Abstract:
The present technology discloses a method for producing aluminum from alumina in an electrolytic cell through the use of carbon monoxide as a partial or total reactant with aluminum oxide. The present technology also discloses the structure of an electrolytic cell configured to house this reaction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to and the benefit of the filing date of U.S. Provisional Application No. 61/871,485, entitled Electrochemical Cell for Aluminum Production Using Carbon Monoxide, filed on Aug. 29, 9013, which is incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    This application relates to the use of carbon monoxide as a partial or total reactant with aluminum oxide in an electrochemical cell used to produce aluminum metal. 
       BACKGROUND 
       [0003]    It is often desirable to obtain aluminum and other metals through processes including electrolysis. A number of processes have been developed to obtain and process such metals. U.S. Pat. Nos. 3,718,550, 4,039,401, 4,430,189, 4,478,693, 4,929,328, 5,505,823, 5,580,437, 5,538,607, 6,719,890, 6,790,337, 7,901,560, and 8,070,921, which are incorporated herein by reference in their entireties, disclose such methods and/or systems for processing metals. One such process is the Hall-Heroult process to produce aluminum in an electrolytic cell. The Hall Heroult process generally involves dissolving alumina in a molten electrolyte, e.g., cryolite, and electrolyzing the molten bath within the electrolytic cell. 
         [0004]    However, none of these disclosures mention the use of carbon monoxide as a substitute for a consumable carbon in an anode. Therefore, a need exists for producing aluminum and other metals without producing consumable carbon anodes for Hall Heroult cells. 
       SUMMARY 
       [0005]    The present technology discloses an electrolytic cell for producing aluminum from alumina. The electrochemical cell may be a pre-existing cell or it may be manufactured specifically for this technology. 
         [0006]    In one embodiment, the cell may comprise a shell comprising an interior and an exterior defined by a top, a bottom, and sides. The cell may include an interior and an exterior defined by the shell. The cell may further comprise an electrolyte solution that may be contained within the interior of the shell. The electrolyte solution may comprise cryolite. The cell may further comprise an anode immersed in the electrolyte solution of the cell. The anode may be powered by an anode bus. The cell may further comprise a cathode affixed to the bottom or sides of the shell and may be immersed in the electrolyte solution of the cell. The cathode may be powered by a cathode bus. The cell may further comprise a gas inlet spanning from the exterior of the cell to the interior of the cell. The gas inlet may have a first opening on the exterior of the cell and a second opening in the electrolyte solution on the interior of the cell. The cell may further comprise a gas outlet spanning from the interior of the cell to the exterior of the cell. The gas outlet may have a first opening in the electrolyte solution on the interior of the cell and a second opening on the exterior of the cell. The cell may further comprise a metal outlet spanning the interior of the cell to the exterior of the cell. The metal outlet may have a first opening in the electrolyte solution on the interior of the cell and a second opening on the exterior of the cell. 
         [0007]    In one embodiment, the present technology provides an electrolytic cell for producing aluminum from alumina where the cell may have an interior and an exterior. The cell may comprise an electrolyte solution contained within the interior of the cell and an anode immersed in the electrolyte solution of the cell. The cell may further comprise a gas inlet spanning from the exterior of the cell to the interior of the cell, the gas inlet having a first opening on the exterior of the cell and a second opening in the electrolyte solution on the interior of the cell. Additionally, the cell may comprise a gas outlet spanning from the interior of the cell to the exterior of the cell. The gas outlet may have a first opening in the electrolyte solution on the interior of the cell and a second opening on the exterior of the cell and a metal outlet spanning the interior of the cell to the exterior of the cell. The metal outlet may have a first opening in the electrolyte solution on the interior of the cell and a second opening on the exterior of the cell. 
         [0008]    In one embodiment, a gas inlet and a gas outlet may be added to a pre-existing electrochemical cell. 
         [0009]    In one embodiment, the present technology discloses a method for using the above described cell. The method further may comprise providing carbon monoxide to the interior of the cell through the gas inlet. The method may comprise completing a reaction in the cell via the following equation: 
         [0000]      2Al 2 O 3 (in electrolyte)+6CO( g )=&gt;4Al(1)+6CO 2 ( g ), 
         [0000]    plating liquid aluminum metal on the cathode, exhausting the aluminum metal from the cell through the metal outlet, and exhausting the carbon dioxide through the cell through the gas outlet. 
         [0010]    The present technology also discloses a method of manufacturing an electrolytic cell as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a cross-sectional view of a modified Hall-Heroult type electrolytic cell. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Reference will now be made in detail to exemplary embodiments of the described herein. It is to be understood that other embodiments may be utilized and structural and functional changes may be made without departing from the respective scope of the disclosure. Moreover, features of the various embodiments may be combined or altered without departing from the scope of the disclosure. As such, the following description is presented by way of illustration only and should not limit in any way the various alternatives and modifications that may be made to the illustrated embodiments and still be within the spirit and scope of the disclosure. 
         [0013]    It is generally known that aluminum may be produced from alumina using an electrolytic cell, i.e., a Hall Heroult cell. Use of such a cell comprises providing a molten salt electrolyte in an electrolytic cell having alumina dissolved therein. An anode extending through the surface into the electrolyte and a cathode on the bottom or sides of the cell are provided. The cathode is comprised of a base material that is reactive with molten aluminum to form a reaction layer that may be wetted by aluminum. An electrical lead from the cathode extends from the cell to a bus bar outside the cell to conduct electricity. Electrical current is passed through the cell, thereby reducing alumina in the electrolyte near the bottom of the anode and depositing aluminum at the cathode. 
         [0014]    In one embodiment, the cell may be any appropriate electrochemical cell with the addition of means to supply carbon monoxide gas to an anode region of the cell. The electrochemical cell may be pre-existing or it may be manufactured specifically for this technology. In one embodiment of the present technology, a solid anode comprises carbon and the electrolyte may be molten cryolite or any other appropriate material at a temperature over 900° C. In one embodiment, the anode may include any one or a combination of carbon or cermet or other suitable materials, which may function as an anode. The cathode may be prepared by providing a base material having high electrical resistivity such as boron carbide and contacting or reacting the surface of the base material to provide a layer such as aluminum boride wettable with molten aluminum. This permits low electrically conductive material having high stability in molten aluminum to function as a cathode. 
         [0015]    The Gibbs Free Energy, denoted as ΔG 0 , of the process may be estimated for the base case in which solid carbon is converted to carbon dioxide as shown in Equation (1): 
         [0000]      2Al 2 O 3  (in cryolite)+3C→4Al(1)+3CO 2 ( g )  (1)
 
         [0016]    The Gibbs Energy is given by H-TS, where H is enthalpy in Joules, T is temperature in Kelvin, and S is entropy in Joules/Kelvin. The values of H and S are provided in Table 1 below. 
         [0000]                                                                                                                                                TABLE 1                   Enthalpy and Entropy of Hall Heroult Reactants and Products.                Compound       Enthalpy of Formation   Entropy                            Al 2 O 3     −1675.7   kJ/mol   0.05092 kJ/(mol K)                C   0   5.6                CO   −110.53   kJ/mol (gas)   0.19766 kJ/(mol K)                Al   0    0.0283 kJ/(mol K)                CO 2     −393.509   kJ/mol (gas)   0.05107 kJ/(mol K)                        
Applying these terms to the Gibbs Free Energy,
 
         [0000]      Δ G= 3 H   CO2 −3 TS   CO2 +4 H   Al −4 TS   Al −2 H   Al2O3 +2 TS   Al2O3 −3 H   C +3 TS   C , or
 
         [0000]      Δ G= 3(−393.509)−3 T (0.05107)+4(0)−4 T (0.0283)−2(−1675.7)+2 T (0.05092)−3(0)+3 T (0.0056),
 
         [0000]      Δ G= 2170.873−0.14777  T KJ/mol.  
 
         [0000]    For a temperature of 1000° C. (1273 K),
 
Since three carbons are involved,
 
         [0000]      ΔG=660.93 KJ/mol.
 
         [0017]    In  FIG. 1 , a modified Hall Heroult electrolytic cell  10  for use in electrolytically reducing alumina to aluminum is shown. Cell  10  may be comprised of a shell  12  having sides  14 , a bottom  16 , and an open top  18 . Alternatively, cell  10  may have a partially-open top or a closed top. Shell  12  may be comprised of steel, nickel, superalloys comprising nickel, metal alloys with a melting temperature of at least 1100° C., or any other appropriate material. Sides  14  and bottom  16  may be provided with a layer of thermal insulation (not shown). A lining (not shown) may also be provided inside the insulation layer to contain an electrolyte solution  20 . The thermal insulation may be comprised of a polymer material, including, but not limited to, an acrylate, polyurethane, polyolefin, polyester, or any other appropriate material. The lining may be comprised of carbon blocks, graphite blocks, or any other appropriate material. Cell  10  may contain an electrolyte solution  20  within its shell  12 , within its thermal insulation, or within its lining. Electrolyte solution  20  may comprise a molten cryolite electrolyte, a fluoride salt, or any other appropriate electrolyte ad/or salt solution. Electrolyte solution  20  may be at any appropriate temperature, including, but not limited to, a temperature above 800° C., above 850° C., above 900° C., above 950° C., even above 1000° C. Here, as elsewhere in the specification and claims, numerical values may be combined to form new or undisclosed ranges. 
         [0018]    Cell  10  also comprises an anode  22  that may be fully or partially immersed in electrolyte solution  20 . Anode  22  may be comprised of any appropriate material, including, but not limited to, carbon, graphite, coke, silicon carbide, boron carbide, niobium carbide, tungsten carbide, hafnium carbide, tantalum carbide, zirconium carbide, molybdenum carbide, titanium carbide, silicon nitride, boron nitride, titanium diboride, or a combination of two or more thereof. Additionally, anode  22  may be solid or may include naturally-occurring pores or man-made pores. For example, anode  22  may be comprised wholly or partially of porous carbon, porous graphite, porous coke, porous silicon carbide, porous boron carbide, porous niobium carbide, porous tungsten carbide, porous hafnium carbide, porous niobium carbide, porous tantalum carbide, porous zirconium carbide, porous molybdenum carbide, porous titanium carbide, porous silicon nitride, porous boron nitride, porous titanium diboride, or a combination of two or more thereof, all of which may allow gases and/or liquids to filter through anode  22 . Further, anode  22  may be operatively connected to an anode bus  24  located fully or partially outside of cell  10 . Anode bus  24  may power cell  10  and anode bus  24  may be comprised of metallic or non-metallic materials, including, but not limited to, copper, brass, aluminum, polymer, graphite, or any other appropriate material or combination of materials. Anode bus  24  provides electrical power to drive reactions to occur in cell  10 . 
         [0019]    Cell  10  further comprises a cathode  26  having a bottom surface  28  that rests on either sides  14  and/or bottom  16  of cell  10  and a top surface  30 . Cathode  26  may be positioned on top of the insulation of cell  10 . Alternatively, cathode  26  may be positioned directly on shell  12 . A layer of aluminum  32  may rest on top surface  30  of cathode  26 . Cathode  26  comprises a base material, including, but not limited to boron carbide, zirconium dioxide, graphite, or any other appropriate material that is stable in molten aluminum. The base material comprises a high electrical resistivity, e.g., greater than 1×10 −4  ohm-m. Here, as elsewhere in the specification and claims, numerical values may be combined to form new or undisclosed ranges. The cathode voltage may be controlled by a cathode bus  25 . 
         [0020]    Cell  10  also includes a metal outlet  34  extending from the interior of cell  10  through sides  14 , bottom  16 , or top  18  to the exterior of cell  10 . Metal outlet  34  may be a tube, pipe or other appropriate shape allowing for material to pass from inside of cell  10  out of cell  10 . Metal outlet  34  may have a first opening near layer of aluminum  32  to an external collection container (not shown) outside of cell  10 . Metal outlet  34  may be formed from any appropriate material, including, but not limited to, carbon, graphite, coke, silicon carbide, boron carbide, niobium carbide, tungsten carbide, hafnium carbide, tantalum carbide, zirconium carbide, molybdenum carbide, titanium carbide, silicon nitride, boron nitride, titanium diboride, or a combination of two or more thereof. 
         [0021]    Cell  10  also includes a gas inlet  36  and a gas outlet  38  extending through top  18  of cell  10  near anode  22 . Alternatively, gas inlet  36  and gas outlet  38  may extend through sides  14  and/or bottom  16  of shell  12 . Gas inlet  36  may be a tube, pipe or other appropriate shape allowing for material to pass from outside of cell  10  into cell  10 . Gas inlet  36  may have a first opening attached to an external source (not shown) outside of cell  10  and a second opening exiting into electrolyte solution  20  of cell  10 . Alternatively, gas inlet  36  may open into porous cathode  26 . The external source may be any appropriate man-made or natural source, including, but not limited to, carbon, graphite, coke, charcoal, char, etc. Gas inlet  36  may be formed from any appropriate material, including, but not limited to, carbon, graphite, coke, silicon carbide, boron carbide, niobium carbide, tungsten carbide, hafnium carbide, tantalum carbide, zirconium carbide, molybdenum carbide, titanium carbide, silicon nitride, boron nitride, titanium diboride, or a combination of two or more thereof. 
         [0022]    Gas outlet  38  may be a tube, pipe or other appropriate shape allowing for material to pass from inside of cell  10  to outside of cell  10 . Gas outlet  38  may have a first opening in electrolyte solution  20  of cell  10  and a second opening attached to a collection container (not shown) outside of cell  10 . The collection container may be a plenum, or any other appropriate container. Gas outlet  38  may be formed from any appropriate material, including, but not limited to, carbon, graphite, coke, silicon carbide, boron carbide, niobium carbide, tungsten carbide, hafnium carbide, tantalum carbide, zirconium carbide, molybdenum carbide, titanium carbide, silicon nitride, boron nitride, titanium diboride, or a combination of two or more thereof. Additionally, gas outlet  38  may further comprise a pressurized system to remove gases and/or liquids out of cell  10  through gas outlet  38 . 
         [0023]    Gas inlet  36  and gas outlet  38  may be comprised of the same materials or of different materials. Similarly, they may be of equal lengths, diameters, and shapes or alternatively, they may be of different lengths, diameters, and sizes. 
         [0024]    In one embodiment of the present technology, cell  10  with a negatively charged shell  12  with respect to positive anode  22  comprised of carbon, may be used to electrolytically reduce alumina to aluminum. Cell  10  is powered by anode bus  24  and cathode bus  25 . Cell  10  contains electrolyte solution  20  comprising cryolite electrolyte at a temperature between 900° C. to 1100° C. A supply of gaseous carbon monoxide is provided through gas inlet  36  near anode  26  into cell  10 . The carbon monoxide may be produced by a reaction of an external source, e.g., a reaction of carbon dioxide with carbon, graphite, charcoal, or char. These materials may be derived from natural sources, including, but not limited to, the atmosphere, byproducts of fire, emissions from organisms, etc. Alternatively, the materials may be produced from man-made sources, including, but not limited to, pollution such as emissions from power plants, chemical plants, factories, automobiles, etc. 
         [0025]    The carbon monoxide exits gas inlet  36  into cryolite electrolyte solution  20  at a temperature between 900° C. to 1000° C. Alternatively, the carbon monoxide may exit gas inlet  36  into porous anode  22  and carbon monoxide may exit anode  22  through the pores and enter electrolyte solution  20 . The carbon monoxide bubbles may come into electrical contact with anode  22 , resulting in a reduction of the aluminum oxide in cryolite electrolyte solution  20  to an aluminum metal. Equation (2) shows this reaction: 
         [0000]      2Al 2 O 3  (in cryolite)+6CO(g)=&gt;4Al(1)+6CO 2 (g)  (2);
 
         [0000]    therefore, 
         [0000]      Δ G =6 H   CO2 −6 TS   CO2 +4 H   Al −4 TS   Al −2 H   Al2O3 +2 TS   Al2O3 −6 H   CO +6 TS   CO ,
 
         [0000]      Δ G =6(−393.509)−6 T (51.07)+4( 0 )−4 T (28.3)−2(−1675.7)+2 T (50.92−6(−110.53)+6 T (197.66),
 
         [0000]      Δ G =1643.526+868.18 T kJ/mol , and
 
         [0000]      ΔG=2715 kJ/mol or 905 kJ.
 
         [0026]    This reaction is more endothermic than the base case discussed above. The very high entropy of carbon monoxide results in a significant temperature-dependent increase in endothermicity. This energy could be supplied via waste heat or combustion heat from another process, whereas the dominant term in Gibbs Free Energy comes from electricity. Since the total number of electrons transferred is the same as the base case, this indicates that a higher voltage must be used in the modified Hall Heroult cell. 
         [0027]    The reversible cell voltage may be calculated as follows: 
         [0000]    
       
         
           
             
               
                 E 
                 r 
               
               = 
               
                 
                   
                     
                       - 
                       Δ 
                     
                      
                     
                         
                     
                      
                     
                       G 
                       0 
                     
                   
                   
                     
                       n 
                       e 
                     
                      
                     F 
                   
                 
                 + 
                 
                   
                     RT 
                     
                       
                         n 
                         e 
                       
                        
                       F 
                     
                   
                    
                   ln 
                    
                   
                       
                   
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                         a 
                         
                           A 
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                            
                           l 
                         
                         4 
                       
                        
                       
                         a 
                         
                           CO 
                           2 
                         
                         3 
                       
                     
                     
                       
                         a 
                         
                           
                             Al 
                             2 
                           
                            
                           
                             O 
                             3 
                           
                         
                         2 
                       
                        
                       
                         a 
                         C 
                         2 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where ΔG 0  is the Gibbs free energy change; F is the Faraday constant, 96,490 C/mol; R is the ideal gas constant, 8.314 J/mol-K, n e  is the number of electrons (equal to 6.0 for conventional carbon anodes) and a Al2O3  is the activity of alumina. The other activities are equal to unity. The decomposition voltage is −1.17 volts at 1000° C. 
         [0028]    Based on the change in Gibbs free energy, carbon monoxide may be used as a substitute for solid carbon, but the cell voltage must be higher if carbon monoxide is used as the anode material in place of carbon. 
         [0029]    The use of a carbon anode in this embodiment may result in decreasing the tendency of solid carbon to be consumed in the above reaction of Equation (2). This may enhance the probability of supplied carbon monoxide participating in the reaction. 
         [0030]    According to Equation (2) above, aluminum 32 may accumulate on cathode  26 , and carbon monoxide oxidizes to create carbon dioxide as a coproduct of the same process. The carbon involved in the reaction may come from the carbon anode, the carbon monoxide, or both. The ratio of carbon anode consumed to aluminum metal produced during the reaction may be less than 1, less than 0.8, less than 0.6, less than 0.4, less than 0.3, less than 0.2, even less than 0.1. Here, as elsewhere in the specification and claims, numerical values may be combined to form new or undisclosed ranges. The ratio of carbon dioxide formed from carbon monoxide may be greater than 0.1, greater than 0.3, greater than 0.5, greater than 0.7, and even greater than 0.9. Here, as elsewhere in the specification and claims, numerical values may be combined to form new or undisclosed ranges. The reaction suggests that increasing the amount of carbon monoxide fed into the modified Hall Heroult cell may skew the equilibrium reaction so that more oxidization of carbon monoxide occurs than oxidation of carbon. Therefore, with this reaction, it is possible to reduce the average rate of carbon consumption in the aluminum synthesis reaction. In one embodiment, non-oxidizing materials are utilized in the anode of a Hall Heroult cell, to eliminate the need to consume solid carbon to produce aluminum. 
         [0031]    The carbon dioxide produced by the reaction described above may exit cell  10  via gas outlet  38 . The carbon dioxide may be released into the atmosphere, or alternatively, the carbon dioxide may be collected and/or pumped into a tank, pipeline, or any other appropriate conduit and/or container. In one embodiment, the carbon dioxide may be recycled and used as a source of carbon monoxide to feed the gas inlet  36  for another cycle of the reactions of cell  10 . If necessary, a pressurized system may aid in the removal of carbon dioxide from cell  10 . The aluminum created at cathode  26  may be removed from cell  10  as a liquid through metal outlet  34  and collected outside the cell. 
         [0032]    The foregoing detailed description has been given for clearness of understanding only and no unnecessary limitations should be understood therefrom as some modifications will be obvious to those skilled in the art without departing from the scope and spirit of the appended claims.