EP0814181A1 - Process for regulating the alumina content of bath in cells for producing aluminium - Google Patents

Abstract

Regulating the alumina content in the cryolite-based bath of an electrolytic aluminium production cell comprises varying the alumina supply rate, as a function of the value and change of cell resistance (R) calculated from the cell terminal potential difference, in alternate phases of under-supply with slow rates (CL) of alumina introduction (phase 1) and phases of over-supply with rapid (CR) or very rapid (CUR) rates of alumina introduction (phase 2) w.r.t. a reference or theoretical rate (CT) corresponding to the mean theoretical alumina consumption of the cell. Each regulation cycle of duration (T) comprises: (a), at the end of each regulation cycle 'i', calculating the average resistance 'R(i)', the resistance change rate or slope 'P(i)', the rate of change of the resistance slope or curve 'C(i)' and a forecasted value of the resistance slope at the instant 't(i+1)' or the extrapolated slope 'PX(i)' (= P(i) + C(i) x T) which is an estimate of the future resistance slope 'P(i+1)' at the end of regulation cycle 'i+1'; (b) comparing the value 'R(i)' with a target value 'Ro' to ascertain whether the anodes require displacement for reducing or for increasing the anode-to-metal distance; and (c) regulating the alumina supply as a function of the values of the slope 'P(i)', the curve 'C(i)' and the extrapolated slope 'PX(i)' to compensate anticipated alumina content changes.

Description

TECHNICAL AREA

The present invention relates to a process for precise regulation of the alumina content in igneous electrolysis cells for the production of aluminum according to the Hall-Héroult process, with a view not only to maintaining the Faraday yield at a high level, but also to reduce emissions of fluorocarbon gases which are particularly harmful and polluting for the environment, this following on from anomalies in the operation of electrolytic cells known as the anode effect.

STATE OF THE ART

In recent years, the operation of aluminum production tanks has been progressively automated, firstly to improve regularity of operation and thereby the Faraday energy balance and yield, but also, with an ergonomic and ecological aim , to limit painful human intervention and increase the yield of fluorinated effluents.

One of the essential factors ensuring the regularity of operation of an aluminum production tank by electrolysis of alumina dissolved in a molten electrolysis bath based on cryolite, is the maintenance of an appropriate content of dissolved alumina in this electrolyte and therefore the adaptation at any time of the quantities of alumina introduced into the bath to the consumption of alumina from the tank.

Thus, an excess of alumina creates a risk of fouling of the bottom of the tank by undissolved alumina deposits which can transform into hard plates electrically insulating part of the cathode. This then promotes formation in the metal of very strong horizontal electric currents which, by interaction with the magnetic fields stir the sheet of metal and cause instability of the bath-metal interface.
Conversely, an alumina defect causes the appearance of the anode effect, resulting in a loss of production and a sudden increase in the voltage at the terminals of the tank, which can increase from 4 to 30 or 40 volts. This overconsumption of energy also has the effect of degrading the energy yield of the tank but also the Faraday yield following the redissolution of the aluminum in the bath and the rise in temperature of the electrolysis bath.

The need to keep the content of alumina dissolved in the electrolyte within precise and relatively narrow limits, and therefore to introduce alumina with the greatest regularity possible, has therefore led those skilled in the art to develop automatic processes. supply and regulation of alumina in electrolytic cells. This need has become an obligation with the use of so-called "acid" electrolysis baths (with a high content of AIF ₃ ) which makes it possible to lower the operating temperature of the tank from 10 to 15 ° C (approximately 950 ° C at instead of 965 ° C usually) and thus achieve Faraday yields of at least 94%. Indeed it is then essential to be able to adjust the alumina content in a very precise and very narrow concentration range (1% to 3.5%), taking into account the decrease in the solubility rate of alumina linked to the new composition as well as the lowering of the bath temperature.

Since the direct measurement of the alumina content of the baths by analysis of samples taken periodically has not proven to be sufficient industrially, most of the known industrial processes have resorted to an indirect evaluation of the alumina contents by following an electrical parameter representative of the alumina concentration of said electrolyte. This parameter is generally the variation of the resistance R across the terminals of the tank supplied with a voltage U, including a counter-electromotive force e evaluated for example at 1.65 volts and traversed by a current I so that R = (U - e) / I.

By calibration, a variation curve of R can be plotted as a function of the alumina content and by measurement of R (at a frequency determined according to well known methods), the concentration of alumina [Al ₂ O ₃ ] can be known at any time. . It is this detection principle that the document FR 1457746 (GB 1091373) adopts for controlling an alumina dispenser associated with a means for piercing the electrolyte crust frozen on the surface of the bath. Likewise US 3400062 implements a measurement of the variation in resistance of the bath using a pilot anode to detect an alumina defect and a tendency to the anode effect and thus act on the rate of introduction of alumina from a hopper fitted with a device for piercing the frozen electrolyte crust.

More recently, precise regulation methods based on the control of the alumina content between an upper limit and a lower limit have been the subject of new patents including US 4126525 and EP 044794 (US 4654129), the latter already being in the name of the plaintiff.

In the first of these patents, the range of alumina contents to be respected is between 2 and 8%. The tank is fed for a predetermined time t1 with an amount of alumina greater than its theoretical consumption until a fixed alumina concentration is obtained (for example 7%, therefore a little less than the admissible maximum of 8%), then the supply is switched to a rate equal to the theoretical consumption for a predetermined time t2, finally the supply is stopped until the appearance of the first symptoms of anode effect. The feeding cycle is then resumed at a rate higher than the theoretical consumption. According to this process, and more precisely the results of its application examples, the Alumina concentration in the bath can vary during a cycle of 3 to 8%, which is still insufficient to regulate the alumina content of an acid bath in a range as low and narrow as 1 to 3 or 4%. This is what the process according to EP 044794 (US 4431491) achieves, in the name of the applicant who calls on the side of the measurement of the resistance R at the terminals of the electrolysis cell with a second adjustment parameter which is the slope P = dR / dt representative of the variation in resistance R caused by a voluntary change in the alumina supply regime of the bath for a determined time. Indeed, the mere knowledge of the resistance R at the terminals of the electrolytic cell is not sufficient to precisely control the alumina content of the bath and consequently to control the quantity or the frequency of the anode effects, because the parameter R at constant bath temperature is a function of 2 variables, on the one hand the image alumina content of the resistivity p of the bath, on the other hand the anode-metal distance (DAM). It is therefore necessary to find another discriminating parameter which is obtained by the slope P = dR / dt, called resistance slope, the only parameter truly representative of the depletion or enrichment of the bath in alumina. By creating, for example, a momentary undernourishment of the alumina bath compared to the theoretical consumption, an increase in the resistivity p is recorded with the lowering of the alumina content of the bath according to a known law of evolution while in the same the DAM of much slower evolution practically did not vary.

It is on the adjustment of these 2 parameters R and dR / dt that the process according to EP 044794 is based which can be summarized as follows: starting from a phase of undernourishment of the bath in alumina, the passage is ordered in the supercharging phase for a predetermined duration T if the resistance R exceeds the upper limit Ro + r where Ro is the setpoint resistance and if the resistance slope P is greater than a setpoint slope Po.

On the other hand, if the slope P remains lower than the setpoint slope Po indicating a sufficient content of alumina in the bath, the underfeeding regime of the bath is kept, but an order of descent of the anode frame or if necessary is given. tightening ”to reduce the DAM and thereby bring R into the setpoint range Ro ± r.

Finally, starting from the supercharging phase of duration T, we go on to a rate of undernourishment at the end of this duration T and if R has become less than the lower limit Ro-r of the setpoint range, an order of rise of the anode frame or "loosening" to increase the DAM and bring R into the setpoint range Ro ± r.
We then start a new cycle.

This regulation mode therefore makes it possible to maintain the alumina content of the bath in a narrow and low range and thus to obtain Faraday yields of the order of 95% with acid baths, simultaneously and significantly reducing the quantity ( or frequency) of anode effects on the tanks which are counted as the number of anode effects per tank and per day (EA / tank / day) under the name “anode effect rate”.

On older generations of side-stitching tanks, the anode effect rate was greater than 2 or even 3 EA / tank / day, while on more recent tanks with punctual stitching this rate is between 0.2 and 0 , 5 EA / tank / day. At this stage, the overconsumption of energy and the Faraday yield loss linked to the anode effects are low and until recent years this level of performance could be considered sufficient.

Recently, however, with the development of very high intensity electrolysis cells and the search for ever higher performance, particularly in terms of Faraday yield and energy efficiency, but also with the consideration of pollution problems by fluorocarbon compounds (CFx), in particular by carbon tetrafluoride CF4, whose high absorption potential of infrared rays promotes the greenhouse effect, the reduction, even elimination, of the anode effects generating fluorocarbon gases has become a priority. In this regard, it should be recalled that the anode effect is a phenomenon of electrolysis of fluoride ions which occurs when there is a defect in oxygen ions in contact with the anodes due in particular to a lack of alumina. Instead of producing carbon dioxide and carbon monoxide according to the normal process, the tank produces fluorocarbon gases whose trapping by the usual means is impossible because of their chemical inertness and their great stability.

PROBLEM

• la construction de nouvelles usines d'électrolyse mettant en oeuvre des cuves de très haute intensité en nombre toujours plus grand,
• l'extension des usines existantes sans accroissement, voire même avec diminution, des rejets gazeux fluorocarbonés.

The development of a precise regulation process for low alumina contents in the electrolysis bath ensuring a high Faraday yield (≥ 95%) with an anode effect rate of less than 0.05 EA / cell / day has become an essential goal for:

• the construction of new electrolysis plants using very high intensity tanks in ever increasing numbers,
• the extension of existing factories without increasing, or even decreasing, fluorocarbon gas emissions.

OBJECT OF THE INVENTION

The method according to the invention makes it possible to solve this pollution problem by lowering the anode effect rate on average to 0.02 EA / tank / day, that is to say well below the rate of 0 , 05 EA / tank / day targeted and a fortiori rates of 0.2 to 0.5 EA / tank / day of the prior art; this even improving the Faraday yield to more than 95%. The method of the invention uses the basic principle of alumina regulation already described in EP 044794 (US 4431491) which implements 2 adjustment parameters, the resistance R and the resistance slope P = dR / dt, which are compared to setpoints to trigger a change in alumina supply regime or an order displacement of the anode frame in order to correct the metal anode distance (DAM).

• la détermination de la résistance et de la pente à chaque fin de cycle de régulation et non plus seulement lorsque la résistance sort de la plage de consigne,
• le déclenchement d'une phase de suralimentation si la teneur en alumine mesurée par la pente de résistance devient très faible et cela quelle que soit la position de la résistance par rapport à la plage de consigne,
• enfin l'affinement des méthodes de détermination de la résistance R et surtout de la pente de résistance P, ainsi que l'utilisation de paramètres auxiliaires qui seront explicités plus loin, assurant à la fois une grande précision et une grande fiabilité au nouveau procédé de régulation.

The method according to the invention is however clearly distinguished from the method previously described by the fact that it implements an entirely different operating sequence at each regulation cycle, in particular with:

• determination of the resistance and the slope at the end of the regulation cycle and no longer only when the resistance leaves the set range,
• the triggering of a supercharging phase if the alumina content measured by the resistance slope becomes very low, regardless of the position of the resistance relative to the set range,
• finally the refinement of the methods for determining the resistance R and above all the resistance slope P, as well as the use of auxiliary parameters which will be explained below, ensuring both great precision and great reliability for the new method of regulation.

It is therefore thanks to the new operating sequence within each cycle taking these different modifications into account, that the method according to the invention has made it possible to divide on average by 10 the anode effect rate obtained with the processes of the prior art chosen, however, from among the most efficient and achieving Faraday yields systematically greater than 95%.

DESCRIPTION OF THE INVENTION

• A/ Au terme de chaque cycle de régulation i, on calcule la résistance moyenne R(i), la vitesse d'évolution de la résistance ou pente de résistance P(i), la vitesse d'évolution de la pente de résistance ou courbure C(i) et une prévision de la valeur de la pente de résistance à l'instant t(i+1) ou pente extrapolée PX(i)=P(i)+C(i)xT qui est une estimation de la future pente de résistance P(i+1) à la fin du cycle de régulation i+1;
• B/ La valeur R(i) est comparée à une valeur de consigne Ro et des ordres de déplacement du cadre anodique sont donnés en conséquence, à savoir : diminution de la distance anode métal ou serrage, augmentation de la distance anode métal ou desserrage;
• C/ L'alimentation en alumine est régulée en fonction des valeurs de la pente P(i), de la courbure C(i) et de la pente extrapolée PX(i), de préférence par rapport à des seuils de référence tels que Po, Co et PXo, de manière à compenser par anticipation les évolutions de la teneur en alumine.

More specifically, the invention relates to a process for regulating the alumina content of the bath in an aluminum production tank by electrolysis of alumina dissolved in a molten salt based on cryolite using an alumina supply at a rate modulated according to the value and the evolution of the resistance R of the tank calculated from the difference in electrical potential measured at the terminals of the tank, alternating phases of undernourishment of alumina with introduction of alumina at slow rate CL (phase 1) and phases of supercharging of alumina with introduction of alumina at rapid rate CR or ultra-fast CUR (phase 2) compared to a reference rate or theoretical rate CT corresponding to the average theoretical consumption of alumina in the tank, characterized by regulation cycles of duration T comprising at each cycle, the following sequence of operations:

• A / At the end of each regulation cycle i, the average resistance R (i) is calculated, the speed of evolution of the resistance or resistance slope P (i), the speed of evolution of the resistance slope or curvature C (i) and a forecast of the value of the resistance slope at time t (i + 1) or extrapolated slope PX (i) = P (i) + C (i) xT which is an estimate of the future resistance slope P (i + 1) at the end of the regulation cycle i + 1;
• B / The value R (i) is compared with a setpoint value Ro and orders to move the anode frame are given accordingly, namely: reduction of the metal anode distance or tightening, increase of the metal anode distance or loosening;
• C / The supply of alumina is regulated as a function of the values of the slope P (i), of the curvature C (i) and of the extrapolated slope PX (i), preferably with respect to reference thresholds such as Po , Co and PXo, so as to compensate in advance for changes in the alumina content.

• Si l'alimentation en alumine est en phase 1, les valeurs P(i), C(i) et PX(i) sont comparées respectivement aux seuils de référence Po, Co et PXo :
  • ° Si P(i) < Po et PX(i) < PXo, la phase 1 se poursuit
  • ° Si P(i) ≥ Po ou PX(i) ≥ PXo, on passe en phase 2 d'alimentation en alumine:
    • . Si C(i) ≥ Co, la phase 2 commence par une alimentation en cadence ultra-rapide pour une durée prédéterminée ou calculée, suivie d'une alimentation en cadence rapide pour une durée prédéterminée ou calculée, le calcul des durées étant effectué en fonction des valeurs calculées en fin du cycle de régulation précédemment défini
    • . Si C(i) < Co, l'alimentation en alumine passe directement en cadence rapide pour une durée prédéterminée ou calculée en fonction des valeurs calculées en fin du cycle de régulation précédemment défini.
• Si l'alimentation en alumine est en phase 2 :
  • la phase 2 se poursuit normalement selon la durée prédéterminée ou calculée à l'issue de la précédente phase 1.

According to an advantageous embodiment of the invention, the regulation of alumina in step C / is carried out under the following conditions:

• If the alumina supply is in phase 1, the values P (i), C (i) and PX (i) are compared respectively to the reference thresholds Po, Co and PXo:
  • ° If P (i) <Po and PX (i) <PXo, phase 1 continues
  • ° If P (i) ≥ Po or PX (i) ≥ PXo, we go to phase 2 of alumina supply:
    • . If C (i) ≥ Co, phase 2 begins with an ultra-fast rate feeding for a predetermined or calculated duration, followed by a fast rate feeding for a predetermined or calculated duration, the durations being calculated as a function values calculated at the end of the previously defined regulation cycle
    • . If C (i) <Co, the supply of alumina goes directly to rapid rate for a predetermined duration or calculated according to the values calculated at the end of the previously defined regulation cycle.
• If the alumina supply is in phase 2:
  • phase 2 continues normally according to the duration predetermined or calculated at the end of the previous phase 1.

During the development of the new process according to the invention, the Applicant was able to observe that it was possible to dramatically reduce the rate of anode effect by switching to a fast-rate feeding regime without waiting that the resistance R has left the setpoint range according to the prior art previously described as soon as the resistance slope P becomes very high, an indication of a very low alumina content in the bath (1 to 2%) and a very high risk of appearance of anode effect. Figure 1 in the appendix which represents the variation of the resistance R at the terminals of an electrolysis cell as a function of the alumina content of the bath for different increasing anode-metal distances DAM ₁ to DAM ₃ , clearly shows that in regulating the alumina content of the bath between 1 and 3.5% we are in the best possible conditions, on the one hand to use acid electrolysis baths at lowered temperature guaranteeing excellent Faraday yields, on the other hand for detect the slightest variation in resistance since we are in the area with the steepest variation slope of R, i.e. in the area of greatest sensitivity. The counterpart of this double advantage implies a very rapid and quantitatively significant reaction capacity in terms of the alumina bath supply regime to prevent the very significant risks of triggering an anode effect which appear as soon as the alumina content of the bath is around 1%.

To solve this problem, which was not fully addressed by the closest prior art regulation method, which only provides for a slope value calculation when the resistance R exceeds a high reference threshold Ro + r, it has been found it is necessary to carry out not only this calculation of the slope at the end of each regulation cycle, but also the calculation of the extrapolated slope planned for the end of the next cycle to compare them with reference thresholds and trigger immediately if necessary and by anticipation of an acceleration of the feed rate in the case of a rapid increase in resistance as shown in the graph in Figure 2.

This new procedure for regulating the alumina content does not exclude the implementation of additional safety procedures.
Thus the regulation procedure is initiated only when the tank is in normal operating conditions (that is to say correctly adjusted, stable and excluding disturbing operations of operation or adjustment such as change of anode, casting of metal or specific regulation procedures) authorizing the transition to phase 1. In the case where the tank is not in normal operating conditions, the supply of alumina is at theoretical rate CT or waiting phase until what it finds normal operating conditions to go to phase 1.
Furthermore, if the supply phase 1 carried out in the normal framework of the regulation procedure is prolonged beyond a predetermined duration and if the number of release orders during this phase 1 exceeds a predetermined threshold of security, we detect that the bath is too rich in alumina and then the alumina supply is very greatly reduced or completely interrupted to purge the bath of its excess alumina.

Conversely, if the number of tightening orders during the same phase 1 exceeds a predetermined safety threshold, the supply phase 2 is started regardless of the values of the resistance slope and the extrapolated slope .

Finally, if the curvature C (i) exceeds a predetermined safety threshold, phase 2 of alumina feeding is started regardless of the values of the resistance slope P (i) and the extrapolated slope PX (i).

• des modifications ont été apportées dans les méthodes de calcul des paramètres connus que sont R et P afin d'en augmenter la précision
• des paramètres complémentaires et nouveaux ont été mis en oeuvre pour en augmenter aussi la fiabilité.

In addition, when determining the adjustment parameters involved in the new regulation process:

• modifications have been made in the methods of calculating the known parameters that are R and P in order to increase the precision thereof
• additional and new parameters have been implemented to also increase reliability.

Thus for the calculation of the resistance R (i) at each end of the regulation cycle i of duration T (between 10 seconds and 15 minutes) at the start of which we give any adjustment commands which modify the resistance level, we divide the regulation cycle i in n elementary cycles of duration t (between 1 second and 15 minutes), the first elementary cycles are eliminated during which the resistance level is modified by the adjustment operations of the anode frame and the mean R (i) over the last na elementary cycles (a <n).

In this case, the average resistance r (k) of this elementary cycle is also calculated at the end of each elementary cycle k of duration t. These r (k) values are stored during the entire supply phase 1 for calculating the slope P (i) keeping the last N values (N being a predetermined number).
Indeed, the resistance slope P (i), the extrapolated slope PX (i) and the curvature C (i) determined at the end of each regulation cycle i of duration T are calculated from the history of average resistances r (k) of the elementary cycles stored since the start of phase 1 of undernourishment within the limit of the last N values and this by any calculation method implementing a smoothing of the raw data r (k) with elimination of the variations of resistance due to adjustment orders of the anode frame.
The computation of the resistance slope and of the auxiliary parameters can be carried out by parabolic regression on the resistances, or by linear regression on the resistance variations, or by any other method equivalent to a nonlinear regression on the resistances.

Preferably, the method used to calculate the resistance slope P (i) consists of a linear regression on the instantaneous resistance variations or slopes dr (k) = r (k) -r (k-1) that we calculates at the end of each elementary cycle k of duration t and after elimination of the elementary cycles during which orders for adjusting the anode frame have been given. This linear regression on the instantaneous slopes dr (k) is equivalent to a parabolic regression on the resistances r (k) after elimination of the variations of resistance due to orders of adjustment of the anode frame.

It should indeed be remembered that resistance evolves along a curve and not along a straight line. However, the slope according to EP 044794 is calculated by directly performing a linear regression on the resistance values measured at regular intervals. As shown in the graph in Figure 3, this necessarily leads to underestimating the real value of the slope. In addition, this default estimation error becomes all the more important as the evolution curve of R is more curved, that is to say that the resistance increases rapidly. Thus according to EP 044794 when the resistance exceeds the high threshold of reference Ro + r of the regulation range, this variation can simply lead to give an order to tighten the anode frame and to extend the power supply at a slow rate while the actual slope P (i) is in fact greater than the slope of reference Po and that an anode effect is then very close.

The new slope calculation method used in the implementation of the present invention is based on the principle of a parabolic regression, which allows a much better approach to the real curve of rise in resistance than a conventional linear regression like the shows the diagram of FIG. 3. If for considerations of complexity and calculation means outside the scope of the invention, the applicant has not implemented exactly this type of regression for the slope calculation, it nevertheless uses a method akin to a parabolic regression consisting in calculating a linear regression line on instantaneous slopes, and the value of the resistance slope P (i) is provided by the ordinate at time t (i) of the line of linear regression on instantaneous slopes.

This new slope calculation procedure also provides additional and new information which is used as auxiliary adjustment parameters in order to optimize the regulation of alumina content.

Knowledge of the linear regression line on instantaneous slopes makes it possible to predict the value of the resistance slope for the cycle i + 1 or extrapolated slope PX (i) which is provided by the ordinate of the regression line extrapolated to l 'instant t (i + 1) = t (i) + T. This extrapolated slope value PX (i) is used to anticipate a rapid rise in resistance and decide to switch to the supply phase at cadence fast CR when this extrapolated slope PX (i) becomes greater than an extrapolated slope of reference PXo so that PX (i) ≥ PXo ≥ Po.

It is also very advantageous to use another auxiliary parameter which is the curvature C (i), that is to say the speed of evolution of the slope of resistance P (i) given by the slope of the straight line linear regression on instantaneous slopes, to trigger and modulate the supercharging itself according to the principle that a high curvature announces a sudden rise in resistance. Thus exceeding the setpoint Co sets off a so-called ultra-fast "CUR" rate supply system. For a curvature lower than Co, the fast-rate supply regime CR controlled by the parameters P (i) and PX (i) is considered sufficient to cause R (i) to drop and avoid an anode effect.

Note that the reference thresholds Po, PXo and Co can take different predetermined or calculated values depending on the operating conditions of the tank (acidity of the bath, temperature, resistance for example).

As an indication, for a 400,000 Amp (400 kA) tank, the value of the reference slope Po is between 10 and 150 pΩ / s, that of the extrapolated slope of reference PXo, is between 10 and 200 pΩ / s, and that of the reference curvature Co is between 0.010 and 0.200 pΩ / s ² ..

• R' = R x 400 / l'
• P'= P x 400 / I'
• C' = C x 400 / I'.

All these operating characteristics valid for a tank of intensity = 400 kA, are easily transposable to tanks of lower intensity knowing that the previous values of resistance R, slope P and curvature C can be defined in relative value compared to at the intensity I '<I traversing these tanks so that

• R '= R x 400 / l'
• P '= P x 400 / I'
• C '= C x 400 / I'.

The invention will be better understood from the detailed description of its implementation below.

EXAMPLE OF IMPLEMENTATION

The method according to the invention was implemented for several months on prototypes of electrolysis cell with precooked anodes supplied with 400,000 amperes under the following conditions:

The alumina is introduced directly into the electrolysis bath fqndu in successive doses of constant mass through several introduction orifices, kept open permanently by a crust breaker. To this end, advantageously, a device for the occasional supply of alumina to the electrolytic cells will be used as described in EP 044794 (= US 4431491) or also in FR 2527647 (= US 4437964) in the name of the applicant.

The resistance R is calculated every tenth of a second from the intensity I and voltage U measurements at the terminals of the tank according to the conventional relationship: $$\text{R ohm =}\frac{{\text{U}}_{\text{volt}}\text{- 1.65}}{{\text{I}}_{\text{Ampere}}}$$

An integrating computer makes it possible to determine the average values of the resistances r (k) every 10 seconds or instantaneous resistances r (k) within a regulation cycle i of duration T = 3 minutes and after elimination if necessary of the first values of the regulation cycle corresponding to the period of the setting orders of the anode frame which modify the resistance level, it calculates the average resistance R (k) of the cycle and the average slopes dr (k) = r (k) - r ( k-1) for the remaining duration of the cycle then determines by linear regression on the stored dr (k) values since the start of phase 1 within the limit of the last N = 360 values, the slope P, the extrapolated slope PX and the curvature C = dP / dt. Then the comparison of the values P, PX and C thus calculated with the respective reference values causes the triggering, via the control-command chain, of the appropriate orders to the alumina dispenser-doser. These reference values are in this case: Po = 66 p Ω / s PXo = 110 p Ω / s Co = 0.065 p Ω / s ²

The average hourly consumption of alumina for a tank of 400,000 amperes is of the order of 230 kg of Al ₂ O ₃ / hour corresponding to the reference rate or theoretical rate of CT supply. In relation to this theoretical rate, we define for example: CL slow rate = CT - 25% or 173 Kg Al ₂ O ₃ / hour used in feed phase 1. CR fast rate = CT + 25% or 288 Kg Al ₂ O ₃ / hour CUR ultra-fast rate = 4 CT or 920 Kg Al ₂ O ₃ / hour used in the feed phase 2.

The tank being under normal operating conditions and the supply being in phase 1, a standard sequence for regulating the alumina level is as follows:
a) We found at the end of cycle i of duration T = 3 minutes R (i) = 5.924 µΩ P (i) = 26 pΩ / s PX (i) = 31 pΩ / s C (i) = 0.028 pΩ / s ² Phase 1 of feeding continues.
b) At the end of cycle i + 1 the values of P (i + 1) and PX (i + 1) remaining below the reference thresholds Po = 65 pΩ / s and PXo = 110 pΩ / s, phase 1 of feeding continues.
c) At the end of cycle i + 2 we have found: R (i + 2) = 5.936 µΩ P (i + 2) = 71 pΩ / s PX (i + 2) = 75 pΩ / s C (i + 2) = 0.022 pΩ / s ² which triggers the transition to phase 2 feeding at a rapid rate CR for a duration of 12 minutes (duration calculated in proportion to the slope at the end of the cycle considered according to the experimentally defined relationship: duration in minutes = 0.083 x P (i) + 6 rounded up to the nearest minute, i.e. in this case: 0.083 x 71 + 6 ∼ 12 minutes).
d) Phase 2 of feeding continues until the start of cycle i + 7 where we return to phase 1 of feeding.
e) At the end of cycle i + 7 we find: R (i + 7) = 5.898 µΩ P (i + 7) = 7 pΩ / s PX (i + 7) = 10 pΩ / sC (i + 7) C (i + 7) = 0.017 pΩ / s ² phase 1 of feeding continues.
f) At the end of cycles i + 8 and i + 9 the values of the slopes P (i + 8) and P (i + 9) and the extrapolated slopes PX (i + 8) and PX (i + 9) remaining less than their reference threshold Po and PXo respective phase 1 supply continues.
g) At the end of cycle i + 10 we have found: R (i + 10) = 5.917 µΩ P (i + 10) = 108 pΩ / s PX (i + 10) = 120 pΩ / s C (i + 10) = 0.067 pΩ / s ² phase 2 of feeding is started with first of all supply in ultra-rapid rate for a predetermined duration of 2 minutes (the duration of feeding in CUR is generally fixed between 1 and 5 minutes to ensure rapid recharging of the bath in alumina without risking its saturation and consequently the clogging of the tank). After 2 minutes phase 2 of feeding goes into rapid rate for a calculated duration of 15 min [0.083 x P (i + 10) + 6 rounded up to the nearest minute].
h) At the end of (2 + 15) = 17 minutes, that is to say during the i + 16 cycle, we return to phase 1 of feeding.
i) At the end of the cycle i + 16 the values of P (i + 16) and PX (i + 16) remaining below the reference thresholds Po and PXo phase 1 of supply continues and more generally the regulation of the rate alumina in the electrolysis bath according to the rules defined above.

The implementation of the process being thus specified, after more than 6 months of application in prototype tanks of 400,000 amperes using an electrolysis bath based on cryolite containing 12% excess of AlF3, therefore of marked acid character. , at a temperature of 950 ° C., the alumina content was permanently adjusted between 1.5% and 3.5% with a central value of 2.1%.

At the same time, the average Faraday yield was 95.6% and the anode effect rate was 0.018 EA / tank / day.

Claims (22)

Method for regulating the alumina content of the bath of an aluminum production tank by electrolysis of alumina dissolved in a molten salt based on cryolite, using an alumina supply at a rate modulated according to the value and of the evolution of the resistance R of the tank calculated from the difference in electrical potential measured at the terminals of the tank, alternating phases of alumina under-supply with introduction of alumina at slow rate CL (phase 1) and alumina supercharging phases with introduction of alumina at a fast CR or ultra-fast CUR rate (phase 2) compared to a reference rate or theoretical rate CT corresponding to the average theoretical consumption of alumina in the tank, characterized by regulation cycles of duration T comprising at each cycle, the following sequence of operations: A / At the end of each regulation cycle i, the average resistance R (i) is calculated, the speed of evolution of the resistance or resistance slope P (i), the speed of evolution of the resistance slope or curvature C (i) and a forecast of the value of the resistance slope at time t (i + 1) or extrapolated slope PX (i) = P (i) + C (i) x T which is an estimate of the future resistance slope P (i + 1) at the end of the regulation cycle i + 1. B / The value R (i) is compared with a setpoint value Ro and orders to move the anode frame are given accordingly, namely reduction of the metal anode distance or tightening, increase of the metal anode distance or loosening. C / The supply of alumina is regulated as a function of the values of the slope P (i), the curvature C (i) and the extrapolated slope PX (i), of so as to compensate in advance for changes in the alumina content. Control method according to claim 1 characterized in that the supply of alumina is regulated, in step C /, as a function of the values of the slope P (i), the curvature C (i) and the extrapolated slope PX (i) compared to reference thresholds Po, Co and PXo. Regulation process according to claim 1 characterized in that the supply of alumina is regulated, in step C /, under the following conditions: • If the supply of alumina is in phase 1, the values P (i), C (i) and PX (i) are compared respectively to reference thresholds Po, Co and PXo: - If P (i) <Po and PX (i) <PXo, phase 1 continues - If P (i) ≥ Po or PX (i) ≥ PXo, we go to phase 2 of alumina supply: . If C (i) ≥ Co, phase 2 begins with an ultra-fast rate feeding for a predetermined or calculated duration, followed by a fast rate feeding for a predetermined or calculated duration, the durations being calculated as a function values calculated at the end of the previously defined regulation cycle . If C (i) <Co, the supply of alumina goes directly to rapid rate for a predetermined duration or calculated according to the values calculated at the end of the previously defined regulation cycle. • If the alumina supply is in phase 2: - phase 2 continues normally according to the duration predetermined or calculated at the end of the previous phase 1. Regulation method according to claim 1 characterized in that the regulation procedure is only authorized when the tank is in normal operating conditions, that is to say correctly adjusted, stable and excluding disturbing operations of operation or adjustment such as anode change, metal casting or specific regulation procedures, and that the regulation procedure begins with a phase 1 of alumina undernourishment. Control method according to one of claims 1 to 4, characterized in that at the end of phase 2 of supply of alumina the tank returns to phase 1 if the tank is in normal operating conditions. Control method according to one of claims 1 to 5, characterized in that at the end of phase 2, the supply of alumina changes to theoretical rate or waiting phase if the tank is not in normal conditions of operation then returns to phase 1 as soon as the tank has returned to normal operating conditions. Control method according to claims 1, 2 or 3, characterized in that if the duration of a phase 1 exceeds a predetermined duration and if the number of release orders during this phase 1 exceeds a predetermined safety threshold, it is detected that the bath is too rich in alumina and then very strongly reduced or completely interrupted the supply of alumina to purge the bath of its excess alumina. Control method according to claims 1, 2 or 3, characterized in that if the number of tightening orders during the same phase 1 exceeds a predetermined safety threshold, phase 2 of alumina feed is started. whatever the values of the resistance slope and the extrapolated slope. Control method according to claims 1, 2 or 3, characterized in that if the curvature exceeds a predetermined safety threshold, phase 2 of alumina supply is started regardless of the values of the resistance slope and the slope extrapolated. Regulation method according to claim 1, characterized in that each regulation cycle i of duration T between 10 seconds and 15 minutes, is divided into n elementary cycles k of duration t between 1 second and 15 minutes. Regulation method according to claims 1 or 10, characterized in that the resistance R (i) calculated at the end of each regulation cycle of duration T is the average of the resistance over the last na elementary cycles of the regulation cycle, c that is to say that the first elementary cycles of the regulation cycle are eliminated during which the regulation can give orders for adjusting the anode frame which modify the resistance level. Regulation method according to claims 10 or 11, characterized in that at the end of each elementary cycle k of duration t is calculated the average resistance r (k) of the elementary cycle and that the successive values r (k) are stored . Method according to claim 12, characterized in that the values r (k) are stored during phase 1, being limited to the last N values. Regulation method according to claims 12 or 13, characterized in that the resistance slope P (i), the extrapolated slope PX (i) and the curvature C (i) determined at the end of each regulation cycle i of duration T are calculated from the history of average resistance r (k) of the cycles elementary by any method using a smoothing of the raw data r (k) with elimination of the resistance variations due to adjustment orders of the anode frame. Regulation method according to claims 1 or 14, characterized in that the calculation of the resistance slope P (i) and of the auxiliary parameters PX (i) and C (i) is carried out by parabolic regression on the resistors or by linear regression on the resistance variations or by any other method equivalent to a nonlinear regression on the resistances. Control method according to claims 1, 14 or 15 characterized in that the method for calculating the resistance slope P (i) and the auxiliary parameters consists of a linear regression on the instantaneous slopes dr (k) = r (k) - r (k-1) after elimination of the cycles during which orders for adjusting the anode frame were given. Regulation method according to claims 1 or 16, characterized in that the value of the resistance slope P (i) is given by the ordinate at time t (i) of the linear regression line on the instantaneous slopes. Regulation method according to claims 1 or 16, characterized in that the forecast of the value of the resistance slope for the cycle i + 1 or extrapolated slope PX (i) is given by the ordinate of the regression line extrapolated to the instant t (i + 1) = t (i) + T. Regulation method according to claims 1 or 16, characterized in that the value of the curvature C (i) is given by the slope of the linear regression line on the instantaneous slopes. Control method according to claims 2 or 3, characterized in that the reference thresholds Po, PXo and Co can take different predetermined or calculated values according to the operating conditions of the tank. Control method according to claims 2 or 3, characterized in that for a 400 kA tank the reference slope Po is fixed between 10 and 150 pΩ / s, the extrapolated reference slope PXo is fixed between 10 and 200 pΩ / s and the reference curvature Co is fixed between 0.010 and 0.200 pΩ / s ² . Control method according to claims 1, 2, 3 or 21, characterized in that the operating characteristics: resistance R, resistance slope P, extrapolated slope PX and curvature C, valid for a tank of intensity I = 400 kA, can be transferred to lower or higher intensity tanks I 'so that: $$\text{R '= R x 400 / I'}$$

$$\text{P '= P x 400 / I'}$$

$$\text{PX '= PX x 400 / I'}$$

and $$\text{C '= C x 400 / I'.}$$

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