Patent Publication Number: US-3878070-A

Title: Apparatus for and method of producing metal

Description:
United States Patent [1 1 Murphy APPARATUS FOR AND METHOD OF PRODUCING METAL [75] Inventor: Joseph A. Murphy, Murraysville,  
 [73] Assignee: Southwire Company and National Steel Corporation, Pittsburgh, Pa.  
 [22] Filed: Feb. 21, I973 [21] Appl. No.: 334,233  
 Related U.S. Application Data [63] Continuation-impart of Ser. No. 298,405, Oct. 18,  
 [52] U.S. Cl 204/64 R; 204/67; 204/225; 204/228; 204/245 [51] Int. Cl. C22d 3/00; C22d 3/02; C22d 3/12 [58] Field of Search 204/67, 243 R-247, 204/228, 64 R, 225  
 [ 1 Apr. 15, 1975 Primary Examiner-John H. Mack Assistant Examiner-D. R. Valentine Attorney, Agent, or Firm-Van C. Wilks; Herbert M. Hanegan; Stanley L. Tate [57] ABSTRACT A method involving the production of metal. in particular, aluminum by providing an electrolytic bath containing dissolved oxide of the metal to be produced in a reduction cell. A direct current flows through the bath and metal is collected on the bottom of the re duction cell. The method includes determining the undesirable process generated noise component of the resistance in a reduction cell. The method preferably involves reducing or eliminating the noise component whenever it exceeds a given level. The apparatus for producing metal includes at least one reduction cell having electrodes for delivering direct current to an electrolytic bath containing dissolved oxide of the metal. A circuit arrangement is operatively arranged to sense the process generated noise component of the resistance on the reduction cell and produces an output signal whenever this component exceeds a given level indicating the existence of a noise level which is detrimental to efficient cell operation. The apparatus preferably includes devices responsive to the output from the circuit arrangement for reducing or eliminating the noise component whenever it exceeds the given level.  
 24 Claims, 2 Drawing Figures APPARATUS FOR AND METHOD OF PRODUCING METAL REFERENCE TO RELATED APPLICATTON This application is a continuation-in-part application of the application of Joseph A. Murphy, entitled A METHOD OF AND APPARATUS FOR PRODUC- lNG METAL, Ser. No. 298,405 which was filed on Oct. 18, 1972.  
 BACKGROUND OF THE INVENTION This invention relates to a method of and to an apparatus for the control of an electrolytic reduction cell or cells for producing molten metal. The invention relates, more particularly, to a method of and to an apparatus for the control of an electrolytic reduction cell or cells in which a metallic compound or solute constituent of a fused electrolyte in an electrolytic cell produces a molten metal. The invention is directed, in its primary adaptation, to the control of an electrolytic cell or cells useful in the production of aluminum.  
  The production of aluminum by electrolysis of an aluminum containing compound is a very old, wellknown process. Commercial aluminum production is carried out by the Hall-Heroult process in which aluminum oxide, refined from bauxite ore, is reduced electrolytically. Alumina, A1 the solute, is dissolved in molten cryolite, NaFlAlF the solvent, at a temperature of about 970 C. The dissolved alumina, when subjected to a high intensity current, in electrolytic cells of either the continuous, self-baking Soderberg anode type or the pre-baked anode type, disassociates into positive aluminum and negative oxygen ions. In practice, a plurality of substantially identical electrolytic reduction cells, for example 28 reduction cells, are arranged in a pot line, that is, they are connected electrically in series. A direct current of from about 50,000 amperes to I60,000 amperes or more, in commercial reduction systems, is usual. The electrical path for the external current source is composed of the carbon anode structure, the electrolytic bath and the cathode structure, usually in the form of collector bars buried in the bottom of the reduction cell. The specific current, in any case being determined by the siize of the electrolytic reduction cells, flows through the bath containing the alumina and electrolyte, a voltage drop of from about four volts to about six volts appearing across each reduction cell during normal volts to about six volts appearing across each reduction cell during normal electrolysis. As the normal electrolysis proceeds, aluminum is deposited at the cathodic bottom of the reduction cell or each of the series-connected reduction cells where it collects as a molten pool of aluminum, a tap being provided in each of the reduction cells so that the aluminum can be periodically removed. The side of the reduction cell which is provided with the tap for removing the molten aluminum is known as the tap side. The oxygen of the alumina combines with the carbon of the anode to form principally carbon dioxide and carbon monoxide, the gases being conventionally led away from each reduction cell by a duct, the duct being positioned near the top of that side of each cell which is opposite to the tap side. This particular side of a reduction cell is referred to generally as the duct side.  
  According to Faradays Law, the pounds of aluminum produced are directly proportional to the quantity of electrical charge passed through each of the reduction cells. An approximate equivalent circuit for an individual reduction cell would show a decomposition voltage of back EMF in series with a resistance having a fixed component and variable component. The fixed component is determined by the electrical resistance of mechanical circuit connections, while the variable component represents the resistance of the bath itself. The bath resistance, in turn, can be expressed as R,. p (D/A), where A, the effective surface area, is essentially constant, but P, the resistivity, varies with the alumina concentration, and D, the anode to cathode spacing, varies with anode consumption as well as with aluminum buildup on the cathodic bottom of the reduction cell. The thermal input to the reduction cell, and hence temperature of the cell, depends on the R losses generated in this resistance.  
  The efficiency of the process, at least in terms of pounds per ampere-hour is determined by the percentage of metallic aluminum which, under the influence of strong magnetic fields, comes into contact with oxygen near the anode to reform the original oxide. The tendency for this to happen increases with temperature and decreases with the distance between the anode and the cathode. Although aluminum production depends on ampere-hours, users of electrical power actually pay on a kilowatt-hour basis; consequently, efficiency in terms of cost depends on maximizing current in the volt-ampere relationship. For efficient operation, the undesirable process generated noise component of the resistance of the reduction cell should be reduced or eliminated, special precautions being taken to assure that this noise component is maintained below a given tolerable level. The resistance of the reduction cell should preferably be regulated to provide additionally a low stable bath temperature with high current and minimum reoxidation.  
  One of the continuing problems encountered in the commercial electrolytic aluminum reduction process is the effective control of the concentration of dissolved alumina in the bath. If the concentration of alumina is depleted from the upper maximum of from about 7% to about 10% down to a certain critical limit, generally considered to be approximately 2.0%, a phenomena known as anode effect occurs, with its consequent wellknown disadvantages and reduced efficiency. The anode effect is a charactetistic of reduction cells in which aluminum is being produced by electrolysis of a cryolite/alumina bath. The anode effect is conventionally extinguished and normal electrolysis restored by the expedient of breaking the frozen top crust of the bath which adds alumina into the bath. Extreme caution must be taken, however, not to charge the bath with too much additional alumina for all of the additional alumina will not dissolve if the amount exceeds a solubility capacity of the electrolyte for alumina at the prevailing temperature, usually about 970 C. If the electrolyte cannot dissolve all of the additionally added alumina, some of the alumina will sink through the electrolyte and through the molten alumina, collecting on the cathodic bottom surface of the reduction cell, with the result that the resistance of the cathode undesirably increases, efficiency declines resulting in what is known as an over-fed or sick reduction cell.  
  In both cases the anode effect, which results from a starvation condition of the bath, and the sick cell phenomena which results from overfeeding the bath, the  
 reduction cell is working under abnormal conditions with the concomitant undesirable decline in overall efficiency. Of the two conditions, the anode effect has been found to be the lesser of the two disadvantages for it can be extinguished more easily than the sick cell condition can be remedied. Consequently, techniques have been developed, involving both intermittent and continuous alumina feeding of an electrolytic bath, which add alumina to the electrolytic bath routinely in amounts adapted to avoid development of a sick cell condition. Such feeding techniques rely on an underfeeding practice, which allows the reduction cell to undergo occasional anode effects, for example, one anode effect per day, which assures against overfeeding alumina into the reduction cell.  
  The US. Pat. to Bruno, et al, No. 3,400,062, issued Sept. 3rd, 1968, discloses a control system for an aluminum reduction cell having an anode, a cathode and a fused electrolytic bath of cryolite and dissolved alumina. A pilot anode is insulatively supported at the reduction cell, with one extending into the electrolytic bath to a given immersion depth. A power supply is provided for supplying direct current energy to the pilot anode. The current density supplied to the pilot anode during a standby, which may be of several minutes duration, is sufficiently high to maintain the auxiliary anode on anode effect. This particular condition, according to the patent specification, is maintained to prevent consumption of the pilot anode by the electrolysis. During a second period of from about 8 to about 10 seconds duration, for example, the current to the pilot anode is reversed in order to eliminate its anode effect. Finally, during a third period of from about ID to about seconds a second or lower voltage, preselected in dependence on the level of alumina control desired, is impressed upon the pilot anode causing the resulting current through it in the forward direction to provide a given control current density in order to determine the alumina concentration of the electrolytic bath by sensing whether or not an anode effect appears on the pilot anode under this lower voltage condition. After a brief pause of a few seconds to allow the current in the pilot anode to stabilize, a sensor associated with the pilot anode is made effective for detecting whether or not an anode effect has appeared. if an anode effect has appeared, it is an indication that the electrolytic bath is in an underfed condition, and the feed rate for the alumina into the bath is increased. On the other hand, if no anode effect appears during this sensing period, the normal slower feed is maintained. Thus, it is clear that the system disclosed in the prior patent to Bruno et al is rather complex and, so far as the reverse current and sensing periods are concerned, requires from about 18 to about 38 seconds to operate.  
  The US. Fat. to Smids, No. 3,539,456, issued Nov. 10th, I970, discloses an electrolytic cell solute determining apparatus for use in the operation of a direct current electrolytic reduction cell in which a pair of alternating current energized auxiliary electrodes, which extend into an aluminum oxide/solute containing bath of the electrolytic reduction cell, serves as a means for sensing the current of cycling anode effects induced thereon during operation of the reduction cell. This particular apparatus, because the alternating current energization of the pilot electrodes, does not require the current reversal in the pilot electrode as was needed in the direct current energized pilot electrode arrangement disclosed in the above-mentioned patent to Bruno, et al. It is to be appreciated, however, that the anode effect sensing apparatus disclosed in the above-mentioned patent to Smids still requires a relatively long period to operate and requires that a special alternating current supply and auxiliary electrodes be provided.  
  The U.S. Pat. to Brown, No. 3,345,273, issued Oct. 3, 1967, discloses a method of and an apparatus for indicating the position of an anode with respect to a cath ode in a multiple anode reduction cell. The relative position is determined by measuring the amplitude of a low frequency [-20 c.p.s.) voltage signal which is superimposed on the direct voltage applied across the reduction cell during operation. These low frequency voltage variations, which may be designated process generated noise, occur whenever the anode or a portion thereof is too close to the anode thereby causing an overload of the anode. The faulty spacing may result from a number of causes. An operator may, in approximating the proper position of a replacement anode block, incorrectly position the replacement block. An anode or anode block, may be inadvertently disturbed during normal operation. Waves may be produced in the bath, a humping or thickening of the aluminum layer under one of the carbon blocks or a portion of the anode structure may occur, a condition which may, at least for a period, drastically reduce the thickness of the cryolite layer resulting in an upset condition.  
  Faulty spacing between the anode and the cathode of a reduction cell may result from portions of an anode or different anode blocks being consumed at different rates. In some instances, pieces of the carbon anode may fall off, resulting in improper spacing. Undesirable process noise may also result from chunks of ore either shorting the anode to the cathode or reducing the resistance of the anode-to-cathode path in the bath.  
  The above-mentioned patent to Brown discloses the concept of sensing the magnitude of the low frequency voltage signal which appears across a reduction cell and using the sensed signal to actuate an alarm whenever it exceeds a predetermined level, indicating too little spacing between the anode and cathode. The sensed low frequency voltage signal may, for example, be fed to a DC. voltmeter causing its pointer to oscillate over a range of 0.2 volts or more, indicating to an operator that the anode is too close to the cathode. An on-line computer may be used to calculate the amplitude from a set of consecutive voltage readings and issue preselected instructions to an operator.  
 SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of producing metal from an electrolytic bath which involves sensing the occurrence of the undesirable process generated noise component of the resistance in a reduction cell.  
  is is another object of the present invention to provide a method of producing metal from an electrolytic bath which involves determining the occurrence of a process generated noise component of the resistance in a reduction cell in excess of a given level, and reducing the noise component below the given level.  
  It is a further object of the present invention to provide, in an apparatus for the reduction of metal from an electrolytic bath, a circuit arrangement for the detec- *ion of the process generated noise component of the .esistance in a reduction cell.  
  It is yet another object of the present invention to rovide, in an apparatus for the reduction of metal &#34;rom an electrolytic bath, a circuit arrangement which :nses the occurrence of the process generated noise component of the resistance and effects the reduction or substantial elimination of the detected noise component whenever it exceeds a tolerable level.  
  It is yet a further object of the present invention to provide, in an apparatus for the reduction of metal from an electrolytic bath, an inexpensive and reliable circuit arrangement for sensing the occurrence of the arocess generated noise component of bath resistance.  
  The foregoing objects, as well as others which are to 1e made apparent from the text below are accomplished according to the present invention in its method aspect by providing an electrolytic bath containing dissolved oxide of the metal to be produced in a reduction cell, causing direct current to flow through the bath and collecting the metal produced on the bottom of the eduction cell. The method includes sensing the process generated noise component of resistance of the lath and determining when this noise component ex- :eeds a given level as an indication of an undesirable noise level. The method may also involve reducing the noise level whenever it exceeds the given level.  
  The foregoing objects, as well as others which are to be made apparent from the text below, are accomplished according to the present invention in its apparatus aspect, in an apparatus which includes at least one reduction cell having electrode means for delivering direct current to the bath. Circuit components are provided for sensing the process generated noise component of bath resistance. Additional circuit components are operatively arranged to be responsive to output from the components for sensing for determiniing the occurrence of noise components in excess of a given level. The apparatus further may include devices responsive to the output from the components for sensing which reduce the noise level whenever it exceeds the given level.  
 BRIEF DESCRIPTION OF THE DRAWING FIGS. 1A and 1B are schematic illustrations of an apparatus for producing metal from an electrolytic bath in accordance with an illustrative apparatus embodiment of the present invention, the apparatus being particularly suitable for carrying out the method according to the present invention.  
 DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT As seen in FIGS. 1A and IB, an alumina reduction cell, generally designated 9, with associated circuitry, suitable for practicing the present invention is shown schematically. The alumina reduction cell 9 includes a steel shell 10 having a carbonaceous lining 11. The conductive lining 11 contains a pool of molten aluminum l2 and a bath 13 of alumina dissolved in a molten electrolyte, the bath 13 being above the pool of molten aluminum 12. Conductive rods, which are embedded in the conductive lining 11, are connected to a cathode conductor or bus 14. It is to be understood that other forms of lining can be used to contain the molten aluminum l2 and the bath 13. A cathode potential can be impressed on the molten aluminum 12 by other conventional means instead of the conductive rods as shown. Suspended above the bath l3, and partially immersed therein, is a carbon anode 15 shown diagrammatically. In practice, the carbon anode 15 may be a multiple bar anode arrangement positioned on a suitable superstructure adjustable as a unit or a conventional vertical or horizontal stub Soderberg-type anode. One multiple bar anode arrangement which can be used for the anode 15 comprises eighteen carbon bars, each weighing about one ton. The molten bath 13 is covered by a hard crust 16 which consists of frozen electrolyte constituents and additional alumina. The anode I5 is connected to a positive bus 17 via a conductor 18. A current sensing device 20 is provided for sensing the current flowing in the conductor 18. The current sensing device 20, which produces a direct voltage directly related to the direct current flowing in the conductor 18, preferably is of a type which does not require a series connection in the conductor 18.  
  On the tap side of the reduction cell 9 (Le, the side from which molten aluminum may be drawn off), a first conventional alumina feeder 24 is provided. A first crust breaking bar 25 is provided in the vicinity of the first feeder 24. A second alumina feeder 26 is provided on the duct side of the reduction cell 9 (Le, the side from which gases may be drawn off) and a second crust breaking bar 27 is provided in the vicinity of the second feeder 26. Two pneumatically or electrically operated motion-producing devices 28 and 30 mechanically connected to the anode 15 are provided respectively for raising and lowering the anode 15 in predetermined increments. A volt meter 31 is connected between the negative bus 14 and the conductor 18.  
  A pulse producing timing circuit 32 is provided for producing two pulse trains, each having an identical pulse repetition rate, for example, a pulse repetition rate of 5 pulses per minute. The two pulse trains are out of phase, one pulse train being displaced from the other by one-half the interval between pulses, for example, by 6 seconds. One of the pulse trains from the timing circuit 32 is fed to the enabling input of a gate circuit 33 and the other pulse train is fed to the enabling input of a gate circuit 34. The signal input of the gate circuit 33 is connected to the current sensing device 20 and receives therefrom a voltage signal directly proportional to the current flowing in the conductor 18. The signal input of the gate circuit 34 is connected to the conductor 18 and receives therefrom a voltage corresponding to the voltage across the reduction cell 9.  
  The respective outputs from the gate circuit 33 and the gate circuit 34 are coupled to the input of a limiting amplifier 35 which is preferably operatively arranged to limit at an input voltage of approximately 10 volts. The limiting amplifier 35 preferably has a gain of one. The output from the limiting amplifier 35 is coupled to an analog to digital converter 36 which produces binary coded digital signal outputs which correspond, at different times, to the current supplied to the reduction cell 9 and to the voltage drop across the reduction cell 9, as determined by which of the two gate circuits 33 and 34 is supplying an input to the limiting amplifier 35.  
  The output from the analog to digital converter 36 is coupled to the first input of an AND circuit 37 and to the first input of an AND circuit 38, second inputs to the AND circuit 37 and to the AND circuit 38 being connected to the timing circuit 32 for receiving the respective pulse trains therefrom. Thus, the AND circuit 37 intermittently passes to its output a binary coded digital signal indicative of the direct current flowing within the reduction cell 9 and the AND circuit 38 intermittently passes into its output a binary coded digital signal indicative of the voltage across the reduction cell 9.  
  The AND circuit 38 has its output coupled to a first input of a subtractor 39. A second input to the subtractor 39 is connected to a binary coded digital signal source 29 which is settable and provides as its signal output a predetermined binary coded digital signal representing the back EMF of the reduction cell 9, this back EMF being nominally 1.6 volts for an alumina/- molten cryolite bath. The output signal from the sub tractor 39 is fed to a first input of an arithmetical circuit, denominated as an arithmetic divider 40. The AND circuit 37 has its output couplled to a second input of the divider 40 via a digital store 41 which stores the binary coded digital signal received from the AND circuit 37 for a sufficiently long period to assure that the divider 40 has present contemporaneously at its two inputs the signals passed by the AND circuits 37 and 38. The divider 40 produces as its output a binary coded digital signal which is the quotient of the digital signal representing the gross voltage across the reduction cell being examined minus the back EMF of the cell divided by the digital signal representing current, its binary coded digital output signal thus corresponds to the resistance of the reduction cell 9, its electrodes and connections thereto.  
  The output from the divider 40 is coupled to a first input of an arithmetic subtractor 43 which is operatively arranged to receive at its second input a predetermined binary coded digital signal from a digital signal source 42, which signal represents the known fixed electrical resistance of the electrical connections to the reduction cell 9. Accordingly, the subtractor 43 produces as its output signal a binary coded signal substantially directly corresponding to the varying resistance of the bath 13.  
  The output from the subtractor 43 is coupled to the signal input of a conventional digital filter 82 which is operatively arranged to separate the process generated noise component of the bath resistance variation (corresponds to a low frequency signal) from the longer term slowly varying component of bath resistance (corresponds substantially to DC); this latter component being determined principally by expected changes in the bath concentrations during normal operation. An output which constitutes the slowly varying component of bath resistance, from the digital filter 82 is coupled to a first input of a first digital signal comparator 44 and a first input of second digital signal comparator 45. A second input to the digital signal comparator 44 is provided from an upper threshold setting circuit 46 which is a source of a binary coded digital signal for establishing an upper resistance value for the bath 13, the alumina concentration being directly related to the resistance of the bath 13. A second input to the second digital comparator 45 is provided from a lower threshold setting circuit 47. The comparator 44 provides an output whenever the digital signal it receives from the digital filter 82 exceeds the digital signal it receives from the upper threshold setting circuit 46, indicating that the resistance of the bath I3 is too high. It is to be understood that the digital signal source 42 and the subtractor 43 are not necessary, the output from the divider 40 could be directly coupled to the input of the digital filter 82 provided that the threshold setting circuits were appropriately set to include the fixed resis tance ofthe electrical connections to the reduction cell 9.  
  The output from the limiting amplifier 35 is also connected to an anode effect detector 48 which is a Zener diode having a voltage switching threshold of approximately volts. Since the anode effect detector 48 has a voltage threshold of 7.5 volts, it will not conduct and will not produce an output signal so long as the voltage of the reduction cell 9 remains within the range below 7.5 volts, the expected range being from about 3.5 volts to about 6.5 volts, 5.0 volts rarely being exceeded, during normal bath conditions. Whenever the voltage across the reduction cell 9 increases above the 7.5 volt level, the anode effect detector 48 conducts producing a logical ONE signal on its output, indicating that the reduction cell 9 is undergoing an anode effect which signals that the concentration of alumina in the bath 13 is much too low for efficient operation. Since anode effect may and often does produce voltages as high as 30 or 40 volts across a reduction cell, the limiting amplifier 35 is arranged to limit at an input of about 10 volts thereby preventing damage to the&#39;analog to digital converter 36 and to the anode effect detector 48 without decreasing&#39;the sensitivity of the circuitry.  
  As discussed above, the circuitry as thus far described in effect determines the resistance of the reduction cell five times every minute. In practical applications of the present invention, the resistance of the re duction cell may be determined at greater intervals, for example, at one minute intervals.  
  A second output from the subtractor 43 is coupled to a conventional digital filter 83, which functions to pass the digital signal representative of the process generated noise component of the bath resistance. An output from the digital filter 83 is coupled to a first input of a third digital signal comparator 84 which has its second input coupled to an output from a third threshold setting circuit 85. The threshold setting circuit 85 is set to provide as its output signal, a digital signal corresponding to that level of process generated noise which is to be tolerated in the apparatus. The comparator 84 provides an output signal whenever the digital signal it receives from the digital filter 83 is greater than that of the digital signal indicative of the level of noise to be tolerated, which it receives from the threshold setting circuit 85.  
  The output from the comparator 84 which, as a practical matter provides an output every minute, for example, is coupled to a three stage shift register 86 which is provided with a shift pulse input from the timing circuit 32, which provides a shift pulse once every minute. Thus the output from the comparator 84 is effectively stored in the shift register 86 once every minute and shifted through the shift register 86 in three steps, appearing as a binary ONE or ZERO at the output of its final stage at the end of a three minute interval, de pending on the signal received from the comparator 84. Likewise binary ONE or ZERO signals appear in the first and second stages of the shift register.  
  In the event that the output signal from each of the three stages of the shift register 86 is a ONE, the AND circuit 87 responds, producing a binary ONE signal which is coupled, as an enabling signal, to the digital signal source 88 and, as a reset signal to the shift register 86. Thus, in order to obtain a binary ONE output from the AND circuit 87, the noise signal must, in effect, have too great a magnitude for at least three consecutive minutes.  
  Four digital sources 50, 51, 52, and 88 are provided. Each of the digital signal sources 50, 51, 52, and 88 include respective stores 53, 54, 55, and 89 which respectively store a regular normal break and feed program, a resistance control, anode position adjusting program, an anode effect extinguishing program, and a noise pot suppression program. The stored programs, in each instance, are respective stored binary coded digital signals in bit parallel and command serial.  
  The digital signal source 50 provides in command sequence and bit parallel a series of binary coded digital command signals from its store 53 to effect, in sequence, the breaking of the crust 16 on the tap side by the breaker bar 24, the feeding of additional alumina to the tap side from the feeder 24, the breaking of the crust 16 on the duct side by the breaker bar 27 and the feeding of additional alumina to the duct side from the feeder 26. The breaking bars 25 and 27 are, in most practical instances, moved up and down several times to assure that the crust 16 is broken, the digital signal source 50 from its store 53 supplying the appropriate command signal or signals for effecting such multiple motions.  
  In a practical instance, the digital signal source 50 supplies the digital command signals, in bit parallel, which effects first a breaking at the tap side, with subsequent feeding of the tap side at a predetermined later time and thereafter, usually approximately 90 minutes later, the breaking and subsequent feeding of the duct side of the reduction cell 9. Since the crust 16 is predominantly alumina, the breaking of the crust 16 enriches the bath 13, resulting in a lowering of the bath resistance. The feeding may also provide, if desired, additional alumina to the bath 13, but is preferably done at a time sufficiently later than the breaking so that the newly fed alumina becomes part of the crust 16 or is supported on its surface. The digital signals, in bit par allel, are supplied from the output of the digital signal source 50 to a command decoder 59 via a series connected negated AND circuit 56, a negated AND circuit 57 and an OR circuit 58.  
  The digital signal source 51 is provided with two enabling inputs which are supplied respectively from the comparator 44 and the comparator 45. ln response to a digital difference signal from the comparator 44, indicating that the upper threshold set point for the resistance of the electrolytic reduction cell 9 has been exceeded, the digital signal source 51 is operatively arranged to supply from its store 54 a binary coded digital signal, in hit parallel, to the command decoder 59 calling for the anode to be lowered by a given increment or increments depending on the magnitude of the digital difference signal supplied from the comparator 44. Thus, the resistance of the reduction cell 9 is lowered until the digital difference signal from the comparator 44 disappears. In response to a digital difference signal from the comparator 45, indicating that the lower threshold set point for the resistance of the reduction cell 9 has been exceeded, the digital signal source 51 is operatively arranged to supply from its store 54 a binary coded digital signal, in bit parallel, to the command decoder 59 calling for the anode 15 to be raised by a given increment or increments depending on the magnitude of the digital difference signal supplied from the comparator 45. Consequently, the resistance of the reduction cell 9 is increased until the digital difference signal from the comparator 45 disappears.  
  The binary coded digital signals from the digital signal source 51 which call for either an incremental lowering or an incremental raising of the anode 15, are supplied to the command decoder 59 via a negated AND circuit 60 and the OR circuit 58. A second output from the digital signal source 5], which simply indicates that the digital signal source 51 is supplying signals to effect anode movement, is coupled to the negated input of the AND circuit 57 thereby interrupting the regular break and feed program fed to the command decoder 59 from the digital signal source 50.  
  The digital signal source 5] as thus far described responds whenever difference signals appear on either the output from the comparator 44 or the comparator 45. The digital signal source 51 is preferably so constructed that it inhibits itself from supplying command signals for a period of five minutes after each of its responses.  
  The output from the anode effect detector 48, which appears as a logical ONE whenever its input exceeds 7.5 volts by virtue of the Zener characteristic of the detector 48, is coupled to the enabling input of the digital signal source 52. Whenever the digital signal source 52 is enabled, it produces from its store 54 a series of binary coded digital command signals, in bit parallel, to effect in sequence the breaking of the crust 16 on both the tap side and the duct side of the reduction cell 9, the lowering of the anode l5, and the subsequent feeding of the reduction cell from both the feeder 24 and the feeder 26. As in the normal breaking and feeding operation, the feeding operations preferably take place during an anode effect extinguishing operation after the crust 16 has hardened. In some instances, it may be sufiicient to break and feed only either the duct side or the tap side to assure anode effect suppression.  
  The output digital command signals, in bit parallel, are coupled to the command decoder 59 via the OR circuit 58.  
  A second output from the digital signal source 52, which simply indicates that the digital signal source 52 is providing an anode effect extinguishing command signal, is coupled to first negated inputs of the AND circuit 60 and of the AND circuit 56 and of an AND circuit for the purpose of disabling feed of the regular break and feed program routine signals, the regular, routine resistance adjusting anode positioning signals and the noisy pot suppression signals to the command decoder 59.  
  Thus, the digital signal sources 50, 51, 52 and 88 supply to the exclusion of each other and on a priority basis coded digital command signals, in bit parallel, to the command decoder 59 which, in turn, produces 6 output signals on its output lines 61-66 which are fed to respective memory circuits 67-72. The memory circuits 67-72 in turn supply signals to respective alternating current solenoid drivers 73-78. The memory circuits 67-72, which may be in the form of long RC time constant circuits, are provided to assure that the output of the command decoder 59 is present sufficiently long to energize their associated respective solenoid drivers 73-78, and at the same time freeing the command decoder 59 for the decoding of additional command signals.  
  The solenoid drivers 73 and 78, which respond respectively to signals stored in the memory circuit 67 and in the memory circuit 72, are arranged to energize respectively the first feeder 24 on the tap side and the second feeder 26 on the duct side of the electrolytic cell 9. The feeders 24 and 27 are of conventional construction and preferably are operated by pneumatically or electrically responsive devices respectively controlled from the solenoid driver 73 and the solenoid driver 78.  
  The solenoid drivers 74 and 77, which respond respectively to signals stored in the memory circuit 68 and in the memory circuit 71, are arranged to energize respectively first and second pneumatically or electrically operated devices 80 and 81 which are mechanically coupled respective to the breaker bars 25 and 27 to effect movement of them.  
  The solenoid drivers 75 and 76, respond respectively to signals stored in the memory circuit 69 and the memory circuit 70, are arranged to energize respectively the pneumatically or electrically operated motion producing device 30 and the pneumatically or electrically operated motion-producing device 28 which are respectively operatively arranged to effect the lowering and the raising of the anode 15.  
  A third output from the digital signal source 88 is fed to each of the threshold setting circuits 46 and 47 for the purpose of adjusting upwardly the high and low thresholds, effectively changing upwardlly the resistant set point for the bath 13 upon initiation of a noise pot suppressing routine, and for returning these thresholds to their original set points after a time interval, preferably in increments.  
  In order to place the apparatus of the present invention in a condition ready for operation, suitable programs in the form of binary coded digital signals for the regular, normal breaking and feeding function, for the resistance control function, for the anode effect extinguishing function, and for the noisy pot suppression are placed respectively in the stores 53, 54, 55 and 89. Having determined, by conventional techniques, the subtantially fixed electrical resistance of the electrical connections to the reduction cell 9, the digital signal source 42 is set to provide, as its output signal, a binary coded digital signal representative of such resistance. The digital signal source 29 is set to provide, as its output signal, a binary coded digital signal representative of the predetermined back EMF of the reduction cell 9, this back EMF being for a suitable alumina/cryolite bath 1.6 volts.  
  The upper threshold setting circuit 46 is set to provide, at its output signal, a fixed binary coded digital signal which corresponds to the upper limit (i.e., l X 10 ohms) of the resistance range for the electrolytic bath 13 during expected normal electrolysis. This set point, for example, corresponds closely to that point at which the gross voltage across the reduction cell 9 would have increased by substantially +0.02 volts at a nominal current of 150,000 amperes. The lower threshold setting circuiit 47 is set to provide, as its output signal, a fixed binary coded digital signal which corresponds to the lower limit (i.e., l9.9X l0 ohms) of the resistance range for the electrolytic bath 13 during expected normal electrolysis. This set point, for example, corresponds closely to that point at which the gross voltage across the reduction cell 9 would havce decreased by substantially 0.02 volts at the nominal current of l50,000 amperes. It is to be understood that different set points could be used if desired, as determined by desired bath conditions and the sensitivity of the control desired in any given case. The threshold setting circuit is set to provide as its output signal, a fixed binary coded digital signal which corresponds to the level of process generated noise which is to be tolerated.  
  The reduction cell 9 is charged with the appropriate amount of solvent, NaF/AIF and alumina, A1 0 which charge forms the electrolytic bath. The reduction process is initiated preferably manually by supplying direct current to the reduction cell 9, with the possible addition of heat form auxiliary heating means, and adjusting manually the position of the anode 15, with respect to the cathode bottom of the reduction cell until the voltage across the reduction cell 9, as readable from the voltmeter 31, and the direct current to the reduction cell 9, as determined by the current sensing de vice 20, are within limits known to provide efficient operation.  
  Once normal electrolysis is progressing, the digital signal source 50 is brought into operation supplying regular break and feed digital command signals to the command decoder 59 which responds to such signals by sequentially signaling, via the memory circuits 68, 67, 71, and 72, the solenoid drivers 74, 73, 77, and 78 which, in turn effect the movement of the breaking bar 25, the feeder 24, the breaking bar 27, and the feeder 26. In normal operation, the tap side of the reduction cell 9 is thus broken and fed every 180 minutes, a delay period being provided between the breaking and feeding. The duct side of the electrodic cell 9 is thus broken and fed also every 180 minutes, the times of each being displaced by minutes from the corresponding breaking and feeding at the tap side of the reduction cell.  
  Electrolysis continues, the circuitry automatically determining the resistance of the bath l3, appropriate signals being produced by the comparator 44 and the comparator 45 which, whenever the resistance of the bath 13 becomes either too high or too low, signal the digital signal source 51 which supplies digital command signals to the decoder 59. The decoder 59 responds by producing, as the case may be, an output signal to either the memory circuit 69 or the memory circuit 70, which cause the anode 15 to be either raised or low ered. This is accomplished by the motion-producing devices 28 and 30 controlled from the solenoid drivers 76 and 75, which respond to the signals stored in the memory circuits 70 and 69 respectively. Whenever the digital signal source 51 is supplying output signals, the output form the digital source 50 is effectively prevented from reaching the command decoder 59 because of the fact that a signal from the digital signal source 51 is coupled to the negated input of the AND circuit 57.  
  During the operation, the voltage across the reduction cell 9 is intermittently sensed, by action of the gate circuit 34, the voltage signal being passed by the limiting amplifier 36, which has a gain of one, its output in turn being supplied to the anode effect detector 48 which conducts whenever the voltage exceeds 7.5 volts, its Zener breakdown voltage. The anode effect detector 48 responds within a few microseconds, much faster than the 20 to 50 millisecond response time of analog to digital converter 36,supplying a logical ONE ignal to the digital signal source 52 which produces a cries of digital command signals to the command decoder 59 to cause, in succession, the crust 16 on the bath 13 to be broken, possibly on both the tap side and ie duct side of the reduction cell 9, the anode to re lowered, and subsequent feeding of one or both ides of the reduction cell. The mechanical movements we effected by the solenoid drivers 75, 74, 73, 77 and &#34;8. The digital signal source 52 also preferably pro =uces a digital command signal which is decoded by the lecoder 59 and fed to the solenoid driver 76, via the nemory circuit 70, to cause the anode 15 to be re- &#39;urned to its earlier position.  
  A separate output from the digital signal source 52 is ed to negated inputs of the AND circuits S6, 60, and 90 to assure that no command signals from the digital signal sources 50, 51,, and 88 are supplied to the command decoder 59 when it is receiving command signals from the digital signal source 52.  
  During the operation of the apparatus, as so far described, the digital filter 83 passes digital signals representing the noise component of bath resistance to the :omparator 84 which provides a binary ONE output signal upon the signal passed by the digital filter 83 exseeding the output from the threshold setting circuit 35. The output from the comparator 84 is stored in the nput stage of the three-stage shift register 86 and shifted, at one minute intervals, through the shift register 86 stage by stage. Since an output from each of the three stages of the shift register 86 are connected to separate inputs of the AND circuit 87, the AND circuit 87 produces a binary ONE output signal only when ONE signals are present at the output of each of the three stages, a condition which prevails only when the comparator 84 has signaled the presence of too high a process generated noise level. The binary ONE output signal form the AND circuit 87 is fed as a reset signal to the shift register 86, thus effecting the resetting of the shift register 87 whenever the noise level has exceeded the threshold level for a three minute interval.  
  The binary ONE signal from the AND circuit 87 is fed to the digital signal source 88 which supplies a digital command signal from its store 89 to the decoder 59, via the AND circuit 90 and the OR circuit 58. The digital command signal is decoded by the decoder 59 and fed via the memory circuit 70 to the solenoid driver 76 which raises the anode 15 a predetermined amount known to be sufficient to reduce the magnitude of noise component of resistance a given amount in most cases. Of course, if the shift register 86 still indicates that the noise level remains too high, further command signals are provided by the dig tal signal source 88 causing the anode 15 to be raised further until the noise level is within an acceptable liznit.  
  So long as the digital source 88 is producing command signals for raising the anode 15, digital signals from the digital signal sources 50 and 51 are blocked from the OR circuit 58 because of negated inputs of the AND circuits 56 and 60 receiving a binary ONE signal from the digital source 88. It is to be appreciated, however, that if the digital signal source 52 were to produce command signals, they would pass to the decoder 59 and the command signals from the digital source 88 would be blocked by virtue of the negated input to the AND circuit 90 from the second output of the digital signal source 52.  
  A further output signal from the digital signal source 88 is fed to the threshold setting circuits 46 and 47 to raise the upper and lower thresholds, effectively setting a higher set point for the resistance of the bath 13.  
  Since it is expected that the cause of a noisy pot will in time be corrected or that the movement of the anode 15 itself may affect suppression of the cause of an anode effect, the digital signal source 88 is operatively arranged to supply to the decoder 59 from its store 89 a command signal or signals which are supplied to the motion-producing device 30 via the solenoid driver and the memory circuit 69 which cause the anode 15 to be lowered to its initial position or toward its initial position in increments, the anode movement ceasing whenever it becomes positioned in its initial position. Of course, the set points of the threshold circuits 46 and 47 are again set to or toward their initial levels, in accordance with command signals reflecting the changing position of the anode 15.  
  In the event the shift register 86 again signals the occurrence of too high a noise level for too long a time, three minutes, during the return of the anode 15 toward its initial position, the noisy pot suppression routine is again activated. This will continue time and again until either the noisy pot phenomena is effectively suppressed or an operator, knowing the phenom ena persists, sets the threshold circuits 46 and 47 to higher values thereby changing the effective set point of the resistance for the bath 13 to a new higher value at which too high a level of process generated noise does not occur.  
  The method of producing metal according to the present invention, it its broadest aspect, involves the steps of providing an electrolytic bath containing dissolved oxide of the metal in a reduction cell, causing direct current of flow through the oath, collecting the metal produced on the bottom of the reduction cell, sensing the process generated noise component of resistance of the bath and determining when the component exceeds a given level as an indication of an undesirable noise level.  
  [n a further aspect, the method according to the present invention includes the step of reducing the noise level whenever it exceeds the given level and, more particularly, when the given level is exceeded for a given period of time, for example, 3 minutes.  
  The method according to the present invention preferably includes reducing the noise component by increasing the spacing between anode and cathode electrodes associated with the bath.  
  The method according to a preferred aspect, involves the production of aluminum. in this cas the electrolytic bath is composed of alumina as the solute and cryolite as the solvent.  
  The method according to a preferred aspect includes the step of reducing the spacing between the cathode and anode after a period subsequent to the increasing step. This is done preferably in discrete increments.  
  In another preferred aspect, the method according to the present invention involves the reestablishment of the original spaceing between anode and cathode.  
  Although the present invention has been described, in its apparatus aspect, in conjunction with a single electrolytic reduction cell, it is to be appreciated that the invention is applicable to systems which involve multiplexing of the command signals in order to control the operating parameters of many reduction cells. in  
 this instance, the circuit arrangement would, of course, also sense the currents supplied to each cell, the voltages across each cell via multiplexing circuits, the feeding of the command signals and the sensing of the cur rents and the voltages being appropriately synchronized.  
  The term digital filter, as is readily understood by those skilled in the art, in the fields of numerical analyses and computers refers to any technique of digital cir cuit which smooths or selectively passes data, while rejecting other data. It is to be appreciated that the digital filters 82 and 83 are conventional and may correspond to convention analog signal filters having resistance, capacitances and/or inductances. It is to be appreciated that the digital filter 83 may be a least squares estimator.  
  [t is also to be appreciated that the invention, in its method aspect, need not be carried out in the illustrated apparatus, but may be carried out by other apparatuses.  
  While one embodiment of the invention has been shown for purposes of illustration, it is to be understood that various changes in the details of construction and arrangement of parts may be made without departing from the spirit and scope of the invention as defined in the method and apparatus claims.  
 It is claimed:  
  1. A mehtod of producing metal comprising providing an electrolytic bath containing dissolved oxide of the metal in a reduction cell, causing direct current to flow through said bath, collecting said metal on the bottom of said reduction cell, electrically sensing the process generated noise component of resistance of said bath and electrically determining when such component exceeds a given level as an indication of an undesirable noise level.  
  2. A method as defined in claim 1 further comprising reducing said process generated noise component of resistance at least to said given level upon determining that said noise component exceeds said given level.  
  3. A method as defined in claim 2 wherein said step of reducing said noise component of resistance is effected by increasing the spacing between anode and cathode electrodes associated with said bath.  
  4. A method as defined in claim 3 further including reducing the spacing between said anode and cathode after a predetermined period subsequent to the step of increasing the spacing.  
  5. A method as defined in claim 4 wherein the step of reducing the spacing is accomplished in a series of incremental steps.  
  6. A method as defined in claim 4 wherein said reducing step effects a reestablishing of substantially the original anode-to-cathode spacing.  
  7. A method as defined in claim 1 wherein the step of providing an electrolytic bath comprises providing an electrolytic bath composed of alumina as the solute and cryolite as the solvent, the metal produced being aluminum.  
  8. A mehtod as defined inclaim 1 wherein the step of electrically determining when said component exceeds a given level determines if said given level has been exceeded for a given period of time.  
  9. An apparatus for producing metal from an electrolytic bath containing dissolved oxide of the metal comprising at least on reduction cell having electrode means for delivering direct current to the electrolytic bath, said electrode means including anode electrode means and cathode electrode means; means for sensing the process generated noise component of the resistance of said bath and means responsive to output from said means for sensing for determining the occurrence of noise components in excess of a given level.  
  10. An apparatus as defined in claim 9 further comprising means responsive to output from said means for determining for reducing the noise component at least to the given level.  
  11. An apparatus as defined in claim 10 wherein said means for reducing the noise component comprise means for increasing spacing between said anode electrode means and said cathode electrode means.  
  12. An apparatus as defined in claim 11 futher comprising means operative subsequently to said means for increasing spacing for reducing the spacing between said anode electrode means and said cathode electrode means.  
  13. An apparatus as defined in cliam 12 wherein said means for reducing the spacing between said anode electrode means and said cathode electrode means is operatively arranged to reestablish the spacing between said anode electrode means and said cathode electrode means substantially to the original spacing.  
  14. An apparatus as defined in claim 12 wherein said means for reducing the spacing is a means for reducing the spacing in incremental distances.  
  15. An apparatus as defined in claim 13 wherein said means for reducing the spacing is a means for reducing the spacing by incremental distances toward the original spacing.  
  16. An apparatus as defined in claim 9 further comprising means for determining the resistance of said bath and means responsive to output from said means for determining resistance for adjusting spacing between said anode electrode means and said cathode electrode means to maintain the resistance of said bath within predetermined limits.  
  17. An apparatus as difined in claim 9 wherein said means for determining the occurrence of noise components in excess of a given level are means for determining the substantially continuous occurrence of noise components in excess of said given level for a given period of time.  
  18. In an apparatus for producing metal from an electrolytic bath containing dissolved oxide of the metal which includes at least one reduction cell having electrode means for delivering direct current to the electrolytic bath, said electrode means including anode electrode means and cathode electrode means, the improvement which comprises:  
 means for producing a resistance signal indicative of the resistance of said electrolytic bath, said resistance signal having a process generated noise component and a normal bath concentration component;  
 means responsive to said resistance signal for separating said process generated noise component and said normal bath concentration component into first and second signals, respectively;  
 means responsive to said first signal for incresing the spacing between said anode electrode and said cathode electrode if said first signal exceeds a predetermined maximum tolerable value for said process generated noise component whereby the latter resistance component of said electrolytic bath will be reduced to at least said tolerable value; and  
 means responsive to said second signal for respectively increasing or decreasing the spacing between said anode electrode and said cathode electrode if said second signal falls below or above predeter&#39; mined minimum or maximum values, respectively, for said normal bath concentration resistance component.  
  19. The apparatus according to claim 18, further comprising means for inhibiting the operation of said means responsive to said second signal in response to the activation of said means responsive to said first signal.  
  20. The apparatus according to claim 19, further comprising means for producing a voltage signal indicative of the voltage across said electrode means, said means for producing a resistance signal being responsive to said voltage signal, and means further responsive to said voltage signal when said voltage signal exceeds a predetermined maximum level for activating means for feeding dissolved oxide to said electrolytic bath whereby said voltage across said electrode means is reduced.  
  21. The apparatus according to claim 20, further comprising means for inhibiting the operation of said means responsive to said first signal and said means responsive to said second signal in response to the activation of said means further responsive to said voltage signal.  
  22. The apparatus according to claim 19, wherein said means responsive to said first signal comprises first threshold setting means for storing said predetermined maximum tolerable value for said process generated noise resistance component; first comparator means for issuing an output signal when said first signal exceeds said maximum tolerabe value; shift register means responsive to said output signal for generating a binary signal, said shift register means having a plurality of stages through which said binary signal is shifted after a periodic time interval; and gating means responsive to the concomitant presence of a binary signal in each of said plurality of stages of said shift register for issuing a command signal, and a digital signal source means responsive to said command signal for effecting said increase in spacing between said electrodes.  
  23. The apparatus according to claim 22, wherein said means responsive to said second signal comprises second threshold setting means for storing said predetermined maximum and minimum values of said normal bath concentration resistance component, and second comparator means for issuing an outut signal when said second signal either falls above or below said maximum or minimum predetermined values.  
  24. The apparatus according to claim 23, wherein said second threshold setting means are responsive to an output from said digital signal source means for resetting said maximum and minimum values of said normal bath concentration resistance component.