EP0085999A1 - Regulation de vice for a group of electrolytic cells with mercury cathode - Google Patents

Abstract

1. Installation for controlling the anode-cathode distances in an assembly of electrolysis cells which each comprise anode units (4, 4', 4", 5, 5', 5") movable with respect to a mercury cathode and motors for moving the anode units individually with respect to the cathode, which installation comprises on the one hand, local control units (8) which are each associated with a cell (1, 2, 3) of the assembly of cells and which each comprise : a device (11) for measuring the electrical conductance, a converter (12) of the electrical conductance into an electrical signal, a detector (14) of local operating conditions of the cell with which it is associated, a converter (15) of the detected local operating conditions into electrical signals, a circuit (18) for comparing the signals of the converter (12) with reference signals representative of reference inputs for the electrical conductance, a comparison memory (19) coupled with the comparison circuit (18), a programmer (20) designed for accomplishing the operation of the local unit in two stages comprising, in a first stage, a selective connection of the anode units to its measuring device (11) and a coupling of its comparison circuit (18) with its comparison memory (19) and, in a second stage, a selective coupling of the anode unit motors with its comparison memory (19) ; on the other hand, a central control unit (9) which comprises : a device (21) for coupling the central control unit (9) with the local control units (8), a local operation memory (22) for storing the signals originating from the converter (15) of the local units (8), a reference circuit (23) designed for defining reference signals from the signals of the local operation memory (22) of the central control unit (9), a reference memory (24) for storing the reference signals originating from the reference circuit (23), a programmer (26) programmed for coupling the central control unit (9) separately and successively with each control unit (8) and connecting the local operation memory (22) and the reference memory (24) of the central unit (9) to the converter (15) and the comparison circuit (18) respectively of the local unit (8) coupled with the central unit (9).

Description

The present invention relates to an installation for regulating a group of electrolysis cells with multiple anodes which can be displaced opposite a mercury cathode, in particular cells for the electrolysis of aqueous solutions of alkali metal halide, and more particularly sodium chloride.

During the operation of a mercury cathode electrolysis cell, it is important to reduce the distance between the anodes and the cathode to as small a value as possible, in order to reduce the consumption of electrical energy and thus make energy efficiency of the optimum electrolysis operation. In particular, in the case of cells equipped with metal anodes, for example anodes of the type described in patent BE-A-811,155 (IMPERIAL CHEMICAL INDUSTRIES LIMITED), it is usual to adjust the anode-cathode distance from the neighborhood and even below 2 mm.

The implementation of small anode-cathode distances means that the position of the anodes in the electrolysis cells must be checked periodically and an adjustment of the anodes may be necessary. In order to obtain optimum energy efficiency, it is indeed necessary to correct the position of the anodes whose distance from the cathode would become exaggeratedly great. It should also be avoided that one or the other anode occasionally comes into contact with the mercury cathode, because the short-circuit which would result therefrom could cause serious degradation at the anode, mainly in the case of titanium anodes carrying an active coating based on noble metal oxide. The causes of an untimely increase in the anode-cathode distances or of a fortuitous contact between an anode and mercury are numerous; they may reside in particular in a deformation or wear of the anode, the formation of agglomerates of large mercury (sometimes called "mercury butter") adhering to the bottom of the cell or floating on the surface of the mercury, an untimely variation of the level of mercury in the cell, a fortuitous turbulence occurring in the flow of mercury.

In patent BE-A-668 236 (IMPERIAL CHEMICAL INDUSTRIES LIMITED), a method for adjusting the position of an anode in a mercury cathode cell is described, according to which the electrical conductance of the separating electrolyte layer is measured. the anode of the cathode, the measured value of the conductance is compared to a set value and the position of the anode is adjusted so that these two values are equal.

In patent BE-A-695,771 (IMPERIAL CHEMICAL INDUSTRIES LIMITED), an apparatus is described implementing this process for the automatic regulation of the position of the anodes of a mercury cathode cell. This known apparatus includes a carriage which is movable above the cell and which carries a member for selective scanning of the individual anodes, a member for measuring the instantaneous value of the electrical conductance of the electrolyte layer between the scanned anode. and the cathode, a transformer of said instantaneous value into an electrical signal, for example a voltage, a circuit for comparing this electrical signal with a setpoint signal representative of a setpoint of electrical conductance and a motorized adjustment device of the position of the scanned anode, coupled to this comparison circuit

The apparatus is programmed so as to selectively and successively scan all the anodes of the cell and execute a complete sequence of process operations, for each anode scanned.

In operating this installation, the set value is a fixed value of the electrical conductance of the electrolyte layer, corresponding to a predetermined anode-cathode distance. It is generally the result of an acceptable compromise between the search for optimum energy efficiency and the search for a sufficient level of safety in the operation of the cell. It must be defined in each particular case according to the operating conditions of the cell.

With the aim of ensuring the regulation of a group of mercury cells, for example of an industrial electrolysis workshop, it has already been imagined to associate with each cell of the group an autonomous regulation apparatus.

Thus, in patent US-A-4,212,721 (HOECHST AG), an installation is described for the regulation of a group of mercury electrolysis cells, comprising a set of local regulation units which are each associated with an individual electrolysis cell and which are linked together to a central control unit designed to collect the general operating data of the group of cells and transfer them to the local regulation units.

This known regulation installation has the particularity of being entirely decentralized, because each local regulation unit collects information on the proper functioning of the cell with which it is associated and regulates only the functioning of this cell, at the start. of this information, without interaction with other local units. This particularity leads to a series of disadvantages, among which a high cost of implementation, a relatively long response time of the local regulation units and the impossibility of ensuring an ordered regulation of the whole group of cells.

The invention seeks to remedy these disadvantages by providing a partially centralized installation which provides automatic, rapid, orderly and reliable regulation of the anode-cathode distances of a group of electrolysis cells.

• - un organe de scrutation sélective des unités locales de régulation,
• - un circuit de consigne pour le traitement des signaux provenant du convertisseur de l'unité locale scrutée et la définition, au moyen de ceux-ci, du signal de consigne,
• - un organe de transmission du signal de consigne vers le circuit de comparaison de l'unité locale scrutée.

To this end, the invention relates to an installation for regulating a group of electrolysis cells with multiple anodes which can be displaced opposite a mercury cathode, comprising local regulation units which are each associated with an electrolytic unit of the group. cells and which each comprise, a member for selective scanning of anode units which can be moved individually in the electrolytic unit, a member for measuring the instantaneous value of the electrical conductance of the electrolyte layer between the scanned anode unit and the cathode, a transformer of said value into an electrical signal, a circuit comparing said signal with a setpoint signal representative of a setpoint of the electrical conductance, and a motorized adjustment device for the position of the scanned anode unit, connected to the comparison circuit; according to the invention, each local regulation unit further comprises a detector of local operating conditions of the electrolytic unit and a converter of the local operating conditions detected into electrical signals, and the local regulation units of the group of cells are coupled to a central regulation unit which includes:

• - a body for selective scrutiny of local regulatory units,
• - a setpoint circuit for processing the signals from the converter of the local unit scanned and defining, by means of these, the setpoint signal,
• - a device for transmitting the setpoint signal to the comparison circuit of the scanned local unit.

In the installation according to the invention, the electrolytic unit generally consists of a group electrolysis cell, a defined set of group cells, for example a pair of cells electrically coupled in series, a defined area of a cell, for example a group of several anodes connected to a common current collector.

The anode unit can be an individual anode or a group of anodes movable together with respect to the mercury cathode, for example a group of anodes fixed together, in bypass, to a rigid and movable current collector.

In each local regulation unit, the organ for scanning the anode units generally consists of a logic circuit designed to connect the anode units of the electrolytic unit successively, separately and in a predetermined order, with the conductance measuring member. and with the local operating conditions detector.

The conductance measuring device, the transformer and the comparison circuit are coupled together and can advantageously be combined in a single device, of the type described. in the aforementioned patent BE-A-668,236, comprising a device for measuring the intensity of the electric current passing through the scanned anode unit, a device for measuring the voltage between this anode unit and the mercury cathode, a computer coupled to these two measuring devices and designed to subtract, from the measured voltage, the reversible potential of the electrolysis reaction, divide the result by the measured value of the current intensity and output a voltage signal representative of the result of the division, and a computer designed to subtract a setpoint voltage from this voltage signal and output an electrical signal representative of the result of this subtraction.

The motorized adjustment device generally consists of an electric motor controlled by the output signal of the comparison circuit to move the scrutinized anode unit away from the cathode or bring it closer according to whether the instantaneous value of the conductance is higher or lower than the setpoint.

The detector may include, for example, thermocouples to measure the temperature of the electrolyte or mercury in the electrolytic unit, densimeters to measure the density of the electrolyte and thus define its concentration, flow meters to measure the flow rates of the electrolyte and mercury in the electrolytic unit, ammeters and voltmeters to measure the overall intensity of the electrolysis current in the electrolytic unit and the voltage across it.

In the central regulation unit, the organ for scanning the local units generally consists of a logic circuit, designed to couple the local regulation units successively and separately with the central unit, in a predetermined order. This circuit is also designed to transfer the signals emitted by the converter from the scanned local unit into the setpoint circuit of the central unit and to transfer the setpoint signals from the central unit to the local scanned unit. .

The setpoint circuit is used to define the setpoint value of the electrolyte conductivity for each anode unit of the electrolytic unit whose local regulation unit is scanned. It consists of an analog or digital computer which is supplied by the signals coming from the converter of the scanned local unit and which is designed to calculate, as a function of local conditions prevailing in the scanned electrolytic unit, the value of the conductance of the electrolyte layer for an imposed anode-cathode distance (conductance setpoint).

In the installation according to the invention, the local regulation units fulfill a double function: on the one hand, they serve to measure the electrical conductance of the electrolyte layer under each anode unit, transform this measurement into an electrical signal, compare this to a setpoint signal and activate the motorized adjustment device of the anode unit, depending on the result of the comparison; on the other hand, they are used to record the local operating conditions relating to each anode unit and to convert them into electrical signals which are transferred to the central regulation unit.

The function of the central regulation unit is to calculate the set values of the anode units, starting from the local operating conditions recorded by the local regulation units, and to transfer these set values to the local regulation units.

In practice, in an electrolysis cell with a mercury cathode, in operation, the optimum value of the anode-cathode distance is rarely identical for all the anodes. It generally differs from one anode to another, depending in particular on the geometry of the anode, its degree of wear or its position in the cell.

Furthermore, all other things remaining equal, the optimum value of the anode-cathode distances in a mercury cell is often influenced by the position of the cell among a group of cells.

Taking into account these observations, in a particularly advantageous embodiment of the invention, the detector associated with each local regulation unit comprises a member for locating the position of the anode unit scanned in the electro unit lytic, and the central regulating unit comprises a member for locating the position, in the group of cells, of the electrolytic unit associated with the scanned local unit.

In this embodiment of the installation according to the invention, the setpoint circuit associated with the central unit is supplied with additional signals defining the position of the scanned anode unit and of the electrolytic unit which contains it and it is thus able to make an additional correction in the definition of the set value.

Generally, in active electrolysis cells, the anode units can be divided into three categories, according to the distance which separates them from the cathode. A first category includes anode units which occupy a position for which this distance is close to the ideal value and which therefore do not require an adjustment; a second category of anode units consists of those which occupy a position which is excessively far from the cathode and which should be adjusted if the aim is to improve the energy efficiency of electrolysis; the third category of anode units includes those which occupy a position which is excessively close to the cathode and which therefore require rapid intervention to avoid a local short-circuit and deterioration of the anode.

In practice, it is desirable to ensure, in order, first an adjustment of the anode units of the third category, then an adjustment of those of the second category.

According to a preferred embodiment of the installation according to the invention, specially designed for the implementation of this ideal mode of regulation, each local regulation unit comprises, on the one hand, between the comparison circuit and the device motorized adjustment, a comparison memory for storing the signals coming from said comparison circuit and, on the other hand, a programmer designed to carry out the operation of the local unit in two phases comprising, in a first phase, a scanning of the units anode and a coupling of the comparison circuit with the comparison memory and in a second phase, a scan of the anode units and a coupling of the comparison memory with an actuator of the motorized adjustment device.

In this embodiment of the invention, the first scanning phase is used to establish the comparison signals of the anode units and the second scanning phase is used exclusively for adjusting the anode units from these signals. It is possible to program the programmer so that, for example, during the second scanning phase, the anode units are scanned successively one after the other in the order of increasing anode-cathode distances.

To this end, according to an advantageous variant of this embodiment of the invention, the comparison memory of each local regulation unit comprises a distributor of the signals stored in an order corresponding to increasing anode-cathode distances and its programmer is coupled to the dispatcher so as to execute the scan in the second phase, in the order of storage of the signals in the comparison memory.

According to a particularly advantageous embodiment of the installation according to the invention, each local regulation unit comprises a memory for the storage of the setpoint signals (which, in the following, will be called "setpoint memory"), a memory for the storage of signals from the transformer (which will hereinafter be designated "conductance memory") and a memory for the storage of signals from the converter (which will hereinafter be designated "local operating memory"); in addition, the central regulation unit comprises a memory for the storage of the signals coming from the converter of the local regulation units and a memory for the storage of the signals coming from the setpoint circuit (in the following, these two memories are called respectively " local operating memory "and" setpoint memory ").

This embodiment of the invention allows greater flexibility in the operation of the installation, in particular allowing the local units and the central regulation unit to perform several functions simultaneously.

• - une étape de scrutation d'une unité locale de régulation avec couplage de la mémoire de marche locale de cette unité locale de régulation à la section afférente à cette unité locale dans la mémoire de marche locale de l'unité centrale;
• - une étape de couplage du circuit de consigne de l'unité centrale de régulation avec la section afférente à une unité locale de régulation, dans la mémoire de marche locale de l'unité centrale;
• - une étape de scrutation d'une unité locale de régulation avec couplage de la mémoire de consigne de cette unité locale à la section afférente à cette unité locale dans la mémoire de consigne de l'unité centrale de régulation.

Thus, according to an interesting variant of this embodiment of the invention, each memory of the central regulation unit is divided into several storage sections each corresponding to a local regulation unit, and the central regulation unit is programmed. so as to carry out successive operating sequences of three stages comprising:

• a step of scanning a local regulation unit with coupling of the local walking memory of this local regulation unit to the section relating to this local unit in the local walking memory of the central unit;
• a step of coupling the reference circuit of the central regulation unit with the section relating to a local regulation unit, in the local operating memory of the central unit;
• a step of scanning a local regulation unit with coupling of the reference memory of this local unit to the section relating to this local unit in the reference memory of the central regulation unit.

In this embodiment of the invention, the three steps of each operating sequence can be carried out on the same local regulation unit or on separate local units, and they can be carried out simultaneously or separately, according to the constructive characteristics of the central unit.

According to a particular embodiment of this embodiment of the invention, it is however preferred that the two steps first mentioned above are executed on the same local control unit, while the third step is performed on a other local unit. It is moreover preferred that the central regulation unit executes the step cited second, after having executed the other two steps.

The installation according to the invention, mainly in its embodiment which has just been described, has the great advantage of shortening the overall duration of the control and adjustment of the anode units and, consequently, of increasing the frequency of these checks and adjustments. It follows that under normal operating conditions of the electrolysis cells, the anode units permanently occupy positions close to the optimum and therefore require only moderate adjustments. This feature allows the use, for the motorized adjustment device, of electric motors with a slow speed of rotation and consequently of low power and small size, the cost, electricity consumption and maintenance costs of which are moderate.

The possibility offered by the installation according to the invention to accommodate electric motors of moderate power brings the additional advantage of greatly simplifying their electrical control circuit. This easily and advantageously accommodates electronic semiconductor components of the thyristor type or of the TRIAC type, depending on whether the motors used are direct current or alternating current, the semi-conductors of the TRIAC type being assemblies equivalent to two opposite thyristors, fitted with a single trigger trigger, (Engineering techniques, electronics E-1022, 3-1972, p. 10).

In a particularly advantageous embodiment of the installation according to the invention, the motorized adjustment device consists of alternating current motors of the synchronous type, possibly fitted with a speed reducer. Because they are characterized by a constant speed of rotation, the use of synchronous motors has the advantage of facilitating the control of the amplitude of movement of the anode units, by simply carrying out a measurement of the operating time of the motors. .

• - une première scrutation avec couplage de la mémoire de comparaison de l'unité locale de régulation avec l'organe d'actionnement du dispositif de réglage motorisé de l'unité anodique : au cours de cette première scrutation, le dispositif de réglage motorisé de l'unité anodique est démarré automatiquement dans le sens adéquat en fonction du signal de comparaison afférent à cette unité anodique ;
• - des scrutations ultérieures répétées, avec couplage de l'organe de mesure, du transformateur, du circuit de comparaison et du dispositif de réglage motorisé de l'unité anodique scrutée : au cours de chacune de ces scrutations, l'unité locale de régulation opère une comparaison entre la conductance instantanée mesurée et la valeur de consigne et elle arrête le dispositif de réglage motorisé si la différence entre ces deux grandeurs tombe, en valeur absolue, sous une valeur de seuil prédéterminée.

The use of synchronous motors has proved to be particularly advantageous in the preferred embodiment described above, where the local regulation units operate in two successive phases respectively comprising a first phase of scanning the anode units used for establishing the signals. of comparison relating to the anode units and a second phase of scanning used for the adjustment of the anode units from these comparison signals. In fact, in a variant of this preferred embodiment, the programmer of each local regulation unit is programmed so that, in the second polling phase, the polling organ operates, to each anode unit, a sequence of successive scans comprising:

• - a first scan with coupling of the comparison memory of the local regulation unit with the actuating member of the motorized adjustment device of the anode unit: during this first scan, the motorized adjustment device of the the anode unit is started automatically in the appropriate direction as a function of the comparison signal relating to this anode unit;
• - repeated subsequent scans, with coupling of the measuring device, the transformer, the comparison circuit and the motorized adjustment device of the scanned anode unit: during each of these scans, the local regulation unit operates a comparison between the instantaneous conductance measured and the set value and it stops the motorized adjustment device if the difference between these two quantities falls, in absolute value, below a predetermined threshold value.

This variant of the invention has the remarkable advantage of allowing the adjustment of several anode units simultaneously on each electrolytic unit, since between two successive scans of an anode unit, the polling member can scan other anode units of the electrolytic unit and start, if necessary, their respective motorized adjustment devices.

Special features and details of the invention will appear during the following description of some embodiments, with reference to the accompanying drawings.

Figure 1 is a schematic plan view of a group of mercury cathode electrolysis cells, connected to a regulating installation according to the invention;

FIG. 2 is a scale of the signals for comparing the anodes of a cell of the group of electrolysis cells.

FIG. 1 shows a group of three mercury cathode 1,2,3 cells for the electrolysis of aqueous solutions sodium chloride. These electrolysis cells are of the horizontal mercury cathode type (JSSconce, Chlorine, its manufacture, properties and uses, 1962, Reinhold Publishing Corporation, New York, pages 181 to 194). They include a movable, slightly inclined mercury cathode, above which anodes such as 4, 4 ', 4 ", 5, 5', 5" are suspended by individual support rods 6. The anodes are distributed in several Pampled rows of anodes coupled in bypass to current supply bus bars 7 (for example the row of anodes 4, 4 'and 4 "and the row of anodes 5, 5' and 5"). The support rods 6 of the anodes are movable vertically and individually to adjust the distance between each anode and the cathode and, for this purpose, an electric motor, not shown, is associated with each anode rod 6. To avoid deterioration motors in contact with the corrosive atmosphere generally prevailing around the cells, each motor is embedded in a mass of synthetic resin poured around the motor and crossed in leaktight manner by the motor shaft.

The installation for regulating the group of cells 1, 2, 3 comprises three local regulation units 8, each associated with an individual electrolysis cell and a central regulation unit 9 associated with the three local units 8.

• - un organe de scrutation schématisé en 10, qui a pour fonction de brancher les anodes de la cellule, l'une après l'autre, sur les organes fonctionnels de l'unité locale ;
• - un organe de mesure 11 de la conductance de la couche d'électrolyte séparant l'anode scrutée de la cathode ;
• - un transformateur 12 couplé à l'organe de mesure 11 et destiné à convertir la valeur numérique de la conductance en un signal de tension électrique :
• - une mémoire de conductance 13, associée au transformateur 12;
• - un détecteur 14 de conditions locales de marche de la cellule associée à l'unité locale ;
• - un convertisseur 15 des valeurs numériques relevées par le détecteur 14, en signaux de tension électrique ;
• - une mémoire de marche locale 16 associée au convertisseur 15;
• - une mémoire de consigne 17;
• - un circuit de comparaison 18 associé aux mémoires 16 et 17;
• - une mémoire de comparaison 19 associée au circuit de comparaison 18 ;
• - un programmateur 20.

Each local unit 8 is designed to scan successively and separately each anode of its cell, in a predetermined logical order, measure the conductance of the electrolyte layer between the scanned anode and the cathode, compare the measured conductance to a set value and modify, if necessary, the position of the scanned anode as a function of the result of the comparison. To this end, the local units 8 each include:

• - A scanning organ schematically at 10, which has the function of connecting the anodes of the cell, one after the other, to the functional organs of the local unit;
• a member 11 for measuring the conductance of the electrolyte layer separating the scanned anode from the cathode;
• - a transformer 12 coupled to the measuring member 11 and intended to convert the digital value of the conductance into an electrical voltage signal:
• - a conductance memory 13, associated with the transformer 12;
• a detector 14 of local operating conditions of the cell associated with the local unit;
• - a converter 15 of the digital values recorded by the detector 14, into electrical voltage signals;
• - a local walking memory 16 associated with the converter 15;
• - a setpoint memory 17;
• - a comparison circuit 18 associated with memories 16 and 17;
• - a comparison memory 19 associated with the comparison circuit 18;
• - a programmer 20.

In each local regulation unit 8, the memories 13, 16, 17 and 19 are each divided into several storage sections 13a, 13b, ..., 16a, 16b, ..., 17a, 17b, ..., 19a , 19b, ..., each storage section, in each memory, corresponding to an individual anode of the electrolysis cell associated with the local regulation unit. For example, in the local unit 8 associated with cell 1, storage sections 13a, 16a, 17a and 19a each correspond to the only anode 4 of this cell 1.

• - un organe de scrutation 21 destiné à brancher sélectivement et successivement chaque unité locale 8 sur l'unité centrale 9 ;
• - une mémoire de marche locale 22, destinée à être couplée sélectivement aux mémoires de marche locale 16 des unités locales 8;
• - un circuit de consigne 23 qui a pour fonction de produire les signaux de consigne nécessaires au fonctionnement des unités locales 8 ;
• - une mémoire de consigne 24 associée au circuit de consigne 23;
• - un organe de repérage 25 de l'unité locale 8 scrutée ;
• - un programmateur 26.

The central unit 9 has the function of calculating the set values relating respectively to the scanned anodes, from information recorded by the detector 14 of the local units 8. It includes, for this purpose:

• - A polling member 21 intended to selectively and successively connect each local unit 8 to the central unit 9;
• a local walking memory 22, intended to be selectively coupled to the local walking memories 16 of the local units 8;
• a setpoint circuit 23 which has the function of producing the setpoint signals necessary for the operation of the local units 8;
• - a setpoint memory 24 associated with the setpoint circuit 23;
• - A locator 25 of the local unit 8 scanned;
• - a programmer 26.

In the central regulation unit 9, the memories 22 and 24 are each divided into three storage sections 22a, 22b, 22c, 24a, 24b, 24c, each section, in each memory, corresponding to an individual local unit 8. Each storage section is further divided into several independent storage areas, not shown, which each correspond to an anode of the electrolysis installation.

In the application example which follows, the programmer 20 of the local regulation units 8 has been programmed so that the local units operate cyclically, independently of each other, each cycle comprising five successive operating phases as described below.

Thus, an operating cycle of the local unit 8 associated with the cell 1 comprises, in chronological order, the following five operational phases.

In a first operational phase, the local unit 8 of the cell 1 is scanned by the central unit 9 and the setpoint memory 17 of this local unit is coupled to section 24a of the setpoint memory 24 of the central unit 9, via a transmission circuit 27a. During this phase, the memory 17 stores setpoint values relating respectively to all the anodes of the cell 1.

At the end of the first operating phase, the central unit 9 is disconnected from the local unit 8 associated with the cell 1.

dans laquelle E désigne la tension réversible de la réaction d'électrolyse pour l'anode considérée.In a second operating phase, the local unit 8 of cell 1 successively scans all the anodes of cell 1 in a pre-established logical order, starting for example with anode 4. While anode 4 is scanned, the detector 14 detects local data on the operation of the cell 1, such as for example the temperature and the concentration of the sodium chloride solution at the inlet and at the outlet of the cell, or at the level of the row of anodes 4 , 4 ', 4 ", the flow of mercury in the cell, the number of the scanned anode 4. These local data are transferred into the converter 15 where they are each converted into a separate electrical voltage signal, which is then stored in section 16a of the local walking memory 16. The measuring device 11 reads the intensity I of the electric current in the anode 4 and the electric voltage U between the anode 4 and the floor of the cell 1, then executes the operation:

in which E denotes the reversible voltage of the electrolysis reaction for the anode considered.

The reversible voltage E is a fixed datum which depends in particular on the nature of the material of the anode and on the position of the latter in the cell. In the case of titanium anodes carrying an active coating formed of a mixture of ruthenium oxide and titanium oxide, E _o is generally fixed between 3.10 and 3.30V, depending on the position of the anode in the cell.

The result of the above operation represents the conductance of the electrolyte layer under the anode 4 in the cell 1; it can be obtained in the manner described in the patents BE-A-668,236 and BE-A-695,771 cited above.

In the transformer 12, the result of the above operation is converted into an electric voltage which is transferred in section 13a of the conductance memory 13.

The aforementioned operations of the second operating phase of the cycle are repeated successively for all the other anodes of cell 1, and the signals obtained are stored respectively in sections 13b, 13c, ... of the conductance memory 13 and in the sections 16b, 16c, ... of the local walking memory 16.

In the third operating phase, the comparison circuit 18 is actuated and connected to the conductance 13, reference 17 and comparison 19 memories. The comparison circuit 18 subtracts the voltage signals from the memory 17 from those of the memory 13 and transfers the resulting signals (comparison signals) into the storage sections 19a, 19b, 19c, ... from the comparison memory 19.

In the fourth operational phase of the operating cycle of local unit 1 of cell 1, the scanning unit 10 operates a new scanning of the cell anodes and, for each scanned anode, it couples the actuation circuit of the motor of the ambdee in section 19a, 19b, ... of the comparison memory 19, which causes the engine to start in the required direction, until cancellation of the signal corresponding to this anode in the memory of tank 19 .

In a fifth phase, the local unit 8 is scanned by the central unit 9 and the local walking memories 16 and 22 of these two regulation units are coupled to one another via a transmission circuit 28a, for transfer the signals from memory 166 to the storage section 22a.

The programmer 26 of the central unit 9 is programmed so that the latter executes a succession of operative sequences of three stages comprising a first stage corresponding to the above-mentioned fifth phase of an operating cycle of a local unit 8 .. a second step corresponding to the second phase of an operating cycle of another local unit 8 and a third step during which the central unit 9 defines, from the information collected in the first step, set values pertaining to a subsequent cycle of operation of the local unit 8 treated in the first step.

• - une première étape au cours de laquelle l'unité centrale 9 scrute l'unité locale 8 de la cellule 2 pour coupler les mémoires de marche locale 16 et 22 de ces deux unités de régulation via le circuit 28b et transférer les signaux de la mémoire 16 dans la section 22b de la mémoire 22;
• - une deuxième étape, au cours de laquelle l'unité centrale 9 scrute l'unité locale 8 de la cellule 3 pour coupler la section 24c de la mémoire de consigne 24 à la mémoire de consigne 17 de cette unité locale, via le circuit 27c; au cours de cette étape, des valeurs de consigne, définies dans l'unité centrale 9 au cours d'une séquence opératoire antérieure, sont transférées vers la mémoire de consigne 17 de l'unité locale 8 de la cellule 3;
• - une troisième étape, au cours de laquelle le circuit de consigne 23 de l'unité centrale 9 est couplé à la mémoire de consigne 24. Pendant cette étape, le circuit de consigne 23 traite les signaux qui ont été stockés dans la section 22b de la mémoire 22 au cours de la première étape et fournit une tension de consigne propre à chaque anode de la cellule 2, et cette tension de consigne est stockée dans la section 24b de la mémoire de consigne 24 jusqu'à une séquence opératoire ultérieure, au cours de laquelle l'unité locale 8 de la cellule 2 sera scrutée à la deuxième étape.

For example, an operating sequence of the central unit 9 will include:

• a first step during which the central unit 9 scans the local unit 8 of the cell 2 to couple the local walking memories 16 and 22 of these two regulation units via the circuit 28b and transfer the signals from the memory 16 in section 22b of memory 22;
• a second step, during which the central unit 9 scans the local unit 8 of the cell 3 to couple the section 24c of the setpoint memory 24 to the setpoint memory 17 of this local unit, via the circuit 27c ; during this step, set values, defined in the central unit 9 during a previous operating sequence, are transferred to the setpoint memory 17 of the local unit 8 of the cell 3;
• a third step, during which the setpoint circuit 23 of the central unit 9 is coupled to the setpoint memory 24. During this step, the setpoint circuit 23 processes the signals which have been stored in section 22b of the memory 22 during the first step and supplies a setpoint voltage specific to each anode of the cell 2, and this setpoint voltage is stored in section 24b of the setpoint memory 24 until a subsequent operating sequence, at during which the local unit 8 of cell 2 will be scrutinized in the second step.

The setpoint voltage is an electrical voltage value, which is representative of the setpoint value of the conductance of the electrolyte layer under the anode considered, for example the anode 4 of cell 2. By definition, this value setpoint is the conductance that the electrolyte layer would have between the anode 4 of the cell 2 and its mercury cathode, under the local operating conditions detected by the detector 14, if the anode 4 occupied an ideal predetermined position .

It is optionally possible to introduce into the setpoint circuit 23 an additional voltage signal coming from the locating member 25, if, in the calculation of the setpoint value, account is taken of the position of the scanned cell 1 in the group electrolysis cells.

In normal operating mode of the electrolysis cells, there is generally only a limited number of anodes which occupy critical positions which are very dangerous (very close to short-circuit with the cathode) or very uneconomical (because excessively far from the cathode), so that in practice there are only a limited number of anodes whose position must be adjusted.

Taking this observation into account, in a preferred variant of the control installation which has just been described, the comparison memory 19 of the local units 8 comprises a distributor of the comparison voltage signals stored therein. The function of the distributor is to distribute the stored signals into three categories of signals which are exemplified in the diagram in FIG. 2, in which the ordinate axis expresses the comparison voltages expressed for example in millivolts.

A first category comprises the comparison signals which lie between two predetermined fixed limit values a and b, situated on either side of the ideal zero value and which thus correspond to the anodes occupying a position close to the optimum with respect to the cathode of their cell; the second category groups together the comparison signals which are lower than the lower limit a, for example the signal c, and which thus correspond to the anodes occupying a position excessively far from the cathode; the third category includes all the comparison signals such as d, which are greater than the upper limit b and which thus correspond to the anodes which are too close to the cathode.

In the second category, the comparison signals are distributed in the order of their decreasing absolute values, which corresponds to a classification of the corresponding anodes in the order of decreasing anode-cathode distances. In the third category, the comparison signals are distributed in the order of their decreasing absolute values, which corresponds to a classification of the corresponding anodes in the order of increasing anode-cathode distances.

The programmers 20 of the local units 8 are also programmed so that, in the fourth operating phase of the operating cycle of the local units 8, the scanning of the anodes of each cell is carried out in the order of distribution of the comparison signals in the comparison memory 19, starting with the third category of signals, then the second category.

This scanning order thus amounts to scanning first the anodes occupying a dangerous position, too close to the cathode, then the anodes occupying a position excessively far from the cathode in relation to an ideal reference position. The anodes of the first category are not scanned during this fourth operating phase.

This preferred variant of the installation according to the invention makes it possible to reduce the time allocated to the fourth operational phase of the operating cycle of the local units 8, to that which is necessary to scan and adjust only part of the anodes of each cell, because in all cases, it is the anodes whose position is the most dangerous that are adjusted first.

This variant embodiment of the invention thus makes it possible to shorten the duration of the operating cycles of the local units and, consequently, to increase the frequency of checks and adjustments of the anodes of the cells.

In a particularly advantageous embodiment of this alternative embodiment of the invention, the anode motors are AC motors of the synchronous type and the programmer 20 of the local regulation units 8 is programmed so that during the fourth operational phase, the scrutinizing organ 10 operates, for each anode, a sequence of successive scans separated from each other by a constant time interval, the duration of which is at most equal to the time taken by the anode to travel a distance equal to that separating position (a) from position (0). Between two consecutive scans of the same anode (for example the anode 4), the polling member 10 operates a series of scans of neighboring anodes (for example anodes 4 ', 4 ", ...).

For example, in the case of cell 1, the polling member 10 will, during the fourth operational phase, scan successively, first the anode 4 and start its engine in the required direction, then the anode 4 'and start its motor and finally the 4 "anode and start the motor of this one. From this moment, the three motors of the three anodes 4,4', 4" rotate continuously, at constant speed, so that the anodes 4, 4 'and 4 "are moved continuously opposite the cathode, each in a defined direction, at constant speed.

The scanning device 10 then returns to the anode 4 and performs a new scanning thereof: during this second scanning, the measuring device 11 reads the instantaneous value of the conductance of the electrolyte layer under the anode 4, the result is transferred, via the transformer 12, to the comparison circuit 18 and the latter sends a signal representative of the difference between the instantaneous conductance measured and the reference value which had been stored during the first operating phase described above. If the emitted comparison signal 18 does not fall below a threshold value corresponding to a correct position of the anode 4, its motor is kept running. The polling member 10 then passes to the anode 4 ′ then to the anode 4 "and the abovementioned operations are executed separately for each of these two anodes. This cycle of scanning of the anodes 4, 4 ′, 4" is repeated repeatedly, at regular time intervals defined above. As soon as the comparison signal 18 detected for an anode (4, 4 ′ or 4 ") falls below the aforementioned threshold value, the motor of this anode is stopped.

The successive scans of the group of anodes 4, 4 ′, 4 "are continued, for example, until a set fixed time is used up or until the three anodes 4, 4 ′ and 4" have been correctly adjusted.

The polling member 10 then passes to a neighboring group of anodes, for example the group of anodes 5, 5 ′ and 5 ″ and starts the same scanning sequences for them.

Claims (10)

1 - Installation for the regulation of a group of electrolysis cells with multiple anodes displaceable opposite a mercury cathode, comprising local regulation units (8) which are each associated with an electrolytic unit (1, 2, 3) of the group of cells and which each comprise: - a selective scanning unit (10) of anode units (4, 4 ', 4 ", 5, 5', 5") which can be moved individually in the electrolytic unit, a member (11) for measuring the instantaneous value of the electrical conductance of the electrolyte layer between the scrutinized anode unit and the cathode, - a transformer (12) of said value into an electrical signal a circuit for comparing (18) said signal with a setpoint signal representative of a setpoint of the electrical conductance, - a motorized adjustment device for the position of the scanned anode unit, connected to the comparison circuit, characterized in that each local regulation unit (8) further comprises a detector (14) of local operating conditions of the electrolytic unit and a converter (15) of the local operating conditions detected into electrical signals, and in that the local regulation units (8) of the group of cells are coupled to a central regulation unit (9) which comprises: - a selective polling unit (21) of the local regulation units (8), a setpoint circuit (23) for processing the signals from the converter (15) of the scanned local unit (8) and defining, by means of these, the setpoint signal, a member for transmitting the setpoint signal to the comparison circuit (18) of the scanned local unit, 2 - Installation according to claim 1, characterized in that the detector (14) associated with each local regulation unit (8) comprises a member for locating the position of the scanned anode unit, in the electrolytic unit and in this that the central unit regulation (9) comprises a member for locating the position, in the group of cells, of the electrolytic unit associated with the local unit (8) scanned. 3 - Installation according to claim 1 or 2, characterized in that each local regulation unit (8) comprises, on the one hand, between the comparison circuit (18) and the motorized adjustment device, a comparison memory (19 ) for the storage of the signals coming from the comparison circuit and, on the other hand, a programmer (20) designed to carry out the operation of the local unit in two phases comprising, in a first phase, a scanning of the anode units and a coupling of the comparison circuit (18) with the comparison memory (19) and, in a second phase, a scanning of the anode units and a coupling of the comparison memory (19) with an actuating member of the motorized adjustment device . 4 - Installation according to claim 3, characterized in that the comparison memory (19) of each local regulation unit (8) comprises a distributor of the signals stored in three categories comprising a first category which groups the signals relating to the anode units for which the anode-cathode distance is between an upper limit (a) and a lower limit (b), a second category which groups the signals relating to the anode units for which the anode-cathode distance is greater than the upper limit (a) and a third category which groups together the anode units for which the anode-cathode distance is less than the lower limit (b), and in that the motorized adjustment device is provided with an actuating member which is designed to actuate the said device only in the case of anode units of the second and third category. 5 - Installation according to claim 3 or 4, characterized in that the motorized adjustment device is an AC motor of the synchronous type. 6 - Installation according to claim 5, characterized in that the programmer (20) is programmed so that, in the aforementioned second phase, the polling member (10) operates, for each anode unit (4, 4 ', 4 ", ..., 5, 5 ', ...), a sequence of successive scans comprising a first scan with coupling of the comparison memory (19) with the actuating member of the motorized adjustment device of the anode unit and repeated subsequent scans with coupling of the measuring member (11), the transformer (12), the comparison circuit (18) and the motorized adjustment device. 7 - Installation according to any one of claims 4 to 6, characterized in that each local regulation unit (8) comprises a setpoint memory (17) for storing the setpoint signals, a conductance memory (13) for storing signals from the transformer (12) and a local run memory (16) for storing signals from the converter (15), and in that the central control unit (9) includes a local run memory (22) for the storage of signals from the converter (15) of the local units and a setpoint memory (24) for the storage of the setpoint signals from the setpoint circuit (24). 8 - Installation according to claim 7, characterized in that each memory (22, 24) of the central regulation unit (9) is divided into storage sections (22a, 22b, 22c, ..., 24a, 24b, 24c, ...) each corresponding to a local regulation unit (8), and the central regulation unit (9) is programmed so as to carry out successive operating sequences of three steps comprising a step of scanning a unit local control (8) with coupling of the local walking memory (16) of this local unit (8) to the section (22a, 22b, 22c, ...) relating to this local unit, in the local walking memory (22) of the central regulation unit (9), a step of coupling the reference circuit (23) of the central regulation unit (9) with the relevant section (22b, 22c, ..., 22a) to this local regulation unit (8) in the local running memory (22) of the central unit (9) and a step of scanning a local regulation unit (8) with coupling of the reference memory (17) of this local unit to the section (24c, ... 24a, 24b) relating to this local unit in the memory setpoint (24) of the central regulation unit (9). 9 - Installation according to any one of claims 1 to 8, characterized in that the detector (14) of local operating conditions of the electrolytic unit (1, 2, 3) comprises bodies for measuring parameters selected in l electrolytic unit among the flow of mercury in the electrolytic unit, the overall intensity of the electric current in the electrolytic unit, the temperature and the concentration of the electrolyte layer under the scanned anode unit. 10 - Installation according to any one of claims 1 to 9, characterized in that the anode unit (4, 4 ', 4 ", 5, 5', 5") is an individual anode of the electrolytic unit (1 , 2, 3) and the motorized adjustment device comprises an individual electric motor for each anode.

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