EP0716165B1 - Process and apparatus for measuring the temperature and the bath level of molten electrolyte in aluminum winning cells - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to temperature and level measurements. electrolyte based on molten cryolite, in production tanks of aluminum by electrolysis of alumina dissolved in said cryolite as well as the application to the determination of the thickness of the bath of molten electrolysis in these same cells.

STATE OF THE ART

The operation of modern electrolysis tanks for the production of aluminum according to the Hall-Héroult process requires constant monitoring of the temperature and volume of the molten electrolysis bath. Most of the electrolysis bath is in the molten state and constitutes the electrolyte in which the carbon anodes are immersed, the rest of the solidified bath forms the lateral slopes and the crust which cover the free surface of the electrolyte. This electrolyte essentially consists of cryolite Na ₃ AlF ₆ and can contain various additives such as CaF ₂ , AlF ₃ , LiF, MgF ₂ , etc ... having the effect of modifying the melting point, the electrochemical properties as well as the ability of the bath to dissolve alumina.

The volume of the electrolyte covering the layer of liquid aluminum at contact of the cathode at the bottom of the tank, or cathode substrate, must be sufficient to ensure rapid dissolution and distribution of alumina which is introduced at the top of the tank. He ... not must not however exceed a certain level beyond which it disturb the thermal equilibrium of the tank and cause corrosion of the steel logs to which the anodes are fixed and by therefore pollution by the iron of the aluminum produced or metal.

The electrolyte level should therefore be checked periodically. representative of its volume, i.e. the level of the interface air / electrolyte. This measure is also useful, in combination with the measurement of the level of the electrolyte / metal interface, to determine by difference the thickness of the electrolyte, i.e. the thickness of the bath molten electrolysis.

Similarly, knowledge and monitoring of the electrolyte temperature are very important, firstly to properly regulate the functioning of the tank in permanent operating mode corresponding to a balance thermal between the power supplied and the power dissipated, other part to optimize the electrolysis process including yield Faraday, knowing that a simple increase in the temperature of the bath about ten degrees celsius can lower the yield by 1 to 2% Faraday, while conversely a lowering of the electrolyte of about ten degrees celsius can in the area of temperature considered (around 950 ° C) reduce already low solubility alumina in the cryolite and promote the "anode effect", that is to say the anode polarization, with sudden rise in voltage at the tank terminals and release in large quantity of products fluorides from the breakdown of the electrolyte.

• des rejets de gaz fluorés dans l'atmosphère environnante lors des ouvertures de la porte ou des capots de la cuve,
• des conditions de travail avec l'exposition de l'opérateur à ces rejets gazeux,
• de la fréquence peu élevée (1 mesure par 24 à 48 h) de ces mesures difficiles à réaliser, qui ne permet pas un contrôle suffisamment suivi et fiable de la température et du niveau de l'électrolyte par rapport aux nouvelles exigences de conduite des cuves à haute intensité.

These temperature and bath level measurements are carried out manually by an operator who periodically opens the door or tank covers and immerses in the electrolyte a pyrometric rod for temperature measurement, then a steel rod for level measurement and the thickness of the electrolyte. One cannot in fact have recourse to a probe immersed continuously in the electrolyte given its very great aggressiveness. This procedure obviously has many drawbacks, in particular from the point of view:

• releases of fluorinated gases into the surrounding atmosphere when the door or tank covers are opened,
• working conditions with the operator's exposure to these gaseous discharges,
• the low frequency (1 measurement per 24 to 48 h) of these difficult-to-perform measurements, which does not allow sufficiently monitored and reliable control of the temperature and level of the electrolyte compared to the new requirements for operating the vessels high intensity.

However, the prior art, even recent, provides only very solutions incomplete to these problems by totally neglecting the measurement aspect of temperature and recommending, for level or thickness measurements electrolyte, methods whose accuracy remains questionable and moreover implying having an individual level adjustment anode on the tanks. Thus the document EP 0195143 describes a method of measurement of the level of the electrolyte in an electrolytic cell according to which one of the anodes traversed by a given current is gradually measured, the decrease in current is measured as a function of the increase of the interpolar distance, therefore of the lifting height and we note the height for which the current has dropped to a fraction predetermined from its initial value. After calibration we can deduce the electrolyte level. For this we add to the distance traveled by the anode, the initial interpolar distance and a correction term geometric.

In fact this method supposes a very great homogeneity of the electrolyte, but its resistivity varies locally and over time with its composition and in particular with the content of dissolved alumina. By elsewhere this method requires significant movements of the anode which can disturb the operation of the tank when this operation is too often repeated.

Likewise, document EP 0288397 describes a process for controlling the solidified bath additions in an electrolysis tank consisting of periodically determine the thickness of the HB electrolyte which is compared to a HC setpoint and then adjusted accordingly. For get HB it is necessary in an intermediate step to measure the level of the bath relative to a fixed mark and this measurement is carried out by means of a probe associated with a level sensor and equipped with a chisel electrically connected to the cathode of the electrolysis tank. When the chisel pin comes into contact with the interface air / electrolyte there is a significant increase in the pointerolle / cathode potential difference. Regardless of whether this process does not give any operational details for this measurement intermediate level (frequency, accuracy and reliability) given in particular the disruptive effect of the solidified bath deposit on the probe, it does not in any way deal with the essential problem of measurement electrolyte temperature.

In summary, no process or device of the prior art solves completely and satisfactorily the problem of precise measurement and reliable temperature and electrolyte level in the tanks aluminum production by electrolysis in order to get rid of classic manual measurements.

OBJECT OF THE INVENTION

• une précision plus grande des mesures de température à ± 2°C (au lieu de ± 5°C en méthode manuelle) et de niveau de l'électrolyte ± 5 mm (au lieu de ± 10 mm en méthode manuelle) associée à une fiabilité accrue de la conduite des cuves d'électrolyse du fait de la plus grande fréquence des mesures, de préférence toutes les 30 minutes à 48 heures au lieu de toutes les 24 à 48 heures, permettant d'éliminer les mesures anormales intervenant notamment en régime de marche transitoire de la cuve.
• un gain de productivité consécutivement à la disparition du poste de mesure manuelle, associé à une amélioration très sensible des conditions de travail au voisinage des cuves avec la suppression de l'ouverture de la porte ou des capots.

The method of the invention and its device for implementing it not only make it possible to overcome the drawbacks of manual temperature and level measurements of the electrolyte, but also have new advantages resulting from their automation, in particular:

• greater accuracy of temperature measurements at ± 2 ° C (instead of ± 5 ° C in manual method) and electrolyte level ± 5 mm (instead of ± 10 mm in manual method) associated with reliability increased handling of the electrolytic cells due to the greater frequency of measurements, preferably every 30 minutes to 48 hours instead of every 24 to 48 hours, making it possible to eliminate the abnormal measurements occurring in particular in transient operation of the tank.
• a gain in productivity following the disappearance of the manual measuring station, associated with a very noticeable improvement in working conditions in the vicinity of the tanks with the elimination of the opening of the door or the covers.

a) Perçage de la croûte de bain solidifié et immersion à une profondeur suffisante par l'orifice ainsi créé, de l'extrémité d'une sonde de température dans l'électrolyte jusqu'à l'obtention d'une température au moins égale à 850°C et de préférence à 920°C, puis maintien de l'immersion de la sonde pendant une durée prédéterminée inférieure à la durée de mise en équilibre thermique de la sonde avec l'électrolyte,

b) Retrait de la sonde et détermination de la température de l'électrolyte par extrapolation des valeurs de température acquises par la sonde au-delà de 850°C et de préférence de 920°C, selon un programme de calcul préétabli,

c) après dégagement éventuel de l'orifice du passage de sonde précédemment créé et enlèvement du dépôt de bain solidifié sur ladite sonde, mesure du niveau d'électrolyte dans la cuve à partir d'un point de cote de référence, par enregistrement de la variation du potentiel entre le substrat cathodique et la sonde dont la position est déterminée par un potentiomètre et dont le potentiel augmente brusquement lorsque l'extrémité inférieure de la sonde ou pointerolle entre en contact avec l'électrolyte,

d) remontée de la sonde et calcul du niveau de l'électrolyte par le capteur après acquisition des signaux potentiel/position de la pointerolle.

More precisely, the invention relates to a process for measuring the temperature and the level of the molten electrolysis bath, or electrolyte, in a tank for the production of aluminum by electrolysis, according to the Hall-Héroult process, of the alumina dissolved in said electrolyte in contact with carbon anodes and resting on the sheet of liquid metal formed on the cathode substrate and the surface of which in contact with air at the top of the tank is covered with a solidified bath crust, characterized in that that, using an appropriate device, integral but electrically isolated from the superstructure of the tank, provided in particular with means for tapping the solidified bath crust, or tapping device, as well as means for measuring the temperature and the electrolyte level, the following sequence of operations is carried out periodically and preferably at a frequency of 30 minutes to 48 hours:

a) Drilling of the solidified bath crust and immersion at a sufficient depth through the orifice thus created, from the end of a temperature probe in the electrolyte until a temperature at least equal to is obtained. 850 ° C and preferably at 920 ° C, then maintaining the immersion of the probe for a predetermined duration less than the duration of thermal equilibration of the probe with the electrolyte,

b) Removal of the probe and determination of the temperature of the electrolyte by extrapolation of the temperature values acquired by the probe above 850 ° C and preferably 920 ° C, according to a pre-established calculation program,

c) after possible clearing of the orifice of the previously created probe passage and removal of the solidified bath deposit on said probe, measurement of the level of electrolyte in the tank from a reference point of measurement, by recording the variation of the potential between the cathode substrate and the probe, the position of which is determined by a potentiometer and the potential of which increases suddenly when the lower end of the probe or pin points in contact with the electrolyte,

d) raising of the probe and calculation of the level of the electrolyte by the sensor after acquisition of the potential / position signals of the chisel.

The invention also relates to the device suitable for setting up implements the process, namely the stitching and measurement device intended for measure, after piercing the surface solidified bath crust, the temperature and level of the electrolyte in a production tank of aluminum by electrolysis of alumina dissolved in the electrolyte, said device, integral but electrically isolated from the superstructure comprising means for pricking, or pricking, the crust, being characterized in that it is provided with means for measuring the temperature and of the level of the electrolyte constituted mainly by a probe cylindrical moving vertically along its major axis inside stitching means by performing automatically, according to a determined operating sequence, the periodic control of this temperature and this level, and that said stitching means ensure also the removal of the solidified bath deposit on the measurement probe.

The invention according to the method and its implementation device is applicable not only to the electrolyte level measurement but also for measuring the metal level at the electrolyte / metal interface liquid and consequently to the automatic determination of the thickness of the electrolyte HB = HT - HM where HT represents the distance from electrolyte level (air / electrolyte interface) relative to a fixed reference level and HM the distance from the metal level (interface electrolyte / liquid metal) with respect to this same fixed level. In this application the invention constitutes another improvement of the process according to EP 0288397 already analyzed in the prior art of the application.

Due to the short life of immersed thermocouple probes in continuous in the electrolyte due to its very high aggressiveness, but also of the need to increase the frequency of temperature performed manually at the same time as the level measurement of electrolyte, led the applicant to study and to point an automatic temperature and level measurement process the electrolyte with a device suitable for its implementation after have found that temperature measurement at high frequency and with good accuracy is possible by intermittent immersion of a probe thermocouple in the electrolyte for a relatively short time does not not requiring the thermal equilibrium of the probe to be obtained with the electrolyte as soon as we can correctly extrapolate its end temperature rise.

1°) La montée en température de la sonde entre 850°C et 1050°C plage habituelle de travail, obéit à une loi d'évolution dans le temps dont l'asymptote peut être calculée par extrapolation de la courbe obtenue sur une courte période de temps.

2°) Seules les N dernières acquisitions de la sonde indiquant une température supérieure ou égale à 850°C et de préférence supérieure ou égale à 920°C doivent être prises en compte pour déterminer par extrapolation la température d'équilibre ou mesure de température de l'électrolyte.

3°) Le nombre N de ces acquistions de température (N ≥ 10), effectuées généralement toutes les 0,1 à 60 secondes, est limité et donc défini par la condition de sortie de l'électrolyte de la sonde au-delà de 850°C et de préférence de 920°C qui est une vitesse de montée en température inférieure à un seuil prédéfini de préférence compris entre 0,1 et 10°C par seconde. Cette limite est généralement atteinte moins de quelques secondes à quelques minutes avant que la sonde n'ait atteint son équilibre thermique c'est-à-dire la température de l'électrolyte. Ainsi pour une mesure de température la durée totale d'immersion de la sonde dans l'électrolyte dont la température est de l'ordre de 950°C, est comprise entre 30 secondes et 30 minutes sans que sa température ne dépasse généralement 940°C.Ces mesures de température de l'électrolyte par extrapolation de la température d'équilibre de la sonde ont pu être validées par des mesures simultanées de température réalisées avec des sondes à thermocouple de même type, immergées en continu dans l'électrolyte jusqu'à leur destruction et à proximité de l'orifice de passage de la sonde à immersion intermittente. Ainsi il a été possible de s'affranchir des hétérogénéités locales de composition et de température de l'électrolyte et de constater que les écarts de températures mesurées selon les 2 méthodes de contrôle étaient compris dans une fourchette de ± 2°C, qui est l'ordre de grandeur de la précision que l'on peut atteindre avec des thermocouples correctement étalonnés.A noter dans le cas présent que le procédé selon l'invention n'est pas lié à une méthode particulière d'extrapolation de la température d'équilibre. Il inclut aussi toute méthode visant à prédéterminer la température d'équilibre de la sonde à partir d'un temps de maintien de la sonde en immersion qui soit inférieur au temps réel de mise en équilibre de la température de la sonde avec celle de l'électrolyte.Par ailleurs d'autres caractéristiques concernant notamment les conditions de mise en oeuvre de la sonde sont à prendre en compte pour obtenir une mesure de température précise et reproductible.

• Il s'agit tout d'abord de la profondeur d'immersion de la sonde qui doit être définie précisément. En effet une erreur importante peut être commise, due aux pertes thermiques par conduction et par rayonnement le long de la sonde, car la température du point de mesure (en bout de sonde) est toujours inférieure à celle de l'électrolyte en régime permanent. La profondeur d'immersion doit être au moins d'l centimètre.
• Il s'agit aussi du nettoyage régulier de la surface externe de la sonde assuré par le piqueur qui entoure ladite sonde et dont le mouvement de translation vertical provoque le décrochement du dépôt de bain solidifié. Il est important en effet que l'extrémité inférieure de la sonde périodiquement immergée soit régulièrement débarrassée du dépôt de bain solidifié sur sa surface externe. Celui-ci, en augmentant à la fois l'épaisseur et la longueur de la sonde, peut fausser d'une part les conditions d'échange thermique électrolyte/sonde et donc la mesure de température et d'autre part le seuil de détection de la pointerolle lors de son entrée dans l'électrolyte et par suite la mesure de niveau d'électrolyte.

Enfin la fréquence relativement élevée des mesures de température, de préférence toutes les 30 minutes à 48 heures, avec possibilité de sélection et d'annulation des mesures anormales, voire même simplement douteuses, quand elles ont été réalisées au cours d'opérations ponctuelles périodiques qui modifient transitoirement l'état d'équilibre de la cuve, contribue à augmenter la fiabilité du procédé de conduite des cuves.Cette sélection est effectuée par le système de commande et de régulation de la cuve reliée au calculateur qui autorise, après un dégagement de l'orifice de passage de sonde et l'enlèvement par raclage du dépôt de bain solidifié, la mise en oeuvre de la mesure du niveau d'électrolyte par immersion de la pointerolle reliée d'une part à un capteur de déplacement et d'autre part au substrat cathodique, dont la différence de potentiel par rapport audit substrat augmente brutalement lorsque la pointerolle entre en contact avec l'électrolyte.Le capteur procède à l'acquisition de 2 signaux position/potentiel à chaque mesure qu'il transforme en niveau d'électrolyte par rapport à un point de référence exprimé en mm. Ces valeurs de niveau sont ensuite transmises au système de commande et de régulation de la cuve pour détermination du niveau moyen de l'électrolyte après élimination des mesures douteuses ou aberrantes.

To do this, the applicant has highlighted in particular that:

1 °) The temperature rise of the probe between 850 ° C and 1050 ° C usual working range, obeys a law of evolution in time whose asymptote can be calculated by extrapolation of the curve obtained over a short period of time.

2 °) Only the last N acquisitions of the probe indicating a temperature greater than or equal to 850 ° C and preferably greater than or equal to 920 ° C must be taken into account to determine by extrapolation the equilibrium temperature or temperature measurement of the electrolyte.

3 °) The number N of these temperature acquisitions (N ≥ 10), generally carried out every 0.1 to 60 seconds, is limited and therefore defined by the condition of exit of the electrolyte from the probe beyond 850 ° C and preferably 920 ° C which is a rate of temperature rise below a predefined threshold preferably between 0.1 and 10 ° C per second. This limit is generally reached less than a few seconds to a few minutes before the probe has reached its thermal equilibrium, that is to say the temperature of the electrolyte. Thus, for a temperature measurement, the total duration of immersion of the probe in the electrolyte, the temperature of which is of the order of 950 ° C., is between 30 seconds and 30 minutes without its temperature generally exceeding 940 ° C. These electrolyte temperature measurements by extrapolation of the equilibrium temperature of the probe could be validated by simultaneous temperature measurements carried out with thermocouple probes of the same type, continuously immersed in the electrolyte up to their destruction and near the orifice of the intermittent immersion probe. Thus it was possible to get rid of the local heterogeneities of composition and temperature of the electrolyte and to note that the temperature differences measured according to the 2 control methods were included in a range of ± 2 ° C, which is l order of magnitude of the precision that can be achieved with correctly calibrated thermocouples. Note in the present case that the method according to the invention is not linked to a particular method of extrapolation of the equilibrium temperature . It also includes any method aiming to predetermine the equilibrium temperature of the probe from a time of maintaining the probe in immersion which is less than the actual time of equilibration of the temperature of the probe with that of the electrolyte. In addition, other characteristics concerning in particular the conditions of implementation of the probe are to be taken into account in order to obtain a precise and reproducible temperature measurement.

• First of all, it is the depth of immersion of the probe which must be defined precisely. Indeed, a significant error can be made, due to thermal losses by conduction and by radiation along the probe, because the temperature of the measuring point (at the end of the probe) is always lower than that of the electrolyte in steady state. The immersion depth must be at least 1 cm.
• It is also a question of regular cleaning of the external surface of the probe provided by the pricker which surrounds said probe and the vertical translational movement of which causes the deposit of the solidified bath to drop. It is indeed important that the lower end of the periodically submerged probe is regularly freed from the deposit of solidified bath on its external surface. This, by increasing both the thickness and the length of the probe, can distort on the one hand the electrolyte / probe heat exchange conditions and therefore the temperature measurement and on the other hand the detection threshold of the chuck when it enters the electrolyte and consequently the measurement of electrolyte level.

Finally, the relatively high frequency of temperature measurements, preferably every 30 minutes to 48 hours, with the possibility of selecting and canceling abnormal, or even simply questionable, measurements when they were carried out during periodic ad hoc operations which transiently modify the state of equilibrium of the tank, contributes to increasing the reliability of the process of driving the tanks. This selection is made by the control and regulation system of the tank connected to the computer which authorizes, after a release of the orifice for the passage of the probe and the removal by scraping of the solidified bath deposit, the implementation of the measurement of the electrolyte level by immersion of the chisel connected on the one hand to a displacement sensor and on the other hand to the cathode substrate, whose potential difference with respect to said substrate increases abruptly when the die comes into contact with the electrol The sensor acquires 2 position / potential signals for each measurement which it transforms into electrolyte level relative to a reference point expressed in mm. These level values are then transmitted to the control and regulation system of the tank to determine the average level of the electrolyte after elimination of doubtful or aberrant measurements.

IMPLEMENTATION OF THE INVENTION

• une représentation schématique de l'ensemble du dispositif de piquage et de mesure avec ses principales connexions (figure 1).
• une vue en coupe longitudinale de la partie inférieure du dispositif de piquage et de mesure, le piqueur étant en position haute et la sonde en position d'immersion Fig. 2a et le piqueur en position basse et la sonde relevée Fig. 2 b.
• différentes configurations de montage des vérins de piquage et de mesure (fig. 3a, 3b, 3c, 3d) qui ne limitent en aucune manière le champ de l'invention à ces seuls modes de réalisation

The invention will be better understood from the detailed description of its implementation by means of the appropriate device called stitching and measurement with reference to FIGS. 1 to 3 concerning respectively:

• a schematic representation of the entire stitching and measuring device with its main connections (Figure 1).
• a view in longitudinal section of the lower part of the stitching and measuring device, the nozzle being in the high position and the probe in the immersion position FIG. 2a and the nozzle in the low position and the probe raised Fig. 2 b.
• different mounting configurations of the stitching and measuring cylinders (fig. 3a, 3b, 3c, 3d) which in no way limit the scope of the invention to these embodiments only

The stitching and measuring device 1 is intended to measure after piercing of the bath crust 2 solidified the temperature and the level of the electrolyte 3 in contact with the carbon anodes 4 and above the sheet of liquid aluminum or metal 5 resting on the cathode substrate 6. It is integral but electrically isolated from the superstructure 7 of the tank and comprises stitching means 8 formed in their part lower by a hollow cylindrical pricker 9 actuated by at least one cylinder 10 driven by a vertical translational movement for drilling then maintain a passage opening in the crust allowing work of the means 11 for measuring the temperature and the level of electrolyte mainly constituted by a cylindrical probe 12. In its vertical translation movement the pricker 9 ensures at the same time, by scraping, the removal of the deposit 18 of solidified bath on the external surface of said probe. In this respect the clearance between the pricker 9 and the probe 12, according to fig. 2a and fig. 2b, must be sufficient (0.5 to 20 mm radius) to allow their relative displacement without friction but do not must not be too large to avoid the progressive formation of a deposit too much solidified bath on the lower part of the probe 12.

The vertical movement of this mobile probe inside the drill 9 which takes place coaxially to the axis of the pricker is ensured by a jack measure 13. A potentiometer 14 makes it possible to determine with precision the position of the probe in height while simultaneously a voltmeter 15 measures the potential difference between probe 12 and the substrate cathodic 6. A level 16 sensor, especially when the end bottom of the probe or pin 20 comes into contact with electrolyte 3, acquires the 2 signals on each descent and reassembly of the probe, calculates the level of the electrolyte / air interface which is transmitted to the command and control system 17.

The probe 12 consists of an external cylindrical sheath 22, by example in stainless steel, from 100 to 600 mm in length, from 7 to 100 mm of external diameter and whose wall thickness does not exceed 40 mm and is preferably between 2 and 10 mm to reduce losses thermal. In the central recess is placed a thermocouple 21 in its sheath 19. This thermocouple is electrically connected to its upper part to the command and control system 17, which by extrapolation of the probe temperature determines the temperature of the electrolyte.

Several variants of the stitching device have been studied and are represented by fig. 3a, 3b, 3c and 3d which cannot be considered however as a limitation of the invention to these only configurations.

Thus in the configuration according to fig. 3a we replaced the cylinder through rod measurement of displacement of probe 12 by a jack simple to reduce the height of the stitching device and measure and increase the power of the measure movement.

In the configuration according to fig. 3b only a central cylinder 10 is used for tapping and an off-center cylinder 13 for measurement (or conversely a central cylinder for measurement and an offset cylinder for tapping). The advantage is to reduce the number and therefore the cost of the cylinders and above all the overall height and width.

Finally the configuration according to fig. 3c the use of a single cylinder versatile 13, 10 for moving the prick and the probe with a mechanism 23 allowing the prick to be locked allows a reduction in the cost of cylinders, a reduction in overall height and width, in increasing the power of movement of the probe.

As for the simplified configuration according to fig. 3d of replacing the stitching function intended to ensure an opening in the crust of bath solidified by a fixed protection 9 'to maintain a hole in the crust, it simplifies the stitching and measuring device with a single measuring cylinder 13.

• par l'intermédiaire des vérins 10 le piqueur 9 est actionné en descente jusqu'au niveau du bain solidifié pour perçage ou dégagement du trou déjà formé dans la croûte 2 puis au bout de 1 à 5 secondes est relevé
• la sonde 12 en position haute dont l'extrémité inférieure 20 est au moins à 50 cm du niveau de l'électrolyte, est alors activée en descente par le vérin 13 jusqu'à la profondeur d'immersion visée, de préférence 8 à 16 cm, de l'extrémité inférieure ou pointerolle 20.

These structural characteristics being specified, the tapping and measuring device 1 for the temperature and the level of the electrolyte 3 is implemented at regular intervals, generally every 30 minutes to 48 hours, in the following manner for carrying out the aluminum production tanks:

• by means of the jacks 10 the pricker 9 is actuated downward to the level of the solidified bath for drilling or clearing the hole already formed in the crust 2 then after 1 to 5 seconds is raised
• the probe 12 in the high position, the lower end 20 of which is at least 50 cm from the level of the electrolyte, is then activated in descent by the jack 13 to the desired immersion depth, preferably 8 to 16 cm , from the lower end or chisel 20.

The immersion time of the probe in the electrolyte, including the temperature according to the composition is about 950 ° C, corresponds to the time acquisition by the probe of at least the temperature of 850 ° C and preferably 920 ° C, plus the time required to obtain, from this temperature, with a very low heating rate of the probe, for example less than 3 ° C / second.

When this threshold is reached, the probe is raised to its initial position and the successive values of temperature measured by the thermocouple 21 are transmitted to the command and regulation system 17 which determines, by extrapolation from the N different pairs of values (ti, Ti) temperature / time, the temperature Tb of the electrolyte.

To carry out the electrolyte level measurement, actuate by safety the pricker 9 downhill to ensure cleaning and passage of probe 12 then its ascent which authorizes the engagement of the electrolyte level measurement sequence. This includes the acquisition by the level sensor 16 of the potential of the probe 12 by relative to the cathode substrate 6 as well as the signal from the potentiometer 14.

When the probe 12 descends the potential with respect to the cathode 6 increases sharply when the chisel 20 comes into contact with the bath 3, then relapse when this same punch leaves the electrolyte when the probe is lifted after a period of immersion not exceeding preferably 20 seconds. These potential variations are recorded by the level sensor which accurately determines when the probe dives into the electrolyte and calculates the thickness of the electrolyte after filtering and smoothing the recording curve in order to eliminate the parasitic effects which could disturb the potentiometer and chisel. The value thus calculated is then transmitted to the control and regulation 17.

ADVANTAGES AND APPLICATIONS OF THE INVENTION

Besides the fact that it is possible to perform with a probe, without manual intervention and without risk of pollution, more than 2000 measurements temperature to ± 2 ° C and this with increased reliability of the pipe tanks due to the increased frequency of temperature and level as well as the choice of the moment to realize them outside the periods of transient regime of the electrolytic cells, the method and device according to the invention can also be adapted to measuring the level of the electrolyte / metal interface. Indeed so analogous you can record by pushing the probe into the sheet of metal a new potential variation between the substrate cathodic and the pointerolle of the probe when this crosses the electrolyte / metal interface. This variation results in a strong decrease in potential probe-metal / cathode difference compared to the previously recorded potential probe-electrolyte / cathode difference due to the significant decrease in resistance of the new medium.

So we can quickly determine from the same origin, by 2 successive series of electrolyte level measurements and metal level, the average level of the HT electrolyte and the average level of the metal HM and deduce therefrom HB = HT - HM the thickness of the electrolyte of which we want to precisely regulate the volume by adding ground bath solid or electrolyte sample. This method of determining the thickness of the electrolyte is obviously faster than that recommended by EP 0288 397 based on the indirect determination of the level metal from the poorly defined anode plane and wear rate anodes. In this regard the application of the method and device of the invention in measuring the thickness of the electrolyte with a view to its regulation is both a complement and an improvement to process according to EP 0288397.

Claims (23)

1. Method for measuring the temperature and the level of the molten electrolysis bath, or electrolyte, in a cell for production of aluminium by electrolysis, according to the Hall-Héroult process, of alumina dissolved in said electrolyte in contact with the carbonaceous anodes and resting on the sheet of liquid metal formed on the cathodic substrate, and the surface of which in contact with the air at the upper part of the cell is covered by a crust of solidified bath, characterised in that with the aid of an appropriate device, integral but electrically insulated from the superstructure of the cell, provided in particular with means for breaking the crust of solidified bath, or a crust-breaker, as well as means for measuring the temperature and the level of the electrolyte, the following sequence of operations is periodically carried out:

   a) Piercing of the crust of solidified bath and immersion into the electrolyte, through the aperture thereby created, of the extremity of a temperature probe to a sufficient depth until a temperature of at least 850°C, and preferably 920°C is obtained, then, from this temperature. maintaining the immersion of the probe for a pre-determined length of time, which is less than the time taken to establish the thermal equilibrium of the probe with the electrolyte,

   b) withdrawal of the probe and determination of the temperature of the electrolyte by extrapolation of the temperature values established by the probe above 850°C and preferably 920°C, according to a pre-established computation program.

   c) after optional clearing of the aperture for the passage of the probe previously created, and removal of the solidified bath deposit from said probe, measurement of the level of electrolyte in the cell from a datum point by recording the variation in potential between the cathodic substrate and the probe, the position of which is determined by a potentiometer, and the potential of which increases rapidly when the lower extremity of the probe or tip comes into contact with the electrolyte,

   d) raising of the probe and calculation of the level of the electrolyte by the sensor after establishment of potential/position signals from the tip.
2. Method according to claim 1, characterised in that the sequence of operations for measuring the temperature and the level of the electrolyte is carried out according to a periodicity of 30 minutes to 48 hours.
3. Method according to claim 1, characterised in that the length of time the probe is kept in the electrolyte at above 850°C and preferably 920°C is defined by the condition of withdrawal of the probe, which is a speed of temperature increase less than a pre-determined threshold, preferably between 0.1 and 10°C per second.
4. Method according to claim 1, characterised in that for a measurement of the temperature, the total immersion time of the probe in the electrolyte is between 30 seconds and 30 minutes.
5. Method according to claim 1, characterised in that the depth of immersion of the extremity of the probe in the electrolyte is at least 1cm, and preferably 8 to 16 cm.
6. Method according to claim 1, characterised in that the removal of the deposit of solidified bath on the external surface of the probe is regularly carried out with the aid of a crust-breaker driven by a vertical translation movement.
7. Method according to claim 6, characterised in that during each measurement of the level the extremity of the probe or tip is immersed in the electrolyte for a duration preferably not exceeding 20 seconds.
8. Device for crust-breaking and measuring (1) intended for measuring, after piercing of the superficial crust (2) of solidified bath, the temperature and the level of the electrolyte (3) in a cell for production of aluminium by the electrolysis of alumina dissolved in the electrolyte, said device, integral with but electrically insulated from the superstructure (7) of the cell, comprising means for breaking the crust, or a crust-breaker (8), being characterised in that it is provided with means (11) for measuring the temperature and the level of the electrolyte (3) principally constituted by a cylindrical probe (12) moving vertically along its main axis inside crust-breaking means (8), carrying out, in an automatic manner according to a determined sequence of operations, the periodic monitoring of this temperature and of this level, and that said crust-breaking means also make possible the removal of the deposit (18) of solidified bath on the measuring probe.
9. Device for crust-breaking and measuring according to claim 8, characterised in that the crust-breaking means (8) are formed, in their lower part, by a cylindrical hollow crust-breaker (9), operated by at least one crust-breaking actuator (10) and driven by a vertical translation movement.
10. Means of crust-breaking and measuring according to claim 8, characterised in that the means (11) for measuring the temperature and the level of the electrolyte are principally constituted by a cylindrical probe (12) moveable inside the crust-breaker (9), the vertical displacement of which coaxially to the axis of the crust-breaker is made possible by a measuring actuator (13).
11. Device for crust-breaking and measuring according to claim 8 or 10, characterised in that a potentiometer (14) is fixed integrally to the rid of the actuator (13) to determine the position of the probe (12).
12. Device for crust-breaking and measuring according to any one of claims 8 or 10, characterised in that a voltmeter (15) measures the difference in potential between the probe (12) and the cathodic substrate (6).
13. Device for crust-breaking and measuring according to any one of claims 8, 10, 11 and 12, characterised in that a level sensor (16) connected electrically to the voltmeter (15) and to the potentiometer proceeds with the establishment of the signals for potential/position of the probe (12) and calculates the level of air/electrolyte interface, or electrolyte level, with each lowering and raising of the probe.
14. Device for crust-breaking and measuring according to any one of claims 8, 10, 11, 12 and 13, characterised in that the probe (12) is constituted by an external cylindrical casing (22), 100 to 600 mm long and 7 to 100 mm in external diameter, with a wall thickness not exceeding 40 mm.
15. Device for crust-breaking and measuring according to claim 14, characterised in that the external cylindrical casing (22) of the probe (12) has a wall thickness of preferably between 2 and 10 mm.
16. Device for crust-breaking and measuring according to one of claims 8, 10, 11, 12, 13 and 14, characterised in that the cylindrical casing (22) contains a thermocouple (21) in its casing (19), connected electrically by its upper part to the control and regulation system (17).
17. Device for crust-breaking and measuring according to claim 9 or 10, characterised in that the clearance between the crust-breaker (9) and the cylindrical probe (12) has a radius of between 0.5 and 20 mm.
18. Device for crust-breaking and measuring according to any one of claims 8, 9 or 10, characterised in that the measuring actuator (13) is central and preferably has a cross rod.
19. Device for crust-breaking and measuring according to any one of claims 8, 9 or 10, characterised in that the measuring actuator (13) is off-centre and that the sole crust-breaking actuator (10) is central.
20. Device for crust-breaking and measuring according to any one of claims 8, 9 or 10, characterised in that it comprises a single polyvalent actuator (13 or 10) for measuring and crust-breaking.
21. Device for crust-breaking and measuring according to any one of claims 8 or 10, characterised in that the measuring actuator (13) is central and that the crust-breaking means (8) intended to produce an opening in the crust (2) are constituted by a permanent fixed protector (9').
22. Application of the method of measuring the level of the electrolyte according to claims 1 to 7 to the measurement of the level of liquid metal in the electrolysis cell.
23. Application of the method of measuring the level of the electrolyte and of the metal according to claims 1 to 7 and 22 to the determination of the thickness of the electrolyte by difference in the measurements of the level of the electrolyte and the level of metal.

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