AU622283B2 - Process for controlling aluminium smelting cells - Google Patents

Description

4 622283 COMPLETE SPECIFICATION FOR OFFICE USE Application Number: Lodged: Complete Specification Priority: Class Int. Class Lodged: Accepted: Published: e4* o 1 1 0 1€ o444 o I 009 Related Art: o 0o #040 4 44 00 4 404 4 0 0 0000 9 a f 000 0 1 i o iC Name of Applicant: Address of Applicant: Actual Inventors: Address for Service: TO BE COMPLETED BY APPLICANT COMALCO ALUMINIUM LIMITED 55 Collins Street, Melbourne, Australia.

Victoria 3000, Geoffrey I. BLATCH, Mark P. TAYLOR and Mark FYFE SMITH SHELSTON BEADLE 207 Riversdale Road Box 410) Hawthorn, Victoria, Australia &tItLr 4 11 4 0 Complete Specification for the invention entitled: PROCESS FOR CONTROLLING ALUMINIUM SMETLING CELLS The following statement is a full description of this invention, including the best method of performing it known to us: Page 1 Our Ref: #2806 TNB:WB 34com i la- 1 3 This invention relates to improvements in the automated 4 control of electrolytic smelting cells for the production of aluminium.

6 Background of the Invention: 7 The control of electrolytic cells in the production of 8 aluminium is influenced by both short term and long term 9 process parameter changes. In the short term, bath superheat, alumina concentration and anode to cathode 11 distance (ACD) need constant monitoring, while longer term 12 control is required for metal depth and the composition and 13 volume of the electrolyte in the cell. Operating 14 abnormalities also require attention, such as sludging, 15 anode effects and their frequency, and the short circuiting t I' 16 of the current between the anodes and the metal pad.

17 The complexity of the interrelationships between the 18 dependent and independent variables in the smelting process 19 are illustrated in Chapter 9 of "Aluminium Smelter Technology" Grjotheim and Welch Aluminium Verlag, A' 21 1988, and this chapter provides a useful summary of the 1 4 22 currently utilised control strategies. This summary and the 23 proliferation of literature on the subject further 24 illustrate the complexity of the problem and the absence of a strategy that provides a satisfactory level of control 26 resulting in constantly high efficiency levels.

27 Numerous examples of control strategy proposals are 28 also to be found in the patent literature. Recent examples 29 include U.S. Patent No. 4,654,129 Leroy which describes a process involving periods of over supply and under supply tc 31 maintain the alumina concentration in the cell within a 32 narrow range by monitoring the rate of change of the 33 resistance of the cell. This process relies for its success 34 on the use of point feeding of alumina to the cell, and it is not therefore useful for cells without point feeders. 36 Also, since in this strategy it is critical to maintain the 37 alumina concentration within a narrow range, the strategy 38 suffers if the concentration moves outside that range and it "d 2 1 2 3 4 6 7 8 9 11 12 13 14 15 16 17 18 19 21 22 23 24 26 27 28 29 31 32 33 34 36 37 38 is often difficult to restore the system to its optimum operating conditions.

A similar control strategy is described in International Patent Application PCT/N086/00017 (W086/050008) Aalbu et al. In common with the above U.S.

patent, the strategy relies heavily on the rate of change of the resistance of the cell to monitor alumina concentration and does not have regard to other important parameters to control the heat and mass balance of the cell. The disclosure similarly does not address the strategy to be adopted during process events, such as alumina feeding, anode movements, anode setting and tapping.

U.S. Patents 4,008,142 and 4,024,034 Doring et al, uses the concept of constant anode-cathode distance to adjust cell resistance according to the known or assumed electrochemical voltage breakdown. Anode-cathode distance adjustment is made in cases where current efficiency (by metal production measurement) is less than expected theoretically. Automatic adjustment of voltage/cell resistance in response to noise on the signal is also indicated. However, no attempt is made to calculate the heat or alumina balances or to make furnace adjustments on this basis, with the exception of adjustment of cell resistance on the basis of long term running metal production figures.

This does not constitute a calculation of the energy balance or process energy requirement.

In U.S. Patent 4,766,552 Aal bu et al, the resistance/alumina concentration curve is used to control alumina concentration on point feed cells. A linear model of the cell resistance variation is set up using the resistance slope as a parameter. By fitting the model to continuous resistance measurements, the slope is estimated.

However, this strategy does not ensure that the resulting slope is related only to alumina concentration, in fact it assumes this one to one relationship. Anode movement is included in the fitted algorithm and other disturbances are filtered by reducing the gain of the fitting functions when they occur. This procedure is very complex and could be A -u 900216, !tbspe.044,comalco. spe, mslF~~

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3 1 prone to error. In addition, the strategy does not attempt 2 to maintain heat balance within the cell.

3 In U.S. Patent 4,333,803 Seger and Haupin, a heat flux 4 sensor is used to measure sidewall heat flow. Cell resistance is adjusted to maintain this at a predetermined 6 value. However, this strategy: 7 1. does not guarantee that heat losses from other portions 8 of the cell are under control (top, bottom); 9 2. does not react to changes inside the cell on a useful time scale (hours or within a day) the cell can be 11 significantly out of heat balance before an adjustment is 12 made, and 13 3. does not provide information about the events/operations 14 occurring in the electrolyte. These events are needed to close the overall energy balance including the continuous 16 changing process requirements and to sense the condition 17 of the liquid electrolyte which is where electrolysis is 18 taking place. Effective bath rciotiv ty oncing in th 1 d c 1 tol-,,4d hor n allo., muh fa. tr rop cn +o a-- I CI

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21 Other control strategies are described in U.S. Patents 22 3,969,669 Brault and Lacroise, 3,829,365 Chandhuri et al, 23 4,431,491 Bonny et al, 4,654,129 Leroy, 4,654,130 Tabereaux 24 et al, 3,622,475 Shiver, 3,878,070 Murphy, 3,573,179 Dirth et al, 4,035,251 Shiver and 4,488,117 Seo. This list is by 26 no means intended to be exhaustive.

27 A primary factor in reduction cell efficiency is the 28 thermal state of the materials in the cell cavity. A control 29 strategy directed at optimizing efficiency should therefore aim to maintain a thermal steady state in the cell. That is, 31 the rate of heat dissipation from the cell cavity should be 32 kept constant. If this is achieved in concert with stable 33 bath and metal inventories, operational stability can be 34 enhanced. The bath superheat will be constant; hence bath volume, chemistry and temperature will be stable due to the 36 absence of ledge freezing or melting. Improved operational 37 stability may allow a cell to be operated with better 38 alumina feed control, at a lower bath ratio, and at a lower 900216,!tbspe.044,comalco.spe, L ?aT

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4i J p 4- 1 2 3 4 6 7 8 9 11 12 13 S 14 4 t 0 16 S 17 18 19 21 S 22 23 0. 24 26 time averaged rate of heat loss. This will improve the process productivity.

A major difficulty in maintaining thermal steady state in a reduction cell is the discontinuous nature of various operations. The energy requirements of alumina feeding and dissolution can vary from minute-to-minute, particularly on breaker-bar cells. This is further exacerbated by the deliberate changes in feed rate required by many feed control techniques. Anode setting in pre-baked cells also introduces a large cyclic energy requirement. Other processes, such as bath additions, anode effects and amperage fluctuations further alter the short-term thermal balance of a cell. Currently available control systems do not address these fluctuating thermal requirements in a comprehensive way. For example, target voltage control has allowed for alumina feeding in some systems. Similarly, anode effects have been used to control the power input.

However, the complete range of variable energy requirements are not treated systematically or quantitatively to maintain a constant rate of heat supply available for dissipation through the cell.

Summary of Invention and Objects: It is an object of the present invention to provide an improved process for controlling aluminium smelting cells in which the heat balance of the cell is comprehensively controlled.

I -7 S 28 controlling the operation of an aluminium smelting ce 29 comprising the steps of: continuously monitoring cell voltage an urrent, 31 (ii) calculating the resistance o the cell from the 32 monitored cell voltage and curr 33 (iii)calculating the r e of change of cell resistance 34 (resistance slopeand providing a smoothed value of said resistance pe, 36 (iv) ilising the smoothed resistance slope values to 37 maintain mass balance in the cell, (V)teitorig cl process operatin including alun 90 0 2 1 6 ,!tbspe.044,comalco.spe,

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ill 3 K'C: ib 2 frequency pot noise while adequately responding to ste 3 changes and the resistance disturbances. This fil ered 4 resistance is used for automatic resistance cont ol. The resistance slope is calculated from raw unfiltered) 6 resistance values as described furtherelow and similar 7 digital filtration is used to co inuously calculate 8 smoothed resistance slope values.

9 The smoothed resistanc slope is searched for values exceeding a predetermine slope which is chosen to indicate 11 concentration polar'sation and alumina depletion. Different 12 forms of alumin search may be used, and these are described r, 13 in greater tail in the following specification.

14 Th invention also provides a system for controlling 15 th operation of an aluminium smelting cell comprising 16 suitable means for performing each of the steps defined a dI 18 -In a second asopt, The invention fErth: provides a 19 process for controlling the operation of an aluminium smelting cell, comprising the steps of: 21 monitoring the cell voltage and current and calculating 22 the resistance of the cell from the monitored voltage and 23 current, S 24 monitoring alumina additions to the cell, monitoring other additions to the cell bath and monitoring operational 26 changes including anode movements, tapping, anode setting l 27 and beam raising, S28 continuously calculating the energy absorbed by the 29 process from thermodynamic energy requirements associated with the cell reaction and the events identified in item (b) 31 above, 32 calculating the heat available for dissipation in the 33 cell from the cell voltage and current and from the 34 continuously calculated process energy requirement determined in item above, 36 calculating from the calculated heat available for 37 dissipation in and from a selected target power ,WAZX 38 dissipation, the integral of the difference between the 900216,!tbspe.044,comalco.spe vaii i i i i i I f

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1 2 3 4 6 7 8 9 11 12 8o84 13 14 15 S 16 8 08 17 18 19 23 8 8' 24 26 s a 27 28 29 31 32 33 34 36 37 S 328 38 heat available and the target power dissipation with respect to time to provide a running heat inventory or integral, calculating from this heat deficit or surplus in the cell the change in power dissipation required in the cell over a predetermined period to restore heat balance (zero heat integral in item establishing an initial target resistanc? for the cell and an allowable band for said target resistance, Calculating the required change in target resistance from the required change in cell power dissipation (item divided by the square of a moving average of the monitored cell currenu, altering the target resistance in accordance with the calculated heat inventory (item (e))and checking that the new target resistance is within said allowable band, and moving the anodes of the cell to achieve said new target resistance.

Preferably alumina concentration control is carried out by continuously calculating the cell resistance, the rate of change of cell resistance and by smoothing the rate of change values to continuously provide smoothed resistance slope values. Base resistance slope and critical threshold slope for the smoothed resistance slope values indicate target and low alumina concentrations respectively.

The above control process will be seen to take account of both the alumina mass balance of the cell and the short term heat balance of'the cell simultaneously.

The calculation of resistance slope and smoothed resistance is preferably delayed for a predetermined time, as described further abv, when any one of the monitored cell process operations occur. Thus the resistance slope and smoothed resistance slope are recalculated after the predetermined time delay on the basis of a stabilized series of raw resistance values, so that the smoothed slope is unaffected by process changes, with the exception of alumina depletion.

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i ii Ii Ii t**t 4 4444 44 4 4 4 4444 4 4 4 lILt 4 44 4 414 *II 4 7- It will be appreciated that the monitored cell process operations cause significant variations in the calculated resistance and the resultant resistance slope such that the latter parameter no longer provides an accurate reflection of the alumina concentration in the cell.

5 By delaying calculation during the process event for a predetermined time sufficient for the resistance value to again become relatively stable, and then recalculating the resistance slope, an "intelligent" smoothed resistance slope can be obtained, and the electrolyte/alumina mass balance may be maintained notwithstanding the effect of the process operation.

The predetermined time delay will vary having regard to the detected operation since different operations have different effects on the stability of the resistance value. In one particular cell (Type VI design), the following delays have been found to be satisfactory after completion of each operation Operation Delay ACD change 60 sec.

Alumina feed 60 sec.

Anode set 120 sec.

20 Beam raise 120 sec.

Bath Addition 300 sec.

In a preferred form, the resistance of the cell is calculated using a known formula which compensates for the continuously calculated back EMF of the cell, as will be described further below. The resistance values are filtered using digital filtration techniques (e.g.

'Ii 44 tb#2806.com 91 1022 a substantially zero heat integral in step (iii) by moving the anodes of the cell to achieve said new target resistance. S../2 ~1 7(a) multiple Kalman filters) in a manner which smooths random and higher frequency pot noise while adequately responding to step changes and the resistance disturbances. This filtered resistance is used for automatic resistance control. The resistance slope is calculated from raw (unfiltered) resistance values as described further below and similar digital filtration is used to continuously calculate smoothed resistance slope values.

The smoothed resistance slope is searched for values exceeding a predetermined slope which is chosen to indicate concentration polarization and alumina depletion. Different forms of alumina search may be used, and these are described in greater detail in the following specification.

The target power dissipation is preferably adjusted

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8 1 2 3 4 6 7 8 9 11 12 13 14 .oo 15 DID 0 16 0 0 17 18 19 S21 no 0 0 0 2 22 23 0 0 24 o oo 26 0.0. 27 0 28 29 31 32 33 34 36 37 38 using bath resistivity data. The bath resistivity and the rate of change of resistivity are calculated and used to adjust the target power dissipation of the cell according to cell response characteristics so that the cell resistivity moves into a target range associated with bath composition and volume.

The cell voltage is preferably monitored to determine the existence of low frequency or high frequency noise in the voltage system.

If the low frequency voltage noise is above a predetermined threshold, the target power dissipation is increased in order to remove cathode sludge deposits. The new target power dissipation value is then used in the control of the cell resistance and hence the heat balance of the cell.

The invention also provides a system for controlling the operation of an aluminium smelting cell comprising suitable means for performing each of the steps defined in the second aspect above.

It will also be noted that if low frequency voltage noise is above a predetermined threshold, the smoothed resistance slope thresholds for low alumina concentration are raised. The critical slope threshold for one pot group under test was 0.025 u-CL/min. at voltage noise levels below the noise threshold of 0.25 When the low frequency noise exceeds the above threshold, the base slope threshold is ramped by an amount proportional to the amount by which the noise signal exceeds the predetermined threshold. The maximum increment of the ramp is 0.05 u-/min. and occurs at a low frequency noise level of 0.50 The filtered slope is again compared with the incremented threshold and if it is fould to be greater than the threshold, the alumina inventory is then considered to determine whether or not the cell is overfed. If this determination is in the negative, the control system instructs a specific form of alumina feeding cycle to be effected this is either an end of search or an anode effect prediction feeding cycle.

Long term heat balance is achieved in a further control i r; ii P I:

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9 1 2 3 4 6 7 8 9 11 0 12 o 13 14 16 o 17 18 19 21 22 0 4 a, 24 26 27 28 29 31 32 33 34 36 37 38 strategy element which causes adjustment of QTARGET' based on the data derived from the resistance measurements monitoring the resistivity of the cell in the manner described in greater detail below.

Brief Description of the Drawings: A presently preferred embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of the three control functions and their interactions, as performed by a preferred embodiment of the control system according to the invention; Figure 2 is a schematic diagram showing the test control system used on an operational pot; Figure 3 is a diagrammatic graph showing one form of alumina concentration search (SFS) and anode effect prediction (AEP) performed by the system embodying the invention; Figure 4A is an operational graph of resistance values against time showing an alternative method of searching for alumina conc-ntration (namely underfeed/overfeed for point feeders) by the control system embodying the invention; Figures 4B to 4E are schematic graphs showing one example of low frequency noise calculation.

Figures 5A and 5B show bath resistivity and rate of change of resistivity QTARGET, daily mean QAVAIL and excess AlF 3 of the bath for two consecutive months.

Figure 5C shows bath resistivity, QTARGET and excess AlF 3 of the bath over a one month period.

Figure 6 is a diagram showing the calculated energy impact or process energy requirement (and hence compensating action) for feeding a test cell; Figure 7 is an operational diagram showing the breakdown of calculated energy absorbed or process energy requirement in a test cell over 24 hours; Figure 8 is an operational diagram showing the test cell response under the control system of the invention over 24 hours, and tbspe.044,comalco.spe,

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c;Y~~I~ 1 t I- r 1 2 3 4 6 7 8 9 11 12 13 01 14 S 16 17 6 4 18 19 .4t4 21 221 22 4 t 23 24 26 27 28 29 31 32 33 34 36 37 38 10 Figure 9 shows operational diagrams illustrating the detail of a stop feed search for alumina control of a test cell.

Description of Preferred Embodiment: In the following description, one embodiment of a control system under test on a working cell will be described in some detail. In describing the control system, it will be assumed that the reader is already aware of the operation of an aluminium reduction cell and the standard methods of monitoring cell voltage and current, and the standard methods of calculating the cell resistance.

Accordingly these aspects will not be described further in this specification.

Referring firstly to Figure 1 of the drawings, the control system embodying the invention is shown in simplified flow diagram form. Before proceeding with a detailed description of the control system, a general overview of the system will be provided.

The aim of the control system is to maintain a cell at thermal steady state. That is, the rate of heat dissipation from the cell should be maintained at a constant, target value. For the control system the heat available for dissipation from the cell (QD' may be defined as:- QD (VC (RE x I 1000)) x I (QF QS QA QM (1) where, VC cell voltage (V) RE metered external resistances (eg rods, b".swork) (uOhm) I line amperage (kamps) QF alumina dissolution power (kW) QS anode setting power (kW) QA power for AlF3/cryolite heating and dissolution (kW) QM remaining process enthalpy requirements (chemical reaction for metal production) (kW) and can be measured readily. The various components of the enthalpy of reaction (QF QS QA QM ii i

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;i i: 1: ri Ij_ D j:- 900216,!tbspe.044,comalco.spe, JL&%JL.L )LV VL GI uIILJkA a All. A-, means for determining whether the smoothed resistance slope is greater than the predetermined threshold and if so determining Ii /6 r- -i I -ii- :I-i i; 1 cl 11 can also be calculated quantitatively using the thermodynamic cycle for reduction of alumina by carbon [see Grjotheim and Welch, Aluminium Smelter Technology 1988 pp 157-161)], the amperage and a specified current efficiency Factors such as the carbon ratio and the AlF 3 consumption vary significantly between plants. This will alter the calculations used. The enthalpy components presented in Table 1 were calculated for the applicant's Bell Bay smelter.

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ENERGY/POWER

COMPONENT REACTI ON REQUI RED QF Al1 2 0 3 )2AI 3 302- 1820 kJ/kg Al 2 0 3 (s,298K) (diss,T) (diss.T) QS C 4> C 1380 kJ/kg carbon (s,298K) (310 Md/anode) AlF 3 ->Al 3F 1470 kJ/kg Air 3 (s,298K) (diss,T) (diss,T) Q a F3 N A3+ 6F 1420 kdlkg cryolite (s,298K) (diss,T) (diss,T) (diss,T) 3 2 3A (diss,T) (diss,T) (0.4352 O.01138X)1 kW QMAl 3 1)CO 2 3) 1 1) COg) N.B. Current Efficiency x Line Current 1 (amps), T =1293K r 1~ 4 i i4

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13 1 2 3 4 6 7 8 9 11 12 13 14 1 5 16 17 Sa 18 19 21 22 23 24 26 27 28 ,29 31 32 33 34 36 37 38 Note that the CE specific for the control system was made based on tapping history.

The time over which energy is consumed by an individual process event must be defined in addition to the amount of energy consumed. In the control system this was achieved by distributing the total energy requirement of setting, feed or additions over predefined periods. Figure 6 illustrates the feed energy distribution for a Bell Bay breaker bar cell. Note that the energy balance was integrated over each 10 minute period and converted to power units.

In addition to the calculations in the previous section, other components were required for the application of the control strategy in practice.

Firstly, the dynamics of the reduction cell and control system meant that maintaining an 'instantaneous' energy balance was not possible. For example, during cell trials the energy absorbed by a cell was calculated over ten minute intervals and anode beam movements were carried out at five minute intervals. Hence responses to events were delayed by up to 15 minutes. Further the rate and range of target resistance changes were limited, and the line current variation for subsequent ten minute periods did not allow accurate elimination of an energy imbalance. As a result, an integral of the power imbalance was used to modify the target resistance of the cell. That is:- Ei (QDi QT) x 0.6 Ei-1 x c (2) where,

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i integral after ith 10 minute interval (MJ) EiI integral after (i-l)th interval (MJ) c integral decay factor QT target heat dissipation (kW) QDi heat available for dissipation for ith 10 min. interval (kW) Cell resistance was increased for E i 0 and reduced for E i 0. Note that a decay factor was included in Eqn This was a recognition that when an energy imbalance in a 4

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vi 900216,!tbspe.044, cmalco. spe, I I A.j >3 14 I~ 1 c: Ii 4I: 4lr ;44 4 4.4 4444 4 44 r3 3 4 43 cell persisted, the energy balance was partly selfcorrecting. (ie A cell loses more heat if it gets hotter.) A second additional component allowed control of the magnitude of the various discontinuous energy responses.

This was necessary in order to model the thermal response of the electrolyte to localised disturbances or material additons. For example, the extra heat needed at an anode after setting is supplied to the bath volume throughout the cell and may have deleterious effects elsewhere. Also the process engineer may wish to reduce the amount of anode beam movement by damping the cell response to individual events.

As a result, coefficients (range 0 to 1) were introduced to tune the instantaneous calculations (thus system responses).

Energy requirements for feed, setting and additions were divided into instantaneous and background (constant) power inputs. The various background power inputs were calculated from:- Feed line amperage, CE (monthly average).

Additions line amperage, CE, addition rate per kg of metal (monthly average).

Anode Setting anode size, number of anodes, setting 'rota' The final necessary component of the control system was a feed control technique which permitted regular anode beam movement while monitoring alumina concentration thereby allowing the cell energy balance to be always under control.

Search techniques were developed with these functions, where the target alumina concentration was detected via a continuously calculated slope of resistance. No scheduled anode effects (AEs) were included in the feed control strategy. The associated large, uncontrolled energy inputs to the process would have been in conflict with the control philosophy, and are difficult to compensate for in the thermal balance.

Referring again to Figure 1 of the drawings, the control system has three basic strings, the first two affecting the short term heat and mass balance of the cell, and the third affecting the medium to long term heat balance 900216,!tbspe.044,comalco.spe,

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15 1 2 3 4 6 7 8 9 10 4 1 12 13 4441 13 14 S 15 16 17 18 19 20 21 22 23 24 S 26 27 28 29 31 32 33 34 of the cell. The control system is implemented using a computer for monitoring the functions of the cell or pot (pot computer), such as a Micromac 6000 computer suitable for the aluminium industry, and a supervisory computer for receiving data from each of a number of -pot computers and for instructing the pot computers to perform various functions.

Initial input data to the computers includes target heat dissipation QT' the specific current efficiency CE for the cell being controlled, the bath resistivity target range for the cell, thermodynamics data, as described in greater detail above, relating to the cell and a 'typical' back emf (EMF) of the cell calculated by regression in a known manner.

The essential operating parameters of the cell are dynamically monitored, and these parameters include: the voltage of the cell V, the current of the cell I, alumina additions, cell bath additions, operations such as anode setting, beam raising, manual alumina addition and oreing up, and anode to cathode distance (ACD) movements. From these dynamic inputs, the resistance of the cell is continually calculated from (V EMF)/I, and the cell resistivity p is calculated from (fR/X ACD)A, where A is the estimated area of the anodes in the cell.

CONTROL STRING 1: ALUMINA FEED CONTROL In the first control string, the pot computer calculates the level of noise in the voltage signal, 0 to 0.1 Hz indicating low frequency noise and 0.1 to 1 Hz indicating higher frequency noise, and further calculates the filtered rate of change of resistance with time (smoothed resistance slope) every second. The basic steps in the filtered slope calculation for each time cycle are: Raw Resistance Slope Calculation.

Raw slope is calculated from the following equation:

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EQ (3) where S raw slope at (t At) RO raw resistance at time (t At) 900216,!tbspe.044,comalco.spe, ~L i 36 absence of ledge freezing or melting. Improved operational 37 stability may allow a cell to be operated with better 38 alumina feed control, at a lower bath ratio, and at a lower r d II 900216,!tbspe.044,comalco.spe, -;1C ie~ i.

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1 single stage filtered resistance at time t At time interval of resistance polling filter constant for filtered resistance (R 1 It should be noted that the denominator in EQ above represents the mean age of the filtered resistance (R 1 (ii) Box filter for out of range raw slopes: The raw slope is checked to determine if it is within the present box filter limits. If this test fails, no further calculations are made in this cycle the slope value is assumed not to be associated with changes in alumina concentration. In the case of the pot under test, the box filter limits were -2.0 and 2.0 micro-ohms/minute.

(iii)Filtered resistance is recalculated (for use in the next time cycle).

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1 (1 Y R EQ 4 (iv) A three-stage filter is used to find the filtered resistance slope (called smoothslope). For the ith stage: Si Si (1 i) YiSi_ EQ where Y i is the pre-set filter constant of the ith stage. In the case of one pot under test, typical filtration constants are 0.100, 0.050 and 0.095 for Y1, Y2 and Y 3 respectively.

The above operations adequately filter high frequency noise from the resistance signal to produce a realistic filtered slope (with some lag from the three stage filter).

In addition, a delay mechanism (discussed above) is included in the calculations to remove the effects of pot operations on the slope, including: break and feed (normal cycles, AEP*) (ii) anode movement (iii)bath additions (iv) tapping* anode setting* (vi) beam raising* Slope calculations are stopped during these operations, and for a pre-set period afterwards. Near the end of these 900216,!tbspe.044,comalco.spe, j-

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;I.-li -i i: _I 1 36 U-tl llsing tne smooUiIeu- I 37 aintain mass balance in the cell, i 900216, !tbspe.044,comalco.spe, 17 1 delay periods, the first stage filtered resistance (R 1 is 2 re-set to the mean of a specified number of raw resistance 3 values. For the cases marked Sl to S3 are also zeroed. In 4 the case of the pot under test, the respective delays following each of the above operations are: 6 i) 60 seconds 7 ii) 120 seconds 8 iii) 300 seconds 9 iv) 10 minutes S 10 v) 120 seconds S11 vi) 120 seconds S 12 Delay periods associated with other operations include: 13 When the pot is put on "manual" for any reason, a delay of 14 30 seconds is introduced.

When alumina is manually added, a delay of 120 seconds is 16 introduced.

17 Similarly when oreing-up is performed, a delay of 60 seconds 18 is introduced.

19 A pre-set delay is also implemented when step ii) of 20 the slope calculation fails to give in-range slopes on a S21 given number of consecutive tests. This is intended to trap 22 the gross resistance disturbances not initiated/expected by 23 the pot computer sludging may cause an unpredictable 24 resistance response).

Different cells will require different delays depending 26 on their operational characteristics and specific bath 27 volumes, and the delay involved for each operation will be 28 empirically determined by a skilled operator for input into 29 the pot computer.

Two alumina search techniques are available on the 31 system, stop feed search (SFS) and feed search (FDS). Both 32 techniques terminate search on a threshold value of 33 increasing resistance slope, implying low end point alumina 34 concentrations and both techniques allow heat balance regulation (anode movement)during the search. The special 36 features of each are described below.

37 i) SFS 38 This technique is essentially a stop feed during which 900216, !tbspe.044,comalco.spe, 36 calculating from the calculated heat avaliaDie zor 37 dissipation in and from a selected target power 38 dissipation, the integral of the difference between the 9 0 0 2 1 6 ,!tbspe.044,comalco.spe, f: I I 18 1 2 3 4 6 7 8 9 11 12 f 13 14 15 16 17 18 19 4 20 21 22 23 24 26 27 28 29 31 32 33 34 36 37 38 the filtered resistance slope is checked every second for values above the critical slope (critslope) indicating alumina depletion. Once the critical slope is attained on a sufficient number of consecutive readings, search is terminated by initiation of an end of search feed followed by the resumption of the previously nominated cycle (see Figure 3).

The search can also be terminated (classed an unsuccessful search) under the following circumstances: Cancelled due to time limitation (max search time).

Cancelled due to anode setting, tapping, oreingup, bath additions.

Cancelled if cell is switched to MANUAL.

The unique features of the SFS with respect to the present invention are: 1. the ability to monitor and interpret the resistance slope through all phases of the search.

2. the ability to move the anodes freely through all phases of the search.

3. the crit slope in the search is a function of the voltage noise in the cell.

The SFS technique has been applied to both breaker bar and point feed cells.

ii) FDS This is a more complex search procedure but one which has the potential for fine alumina concentration control on point feed cells. The strategy involves following resistance slope before and during underfeeding and overfeeding periods until a target alumina concentration is achieved.

The stages of the searching routine are as follows: After commencement of searching, the filtered resistance slope is monitored for a short time period and compared with a parameter, base search slope, near the minimum point on the resistance-time curve in Figure 4A. The objective is to adjust alumina concentration to this base level.

As the alumina concentration of the cell decreases, the resistance slope increases from a negative value up to the 900216,!tbspe.044,comalco.spe 19 1 2 3 4 6 7 8 9 12 0 13 o 16 17 18 19 0. 20 21 22 23 24 25 26 27 28 29 31 32 33 34 36 37 38 value of the base search slope. Thus slopes more negative than base search slope indicate a higher than 'base-level' alumina concentration and are actioned by changing to x underfeed. Slopes more positive than base search slope indicate a lower than base-level concentration and cause a y overfeed cycle to begin.

When the filtered slope passes through base search slope (or the under/overfeeding period times out whichever is first), a feedrate of x underfeed is selected for the remainder of the search period.

The filtered slope is then monitored until its value increases positively to target search slope. At this stage the alumina concentration has been adjusted to its target operating level. FDS is terminated and the previously selected (nominal or fixed) feedrate is resumed immediately.

By gradually increasing base search slope towards the target value (target search slope), it is possible to minimize the absolute variation in alumina concentration during FDS under point feeding of alumina. Additionally, if the percent under and overfeed are decreased to small values (such as the proportion of time spent on search will increase allowing very close feed control for most of the operation.

ANODE EFFECT PREVENTION MECHANISM Anode effect prediction (AEP) is provided by a check on the filtered resistance slope every second during normal feeding of the cell (Fig. If it exceeds a pre-set AEP slope an AEP feed cycle is initiated immediately to avoid an anode effect.

This high resistance slope results from the critical depletion of alumina concentration in the cell during periods when alumina searching is not occurring. Resistance changes due to operations like setting, tapping and bath additions are removed by the filtered slope calculation.

However, resistance changes due to metal pad instability are included in the filtered slope. Hence the pre-set AEP slope is increased if excessive low frequency noise is detected, as discussed further below, to reduce the likelihood that

I

I

90 0 216,!tbspe.044,comalco.spe, i f I:ii back EMF of the cell, as will be described turtner Delow.

resistance values are filtered using digital filtration techniques (e.g.

tb#2806.com 91 1022

A

~ia 20 1 2 3 4 6 7 8 9 *cat .o0 12 0 0 13 0044' 13 14 o 15 16 17 18 o" 19 oo 0 0 20 0 21 00 0 0" 22 0 0 o e a 23 24 25 26 4t 27 28 29 30 31 32 33 34 36 37 38 the system will trigger an AEP feeding cycle due to low frequency noise. It will be appreciated that low frequency, cyclic voltage variations (of less than one cycle per second) are sometimes observed due to instability in the liquid aluminium pad. The rates of resistance increase associated with these cycles can, in the case of severe instability, exceed the resistance slope thresholds above, triggering alumina feeding when this is not warranted. To guard against this occurrence the slope thresholds for both end of search and AEP are increased by a predetermined amount when low frequency voltage noise is detected above a certain amplitude (in micro-ohms). The critical slope threshold for one pot under test was 0.035 uSL/min. and the voltage noise threshold was 0.25 u-J. When the low frequency noise exceeds the above threshold, the critical slope threshold is ramped by an amount proportional to the amount by which the noise signal exceeds the predetermined threshold. The maximum increment of the ramp is 0.05 u-n./min. and occurs at a low frequency noise level of 0.50 uA. The filtered slope is again compared with the threshold and if it is found to be greater than the threshold, the alumina inventory is then considered to determine whether or not the cell is overfed. If this determination is in the negative, the control system instructs an AEP alumina feeding cycle to be effected. The operation of AEP can also be stopped for a defined period after an AEP prediction as further protection against excessive AEP triggered feeds during periods of cell instability.

Both high and low frequency noise calculations are performed continuously in this module. While the high frequency calculation is a simple 1 Hz, minimum R/maximum R relationship, the low frequency characteristic needs further explanation and this is given below.

LOW FREQUENCY NOISE CALCULATION The main function of the low-frequency noise calculation is to detect noise generated by metal pad instability. In this novel formulation, a group of consecutive resistances are summed, then averaged. A ring 900216,!tbspe.044,comalco.spe, i i /4/ 0 i u^A/ 7"

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rm, Mo tb#2806.com 91 10 18 Qb I 21 1 2 3 4 6 7 8 9 O.,0 10 o e 0 11 12 13 *1 14 too 0 0 15 16 17 18 19 20 o a 21 f° 22 23 24 25 4 I S 26 27 28 29 31 32 33 34 36 37 38 buffer containing a time sequence of these averages are then stored for some period of time (usually less than 2 AVC periods). Figure 4B is an example of the resulting data in a computer; essentially it is 'i resistance vs time plot with the high-frequency noise removed. The low frequency noise is the sum of absolute differences in adjacent resistance averages minus the absolute difference between the newest and oldest averages, divided by the time interval.

k 1 i.e. NOISE ARi AR i ARo ARk (to-tk) S= 0 where AR i is the average resistance at time t i Examples of idealized curves and their noise are shown in Figures 4C to E.

The calculation of noise with the addition i .ach new mean resistance (and the elimination of the oldest resistance) requires less calculation time than standard noise calculations. In the case of one test pot trial the mean resistances are calculated over 10 seconds, and values (5 minutes history) are stored.

CONTROL STRING 2: SHORT RANGE HEAT BALANCE CONTROL In the second control string the heat supplied and the heat required for aluminium production are calculated from the dynamic inputs described above (cell voltage and current, alumina additions, bath chemistry additions, operations and anode movements) and the heat available (QAVAIL) for dissipation by the cell is also calculated. The difference between available heat and the previously determined target heat (QT) is integrated with respect to time and from this integral a running heat inventory is calculated. The target resistance (RTARGET), derived in the manner described above from QTARGET' is regularly updated on the pot computer to adjust the heat balance of the cell to minimize the imbalance represented by the heat inventory integral. The target resistance must lie between the specified minimum and maximum allowable limits. These limits are, for example, 32-40 for a typical pot under test, i.e. a band of about 6 to 8 If the average actual 90 0 216,!tbspe.044,coalc.s.8pe, ii o o0o0 0 .O 0 04 0 0 30 0 -&0

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22 1 resistance over the resistance regulation (AVC) period is 2 significantly different (outside a specified dead band) from 3 the new target resistance, the pot computer then issues a 4 beam raise or lower signal to move the cell resistance back into the dead band. This instruction is limited to a pre-set 6 amount (AR max.).

7 If the updated resistance target consistently falls 8 above or below one of the allowable limits, disallowing the 9 regulation of resistance as described above, the operating 10 amperage, ore cover level, or bath and metal levels are 11 reviewed so that a more flexible region of the operating 12 envelope can be chosen for the cell.

0000 0ooo 13 0o00 14 I 15 16 17 18 19 000 20 21 22 23 24 25 0 0 0 0a 27 28 29 31 32 33 34 36 37 38 The set point, RTARGET, is updated at regular intervals on the basis of short range heat balance calculations. The short range calculations require the following information: Real time clock for scheduling and distributing intermittent power absorbed functions during operations.

-Vi, Ii, R i one minute average voltage, current and resistance.

-PCELL Cell Power input (heat balance interval average).

-Current efficiency based on cumulative metal tap.

-Software switches indicating commencement of a cell operation.

-Alumina dump counters metering alumina actually fed to the cell.

PABSORB power absorbed calculation This information is used to calculate three parameters: QAVAIL The available power dissipation over the previous period.

R The average actual cell resistance over the previous period.

I The average cell amperage over a longer time period (default period is one hour).

The calculated value of the available power dissipation is compared to the target value for the cell and the thermal imbalance AQ obtained. (The target value (QTARGET) is initially determined from a steady-state computer thermal model prediction and cell operating diagram and then updated 900216,!tbspe.044,comalco.spe, i' i I~ ~-l-Tii- i~lr~ I~I B.I"

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23 1 2 3 4 6 7 8 9 S4 11 12 13 00@0 14 0 15 16 17 18 0 4 19 4" 21 22 23 24 26 27 28 29 31 32 33 34 36 37 38 imbalance is integrated and converted directly into a A R and an RTARGET using the average value of amperage and the previous target resistance respectively.

As mentioned earlier, resistance regulation maintains cell resistance at or near the target value calculated in the heat balance program. Also, as will be discussed, anode movements do not in any way affect the mechanics of feed control on the cell. Functionally, the implications of these strategy requirements are as follows: i) resistance regulation is prohibited on three occasions only: during beam raising during anode setting during tapping when TVC is operative.

ii) resistance regulation frequency is increased so that the interval between resistance regulation is reduced to five minutes or less.

iii) The proportionality constants for resistance regulation buzz time (decisec/micro-ohm) are set as close as possible to the reciprocal product of resistance/cm of ACD and beam speed (up or down). This ensures that jle resistance regulation attempt moves the resistance to its target value eliminating kilowatt errors from this source.

iv) The dead band for resistance regulation is tight 0.20 micro-ohm).

CONTROL STRING 3: MEDIUM-LONG RANGE HEAT BALANCE CONTROL In the final control string, long term heat balance control is used to continually update the target power dissipation QTARGET through trends in bath resistivity data.

This prevents longer term changes in bath thermal conditions and chemistry which occur through breakdown of ore cover, changes in current efficiency or amperage, and variations in anode carbon quality with respect to reactivity, thermal conductivity and anode spike formation.

Bath resistivity data is used to detect all chemistry and thermal conditions in real time.

Bath resistivity is calculated at approximately hourly intervals, using controlled beam movements, with beam 900216,!tbspe.044,comalco.spe, i' 37 V and can be measured readily. The various 38 components of the enthalpy of reaction (QF QS QA QM) 900216,!tbspe.044,comalco.spe,

V

I i j 8 9 11 12 S13 14 15 16 17 18 19 21 4 0 22 23 24 26 27 28 29 31 32 33 34 36 37 38 24 movement measured in the usual manner by a shaft counter.

Using the average change in cell resistance before and after the beam movement sequence, bath resistivity is calculated from the known relationship.

RTOT /eff AGD

+ZRFIXED

AA

'R.AA

Thus bath resistivity Oeff JdACD TRFIXED is the sum of the contribution of resistance values due to ohmic effects and possible reaction decomposition effects. This value is assumed to be constant for changes in ACD.

AA is the nominal area of the anode and is assumed to be constant JACD is measured using the shaft counter fAR is the difference between cell resistance before and after the 20 decisecond buss-up.

The bath resistivity and its rate of change is a good indication of the concentration of A1F 3 There is a lag time between a rise in XS AlF 3 and a rise in bath resistivity. This characteristic depends on liquid bath volume, anode and cathode condition, and other pot characteristics. Freezing in a cell occurs when the bath super->heat drops below a certain level and is identifiable by an increase in XS AIF 3 After taking the lag time into consideration, if the bath resistivity is increasing to a level where electrolyte freezing and increases in XS Alf 3 are occurring, the Q TARGET is adjusted in the system so that more power is supplied to the cell. This causes a greater rate of heat dissipation throuth the electrolyte and increases its superheat, reducing its tendency to freeze.

The response must be tuned to the lag time of the resistivity measurement as well as to the QDISS/Superheat relationship, so that QTARGET does not overshoot its correct value.

The initial or starting value for the target heat dissipation QT is derived as follows.

900216,!tbspe.044,comalco.spe, I1:4 900216,!tbspe.044,comalco.spe, r 25 1 2 3 4 6 7 8 9 10 If I 11 12 13 14 16 17 18 19 21 22 23 24 26 27 28 29 31 32 33 34 36 Thermal model calculations (Finite element prediction of isotherms and flows within the cell in question) are used to determine the steady-state level, of heat loss required from a particular cell design (eg the test pot referred to above is a Type VI cell design and requires 220 230 kW depending on metal level and alumina cover). This target or 'design heat loss' is QCELL' The process energy requirement for aluminium production can be calculated in a known manner for the cell once the line amperage is known: PABSORB PCONTINUOUS PINTERMITTENT =L(a b.CE) {PFEED PSETTING ADDITIONS

+/PAE

o ideally J In Table 1 this is calculated to be 1.956 Volts x I at 95% current efficiency (CE) for a typical test cell at Bell Bay (At 90% efficiency this figure is 1.841 Volts x I).

Adding to this the power loss from the bus bar around the cell: REXTERNAL x 12 we have the total power input required for the cell: PTOTAL ABSORB QCELL PEXTERNAL VABSORB.I QCELL REXTERNAL1 2 TABLE 1 At I 87 kA PTOTAL 170.2 kW 225 kW 18.2 kW and 95% CE 413.4 kW REXT 2.4 on Pot under test QCELL 225kW V ABSRB 1.956V

ABSORB

This power input equates to a cell voltage of VTOTAL PTOTAL

I

4.75 V This cell voltage equates to a target (initial) cell resistance of RTARGET (VTOTAL 1.65) I 900216,!tbspe.044,comalco.spe, j i: 26 1 =35.65 u -CL 2 Typically this resistance will be used as a back-up or 3 start-up value on the pot computer. It will also lie in the 4 mid-range of the allowable target resistance band.

Initial Settings are therefore: 6 QTARGET =225 kW 7 RTARGET =35.65 u1L.- 8 However RTARGET will change every ten minutes by R as the 9 PABS0RB term is continuously recalculated according to pot requirements (feeding, anode setting etc.).

11 Figures 5A and 5B show selected pot parameters over 2 OJ, 12 months operation of a reduction l!1, with constant QTARGET* 13 The XS AlF 3 varied significantly over this period and 14 (a nd I are seen to be good indicators of this. Twice during the period shown, manual increases to the power input 16 were made to increase the cell superheat and reduce the XS 17 AlF 3 (times and In both cases high values of and 18 ±L were evident before manual intervention.

19 Such observations resulted in the development and testing of a closed-loop control system in which the target 21 energy input to the cell (QTARGET) was changed based on 22 (and/or .For the 1 month period in Figure 23 control Of QTARGET was based on only. (Both the manually 24 set 'nominal' TAGTand 'actual' TAGTare shown in this 9 4 TRE TRE 25 Figure.) Note that the high XS AlF 3 on days 4, 11, 19 and 26 29 correspond with high fPvalues. The resultant increased 27 power inputs controlled the high XS AlF 3 excursions, 28 making manual intervention unnecessary.

29 SYSTEM TESTING Frequent VAVC action maintains the actual resistance 31 close to the continually updated target value, and the 32 magnitude of its allowable resistance changes are specified 33 as a heat balance parameter within absolute resistance 34 -limits as discussed earlier. More importantly, AVG will not be disallowed during operations unless it is physically 36 unreasonable to perform beam movement. These occasions are 37 during tapping, anode setting and beam raising.

38 An extended trial of the above described control system I 900216,C-disk 20-meg,comalco.spe,

R

i 27 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 26 27 28 29 31 32 33 34 36 37 38 has been made on a group of cells at one of the applicant's smelters. For the trial the CE and 'QT' for each cell were selected based on long-term data computer modelling and cell condition. It should be noted that cell condition fluctuates due to factors such as cell ore cover, seasonal temperatures, alumina properties, bath composition and cell age. Hence the parameters should be updated on a regular basis.

Calculation of the power absorbed for the control system used the following hardware inputs:- -voltage and amperabe (iHz) -a switch to indicate anode setting (at cell) -keyboard input for bath additions in kg (adjacent to cell) -keyboard input for manual alumina addition and oreing-up The results presented in Figures 7 and 8 show the behaviour of a cell under the control system over 24 hours.

Figure 7 shows the calculated heat absorbed by the cell, broken down into it's four operational components.

Fluctuations in the power required for reaction (metal production) (Fig. 7a) were due to line amperage variations.

The power absorbed by alumina feeding (Fig. 7b) had a strong cyclic pattern. This pattern is accentuated because the alumina searches (SFS) included cessation of feeding (for the day shown). Figure 7c shows the effect of replacing two anodes. For setting, the energy distribution was spread over 5 hours; this was based on trial data and computer modelling of the heat absorbed by the new blocks. Figure 7d includes the energy input fc a 15kg bag of AlF3. Note that 50% of feed power, 50% of setting power, and 20% of the additions power were supplied as constant background inputs, while the remainder in each case was triggered by the respective events.

The calculation of the total absorbed energy is shown in Figure 8a. Figure 8b shows the power available for dissipation from the cell as heat (Eqn Note the target dissipation rate of 240kW for this cell. The target and calculated actual heat dissipation clearly show the heat deficit/excess in Figure 8c. The cell had an energy 900216,C-disk 2 0-meg,comalco.spe,

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i (f -i 1 28 1 2 3 4 6 7 8 9 0t12 13 0 14 16 17 18 19 0 20 o0 21 22 23 24 t 4 25 S' 26 27 28 29 31 32 33 34 36 37 38 imbalance for periods up to 2 hours. This was primarily due to the power input constraints imposed by the cell resistance control band. Figure 8d shows the control band of 32.5 to 38 uOhm used over the 24 hour period. Anode beam movements are clearly larger, and more frequent, than for control systems previously reported in the literature. This reflects the extent of thermal disturbance which is imposed on most reduction cells in a single day.

Figure 9 illustrates the behaviour of the alumina feed control component of the system during a typical, successful stop feed search (SFS). (The search period is marked in Fig.

8d.) One minute averages of anode cathode distance (ACD), cell resistance and slope of resistance are shown. The centre channel bath temperature, measured at ten minute intervals, is also presented. The change in ACD was transduced using the rotation shaft counter (proximity switches) on the anode beam drive shaft. The resistance slope (Fig. 9d) was zeroed at the start and end of the SFS; the end of search slope was 0.025 uOhm/min. The search lasted approximately 90 minutes, and there was substantial beam movement throughout. The high resistance/ACD at the start of searching was due to the energy requirement of a 23kg alumina feed immediately beforehand. Once this energy was supplied, the control system reduced the power input.

The control approach allowed long SFSs to be scheduled without the bath temperature or superheat increasing substantially. This allowed back-feeding and depletion of alumina to the target level. The stable bath temperature is clearly shown in Figure 9c, although there was a temperature fall caused by the feed before SFS. Typically, a bath temperature change of only was measured during SFS.

While there is some fluctuation in the dynamics of the resistance slope, the underlying trend and threshold values were reliable. The SFS technique achieved good feed control, consistently, with 0.3 AEs/day.

The trial results demonstrate a number of inherent advantages in the control system. Since the energy requirements were calculated from basic information (eg line 9 0 0216,C-disk 20-meg,comalco.spe 1j: 29 0 n Ii: 1 2 3 4 6 7 8 9 10 11 12 13 I t S. 14

I

16 17 18 t 19 21 22 23 24 26 27 28 29 31 32 33 34 36 37 38 amperage, alumina dumps, thermodynamic data), changes to the operating environment were catered for automatically. If a variation in potline amperage occurred, the control system automatically adjusted the resistance targets of the cell.

The mean resistance at which the cell operated over longer periods were also varied if the long-term amperage was changed. Similarly, any decision to change the number of dumps for each feed, the timing of SFSs or the number/size of anodes set was catered for easily. Fundamentally, this was due to the control system being based on the real operating target and component energy requirements of the smelting process rather than the less direct measures of target voltage or resistance. This same mechanistic approach can also reinforce the understanding of the process for those operating it.

There are, of course, some practical constraints imposed on the control system by the process. If the potline amperage is reduced significantly for a sufficient period, each cell will experience a substantial energy deficit. Thus all cells in the potline will attempt to operate at their maximum target resistance simultaneously. The potline voltage may then exceed the rectifier limits. This problem can be overcome by including safety factors in the control system which limit the closure of energy balance attempted under extreme potline conditions. On an individual pot basis there may also be variations in heat dissipation, current efficiency and the integrity of the top cover/crust, requiring individualization of the QT targets for each cell.

The control system embodying the invention maintains a target rate of heat loss from a reduction cell via calculation of the energy absorbed by the process. The trial results show that the system made regular anode beam movements while maintaining good thermal balance on the cell. The control system described here is a building block for the optimization of reduction cell efficiency via understanding and reducing variations in the cell thermal balance.

The overall configuration of a typical control system 900216,C-disk i IIl t I 30 0043 o 0 0400 0 0000 000 *00 0 is shown in Figure 2. The physical location of each control module on the system in this implementation has been determined by the computing power available at the pot computer and supervisory computer levels respectively. Thus the more complex heat balance control module has been placed on a Microvax supervisory computer. This also has the advantage of providing an interactive human interface to the control function for diagnostics and further development. As a general strategy, however, all essential control functions in a distributed potline system should be located at the lowest intelligent level the pot computer in this case so that maximum safety and redundancy can be built into the system.

The computer control functions detailed in Figure 2 will be recognised by persons of skill in the art and since many of the functions are not critical to the invention, they will not be further described in this specification.

The claims form part of the disclosure of this specification.

0009 00 0 0a 0 0040(9 K.s 900216,C-disk L -1

Claims (17)

1. A process for controlling the operation of an aluminium smelting cell comprising the steps of: maintaining the mass balance of the cell at a predetermined level by calculating and monitoring a smoothed rate of change of the resistance of the cell to detect a predetermined slope threshold indicative of low alumina concentration in the cell, and maintaining the heat balance of the cell by calculating a target heat dissipation for the cell; (ii) calculating the heat available for dissipation by the cell; (iii) calculating a running heat inventory from the integral of the heat available minus the target heat, and (iv) modifying a target resistance value for the cell to achieve a substantially zero heat integral in step (iii) by moving the anodes of the cell to achieve said new target resistance,

2. The process of claim 1, further comprising monitoring cell operations including alumina dumps, cell bath additions, process operations and anode movements and delaying the calculation of the smoothed rate of change of resistance in the cell for a predetermined time when any one of said cell operations takes place, and recalculating said smoothed resistance slope after said predetermined time delay so that said smoothed slope is unaffected by process changes with the exception of alumina depletion.

3. The process of claim 1 or 2, wherein the step of calculating a 1 I: r :r e r -ilr tb#2806.com 91 10 22 J/is increasei it excessive low trequency noibu .L6 uaeL--1 38 as discussed further below, to reduce the likelihood that I 900216, !tbspe.044,comalco.spe, 32 smoothed value of said resistance slope includes the steps of calculating a raw resistance slope checking to determine whether the raw slope falls within predetermined limits and rejecting any value falh-ag outside such limits, and calculating the filtered resistance slope.

4. The process of claim 3, wherein the raw resistance slope is calculated from the equation: so (Ito R 1 (At(1 1/8)) where so raw slope at time (t At) =o raw resistance at (t At) R, single stage filtered resistance at time t At time interval of resistance polling 6 filter constant for filtered resistance (R 1 and said filtered resistance slope is calculated from the equation R, R, (1 81) 8 1 R., 15 -and Si i S(1 6) 8iSi- 1 where 8i is a predetermined filter constant of the ith stage.

The process of any preceding claim, wherein step includes the step of searching the smoothed resistance slope for values exceeding a predetermined slope chosen to indicate alumina depletion.

6. The process of any preceding claim wherein the voltage or resistance of the cell is monitored to detect the presence of low frequency or high frequency noise in the voltage signal, and in the event that the low frequency noise in the voltage signal is above a predetermined threshold, the slope threshold for low alumina 4.4, r I 4444 4444 t k 4 L A NT tb#2806.com 9 02 91 1022 38 consecutive resistances are summed, then averaged. A ring r 900216,!tbspe.044,comalco.spe, 33 concentration detection is increased by a predetermined amount.

7. The process of any preceding claim wherein the resistivity of the cell bath is calculated and the resistivity and rate of change of resistivity with time are monitored to detect values greater than predetermined limits indicative of the cell superheat being out of range, and adjusting the target heat dissipation of the cell to return the cell super heat to within a predetermined range.

8. A process for controlling the operation of an aluminium smelting cell, comprising the steps of: monitoring the cell voltage and current, monitoring alumina additions to the cell, monitoring other additions to the cell bath and monitoring operational changes such as anode movements, tapping, anode setting and beam raising, continuously calculating the resistance of the cell, continuously calculating the rate of change of cell resistance, and smoothing the rate of change values so calculated to continuously provide smoothed resistance slope values, continuously monitoring cell voltage or resistance to determine the existence of low frequency or high frequency noise in the voltage signal, continuously calculating the energy absorbed by the process from thermodynamics and the events identified in item above, calculating the heat available for dissipation in the cell from the cell voltage and current and from the continuously calculated I VII l U tb#2806.com 91 1022 VO r 0 Ij are, ror example, JZ-4U u-nl, tor a typica± pUL ullueL uc-, 38 i.e. a band of about 6 to 8 u If the average actual 90 0 2 1 6 ,!tbspe.044,comalci.spe, I 34 process energy requirement determined in item above, establishing a base threshold and a critical threshold for said smoothed resistance slope value indicating target and low alumina concentrations respectively, determining whether low frequency voltage noise exists above a predetermined threshold and increasing the smoothed resistance slope threshold in the event that said low frequency voltage noise is above said threshold, calculating the alumina inventory of the cell from the monitored alumina additions, determining whether the smoothed resistance slope is greater than the predetermined threshold and if so determining whether the cell has been overfed from the calculated alumina inventory, and if not causing an alumina feed to occur. 15 calculating from the calculated heat available for dissipation and Sttl !from a selected target power dissipation, the integral of the d difference between the heat available and the target power dissipation with respect to time, calculating from this heat deficit or surplus in the cell the change I. 20 in power dissipation required in the cell over a predetermined period to restore heat balance (zero heat integral in item establishing an initial target resistance and an allowable band for said target resistance, calculating the required change in target resistance from the 25 required change in cell power dissipation (item divided by tb#2806.com 91 10 22 V* T I f( 1 37 initially determined from a steady-state computer LlrteLii a 38 model prediction and cell operating diagram and then updated 1 1• 900216, !tbspe.044,comalco.spe, i i 35 the square of the moving average of the cell current, altering the target resistance in accordance with the calculated heat inventory (item and checking that the new target resistance is within said allowable band, and I 5 moving the anodes of the cell to achieve said new target resistance.

9, The process of claim 8, further comprising the step of monitoring the resistivity of the cell bath and the rate of change of said resistivity with respect to time to detect values greater than predetermined limits indicative of cell superheat being out of range, and adjusting the target 'i i heat dissipation of the cell to return the cell superheat to within a predetermined range.

The process of claim 8 or 9, wherein said bath resistivity is measured by measuring the resistance of the cell over a predetermined i 15 period, adjusting the anode to cathode distance by a predetermined amount, measuring the resistance of the cell over a predetermined period, and calculating the resistivity of the bath from the formula C STOT= pff ACD RFIXED AA

11. The process of claim 8, 9 or 10, further comprising the step of determining whether the low frequency voltage noise in the cell is above a predetermined threshold, and if so increasing the target power dissipation in the control of the heat balance of the cell to remove tb#2806.com 91 10 22 NT 0 i -36- cathode sludge deposits.

12. A system for controlling the operation of an aluminium smelting cell comprising: means for maintaining the mass balance of the cell at a predetermined level by calculating and monitoring a smoothed rate of change of the resistance of the cell to detect a predetermined slope threshold indicative of low alumina concentration in the cell, and means for maintaining the heat balance of the cell including means for calculating a target heat dissipation for the cell; (ii) means for estimating the heat available for dissipation by the cell; (iii) means for calculating a running heat inventory from the integral of the heat available minus the target heat, and (iv) means for modifying a target resistance value for the cell to achieve a substantially zero heat integral in step (iii) by iA moving the anodes of the cell to achieve said new target resistance. I

13. A system for controlling the operation of an aluminium smelting cell, comprising the steps of: means for monitoring the cell voltage and current, means for monitoring alumina additions to the cell, monitoring other additions to the cell bath and monitoring operational changes such as anode movements, tapping, anode setting and beam raising, I A tb#2806.com 91 1022 j/ Ine iniLia± o rL- i L.Li -n V 1 L 38 dissipation QT is derived as follows. 900216,!tbspe.044,comalco.spe, I 37 means for continuously calculating the resistance of the cell, means for continuously calculating the rate of change of cell resistance, and smoothing the rate of change values so calculated to continuously provide smoothed resistance slope values, means for continuously monitoring cell voltage or resistance to determine the existence of low frequency or high frequency noise in the voltage signal, means for continuously calculating the energy absorbed by the process from thermodynamics and the events identified in item above, means for calculating the heat available for dissipation in the cell from the cell voltage and current and from the continuously calculated process energy requirement determined in item (f) above, means for establishing a base threshold and a critical threshold for said smoothed resistance slope value indicating target and low alumina concentrations respectively, means for determining whether low frequency voltage noise exists above a predetermined threshold and increasing the smoothed resistance slope threshold in the event that said low frequency voltage noise is above said threshold, means for calculating the alumina inventory of the cell from the monitored alumina additions, means for determining whether the smoothed resistance slope is greater than the predetermined threshold and if so determining U .f tb#2806.com 91 1022 'y -38- whether the cell has been overfed from the calculated alumina inventory, and if not causing an alumina feed to occur, means for calculating from the calculated heat available for dissipation and from a selected target power dissipation, the integral of the difference between the heat available and the target power dissipation with respect to time, (in) means for calculating from this heat deficit or surplus in the cell the change in power dissipation required in the cell over a predetermined period to restore heat balance (zero heat integral in item means for establishing an initial target resistance and an allowable hand for said target resistance, means for calculating the required change in target resistance o a from the required change in cell power dissipation (item (mn)) divided by the square of the moving average of the cell current, means for altering the target resistance in accordance with the 0*~~~calculated heat inventory (item and checking that the new target resistance is within said allowable band, and a(q) means for moving the anodes of the cell to achieve said new ,1*20 target resistance.

14. The system of claim 12 or 13, further comprising means for monitoring the resistivity of the cell bath and the rate of change of said resistivity with respect to time to detect values greater than predetermined limits indicative of cell super heat being out of range, 25 and means for adjusting the target heat dissipation of the cell to return t~i~ttb#2806.com 91 10 22 Li 37 during tapping, anode setting and beam raising. 38 An extended trial of the above described control system 9 0 0216,C-disk I~ It, 11 II i -39- the cell super heat to within a predetermined range.

The system of claim 14, further comprising means for determining whether low frequency voltage noise in the cell is above a predetermined threshold, and means for increasing the target power dissipation in the control of the heat balance of the cell to remove cathode sludge deposits if said low frequency voltage noise is above predetermined threshold.

16. The process according to any one of claims 1 to 11 substantially as hereinbefore described with reference to the accompanying drawings.

17. A system for controlling an aluminium smelting cell substantially as hereinbefore described with reference to the accompanying drawings. DATED this 18th October 1991 CARTER SMITH BEADLE Fellows Institute of Patent Attorneys of Australia Patent Attorneys for the Applicant: COMALCO ALUMINIUM LIMITED rrr r I rr ~I. c r rr t I tb#2806.com 91 10 22