Aluminium electrowinning with metal-based anodes

Abstract

A process for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte in a cell operating at reduced temperature, typically below 870° C., utilising nickel-alloy based anodes, in particular nickel-iron alloy anodes. The electrolyte contains AlF 3 in such a high concentration, usually above 20 weight %, in addition to cryolite, that fluorine-containing ions rather than oxygen ions are oxidised on the anodes. However, only oxygen is evolved, the evolved oxygen being derived from the dissolved alumina present near the anodes. The anodes may be porous at the surface so as to provide a high active surface area for operation at low current density.

1 . A process for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte in a cell operating at reduced temperature and utilising metal-based anodes comprising an alloy of nickel and an alloying metal having an outer part consisting predominantly of nickel which forms an electrochemically active surface for the oxidation of ions, in which the electrolyte contains AlF 3  in such a high concentration that fluorine-containing ions predominantly rather than oxygen ions are oxidised on the electrochemically active surfaces, however, only oxygen is evolved, the evolved oxygen being derived from the dissolved alumina present near the electrochemically active anode surfaces.  
     
     
         2 . The process of  claim 1 , wherein dissolved alumina predominantly combines with oxidised fluorine ions to produce aluminium fluoride and oxygen.  
     
     
         3 . The process of  claim 2 , wherein dissolved alumina combines with monoatomic nascent fluorine formed by oxidation of fluorine ions to produce oxygen gas and partly dissociated aluminium fluoride.  
     
     
         4 . The process of  claim 1 , wherein aluminium oxyfluoride ions predominantly rather than oxygen ions are oxidised.  
     
     
         5 . The process of  claim 4 , wherein aluminium oxyfluoride ions resulting from the combination of aluminium fluoride and alumina rather than oxygen ions are oxidised on the electrochemically active surfaces into transient aluminium oxyfluoride which decomposes into oxygen and aluminium fluoride.  
     
     
         6 . The process of  claim 1 , wherein the operating temperature of the electrolyte is below 900° C., preferably below 880° C., and even more preferably below 870° C.  
     
     
         7 . The process of  claim 1 , wherein the electrolyte contains cryolite and, in addition to cryolite, an excess of AlF 3  in an amount of at least 20 weight % of the electrolyte, preferably between 25 and 35 weight % of the electrolyte.  
     
     
         8 . The process of  claim 1 , wherein the electrolyte further contains CaF 2  and/or MgF 2 .  
     
     
         9 . The process of  claim 1 , wherein said alloying metal of the nickel alloy is iron.  
     
     
         10 . The process of  claim 1 , wherein the outer part of the anode comprises more than 75 weight % nickel, preferably between 85 and 95 weight % nickel.  
     
     
         11 . The process of  claim 1 , wherein the outer part has an open porosity defining a high surface area electrochemically active surface, current being passed at a low current density on the high surface area electrochemically active surface.  
     
     
         12 . The process of  claim 11 , wherein part of said alloying metal of the nickel alloy dissolves into the electrolyte to form said open porosity.  
     
     
         13 . The process of  claim 1 , comprising circulating electrolyte containing dissolved aluminium to constantly maintain dissolved alumina near the electrochemically active anode surfaces.  
     
     
         14 . A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte operating at reduced temperature and utilising metal-based anodes comprising an alloy of nickel and an alloying metal having an outer part consisting predominantly of nickel which forms an electrochemically active surface for the oxidation of ions, in which the electrolyte contains AlF 3  in such a high concentration that fluorine-containing ions predominantly rather than oxygen ions are oxidised on the electrochemically active surfaces, however, only oxygen is evolved, the evolved oxygen being derived from the dissolved alumina present near the electrochemically active anode surfaces.  
     
     
         15 . The cell of  claim 14 , wherein the temperature of the electrolyte is below 900° C., preferably below 880° C., even more preferably below 870° C.  
     
     
         16 . The cell of  claim 14 , wherein the electrolyte contains cryolite and, in addition to cryolite, an excess of AlF 3  in an amount of at least 20 weight % of the electrolyte, preferably between 25 and 35 weight % of the electrolyte.  
     
     
         17 . The cell of  claim 14 , wherein the electrolyte further contains CaF 2  and/or MgF 2 .  
     
     
         18 . The cell of  claim 14 , wherein said alloying metal of the nickel alloy is iron.  
     
     
         19 . The cell of  claim 14 , wherein the outer part of the anodes comprises more than 75 weight % nickel, preferably between 85 and 95 weight % nickel.  
     
     
         20 . The cell of  claim 14 , wherein the nickel alloy has a decreasing concentration of said alloying metal towards the electrochemically active surface layer.  
     
     
         21 . The cell of  claim 20 , wherein the nickel alloy has a nickel metal rich outer part with a porosity defining a high surface area electrochemically active surface, said porosity containing cavities which are partly or completely filled during use with fluorides of at least one metal selected from nickel, said alloying metal and aluminium.  
     
     
         22 . The cell of  claim 20 , wherein the nickel metal rich outer part comprises nickel metal and said alloying metal in a nickel/alloying metal atomic ratio of more than 3 where it reaches the electrochemically active surface.  
     
     
         23 . The cell of  claim 14 , wherein the alloy of nickel with said alloying metal has before use a nickel/alloying metal ratio below 1.  
     
     
         24 . The cell of  claim 14 , wherein the alloy of nickel with said alloying metal has before use a nickel/alloying metal ratio of at least 1, in particular from 1 to 4.  
     
     
         25 . The cell of  claim 14 , wherein the alloy of nickel with said alloying metal contains one or more additives, the alloy before use containing nickel with said alloying metal in a total amount of at least 85 weight %, preferably at least 95 weight %, and the balance said additive(s).  
     
     
         26 . The cell of  claim 24 , wherein one or more additives are selected from chromium, copper, cobalt, silicon, titanium, tantalum, tungsten, vanadium, yttrium, molybdenum, manganese, aluminium and niobium in a total amount of up to 10 weight % in particular up to 5 weight %, of the alloy before use.  
     
     
         27 . The cell of  claim 25 , wherein one or more additives are catalytically active and selected from iridium, palladium, platinum, rhenium, 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 before use.  
     
     
         28 . The cell of  claim 14 , wherein before anodic polarisation the nickel alloy is covered with an integral oxide film obtainable by oxidising the alloy in an oxidising atmosphere.  
     
     
         29 . The cell of  claim 14 , wherein each anode is a nickel iron alloy-based anode.