High stability flow-through non-carbon anodes for aluminium electrowinning

Abstract

A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprises a non-carbon metal-based anode having an electrically conductive metallic structure. This anode structure comprises an outer part with an electrochemically active anode surface on which, during electrolysis, oxygen is anodically evolved, and which is suspended in the electrolyte substantially parallel to a facing cathode. The anode structure has one or more flow-through openings extending from the active anode surface through the metallic structure, the flow-through opening(s) being arranged for guiding a circulation of electrolyte driven by the fast escape of anodically evolved oxygen. The outer part of the anode comprises a layer that contains predominantly cobalt oxide CoO to enhance the stability of the anode.

1 . A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprising at least one non-carbon metal-based anode having an electrically conductive metallic structure that comprises an outer part with an electrochemically active anode surface on which, during electrolysis, oxygen is anodically evolved, and which is suspended in the electrolyte substantially parallel to a facing cathode, said metallic structure having one or more flow-through openings extending from the active anode surface through the metallic structure, said flow-through opening(s) being arranged for guiding a circulation of electrolyte driven by the fast escape of anodically evolved oxygen, wherein said outer part of the anode comprises a layer that contains predominantly cobalt oxide CoO. 
     
     
         2 . The cell of  claim 1 , wherein the anode structure is a foraminate structure. 
     
     
         3 . The cell of  claim 2 , wherein the anode structure comprises a series of parallel anode members, in particular horizontal anode members having electrochemically active surfaces in a generally coplanar arrangement to form said active anode surface, the anode members being spaced apart to form longitudinal flow-through openings for the circulation of electrolyte driven by the fast escape of anodically evolved oxygen. 
     
     
         4 . The cell of  claim 3 , wherein the anode members are blades, bars, rods or wires. 
     
     
         5 . The cell of  claim 1 , wherein the active anode surface is substantially horizontal. 
     
     
         6 . The cell of  claim 1 , wherein the active anode surface is substantially vertical or inclined to the horizontal. 
     
     
         7 . The cell of  claim 1 , wherein the molten electrolyte is at a temperature below 950° C., in particular in the range from 910° to 940° C., and consists of:
 6.5 to 11 weight % dissolved alumina, in particular 7 to 10 weight %; 
 35 to 44 weight % aluminium fluoride, in particular 36 to 42 weight % aluminium fluoride, such as 36 to 38 weight; 
 38 to 46 weight % sodium fluoride, in particular 39 to 43 weight %; 
 2 to 15 weight % potassium fluoride, in particular 3 to 10 weight % potassium fluoride, such as 5 to 7 weight %; 
 0 to 5 weight % calcium fluoride, in particular 2 to 4 weight % calcium fluoride; and 
 0 to 5 weight % in total of one or more further constituents, in particular up to 3 weight %. 
 
     
     
         8 . The cell of  claim 7 , wherein the electrolyte contains as further constituent(s) 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  claim 1 , wherein the electrolyte contains alumina at a concentration near saturation on the active anode surface. 
     
     
         10 . The cell of  claim 1 , wherein the CoO-containing layer is integral with a core made of cobalt or a cobalt alloy. 
     
     
         11 . The cell of  claim 1 , wherein the anode comprises an electrically conductive substrate that is covered with an applied electrochemically active coating that comprises the CoO-containing layer. 
     
     
         12 . The cell of  claim 11 , wherein the CoO-containing layer is a layer of sintered particles. 
     
     
         13 . The cell of  claim 11 , wherein the CoO-containing layer is an integral oxide layer on an applied Co-containing metallic layer of the coating. 
     
     
         14 . The cell of  claim 11 , which comprises an oxygen barrier layer between the CoO-containing layer and the electrically conductive substrate. 
     
     
         15 . The cell of  claim 14 , wherein the oxygen barrier layer contains at least one metal selected from nickel, copper, tungsten, molybdenum, tantalum, niobium and chromium, or an oxide thereof. 
     
     
         16 . The cell of  claim 15 , wherein the oxygen barrier layer further contains cobalt. 
     
     
         17 . The cell of  claim 16 , wherein the oxygen barrier layer is a cobalt alloy containing at least one metal selected from nickel, tungsten, molybdenum, tantalum and niobium. 
     
     
         18 . The cell of  claim 17 , wherein the cobalt alloy contains:
 at least one of nickel, tungsten, molybdenum, tantalum and niobium in a total amount of 5 to 30 wt %, in particular 10-20 wt %; and   one or more further elements and compounds in a total amount of up to 5 wt %,   
       the balance being cobalt. 
     
     
         19 . The cell of  claim 18 , containing as said further elements at least one of aluminium, silicon and manganese. 
     
     
         20 . The cell of  claim 14 , wherein the CoO-containing layer is integral with the oxygen barrier layer. 
     
     
         21 . The cell of  claim 14 , wherein the oxygen barrier layer is integral with the electrically conductive substrate. 
     
     
         22 . The cell of  claims 14 , wherein the oxygen barrier layer and the CoO-containing layer, or precursors thereof, are distinct applied layers. 
     
     
         23 . The cell of  claim 13 , wherein the Co-containing metallic layer contains cobalt in an amount of at least 95 wt %, in particular more than 97 wt % or 99 wt %. 
     
     
         24 . The cell of  claim 13 , wherein the Co-containing metallic layer contains at least one additive selected from silicon, manganese, nickel niobium, tantalum and aluminium in a total amount of 0.1 to 2 wt %. 
     
     
         25 . The cell of  claim 11 , wherein the electrically conductive substrate comprises at least one metal selected from chromium, cobalt, hafnium, iron, nickel, copper, platinum, silicon, tungsten, molybdenum, tantalum, niobium, titanium, tungsten, vanadium, yttrium and zirconium, or a compound thereof, in particular an oxide, or a combination thereof. 
     
     
         26 . The cell of  claim 25 , wherein the electrically conductive substrate has an outer part made of cobalt or a cobalt-rich alloy to which the coating is applied. 
     
     
         27 . The cell of  claim 26 , wherein the outer part is made of a cobalt-rich alloy containing at least one of nickel, tungsten, molybdenum, tantalum and niobium, said cobalt alloy containing in particular:
 at least one of nickel, tungsten, molybdenum, tantalum and niobium in a total amount of 5 to 30 wt %, in particular 10-20 wt %; and   one or more further elements and compounds in a total amount of up to 5 wt %,   
       the balance being cobalt. 
     
     
         28 . The cell of  claim 11 , wherein the electrically conductive substrate contains or consists essentially of one or more oxidation-resistant metals. 
     
     
         29 . The cell of  claim 28 , wherein said one or more oxidation-resistant metals is/are selected from nickel, cobalt, chromium and niobium. 
     
     
         30 . The cell of  claim 25 , wherein the electrically conductive substrate is an alloy of nickel, iron and copper, in particular an alloy containing: 65 to 85 weight % nickel; 5 to 25 weight % iron; 1 to 20 weight % copper; and 0 to 10 weight % further constituents. 
     
     
         31 . The cell of  claim 10 , wherein the core is made of the same material as: the oxygen barrier layer of  claim 16 , the Co-containing metallic layer of  claim 23 , or the cobalt-rich alloy of  claim 27 . 
     
     
         32 . The cell of  claim 1 , wherein the CoO-containing layer has an open porosity of up to 12%, in particular up to 7%. 
     
     
         33 . The cell of  claim 1 , wherein the CoO-containing layer has a porosity with an average pore size below 7 micron, in particular below 4 micron. 
     
     
         34 . The cell of  claim 1 , wherein the CoO-containing layer contains cobalt oxide CoO in an amount of at least 80 wt %, in particular more than 90 wt % or 95 wt %. 
     
     
         35 . The cell of  claim 1 , wherein the CoO-containing layer is substantially free of Co 2 O 3  and substantially free of Co 3 O 4 . 
     
     
         36 . The cell of  claim 1 , wherein the CoO-containing layer is electrochemically active for the oxidation of oxygen ions and is uncovered or is covered with an electrolyte-pervious layer. 
     
     
         37 . The cell of  claim 1 , wherein the CoO-containing layer is covered with an applied protective layer, in particular an applied oxide layer. 
     
     
         38 . The cell of  claim 37 , wherein the applied protective layer contains cobalt oxide. 
     
     
         39 . The cell of  claim 37 , wherein the applied protective layer contains iron oxide. 
     
     
         40 . The cell of  claim 39 , wherein the applied protective layer contains oxides of cobalt and of iron, in particular cobalt ferrite. 
     
     
         41 . The cell of  claim 37 , wherein the applied protective layer contains a cerium compound, in particular cerium oxyfluoride. 
     
     
         42 . The cell of  claim 37 , wherein the applied protective layer is electrochemically active for the oxidation of oxygen ions and is uncovered or is covered with an electrolyte pervious-layer. 
     
     
         43 . The cell of  claim 1 , which has an electrochemically active surface that contains at least one dopant, in particular at least one dopant selected from iridium, palladium, platinum, rhodium, ruthenium, silicon, tungsten, molybdenum, tantalum, niobium, tin or zinc metals, Mischmetal, metals of the Lanthanide series, as metals and compounds, in particular oxides, and mixtures thereof. 
     
     
         44 . The cell of  claim 43 , wherein the electrochemically active surface is made of an active material containing the dopant(s) in a total amount of 0.1 to 5 wt %, in particular 1 to 4 wt %. 
     
     
         45 . The cell of  claim 1 , comprising a cathode that has an aluminium-wettable surface, in particular a horizontal or inclined drained surface. 
     
     
         46 . The cell of  claim 45 , wherein the cathode has an aluminium-wettable coating that comprises a refractory boride and/or an aluminium-wetting oxide. 
     
     
         47 . The cell of  claim 1 , wherein the anode is suspended in the electrolyte by a stem, in particular a stem having an outer part comprising a layer that contains predominantly cobalt oxide CoO. 
     
     
         48 . A method of producing aluminium in a cell as defined in  claim 1 , comprising passing an electric current through the or each active anode structure to its active anode surfaces as electronic current and through the electrolyte to the cathode as ionic current, thereby electrolysing the dissolved alumina to produce aluminium on the cathode(s) and oxygen on the active anode surface(s) which is released through said flow-through openings. 
     
     
         49 . The method of  claim 48 , wherein oxygen ions are oxidised on the anode's layer that contains predominantly cobalt oxide CoO. 
     
     
         50 . The method of  claim 48 , wherein oxygen ions are oxidised on an active layer applied to the anode's layer that contains predominantly cobalt oxide CoO. 
     
     
         51 . A non-carbon metal-based anode for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprising an electrically conductive metallic structure that comprises an outer part with an electrochemically active anode surface on which oxygen is anodically evolved and which is suspended in the electrolyte substantially parallel to a facing cathode during use, said metallic structure having one or more flow-through openings extending from the active anode surface through the metallic structure, said flow-through opening(s) being arranged for guiding during use a circulation of electrolyte driven by the fast escape of anodically evolved oxygen, wherein said outer part of the anode comprises a layer that contains predominantly cobalt oxide CoO.