Abstract:
An improved method of producing aluminum in an electrolytic cell containing alumina dissolved in an electrolyte, the method comprising the steps of providing a molten salt electrolyte at a temperature less than 900° C. having alumina dissolved therein in an electrolytic cell having a liner for containing the electrolyte, the liner having a bottom and walls extending upwardly from the bottom, the liner being substantially inert with respect to the molten electrolyte. A plurality of non-consumable anodes and cathodes are disposed in the electrolyte and an electric current is passed through the anodes and through the electrolyte to the cathodes depositing aluminum on the cathodes and generating oxygen bubbles at the anodes, the bubbles stirring the electrolyte. Periodically, the electric current flow to the cell is reduced for extended periods. The electrolyte and aluminum in the cell is maintained in a molten condition during the extended periods of reduced current flow by application of heat to the bottom for purposes of heating the cell.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to aluminum reduction cells and more particularly, it relates to a method for controlling the temperature in low temperature electrolytic reduction cells used for production of aluminum from alumina dissolved in a molten salt electrolyte.  
           [0002]    The use of low temperature (less than about 900° C.) electrolytic cells for producing aluminum from alumina have great appeal because they are less corrosive to cermet or metal anodes and other materials comprising the cell. The Hall-Heroult process, by comparison, operates at temperatures of about 950° C. This results in higher alumina solubility but also results in greater corrosion problems. Also, in the Hall-Heroult process, carbon anodes are consumed during the process and must be replaced on a regular basis. In the low temperature cells, non-consumable anodes are used and such anodes evolve oxygen instead of carbon dioxide which is produced by the carbon anodes.  
           [0003]    However, operation of low temperature electrolytic cells is not without problems. For example, before operating the cell, there is the need to heat the cell to operating temperature. In a conventional Hall-type cell, the temperature of the cell is raised at startup by electrical resistance heating of a coke bed in the cell cavity or by directing gas flames into the cell cavity. It will be appreciated that these methods of heating are not suitable for low temperature cells.  
           [0004]    Another problem with low temperature cells includes controlling temperature of the cell during operation. In conventional cells, temperature of operation can be controlled by changing the distance between the anode and the cathode. However, this method is not applicable in a low temperature cell employing fixed inert anodes and dimensionally stable cathodes because the anode-cathode distance is usually set at startup. U.S. Pat. Nos. 5,415,742 and 5,279,715 suggest varying the power input to an electrolytic cell to control temperature by varying the amount of electrolyte between the electrodes or varying the extent of cross-sectional area available for current flow between the anodes and the cathodes.  
           [0005]    U.S. Pat. No. 4,333,803 discloses a method and apparatus for maintaining a predetermined energy balance in a device, such as an aluminum reduction cell. The apparatus includes a relatively short and thin heat flow sensor having a first and second thermocouple located within opposite closed ends of a hollow thermally conductive body. Each thermocouple is composed of two wires of the same dissimilar metals. The sensor is secured by one closed end of the sensor body to an outside surface of the wall member to extend substantially perpendicular to the location on the wall without significantly affecting the heat flow from the wall surface being measured.  
           [0006]    U.S. Pat. No. 5,882,499 discloses a process for regulating the temperature of electrolytic cells. It involves acting on the temperature of the pot by means of the setpoint resistance Ro which is modulated so as to correct the temperature both by anticipation and by reversed feedback. Correction by anticipation, known as “a priori” correction, allows for known, quantified disturbances and allows their effect on the temperature of the pot to be compensated in advance. Reversed feedback correction, known as “a posteriori” correction, involves determining, from direct measurement at regular time intervals of the temperature of the electrolytic bath, a mean temperature corrected as a function of periodic operating procedures and allows the variations and deviations from the setpoint temperature to be compensated.  
           [0007]    U.S. Pat. No. 466,460 discloses recovering aluminum from aluminum chloride by electrolysis when the aluminum chloride is heated to a high temperature and pressure.  
           [0008]    U.S. Pat. No. 473,866 discloses employing an electric current to effect electrolytic decomposition, and maintaining the state of fusion by the combined heating effects of such current and a flame or like auxiliary source of heat which, like the heat due to the current, acts directly on the ore next the electrodes rather than through the walls of a furnace or crucible.  
           [0009]    U.S. Pat. No. 3,632,488 discloses a method of controlling an aluminum reduction cell in which the heat flow coefficient for the bath is determined and used with a desired bath temperature to calculate the bath&#39;s heat loss energy. Calculations are also made of the cell&#39;s power requirements for purposes other than heating the bath such as the energy required to reduce the cell&#39;s alumina. The sum of the bath heat loss and other energy requirements is divided by the cell&#39;s base amperage to determine a set voltage and the cell&#39;s anode is adjusted to keep the cell voltage within predetermined limits of the set voltage.  
           [0010]    U.S. Pat. No. 4,045,309 discloses that the energy balance in an aluminum reduction cell is controlled by measuring the temperature of the side lining of the cell, preferably at the level of the surface of the electrolyte, comparing the measured temperature with a reference temperature, and when the difference between the measured and reference temperatures exceeds a given value, adjusting the depth of immersion of the cell anodes within the electrolyte.  
           [0011]    U.S. Pat. No. 4,146,444 discloses a method for preheating a molten salt electrolysis cell having an electrode which includes at least one element protruding into the interior of the cell. The method disclosed includes the distribution of a carbonaceous aggregate around such an element, and the ignition of this aggregate, so that the element may be brought to an elevated temperature without breaking due to the effects of thermal gradients.  
           [0012]    U.S. Pat. No. 4,181,584 discloses a method for heating an electrolytic cell wherein holes are drilled in the solid electrolyte, e.g., to the floor of the cell, to provide space for supporting blocks. The holes are spaced a predetermined distance apart to position at least one anode between them. Supporting blocks with a length sufficient to extend from the floor to at least the level of the solid electrolyte beneath the anode are placed in the holes and a resistance heater, preferably one having a positive change in resistivity with temperature, is disposed between the supporting blocks at least one of which is electrically conductive. The anode of the cell is lowered into electrical contact with the resistance heater and current sufficient to heat the resistance heater to at least the melting temperature of the electrolyte is passed from the anode through the resistance heater. Heating of the resistance heater is continued until the solid electrolyte in the cell has melted.  
           [0013]    U.S. Pat. No. 4,608,135 relates to an improvement in a Hall cell. The improvement comprises an air passageway between the insulating layer and the outer surface of the carbon lining sidewall and an air inlet port adjacent the bottom of the passageway for passing air into the air passageway and along the outer surface of the carbon lining sidewall whereby the carbon sidewall may be cooled sufficiently to permit the formation of a protective layer of frozen bath on the inner surface thereof. The heated air then flows across the top of the cell whereby the cell retains at least a part of the heat exchanged through the sidewall.  
           [0014]    U.S. Pat. No. 4,865,701 discloses that alumina is reduced to molten aluminum in an electrolytic cell containing a molten electrolyte bath composed of halide salts and having a density less than alumina and aluminum and a melting point less than aluminum. The cell comprises a plurality of vertically disposed, spaced-apart, non-consumable, dimensionally stable anodes and cathodes. Alumina particles are dispersed in the bath to form a slurry. Current is passed between the electrodes, and oxygen bubbles form at the cathodes. The oxygen bubbles agitate the bath and enhance dissolution of the alumina adjacent the anodes and inhibit the alumina particles from settling at the bottom of the bath. The molten aluminum droplets flow downwardly along the cathodes and accumulate at the bottom of the bath.  
           [0015]    In spite of these disclosures, there is still a great need for an improved low temperature electrolytic cell for producing aluminum from alumina which includes means for bringing the cell to operating temperature, a method of controlling the temperature of the cell during operation and means for maintaining the cell at temperature while reducing the electric current input for load leveling purposes during peak power demand periods.  
         SUMMARY OF THE INVENTION  
         [0016]    It is an object of the invention to provide a method for producing molten aluminum in an electrolytic cell.  
           [0017]    It is another object of the invention to provide an improved method and apparatus for raising the temperature of a low temperature, electrolytic cell for the production of aluminum to operating temperature.  
           [0018]    It is another object of the invention to provide a method for controlling the temperature of a low temperature electrolytic cell during the production of aluminum from alumina.  
           [0019]    It is still another object of the invention to provide a method and apparatus for maintaining a low temperature cell used for production of aluminum at temperature during periods of reduced electric current input to the cell during peak power demand.  
           [0020]    These and other objects will become apparent from a reading of the specification and claims appended hereto.  
           [0021]    In accordance with these objects, there is provided an improved method of producing aluminum in an electrolytic cell containing alumina dissolved in an electrolyte, the method comprising the steps of providing a molten salt electrolyte at a temperature less than 900° C. having alumina dissolved therein in an electrolytic cell having a liner for containing the electrolyte, the liner having a bottom and walls extending upwardly from the bottom, the liner being substantially inert with respect to the molten electrolyte. A plurality of non-consumable anodes and cathodes are disposed in the electrolyte and an electric current is passed through the anodes and through the electrolyte to the cathodes depositing aluminum on the cathodes and generating oxygen bubbles at the anodes, the bubbles stirring the electrolyte. Periodically, the electric current flow to the cell is reduced for extended periods. The electrolyte and aluminum in the cell is maintained in a molten condition during the extended periods of reduced current flow by application of heat to the bottom for purposes of heating the cell. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a plan view illustrating an embodiment of the invention which may be used in the practice of the invention.  
         [0023]    [0023]FIG. 2 is a cross-sectional view of an electrolytic cell along line A-A of FIG. 1.  
         [0024]    [0024]FIG. 3 is a cross-sectional view of an electrolytic cell along line B-B of FIG. 1.  
         [0025]    [0025]FIGS. 4A and 4B are cross-sectional views of a channel used for delivering molten aluminum.  
         [0026]    [0026]FIG. 5 is a cross-sectional view of an electrolytic test cell showing a conduit or collector in connection with cathodes for delivering molten metal to a reservoir.  
         [0027]    [0027]FIG. 6 is a view along line C-C of FIG. 5.  
         [0028]    [0028]FIG. 7 is a cross-sectional view along line D-D of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    In FIG. 1, there is shown a top or plan view of an embodiment of the invention which illustrates an electrolytic cell  2  for the electrolytic production of aluminum from alumina dissolved in an electrolyte contained in the cell. Cell  2  comprises a metal or alloy liner  4  having bottom and sides for containing electrolyte. Non-consumable or inert anode  6  is shown mounted vertically inside liner  4  which preferably has the same composition as anode  6 . Further, as shown in FIG. 1, anode  6  is connected to liner  4  by means of straps  8  to provide an electrical connection therebetween. Also, liner  4  is shown connected to bus bar  14 A by electrical conducting strap  9 . Cathodes  10  are shown positioned on either side of anode  6 . Cathodes  10  are electrically connected to bus bar  14 B by appropriate connection means such as strap  16 . Liner  4  is layered with thermal insulating material  18  such as insulating fire brick which is contained with a metal shell  20 .  
         [0030]    In operation, electrical current from bus bar  14 A flows through electrical strap  9  into anodic liner  4 . Current from liner  4  flows through conducting straps  8  to anodes  6  then through an electrolyte to cathodes  10 . The current then flows from cathodes  10  along connection means  16  to a second bus bar  14 B. Additional electrolytic cells may be connected in series on each side of cell  2 .  
         [0031]    While any inert anode including cermets or metal alloys may be used in the electrolytic cell of the invention, it is preferred that the anode material including the anodic liner be comprised of Cu—Ni—Fe compositions that have resistance to oxidation by the electrolyte. Suitable anode compositions are comprised of 25-70 wt. % Cu, 15-60 wt. % Ni and 1-30 wt. % Fe. Within this composition, a preferred anode composition is comprised of 35-70 wt. % Cu, 25-48 wt. % Ni and 2-17 wt. % Fe with typical compositions comprising 45-70 wt. % Cu, 28-42 wt. % Ni and 13-17 wt. % Fe.  
         [0032]    It will be noted that a number of anodes and cathodes is employed with the anodes and cathodes used in alternating relationship.  
         [0033]    In the plan view in FIG. 1, there is shown a schematic of conduit  30  (see also FIGS. 2 and 3) for conveying molten aluminum from cathodes  10  to a molten aluminum reservoir  34 . In FIG. 1, molten aluminum reservoir  34  is shown contained within liner  4 . Thus, aluminum produced at cathodes  10  is collected in conduit  30  and is conveyed to molten aluminum reservoir  34  for removal from the cell.  
         [0034]    [0034]FIG. 2 is a cross-sectional view along line A-A of FIG. 1 showing anodic liner  4 , straps  8  connecting anodes to the liner, cathode  10 , strap  9  connecting liner  4  to bus bar  14 A and insulation  18  contained between anodic liner  4  and metal shell  20 . Also, shown in FIG. 2 is electrical connection means  16  used to connect cathodes  10  to bus bar  14 B. Connection means  16  may be comprised of a flexible metal strap  22  which is connected to bus bar  14 B. Flexible metal strap  22  is connected to cathode  10  by collector bars  24  which are slotted on the bottom and straddle or fit over cathode  10 . Strap  22  is connected to collector bar  24  utilizing an aluminum cap  26 . That is, aluminum cap  26  is cast on collector bar  24  and strap  22  is welded thereto. Electrical connection between the cathode and collector bar may be provided by using aluminum metal at the connection. That is, aluminum metal becomes molten at cell operating temperature and wets both the cathode and collector bar, particularly if both cathode and collector bar are fabricated from titanium diboride. To guard against air burn of collector bar  24  during operation, a sleeve or tube of alumina  28  or like material may be used to cover or surround collector bar  24 .  
         [0035]    Referring further to FIG. 2, it will be seen that anodic liner  4  has vertical sides  32  and bottom referred to generally as  36 . Bottom  36  has two sides  38  which are contiguous with walls or sides  32 . Sides  38  of bottom  36  are sloped downwardly towards a central trough or channel  40 . Channel  40  is filled with an electrical insulating material  42 , substantially non-reactive with bath or aluminum. Electrical insulating material  42  may be selected from alumina and boron nitride or other suitable non-reactive material. A tube  44  of refractory material, e.g., titanium diboride, is positioned in insulating material  42  to carry molten aluminum away from cathodes  10  to reservoir  34 .  
         [0036]    Cathodes  10  are shown positioned under surface  46  of electrolyte  45  and spaced substantially equally from sides  32  of liner  4 . Cathodes  10  have a lower surface or edge  48  which rest on blocks  50  which are electrically insulating blocks, e.g., alumina or boron nitride blocks. Lower surface or edges  48  are shown positioned parallel to sides  38  of liner  4 . Cathodes  10  terminate in a point or end  52  provided in slotted opening  58  in tube  44  (see FIG. 3). In operation of the cell, aluminum deposited on the cathode flows towards point or end  52  and into tube  44  from where it is removed to reservoir  34 . Grooves  54  may be provided in cathode  10  to aid in the flow of molten aluminum on the cathode surface towards point or end  52  for purposes of collection.  
         [0037]    [0037]FIG. 3 is a cross-sectional view along line B-B of FIG. 1 showing liner  4 , anodes  6 , cathodes  10 , molten aluminum reservoir  34 , and refractory tube  44  for transferring or carrying molten aluminum from cathodes  10  to molten aluminum reservoir  34 . It will be noted that refractory tube  44  has a central bore  56  having slotted openings  58  therein approximate or adjacent cathodes  10 . Openings  58  permit molten aluminum collected at the cathodes to pass into bore  56  and flow towards molten aluminum reservoir  34 . Molten aluminum in bore  56  passes through opening  60  into molten aluminum reservoir  34  where a body  62  of molten aluminum collects therein. A layer  64  of electrolyte  45  may be provided on top of body  62  to protect against oxidation of molten aluminum with air. The head of electrolyte or bath contained by liner  4  forces aluminum from the cathodes into bore  56  and therefrom into reservoir  34 . The aluminum produced is collected continuously from all the cathodes and directed to body  62  which is contained in an electrically insulated vessel or reservoir.  
         [0038]    While not wishing to be bound by any theory of invention, the collection of body  62  of aluminum is explained as follows. That is, with reference to FIG. 3, there is shown the heads of electrolyte in cell  2 . Also shown is the head of aluminum reservoir  34 . The top of tube  44  is used as the reference plane. The head of electrolyte in cell  2  is denoted as h b1  and the total head in collection vessel or reservoir  34  is denoted as h a +h b2 . The pressure from the heads h a +h b2 must be less than the pressure from the electrolyte or bath head h b1  to prevent aluminum leaking out of joints or openings  58  between cathodes  10  and tube  44 . This concept may be represented by the following formula: 
           h   b1 ρ b1   ≧h   a ρ a   +h   b2 ρ b2   Eq.(1) 
         [0039]    If equality is used in Eq.(1) and the following values are assumed,  
         [0040]    h b1 =45 cm (i.e., 18 inch high cathodes)  
         [0041]    h b2 =5 cm  
         [0042]    ρ b1 =1.97 g/cm 3    
         [0043]    ρ b2 =1.97 g/cm 3    
         [0044]    ρ a =2.36 g/cm 3    
         [0045]    these values give h a  (max.)=33 cm, or a total maximum head (h b2 h a ) in the collection vessel of 38 cm.  
         [0046]    Aluminum  62  is removed from reservoir  34  by periodic siphoning. When the aluminum is tapped from collection vessel  34 , the head difference between the bath and the vessel is 45-5=40 cm. Bath then has to be excluded from tube  44  by interfacial tension of aluminum/bath in slots or openings  58  between the cathodes  10  and tube  44 . The width of slot or opening  44  can be calculated by: 
           t≦ 2 γ/Δhρg , where t is the width of opening  58   Eq.(2) 
         [0047]    Using the following values:  
         [0048]    γ=500 dyne/cm  
         [0049]    Δh=40 cm  
         [0050]    ρ=1.97 g/cm 3    
         [0051]    g=980 dyne/gm  
         [0052]    gives t (max.)=0.013 cm (0.13 mm or 130 μm).  
         [0053]    Thus, for a cell of this size, the width of opening  58  would have be on the order of 130 μm.  
         [0054]    During startup of a cell, there is a substantial increase in temperature. Thus, it may be necessary to accommodate the differential expansion between lining  4  and refractory tube  44 . FIGS. 4A and 4B illustrates joints which may be used in conjunction with refractory tube  44 . These joints permit differential expansion between lining  4  and refractory tube  44  during cell startup. It will be seen from FIG. 4A that refractory tube  44  is comprised of joints  68  where the one end of tube  44  fits into another part of tube  44 . A space is provided at joint  68  to care for any differential expansion which may occur between lining  4  and refractory tube  44 . In FIG. 4B, another type of joint is disclosed to accommodate differential expansion during startup of cell  2 . That is, at joint  70 , a tubular member  72  is provided inside refractory tube  44  overlapping joint  70  to ensure against leakage and yet provide for differential thermal expansion. Tubular member  72  may be comprised of the same or similar material as refractory tube  44 .  
         [0055]    This invention was tested in a 300A cell having configuration as shown in FIGS. 5 and 6. In FIG. 5, the cell was comprised of anodic liner  4 , anodes  6  and cathodes  10 . A molybdenum tube  44  was passed through openings  76  in the bottom of cathodes  10  (see FIG. 6) and inserted into alumina reservoir  34 . Openings or slits  58  were provided adjacent cathode faces to receive molten aluminum deposited at the cathode during cell operation. Opening  74  in alumina reservoir  34  was provided with less than 0.25 mm clearance for tube  44 . It was found that if opening  74  was coated or sprayed with a material wettable with aluminum, e.g., molybdenum, a seal was facilitated to exclude bath. The openings  76  are shown in bottom of cathodes  10  in FIG. 6 which is a cross-sectional view along line C-C of FIG. 5. The cathodes were comprised of TiB 2  and the anodes were comprised of Fe—Ni—Cu alloy. A layer of bath  45  was provided in reservoir  34  to avoid oxidation of molten aluminum  62 . The electrolyte in cell  4  consisted essentially of NaF:AlF 3  eutectic, about 45 mol. % AlF 3  and had 6 wt. % excess alumina dispersed therein. The cell was operated for 4-6 hours at a temperature of 760° C. and a current density of 100 amps. After operation, it was found that aluminum was collected in reservoir  34 .  
         [0056]    While reference herein has been made to TiB 2  cathodes, it will be understood that the cathodes can be comprised of any suitable material that is substantially inert to the molten aluminum such as zirconium boride, molybdenum, titanium carbide and zirconium carbide.  
         [0057]    The anode can be any non-consumable anode selected from cermet or metal alloy anodes inert to electrolyte at operating temperatures. The cermet is a mixture of metal such as copper and metal oxides and the metal alloy anode is substantially free of metal oxides. A preferred oxidation-resistant, non-consumable anode for use in the cell is comprised of iron, nickel and copper, and containing about 1 to 50 wt. % Fe, 15 to 50 wt. % Ni, the remainder consisting essentially of copper. A further preferred oxidation-resistant, non-consumable anode consists essentially of 1-30 wt. % Fe, 15-60 wt. % Ni and 25 to 70 wt. % Cu. Typical oxidation-resistant, non-consumable anodes can have compositions in the range of 2 to 17 wt. % Fe, 25 to 48 wt. % Ni and 45 to 70 wt. % Cu.  
         [0058]    The electrolytic cell can have an operating temperature less than 900° C. and typically in the range of 660° C. (1220° F.) to about 800° C. (1472° F.). Typically, the cell can employ electrolytes comprised of NaF+AlF 3  eutectics, KF+AlF 3  eutectic, and LiF. The electrolyte can contain 6 to 26 wt. % NaF, 7 to 33 wt. % KF, 1 to 6 wt. % LiF and 60 to 65 wt. % AlF 3 . More broadly, the cell can use electrolytes that contain one or more alkali metal fluorides and at least one metal fluoride, e.g., aluminum, fluoride, and use a combination of fluorides as long as such baths or electrolytes operate at less than about 900° C. For example, the electrolyte can comprise NaF and AlF 3 . That is, the bath can comprise 62 to 53 mol. % NaF and 38 to 47 mol. % AlF 3 .  
         [0059]    As noted, thermal insulation  18  is provided around liner  4 . Also, a lid  3  shown in FIG. 2 is provided having insulation sufficient to ensure that the cell can be operated without a frozen crust and frozen sidewalls.  
         [0060]    The vertical anodes and cathodes in the cell are spaced to provide an anode-cathode distance in the range of ¼ to 1 inch. Electrical insulative spacers  5  (FIG. 3) can be used to ensure maintenance of the desired distance between the anode and cathode. In addition, bottom edge  54  of cathode  10  should be maintained at a distance of ¼ to 1 inch from bottom  38  of anode liner  4  in order to ensure adequate current density and gas evolution on the bottom to keep alumina suspended.  
         [0061]    In the present invention, the anodes and cathodes have a combined active surface ratio in the range of 0.75 to 1.25.  
         [0062]    In the low temperature electrolytic cell of the invention, alumina has a lower solubility level than in conventional Hall-type cells operated at a much higher temperature. Thus, in the present invention, solubility of alumina ranges from about 2 wt. % to 5 wt. %, depending to some extent on the electrolyte and temperature used in the cell. Higher temperatures will result in higher solubility levels for alumina. To ensure against anode effect, an excess of alumina over solubility may be maintained in the electrolyte. Thus, the cell can be operated with a slurry of alumina (undissolved alumina) in the electrolyte in the range of 0.2 to 30 wt. % with a preferred slurry containing undissolved alumina in the range of 5 to 10 wt. % alumina. The ranges provided herein include all the numbers within the range as if specifically set forth. Alumina can be added from hopper  70  (FIG. 2) to the space between electrodes and wall of sides  32  of liner  4 . The alumina is added in an amount such that the density of the slurry does not exceed 2.3 g/cm 3 , which is the density of molten aluminum.  
         [0063]    During operation of the cell, oxygen is produced at the anode surfaces and bubbles upwardly through electrolyte slurry  45  producing stirring in the cell. The stirring resulting from the evolution of gas bubbles provides dissolution of alumina in the electrolyte and aids in maintaining saturation of dissolved alumina. The stirring also ensures a constant supply of dissolved alumina to the anode surface. Further, the gas bubbles maintain undissolved alumina particles in suspension in the cell and prevents or inhibits alumina particles from settling to the bottom of the cell. Thus, it will be seen that the anodic liner importantly contributes to evolution of gaseous bubbles to enhance the performance of the cell, and thus suspended alumina particles are maintained substantially uniformly distributed throughout the electrolyte. Bayer-type alumina particles may be used and are approximately 100 μm in diameter, but composed of an agglomeration of smaller particles. Ground alumina with 1 μm particles has been used in laboratory tests.  
         [0064]    Alumina useful in the cell can be any alumina that is comprised of finely divided particles. Usually, the alumina has a particle size in the range of about 1 to 100 μm with a preferred size being in the range of 1 to 10 μm.  
         [0065]    In the present invention, the cell can be operated at a current density in the range of 0.1 to 1.5 A/cm 2  while the electrolyte is maintained at a temperature in the range of 660° to 800° C. A preferred current density is in the range of about 0.4 to 0.6 A/cm 2  of anode surface. The lower melting point of the bath (compared to the Hall cell bath which is above 950° C.) permits the use of lower cell temperatures, e.g., 730° to 800° C., which increases current efficiency in the cell and reduces corrosion of the anodes and cathodes.  
         [0066]    In another embodiment of the invention, the temperature of the cell may be controlled during operation of the cell or during reduced current usage at peak power demand periods when the cost of electricity is high. Thus, referring to FIG. 1, there is shown means  80  for heating cell  2 , for example, during startup when no current is flowing or during periods of reduced operation such as reduced current flow during periods of peak power usage. Means  80  is comprised of gas burner tubes  82  (shown in outline form) which extend underneath sides  38  of bottom  36  (see FIGS. 2 and 7). In FIG. 1, means  80  for heating cell  2  is provided with a gas inlet  84  and an exhaust gas outlet  86 . That is, when cell  2  is required to be heated, gas is introduced at  84  and burned with air in burners  82 . The exhaust gases are removed at end  86  of the gas burner tube. By reference to FIG. 7, it will be seen that gas burner tubes  82  are provided with openings  88  which emit gas for purposes of controlled burning with air. The flow of gas to burner tube  82  may be controlled or regulated using a gas control  98  well known in the gas industry. The gas control also determines the amount of heat being applied to the cell. By references to FIG. 2, it will be noted that tubes  82  are provided with openings  90  which permit air to flow underneath bottom  36  of cell  2  for purposes of burning when the cell is being heated. As noted, exhaust gases are removed at exit  86  during heating. An air pump  94  may be used to forcibly introduce air through air inlet  92  (FIG. 7) to the burners and remove gases after burning.  
         [0067]    The use of metal liner  4  permits the addition of heat to the cell as described.  
         [0068]    The use of external heat applied through liner  4  in the manner described is useful in cell startup situations. For purposes of startup, electrolyte or bath is added to the cell in powdered form and placed between the anodes  6  and cathodes  10 . Gas burner tube  82  is lighted and heat is transferred through metal liner  4  or bottom  36  to melt the powdered electrolyte. As the electrolyte melts, additional electrolyte is added to bring the bath to operating level  46 . Alumina is dissolved in the electrolytic melt. Thereafter, electrical current is passed through the cell and the electrolyte to produce aluminum. The electrolytic production of aluminum generates heat and thus the gas heaters are turned off.  
         [0069]    When the cell is operational, heat must be removed. The cell is designed so that heat lost therefrom during operation is less than the amount of heat generated by the electrochemical reaction within the cell. That is, during operation of the cell to produce aluminum, the heat loss through the sides, lid and bottom of the cell is less than heat generated in the cell as a result of the electrochemical reaction, resulting in an accumulation of extra heat in the cell. The extra heat must be removed at a controlled rate to maintain the cell at steady state during operation. Thus, for purposes of the present invention, air is introduced to tube  92  using air pump  94 . From tube  92 , air is directed underneath bottom  36  and exhausted through exit  86  to remove heat from the cell. As noted, air is circulated underneath the bottom of the cell using air pump  94  to control the rate of flow across the bottom of the cell to remove heat therefrom. Air pump  94  is controlled by an electronic controller such as a programmable logic controller (PLC). The PLC receives readings or signals from thermocouple  96  which monitors the temperature of electrolyte  45 . In the controller, the reading or signal from the thermocouple is compared to a set signal or reading. In response to the comparison, the air flow rate can be increased, decreased, or maintained. That is, the PLC can be programmed to increase or decrease the flow of air from air pump  94 , depending on the temperature of the electrolyte, and thus the electrolyte is maintained in a controlled temperature range.  
         [0070]    Thermocouple  96  may be protected from bath  45  by any suitable protective sleeve which is resistant to attack by molten electrolyte when the thermocouple is immersed in the bath. The protective sleeve may be comprised of graphite or molybdenum, for example. If excess alumina is used in the bath to provide a slurry, a protective alumina sleeve may be used. In another embodiment, the thermocouple may be placed in a wall such as a side wall to monitor the temperature of liner  4  to maintain temperature control. In this embodiment, a protective sheath is not required. Thus, the temperature of cell  2  may be controlled in this manner without the need for anode and cathode adjustments and thus the cell can be set for optimum production.  
         [0071]    The electrolytic production of aluminum requires considerable use of electric power and thus, the cost of such power has a large impact on the cost of producing aluminum metal. Accordingly, it is extremely important to find the lowest cost electric power. For example, it is important to avoid the usage of electric power during peak power periods because the cost of the power is much higher than off-peak power rates. Thus, it can be seen that there is great economic incentive to reduce cell current during peak power periods. In the present invention, cell current can be reduced or even stopped during peak power periods without concern for freezing electrolyte and molten aluminum contained in the cell. When the electric current is reduced, heat can be added to the cell through bottom  36  using gas heater  82 . That is, when the PLC detects a sufficient drop in temperature of electrolyte  45  by thermocouple  96 , it sends a signal to gas controller  98  to introduce sufficient gas along with air to gas heater  82  to heat cell  2 . An electronic ignition can be used to ignite the gas and the PLC can be used to continuously adjust the gas and air flow rates to maintain a temperature such that the electrolyte and aluminum contained in the cell remain in molten condition. At the end of the peak power period, full cell current is used again, gas is turned off and the cell returned to normal operation. In this way, the cell can be operated to effectively avoid daily peak power rates.  
         [0072]    In another aspect of the invention, the effect of peak power rates can be avoided in the low temperature cell in another way. That is, current flow to the cell can be reduced during peak power rates from operating level to a level sufficient to maintain the cell in molten condition. Thus, voltage of the cell can be reduced from a normal operating voltage of about 3.5 volts and heat must be added. At about 3.1, e.g., 3.062 volts, zero heat is generated. The difference between 3.5 and 3.1 volts, i.e., 0.4 volts, produces the designed heat loss through the insulation. Reducing the voltage to 2.3 volts stops the current and metal production. Heat must be added to the cell using an outside source, as described herein at a voltage and current below the design level. Thus, it will be appreciated that a combination of reduced voltage and outside heat as described may be used and such is contemplated. After the period of peak power rates, the flow is returned to normal operating conditions.  
         [0073]    The following example is further illustrative of the invention.  
       EXAMPLE  
       [0074]    An apparatus was used comprising the liner for the 300A cell and a single molybdenum (Mo) cathode. The cathode was located beneath the electrolyte and was a flat plate, ⅛″ (0.32 cm) thick, of rectangular cross section except at the bottom. The bottom edge was brought to a point in the center of the cross section, with the bottom edges at angles of about 7 degrees from horizontal. Under the electrolyte, this cathode plate measured 4″ (10.2 cm) across, 4″ (10.2 cm) high along each outside edge, and 4.25″ (10.8 cm) height in the center (at the point). These two sloped-bottom edges meeting at the point had attached to them Mo tubing. The tubing outside diameter (OD) was ¼″ (0.64 cm), and the inside diameter (ID) was ⅛″ (0.32 cm). Each piece was about 2.01″ (5.1 cm) long. This tubing was slotted over each length such that the bottom edges of the cathode each resided within the corresponding piece of tubing, with a clearance between the side of the cathode and the closest edges of tubing meeting the criteria of Eq. (2). The two pieces of tubing were butted together at the bottom point of the cathode, where they met. A hole was provided in one side of these tubes to allow connection to another such tub of Mo of the same ID and OD, which passed from that face of the cathode perpendicularly to that face, and at an angle of about 15 degrees downward from horizontal. This served as the conveyance from the cathode to a collection chamber, and had a total length of 2″ (5.1 cm).  
         [0075]    The collection chamber comprised a length of closed-end round bottom alumina tubing. The chamber was situated such that it was about 1.5″ (3.8 cm) from the face of the cathode. Thus, about ½ inch of the conveyance tube resided within the walls and internal space of this tubing.  
         [0076]    The alumina tubing had an ID of b  1 {fraction ( 3 / 8 )}″ (3.50 cm) and an OD of 1⅝″ (4.13 cm). The curvature for the closest end began about 11⅜″ (28.9 cm) from the open end, and the total length of the piece as 12″ (30.5 cm). At a distance of about 11⅛″ (28.3 cm) from the open end, a hole was centered in the tubing. This hole had a diameter of about {fraction (5/16)}″ (0.80 cm). On the alumina circumference of this hole, and on the outside of the tubing around the hole in a roughly circular area of about 1″ (2.54 cm) in diameter, Mo was applied by a flame-spray method. The conveyance tube was then placed to enter the chamber through this Mo-coated hole. The distance between the hole coating and the outer surface of the conveyance tube met the condition of Eq. 2. With this arrangement, the point of the cathode was about 1⅜″ (3.50 cm) from the bottom of the anode liner of the cell while the bottom of the alumina tubing rested on the bottom of the anode liner, and the minimum distance from the bottom of the liner to any cathode metal (in particular, the lowest point of the flame-sprayed Mo) was about ⅝″ (1.6 cm).  
         [0077]    Because Mo oxidizes readily in air at elevated temperatures, the above assembly was lowered into already-molten electrolyte prior to the electrolysis test described below. The anode liner holding the electrolyte, which was the only anode in this test, was of an investment cast 10:15:15 Cu:Ni:Fe alloy.  
         [0078]    The electrolyte was about 45 mol. % aluminum fluoride (AlF 3 ) and 55 mol. % sodium fluoride (NaF). 3000 g were used at an operating temperature of about 760° C. The electrolyte was maintained as a slurry with undissolved alumina, above saturation. The weight percent excess undissolved alumina was about 6%, and the alumina particle size was nominally one micron. Electrolysis was conducted at 100 amperes for a total of 5.1 hours in this test.  
         [0079]    In this test, the cathode itself, conveyance tube and flame-sprayed Mo had been wetted with aluminum (Al) in a previous test. When the apparatus was inserted into the melt, the Al melted quickly and a seal performed. A heated stainless steel siphon tube connected through a valve to a vacuum was inserted into the collection chamber to a depth about ½″ (1.27 cm) above the top of the hole in the chamber.  
         [0080]    After about one hour of electrolysis at 100 amperes, a length of tungsten (W) wire was inserted into the chamber until it touched the closed end at the bottom thereof. This was then pulled out and inspected; such procedure constituting a measurement of the depth of both Al and electrolyte in the chamber. The Al depth was determined to be 1.8″ (4.6 cm), and the electrolyte layer above this appeared to be quite thin, about 0.04″ (0.1 cm). This depth represented more Al than would be produced in the one hour of electrolysis, and included Al previously present on the cathode assembly.  
         [0081]    After another 1.38 hours of continued electrolysis, the Al depth was measured again and found to be about 2.3″ (5.8 cm) deep. The increase in depth corresponds to an addition of about 12.2 ml of liquid Al, which was about 28 g at 760° C. This is about 61% of the total amount of Al the electrolysis would produce during this time.  
         [0082]    After an additional ¾ hour, the Al depth had climbed only another 0.1″ (0.25 cm). At this point in the test, a vacuum was applied to the siphon tube. Once the siphon was drawing only air, the depth was measured in the collection vessel, and found to be 1.1″ (2.8 cm). When the vacuum was subsequently removed, this level climbed an additional 0.2″ (0.5 cm). Assuming that this siphon procedure collected 1.1″ (2.8 cm) of Al depth, a total of about 61.6 g of Al was collected with the siphon.  
         [0083]    After these procedures, electrolysis was maintained for an additional two hours. The depth at the end of this period was measured to be only 1.5″ (3.8 cm). Thus, little Al was collected in the chamber after the initial siphoning.  
         [0084]    After the test, a total of 119.8 g of Al was recovered, representing a current efficiency of about 60% based on this recovered metal. Of the total recovered, about 62 g was found to have been collected with the siphon. It was noted that the region that had been sprayed with Mo now had a significant amount of the intermetallic material that forms at the interface of Al and Mo phases. This material is mushy at temperature and does not flow readily. It is believed that the reason the Al depth ceased to climb in the collection chamber after the measurement taken 1.38 hours into the test is that the mushy material impeded the free flow of liquid Al into the chamber.  
         [0085]    This test showed that (a) the principles of Eq. (2) function to form an effective seal between the chamber and the electrolyte, (b) the liquid Al formed on the cathode can be conveyed to a collection chamber driven by the difference in hydrostatic head at the bottom of the cathode and in the chamber, and (c) liquid Al can be siphoned from such a chamber once it has collected there.  
         [0086]    While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass other embodiments which fall within the spirit of the invention.