Aluminium production cells with iron-based metal alloy anodes

Abstract

An iron-based metal anode for the electrowinning of aluminium by the electrolysis of alumina in a molten fluoride electrolyte has an electrochemically active integral outside oxide layer on an iron-based alloy that consists of 75 to 90 weight % iron; 0.5 to 5 weight % in total of at least one rare earth metal, in particular yttrium; 1 to 10 weight % aluminium; 0 to 10 weight % copper; 0 to 10 weight % nickel; and 0.5 to 5 weight % of other elements. The total amount of aluminium, copper and nickel is in the range from 5 to 20 weight %; and the total amount of rare earth metal(s), aluminium and copper is also in the range from 5 to 20 weight %. The electrochemically active surface layer is predominantly of iron oxide that slowly dissolves into the electrolyte during operation and is maintained by progressive slow oxidation of iron at the interface of the bulk metal of the alloy with the oxide layer. This progressive slow oxidation of iron corresponds to the dissolution of iron into the electrolyte which remains at or below saturation level at the operating temperature, the operating temperature being maintained sufficiently low to limit the contamination of the product aluminium to an acceptable level, and the electrolyte being circulated to maintain a sufficient concentration of alumina in the anode cathode gap.

1 . An iron-based metal anode for the electrowinning of aluminium by the electrolysis of alumina in a molten fluoride electrolyte, having an electrochemically active integral outside oxide layer on an iron-based alloy that consists of: 
 75 to 90 weight % iron;    0.5 to 5 weight % in total of one or more rare earth metals, in particular yttrium;    1 to 12 weight % aluminium;    0 to 10 weight % copper;    0 to 10 weight % nickel; and    0 to 5 weight % of other elements,    wherein the total amount of aluminium, copper and nickel is in the range from 5 to 20 weight %; and the total amount of rare earth metal(s), aluminium and copper is in the range from 5 to 20 weight %.    
     
     
         2 . The anode of  claim 1 , wherein the iron-based alloy contains yttrium in an amount of 0.5 to 3 weight %.  
     
     
         3 . The anode of  claim 1  or  2 , wherein the iron-based alloy contains aluminium in an amount of 2 to 10 weight %, preferably 4 to 8 weight %.  
     
     
         4 . The anode of  claim 1 ,  2  or  3 , wherein the iron-based alloy contains copper in an amount of 0.5 to 8 weight %.  
     
     
         5 . The anode of any preceding claim, wherein the iron-based, alloy contains nickel in an amount of 0.5 to 8 30 weight %.  
     
     
         6 . The anode of any preceding claim, wherein the iron-based alloy contains, as said other element(s), at least one of molybdenum, manganese, titanium, tantalum, tungsten, vanadium, zirconium, niobium, chromium, cobalt, silicon and carbon.  
     
     
         7 . The anode of any preceding claim, wherein the iron-based alloy contains said other element(s) in an amount up to 2 weight %.  
     
     
         8 . The anode of any preceding claim, wherein the iron-based alloy contains copper in an amount of 2 to 6 weight % and/or nickel in an amount of 2 to 8 weight %.  
     
     
         9 . The anode of any preceding claim, wherein the iron-based alloy contains aluminium in an amount of 4 to 6 weight %.  
     
     
         10 . The anode of any preceding claim, wherein the total amount of aluminium, copper and nickel is in the range from 8 to 18 weight %.  
     
     
         11 . The anode of any preceding claim, wherein the total amount of rare earth metal, aluminium and copper is in the range from 8 to 18 weight %.  
     
     
         12 . The anode of any preceding claim, wherein the iron-based alloy consists of: 
 80 to 90 weight % iron;    0.5 to 3 weight % yttrium;    2 to 6 weight % aluminium;    1 to 8 weight % copper;    1 to 8 weight % nickel; and    0 to 5 weight % of other elements.    
     
     
         13 . The anode of any preceding claim wherein the iron-based alloy contains copper and nickel in a weight ratio Cu:Ni in the range 1:3 to 3:1.  
     
     
         14 . The anode of any preceding claim, wherein the iron-based alloy is made by casting iron together with said metals as additives.  
     
     
         15 . A cell for the electrowinning of aluminium by the electrolysis of alumina in a molten fluoride electrolyte utilising an iron-based metal anode having an ectrochemically active integral outside oxide layer according to any one of the preceding claims.  
     
     
         16 . The cell of  claim 15 , wherein during operation the electrochemically active integral outside oxide layer of the anode slowly dissolves into the electrolyte and is maintained by progressive slow oxidation of iron at the interface of the metal bulk of the alloy with the oxide layer.  
     
     
         17 . The cell of  claim 15  or  16 , wherein the concentration of alumina dissolved in the electrolyte is below 10 weight %, preferably between 5 weight % and 8 weight %.  
     
     
         18 . The cell of claim any one of  claims 1  to  17 , comprising an aluminium-wettable cathode.  
     
     
         19 . The cell of any one of  claims 16  to  18 , wherein the progressive slow oxidation of iron at the interface of the bulk of the alloy with the oxide layer corresponds to the dissolution of iron into the electrolyte at a rate such that the maximum concentration of iron species in the electrolyte is at or below the saturation level of iron species in the electrolyte at the operating temperature.  
     
     
         20 . The cell of  claim 19 , wherein the operating temperature is maintained sufficiently low to control the dissolution of iron into the electrolyte and limit the contamination of the product aluminium to an acceptable level.  
     
     
         21 . The cell of  claim 20 , wherein the operating temperature is below 930° C., preferably between 840° C. and  
     
     
         22 . The cell of any one of  claims 15  to  21 , wherein the electrolyte contains NaF and AlF 3  in a molar ratio in the range from 1.2 to 2.4.  
     
     
         23 . The cell of any one of  claims 15  to  22 , which is arranged to circulate alumina-depleted electrolyte away from the electrochemically active oxide layer of the anode(s), enrich the electrolyte with alumina, and circulate alumina-enriched electrolyte towards the electrochemically active oxide layer of the anode(s).  
     
     
         24 . A method for the electrowinning of aluminium by the electrolysis of alumina in a molten fluoride electrolyte, comprising dissolving alumina in the electrolyte and electrolysing the alumina-containing electrolyte to produce aluminium on a cathode and oxygen on an iron-based metal anode as claimed in any one of  claims 1  to  14 .  
     
     
         25 . The method of  claim 24 , wherein the electrochemically active integral outside oxide layer of the anode slowly dissolves into the electrolyte and is maintained by progressive slow oxidation of iron at the interface of the metal bulk of the alloy with the oxide layer providing a dissolution of iron into the electrolyte at a rate such that the maximum concentration of iron species in the electrolyte is at or below the saturation level of iron species in the electrolyte at the operating temperature.  
     
     
         26 . The method of  claim 24  or  25 , wherein the operating temperature is maintained sufficiently low to control the dissolution of iron into the electrolyte and limit the contamination of the product aluminium to an acceptable level.  
     
     
         27 . The method of  claim 26 , wherein the operating temperature is below 930° C., preferably between 840° C. and 890° C.  
     
     
         28 . The method of any one of  claims 24  to  27 , wherein the electrolyte contains NaF and AlF 3  in a molar ratio in the range from 1.2 to 2.4.  
     
     
         29 . The method of any one of  claims 24  to  28 , wherein the concentration of alumina dissolved in the electrolyte is below 10 weight %, preferably between 5 weight % and 8 weight %.  
     
     
         30 . The method of any one of  claims 24  to  29 , wherein alumina-depleted electrolyte is circulated away from the electrochemically active. oxide layer of the anode(s), enriched with alumina, and alumina-enriched electrolyte is circulated towards the electrochemically active oxide layer of the anode(s).  
     
     
         31 . The method of any one of  claims 24  to  30 , wherein aluminium is produced on an aluminium-wettable cathode.  
     
     
         32 . The method of  claim 31 , wherein the product aluminium is continuously drained from the aluminium-wettable cathode.