CA1050474A - Sensing and controlling resistance noise component in electrolytic reduction cell - Google Patents
Sensing and controlling resistance noise component in electrolytic reduction cellInfo
- Publication number
- CA1050474A CA1050474A CA192,693A CA192693A CA1050474A CA 1050474 A CA1050474 A CA 1050474A CA 192693 A CA192693 A CA 192693A CA 1050474 A CA1050474 A CA 1050474A
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- bath
- reduction cell
- resistance
- electrode means
- determining
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
- C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
- C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
- C25C3/20—Automatic control or regulation of cells
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Electrolytic Production Of Metals (AREA)
- Adhesives Or Adhesive Processes (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
This disclosure concerns 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 reduction cell. The invention includes a method and apparatus for determining the undesirable process generated noise component of the resistance in the reduction cell and, preferably, reducing or eliminating the noise component whenever it exceeds a given level. More particularly, a circuit arrangement is operatively arranged to sense the process generated noise component and produce an output signal whenever this component exceeds a given level indicating the existence of a noise level which is detrimental to efficient cell operation.
This disclosure concerns 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 reduction cell. The invention includes a method and apparatus for determining the undesirable process generated noise component of the resistance in the reduction cell and, preferably, reducing or eliminating the noise component whenever it exceeds a given level. More particularly, a circuit arrangement is operatively arranged to sense the process generated noise component and produce an output signal whenever this component exceeds a given level indicating the existence of a noise level which is detrimental to efficient cell operation.
Description
~-~0504'74 BACKGROUND OF THE INVENTION
This invention relates to a method of and to an apparatus for the control of an el~ctrolytic 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 such as aluminum.
In the production of aluminum by the electrolytic reduction of alumina dissolved in a molten cryolite bath, one of the continuing problems 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 well-known disadvantages and reduced efficiency. The anode effect is a characteristic 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 970C. 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 ~050474 on the cathodic bottom surf'ace'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 of the anode effect, which results from a staTvation condition of the bath, and of the sick cell phenomen which results from over-feeding the bath, the re-duction 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 extinguish-ed more easily than the sick cell condition can be remedied.
Consequently, techniques have been developed, involYing both intermittent and continuous alumina feeding of an electroly-tic bath, which add alumina to the electrolytic bath routine-~ ly in amount adapted to avoid development of a sick cell: condition. Such feeding techniques rely on an under-feeding practice, which allows the reduction cell to undergo occa-sional anode effects, for example, one anode effect per day, which assures agalnst over-feeding alumina into the reduction cell.
Another problem associated with the electrolytic redùction of aluminum is loss of efficiency caused by the occurrence of low frequency voltage signals which are super-imposed on the direct voltage applied across the reduction cell during operation. These low frequency variations, which may be:designated process generated noise, .occur s ~ ~ whenever the an.ode or a portion thereof is too close to the cathode thereb'y causing an overload of the anode. The faul-~3Q ~ ty s:pacing~may res'ult from a number of causes. An operator may, in approximating the proper ::
~ ~ , ~050~74 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. Ln 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.
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.
105047~
STATEMENT OF THE INVENTION
The method of producing metal according to the present invention, in its broadest aspect, invol~es the steps of providing an electrolytic bath containing dissolved oxide of the metal in a reduction cell, causing direct current to ~low through the bath, 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.
In 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 timeJ for example, three minut es.
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, in~rolves the production of aluminum. In this case the electrolytic bath is composed oi aIumina 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 .
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105047~
In another preferred aspect, the method according to the present invention involves the reestablishment of the original spacing between anode and cathode.
Apparatus for producing metal according to the present invention, in its broadest aspect, 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 determining 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
Figures lA and lB 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 Figures lA and lB, 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 12 and a bath 13 of alumina ~OS0474 dissolved in a molten' el'ectrolyte, the bath 13 being above the pool of molten aluminum 12.' Conductive rods, which are em~edded in the conductive'lining 13, are connected to a cathode conductor OT bus 14. It is to be understood that other forms of lining can be used to contain the molten al-uminum 12 and the bath 13. A cathode potential can be im-pressed on the molten aluminum 12 by other conventional means instead of the conductive rods as shown. Suspended above the ~-bath 13, 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 suit-able superstructure adjustable as a unit o,r a conven~ional vertical or horizontal stub Soderberg-type anode. One multi-ple bar anode arrangement which can be used for the,anode 15 comprises eighteen carbon bars, each weighing about 1 ton.
The molten bath 13 is covered by a hard crust 16 which con-sists of frozen electrolyte constituents and additional alu-mina. The anode l5 is connected to a positive bus 17 via a ~-; conducto~ 18. A current sensing device 20 is provided forsensing 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 con-nection in the conductor 18.
On the tap side of the reduction cell 9 (ie. the side ' from~which molten aluminum may be drawn off), a firs* conven-ti~onal alumina feeder 24 is provided. A first crust breaking bar 25 is provided in the vicinity of the firs~ feeder 24.
A second alumina feeder 26 is provided on the duct side of -~, 30 ~ the reduction cell 9 ~ie. the side from which gases may be drawn ,off) and a second crust breaking bar 27 is provided in . ~
the - - ~ - 7 -~ .
~OS04~4 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 respecti~ely 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 repitition rate, for example, a pulæ 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 therefror21 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 conduct or 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 ten volts. The limiting amplifier 35 preferably has a gain of one. The output from the limiting ampliIier 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 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 putput 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 subtractor 39 is fed to a first input of an arithmetal circuit, denominated a~ an arithmatic divider 40. ~he AND circuit 37 has its output coupled 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 con-tcmporaneously at its two inputs the signals passed by the AND c`ircuits : 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 arithmatic 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. Accord-ingly, 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 D. C. ); 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, ~(~50474 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 ]3 is too low. It is to be understood that the digital signal source 42 and the subtractor 43 are not necessary, the output frcm 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 resistance of the 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 7. 5 volts. Since the anode effect detector 48 has a voltage threshold of 7. 5 voltsJ 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 vclts 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 con-centration of alumina in the bath 13 is much too low for efficient operation.
Since anode effect may and often d~es produce voltages as high as 30 or 40 volts across a reduction cell, the limiting amplifier 35 is arranged ~050474 to limit at an input of about 10 volts thereby preventing damage to the analog digital converter 36 and to the anode effect detector 48 without decreasing the sensitivity of the circuitry.
AB 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 reduction cell may be determined at greater intervalsf for example, at one minute intervals.
A second output from the sub~ra~tor 43 is coupled to a conventional digital filter 83, which functions to pass the digital sign~l 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 thres~iold 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 ~e apparatus. ~he 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 receiYes from the threshold setting circuit 85.
The output from the comparator 84 which, as a practical matter provides an output euery 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.
Ihus the output from the comparator 84 is effectively stored in the shift . .
1~S0474 register 86 once every minute and shifted through the shift reglster 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, depending on the signal received from the comparator 84. Likewise binary ONE or 2~ERO 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 ~inary 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 25, 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 10504~74 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 conlmand 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 parallel, are supplied from the output of the digital signal source 50 to a command decoder 55 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 t comparator 45, In 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 bit parallel, to the command , 105047~
decoder 55 calling for the anode 15 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 com-parator 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 arrang~d to supply from its store 54 a binary coded digital signal, in bit parallel, to the command decoder 55 calling for the anode lS to be raised by a given increment or mcrements 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.
.I 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 lS, are supplied to the command decoder 55 via a negated AND circuit 60 and the OR circuit 58. A second output from the digital signal source 51, 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 55 from the digital signal source 50.
The digital signal source 51 as thus far described responds whenever difference signals appear on either the output from the com-parator 44 or the comparator 45. The digital signal source 51 is ~15-, . : . ; : :, - : , ~ . . :
preferably so constructed that it inhibits itself from supplying command signals for a period of five minuts 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 15, 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 sufficient 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 55 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 signals, is coupled to first negated inputs of the AND circuit 60 and of the AND circuit 56 and of an AND circuit 90 for the purpose of disabling feed of the regular break and feed program ;~ routine signal s, the regular, routine resistance adjusting anode position-;~ ing signals and the noisy pot suppression signals to the command decoder 55.
~:
.:. -. . ~ , .: ~ . . , ~ , .
~ 0~74 ~hus7 the digital signal sources 509 51, 52 and 88 supply . to the ex~lusion of each other and on a priority basis binary coded digital command signals, in bit parallelj to the command decoder 55 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 55 is present sufficiently long to energize their associated respective solenoid drivers 73-78, and at the same time frees the command decoder 55 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 æolenoid 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, which respond respectively to signals stored in the memory circuit 69 and the memory circuit 70, - ~ :
1 ~OS0474 are arranged to energize respectively the pneumatically or electrically operated motion-producing devices 28 and 30 which are respectively operatively arrangéd 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 clrcuits 46 and 47 for the purpose of adjusting upwardly the high and low thresholds, effectively changing upwardly the resistant set point for the bath 13 upon initiation of a noisy 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 apparatuæ of the present invention in a condition ready for operation, suitable programs in the form of binary i coded digital ~ignals 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 substantially fixed electrical resistance of the electrical connections to the reduetion cell 9, the digital signal source 42 is set to provide, as its output signai, a binary coded digital signal representative of such ,~ resistance. The digital signal source 29 is set to provide, as its output '~0 signal, a binary coded digital signal representative of the predetermined 1 ~ ~ 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, as , j ` its o utput signal, a ~ixed binary coded digital signal which corresponds to the upper limit (i. e., 20. I x 10 ohms) of the resistance range for the ~.. .
~OSOg74 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 circuit 47 is set to provide, as its output signal, a fixed binary coded digital signal which corresponds to the lower limit (i. e., 19. 9 x 10 ohms) ~f 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 decreased by substantially -0. 02 volts at the nominal current of 150, 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. 'rhe threshold setting circuit 85 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/AlF3, and alumina, A12O3, which charge forms the electrolytic bath. The reduction process is initiated preferably manually by supplying direct current to the reduction cell 9J with the possible addition of heat from 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 ., I
from the voltmeter 31, and the direct current to the reduction cell 9, as determined by the current s~nsing device 20, are within limits known to provide efficient operation.
. . .
: l ., Once no~lal electrolysicr is progressing~ t~e d~gital ~ -s~gnal source 5U is broug~t into operat~on suppl~ng regular break and f~ed digital co~mland signals to t~e command decoder 55 ~c~ responds ~o SllCh signals ~y~sequent~ally~ slgnaling, vla t~e memor~ circuits 68, 67, 71 and 72, the soleno~d drivers 74, 73, 77, and 78 ~rhich, ~n turn e~fect the movement of the ~reaking bar 25, t~e feeder 24, the ~reaking ~ar 27, and the $eeder 26. In normal operation, the tap slde o the redwction cell 9 is thus broken and fed e~er~ 180 minutes, a dela~ per~od ~e~llg ~rovided between the breaking and ~eeding.
The duct side o ~he electrodic cell 9 is thus broken and fed also ever~ 180 mtnutes, the times of each being displaced by gO minutes from t~e corresponding ~reaking and feeding at the tap side of the reduction cell.
Electrolrsis continues, the circuitry automatically determining the resistance of the bath 13, appropriate signals being produced b~ 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 dlgital command signals to the decoder 55. The de-coder 55 responds ~r producing, as the case may ~e, an output signal to either the memorr circuit 69 or the memor~ circuit 70, which cause the anode 15 to be either ralsed or lowered~
This is accomplished by the motion-producing devices 28 and 30 controlled from the solenoid drivers 75 and 76, which re-spond to the signals stored in the memory circuits 70 and 69 respectivel~. Dlgital signal source 50 is effectively pre-vented from reaching the co~mand decoder 55 ~ecause of the fact th~t ~ sxgnal ro~ t~e d~gital sign~l source 51 ~s- cQu~led to t~e neg~ted input of t~e AND c~cuit 57, .
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`-` lOS0474 During 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 limi-ting 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 sig-lQ nal to the digital signal source 52 which produces a series of digital command signals to the command decoder 55 to cause, in succession, the crust 16 on the bath 13 to be broken, possibly on both the tap side and the duct side of the re-duction cell 9, the anode 15 to be lowered, and subsequent feeding of one or both sides of the reduction cell. The mechanical movements are effected by the solenoid drivers 75, 74, 73, 77 and 78. The digital signal 52 source also preferably produces a digital command signal which is decoded l by the decoder 55 and fed to the solenoid driver 76, via the `,`j ;~ 2a memory circuit 70, to cause the anode 15 to be returned to its earlier position.
A separate output from the digital signal source 52 . ., is fed to negated inputs of the AND circuits 56, 60, and 90 j~ to assure that no command signals from the digital signal , sources 50, 51,and 88 are supplied to the command decoder `~¦ 55 while 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 rep-resenting the noise component of bath resistance to the com-parator 84 which provides a binary ONE output signal upon the ~ signal passed by the digital filter 83 exceed the output $
~; - 21 -,,,, ~
~S0474 - from the threshold setting circuit 85. The output from thecomparator 84 is stored in the input stage of the three-stage shift register 86 and shifted, at one minute intervals, through the shift register 86 stage by stage. Since an out-put from each o~ 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 , 10 84 has signaled the presence of too high a process generated noise level. The binary ONE outp~t signal from the AND cir-cuit 87 is fed as a reset signal to the shift register 86, thus effecting the resetting of the shift register 87 when-i ever the noise level has exceeded the threshold level for a '~ three minute interval.
! The bin,ary ONE signal rom the AND circuit 87 is fed tothe digital signal source 88 which supplies a digital command signal from its store 89 to the decoder 55, Yia the AND cir-. cuit 90 and the OR circuit 58. The digital command signal ,,, 20 decoded by the decoder 55 and fed via the memory circuit 70to the solenoid driver 76 which raises the anode 15 a prede-' termined amount known to be sufficient to reduce the magni-tude 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 digital signal source 88 causing the anode 15 to be raised further until the noise level is within an acceptable limit.
S~ long as the digital signal source 88 is producing com-mand slgnals for raising the anode 15, digital signals f~om the digital signal sources 50 and 51 are blocked from the O~
cir~uit 58 because of negated
This invention relates to a method of and to an apparatus for the control of an el~ctrolytic 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 such as aluminum.
In the production of aluminum by the electrolytic reduction of alumina dissolved in a molten cryolite bath, one of the continuing problems 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 well-known disadvantages and reduced efficiency. The anode effect is a characteristic 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 970C. 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 ~050474 on the cathodic bottom surf'ace'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 of the anode effect, which results from a staTvation condition of the bath, and of the sick cell phenomen which results from over-feeding the bath, the re-duction 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 extinguish-ed more easily than the sick cell condition can be remedied.
Consequently, techniques have been developed, involYing both intermittent and continuous alumina feeding of an electroly-tic bath, which add alumina to the electrolytic bath routine-~ ly in amount adapted to avoid development of a sick cell: condition. Such feeding techniques rely on an under-feeding practice, which allows the reduction cell to undergo occa-sional anode effects, for example, one anode effect per day, which assures agalnst over-feeding alumina into the reduction cell.
Another problem associated with the electrolytic redùction of aluminum is loss of efficiency caused by the occurrence of low frequency voltage signals which are super-imposed on the direct voltage applied across the reduction cell during operation. These low frequency variations, which may be:designated process generated noise, .occur s ~ ~ whenever the an.ode or a portion thereof is too close to the cathode thereb'y causing an overload of the anode. The faul-~3Q ~ ty s:pacing~may res'ult from a number of causes. An operator may, in approximating the proper ::
~ ~ , ~050~74 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. Ln 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.
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.
105047~
STATEMENT OF THE INVENTION
The method of producing metal according to the present invention, in its broadest aspect, invol~es the steps of providing an electrolytic bath containing dissolved oxide of the metal in a reduction cell, causing direct current to ~low through the bath, 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.
In 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 timeJ for example, three minut es.
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, in~rolves the production of aluminum. In this case the electrolytic bath is composed oi aIumina 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 .
,, i ... -- - - . - . - . . . . .. . .
105047~
In another preferred aspect, the method according to the present invention involves the reestablishment of the original spacing between anode and cathode.
Apparatus for producing metal according to the present invention, in its broadest aspect, 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 determining 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
Figures lA and lB 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 Figures lA and lB, 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 12 and a bath 13 of alumina ~OS0474 dissolved in a molten' el'ectrolyte, the bath 13 being above the pool of molten aluminum 12.' Conductive rods, which are em~edded in the conductive'lining 13, are connected to a cathode conductor OT bus 14. It is to be understood that other forms of lining can be used to contain the molten al-uminum 12 and the bath 13. A cathode potential can be im-pressed on the molten aluminum 12 by other conventional means instead of the conductive rods as shown. Suspended above the ~-bath 13, 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 suit-able superstructure adjustable as a unit o,r a conven~ional vertical or horizontal stub Soderberg-type anode. One multi-ple bar anode arrangement which can be used for the,anode 15 comprises eighteen carbon bars, each weighing about 1 ton.
The molten bath 13 is covered by a hard crust 16 which con-sists of frozen electrolyte constituents and additional alu-mina. The anode l5 is connected to a positive bus 17 via a ~-; conducto~ 18. A current sensing device 20 is provided forsensing 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 con-nection in the conductor 18.
On the tap side of the reduction cell 9 (ie. the side ' from~which molten aluminum may be drawn off), a firs* conven-ti~onal alumina feeder 24 is provided. A first crust breaking bar 25 is provided in the vicinity of the firs~ feeder 24.
A second alumina feeder 26 is provided on the duct side of -~, 30 ~ the reduction cell 9 ~ie. the side from which gases may be drawn ,off) and a second crust breaking bar 27 is provided in . ~
the - - ~ - 7 -~ .
~OS04~4 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 respecti~ely 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 repitition rate, for example, a pulæ 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 therefror21 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 conduct or 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 ten volts. The limiting amplifier 35 preferably has a gain of one. The output from the limiting ampliIier 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 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 putput 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 subtractor 39 is fed to a first input of an arithmetal circuit, denominated a~ an arithmatic divider 40. ~he AND circuit 37 has its output coupled 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 con-tcmporaneously at its two inputs the signals passed by the AND c`ircuits : 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 arithmatic 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. Accord-ingly, 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 D. C. ); 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, ~(~50474 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 ]3 is too low. It is to be understood that the digital signal source 42 and the subtractor 43 are not necessary, the output frcm 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 resistance of the 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 7. 5 volts. Since the anode effect detector 48 has a voltage threshold of 7. 5 voltsJ 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 vclts 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 con-centration of alumina in the bath 13 is much too low for efficient operation.
Since anode effect may and often d~es produce voltages as high as 30 or 40 volts across a reduction cell, the limiting amplifier 35 is arranged ~050474 to limit at an input of about 10 volts thereby preventing damage to the analog digital converter 36 and to the anode effect detector 48 without decreasing the sensitivity of the circuitry.
AB 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 reduction cell may be determined at greater intervalsf for example, at one minute intervals.
A second output from the sub~ra~tor 43 is coupled to a conventional digital filter 83, which functions to pass the digital sign~l 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 thres~iold 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 ~e apparatus. ~he 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 receiYes from the threshold setting circuit 85.
The output from the comparator 84 which, as a practical matter provides an output euery 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.
Ihus the output from the comparator 84 is effectively stored in the shift . .
1~S0474 register 86 once every minute and shifted through the shift reglster 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, depending on the signal received from the comparator 84. Likewise binary ONE or 2~ERO 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 ~inary 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 25, 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 10504~74 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 conlmand 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 parallel, are supplied from the output of the digital signal source 50 to a command decoder 55 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 t comparator 45, In 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 bit parallel, to the command , 105047~
decoder 55 calling for the anode 15 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 com-parator 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 arrang~d to supply from its store 54 a binary coded digital signal, in bit parallel, to the command decoder 55 calling for the anode lS to be raised by a given increment or mcrements 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.
.I 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 lS, are supplied to the command decoder 55 via a negated AND circuit 60 and the OR circuit 58. A second output from the digital signal source 51, 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 55 from the digital signal source 50.
The digital signal source 51 as thus far described responds whenever difference signals appear on either the output from the com-parator 44 or the comparator 45. The digital signal source 51 is ~15-, . : . ; : :, - : , ~ . . :
preferably so constructed that it inhibits itself from supplying command signals for a period of five minuts 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 15, 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 sufficient 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 55 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 signals, is coupled to first negated inputs of the AND circuit 60 and of the AND circuit 56 and of an AND circuit 90 for the purpose of disabling feed of the regular break and feed program ;~ routine signal s, the regular, routine resistance adjusting anode position-;~ ing signals and the noisy pot suppression signals to the command decoder 55.
~:
.:. -. . ~ , .: ~ . . , ~ , .
~ 0~74 ~hus7 the digital signal sources 509 51, 52 and 88 supply . to the ex~lusion of each other and on a priority basis binary coded digital command signals, in bit parallelj to the command decoder 55 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 55 is present sufficiently long to energize their associated respective solenoid drivers 73-78, and at the same time frees the command decoder 55 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 æolenoid 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, which respond respectively to signals stored in the memory circuit 69 and the memory circuit 70, - ~ :
1 ~OS0474 are arranged to energize respectively the pneumatically or electrically operated motion-producing devices 28 and 30 which are respectively operatively arrangéd 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 clrcuits 46 and 47 for the purpose of adjusting upwardly the high and low thresholds, effectively changing upwardly the resistant set point for the bath 13 upon initiation of a noisy 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 apparatuæ of the present invention in a condition ready for operation, suitable programs in the form of binary i coded digital ~ignals 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 substantially fixed electrical resistance of the electrical connections to the reduetion cell 9, the digital signal source 42 is set to provide, as its output signai, a binary coded digital signal representative of such ,~ resistance. The digital signal source 29 is set to provide, as its output '~0 signal, a binary coded digital signal representative of the predetermined 1 ~ ~ 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, as , j ` its o utput signal, a ~ixed binary coded digital signal which corresponds to the upper limit (i. e., 20. I x 10 ohms) of the resistance range for the ~.. .
~OSOg74 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 circuit 47 is set to provide, as its output signal, a fixed binary coded digital signal which corresponds to the lower limit (i. e., 19. 9 x 10 ohms) ~f 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 decreased by substantially -0. 02 volts at the nominal current of 150, 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. 'rhe threshold setting circuit 85 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/AlF3, and alumina, A12O3, which charge forms the electrolytic bath. The reduction process is initiated preferably manually by supplying direct current to the reduction cell 9J with the possible addition of heat from 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 ., I
from the voltmeter 31, and the direct current to the reduction cell 9, as determined by the current s~nsing device 20, are within limits known to provide efficient operation.
. . .
: l ., Once no~lal electrolysicr is progressing~ t~e d~gital ~ -s~gnal source 5U is broug~t into operat~on suppl~ng regular break and f~ed digital co~mland signals to t~e command decoder 55 ~c~ responds ~o SllCh signals ~y~sequent~ally~ slgnaling, vla t~e memor~ circuits 68, 67, 71 and 72, the soleno~d drivers 74, 73, 77, and 78 ~rhich, ~n turn e~fect the movement of the ~reaking bar 25, t~e feeder 24, the ~reaking ~ar 27, and the $eeder 26. In normal operation, the tap slde o the redwction cell 9 is thus broken and fed e~er~ 180 minutes, a dela~ per~od ~e~llg ~rovided between the breaking and ~eeding.
The duct side o ~he electrodic cell 9 is thus broken and fed also ever~ 180 mtnutes, the times of each being displaced by gO minutes from t~e corresponding ~reaking and feeding at the tap side of the reduction cell.
Electrolrsis continues, the circuitry automatically determining the resistance of the bath 13, appropriate signals being produced b~ 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 dlgital command signals to the decoder 55. The de-coder 55 responds ~r producing, as the case may ~e, an output signal to either the memorr circuit 69 or the memor~ circuit 70, which cause the anode 15 to be either ralsed or lowered~
This is accomplished by the motion-producing devices 28 and 30 controlled from the solenoid drivers 75 and 76, which re-spond to the signals stored in the memory circuits 70 and 69 respectivel~. Dlgital signal source 50 is effectively pre-vented from reaching the co~mand decoder 55 ~ecause of the fact th~t ~ sxgnal ro~ t~e d~gital sign~l source 51 ~s- cQu~led to t~e neg~ted input of t~e AND c~cuit 57, .
,: .
.. ~o ~' ~
.~" ., ,~
-' . ~ ',. ' . . , ' ' ' - ' . '. ' - . . : , , .
`-` lOS0474 During 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 limi-ting 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 sig-lQ nal to the digital signal source 52 which produces a series of digital command signals to the command decoder 55 to cause, in succession, the crust 16 on the bath 13 to be broken, possibly on both the tap side and the duct side of the re-duction cell 9, the anode 15 to be lowered, and subsequent feeding of one or both sides of the reduction cell. The mechanical movements are effected by the solenoid drivers 75, 74, 73, 77 and 78. The digital signal 52 source also preferably produces a digital command signal which is decoded l by the decoder 55 and fed to the solenoid driver 76, via the `,`j ;~ 2a memory circuit 70, to cause the anode 15 to be returned to its earlier position.
A separate output from the digital signal source 52 . ., is fed to negated inputs of the AND circuits 56, 60, and 90 j~ to assure that no command signals from the digital signal , sources 50, 51,and 88 are supplied to the command decoder `~¦ 55 while 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 rep-resenting the noise component of bath resistance to the com-parator 84 which provides a binary ONE output signal upon the ~ signal passed by the digital filter 83 exceed the output $
~; - 21 -,,,, ~
~S0474 - from the threshold setting circuit 85. The output from thecomparator 84 is stored in the input stage of the three-stage shift register 86 and shifted, at one minute intervals, through the shift register 86 stage by stage. Since an out-put from each o~ 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 , 10 84 has signaled the presence of too high a process generated noise level. The binary ONE outp~t signal from the AND cir-cuit 87 is fed as a reset signal to the shift register 86, thus effecting the resetting of the shift register 87 when-i ever the noise level has exceeded the threshold level for a '~ three minute interval.
! The bin,ary ONE signal rom the AND circuit 87 is fed tothe digital signal source 88 which supplies a digital command signal from its store 89 to the decoder 55, Yia the AND cir-. cuit 90 and the OR circuit 58. The digital command signal ,,, 20 decoded by the decoder 55 and fed via the memory circuit 70to the solenoid driver 76 which raises the anode 15 a prede-' termined amount known to be sufficient to reduce the magni-tude 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 digital signal source 88 causing the anode 15 to be raised further until the noise level is within an acceptable limit.
S~ long as the digital signal source 88 is producing com-mand slgnals for raising the anode 15, digital signals f~om the digital signal sources 50 and 51 are blocked from the O~
cir~uit 58 because of negated
- 2~ -inputs of the AND circuits 56 and 60 receiving a binary ONE
s.ignal from the digital singal source 88. It is to be appre-ciated, however, that if the digital source 52 were to produce command signals, they would pass to the decoder 55 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 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 effect suppression of the cause of an anode effect, the digital signal source 88 is operatively arranged to supply to the decoder 55 from its store 89 a command signal or signals which are supplied to the motion-producing device 30 via the solenoid driver 75 and the memory circuit 69 which cause the anode 15 to be lowered to its inital ,j 20 position or toward its initial position in increments, . the anode movement ceasing whenever the anode becomes ~i positioned in its initial position. Of course~ the set I points of the threshold circuits 46 and 47 are again set .,1 .
.J to or toward their initial levels, in accordance with command signals reflecting the changing position of the ~ anode 15.
-'l 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 1:
toward its initial position, the noisy pot suppression , ~
, .
10504'74 routine is again activatea. This ~ill continue time and again until either the noisy pot phenomena is effectively suppressed or an operator, knowing the phenomena persists, sets the threshold circuits 46 and 47 to higher values there-by changing *he 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.
Although the present invention has been described, in its apparatus aspect, in conjunction with a single electro-lytic reduction cell, it is to be appreciated that the inven-tion is applicabie to systems which involve multiplexing of ^, the command signals in order to control the operating parame-ters 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 circui~s, the feeding of the command signals and the sensing of the currents and the voltages being appropriately synchro-nized.
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 or digital circuit I 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 . 1 analog signal filters having resistances, capacitances and/or inductances. It is to be appreciated that the digital filter 83 may be a least squares estimator.
` ~ It is also to be appreciated that-the invention~ in its [-~ ~ method aspect, need not be carried out in the illustrated ~ ~ .~
appa~atus, but may be carried out by other apparatuses , .' j -c ' ,.,., . ~ ' : ': . . , ' "
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 claimq.
.' :.
:
l .
,,, ~ .
'1~
1:
'':1 ~ .
$~
:~ :t: : :
: !~1 ~' ~
-fl~. I
s.ignal from the digital singal source 88. It is to be appre-ciated, however, that if the digital source 52 were to produce command signals, they would pass to the decoder 55 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 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 effect suppression of the cause of an anode effect, the digital signal source 88 is operatively arranged to supply to the decoder 55 from its store 89 a command signal or signals which are supplied to the motion-producing device 30 via the solenoid driver 75 and the memory circuit 69 which cause the anode 15 to be lowered to its inital ,j 20 position or toward its initial position in increments, . the anode movement ceasing whenever the anode becomes ~i positioned in its initial position. Of course~ the set I points of the threshold circuits 46 and 47 are again set .,1 .
.J to or toward their initial levels, in accordance with command signals reflecting the changing position of the ~ anode 15.
-'l 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 1:
toward its initial position, the noisy pot suppression , ~
, .
10504'74 routine is again activatea. This ~ill continue time and again until either the noisy pot phenomena is effectively suppressed or an operator, knowing the phenomena persists, sets the threshold circuits 46 and 47 to higher values there-by changing *he 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.
Although the present invention has been described, in its apparatus aspect, in conjunction with a single electro-lytic reduction cell, it is to be appreciated that the inven-tion is applicabie to systems which involve multiplexing of ^, the command signals in order to control the operating parame-ters 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 circui~s, the feeding of the command signals and the sensing of the currents and the voltages being appropriately synchro-nized.
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 or digital circuit I 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 . 1 analog signal filters having resistances, capacitances and/or inductances. It is to be appreciated that the digital filter 83 may be a least squares estimator.
` ~ It is also to be appreciated that-the invention~ in its [-~ ~ method aspect, need not be carried out in the illustrated ~ ~ .~
appa~atus, but may be carried out by other apparatuses , .' j -c ' ,.,., . ~ ' : ': . . , ' "
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 claimq.
.' :.
:
l .
,,, ~ .
'1~
1:
'':1 ~ .
$~
:~ :t: : :
: !~1 ~' ~
-fl~. I
Claims (8)
1. An apparatus for producing metal from an electro-lytic bath containing dissolved oxide of the metal com-prising at least one reduction cell having electrode means for delivering direct current to the electrolytic bath, said electrode means including anode electrode means and ca-thode electrode means; the improvement therein comprising 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, and comprising means responsive to output from said means for determining for reducing the noise component at least to the given level.
2. The apparatus of claim 1 wherein said means for reducing the noise component comprise means for increasing spacing between said anode electrode means and said cathode electrode means.
3. The apparatus of claim 2 and further comprising means operative subsequently to said means for increasing spacing for reducing the spacing between said anode electrode means and said cathode electrode means, in a series of incre-mental distances, to thereby reestablish the spacing between said anode electrode means and said cathode electrode means substantially to the original spacing.
4. The apparatus of either claim 1, 2 or 3, wherein said means for determining the occurrence of noise components in excess of a given level are means for determining the sub-stantially continuous occurrence of noise components in ex-cess of said given level for a given period of time.
5. An apparatus for producing aluminum from an electro-lytic bath containing alumina dissolved in cryolite, the apparatus comprising 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 improve-ment therein comprising means for determining the resistance of said bath including means for sensing and determining the process generated noise component of said resistance, and means responsive to output from said means for deter-mining resistance for adjusting spacing between said anode electrode means and said cathode electrode means to main-tain the resistance of said bath within predetermined limited.
6. A method 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, and collecting said metal on the bottom of said reduction cell; the improvement therein comprising sensing the process generated noise component of resistance of said bath and determining when such component exceeds a given level as an indication of an undesirable noise level, and thereafter reducing the process generated noise component to at least the given level.
7. A method as claimed in claim 6, wherein said electro-lytic bath is composed of alumina as the solute and cryolite as the solvent, the metal produced being aluminum.
8. The apparatus of either claim 1, 2 or 3 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, wherein said electrolytic bath is composed of alumina as the solute and cryolite as the solvent, the metal produced being aluminum.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US334233A US3878070A (en) | 1972-10-18 | 1973-02-21 | Apparatus for and method of producing metal |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1050474A true CA1050474A (en) | 1979-03-13 |
Family
ID=23306232
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA192,693A Expired CA1050474A (en) | 1973-02-21 | 1974-02-15 | Sensing and controlling resistance noise component in electrolytic reduction cell |
Country Status (12)
| Country | Link |
|---|---|
| JP (1) | JPS5741557B2 (en) |
| AT (1) | AT344407B (en) |
| BR (1) | BR7401316D0 (en) |
| CA (1) | CA1050474A (en) |
| CH (1) | CH594065A5 (en) |
| DE (1) | DE2408150A1 (en) |
| FR (1) | FR2219239B1 (en) |
| GB (1) | GB1440694A (en) |
| IN (1) | IN139544B (en) |
| IT (1) | IT1013057B (en) |
| NO (1) | NO143877C (en) |
| ZA (1) | ZA74950B (en) |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3345273A (en) * | 1964-02-04 | 1967-10-03 | Reynolds Metals Co | Method of and apparatus for indicating anode positions |
| FR1457746A (en) * | 1964-09-29 | 1966-01-24 | Reynolds Metals Co | Improvements made to control means for reduction tanks |
| US3712857A (en) * | 1968-05-20 | 1973-01-23 | Reynolds Metals Co | Method for controlling a reduction cell |
| RO59532A (en) * | 1970-10-13 | 1976-03-15 | ||
| JPS5244286A (en) * | 1975-09-30 | 1977-04-07 | Iwashiya Sawadakenzou Shoten:Kk | Shaking incubator |
-
1974
- 1974-02-13 ZA ZA00740950A patent/ZA74950B/en unknown
- 1974-02-15 CA CA192,693A patent/CA1050474A/en not_active Expired
- 1974-02-18 CH CH221374A patent/CH594065A5/xx not_active IP Right Cessation
- 1974-02-19 AT AT132674A patent/AT344407B/en not_active IP Right Cessation
- 1974-02-20 IN IN354/CAL/74A patent/IN139544B/en unknown
- 1974-02-20 NO NO740558A patent/NO143877C/en unknown
- 1974-02-20 DE DE19742408150 patent/DE2408150A1/en not_active Withdrawn
- 1974-02-21 FR FR7405896A patent/FR2219239B1/fr not_active Expired
- 1974-02-21 JP JP49020974A patent/JPS5741557B2/ja not_active Expired
- 1974-02-21 GB GB791574A patent/GB1440694A/en not_active Expired
- 1974-02-21 BR BR741316A patent/BR7401316D0/en unknown
- 1974-02-25 IT IT48670/74A patent/IT1013057B/en active
Also Published As
| Publication number | Publication date |
|---|---|
| IT1013057B (en) | 1977-03-30 |
| ATA132674A (en) | 1977-11-15 |
| CH594065A5 (en) | 1977-12-30 |
| GB1440694A (en) | 1976-06-23 |
| NO740558L (en) | 1974-08-22 |
| FR2219239B1 (en) | 1978-10-27 |
| DE2408150A1 (en) | 1974-08-22 |
| FR2219239A1 (en) | 1974-09-20 |
| AT344407B (en) | 1978-07-25 |
| JPS5035013A (en) | 1975-04-03 |
| NO143877B (en) | 1981-01-19 |
| NO143877C (en) | 1981-04-29 |
| ZA74950B (en) | 1975-01-29 |
| JPS5741557B2 (en) | 1982-09-03 |
| BR7401316D0 (en) | 1974-10-29 |
| IN139544B (en) | 1976-06-26 |
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