Patent Publication Number: US-2007095672-A1

Title: Method of controlling aluminum reduction cell with prebaked anodes

Description:
FIELD OF THE INVENTION  
      This invention relates to aluminum production by electrolysis of alumina. More particularly, the invention relates to control processes for electrolytic cells utilizing prebaked anodes and improving the respective control of alumina feeding systems.  
     BACKGROUND OF THE INVENTION  
      In the conventional operation of electrolytic cells in which alumina, Al 2 O 3  is reduced to aluminum, Al, the alumina is added to the cell on a prescribed time schedule. In recent years, operation of aluminum production cells and control of the alumina concentration in the electrolytic bath have been significantly and progressively automated. This automation is necessary to obtain the desired high current efficiency operational process flow of the cell.  
      An essential factor affecting process flow of an aluminum electrolytic cell, which produces aluminum by the electrolysis of alumina dissolved in molten cryolite, is the rate of introduction of the alumina into the bath. When an excess quantity of alumina is added over a length of time, sludge (undissolved alumina and bath deposits) accumulates at the bottom of the cell. Operating with high sludge accumulations eventually results in decreased metal production. On the other hand, operating with a deficiency of alumina or with too little alumina being consistently added to the bath requires extra energy. An alumina deficiency causes the occurrence of the ‘anodic effect’ or ‘racing’ phenomenon, which causes an abrupt increase in the voltage at the terminals of the cell, which can go from 4 to 30 or 40 volts, and which has repercussions on the entire production process. This occurs because of the increased anode over potential and therefore, the metal production in all of the cells in the potline is lowered. Thus, the control of aluminum feeding systems needs a rate of feed which is neither too rich nor too lean.  
      Control of alumina concentration is a critical task in electrolysis control technology and rate of control of existing alumina feeding systems are necessary to maintain feed within preset limits. Such control systems must allow no deviation because both repetitive occurrences of anode effects and excessive sludge interfering with the cathode operation are technologically unacceptable.  
      USSR Inventor&#39;s Certificate No. 1724713 discloses a prior art method for automatically controlling an aluminum reduction cell. According to this method cell voltage and potline current are measured. Then, calculations determine the “normalized” cell voltage, the rate of change thereof, and the alumina concentration in the bath. The control “normalized” voltage within the preset limits of the anode movement and with respect to the variation of the feed rate of alumina into the cell. Simultaneously the frequent overfeed and rare underfeed modes are alternated. The feeding modes are changed depending upon variations of the normalized voltage. During this process, either alumina feeding into the cell is completely terminated or one or the other feeding mode is turned on for a certain time.  
      The disadvantage of the above discussed method is that the control algorithm is too undeveloped resulting in low quality stabilization of the thermal energy and the electrochemical conditions of the process. This result occurs as both in the cathode-anode distance and the alumina feed rate adjustments are executed based only on the “normalized” cell voltage data.  
      Another method of controlling process in an aluminum reduction cell is disclosed by the Russian Patent RU 2,113,552. According to this method, the actual cell voltage and current values are measured. Then based on such measurements, the normalized voltage, the rate of change of the latter in time and the alumina concentration within the cell are determined. Further steps of this method are as follows: comparison of actual values of these characteristics with their preset values, maintaining of normalized cell voltage within the preset limits by moving the anode and regulating the amount of alumina fed into the cell by alternating the overfeed and underfeed modes.  
      The above-discussed prior art control method is based on the known relationship between the cell voltage U cell  and the alumina concentration within the bath C Al . When other characteristics of the method are invariable, every change in the voltage is subject only to change in the alumina concentration of the bath. Thus, it is possible to use the rate of change of voltage dU cell /dt in order to approximately evaluate C Al .  
      A substantial drawback of this method is that of depending upon U cell =f (C Al ), which is a function nonlinear in nature over (minimum) the range of C Al =3.5-4.5%. In this method, when in the area of high alumina concentrations (more than 4%), the increase in the cell voltage indicates an increase in C Al . Conversely, an increase in the cell voltage in the area of low alumina concentrations (less than 4%), indicates a decrease in C Al . The latter also predicts an upcoming anode effect. In actual industrial practice the low concentration range is called “left branch of the concentration curve”, and the high concentration range is called “right branch of the concentration curve”.  
      Operational experience suggests that the use of the above-discussed prior art method does not always bring positive results. In order to properly control the process, it is necessary to initially define the side of the concentration the curve utilized in the current operation of the cell. When the branch is defined incorrectly, the effect of the controlling action, such as the cathode-anode distance adjustment or the amount of alumina charged into the cell is directly opposite to the expected result.  
     SUMMARY OF THE INVENTION  
      The method of the invention can be utilized to control processes in the aluminum reduction cells with prebaked anodes, so as to improve the quality of controlling the alumina feeding systems. The method consists of the steps of measuring present electric current, comparing the value of the present electrical current with the predetermined values thereof, and maintaining such present values within preset limits by regulating the amount of alumina charged into the cell. Further, in the method of the invention, an area of reduced alumina concentration is identified in the cell by measuring the existing, present electrical current values. The area of reduced alumina concentration can be also identified by reviewing the rate of increase of the back EMF value at each anode independently of other anodes in the cell. Then, a control a signal is generated by the control system to a specific feeder of the alumina feeding system to raise alumina concentration in the area of reduced alumina concentration to predetermined levels.  
      One object of the invention is to automatically control the level of alumina concentration C al  within the cell and to stabilize the same at a preset level within the cell volume.  
      It is a further object of the invention to eliminate unplanned anode effects and alumina aggregation of the cathode bottom and to ultimately improve performance of the aluminum reduction process.  
    
    
     DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is front elevational view showing partially in a cross-section an aluminum reduction cell with a measuring system of the invention;  
       FIG. 2  is a diagram of anode current load prior to and during the occurrence of the anode effect; and  
       FIG. 3  is a diagram of algorithm illustrating operation of the control system of the reduction cell, wherein Block  1  is the diagram reflecting operation of the control system according to prior art, and Block  2  is the diagram illustrating operation of the control system of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring now to  FIG. 1  showing a partial cross-sectional view of an aluminum reduction cell with a measuring system of the invention. A plurality of prebaked anodes  1  are disposed within the cell and are combined with a system for measuring electric current. Each anode  1  is provided with electric load measuring transducer  2  for detecting and directing signals to a control and data acquisition unit  3 . In this manner, the current at each individual anode is measured and evaluated independently of other anodes in the system. In order to determine within an aluminum reduction cell an area having the reduced level of alumina concentration, information about the electrical current load at each individual anode is collected, accumulated and analyzed. These data form the basis for the dynamic analysis of the electrical current load change at each anode.  
      Each anode of the cell is assigned to a predetermined area. The concentration of alumina in this area is maintained by a predetermined feeder of the alumina feeding system. Reduction of the electrical current load in a single anode or in the group of the anodes (with the current load in other anodes being invariable) is considered as the local reduction of alumina concentration. Referring now to  FIG. 2  which is a diagram reflecting the anode current load prior to and during the occurrence of the anode effect. The diagram illustrates considerable reduction of the electrical load in individual anodes relative to other anodes of the cell (see for example, anode No.  4  and anode No.  12 ). This load reduction is indicative of isolation of the anode working surface by a gas film. As a consequence, the alumina concentration decreases in this bath area in the vicinity of the respective anodes. As the alumina concentration in the bath decreases the wetting of the anode by the bath declines, and as a result, gas bubbles on the anode surface enlarge causing at least partial isolation of the anode surface from the bath. As the isolation increases, so does the current density on the free surface of the anode. The data reflecting this condition is predicative of the anode effect, as further deterioration of this process generally results in just such an occurrence. In the diagram of  FIG. 2  this undesirable development takes place at the time spot 12:52. To obviate this problem, a responsive signal is generated by the control system of the invention for the control of the feeder associated with the area of the cell adapted to accommodate the monitored anodes. Operational mode of the respective feeder is chosen on the basis of the existing, present mode of the cell operation. However, this feeder functions with increased operational frequency compare to other feeders of the system. The increased rate of alumina feeding is carried out for a predetermined time interval or until the electrical load in the respective anodes starts to increase.  
      The method of controlling an aluminum reduction cell with prebaked anodes of the invention consists of the steps of measuring the present values of the current, comparing the present current values with preset values thereof and maintaining such values within preset limits by regulating the amount of alumina charged into the cell. According to the invention an area of reduced alumina concentration within the cell is defined by measuring the existing, present values of the current in each anode of the cell independently of other anodes. During this procedure a specific automatic alumina feeder is associated with the group of neighboring anodes. The electric current reduction in an individual anode or a group of anodes is detected and measured. Such current reduction is being evaluated comparative to the electric current in other anodes or a group of anodes of the cell. Upon detection of the reduced level of electric current in the individual anode or group of anodes, the rate of feeding of alumina is increased into the bath area in the vicinity of the reduced alumina concentration. The rate of feeding of alumina is increased relative to the typical feeding rate of alumina into the bath.  
      As to  FIG. 3 , it reflects a combined diagram of algorithm illustrating operation of the control system of the reduction cell, wherein Block  1  is the diagram showing operation of the control system according to prior art, and Block  2  is the diagram illustrating operation of the control system of the invention. These diagrams will be discussed in detail with respect to the embodiment of Example 1.  
      The area of reduced alumina concentration is identified by verifying the rate of increase of back EMF value in each anode or group of anodes of the cell. The value of the back EMF is determined by reviewing changes of the total electric current passing through all anodes of the cell. Utilization of the back EMF value for the purposes of the invention are discussed in detail with respect to the embodiment of Example 2.  
      According to the prior art method to maintain the alumina concentration within preset limits cell voltage and line current are measured. Furthermore, calculations are conducted to define the present normalized voltage values (Unorm) and the rate of change in time (dUnorm/dt) so as to compare them with preset values and form cycles consisting of the sequence of basic feeding mode, underfeed mode and overfeed mode of the cell. As to the method of the invention, the area of reduced alumina concentration within the cell is defined by measuring present electrical current values in all anodes of the cell independently of each other. Upon detection of such area, the alumina concentration in this area is increased until a predetermined level of concentration is reached. To carry out this task a signal is generated by the control system of the cell to respective feeder of the alumina feeding system. As a result the rate of alumina feeding is increased  
     EXAMPLE 1  
      Frequent and unplanned anode effects are taking place in the prior art aluminum reduction cells utilizing control systems where the level of alumina concentration in the bath melt is controlled by evaluation of changes in the values of Unorm and dUnorm/dt. Such anode effects are typically caused by the lag in response time of the feeding system to reduction in the alumina concentration. This results when the review of the normalized voltage is too close in time to the occurrence of an anode effect and the corrective action taken cannot forestall the anode effect. In other words, such a method does not have the sensitivity to detect an approaching anode effect. By the method of the invention, however, it is possible to predict the occurrence of an anode effect and to anticipate the situation by providing earlier warning signals of the need to change the alumina concentration. Furthermore, the method of the invention is reflected in the improved alumina feeding control algorithm. An example of such algorithm is illustrated in the Block  2  of the diagram illustrated in  FIG. 3 . According to the Block  2  diagram, the control system of the invention operates as follows. The existing current load at a specific anode of the cell is measured and compared to the previous value thereof.  
      Then the difference ΔIi (where i is the number of the respective anode) between the existing and previous current load values is calculated. When the difference between the loads exceeds a preset value, a signal is generated by the control system, so as to regulate the alumina concentration in the cell. In this manner, the increased rate of alumina feeding into the cell by one of the feeders of the feeding system is initiated. This feeder is directly associated with the area of the cell having reduced alumina concentration.  
     EXAMPLE 2  
      A method of evaluation of the reverse EMF value of the cell based on analysis of the changes in the line current is known in the art. According to this method at the time of considerable changes in the cell current, the values of the cell current and voltage are monitored before and after such changes are taking place. Then, the reverse EMF of the cell is calculated by the following formula:  
                 U   ⁢           ⁢     1   cell       -     E   back         I   ⁢           ⁢     1   cell         ⁢     (   before   )       =           U   ⁢           ⁢     2   cell       -     E   back         I   ⁢           ⁢     2   cell         ⁢     (   after   )         ;   where       
 
 U 1   cell , I 1   cell  are the cell voltage and cell current values before to the change of amperage; 
 
 U 2   cell , I 2   cell  are the cell voltage and cell current values after the change of amperage. 
 
      According to the method of the invention the area with the reduced level of alumina concentration is evaluated according to the change of the reverse EMF which is calculated for each anode of the cell. The reverse EMF values for each individual anode are calculated by the same method as for the entire cell. However, in this instance the electrical current values of each anode are determined by the following formula:  
                 U   ⁢           ⁢   1   ⁢     i   cell       -     E   back         I   ⁢           ⁢   1   ⁢     i   anode         ⁢     (   before   )       =           U   ⁢           ⁢   2   ⁢     i   cell       -     E   back         I   ⁢           ⁢   2   ⁢     i   anode         ⁢     (   after   )         ,   where       
 
 U 1 i cell , I 1 i cell  are the values of cell voltage, and i th  is the anode current value before the change of amperage; 
 
 U 2 i cell , I 2 i cell  are the values of cell voltage, and i th  is the anode current value after the change of amperage. 
 
      The values of the reverse EMF of the anodes are compared to each other and the area with reduced alumina concentration is determined. A respective signal is generated for the feed control system of the invention to regulate to the level of alumina concentration in the cell. To accomplish this task the alumina feeding rate by one of the feeders is increased.  
      Utilization of the method of the invention results in the improved distribution of alumina concentration in the cell bath volume. Evaluation of the reverse EMF values provides reliable and earlier diagnostic of the forthcoming anode effect. All of the above result on the improved quality control and stability of the cell operation and enhances the anticipatory aspects of the feeding system control functions.