Patent Publication Number: US-2005121333-A1

Title: Electrolytic reduction of metal oxides

Description:
1. FIELD OF THE INVENTION  
      The present invention relates to electrolytic reduction of metal oxides.  
     2. BACKGROUND OF AND PRIOR ART TO THE INVENTION  
      The present invention was made during the course of an on-going research project on the electrolytic reduction of titania (TiO 2 ) carried out by the applicant.  
      During the course of the research project the applicant carried out experimental work on an electrolytic cell that included a graphite crucible that formed an anode of the cell, a pool of molten CaCl 2 -based electrolyte in the crucible, and a cathode that included solid titania.  
      The CaCl 2 -based electrolyte was a commercially available source of CaCl 2 , namely calcium chloride dihydrate, that decomposed on heating and produced a very small amount of CaO.  
      The applicant operated the electrolytic cell at a potential above the decomposition potential of CaO and below the decomposition potential of CaCl 2 .  
      The applicant found that the cell could electrolytically reduce titania to titanium with very low concentrations of oxygen.  
      The applicant does not have a clear understanding of the electrolytic cell mechanism at this stage. Nevertheless, whilst not wishing to be bound by the comments in this paragraph, the applicant offers the following comments by way of an outline of a possible cell mechanism. The applicant believes that operating the experimental electrolytic cell above a potential at which the CaCl 2 -based electrolyte partially decomposed had the result of producing Ca ++  cations that migrated to the vicinity of the titania in the cathode and provided a driving force that facilitated extraction of O −−  anions produced by electrolytic reduction of titania to titanium in the cathode. The applicant also believes that the O −−  anions, once extracted from the titania, migrated to the anode and reacted with anode carbon and produced CO and released electrons that facilitated electrolytic reduction of titania to titanium in the cathode. In addition, the applicant believes that carbon in the anode reacted with Ca ++  cations and produced a complex calcium carbide. The experimental worked carried out by the applicant produced evidence of Ca metal in the electrolyte. The applicant believes that the Ca metal was the result of electrodeposition of Ca ++  cations as Ca metal on electrically conductive sections of the cathode and that at least part of the Ca metal dissolved in the electrolyte and migrated to the vicinity of the titania in the cathode and participated in chemical reduction of oxides.  
      However, notwithstanding that the cell could electrolytically reduce titania to titanium with very low concentrations of oxygen, the applicant also found that there were relatively significant amounts of carbon transferred from the anode to the electrolyte and to the titanium produced at the cathode under a wide range of cell operating conditions. Carbon in the titanium is an undesirable contaminant. In addition, carbon transfer was responsible for low energy efficiency of the cell. Both problems are significant barriers to commercialisation of electrolytic reduction technology.  
      The applicant carried out experimental work to identify the mechanism for carbon transfer and to determine how to minimise carbon transfer and/or to minimise the adverse effects of carbon transfer.  
     3. SUMMARY OF INVENTION  
      Broadly, the invention resides in replacing the carbon anode with a molten metal anode.  
      According to the present invention there is provided an electrolytic cell for electrolytic reduction of a metal oxide in a solid state, which electrolytic cell includes (a) a molten electrolyte, (b) a cathode formed at least in part from the metal oxide in contact with the electrolyte, and (c) a molten metal anode in contact with the electrolye.  
      Preferably the metal of the molten metal anode has a relatively high saturation level for oxygen at the operating temperature of the cell.  
      Preferably the metal is chosen such that its melting point is within the operating temperature ranges of the electrolyte.  
      Preferably the melting point of the metal of the molten metal anode is higher than the melting point of the electrolyte and lower than the vaporisation and/or decomposition temperature of the electrolyte in order to prevent electrolyte consumption and removal through vaporisation.  
      Preferably the metal of the molten metal anode has a very low solubility in the molten electrolyte at the cell operating temperatures, as high solubility is detrimental because the anode metal will deplete and deposit on the cathode. The latter might not be a serious problem where there is low solubility and reactability of the metal with the cathode metal at the operating temperature.  
      Preferably the metal of the molten metal anode is silver or copper.  
      The solubility of oxygen in the Ag—O system at 1000° C. is around 0.32% by weight. Ag has a melting point of 960° C., which is about 300 to 100° C. above the melting point of alkali and alkaline earth halides that provide suitable electrolytes.  
      The solubility of oxygen in the Cu—O system at 1100° C. is 0.39% by weight. The melting point of copper is 1083° C., which is well below the boiling points of the above mentioned electrolytes.  
      Preferably the electrolytic cell further includes a means for removing oxygen that has diffused into the molten metal anode from the cell.  
      Such an “oxygen scavenging pump” means can have a number of different forms.  
      One option includes a duct that communicates with the molten metal anode and a device to create a partial pressure reduction between the molten metal anode and a head of molten metal within the duct.  
      An advantage of an “oxygen scavenging pump” means is that the amount of the molten metal anode required can be minimised, since the saturation wt % limits of oxygen within the molten anode metal are no longer the sole determining parameter of oxygen uptake by the anode.  
      For example, in order to reduce log of titania to pure titanium, 1 kg Ag would be required in the absence of an oxygen scavenging pump means to remove substantially all of the oxygen from the molten metal anode. Continuous removal of oxygen from the molten metal anode facilitated by the means allows the process to be performed continuously, as compared with batch processing.  
      According to the present invention there is also provided a method of electrolytically reducing a metal oxide in a solid state in an electrolytic cell, which electrolytic cell includes (a) a molten electrolyte, (b) a cathode in contact with the electrolyte, the cathode being formed at least in part from the metal oxide, and (c) a molten metal anode in contact with the electrolye, which method includes applying a cell potential across the anode and the cathode.  
      Preferably the method includes maintaining the cell temperature above the melting points of the electrolyte and the metal of the metal anode.  
      Preferably the method includes maintaining the cell temperature below the vaporisation and/or decomposition temperatures of the electrolyte.  
      Preferably the method includes applying a cell potential above a decomposition potential of at least one constituent of the electrolyte so that there are cations of a metal other than that of the cathode metal oxide in the electrolyte.  
      Preferably the metal oxide is a titanium oxide.  
      It is preferred that the metal oxide be titania.  
      In a situation in which the metal oxide is titania it is preferred that the electrolyte be a CaCl 2 -based electrolyte that includes CaO as one of the constituents.  
      In such a situation it is preferred that the method includes maintaining the cell potential above the decomposition potential for CaO.  
      It is also preferred that the method includes maintaining the cell potential below the decomposition potential for CaCl 2 .  
      It is preferred that the method includes maintaining the cell potential less than 3.0V.  
      It is preferred particularly that the method includes maintaining the cell potential below 2.5V.  
      It is preferred more particularly that the method includes maintaining the cell potential below 2.0V.  
      It is preferred that the method includes maintaining the cell potential at least 1.5V.  
      The following example illustrates an application of the invention in the process of reducing titania into substantially pure titanium using an electrolytic cell constructed in accordance with the present invention and as illustrated schematically in  FIG. 1 . 
    
    
     4. DESCRIPTION OF EXEMPLARY EMBODIMENT  
       FIG. 1  is a schematic illustration of a electrolytic cell that can be scaled-up in application of the present invention. 
    
    
      Whilst the example described below relates to the electrolytic reduction of titania, the basic principle is equally applicable to other metal oxides, in particular oxides of Si, Ge or alloys containing these metals.  
      With reference to the figure, the electrolytic cell  5  includes a graphite-free crucible  10  made of a suitable refractory material that is essentially inert as regards reaction with the electrolyte and electrode materials described below at cell operating temperatures of between 1000° C. and 1200° C.  
      The electrolytic cell further includes a pool  18  of molten CaCl 2  electrolyte within the crucible  10 .  
      The electrolytic cell  5  further includes a pool  14  of molten silver or copper (within the crucible  10 . The molten Ag or Cu forms the anode  14  of the cell. In view of the different densities, the molten metal anode  14  is below the molten electrolyte pool  18 .  
      The electrolytic cell  5  further includes a titania plate  12  positioned within a cage  12   b . The cage  12   b  (and therefore the plate  12 ) is suspended into the crucible  10  by means of a lead  12   a . This assembly forms the cathode  20  of the cell.  
      The electrolytic cell  5  further includes a power source  16  and electrical connections between the power source  16  and the anode  14  and the cathode  20 . The connections include electrical leads  17  and  12   a  and a suitable high-temperature resistant plate member  15 , preferably of stainless steel, that provides electric connection between the interior of crucible  10  (and thus anode  14 ) and the lead  17 .  
      In use, power source  16  provides for constant potential (voltage) settings thereby allowing the cell  5  to draw the amount of charge required during the electrolytic refining of the metal oxide body at a selectable potential.  
      The electrolytic cell  5  further includes type B thermocouples contained in heat-resistant, inert sheaths (not illustrated) for monitoring temperature in the molten metal anode  14  and the molten electrolyte  18 .  
      The electrolytic cell  5  further includes a refractory tube  20  that connects the interior of the crucible  10 , below the molten metal anode bath level (a), with a device for imparting a negative pressure differential between anode bath  14  and the head (b) of molten Ag suctioned into the tube  20 . The pressure differential need only be slight to provide a driving force for diffusion and transport of oxygen that is dissolved into the metal anode bath  14  into the tube  20  which is preferably vented to atmosphere.  
      In use, the above-described electrolytic cell  5  is positioned in a suitable furnace to maintain the electrolyte and the anode metal in their respective molten states. The atmosphere around the crucible  10  is preferred to be an inert gas, such as argon, that does not react with the molten electrolyte.  
      Once the cell reaches its operating temperature, about 1150 to 1200° C., depending on the anode metal employed, a constant voltage of around 2.5-3 V is applied between the cell electrodes  12  and  14 , the cell potential being above the decomposition potential of CaO in the electrolyte but below the decomposition potential of CaCl 2 , whereby reduction of the titania in the cathode is carried out as described above.  
      The oxygen that passes into the electrolyte  18  is subsequently transported to the metal bath anode  14  where it dissolves. The dissolved oxygen then diffuses through the molten anode bath  14  under the pressure differential imparted through duct  20  and is released as O 2  into the surrounding atmosphere.  
      As will be noted, this transport mechanism is effective for as long as oxygen in the molten metal anode is below the saturation level.  
      By way of example, it is noted that other shapes and configurations of the titania cathode  20  are equally employable, bearing in mind the need to ensure proper electric contact between the power source  16  and the titania to be reduced within the cell.