conductive ceramics, which is covered with the cobalt-iron alloy.  
     
     
         75 . The anode of  claim 74 , wherein the core is a non-porous cobalt rich cobalt-iron alloy.  
     
     
         76 . A method of manufacturing an anode according to  claim 54  for use in a cell for the electrowinning of aluminium, comprising forming the cobalt-rich openly porous outer portion which consists predominantly of cobalt metal by providing a cobalt-iron alloy having an outer portion and selectively removing at least part of the iron from the outer portion.  
     
     
         77 . The method of  claim 76 , wherein the cobalt-rich openly porous outer portion is formed by selectively removing iron from a cobalt iron alloy by electrolytic dissolution.  
     
     
         78 . The method of  claim 76 , wherein the cobalt-rich openly porous outer portion is formed by selectively oxidising and diffusing iron from a cobalt-iron alloy.  
     
     
         79 . The method of  claim 78 , wherein an external integral cobalt-iron oxide containing layer pervious to molten electrolyte is formed from the diffused oxidised iron rather than cobalt, the oxide surface layer adhering to the openly porous cobalt rich outer portion, the oxidation of the cobalt-iron alloy comprising one or more steps at a temperature of 800° to 1200° C. for up to 60 hours in an oxidising atmosphere.  
     
     
         80 . The method of  claim 78 , wherein the cobalt-iron alloy is oxidised in an oxidising atmosphere for 0.5 to 10 hours.  
     
     
         81 . The method of  claim 79 , wherein the oxidising atmosphere consists of oxygen or a mixture of oxygen and one or more inert gases having an oxygen content of at least 10 molar/o of the mixture.  
     
     
         82 . The method of  claim 79 , wherein the oxidising atmosphere is air.  
     
     
         83 . The method of  claim 78 , comprising oxidising the cobalt-iron alloy at a temperature of 1050° to 1150° C.  
     
     
         84 . The method of  claim 78 , comprising subjecting the cobalt-iron alloy to a thermal-mechanical treatment to modify its microstructure before oxidation.  
     
     
         85 . The method of  claim 78 , comprising casting the cobalt-iron alloy with additives to provide a microstructure for enhancing oxidation.  
     
     
         86 . The method of  claim 81 , wherein oxidation in the oxidising atmosphere is followed by a heat treatment in an inert atmosphere at a temperature of 800° to 1200° C. for up to 60 hours.  
     
     
         87 . The method of  claim 77 , wherein the selective removal of iron, in particular by oxidation in the oxidising atmosphere, is carried out partly before use of the anode and is continued in-situ by iron dissolution at electrolysis start-up.  
     
     
         88 . The method of  claim 76 , comprising forming a cobalt-iron alloy layer on a core made of an electronically conductive material, such as a cobalt-rich cobalt-iron alloy.  
     
     
         89 . The method of  claim 88 , comprising depositing cobalt and iron metal on the core.  
     
     
         90 . The method of  claim 88 , comprising depositing cobalt and iron compounds on the core and then reducing the compounds.  
     
     
         91 . The method of  claim 90 , wherein the cobalt and iron compounds are Fe(OH) 2  and Co(OH) 2  which are reduced in a hydrogen atmosphere to form an openly porous cobalt-iron alloy layer.  
     
     
         92 . The method of  claim 88 , comprising co-depositing cobalt and iron and/or compounds thereof onto the core.  
     
     
         93 . The method of  claim 88 , comprising depositing at least one layer of iron and/or an iron compound and at least one layer of cobalt and/or a cobalt compound onto the core, and then interdiffusing the layers.  
     
     
         94 . The method of  claim 88 , comprising depositing electrolytically or chemically at least one of cobalt, iron and compounds thereof onto the core.  
     
     
         95 . The method of  claim 88 , comprising arc spraying or plasma spraying at least one of cobalt, iron and compounds thereof onto the core.  
     
     
         96 . The method of  claim 88 , comprising applying at least one of cobalt, iron and compounds thereof by painting, dipping or spraying onto the core.  
     
     
         97 . The method of  claim 76 , wherein the cobalt-rich openly porous outer portion is formed by sintering a powder precursor.  
     
     
         98 . A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, the cell comprising at least one anode as defined in  claim 54  facing and spaced from at least one cathode.  
     
     
         99 . A method of producing aluminium in a cell according to  claim 98  containing alumina dissolved in a molten electrolyte, the method comprising passing an ionic current in the molten electrolyte between the cathode(s) and the electrochemically active surface layer of the anode(s), thereby evolving at the anode(s) oxygen gas derived from the dissolved alumina and producing aluminium on the cathode(s).  
     
     
         100 . The method of  claim 99 , wherein at least part of the iron rather than cobalt of the cobalt-rich openly porous outer portion of at least one anode is selectively removed by electrolytic dissolution in-situ.  
     
     
         101 . The method of  claim 99 , wherein at least part of the iron rather than cobalt of the cobalt-rich openly porous outer portion of at least one anode is selectively removed by oxidising said outer portion in-situ by atomic and/or molecular oxygen formed on the electrochemically active surface until the electrochemically active surface forms a barrier impervious to oxygen.  
     
     
         102 . The method of  claim 99 , comprising permanently and uniformly substantially saturating the molten electrolyte with alumina and species of at least one major metal present in the cobalt-rich openly porous outer portion of the anode(s) to inhibit dissolution of the anode(s).  
     
     
         103 . The method of  claim 102 , wherein the cell is operated with the molten electrolyte at a temperature sufficiently low to limit the solubility of said major metal species thereby limiting the contamination of the product aluminium to an acceptable level.  
     
     
         104 . The method of  claim 99 , wherein the cell is operated with the molten electrolyte at a temperature from 730° to 910° C.  
     
     
         105 . The method of  claim 99 , wherein aluminium is produced on an aluminium-wettable cathode, in particular a drained cathode.