EP3256623B1 - Aluminium smelter and method to compensate for a magnetic field created by the circulation of the electrolysis current of said aluminium smelter - Google Patents

Description

The present invention relates to an aluminum smelter, intended for the production of aluminum by electrolysis, and to a method for compensating the vertical and horizontal components of a magnetic field generated by the circulation of an electrolysis current in this aluminum smelter.

It is known to produce aluminum industrially from alumina by electrolysis according to the Hall-Héroult process. For this purpose, an electrolysis tank is provided comprising a steel box inside of which a coating of refractory materials is arranged, a cathode of carbonaceous material, crossed by cathode conductors intended to collect the electrolysis current at the cathode to lead it to cathode outputs passing through the bottom or sides of the box, routing conductors extending substantially horizontally to the next tank from the cathode outputs, an electrolytic bath in which the alumina is dissolved , at least one anode assembly comprising at least one anode immersed in this electrolytic bath, an anode frame to which the anode assembly is suspended, and conductors for increasing the electrolysis current, extending from bottom to top, connected to the conductors routing of the previous electrolysis tank to route the electrolysis current from the cathode outlets to the anode frame e and to the anode assembly and the anode of the next tank. The anodes are more particularly of the prebaked anode type with prebaked carbon blocks, that is to say baked before introduction into the electrolysis tank.

Aluminum production plants, or aluminum smelters, traditionally include several hundred electrolytic cells, aligned transversely in parallel lines and connected in series.

These electrolytic cells are traversed by an electrolysis current of the order of several hundreds of thousands of amperes, which creates a large magnetic field. The vertical component of this magnetic field, mainly generated by the conductors carrying the current from one electrolysis cell to the next, is known to cause instabilities called magnetohydrodynamic instabilities (MHD).

These MHD instabilities are known to degrade the yield of the process. The more unstable a tank, the greater the interpolar distance between the anode and the metal sheet. However, the greater the interpolar distance, the higher the energy consumption of the process because it is dissipated by the Joule effect in the interpolar space.

On the other hand, the horizontal components of the magnetic field, generated by the entire path of the electric current, both in the conductors inside the tank and those outside, interact with the electric current passing through liquids, which causes stationary deformation of the sheet of metal. The unevenness of the metal sheet caused must remain low enough so that the anodes are consumed uniformly with little waste. To obtain a small difference in level, it is necessary that the horizontal components of the magnetic field are as asymmetrical as possible in liquids (electrolytic bath and sheet of metal). For the longitudinal and transverse components of the magnetic field which constitute the horizontal components, by antisymmetric it is meant that when one moves perpendicular to the central axis of the tank, parallel to the considered component of the field, and when one located at equal distance on either side of this central axis, the value of the component considered is opposite. The asymmetry of the horizontal components of the magnetic field is the configuration providing the most symmetrical and flat interface distortion possible in the tank.

It is known, in particular documents of patent FR1079131 and FR2469475 , to fight against MHD instabilities by compensating for the magnetic field created by the circulation of the electrolysis current, thanks to a particular arrangement of the conductors conducting the electrolysis current. For example, according to the document of patent FR2469475 , the routing conductors bypass laterally the ends or heads of each electrolysis tank. We are talking about self-compensation. This principle is based on local balancing of the magnetic field, on the scale of an electrolysis cell.

The main advantage of self-compensation is the use of the electrolysis current itself to compensate for MHD instabilities.

However, self-compensation can create significant lateral bulk since the electrical conductors bypass the heads of electrolytic cells.

Above all, the long length of the routing conductors for the implementation of this solution generates electrical loss online by resistive effect of the conductors, therefore an increase in operating costs, and requires a lot of raw material, therefore high manufacturing. These drawbacks are all the more marked as the electrolytic cells have large dimensions and operate with high intensities.

Also, the design of an aluminum smelter with a self-compensated electrical circuit is frozen. However, during life, it may become necessary to increase the intensity of the current electrolysis, beyond the intensity expected during design. This also in fact modifies the distribution of the magnetic field of the self-compensated electric circuit, not designed for this new distribution, which no longer makes it possible to optimally compensate for this magnetic field. There are solutions to overcome this lack of scalability and find a magnetic compensation close to the optimum, but these solutions are particularly complex and costly to implement.

Another solution to reduce MHD instabilities, known in particular from the patent document FR2425482 , consists in using a secondary electrical circuit, or external loop, running along the rows of electrolytic cells, on the sides. This secondary electrical circuit is traversed by a current whose intensity equals a predetermined percentage of the intensity of the electrolysis current. Thus, the external loop generates a magnetic field compensating for the effects of the magnetic field created by the electrolysis current of the neighboring line of electrolysis cells.

It is also known from the patent document EP0204647 the use of a secondary circuit along the rows of electrolytic cells on the sides to reduce the effect of the magnetic field generated by the routing conductors, the intensity of the current flowing through the electrical conductors of this secondary circuit being on the order of 5 to 80% of the intensity of the electrolysis current, and this current flowing in the same direction as the electrolysis current.

The compensation solution by external loop has the advantage of having a secondary circuit independent of the main circuit traversed by the electrolysis current.

The arrangement of the secondary circuit, located on the sides of the tank rows near the short sides of the boxes, at the height of the bath-metal interface, allows compensation of the vertical component without impacting the horizontal components of the magnetic field.

The compensation solution by external loop significantly reduces the length, mass and electrical losses of the routing conductors, but requires an additional electrical supply station and an additional independent secondary electrical circuit.

It will also be noted that the compensation solution by external loop involves a plurality of magnetic fields, with the current of the series, creating a very strong total ambient field, so that this implies constraints on the operations and the material (for example shielding necessary vehicles), and so that the magnetic field of a queue impacts the stability of the tanks of the neighboring queue. To limit the influence of a queue on the neighboring queue, it is necessary to move them away from each other, which constitutes a significant spatial constraint and therefore involves sheltering each row of electrolytic cells in a separate hangar.

Furthermore, the junction portion of the electrolysis circuit and the secondary circuit joining the ends of two adjacent rows of electrolysis cells tends to destabilize the end-of-line cells. To avoid having unstable end-of-line tanks, it is possible to configure this portion of the secondary circuit according to a predetermined route, as is known from the patent. FR2868436 , in order to correct the magnetic field so that the impact on the end-of-line tanks becomes acceptable. However, this route notably lengthens the length of the secondary circuit, therefore the material cost. It should be noted that the usual solution consists in moving the junction portion of the secondary circuit and the electrolysis circuit away from the cells located at the end of the line, but this increases the size in addition to increasing the length of the electrical conductors, therefore the material and energy cost.

It will therefore be noted that the known solutions for compensation by external loop generate relatively high structural costs.

Also, the present invention aims to overcome all or part of these drawbacks by proposing an aluminum smelter with a magnetic configuration making it possible to have very magnetically stable tanks, and offering improved compactness. The present invention also relates to a method of compensating for a magnetic field created by the circulation of an electrolysis current in this aluminum smelter.

• au moins un premier circuit électrique de compensation s'étendant sous les cuves d'électrolyse, ledit au moins un premier circuit électrique de compensation pouvant être parcouru par un premier courant de compensation destiné à circuler sous les cuves d'électrolyse en sens inverse du sens de circulation global du courant d'électrolyse,
• au moins un deuxième circuit électrique de compensation s'étendant sur au moins un côté de ladite au moins une file de cuves d'électrolyse, ledit au moins un deuxième circuit électrique de compensation pouvant être parcouru par un deuxième courant de compensation destiné à circuler dans le même sens que le sens de circulation global du courant d'électrolyse.

To this end, the subject of the present invention is an aluminum smelter comprising at least one row of electrolytic cells arranged transversely to the length of said at least one row, one of the electrolytic cells comprising anode assemblies and electrical conductors for mounting and connection to the anode assemblies, characterized in that the electrical conductors for mounting and connection extend upward along two opposite longitudinal edges of the electrolysis tank to conduct the electrolysis current to the assemblies anodic, and in that the smelter includes

• at least a first electrical compensation circuit extending under the electrolytic cells, said at least one first electrical compensation circuit being able to be traversed by a first compensating current intended to flow under the electrolytic cells in opposite direction overall circulation of the electrolysis current,
• at least one second electrical compensation circuit extending on at least one side of said at least one row of electrolytic cells, said at least one second electric compensation circuit which can be traversed by a second compensation current intended to flow in the same direction as the overall direction of circulation of the electrolysis current.

Thus, the smelter according to the invention offers the advantage of having very stable tanks magnetically, since it compensates for both the horizontal and vertical components of the magnetic field generated by the circulation of the electrolysis current, which makes it possible to improve the overall yield, and this without negative impact on the size of the smelter according to the invention since the first electrical compensation circuit extends under the electrolytic cells.

According to a preferred embodiment, the electrical rise and connection conductors comprise electrical rise and upstream connection conductors, adjacent to the upstream longitudinal edge of the electrolytic cell, and electrical rise and downstream connection conductors, adjacent at the longitudinal downstream edge of the electrolysis tank, and the smelter is configured so that the distribution of the electrolysis current is asymmetrical between the upstream and downstream connection and upstream electrical conductors, the intensity of the upstream electrolysis current intended to run through all of the upstream and upstream electrical conductors of the electrolysis tank being equal to] 50-100 [% of the intensity of the electrolysis current, and the intensity of the downstream electrolysis current intended to run through all the electrical conductors for rising and connecting downstream of the electrolytic cell being equal to] 0-50 [% of the current intensity electrolysis, the sum of the intensities of the upstream and downstream electrolysis currents being equal to the intensity of the electrolysis current.

An advantage of these characteristics is that it makes it possible to effectively compensate the magnetic field for a large-sized electrolysis cell, in particular of large width, without any additional cost of raw materials.

Indeed, if the distribution of the upstream downstream electrolysis current is symmetrical, that is to say if this distribution is 50% upstream and 50% downstream, and the width of the tanks electrolysis is increased, to have a better efficiency, it is created, due to the increase in the path traveled by the electrical conductors of routing under the electrolysis tank to supply the electrical conductors of rise and downstream connection, an imbalance detrimental to the proper functioning of the electrolysis tank. To restore balance, the section of these electrical conductors should be increased under the electrolytic cell. However, this increase in section implies a significant additional cost in raw materials. On the other hand, the Applicant has observed that the smelter according to the present invention makes it possible to introduce an asymmetry in the distribution of the electrolysis current between the upstream and downstream of the electrolytic cells without damaging increase in the cross-section of the electrical conductors, while having very magnetically stable electrolytic cells.

The choice of the distribution between intensities of the upstream and downstream electrolysis currents is made by economic study. This choice mainly depends on the distance between two tanks and the height of the tanks. This distribution is achieved by adjusting the sections of the electrical conductors of the upstream and downstream electrical circuits, taking into account their length.

According to a preferred embodiment, the smelter comprises a power station configured to circulate through said at least one first electrical compensation circuit a first compensation current of intensity equal to twice the intensity of the current of downstream electrolysis, to within plus or minus 20%, and preferably to plus or minus 10%.

An advantage of this characteristic is that for this value of the intensity of the first compensation current, which is a direct function of the distribution of the electrolysis current between the upstream and downstream of the electrolysis cells, the Applicant has observed that the horizontal magnetic field generated by the first electric compensation circuit precisely corrects the asymmetry of the horizontal magnetic field resulting from the asymmetry between the upstream and downstream electrolysis current, in order to have an asymmetric distribution of the horizontal components of the field magnetic This first compensation current also makes it possible to partially correct the vertical magnetic field, as a function of the distribution between upstream and downstream electrolysis current of the cell, in order to reduce the MHD instabilities in the cell.

According to a preferred embodiment, the smelter includes a power station configured to circulate through said at least one second electrical compensation circuit a second intensity compensation current between 50% and 100% of the difference d intensity between the upstream and downstream electrolysis currents, and preferably between 80% and 100% of the difference in intensity between the upstream and downstream electrolysis currents.

By intensity of the second compensation current is meant the sum of the intensities flowing in the conductors forming the second compensation circuit, in particular when the second compensation circuit consists of two conductors (or loops) arranged on either side of the electrolysis tank.

The Applicant has observed that for this intensity value of the second compensation current, which is also a direct function of the distribution of the electrolysis current between the upstream and downstream of the electrolysis cells, the vertical magnetic field generated by the second electrical compensation circuit corrects the vertical magnetic field generated by the electrolysis current flowing in the main electrical circuit (cell to cell circuit) and already partly corrected by the current flowing in the first compensation circuit.

Note that this feature is particularly advantageous when used in combination with the previous one.

According to a preferred embodiment, the electrical rise and connection conductors are distributed at regular intervals along the longitudinal edge of the electrolytic cell to which these electrical rise and connection conductors are adjacent.

An advantage of this characteristic is to have a uniform distribution over the entire length of the cell of the longitudinal horizontal component of the magnetic field (that is to say parallel to the length of the electrolysis cell), which allows to facilitate its compensation via the first compensation circuit.

The electrical rise and connection conductors are advantageously arranged symmetrically with respect to the transverse median plane XZ of the electrolytic cells, which makes it possible to obtain an antisymmetric distribution of the transverse component of the magnetic field along X.

According to a preferred embodiment, the upstream electrical conductors and upstream connection and the upstream electrical conductors and downstream connection are located equidistant from a longitudinal median plane YZ of the electrolysis cell.

According to a preferred embodiment, the upstream electrical conductors and upstream connection and the upstream electrical conductors and downstream connection are arranged substantially symmetrically with respect to said longitudinal median plane YZ of the electrolysis cell.

This configuration, combined with the first compensation circuit, ensures perfect asymmetry of the longitudinal component of the magnetic field along Y.

According to a preferred embodiment, said at least one first electrical compensation circuit comprises electrical conductors extending under the electrolytic cells, together forming a sheet made up of a plurality of parallel electrical conductors, typically from two to twelve, and preferably from three to ten parallel electrical conductors.

The number of parallel conductors required depends in part on the distance between the liquids and these same conductors. The greater the distance, the smaller the number of conductors, the shorter the distance, the greater the number of conductors.

An advantage of this characteristic is a compensation distributed over the entire length of the electrolysis tank, thus producing better results. Note that the first electrical compensation circuit is configured so that the first compensation current flows in the same direction through all the electrical conductors of the ribbon cable.

The intensity of the first compensation current corresponds to the sum of the intensities of the currents flowing in each of the parallel electrical conductors of the sheet extending under the tanks.

According to a preferred embodiment, the electrical conductors of said sheet are arranged at regular intervals from one another in a longitudinal direction Y of the electrolysis cells.

According to a preferred embodiment, the electrical conductors of said sheet are arranged in a substantially symmetrical manner with respect to a transverse median plane XZ of the electrolysis cells.

According to a preferred embodiment, the electrical conductors of said sheet are arranged in the same horizontal plane XY.

An advantage of these characteristics is to further improve the compensation of the unfavorable magnetic field.

According to a preferred embodiment, said at least one second electrical compensation circuit comprises electrical conductors extending on each side of said at least one row of electrolytic cells, and the second compensation current flows in the same direction as the overall direction of flow of the electrolysis current on each side of the electrolysis cells.

Thus, the electrical conductors of said at least one second electrical compensation circuit form an internal loop and an external loop, and thus offer improved compensation of the magnetic field. By internal loop is meant the loop being the closest to the neighboring queue and by external loop, the loop being the most distant.

According to a preferred embodiment, the intensity of a second compensation current flowing in an internal loop of said at least one second compensation circuit differs from the intensity of a second compensation current flowing in an external loop of said at least one second compensation circuit.

This characteristic makes it possible to compensate for the residual vertical magnetic field of the neighboring file.

The intensity of the second compensation current corresponds to the sum of the intensities of the currents flowing in each of the loops.

According to a preferred embodiment, the intensity of the second compensation current flowing in the internal loop is greater than the intensity of the second compensation current flowing in the external loop.

This corrects the magnetic field created by the neighboring queue. This neighboring file creates a magnetic field proportional to a current in the series from which the downstream electrolysis current is subtracted twice, while a “conventional” electrolysis series will undergo a magnetic field directly proportional to the totality of the current. electrolysis. Thus, thanks to the first compensation circuit, the disturbing field created by the neighboring queue is much weaker and requires much less correction. Consequently, concerning the second compensation circuit, the difference between the intensity of the inner loop and that of the outer loop will be much smaller than in the case of the patent. EP0204647 and the gap between the two rows of tanks can be minimized.

According to a preferred embodiment, the electrical conductors forming the second electrical compensation circuit are substantially symmetrical with respect to a median transverse XZ plane of the electrolysis cells.

This improves compensation for the deleterious magnetic field.

According to a preferred embodiment, the electrical conductors of the second electrical compensation circuit extend in the same horizontal plane XY, preferably at the height of a sheet of liquid aluminum formed inside the electrolysis cells during of the electrolysis reaction.

This arrangement improves the compensation of the vertical magnetic field without impacting the horizontal component of the field already compensated by the first compensation circuit.

Preferably, the smelter comprises two consecutive and parallel rows of electrolytic cells, and the circuit of the internal loop forms, at the end of the row, means for compensating for the “end of the row” effects caused by the connecting conductors between lines, which provides more magnetic stability and therefore improves the efficiency of the line end tanks.

According to a preferred embodiment, said at least one first electrical compensation circuit is independent of the main electrical circuit through which the electrolysis current flows.

This characteristic has the advantage of limiting the consequences of damage such as piercing of the electrolysis cell by the liquids contained in this electrolysis cell. In addition, this characteristic is advantageous in terms of scalability since it makes it possible to vary the intensity of the first compensation current to adjust the magnetic compensation. An adjustment of the magnetic compensation is useful when the electrolytic cells are modified, because the magnetic configuration of these electrolytic cells is modified, or to adapt the stirring of the alumina to the quality of this alumina (which allows maintain an optimal yield despite the different quality of the alumina).

According to a preferred embodiment, said at least one second electrical compensation circuit is independent of the main electrical circuit traversed by the electrolysis current.

As explained above, this has an advantage in terms of scalability since it makes it possible to vary the intensity of the first compensation current to adjust the magnetic compensation.

According to a preferred embodiment, the electrolysis cell has a modular electrical construction in N modules repeated in the direction of its length, each module comprising electrical conductors configured to generate the same predetermined magnetic configuration.

This characteristic is advantageous in terms of scalability: it allows modifications to the electrolysis tank, for example an enlargement by adding one or more modules, without modifying the principle of magnetic balancing of the electrolysis tank.

To obtain the same magnetic configuration, each electrical module has the same arrangement of electrical conductors, each electrical conductor of an electrical module being traversed by the same intensity and the same direction of current as the corresponding electrical conductor of an adjacent electrical module. The electrical conductors of each module are in particular the electrical rise and connection conductors, the anode assemblies, the cathodes, the cathode conductors, the cathode outputs, the electrical routing conductors, and conductors the electrical conductor layer of the first electrical compensation circuit. These electrical conductors are therefore arranged in relation to each other in the same way from one module to another. In particular, each electrical module comprises the same number of electrical conductors of the sheet of electrical conductors of the first electrical compensation circuit.

It is specified that the electrolysis cells of the aluminum smelter include all or part of the abovementioned characteristics of the electrolysis cell.

• la circulation, en sens contraire du sens de circulation global du courant d'électrolyse, d'un premier courant de compensation à travers ledit au moins un premier circuit électrique de compensation,
• la circulation, dans le même sens que le sens de circulation global du courant d'électrolyse, d'un deuxième courant de compensation à travers ledit au moins un deuxième circuit électrique de compensation.

The invention also relates to a method for compensating for a magnetic field created by the circulation of an electrolysis current in a plurality of electrolysis cells of an aluminum smelter having the aforementioned characteristics, the method comprising:

• the circulation, in the opposite direction of the overall direction of circulation of the electrolysis current, of a first compensation current through said at least one first electrical compensation circuit,
• the circulation, in the same direction as the overall direction of circulation of the electrolysis current, of a second compensation current through said at least one second electrical compensation circuit.

Thus, this method offers effective magnetic compensation of the magnetic field generated by the circulation of the electrolysis current in the series of electrolysis cells of the smelter, by limiting the size.

According to a preferred embodiment, the method comprises an asymmetrical distribution of the electrolysis current between the upstream and downstream of the electrolytic cells, all of the electrical conductors for raising and connecting upstream of the electrolytic cells. electrolysis being traversed by an upstream electrolysis current of intensity between] 50-100 [% of the intensity of the electrolysis current, and all of the electrical conductors for rising and connecting downstream of the cells of electrolysis being traversed by a downstream electrolysis current of intensity between] 0-50 [% of the intensity of the electrolysis current, the sum of the intensities of the upstream and downstream electrolysis currents being equal to the intensity of the electrolysis current.

This process makes it possible to obtain magnetically stable electrolytic cells, including when the electrolytic cells are large, in particular very large. The yield can thus be significantly increased.

According to a preferred embodiment, the intensity of the first compensation current is equal to twice the intensity of the downstream electrolysis current, to within plus or minus 20%, and preferably to plus or minus 10%.

An advantage of this characteristic is that for this value of the intensity of the first compensation current, which is a direct function of the distribution of the electrolysis current between the upstream and downstream of the electrolysis cells, the Applicant has observed that the horizontal magnetic field generated by the first electrical compensation circuit precisely corrects the asymmetry between the upstream and downstream current, in order to have an asymmetric distribution of the horizontal components of the magnetic field. This first compensation current also makes it possible to correct all or part of the vertical magnetic field, according to the distribution between upstream and downstream electrolysis current of the cell, in order to reduce the MHD instabilities in the cell. The entire vertical magnetic field is corrected if the distribution between upstream and downstream is 50%.

According to a preferred embodiment, the intensity of the second compensation current is between 50% and 100% of the difference in intensity between the upstream and downstream electrolysis currents, and preferably between 80% and 100% of the difference in intensity between the upstream and downstream electrolysis currents.

Likewise, the Applicant has observed that for this intensity value of the second compensation current, which is also a direct function of the distribution of the electrolysis current between the upstream and downstream of the electrolysis cells, the vertical magnetic field generated by the second electric compensation circuit precisely corrects the remaining vertical magnetic field, resulting from the sum of the vertical magnetic field of the electrolysis current (cell to cell circuit) and the first compensation circuit.

According to a preferred embodiment, said at least one second electrical compensation circuit comprises an internal loop and an external loop, and in which the intensity of a second compensation current flowing in the internal loop differs from the intensity of a second compensation current flowing in the external loop.

According to a preferred embodiment, the intensity of the second compensation current flowing in the internal loop is greater than the intensity of the second compensation current flowing in the external loop.

According to a preferred embodiment, the method comprises a step of analyzing at least one characteristic of the alumina in at least one of the electrolysis cells of said aluminum smelter, and determining the intensity values of the first current of compensation and the second compensation current to be circulated as a function of said at least one characteristic analyzed.

Thus, the method makes it possible to modify the magnetic compensation, to voluntarily induce, in special cases, a modification of the flow in the liquids and flow velocities while controlling (slightly degrading) the MHD instabilities of the bath / metal interface. The flow of liquids (bath + aluminum) indeed helps to stir the alumina, which, depending on the speed and shape of the flow as well as the quality of the alumina, improves the yield. This preferred embodiment therefore makes it possible to improve the yield by optimizing the flow to dissolve the alumina while controlling the level of "degradation" of the MHD stability of the bath / metal interface.

• La figure 1 est une vue schématique d'une aluminerie selon l'état de la technique,
• La figure 2 est une vue schématique de côté de deux cuves d'électrolyse successives de l'état de la technique,
• La figure 3 est une vue schématique en filaire du circuit électrique parcouru par le courant d'électrolyse dans les deux cuves d'électrolyse de la figure 2,
• La figure 4 est une vue schématique en coupe selon un plan longitudinal vertical d'une cuve d'électrolyse de l'état de la technique,
• La figure 5 est une vue schématique d'une aluminerie selon un mode de réalisation de l'invention,
• La figure 6 est une vue schématique de côté de deux cuves d'électrolyse successives d'une aluminerie selon un mode de réalisation de l'invention,
• La figure 7 est une vue schématique en coupe selon un plan longitudinal YZ d'une cuve d'électrolyse d'une aluminerie selon un mode de réalisation de l'invention,
• La figure 8 est une vue schématique filaire du circuit électrique parcouru par le courant d'électrolyse dans une cuve d'électrolyse d'une aluminerie selon un mode de réalisation de l'invention,
• La figure 9 est un tableau montrant l'intensité du courant d'électrolyse parcourant chaque segment de la figure 8,
• Les figures 10 à 12 sont des vues schématiques filaires du circuit électrique parcouru par le courant d'électrolyse dans une cuve d'électrolyse d'une aluminerie selon un mode de réalisation de l'invention, montrant pour cette cuve d'électrolyse les zones générant un champ magnétique significatif,
• La figure 13 est un tableau montrant la contribution de chaque segment des figures 10 à 12 dans le calcul de la composante verticale du champ magnétique généré par la circulation du courant d'électrolyse,
• La figure 14 est un tableau montrant la contribution de chaque segment des figures 10 à 12 dans le calcul de la composante horizontale longitudinale du champ magnétique généré par la circulation du courant d'électrolyse.

Other characteristics and advantages of the present invention will emerge clearly from the description below of a particular embodiment, given by way of nonlimiting example, with reference to the appended drawings in which:

• The figure 1 is a schematic view of an aluminum smelter according to the state of the art,
• The figure 2 is a schematic side view of two successive electrolytic cells of the state of the art,
• The figure 3 is a schematic wire-frame view of the electrical circuit traversed by the electrolysis current in the two electrolysis cells of the figure 2 ,
• The figure 4 is a schematic sectional view along a vertical longitudinal plane of an electrolytic cell of the state of the art,
• The figure 5 is a schematic view of an aluminum smelter according to one embodiment of the invention,
• The figure 6 is a schematic side view of two successive electrolytic cells of an aluminum smelter according to an embodiment of the invention,
• The figure 7 is a schematic sectional view along a longitudinal plane YZ of an electrolysis cell of an aluminum smelter according to an embodiment of the invention,
• The figure 8 is a wired schematic view of the electric circuit traversed by the electrolysis current in an electrolysis tank of an aluminum smelter according to an embodiment of the invention,
• The figure 9 is a table showing the intensity of the electrolysis current flowing through each segment of the figure 8 ,
• The figures 10 to 12 are schematic wire-frame views of the electric circuit traversed by the electrolysis current in an electrolysis tank of an aluminum smelter according to an embodiment of the invention, showing for this electrolysis tank the zones generating a significant magnetic field,
• The figure 13 is a table showing the contribution of each segment of figures 10 to 12 in the calculation of the vertical component of the magnetic field generated by the circulation of the electrolysis current,
• The figure 14 is a table showing the contribution of each segment of figures 10 to 12 in the calculation of the longitudinal horizontal component of the magnetic field generated by the circulation of the electrolysis current.

The figure 1 shows an aluminum plant 100 of the state of the art. The aluminum smelter 100 comprises electrolytic cells arranged transversely to the length of the line which they form. The electrolytic cells are here aligned in two parallel lines 101, 102. These electrolytic cells are traversed by an electrolysis current I ₁₀₀ . Two electrical compensation circuits 104, 106 extend on the sides of the lines 101, 102 to compensate for the magnetic field generated by the circulation of the electrolysis current I ₁₀₀ from one electrolysis cell to another and in the neighboring line . The electrical compensation circuits 104, 106 are traversed respectively by currents I ₁₀₄ , I ₁₀₆ flowing in the same direction as the electrolysis current I ₁₀₀ . Feed stations 108 supply the series of electrolytic cells and the electrical compensation circuits 104, 106. According to this example, for an electrolysis current of intensity 500kA, and taking into account the magnetic disturbances at the "end of the line", the distance D ₁₀₀ between the electrolysis cells closest to the supply stations 108 and the 108 supply stations is around 45m, and the distance D ₃₀₀ over which the electrical compensation circuits 104, 106 extend beyond the end of the line is around 45m, while the distance D ₂₀₀ between the two queues 101, 102 is around 85m to limit the magnetic disturbances from one queue to the other.

The figure 2 shows two consecutive traditional electrolytic cells 110 from the same row of electrolytic cells. As we can see on the figure 2 , the electrolysis tank 110 comprises a box 112 internally lined with refractory materials 114, a cathode 116 and anodes 118 immersed in an electrolytic bath 120 at the bottom of which is formed a sheet 122 of aluminum. The cathode 116 is electrically connected to cathode conductors 124 which cross the sides of the box 112 at cathode outputs 126. The cathode outputs 126 are connected to conductors 128 which convey the electrolysis current to the conductors 130 for mounting and connecting a next electrolysis tank. As we can see on the figure 2 , these rise and connection conductors 130 extend, obliquely, on one side only, the upstream side, of the electrolytic cells 110 and extend above the anodes 118, up to the longitudinal central part electrolysis tanks 110.

The electrolytic cell comprises a superstructure 132 which crosses it longitudinally, above the box 112 and anodes 118. The superstructure 132 in particular comprises a beam resting on feet (not shown) at each of its longitudinal ends. The beam supports an anode frame 134, this anode frame 134 also extending longitudinally above the box 112 and anodes 118. The anode frame 134 supports the anode assemblies, the latter being electrically connected to the anode frame 134.

The figure 3 schematically illustrates the path traveled by the electrolysis current I ₁₀₀ in each of the electrolysis cells 110 and between two adjacent electrolysis cells 110 like those shown on the figure 2 . We note in particular that the rise of the electrolysis current I ₁₀₀ to the anode assembly of an electrolysis tank 110 is asymmetrical since this rise is carried out only upstream of the electrolysis tanks 110 in the direction of overall circulation of the electrolysis current I ₁₀₀ in the queue (to the left of the cells on the figures 2 and 3 ).

The figure 4 shows the arrangement on the sides of the tanks 110 of the state of the art of electrical conductors forming the electrical compensation circuits 104, 106, these electrical conductors being traversed respectively by the compensation currents I ₁₀₄ , I ₁₀₆ flowing in the same sense that the electrolysis current I ₁₀₀ here passing through the routing conductors 128 positioned below the tank.

The figure 5 shows an aluminum smelter 1 according to an embodiment of the invention. The aluminum smelter 1 is intended for the production of aluminum by electrolysis according to the Hall-Héroult process.

The aluminum smelter 1 comprises a plurality of substantially rectangular electrolytic cells intended for the production of aluminum by electrolysis, these electrolytic cells being able to be aligned in one or more rows 2 which can be substantially parallel. Where appropriate, the lines 2 are electrically connected in series and supplied with electrolysis current IE. The aluminum smelter 1 also comprises a first electrical compensation circuit 4, which extends under the row or rows of electrolytic cells, and a second electrical compensation circuit 6, which extends over at least one side of the or rows 2 of electrolytic cells. According to the example of the figure 5 , the second electrical compensation circuit 6 extends on both sides of each row 2 of electrolysis cells. Still following the example of the figure 5 , the smelter has two rows of cells arranged parallel to each other, supplied by the same supply station 8, and electrically connected in series so that the electrolysis current IE flowing in the first of two rows 2 of electrolytic cells then circulates in the second of the two rows 2 of electrolytic cells. Electrolysis tanks are arranged transversely to each row 2 that these electrolytic cells form. It will be noted that by electrolysis tank 2 arranged transversely is understood electrolysis tank 2, the largest dimension of which, the length, is substantially perpendicular to the overall direction in which the electrolysis current IE flows.

In the present description, upstream and downstream are defined with respect to the overall direction of circulation of the electrolysis current IE, that is to say the direction of circulation of the electrolysis current IE at the scale of queue 2 of electrolysis tanks.

It should also be noted that the description is made in relation to a Cartesian frame of reference linked to an electrolytic cell, the X axis being oriented in a transverse direction to the electrolytic cell, the Y axis being oriented in a longitudinal direction of the electrolytic cell, and the axis Z being oriented in a vertical direction of the electrolytic cell. Longitudinal, transverse, vertical orientations, directions, planes and displacements are thus defined in relation to this reference system.

It will be noted that the electrolysis cells of the aluminum smelter are preferably large electrolysis cells, the use of large electrolysis cells being made possible by the particular configuration of the electrolysis cells of the aluminum smelter according to the invention, as described in more detail below. The dimensions of an electrolytic cell are defined by the surface on the ground that this electrolytic cell represents. For this we consider that the dimensions of the tank are defined by the external dimensions of its box. By large electrolysis cell means electrolysis cell having a width greater than 4 m, preferably greater than or equal to 5 m, and in particular greater than or equal to 6 m, and / or having a length greater than 15 m, preferably greater than or equal to 20 m, and in particular greater than or equal to 25 m.

The figure 6 shows in more detail the electrolytic cells 10 of the aluminum smelter 1 according to one embodiment. As illustrated in this figure, the electrolysis tanks 10 of the aluminum smelter 1 comprise a box 12, anode assemblies 14, a cathode 16 through which cathode electrical conductors 18 pass for collecting the electrolysis current IE at the cathode 16 to conduct it to other electrical conductors called cathode outputs 20 outside the box 12, electrical rise and connection 22 conductors to the anode assemblies 14 to conduct the electrolysis current IE to the anode assemblies 14, and conductors electrical routing 24 connected to the cathode outputs 20 and intended to conduct the electrolysis current IE from the cathode outputs 20 to the electrical rise and connection conductors 22 of the next electrolysis tank 10.

The box 12 includes an inner coating 26 made of refractory materials. As illustrated on figures 6 and 7 , the box 12 preferably comprises cradles 28 of reinforcements. The box 12 can be metallic, for example steel.

The anode assemblies 14 comprise a support 30 and at least one anode 32. The anode or anodes 32 are in particular made of carbonaceous material and more particularly of the precooked type. The support 30 comprises a first electrically conductive part 34, for example a cross member, extending essentially in a transverse direction X of the electrolytic cells 10, and a second electrically conductive part 36 formed of several electrically conductive elements capable of be called "logs", the logs comprising a distal end electrically connected to the first part 34 of the support 30 and a proximal end electrically connected to the anode (s) 32 in order to conduct the electrolysis current IE from the first part 34 of the support 30 to this or these anodes 32. The anode assemblies 14 are intended to be removed and replaced periodically when the anode or anodes 32 are worn.

The cathode 16 can be formed from several cathode blocks of carbonaceous material. The cathode 16 is crossed by the cathode conductors 18 intended to collect the electrolysis current IE at the cathode 16 to lead it to the cathode outlets 20 advantageously exiting through the bottom of the box 12, as illustrated in the figure 6 .

The electrical rise and connection conductors 22 extend upwards along two opposite longitudinal edges 38 of each electrolytic cell 10, to conduct the electrolysis current IE towards the anode assemblies 14. It is specified that the edges longitudinal 38 of the electrolytic cells 10 correspond to the edges of larger dimension, that is to say the edges of the electrolytic cells 10 which are substantially parallel to the longitudinal direction Y. By way of example, an electrolytic cell 10 operating with an intensity of 400 to 1000 k amperes can for example preferably comprise from 4 to 40 rise and connection conductors 22 distributed regularly over the entire length of each of its two edges longitudinal 38. The electrical rise and connection conductors 22 include electrical rise and upstream connection conductors 22A, that is to say adjacent to the longitudinal edge 38 upstream of the electrolysis tank 10, and electrical conductors of rise and downstream connection 22B, that is to say adjacent to the longitudinal edge 38 downstream of the electrolysis tank 10. The upstream connection and upstream electrical conductors 22A are electrically connected to an upstream end of the first part 34 of the support 30, and the upstream connection and upstream connection conductors 22B are electrically connected to a downstream end of this first part 34 of the support 30.

The electrical routing conductors 24 are connected to the cathode outputs 20 and are intended to conduct the electrolysis current IE from these cathode outputs 20 to the electrical rise and connection conductors 22 of the next electrolysis tank 10 of the series.

The cathode conductors 18, the cathode outputs 20 and / or the routing conductors 24 may be metal bars, possibly composite, for example made of aluminum, copper and / or steel.

A sheet of liquid aluminum 40 is formed during the electrolysis reaction.

It will be noted that the electrolysis cells 10 of the aluminum smelter 1 according to the invention are preferably electrolysis cells 10 of the anode replacement type by upward vertical traction of the anode assemblies 14 above the cell 10 of electrolysis, as shown by the electrolysis tank 10 right on the figure 6 . The rise and connection conductors 22 extend on either side of the box 12 without extending in line with the anodes 32, that is to say without extending in a volume obtained by vertical projection of the area of the anodes 32 projected in a horizontal plane. In addition to the advantage that this represents for allowing an anode change 32 by vertical upward traction, this also makes it possible to reduce the length of the rise and connection conductors 22 compared to the use of rise and connection conductors 130 of the type classic, visible on the figure 2 , which typically extend above the electrolysis tank 110 up to the longitudinal central part of the electrolysis tank 110. This helps to reduce manufacturing costs. It is also noted that the horizontal part 34 of the support 30 is supported and connected at each of the two longitudinal edges 38 of each electrolytic cell 10.

Thus, the anode assembly is no longer supported and electrically connected above the box and the anodes by means of a superstructure 132, as is the case for the electrolysis tanks of the state of the art illustrated on the figure 2 . The electrolysis cells 10 of the aluminum smelter 1 according to this embodiment of the invention are therefore free of superstructure. The absence of superstructure makes it possible to widen and / or lengthen the electrolytic cells 10, in order to benefit from large electrolytic cells 10, as mentioned previously. Such a widening or lengthening of the electrolytic cells 110 of the prior art is not possible due to the superstructure 132, since this widening and / or lengthening would result in a widening and / or lengthening of the superstructure 132 itself even, therefore the span of the beam between the feet supporting the beam and the weight to be supported by this superstructure 132. There are superstructures comprising one or more intermediate arches of support of the beam, but such intermediate arches, extending transversely above the box 112 and the anodes 118, are bulky and complicate the operations on tanks, in particular the changes of anodes.

The fact of being able to increase the dimensions of the electrolysis cells, combined with an increase in the intensity of the electrolysis current IE, without creating MHD instabilities due to the particular magnetic configuration of the aluminum smelter 1 according to the The invention described in more detail below, makes it possible to substantially improve the efficiency of the aluminum smelter 1 in comparison with the state of the art.

The electrical conductors of the aluminum smelter 1 (in particular electrical rise and connection conductors 22, support 30, cathode outputs 20, routing conductors 24, electrical conductors of the first and second electrical compensation circuits 4, 6) are indeed configured to obtain effective compensation of the horizontal and vertical components of the magnetic field generated by the circulation of the electrolysis current IE and, in so doing, a limitation of the instabilities MHD, therefore an improved efficiency.

More particularly, the distribution of the electrolysis current IE passing through the electrical rise and connection conductors 22 is asymmetrical between the electrical rise and connection upstream 22A and downstream 22B conductors 22B. The electrolysis current IE is divided into an upstream electrolysis current IEA, which runs through all of the upstream electrical conductors upstream and upstream connection 22A of the electrolysis cells, and a downstream electrolysis current IEB, which runs through all of the upstream and downstream connection electrical conductors 22B of the electrolysis cells 10. The intensity of the upstream electrolysis current IEA is equal to] 50-100 [% of the intensity of the electrolysis current IE, while the intensity of the downstream electrolysis current IEB is equal to] 0-50 [ % of the intensity of the electrolysis current IE, it being specified that the upstream electrolysis currents IEA and downstream IEB are complementary, that is to say that the sum of the intensities of the upstream electrolysis currents IEA and downstream IEB is equal to the intensity of the electrolysis current IE.

This asymmetrical distribution with preponderance of the upstream relative to the downstream is particularly advantageous when the electrolysis tanks 10 of the smelter are large-scale electrolysis tanks. Indeed, the asymmetry upstream / downstream of the electrolysis current IE makes it possible to avoid resorting to an excessive increase in cross section of the routing conductors 24 under the electrolysis tank 10, so that savings in materials and in space are made, and this without prejudice to the magnetic stability of the electrolytic cell 10.

The choice of the distribution between intensities of the upstream and downstream electrolysis currents IEA, IEB is made by economic study. This choice mainly depends on the distance between two tanks and the height of the tanks. This distribution is achieved by adjusting the sections of the electrical conductors of the upstream and downstream electrical circuits, taking into account their length.

The rise and connection conductors 22 extend substantially vertically, and preferably only vertically, so that the path of the electrolysis current IE through the rise and connection conductors 22 generates a magnetic field with only horizontal components, but no vertical component.

Likewise, the second part 36 of the support 30 of the anode assembly 14, and / or the cathode outputs 20, advantageously extend in a vertical direction, and preferably only vertically, so that the flow of the current IE electrolysis through this second part 36 and / or through the cathode outputs 20 generates a magnetic field having only horizontal components, but no vertical component.

It will be noted that the cathode outputs 20 advantageously pass through the bottom of the box 12. The fact of having cathode outputs 20 from the bottom, instead of cathode outputs on the sides of the electrolysis tank as in the prior art ( figure 2 ), makes it possible to reduce the length of the routing conductors 24. The reduction in length of the routing conductors 24 allows, in addition to saving raw materials, a substantial reduction in horizontal currents in liquids and, therefore, better stability MHD .

Furthermore, still with a view to effectively compensating for the magnetic field created by the circulation of the electrolysis current IE, the first part 34 of the support 30 of the anode assembly 14 extends, preferably only, in a substantially horizontal and parallel manner to the transverse direction X of the electrolysis cells 10.

Likewise, the routing conductors 24 advantageously extend in a substantially rectilinear manner and parallel to the transverse direction X of the electrolysis cells 10, up to the rise and connection conductors 22 of the next electrolysis cell 10. The cost of the electrical routing conductors 24 is thus limited, by minimizing their length. The magnetic fields generated by these electrical conductors 24 of routing are also limited with respect to the state of the art, and in particular with respect to the self-compensated electrolysis cells of the state of the art.

The electrical rise and connection conductors 22 are preferably distributed at regular intervals over substantially the entire length of the longitudinal edge 38 to which they are adjacent. In other words, the same distance separates two consecutive rise and connection electrical conductors 22 in the longitudinal direction Y. This improves the balance of the longitudinal horizontal component of the magnetic field (that is to say parallel to the length of the electrolysis tank 10).

The electrical upstream and upstream connection conductors 22A and the electrical upstream and downstream connection conductors 22B may be arranged equidistant from a longitudinal median plane YZ of each electrolytic cell 10, that is to say a plane substantially perpendicular to the transverse direction X and separating each electrolytic cell 10 into two substantially equal parts. In other words, the upstream electrical conductors and upstream connection 22A are at the same distance from this longitudinal median plane YZ as the upstream electrical conductors and downstream connection 22B. In addition, the upstream electrical conductors and upstream connection 22A are advantageously arranged in a manner substantially symmetrical to the upward electrical conductors and downstream connection 22B, with respect to this longitudinal median plane YZ. The advantageously substantially asymmetrical characteristic of the distribution of the horizontal magnetic field in liquids is thus further improved.

To limit the magnetic field generated by the circulation of the electrolysis current through the electrical rise and connection conductors 22, these electrical rise and connection conductors advantageously extend above liquids (electrolytic bath) at a height h between 0 and 1.5 meters. The length of the rise and connection conductors 22 is thus greatly reduced compared to the rise and connection conductors 130 of conventional type which extend to heights greater than two meters for the electrolysis tanks 130 in the state of the technique.

To improve the compactness of the aluminum smelter 1 and limit the costs of raw materials, the upstream conductors and upstream connection 22A of the electrolytic cells 10 can be staggered with respect to the upstream conductors and downstream connection 22B of the electrolytic cell 10 preceding it in the queue 2. This in fact allows the electrolytic cells 10 to be brought as close as possible to one another, that is to place more electrolytic cells 10 in series over the same distance, which increases the yield, that is to reduce the length of a line 2 of electrolytic cells 10, therefore saving space and achieving structural savings.

For effective compensation of the horizontal components of the magnetic field generated by the circulation of the electrolysis current IE, that is to say to have anti-asymmetrical horizontal components, the first part 34 of the support 30 of the anode assembly 14 and the second part 36 of the support 30 of the anode assembly 14 are configured so that the intensity of the fraction of electrolysis current flowing through an upstream half of this second part 36 is substantially equal to the intensity of the fraction of electrolysis current flowing through a downstream half of this second part 36. In other words, and as shown in the figure 8 , the intensity of the fraction of electrolysis current passing through all of the logs situated on the upstream side of a longitudinal median plane YZ of the electrolysis tank is substantially equal to the intensity of the fraction of electrolysis current crossing all the logs located downstream side of this longitudinal median plane YZ. In particular, as can be seen from the S9 segment of the figure 8 read in combination with the table of figure 9 , part of the upstream electrolysis current IEA reaches the logs located on the downstream side of the median plane YZ of the electrolysis tank 10. This is achieved by an overall electrical balancing of the different sections of conductors.

The principle of magnetic compensation or balancing of the aluminum smelter 1 according to the invention makes it possible to obtain for the aluminum smelter 1 a circuit of conductors which can be produced in a modular fashion, as illustrated on the figure 7 . Each module M can for example comprise an electrical conductor of the first electrical compensation circuit 4 and a certain number of conveying conductors 24 and rising and connecting conductors 22 associated for each electrolytic cell 10. The fact is that the electrical conductors included in each module M (rise and connection conductors 22, anode assembly 14, cathode 16, cathode conductors 18, cathode outputs 20, routing conductors 24, electrical conductors of the first compensation circuit 4 ) are configured to generate the same predetermined magnetic configuration. In other words, the electrical conductors of each module M are arranged and traversed by currents such that each module M generates the same vertical and horizontal components of magnetic field.

The circuit of conductors, and therefore each electrolytic cell 10, can be composed of a certain number N of modules M, determining the length of the electrolytic cells 10 and the intensity of the current passing through the electrolytic cells ( the intensity of the electrolysis current IE circulating in the series of electrolysis cells being equal to the intensity of the fraction of electrolysis current passing through each module M multiplied by the number N of modules M).

It is important to specify that, taking into account the magnetic configuration of each module M, the choice of the number N of modules M per cell 10 of electrolysis, compensated by the secondary compensation circuit 6 on the ends of cell, does not disturb that little the magnetic equilibrium of the electrolysis cells 10. This makes it possible to obtain an optimal magnetic configuration, and this, for amperages above 1000 kA or even 2000 kA during the design or an extension of the length of the electrolysis cells 10 by the addition of such modules. Conversely, the elongation of self-compensated or compensated type electrolysis cells by magnetic compensation circuits arranged on the sides of the cells known from the prior art means that the conductor circuits must be completely redesigned. Also, the ratio of the quantity of material forming the circuit of conductors brought back to the production surface of the electrolytic cells 10 does not deteriorate when the electrolytic cells 10 are lengthened, it increases in proportion to the number N of modules M and the intensity passing through the electrolysis cells 10. Thus, the electrolytic cells 10 can be extended simply as required and the intensity of the current passing through them is not limited. The modular construction of the electrical conductors of the electrolysis cells 10 therefore offers an advantage in terms of scalability, since this modular construction, combined with a simple adjustment of the amperage of the secondary compensation circuit, makes it possible to modify the cells 10 d ' electrolysis without affecting their magnetic and electrical balancing.

The table of the figure 9 , read in combination with the figure 8 , shows for a module the intensity values traversing the different electrically conductive elements of the electrolytic cells 10, these conductive elements being symbolized by segments: S1 for the rise and upstream connection conductors 22A; S2, S5 and S8 for the first part 34 of the support 30; S3 and S9 for the second part 36 of the support 30, the anode (s) 32, the electrolytic bath, the aluminum sheet 40, the cathode 16, the cathode conductors 18 and the cathode outputs 20; S4, S6 and S10 for routing conductors 24; S7 for upstream and downstream connection conductors 22B.

It is specified that the sum of the intensities i and ia indicated in the table of figures 9 , 13 and 14 is equal to the intensity of the upstream electrolysis current IEA divided by the number N of modules of the electrolysis tank 10; the intensity ib is equal to the intensity of the downstream electrolysis current IEB divided by the number N of modules of the electrolysis tank 10; the sum of ia and ib is equal to i; the sum of the upstream and downstream electrolysis currents IEA, IEB is therefore equal to 2i multiplied by the number N of modules; and the intensity of the electrolysis current IE flowing through the series of electrolysis cells is equal to the sum of the intensity of the upstream electrolysis current IEA traversing the entire upstream part of the electrolysis cell and l intensity of the downstream electrolysis current IEB passing through the entire downstream part of the electrolysis tank, that is to say the product of 2i and the number N of modules of the electrolysis tank.

The figures 10 to 12 are schematic wire-frame views of the electrical circuit traversed by the electrolysis current in a module of an electrolysis tank 10 of the aluminum smelter 1, and showing for this electrolysis tank 10 the three main zones P1, P2, P3 generating a significant disturbing magnetic field: an upstream area P1, a central intermediate area P2, and a downstream area P3 symmetrical with the upstream area P1 with respect to a longitudinal median plane YZ of the electrolysis cells 10.

The table of the figure 13 , read in combination with figures 10, 11 and 12 , schematically shows the vertical component of the magnetic field generated by the electrical conductors (represented schematically by segments) of the electrolytic cell 10, respectively in the three zones P1, P2, P3 of the electrolytic cells, by the first and second compensation circuits 4, 6. By adding the contributions of each of these electrical conductors, and that of the first and second compensation circuits 4, 6, it can be seen that the vertical component Bz of magnetic field generated by the circulation of the current d electrolysis is zero, that is to say perfectly compensated. Thus, MHD instabilities are minimized; this offers the possibility of substantially improving the yield.

In addition, the table of the figure 14 , also read in combination with figures 10, 11 and 12 , schematically shows the longitudinal horizontal component of the magnetic field generated by the circulation of the electrolysis current through the electrical conductors (symbolized by segments) of the electrolysis tank 10, zone by zone, and through the first and second circuits 4, 6. The horizontal transverse component of the magnetic field is, for its part, well asymmetrical because the conductors are symmetrical with respect to the plane XZ. By adding the contributions of each segment, and those of the first and second compensation circuits 4, 6, it can be seen that the longitudinal horizontal component By of the magnetic field is asymmetric (opposite in the upstream and downstream areas P1, P3, and zero in the central P2 zone). This antisymmetry eliminates the deleterious effects linked to the horizontal components of the magnetic field.

The first electrical compensation circuit 4 is described in more detail below.

The first electrical compensation circuit 4 extends under the electrolytic cells 10. This first electrical compensation circuit 4 is intended to be traversed by a first compensation current IC1, in the opposite direction to the overall direction of circulation of the electrolysis current IE, as can be seen on the figures 5 and 7 . It will be recalled that by overall direction of circulation of the electrolysis current IE is meant the direction of circulation of the electrolysis current IE on the scale of the aluminum smelter 1 or of the row or rows 2 of electrolysis cells 10.

The first electrical compensation circuit 4 comprises electrical conductors which may be metal bars, for example made of aluminum, copper or steel, or, advantageously, electrical conductors made of superconductive material, the latter making it possible to reduce energy consumption and, because of their lower mass than that of equivalent metal conductors, reduce the structural costs for supporting them or for protecting them from possible metal flows by means of metal deflectors 42 ( figure 7 ) or by burying them. Advantageously, these electrical conductors made of superconductive material can be arranged to perform several turns in series under the row or rows of tanks, as described in the patent application. WO2013007893 on behalf of the plaintiff.

The aluminum smelter 1 comprises a supply station 44 configured to circulate through the first electrical compensation circuit 4 a current intensity IC1 equal to twice the intensity of the downstream electrolysis current IEB, more or less 20% close, and preferably within 10%.

This supply station 44 can be a clean electrical supply station, that is to say distinct from the supply station 8 supplying the electrolysis cells 10 with electrolysis current IE. The power supply station 44 of the first compensation circuit 4 is therefore exclusively dedicated to supplying this first compensation circuit 4.

The first electrical compensation circuit 4 is thus also independent of the main electrical circuit traversed by the electrolysis current IE comprising in particular the row or rows 2 of electrolysis cells 10. If the first electrical compensation circuit 4 is damaged, for example a piercing of one of the electrolysis cells 10 by the liquids contained in the electrolysis cells, the temperature of which is close to 1000 ° C., the reaction d electrolysis can continue, with a lower yield however since the magnetic compensation is impacted. In addition, the intensity of the first compensation current IC1 can be modified independently of the electrolysis current IE. This is of paramount importance in terms of scalability and adaptability. On the one hand because this allows, in the event of an increase in the intensity of the electrolysis current IE during the lifetime of the aluminum smelter 1, to adapt the magnetic compensation to this development, by varying the intensity of the first compensation current IC1 as required. On the other hand because it makes it possible to adapt the amperage of the first compensation current IC1 to the characteristics and the quality of the available alumina. This makes it possible to control the speed of the MHD flows to promote or limit the stirring of the liquids and the dissolution of the alumina in the bath according to the characteristics of alumina available, which ultimately contributes to the best possible yield given the supply of alumina.

The electrical conductors of the first electrical compensation circuit 4 extend under the electrolytic cells, together forming a sheet of parallel electrical conductors, advantageously from two to twelve, and preferably from three to ten parallel electrical conductors. In other words, in longitudinal section of an electrolytic cell 10, that is to say in a longitudinal plane YZ of the electrolytic cell 10, as shown in the figure 7 , the first electrical compensation circuit 4 extends in several places of the electrolysis tank 10. It will be noted that the first compensation current IC1 flows in the opposite direction to the overall direction of circulation of the electrolysis current IE, this through all the electrical conductors forming the sheet. The sheet can be formed by the same electrical circuit forming several turns or loops in series under the electrolytic cells 10, each loop corresponding to an electrical conductor of the sheet. Alternatively, the sheet may be formed by a division into a bundle of parallel electrical conductors of the first electrical compensation circuit 4, the latter possibly being able to form a single loop under the electrolytic cells 10 if necessary.

The intensity of the first compensation current IC1 is equal to the sum of the intensities of the compensation current flowing through each electrical conductor of the sheet. Preferably, the intensity of the first compensation current IC1 in each electrical conductor of the sheet is equal to the intensity of the first compensation current IC1 divided by the number of electrical conductors of this sheet.

The electrical conductors of the sheet are advantageously equidistant from each other. The same distance therefore separates two adjacent electrical conductors from the sheet. This also improves the compensation for the unfavorable magnetic field.

The electrical conductors of the sheet can extend parallel to each other. They preferably extend parallel to the transverse direction X of the electrolytic cells 10. Furthermore, the electrical conductors forming the sheet can all be arranged in the same horizontal plane XY. This also makes it possible to improve the compensation of the magnetic field generated by the circulation of the electrolysis current.

In addition, the electrical conductors of the sheet may extend substantially symmetrically with respect to the transverse median plane XZ of the electrolytic cells, that is to say with respect to the plane perpendicular to the longitudinal direction Y, this plane separating the electrolytic cells 10 in two substantially equal halves.

According to the example of the figure 7 , the first electrical compensation circuit 4 forms a sheet of three substantially equidistant conductors and arranged in the same substantially horizontal XY plane. This layer comprises as many electrical conductors as the electrolysis tank 10 comprises modules M.

In fact, the sheet is advantageously configured so that each module M of the electrolytic cell 10 includes the same number of electrical conductors of the first electrical compensation circuit 4. This makes it possible to obtain compensation for the magnetic field per module, which produces better effects and offers a significant advantage in terms of implementation and scalability.

The second electrical compensation circuit 6 is described in more detail below.

The second electrical compensation circuit 6 extends on at least one transverse side of the electrolytic cells 10, substantially parallel to the transverse direction X of the electrolytic cells 10, that is to say parallel to the row or rows 2 of electrolytic cells 10. The second electrical compensation circuit 6 is intended to be traversed by a second compensation current IC2, in the same direction as the overall direction of circulation of the electrolysis current IE.

Preferably, the second electrical compensation circuit 6 extends along the two transverse sides of the electrolysis cells 10, as illustrated in the figure 5 . In this case, the internal conductor 61 designates the electrical conductors of the second electrical compensation circuit 6 which are located between the first two adjacent rows 2 of electrolytic cells 10, and the external conductor 62 designates the electrical conductors of the second electrical circuit of compensation 6 which are located on the outer side of the rows 2 of electrolytic cells 10, that is to say which are on the other side of the electrolytic cells 10 with respect to the electrical conductors forming the internal loop 61. The loop internal 61 is traversed by a second compensation current IC21 and the external loop 62 is traversed by a second compensation current IC22. The second compensation currents IC21 and IC22 flow in the same direction. The sum of the currents IC21 and IC22 flowing respectively in the internal loop 61 and in the external loop 62 is equal to the compensation current IC2. The internal loop 61 and / or the external loop 62 can possibly make several turns in series; if necessary the intensity of the current IC21, respectively IC22, is the product of the number of revolutions in series by the intensity of the current flowing in each revolution in series.

The aluminum smelter 1 comprises a supply station 46 which is advantageously configured to circulate through the second electrical compensation circuit 6 (internal loop 61 and / or external loop 62) a total intensity (if necessary loop internal 61 plus external loop 62) of compensation current IC2 of between 50% and 100% of the difference in intensity between the upstream and downstream electrolysis currents, and preferably between 80% and 100% of the difference of intensity between the upstream and downstream electrolysis currents. This intensity value, fixed as a function of the asymmetrical distribution of the electrolysis current IE in each electrolysis tank 10, offers, in synergy with the choice of the asymmetric distribution value IEA, IEB and of the intensity of the first compensation current IC1, the best magnetic field compensation results, effectively applicable to large size electrolytic cells 10.

Preferably, the intensity of the current IC21 flowing in the internal loop 61 differs from the intensity of the current IC22 flowing in the external loop 62. More particularly, the intensity of the current IC21 flowing in the internal loop 61 is advantageously greater than 1 intensity of the current IC22 flowing in the external loop 62.

The current flowing through the internal loop 61 may be increased in order to compensate for the impact of the neighboring file on the vertical magnetic field. This increase will have a typical value close to (within 50%) IE2xD61 / DP2, where IE2 = IE-IC1 + IC2 = IE + IEA - 3 IEB and DP2 is the distance from the neighboring line to the center of the tank and D61 is the distance from the internal loop 61 to the center of the tank. For a conventional electrolysis series IE2 is greater than or equal to IE. It can be noted that IE + IEA - 3 IEB is much lower than IE. This is a gain from this design which allows the neighboring queue to be brought closer because the creation of the magnetic field by the neighboring queue is much weaker at no additional cost compared to what is known to those skilled in the art.

The supply station 46 supplying the second compensation circuit 6 can be a clean electrical supply station, that is to say separate from the supply station 8 supplying the electrolysis cells 10 with electrolysis current IE and distinct from the supply station 44 supplying the first electrical compensation circuit 4. The electrical supply station 46 of the second compensation circuit 6 is therefore exclusively dedicated to supplying this second compensation circuit 6. The second electrical compensation circuit 6 is thus also independent of the main electrical circuit traversed by the electrolysis current IE. The intensity of the second compensation current IC2 can be modified independently of the electrolysis current IE, thus offering substantial advantages in terms of scalability and adaptability of the aluminum smelter 1, as explained previously concerning the first electrical compensation circuit 4 Advantageously, the second compensation circuit 6 can also be distinct from the first compensation circuit 4.

When the second electrical compensation circuit 6 extends on both sides of the electrolysis cells 10, the electrical conductors forming this second electrical compensation circuit 6 can advantageously be symmetrical with respect to a median transverse plane XZ of the electrolysis cells 10 . This improves compensation for the deleterious magnetic field.

Furthermore, still with the aim of effectively compensating this magnetic field, created by the circulation of the electrolysis current IE, the electrical conductors of the second electrical compensation circuit 6 advantageously extend in the same horizontal plane XY. Preferably, this horizontal plane XY is located at the height of the sheet of liquid aluminum 40 formed inside the electrolysis cells 10 during the electrolysis reaction.

It will be noted that the electrical conductors forming the second electrical compensation circuit 6 can advantageously be configured so as to limit the “end of line” effects, as shown on the figure 5 .

The electrical conductors forming the second electrical compensation circuit 6 can be metal bars, for example made of aluminum, copper or steel, or, advantageously, to electrical conductors made of superconductive material, the latter making it possible to reduce energy consumption and, because of their lower mass than that of equivalent metal conductors, reduce the structural costs for supporting them. Advantageously, these electrical conductors made of superconductive material can be arranged to perform several turns in series on the side or sides of the rows 2 of electrolytic cells 10, as described in the patent application. WO2013007893 on behalf of the plaintiff.

• le fait de faire circuler, en sens contraire du sens de circulation global du courant d'électrolyse IE, le premier courant de compensation IC1 à travers le premier circuit électrique de compensation 4,
• le fait de faire circuler, dans le même sens de circulation que le sens de circulation global du courant d'électrolyse IE, le deuxième courant de compensation IC2 à travers le deuxième circuit électrique de compensation 6.

The invention also relates to a method of compensating for the magnetic field created by the circulation of an electrolysis current IE in the electrolytic cells 10 of the aluminum smelter 1 described above. This process includes:

• circulating, in the opposite direction to the overall direction of circulation of the electrolysis current IE, the first compensation current IC1 through the first electrical compensation circuit 4,
• circulating, in the same direction of circulation as the overall direction of circulation of the electrolysis current IE, the second compensation current IC2 through the second electrical compensation circuit 6.

The method also advantageously comprises the fact of asymmetrically distributing the electrolysis current IE between the upward electrical conductors and upstream connection 22A and the upward electrical conductors and downstream connection 22B.

This step of asymmetric distribution of the electrolysis current between upstream and downstream of the electrolysis tanks 10 comprises the separation of the electrolysis current IE into an upstream electrolysis current IEA, which flows through all of the upstream and upstream electrical connection conductors 22A of each electrolysis tank 10, so that the intensity of the upstream electrolysis current IEA is between] 50-100 [% of the intensity of the electrolysis current IE, and in a downstream electrolysis current IEB, which flows through all of the electrical upstream and downstream connection conductors 22B of each electrolysis tank 10, so that the intensity of the downstream electrolysis current IEB is included between] 0-50 [% of the intensity of the electrolysis current IE, the sum of the intensities of the upstream and downstream electrolysis currents IEA, IEB being equal to the intensity of the electrolysis current IE.

The step of circulating the first compensation current IC1 is advantageously such that the intensity of the first compensation current IC1 is equal to twice the intensity of the downstream electrolysis current IEB, to within plus or minus 20%, and preferably within 10%.

The step of circulating the second compensation current IC2 is advantageously such that the total intensity (internal loop 61 + external 62) of the second compensation current IC2 is between 50% and 100% of the difference in intensity between the upstream IEA and downstream IEB electrolysis currents, and preferably between 80% and 100% of the difference in intensity between the upstream and downstream electrolysis currents.

For these intensity values of the upstream electrolysis current IEA, of the downstream electrolysis current IEB, of the first compensation current IC1 and of the second compensation current IC2, the Applicant has observed that the magnetic field generated by the circulation of the current is most effectively compensated.

In addition, the intensity of the current IC21 flowing in the internal loop 61 may differ from the intensity of the current IC22 flowing in the external loop 62. More particularly, the intensity of the current IC21 flowing in the internal loop 61 is advantageously greater than the intensity of the current IC22 flowing in the external loop 62.

Furthermore, the method can advantageously include a step of analyzing at least one characteristic of the alumina in at least one of the electrolysis tanks 10 of the aluminum smelter 1 described above, and determining a distribution of values. of the intensity of the upstream and downstream electrolysis currents IEA, IEB to be circulated as a function of this analyzed characteristic, which also defines, if necessary, the intensity values of the first and second compensation currents IC1, IC2 and if necessary upstream and downstream electrolysis currents IEA, IEB. The intensity values of the first and second compensation currents IC1, IC2, and if necessary upstream and downstream electrolysis currents IEA, IEB, can then be modified up to the values determined previously if the intensity values of the first and second compensation currents IC1, IC2 and initial upstream and downstream electrolysis currents IEA, IEB differ from the values thus determined. Thus, the method makes it possible to modify the magnetic compensation, in order to increase or reduce the mixing of the liquids while controlling the MHD instabilities. In general, the stronger the mixing (or flow) of liquids, the more efficient the alumina dissolution will be but the more the bath / metal interface will be unstable (= MHD instability), which can degrade the efficiency of the tanks. Such a method is particularly advantageous with the configuration of the electrical conductors described above because it makes the electrolytic cells 10 magnetically very stable and therefore offers a greater range for modulating / optimizing the mixing according to the quality of the alumina. . The characteristics of the alumina analyzed can in particular be the ability of the alumina to dissolve in the bath, the fluidity of the alumina, its solubility, its fluorine content, its humidity, etc.

The determination of a distribution of intensity values of the upstream and downstream compensation currents IEA, IEB and / or of intensity values of the first and second compensation currents IC1, IC2 according to the characteristics of the alumina analyzed can be in particular carried out by the use of an abacus, for example produced by a person skilled in the art by calculation, experimentation and recording of the optimal correspondences intensities of the upstream and downstream electrolysis currents IEA, IEB / characteristics of alumina. This involves quantifying the intensity of the mixing of the desired liquids with regard to the level of MHD instabilities.

It may happen that the alumina available for continuous operation of the smelter is of different quality, in particular more or less pasty, and therefore having different abilities to dissolve in the electrolysis bath. In this case, the movements of the liquids in the electrolytic cells 10 are an asset, because they allow this alumina to be stirred to promote its dissolution. However, in the case of self-compensation in particular (used in the state of the art), the magnetic field at the origin of the movements of liquids is directly compensated via the electrolysis current itself, with a distribution of the magnetic field imposed and frozen by the path of the routing conductors. It is therefore not possible in self-compensating aluminum smelters to introduce a voluntary and temporary imbalance in the compensation of the magnetic field in order to increase the intensity of the mixing of alumina in the tanks, in order to increase the efficiency of dissolution. So when the available alumina is only alumina more difficult to dissolve than typically, the efficiency of self-compensating smelters can be significantly affected.

Of course, the invention is in no way limited to the embodiment described above, this embodiment having been given only by way of example. Modifications are possible. Thus, the present invention is for example compatible with the use of anodes of the "inert" type at the level of which oxygen forms during the electrolysis reaction.

Claims (18)

1. Aluminum smelter (1) comprising at least one row (2) of electrolytic cells (10) arranged transversely in relation to the length of said at least one row (2), one of the electrolytic cells (10) comprising anode assemblies (14) and rising and connecting electrical conductors (22) to the anode assemblies (14), characterized in that the rising and connecting electrical conductors (22) extend upwardly along two opposite longitudinal edges (38) of the electrolytic cell (10) for conducting electrolysis current (IE) to the anode assemblies (14), and in that the aluminum smelter (1) includes:

   - at least one first electrical compensation circuit (4) extending beneath the electrolytic cells (10), said at least one first electrical compensation circuit (4) being possibly traversed by a first compensation current (IC1) designed to flow under the electrolytic cells (10) in the opposite direction to the global direction of flow of the electrolysis current (IE),

   - at least one second electric compensation circuit (6) extending over at least one side of said at least one row (2) of electrolytic cells (10), said at least one second electric compensation circuit (6) being possibly traversed by a second compensation current (IC2) designed to flow in the same direction as the global direction of flow of the electrolysis current (IE).
2. Aluminum smelter (1) according to claim 1 in which the rising and connecting electrical conductors (22) comprise upstream rising and connecting electrical conductors (22A), adjacent to the upstream longitudinal edge (38) of the electrolytic cell (10), and downstream rising and connecting electrical conductors (22B), adjacent to the downstream longitudinal edge (38) of the electrolytic cell (10), and the aluminum smelter (1) is laid out so that the distribution of the electrolysis current (IE) is asymmetrical between the upstream (22A) and downstream (22B) rising and connecting electrical conductors, the intensity of the upstream electrolysis current (IEA) designed to run through all of the rising and connecting electrical conductors upstream (22A) of the electrolytic cell (10) being equal to] 50-100 [% of the intensity of the electrolysis current (IE), and the intensity of the downstream electrolysis current (IEB) designed to run through all of the rising and connecting electrical conductors downstream (22B) of the electrolytic cell (10) is equal to] 0-50 [% of the intensity of the electrolysis current (IE), the total intensity of the upstream and downstream electrolysis currents (IEA), (IEB) being equal to the intensity of the electrolysis current (IE).
3. Aluminum smelter (1) according to claim 2 in which the aluminum smelter comprises a power station (44) configured to cause to flow through said at least one first compensating electrical circuit (4) a first compensating current (IC1) of intensity equal to twice the intensity of the downstream electrolysis current (IEB) to the nearest 20%, and preferably to the nearest 10%.
4. Aluminum smelter (1) according to claim 2 or 3 in which the aluminum smelter (1) includes a power station (46) configured to cause to flow through said at least one second electrical compensating circuit (6) a second compensating current (IC2) of intensity between 50% and 100% of the difference in intensity between the upstream and downstream electrolysis currents (IEA, IEB), and preferably between 80% and 100% of the difference in intensity between the upstream and downstream electrolysis currents (IEA, IEB).
5. Aluminum smelter (1) according to claims 1 to 4 in which the rising and connecting electrical conductors (22) are distributed at regular intervals along the longitudinal edge (38) of the electrolytic cell (10) to which these rising and connecting electrical conductors (22) are adjacent.
6. Aluminum smelter (1) according to claims 1 to 5 in which said at least one first electrical compensating circuit (4) includes electrical conductors extending under the electrolytic cells (10) together forming a layer made up of a plurality of parallel electrical conductors, typically from two to twelve, and preferably three to ten parallel electrical conductors.
7. Aluminum smelter (1) according to claim 6 in which the electrical conductors of said layer are arranged at regular intervals from each other along a longitudinal direction (Y) of the electrolytic cells (10).
8. Aluminum smelter (1) according to one of claims 6 to 7 in which the electrical conductors of said layer are arranged in the same horizontal plane (XY).
9. Aluminum smelter (1) according to one of claims 1 to 8 in which said at least one second electric compensating circuit (6) includes electrical conductors extending from each side of said at least one row (2) of electrolytic cells (10), and the second compensating current (IC2) flows in the same direction as the direction of the overall flow of the electrolysis current (IE) on each side of the electrolytic cells (10).
10. Aluminum smelter (1) according to claim 9 in which the electrical conductors of the second compensating electrical circuit (6) extend in the same horizontal plane (XY), preferably at the height of a layer of liquid aluminum (40) formed inside the electrolytic cells (10) during the electrolysis reaction.
11. Aluminum smelter (1) according to one of claims 1 to 10 in which said at least one first electric compensating circuit (4) and/or said at least one second electric compensating circuit (6) are independent of the main electrical circuit through which the electrolysis current (IE) flows.
12. Method of compensating for a magnetic field created by the flow of an electrolysis current (IE) in a plurality of electrolytic cells (10) of an aluminum smelter (1) according to one of claims 1 to 11, the method comprising:

    - flow, in the opposite direction to the direction of overall flow of the electrolysis current (IE), of a first compensating current (IC1) through said at least one first electrical compensating circuit (4),

    - flow, in the same direction as the direction of overall flow of the electrolysis current (IE), of a second compensating current (IC2) through said at least one second electrical compensating circuit(6).
13. Method according to claim 12 in which the method comprises an asymmetric distribution of the electrolytic current (IE) between the upstream and the downstream of the electrolytic cells (10), the set of rising and connecting electrical conductors (22) upstream of the electrolytic cells (10) being traversed by an upstream electrolysis current (IEA) of intensity between ]50-100[% of the intensity of the electrolysis current (IE), and the set of rising and connecting electrical conductors (22) downstream of the electrolytic cells (10) being traversed by a downstream electrolysis current (IEB) of intensity between ]0-50[% of the intensity of the electrolysis current (IE), the sum of intensities of the upstream and downstream electrolysis currents (IEA), (IEB) being equal to the intensity of the electrolysis current (IE).
14. Method according to claim 13 in which the intensity of the first compensating current (IC1) is equal to twice the intensity of the downstream electrolysis current (IEB), to the nearest 20%, and preferably to the nearest 10%.
15. Method according to claim 12 or 13 in which, the intensity of the second compensating current (IC2) is between 50% and 100% of the difference in intensity between the upstream and downstream electrolysis currents (IEA, IEB), and preferably between 80% and 100% of the difference in intensity between the upstream and downstream electrolysis currents (IEA, IEB).
16. Method according to one of claims 12 to 15 in which said at least one second electric compensating circuit (6) comprises an inner loop and an outer loop, and wherein the intensity of a second compensating current (IC21) flowing in the inner loop is different from the intensity of a second compensating current (IC22) flowing in the outer loop.
17. Method according to claim 16 in which the intensity of the second compensating current (IC21) flowing in the inner loop is greater than the intensity of the second compensating current (IC22) flowing in the outer loop.
18. Method according to one of claims 12 to 17 in which the method comprises a step of analyzing at least one characteristic of the alumina in at least one of the electrolytic cells (10) of said aluminum smelter (1), and determining the intensity values of the first compensating current (IC1) and the second compensating current (IC2) to be made to flow as a function of said at least one characteristic analyzed.

Images