Abstract:
A spent anode is replaced with a new anode in an electrolysis cell having an anode bus bar and an anode rod contacting the bus bar. A desired distance (D 4 ) from the bus bar to a reference point on or adjacent to an anode rod for the new anode is calculated, the spent anode is replaced with a new anode so that the reference point on the new anode rod is spaced from the bus bar by an actual distance (D 5 ), and the actual distance (D 5 ) is measured at least once by means of a vision system. The actual distance (D 5 ) is preferably adjusted using a feedback control loop in a computer so that D 5  approaches the desired distance (D 4 ).

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a process and apparatus for the periodic replacement of anodes in electrolytic cells. More specifically, the invention relates to an improved process and apparatus for automatically and accurately positioning the height of new carbon anodes in cells producing aluminum by electrolysis of alumina in a molten salt bath.  
         BACKGROUND OF THE INVENTION  
         [0002]    The well-known Hall-Heroult process produces aluminum by electrolysis of alumina dissolved in a molten fluoride salt bath maintained at temperatures of 900-1000° C. Alumina (Al 2 O 3 ) produces aluminum and oxygen when it breaks down. Aluminum is collected in a molten layer below the anode and oxygen is released adjacent the anode.  
           [0003]    Carbon is used as the anode material because oxidation-resistant anodes are not yet commercially available. Carbon is consumed in relatively large quantities in the process, generally about 420 to 550 kg. carbon per metric ton of aluminum produced.  
           [0004]    A new anode includes a carbon block joined by stubs and an iron yoke to an aluminum or copper anode rod. The height of the carbon block in a new anode is about 62 cm. Its life span in a cell is about 27 days after which the height of the carbon block is reduced to about 15 cm. The spent anode must be replaced before it is completely consuned in order to avoid the risk of contaminating aluminum with steel from the stubs or with cast iron used for joining stubs into the carbon block. A small aluminum plant having 264 cells may replace close to 400 anodes per day, requiring about 150,000 anode replacements per year.  
           [0005]    When a new anode replaces a spent anode in a cell, its height must be positioned accurately in order to assure efficient operation of the cell. The new anode should also be positioned quickly in order to minimize gas emission and cell perturbations. Several processes and apparatus for replacing anodes have been developed in the prior art. Some prior art patents covering various aspects of anode changing include Messina U.S. Pat. No. 3,850,305; Kato et al. U.S. Pat. No. 4,032,020; Duclaux U.S. Pat. No. 4,465,578; Skaar et al. U.S. Pat. No. 4,992,146; Marttila et al. U.S. Pat. No. 5,151006; Luebke et al. U.S. Pat. No. 5,730,855; and Zannini U.S. Pat. No. 5,435,897. However, there still remains a need for an efficient and economical process and apparatus for positioning new anodes accurately and quickly in an aluminum electrolysis cell.  
           [0006]    A principal objective of the present invention is to provide an efficient and economical process and apparatus for automatically positioning the height of new anodes in an aluminum electrolysis cell.  
           [0007]    A related objective of the invention is to provide a process and apparatus for reducing variations in the height of new anodes among different individuals operating the electrolysis cell.  
           [0008]    An advantage of the present invention is that vertical positioning of new anodes is minimally subject to variations in position of the overhead crane supporting the anode changing apparatus.  
           [0009]    Additional objectives and advantages of the invention will become readily apparent to persons skilled in the art from the following detailed description of some particularly preferred embodiments.  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with the present invention there is provided a process and apparatus for automatically positioning replacement anodes in an electrolysis cell for producing a metal, preferably aluminum. Other metals produced by electrolytic processes include lead, magnesium, zinc, zirconium, titanium, and silicon. Electrolysis cells producing aluminum include at least one anode having an anode rod connected with a bus bar, a molten salt bath contacting the anode, and a cathode spaced from the anode. The molten salt bath includes a cryolite electrolyte and alumina dissolved in the electrolyte. An electric current passing through the electrolyte breaks down alumina into aluminum collected in a liquid layer below the anode and oxygen released adjacent the anode.  
           [0011]    The anodes generally include a carbon block, a metal device anchored in the carbon block, and a metal rod connected with the device. The device is generally made of steel. The anode rod is made of aluminum or copper. The metal device may have 1, 2, 3, or 6 stubs anchored in the carbon block and preferably includes 3 stubs so that it is called a “tripod”. The tripod is connected with the carbon block by a cast iron material called “rodding”. The tripod is connected with the anode rod by an explosion welded joint called a “clad”.  
           [0012]    An upper portion of the anode rod preferably defines an opening called a “lifting slot” for connecting the anode rod with a pulling tool. A pin extends through the lifting slot and metal hooks (called “snugs”) engage with the pin to raise and lower the anode rod. The snugs are connected by a device called a “connector” with a lifting tool supported by an overhead crane extending downwardly from an apparatus (called a “pot tending machine” or “PTM”) extending between 2 main steel beams overhead. The PTM also includes a cabin or turret for housing an operator, and a crane supporting tools for replacing anodes, for siphoning metal from the cell, and for feeding aluminum fluoride to the cell.  
           [0013]    Optionally, the PTM may also support one or more digital cameras, a computer, and a programmable logic controller (“PLC”) for carrying out the process of the invention, as described below in greater detail.  
           [0014]    Carbon in the anode blocks is consumed as aluminum is produced. Accordingly the spent carbon anodes must be replaced with new anodes approximately every 27 days. Because heat is lost from the cell while anodes are being exchanged, it is desirable to change the anodes quickly consistent with safety and other objectives of the plant. The new anodes must be positioned accurately to optimize aluminum production and to avoid anode effects. Positioning of the anodes is measured with reference to a bottom of the spent anodes and a bottom of the new anodes. The anode bus bar or a plane adjacent thereto is chosen as an absolute reference for vertical positioning. One advantage of the present invention is that distance measurements carried out for purposes of positioning new anodes do not rely upon a reference point on the overhead crane. Accordingly, variations in position of the overhead crane have little or no effect upon measurements of actual distances.  
           [0015]    When replacing a spent anode with a new anode, the bottom of the new anode is positioned higher than the bottom of the spent anode by a predetermined distance X that is chosen to optimize cell performance. X may vary between about 10 and 20 mm. and is about 15 mm. in a particularly preferred embodiment.  
           [0016]    In a preferred embodiment of the present invention, several measurements are performed to position the new anodes accurately. Before measuring, a first reference point named reference one (R 1 ) is chosen. The reference one is related to the anode bus bar. A second reference point named reference two (R 2 ) is chosen on the anode rod or somewhere else to link up with the anode rod.  
           [0017]    In a first step, before a spent anode is removed from its connection with the anode bus bar a measurement is taken of the vertical distance between the reference one and the reference two. This distance, called the first distance or D 1 , is measured by a vision system that is preferably at least one digital camera or a digital laser distance detector, each being connected to a computer including an image processing algorithm locating the reference points and the vertical distance between them. The laser distance detector may be either a sweeping laser or a fixed beam.  
           [0018]    In a second step, a crust above the carbon block is broken, connections between the anode rod and the bus bar are removed, and the spent anode is lifted from the cell. A second measurement is taken of the spent anode to determine the distance between the reference two and the bottom of the spent anode. This measured distance is called the second distance or D 2 . The spent anode is then placed in a storage rack for spent anodes. In a particularly preferred embodiment of the invention distances are measured by combining images obtained from 3 separate digital cameras installed on a mobile rigid arm. Digital cameras with images of 1,300 pixels×1,100 pixels are quite suitable for practice of the invention.  
           [0019]    In a third step, a new anode is procured and lifted by a pulling tool supported by the overhead crane. The pulling tool preferably includes a load cell. A third measurement is taken of the distance between the bottom of the new anode and the reference two. The result is called the third distance or D 3 . An advantage of the present invention is that measurements D 2  and D 3  can be taken even if anodes are swinging.  
           [0020]    By using the distances D 1 , D 2  and D 3 , the computer calculates a desired distance D 4  between references one and two for the new anodes. This calculation is in accordance with the formula:  
             D   4 = D   3 − D   2 + D   1 + X    
           [0021]    where D 1 , D 2 , and D 3  are defined above and X is 15 mm. in the most preferred embodiment. This value of X corresponds to the optimum distance for the bottom of the new anode to lie above the bottom of the spent anode.  
           [0022]    After the desired distance (D 4 ) is calculated, a new anode is positioned in the cell and the distance D 5  between references one and two is measured. This measurement is carried out by at least one digital camera or by a digital laser distance detector and the resulting signal is sent to the PLC. The PLC compares D 5  with desired distance D 4  and if there is detectable difference, the PLC sends a signal to the overhead crane instructing the pulling tool to raise or lower the new anode as needed to minimize the difference between D 5  and D 4 . Measurements of D 5  and movements of the pulling tool are repeated as many times as needed to reduce the difference between D 5  and D 4  to an acceptable value. Then, the new anode is positioned and connected to the anode bus bar. A feedback of the difference between D 4  and D 5  can also be given to the crane operator to manually lower or raise the lifting tool. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a front elevational view of a new anode for an aluminum electrolysis cell.  
         [0024]    [0024]FIG. 2 is a front elevational view of a spent anode removed from an aluminum electrolysis cell.  
         [0025]    [0025]FIG. 3 is a front elevational view of an apparatus for replacing spent anodes with new anodes in accordance with the invention.  
         [0026]    [0026]FIGS. 4A and 4B are schematic illustrations of an aluminum electrolysis cell.  
         [0027]    [0027]FIGS. 5-8 are schematic illustrations of distance measurements to be made in an aluminum electrolysis cell in accordance with a preferred embodiment of the invention.  
         [0028]    [0028]FIG. 9 is a fragmentary, front elevational view of an alternative embodiment for the implementation of reference  2 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    In accordance with a particularly preferred embodiment of the present invention there is provided a process and an apparatus for replacing a spent anode with a new anode in an electrolysis cell for making aluminum. As shown in FIG. 1, a new anode  10  includes a large carbon block  11 , a steel tripod  12  having 3 prongs anchored in the carbon block, and a metal rod  13  extending upwardly of the tripod. The tripod  12  is connected with the carbon block  11  by cast iron rodding  14 , a small portion of which is shown extending upwardly of a top surface of the carbon block  11 . A clad  15  comprising an explosion welded joint connects the tripod  12  with the rod  13 . The rod  13  defines a lifting slot  16  for connecting the rod  13  with a lifting tool, as described below. A bottom surface  18  of the anode block  11  lies in the anode plane of the new anode  10 .  
         [0030]    The height of the anode block  11  in the new anode  10  of FIG. 1 is about 62 cm. In FIG. 2 there is shown a spent anode  20 , removed from an aluminum electrolysis cell. The spent anode  20  includes a carbon block  21 , a steel tripod  22 , and an anode rod  23 . In the spent anode  20 , the height of the carbon block  21  is reduced to about 15 cm.  
         [0031]    As shown in FIG. 3, an electrolysis cell  30  for producing aluminum includes anodes  31  each having a carbon block  32 , a tripod  33 , and an anode rod  34 . The anodes  31  are suspended in a molten salt bath or cryolite electrolyte  35  above a molten metal pad  36  supported by a carbon cathode  37 . A removable metal hood  38  prevents fumes from escaping a cell chamber  39  above the molten salt bath  35 .  
         [0032]    A pot tending machine or PTM  40  above the cell  30  is supported by 2 steel guide rails (not shown). The PTM  40  includes a cabin or turret  42  for housing an operator, an overhead crane  43  supporting a pulling tool  44  for raising and lowering the anodes  31  by gripping their rods  34 , at least one digital camera  46  for measuring distances, and a programmable logic controller (PLC)  48  linked with the pulling tool  44  and camera  46 . The pulling tool  44  positions the anode rods  34  adjacent an anode bus bar  50 .  
         [0033]    Referring now to FIG. 4A, there are shown schematically 2 spent anodes  20  in an aluminum electrolysis cell  30 . FIG. 4B shows 2 new anodes  10  each including a carbon block  11 , a tripod  12 , and an anode rod  13  extending upwardly above a bus bar  50  (reference number one or R 1 ). A lifting slot  16  in the anode rod  13  can serves as a reference point for distance measurements (reference number two or R 2 ). The new anode plane  18  is a bottom horizontal surface of the carbon block  11 .  
         [0034]    Referring again to FIG. 4A, the spent anode  20  includes a carbon block  21 , a tripod  22 , and an anode rod  23  extending above the bus bar  50 . The rod  23  defines a lifting slot  26 . The spent anode plane  28  is a bottom horizontal surface of the carbon block  21 .  
         [0035]    In FIG. 4B, the distance between the bottom  18  of the new anode and the anode bus bar  50  is called DA. Similarly in FIG. 4A the distance between the bottom  28  of the spent anode and the bus bar  50  is called DM. The cell  30  operates more efficiently after a new anode  10  is installed if the new anode bottom  18  is about 15 mm. higher than the spent anode bottom  28 . In other words, the relation between DA and DM is preferably in accordance with the following formula:  
           DA=DM− 15 mm.  
         [0036]    Positioning a new anode in an electrolysis cell in accordance with a preferred embodiment of the present invention involves 4 distance measurements. Referring first to FIG. 5, before 2 spent anodes  20   a ,  20   b  are removed from the cell a digital camera takes a picture of the anode rods  23   a,    23   b  either singly or both at the same time. The picture must show the anode bus bar  50  and the reference points  60   a,    60   b  adjacent the lifting slots. An image processing algorithm locates the reference points  60   a,    60   b  and the bus bar  50  to evaluate the vertical distances (D 1 , D 1 ′) between them.  
         [0037]    Before conducting the second measurement step the crust is broken, connections between the anode rods  23   a,    23   b  and the bus bar  50  are removed, and the spent anodes  20   a,    20   b  are lifted out from the cell. A second digital picture is taken of each spent anode  20   a,    20   b  singly or both at the same time, showing the distances (D 2 , D 2 ′) between the reference points  60   a,    60   b  and the anode planes  28   a,    28   b  for each spent anode as shown in FIG. 6. The picture may be taken at any time after the spent anodes  20   a,    20   b  are lifted from the cell and until they are placed on the spent anode rack. An image processing algorithm locates the reference points  60   a,    60   b  and the anode planes  28   a,    28   b  to evaluate the vertical distances (D 2 , D 2 ′) between them.  
         [0038]    The spent anodes  20   a,    20   b  are places on an anode rack (not shown) and 2 replacement anodes  10   a,    10   b  are lifted as shown in FIG. 7. A third picture is taken by the digital camera of each new anode  10   a,    10   b  individually or both at the same time. This picture can be taken anywhere on the path taken by the new anodes  10   a,    10   b  from the time they are raised above the rack and the time they are above the cell. An image process algorithm locates the reference points  60   a,    60   b  on the new anodes and their anode planes  18   a,    18   b,  to evaluate the distances (D 3 , D 3 ′) between them.  
         [0039]    The desired distances (D 4 , D 4 ′) between the bus bar  50  and the reference points  60   a,    60   b  are now calculated according to the formula:  
           D   4 = D   3 − D   2 + D   1 +15 mm.  
         [0040]    The new anodes  10   a,    10   b  are then lowered into the cell  30  and positioned at a height selected by the operator. The connectors are put back in place without tightening them. As shown in FIG. 8, a picture is then taken of both new anodes  10   a,    10   b,  showing their reference points  60   a,    60   b  and the anode bus bar  50  to evaluate the vertical distance (D 5 , D 5 ′) between them. The algorithm takes measurements about 2-5 times per second and communicates them to the PLC. The measurements are used as a feedback to a control loop on the vertical positions of the new anodes  10   a,    10   b,  using the calculated values (D 4 , D 4 ′) as set points, and vertical positions of the reference points  60   a,    60   b  are adjusted accordingly. After this control loop completes its action, the bottoms  18   a,    18   b  of the new anodes  10   a,    10   b  are each located 15 mm. above where the bottoms  28   a,    28   b  on the spent anodes  20   a,    20   b  were located.  
         [0041]    An alternative embodiment of an apparatus of the invention shown in FIG. 9 includes a load cell  70  above a pulling tool  44  gripping the anode rod  34 . A tag  75  extending laterally of the anode rod  34  is substituted for a slot in the anode rod as reference two (R 2 ). A target inscribed onto the tag  75  provided a convenient and readily visible reference point for measuring distances D 1 , D 2 , D 3 , and D 5  in accordance with the procedures described above.  
         [0042]    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.