Abstract:
A hollow partition wall is employed to feed carbon material to an underflow of a carbothermic reduction furnace used to make aluminum. The partition wall divides a low temperature reaction zone where aluminum oxide is reacted with carbon to form aluminum carbide and a high temperature reaction zone where the aluminum carbide and remaining aluminum oxide are reacted to form aluminum and carbon monoxide.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a process for the production of aluminum by carbothermic reduction of alumina and to a reactor for the production of aluminum by carbothermic reduction of alumina.  
         BACKGROUND ART  
         [0002]    The direct carbothermic reduction of alumina has been described in U.S. Pat. No. 2,974,032 (Grunert et al.) and it has long been recognized that the overall reaction: Al 2 O 3 +3C=2Al+3CO (1) takes place, or can be made to take place, in two steps: A 1   2 O 3 +9C=Al 4 C 3 +6CO (2); and Al 4 C 3 +Al 2 O 3 =6Al+3CO (3).  
           [0003]    Reaction (2) takes place at temperatures below 2000° C. Reaction (3), which is the aluminum producing reaction, takes place at appreciably higher temperatures of 2200° C. and above; the reaction rate increases with increasing temperature. In addition to the species stated in reactions (2) and (3), volatile species including gaseous Al, gaseous aluminum suboxide (Al 2 O) and CO are formed in reactions (2) and (3) and are carried away with the off gas. Unless recovered, these volatile species will represent a loss in the yield of aluminum. Both reactions (2) and (3) are endothermic.  
           [0004]    U.S. Pat. No. 6,440,193 relates to such a process for carbothermic production of aluminum where aluminum carbide is produced together with molten aluminum oxide in a low temperature compartment. The molten bath of aluminum carbide and aluminum oxide flows from the low temperature compartment into a high temperature compartment where the aluminum carbide (Al 4 C 3 ) is reacted with the aluminum oxide (Al 2 O 3 ) to produce aluminum. In the high temperature compartment, aluminum forms a layer on top of a molten slag layer and is tapped from the high temperature compartment. The off-gases from the low temperature compartment and from the high temperature compartment, which contain Al vapor and volatile aluminum suboxide (Al 2 O) are reacted to form Al 4 C 3 . The low temperature compartment and the high temperature compartment are located in a common reaction vessel, with the low temperature compartment being separated from the high temperature compartment by an underflow partition wall. The molten bath containing aluminum carbide and aluminum oxide produced in the low temperature compartment continuously flows under the partition wall and into the high temperature compartment by means of gravity flow which is regulated by tapping of aluminum from the high temperature compartment. The energy needed to maintain the temperature in the low temperature compartment and in the high temperature compartment is provided by separate energy supply systems.  
           [0005]    In the second step, reaction (3), excess carbon is necessary to promote the production of aluminum. In order to maintain a sufficient carbon content in the high temperature compartment, it is necessary to add additional carbon to the high temperature compartment. According to U.S. Pat. No. 6,440,193 the additional carbon is added through a supply means arranged in the roof of the high temperature compartment thereby requiring the additional carbon to pass through the top layer of molten aluminum in the high temperature compartment and into the molten bath in the high temperature compartment.  
         SUMMARY OF THE INVENTION  
         [0006]    It has been discovered that the addition of carbon material to the top of the molten aluminum can cause a reverse reaction of the aluminum as well as poor distribution of the carbon in the high temperature reaction zone. In order to overcome this problem, it has been discovered that the additional carbon material should be added directly into the slag layer and below the upper aluminum layer, thereby keeping the composition of the slag layer more uniform during the formation of aluminum in the high temperature compartment. It has been further discovered that the additional carbon material should be distributed as evenly as possible in the slag layer in the high temperature compartment. Finally, it has been discovered that the additional carbon material should be added in a controllable manner.  
           [0007]    In order to take advantage of these discoveries, a process and a reactor have been invented. Specifically, the process of the present invention comprises adding additional carbon material to the slag as it flows below the partition wall from the low temperature compartment to the high temperature compartment. The reactor of the present invention comprises a means for supplying the additional carbon material to the slag as it flows below the partition wall from the low temperature compartment to the high temperature compartment.  
           [0008]    In a preferred embodiment of the present invention, the means for supplying the additional carbon material to the slag layer is an opening in the lower portion of the partition wall. More specifically, the partition wall is hollow with an opening in the bottom that allows additional carbon material to flow out the bottom of the partition wall and into the underflow of slag as it moves from the low temperature compartment to the high temperature compartment of the reactor. A transport means, such as a screw or ram or a combination of a screw and a ram, is employed to move the additional carbon through the wall. Preferably, the hollow partition wall is vertically movable so as to vary the height of the opening in the slag underflow.  
           [0009]    By adding the additional carbon material to the underflow of slag at the partition wall, the additional carbon material is added directly into the slag, below the level of the upper aluminum layer, and the amount of added carbon material can be evenly distributed throughout the slag in the high temperature compartment. Since the partition wall is vertically movable, the point of addition for the additional carbon material can be varied. Normally the vertical position of the wall is only adjusted when the furnace is not in operation. Furthermore, the amount of carbon added to the slag can be controlled by the speed at which the transport means moves the additional carbon material through the wall.  
           [0010]    Preferably, the hollow area and the opening in the partition wall extend across the entire wall. Alternatively, the hollow area can be divided into a series of channels or into vertically oriented conduits. Each conduit has an opening at the base of the wall to conduct additional carbon material downward and feed the additional carbon material into the underflow of slag.  
           [0011]    Broadly, the present invention is a process for supplying additional carbon material to a reactor for carbothermic production of aluminum wherein the reactor is divided into a low temperature compartment and a high temperature compartment by a hollow underflow partition wall. A molten bath or slag comprising aluminum carbide and aluminum oxide is produced in the low temperature compartment. The molten bath of aluminum carbide and aluminum oxide flows under the hollow underflow partition wall into the high temperature compartment where the aluminum carbide is reacted with alumina to produce aluminum which forms a layer on top of the molten slag bottom layer and where aluminum is tapped from the high temperature compartment. The additional carbon material is supplied to the molten bath of aluminum carbide and aluminum oxide through at least one opening in the hollow underflow partition wall, said opening being at a level below the layer of molten aluminum in the high temperature compartment. In other words, the opening is positioned in the wall at the level of the slag as it flows under the wall.  
           [0012]    The reactor of the present invention is a reactor for carbothermic production of aluminum which comprises a reaction vessel comprising a low temperature reaction compartment and a high temperature reaction compartment. The low temperature compartment has means for supply of materials to said compartment and one or more electrodes for supplying electric operating current to said compartment, said electrode or electrodes being positioned for submersion in a molten bath which is produced in the low temperature compartment. The high temperature reaction compartment is separated from the low temperature compartment by means of a hollow partition wall. The hollow partition wall has at least one opening into the underflow of the molten bath which allows underflow of the molten bath from the low temperature reaction compartment to the high temperature compartment. A plurality of pairs of substantially horizontally arranged electrodes are arranged in the sidewall of the high temperature compartment of the reaction vessel for supply of electric current to said compartment. The high temperature compartment has an outlet for continuously tapping molten aluminum. The molten bath produced in the low temperature compartment flows into the high temperature compartment by gravity flow effected by tapping the top aluminum layer in the high temperature compartment. The at least one opening in the partition wall is positioned at a level below the layer of molten aluminum in the high temperature compartment.  
           [0013]    In accordance with the present invention, the additional carbon material can take the form of coke, coal, agglomerated carbon powder or any other form. Also, additional carbon material can take the form of Al 4 C 3 , which is preferred in order to reduce the amount of CO gas produced in the high temperature compartment as well as to recycle Al 4 C 3  from off-gas reactors connected to the high and low temperature compartments. Finally, Al 4 C 3  filtered off from the produced aluminum tapped from the reactor can also be used as a form of additional carbon material.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    These and other aspects of the present invention may be more fully understood with reference to the drawings wherein:  
         [0015]    [0015]FIG. 1 is a cross-sectional view of a preferred embodiment of a reactor vessel according to the present invention,  
         [0016]    [0016]FIG. 2 is a cross-sectional view of a hollow partition wall,  
         [0017]    [0017]FIG. 3 is a top view of the hollow partition wall of FIG. 2 taken along line III-III,  
         [0018]    [0018]FIG. 4 is a top view of a partition wall with a plurality of conduits therein; and  
         [0019]    [0019]FIG. 5 is a side view of the partition wall of FIG. 4 taken along line V-V. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0020]    [0020]FIG. 1 shows a generally rectangular-shaped gas tight reaction vessel  1  divided into a low temperature compartment  2  and a high temperature compartment  3  by means of a hollow underflow partition wall  4  that allows flow of a molten bath from the low temperature compartment  2  to the high temperature compartment  3  and the addition of additional carbon material to the flow of molten bath as it passes under partition wall  4 . At the end of the high temperature compartment  3  opposite the low temperature compartment  2  there is arranged an outlet  5  for tapping or removing a layer of molten aluminum  31 . The molten bath flows from the low temperature compartment  2  to the high temperature compartment  3  by gravity. The flow is effected and regulated by the tapping of aluminum  31  at outlet  5 . When aluminum is tapped from the high temperature compartment, a corresponding amount of molten bath flows under the partition wall from the low temperature compartment to the high temperature compartment. The two compartments are not connected by separate ducting.  
         [0021]    In the low temperature compartment  2  there are arranged a plurality of electrodes  6 , usually two to four, extending through the roof of the reaction vessel  1 . The electrodes  6  are, during the operation of the reaction vessel  1 , intended to pass through the bath and to be submerged in the molten bath in the low temperature compartment  2  to supply energy by resistance heating. The electrodes  6  may have conventional means (not shown) for supply of electric current and conventional means (not shown) for regulating the electrodes  6 . The electrodes  6  are preferably consumable graphite electrodes, although any other material suitable for such use can also be employed.  
         [0022]    In the high temperature compartment  3  there are arranged a plurality of pairs of electrodes  7  along the sidewalls of the reaction vessel  1 . In FIG. 1 the side view electrodes are depicted as circles as they protrude from one wall and so only one electrode of each set is shown. The electrodes  7  can be consumable graphite electrodes or non-consumable inert electrodes. Each pair of electrodes  7  is individually supplied with electric current. By using a plurality of pairs of electrodes  7  in the sidewall of the reaction vessel  1 , an even temperature is reached in the molten bath in the high temperature compartment  3 . As shown, the electrodes  7  do not pass through the top of the bath and are disposed below the level of the aluminum layer  31 , providing advantages described previously. In the roof of the low temperature compartment  2  there is arranged supply means  8  for supply of alumina  32  from hopper  34  and carbonaceous reduction material  36  to the low temperature compartment  2 . The supply means  8  is preferably gas tight so that raw materials can be supplied without the escape of reactor off-gases through the supply means  8 .  
         [0023]    Over the roof in the low temperature compartment  2  there is further arranged a first gas exit  9 . The gas exit  9  can pass to reactor  10  to recover Al 4 C 3 .  
         [0024]    Over the roof in the high temperature compartment  3  there is arranged a second gas exit  19  which is identical to the gas exit  9  arranged on the roof over the low temperature compartment  2 . Off-gases from the high temperature compartment  3  can pass to another rector  10  to recover Al 4 C 3 . Gases flowing through exits  9  and  19  could also both pass through the same reactor  10 .  
         [0025]    Hollow partition wall  4  has hopper  30  positioned on top to hold additional carbon material and to feed additional carbon material down through hollow partition wall  4  into the underflow molten bath. Recovered Al 4 C 3  from reactor  10  is preferably recycled to hopper  30  for use as additional carbon material. Hopper  30  and hollow partition wall  4  are preferably gas tight so that additional raw material can be supplied to the reactor without the escape of reactor off-gases.  
         [0026]    [0026]FIG. 2 illustrates a cross-sectional view of a preferred embodiment of hollow partition wall  4 ′ while FIG. 3 shows a top view of the wall taken along line III-III of FIG. 2. Wall  4 ′ comprises sides  4 ′ a  and  4 ′ b  and space  4 ′ c  for holding carbon material and housing a screw  4 ′ d  to transport additional carbon material down through space  4 ′ c  and out opening  4 ′ e  at the bottom of wall  4 ′. Preferably, cooling system  4 ′ f  is provided on the outside of wall  4 ′. Cooling system  4 ′ f  is a conventional cooling system operated in conventional manner. A rack and pinion system  4 ′ g  is used to vertically move wall  4 ′. By moving wall  4 ′, the level of opening  4 ′ e  varies thereby allowing for control of the height of addition of the additional carbon material into the underflow slag. The speed at which screw  4 ′ d  is operated controls the amount of additional carbon material fed through opening  4 ′ e.    
         [0027]    Rack and pinion system  4 ′ g  is a conventional system operated in a conventional manner to move wall  4 ′ and adjust the height at which additional carbon material is fed to the slag.  
         [0028]    Cooling system  4 ′ f  also aids in guiding the movement of wall  4 ′.  
         [0029]    [0029]FIGS. 4 and 5 illustrate another embodiment wherein the hollow area has been divided into a plurality of conduits. Such conduits can also be seen as circular spaces or hollows. Partition wall  4 ″ has spaces  4 ″ c  and screws  4 ″ d  positioned therein to feed carbon material downward through space  4 ″ c  to the underflow slag. The amount of additional carbon material added to the underflow slag is controlled by the speed at which screws  4 ″ d  are turned in spaces  4 ″ c . The faster the speed, the more additional carbon material is added to the underflow slag. Additional carbon material passes out of wall  4 ″ through openings  4 ″ e . Cooling/protective layer  4 ″ f  is also provided on wall  4 ″.  
         [0030]    Screws  4 ′ c  and  4 ″ c  are conventional devices operated in a conventional manner to move the solid particulate additional carbon material down through spaces  4 ′ c ,  4 ″ c  and out openings  4 ′ e ,  4 ″ e , respectively. Preferably, the motors used to turn screws  4 ′ c ,  4 ″ c  are variable to provide for a change of speed and control of the amount of additional carbon material added to the underflow slag.  
         [0031]    A preferred embodiment providing an example for carrying out the process according to the present invention will now be described in connection with FIG. 1. A charge of alumina and carbon is supplied through the supply means  8  to the low temperature compartment  2 . Electric energy is supplied through the electrodes  6  to provide and maintain a molten slag bath of alumina and Al 4 C 3  at a temperature of about 2000° C. The electrodes  6  are submerged in the molten slag bath whereby the energy is transferred to the molten slag bath by resistance heating. The off gas from the low temperature compartment  2 , which usually will contain CO, Al 2 O and some Al vapor, is withdrawn through an off gas duct and into the lower part of the off gas exit  9 . The Al 4 C 3  which is recovered in reactor  10  is preferably recycled to the reactor through hopper  30  and hollow partition wall  4 .  
         [0032]    The molten slag consisting of aluminum carbide and alumina produced in the low temperature compartment  2  will continuously flow under hollow partition wall  4  and into the high temperature compartment  3 . Additional carbon material from hopper  30  will flow down through hollow partition wall  4  and into the molten slag flowing under wall  4 .  
         [0033]    As shown in FIGS. 2-5, screws  4 ′ d ,  4 ″ d  are rotated to transport additional carbon material through walls  4 ′,  4 ″ and out openings  4 ′ e ,  4 ″ e , respectively. Rack and pinion system  4 ′ g  is employed to raise and lower wall  4 ′ thereby varying the height of opening  4 ′ e  in the slag. The speed of screws  4 ′ d ,  4 ″ d  is varied to control the amount of additional carbon material that flows down from hopper  30  and into the underflow slag.  
         [0034]    In the high temperature compartment  3  the temperature of the molten slag is increased to 2100° C. or more by supply of electric current to the plurality of sidewall electrodes  7 , which heat the slag bath by resistance heating. By using a plurality of pairs of electrodes  7  arranged along the sidewalls of the high temperature compartment  3 , below rather than through molten aluminum layer  31 , very importantly, the temperature can be controlled in slag bath along the length of the high temperature compartment  3 , and localized superheating is reduced or avoided. This process involves essentially horizontal flow of the molten slag into high temperature compartment  3 , as shown by the arrows  38  in compartment  2 , without need of a separate heating duct or use of gases to effect slag flow.  
         [0035]    By maintaining the temperature in the slag bath in the high temperature compartment  3  at a temperature above about 2100° C., aluminum carbide will react with alumina to produce Al and CO gas. The additional carbon will replace carbon consumed during the Al producing reaction. Due to the high temperature, an appreciable amount of produced Al will vaporize together with Al 2 O and will leave the furnace with the off gas. The liquid Al produced in the high temperature compartment  3  will, due to its low density, form a molten layer  31  on top of the molten slag bottom layer and it is tapped from the furnace through the overflow outlet  5 . There is no need to recirculate the remaining slag back into the low temperature compartment  2  by separate ducting, saving substantial costs and simplifying the process. During the reaction of aluminum carbide and alumina, the molten slag bath in the high temperature compartment will be depleted of carbon. Additional carbon material is therefore supplied to the high temperature compartment  3  through hollow partition wall  4 . In addition to carbon material, solid alumina can be charged to the high temperature compartment  3  through hollow partition wall  4 .  
         [0036]    The aluminum produced in the high temperature compartment  3  will be saturated with molten aluminum carbide. The superheated aluminum in the high temperature compartment  3  is continuously tapped through the over/underflow outlet  5  and can be passed to downstream operations. The aluminum is then cooled to form a stream  40 , preferably by addition of aluminum scrap  42  in cooling vessel  44 , to a temperature above the melting point for aluminum. When the aluminum is cooled, a major part of the aluminum carbide dissolved in the aluminum will precipitate as solid aluminum carbide  46  and can be skimmed off from the cooled molten aluminum in purification vessel  48 . Vessels  44  and  48  can be combined. The remaining aluminum carbide  50  can be removed by conventional means, such as by passing stream  49  through filter  52 . The aluminum carbide removed from the aluminum after tapping is preferably recycled to the low temperature compartment  2  and/or to hollow partition wall  4 . The cooling vessel, purification vessel and filter may be of any type useful to perform its function.  
         [0037]    The purified aluminum stream  54  may then be passed to any number of apparatuses, such as degassing apparatus  56  to remove, for example, H 2 , fluxing apparatus  58  to scavenge oxides from the melt and eventually to casting apparatus  60  to provide unalloyed primary shapes such as ingots  62  or the like of about 50 lb. (22.7 kg) to about 750 lb. (341 kg). These ingots may then be remelted for final alloying in a holding or blending furnace or the melt from fluxing apparatus may be directly passed to a furnace for final alloying and casting as alloyed aluminum shapes. Elements such as Cu, Fe, Si, Mg, Ni, Cr, etc. may be added to the blending furnace as rich alloy ingots such as 82% Al/18% Cu since addition in pure form may not be feasible. These operations are well known and described, for example, in  Aluminum , Vol. III, Ed. Kent R. Van Horn, Amer. Soc. of Metals (1967), pp. 18-36, herein incorporated by reference.  
         [0038]    The amount and location of carbon in the slag layer of the high temperature compartment  3  can be measured by sensor  70  or by measuring the electric resistance of the slag. This helps to determine both the amount of carbon present and whether the carbon is evenly distributed in the slag layer. Sensor  70  is a conventional sensor operated in a conventional manner.  
         [0039]    Sensor  70  communicates with screw motor  72  and rack and pinion system  4 ′ g  to control the amount of carbon material added as well as the height in the slag layer where the carbon material is to be added. Individual motors of each screw conveyor  4 ′ d ,  4 ″ d  are independently controlled to control the addition of carbon material in a third dimension. In particular, if additional carbon material is needed along the sides of the furnace, only screws  4 ′ d ,  4 ″ d  at the ends of walls  4 ′,  4 ″ are operated while the screws  4 ′ d ,  4 ″ d  in the middle of wall  4 ′,  4 ″ are stopped. As will be appreciated, independent control of each of screws  4 ′ d ,  4 ″ d  along with rack and pinion system  4 ′ g  allows for three-dimensional control of the addition of carbon material through walls  4 ′,  4 ″.  
         [0040]    It will be understood that the claims are intended to cover all changes and modifications of the preferred embodiments of the invention herein chosen for the purpose of illustration which do not constitute a departure from the spirit and scope of the invention.  
         [0041]    Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.