Aluminum smelter and method to compensate for a magnetic field created by the circulation of the electrolysis current of said aluminum smelter

Abstract

This aluminum smelter comprises a line of electrolytic cells arranged transversely to the line, one of the cells comprising anode assemblies and electrical conductors mounted and connecting the anode assemblies. Rising and connecting conductors extend upwardly along two opposite longitudinal edges of the cell. In addition, the aluminum smelter comprises a first electrical compensating circuit extending under the cell and which can be traversed by a first compensating current in the opposite direction to that of the electrolysis current, a second electrical compensating circuit extending on one side of the line that can be traversed by a second compensating current in the same direction as the electrolysis current.

Claims

1. Aluminum smelter comprising at least one row of electrolytic cells arranged transversely in relation to a length of said at least one row, a first electrolytic cell of the row of electrolytic cells comprising anode assemblies and rising and connecting electrical conductors to the anode assemblies, characterized in that the rising and connecting electrical conductors extend upwardly along two opposite upstream and downstream longitudinal edges of the first electrolytic cell for conducting electrolysis current to the anode assemblies, and in that the aluminum smelter includes: at least one first electrical compensation circuit extending beneath the electrolytic cells, said at least one first electrical compensation circuit being configured to be traversed by a first compensation current designed to flow under the electrolytic cells in an opposite direction to a global direction of flow of the electrolysis current, at least one second electric compensation circuit extending over at least one side of said at least one row of electrolytic cells, said at least one second electric compensation circuit being configured to be traversed by a second compensation current designed to flow in a same direction as the global direction of flow of the electrolysis current.

2. Aluminum smelter according to claim 1 in which the rising and connecting electrical conductors comprise upstream rising and connecting electrical conductors, adjacent to the upstream longitudinal edge of the first electrolytic cell, and downstream rising and connecting electrical conductors, adjacent to the downstream longitudinal edge of the first electrolytic cell, and the aluminum smelter is laid out so that a distribution of the electrolysis current is asymmetrical between the upstream and downstream rising and connecting electrical conductors, an intensity of an upstream electrolysis current designed to run through all of the rising and connecting electrical conductors upstream of the first electrolytic cell being equal to 50-100% of an overall intensity of the electrolysis current, and an intensity of a downstream electrolysis current designed to run through all of the rising and connecting electrical conductors downstream of the first electrolytic cell is equal to 0-50% of the overall intensity of the electrolysis current, a total intensity of the upstream and downstream electrolysis currents being equal to the overall intensity of the electrolysis current.

3. Aluminum smelter according to claim 2 in which the aluminum smelter comprises a power station configured to cause to flow through said at least one first compensation electrical circuit a first compensating current of intensity equal to twice the intensity of the downstream electrolysis current (IEB) to the nearest 20%.

4. Aluminum smelter according to claim 2 in which the aluminum smelter includes a power station configured to cause to flow through said at least one second electrical compensation circuit the second compensation current of an intensity between 50% and 100% of a difference in intensity between the upstream and downstream electrolysis currents.

5. Aluminum smelter according to claim 1 in which the rising and connecting electrical conductors are distributed at regular intervals along the upstream and downstream longitudinal edges of the first electrolytic cell to which the rising and connecting electrical conductors are adjacent.

6. Aluminum smelter according to claim 1 in which the rising and connecting electrical conductors along the upstream and downstream longitudinal edges are equidistant from a longitudinal central plane of the first electrolytic cell.

7. Aluminum smelter according to claim 6 in which the rising and connecting electrical conductors along the upstream and downstream longitudinal edges are arranged substantially symmetrically relative to said longitudinal central plane of the first electrolytic cell.

8. Aluminum smelter according to claim 1 in which said at least one first electrical compensation circuit includes electrical conductors extending under the electrolytic cells together forming a layer made up of a plurality of parallel electrical conductors.

9. Aluminum smelter according to claim 8 in which the electrical conductors of said layer are arranged at regular intervals from each other along a longitudinal direction of the electrolytic cells.

10. Aluminum smelter according to claim 8 in which the electrical conductors of said layer are arranged substantially symmetrically with respect to a transverse median plane of the electrolytic cells.

11. Aluminum smelter according to claim 8 in which the electrical conductors of said layer are arranged in a same horizontal plane.

12. Aluminum smelter according to claim 8 in which said at least one second electric compensation circuit includes electrical conductors extending from each side of said at least one row of electrolytic cells, and the second compensation current flows in the same direction as the global direction of flow of the electrolysis current on each side of the electrolytic cells.

13. Aluminum smelter according to claim 12 in which an intensity of an inner second compensation current flowing in an inner loop of said at least one second compensation circuit differs from an intensity of an outer second compensation current flowing in an outer loop of said at least one second compensation circuit.

14. Aluminum smelter according to claim 13 in which the intensity of the inner second compensating current flowing in the inner loop is greater than the intensity of the outer second compensating current flowing in the outer loop.

15. Aluminum smelter according to claim 12 in which the electrical conductors forming the at least one second compensating electrical circuit are substantially symmetrical with respect to a median transverse plane of the electrolytic cells.

16. Aluminum smelter according to claim 12 in which the electrical conductors of the second compensating electrical circuit extend in a same horizontal plane, at a height of a layer of liquid aluminum formed inside the electrolytic cells during an electrolysis reaction.

17. Aluminum smelter according to claim 1 in which said at least one first electric compensation circuit is independent of a main electrical circuit through which the electrolysis current flows.

18. Aluminum smelter according to claim 1 in which said at least one second electric compensating circuit is independent of a main electrical circuit through which the electrolysis current flows.

19. Aluminum smelter according to claim 1 in which the first electrolytic cell is of modular electrical construction in N modules repeated in a direction of length of the first electrolytic cell, each module comprising electrical conductors configured to generate a same predetermined magnetic configuration.

20. Aluminum smelter according to claim 1, characterized in that the at least one first electrical compensation circuit comprises electrical conductors extending beneath the electrolytic cells, and wherein the first compensation current is designed to flow through all of the electrical conductors of the at least one first electrical compensation circuit in the opposite direction to the global direction of flow of the electrolysis current.

21. Method of compensating for a magnetic field created by the flow of an electrolysis current in the aluminum smelter according to claim 1, the method comprising: causing a flow, in the opposite direction to the global direction of flow of the electrolysis current, of the first compensation current through said at least one first electrical compensation circuit, causing a flow, in the same direction as the global direction of flow of the electrolysis current, of a second compensation current through said at least one second electrical compensation circuit.

22. Method according to claim 21 in which the method comprises causing an asymmetric distribution of the electrolysis current between the upstream and the downstream of the electrolytic cells, an upstream set of the rising and connecting electrical conductors upstream of the electrolytic cells being traversed by an upstream electrolysis current of an intensity between 50-100% of an overall intensity of the electrolysis current, and a downstream set of the rising and connecting electrical conductors downstream of the electrolytic cells being traversed by a downstream electrolysis current of an intensity between 0-50% of the overall intensity of the electrolysis current, a sum of the intensities of the upstream and downstream electrolysis currents being equal to the overall intensity of the electrolysis current.

23. Method according to claim 22 in which an intensity of the first compensating current is equal to twice the intensity of the downstream electrolysis current, to the nearest 20%.

24. Method according to claim 22 in which an intensity of the second compensating current is between 50% and 100% of a difference in intensity between the upstream and downstream electrolysis currents.

25. Method according to claim 21 in which said at least one second electric compensating circuit comprises an inner loop and an outer loop, and wherein an intensity of a second inner compensating current flowing in the inner loop is different from an intensity of a second outer compensating current flowing in the outer loop.

26. Method according to claim 25 in which the intensity of the second inner compensating current flowing in the inner loop is greater than the intensity of the second outer compensating current flowing in the outer loop.

27. Method according to claim 21 in which the method comprises analyzing at least one characteristic of alumina in at least one of the electrolytic cells of said aluminum smelter, and determining intensity values of the first compensating current and the second compensating current to be made to flow as a function of said at least one characteristic analyzed.

Description

(1) Other characteristics and advantages of this invention will be clearly apparent from the following description of a particular embodiment provided by way of a non-limiting example with reference to the appended drawings, in which:

(2) FIG. 1 is a schematic view of an aluminum smelter according to the state of the art,

(3) FIG. 2 is a schematic view from the side of two successive electrolytic cells according to the state of the art,

(4) FIG. 3 is a line diagram of the electrical circuit through which the electrolysis current flows in the two electrolytic cells in FIG. 2,

(5) FIG. 4 is a schematic view in cross-section along a longitudinal vertical plane of an electrolytic cell according to the state of the art,

(6) FIG. 5 is a schematic view of an aluminum smelter according to one embodiment of the invention,

(7) FIG. 6 is a schematic view from the side of two successive electrolytic cells in an aluminum smelter according to one embodiment of the invention,

(8) FIG. 7 is a schematic view from the side of two successive electrolytic cells in an aluminum smelter according to one embodiment of the invention,

(9) FIG. 8 is a schematic view from the side of two successive electrolytic cells in an aluminum smelter according to one embodiment of the invention,

(10) FIG. 9 is a table showing the intensity of the electrolysis current through each segment of FIG. 8,

(11) FIGS. 10 to 12 are schematic wiring diagrams of the electric circuit through which the electrolysis current flows in an electrolytic cell of an aluminum smelter according to one embodiment of the invention, showing for this electrolytic cell zones generating a significant magnetic field,

(12) FIG. 13 is a table showing the contribution of each segment in FIGS. 10 to 12 in computing the vertical component of the magnetic field generated by the flow of the electrolysis current,

(13) FIG. 14 is a table showing the contribution of each segment in FIGS. 10 to 12 in computing the longitudinal horizontal component of the magnetic field generated by the flow of the electrolysis current,

(14) FIG. 1 shows an aluminum smelter 100 according to prior art. Aluminum smelter 100 comprises electrolytic cells arranged transversely in relation to the length of the row which they form. The electrolytic cells are here aligned in two parallel rows 101, 102. These electrolytic cells are traversed by an electrolysis current I.sub.100. Two compensating electrical circuits 104, 106, run along the sides of rows 101, 102 to compensate for the magnetic field generated by the flow of electrolysis current I.sub.100 from one electrolytic cell to another and in the adjacent row. Electrical compensating circuits 104, 106 respectively are traversed by currents I.sub.104, I.sub.106 flowing in the same direction as electrolysis current I.sub.100. Power stations 108 provide power to the series of electrolytic cells and compensating electrical circuits 104, 106. According to this example, for an electrolysis current of intensity 500 kA, and taking into account end-of-row magnetic disturbances, the distance D.sub.100 between the electrolytic cells closest to power stations 108 and power stations 108 is of the order of 45 m, and the distance D.sub.300 over which compensating electrical circuits 104, 106 extend beyond the ends of the row, is of the order of 45 m, while the distance D.sub.200 between the two rows 101, 102 is of the order of 85 m in order to limit magnetic disturbances between one row and another.

(15) FIG. 2 shows two consecutive conventional electrolytic cells 110 in one row of electrolytic cells. As shown in FIG. 2, electrolytic cell 110 comprises a pot shell 112 lined internally with refractory materials 114, a cathode 116 and anodes 118 immersed in an electrolyte bath 120, at the bottom of which a layer 122 of aluminum forms. Cathode 116 is electrically connected to cathode conductors 124 which pass through the sides of pot shell 112 at the level of cathode outputs 126. Cathode outputs 126 are connected to linking conductors 128 which route the electrolysis current to the rising and connecting conductors 130 of the next electrolytic cell. As shown in FIG. 2, these rising and connecting conductors 130 extend obliquely along a single side, the upstream side, of electrolytic cells 110 and extend above anodes 118 as far as the central longitudinal part of electrolytic cells 110.

(16) The electrolytic cell comprises a superstructure 132 extending there through longitudinally above pot shell 112 and anodes 118. The superstructure 132 in particular includes 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 pot shell 112 and anodes 118. The anode frame 134 supports the anode assemblies, the latter being electrically connected to the anode frame 134.

(17) FIG. 3 schematically illustrates the path traveled by electrolysis current I.sub.100 in each of electrolytic cells 110 and between two adjacent electrolytic cells 110, such as those shown in FIG. 2. It will in particular be noted that the electrolysis current I.sub.100 rises up to the anode assembly of an electrolytic cell 110 asymmetrically because this rise takes place only upstream of the electrolytic cells 110 in the overall direction of flow of electrolysis current I.sub.100 within the row (to the left of the cells in FIGS. 2 and 3).

(18) FIG. 4 shows the arrangement on the sides of cells 110 of state-of-the-art electrical conductors forming electrical compensation circuits 104, 106, these electrical conductors being traversed respectively by compensation currents I.sub.104, I.sub.106 flowing in the same direction as the electrolysis current I.sub.100 flowing here through routing conductors 128 positioned below the cell.

(19) FIG. 5 shows an aluminum smelter 1 according to one embodiment of the invention. Aluminum smelter 1 is designed for aluminum production using electrolysis by means of the Hall-Hroult process.

(20) Aluminum smelter 1 comprises a plurality of electrolytic cells, which are substantially rectangular, designed to produce aluminum by electrolysis. These electrolytic cells can be aligned in one or more rows 2, which may be substantially parallel. Where appropriate, rows 2 are electrically connected in series and supplied with electrolysis current IE. Aluminum smelter 1 also comprises a first electrical compensating circuit 4 which extends under the row or rows of electrolytic cells, and a second electric compensating circuit 6, which extends over at least one side of the row or rows 2 of electrolytic cells. According to the example shown in FIG. 5, the second electrical compensating circuit 6 extends on both sides of each row 2 of electrolytic cells. Still according to the example shown in FIG. 5, the aluminum smelter comprises two rows of cells arranged in parallel relative to one another, fed by a single power station 8, and electrically connected in series so that the electrolysis current IE flowing in the first two rows 2 of electrolytic cells then flows into the second of the two rows 2 of electrolytic cells. The electrolytic cells are arranged transversely in relation to each row 2 that these electrolytic cells form. It will be noted that by a transversely arranged electrolytic cell 2, is meant an electrolytic cell 2 whose largest dimension, its length, is substantially perpendicular to the overall direction in which electrolysis current IE flows.

(21) In the present description, upstream and downstream are defined with respect to the overall direction of flow of the electrolysis current IE, i.e. the direction of flow of IE electrolysis current at the level of the row 2 of electrolytic cells.

(22) It is also pointed out that the description is provided in relation to a Cartesian frame of reference relating to an electrolytic cell, the X axis being orientated in a transverse direction of the electrolytic cell, the Y axis being orientated in a longitudinal direction of the electrolytic cell and the Z axis being orientated in a vertical direction of the electrolytic cell. Longitudinal, vertical and transverse orientations, directions, plans and movements are defined relative to this standard.

(23) Note that the electrolytic cells in the aluminum smelter are preferably electrolytic cells of large dimensions, the use of large electrolytic cells being made possible by the particular layout of the electrolytic cells in the aluminum smelter according to the invention, as described in more detail below. The dimensions of an electrolytic cell are defined by the floor area that the electrolytic cell represents. For this, it is considered that the dimensions of the cell are defined by the outer dimensions of its pot shell. A large electrolytic cell means an electrolytic cell having a width greater than 4 m, preferably greater than or equal to 5 m, and especially 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 especially greater than or equal to 25 m

(24) FIG. 6 shows in more detail electrolytic cells 10 in aluminum smelter 1 according to one embodiment. As illustrated in this figure, electrolytic cells 10 of aluminum smelter 1 comprise a pot shell 12, anode assemblies 14, a cathode 16 through which pass the cathode electrical conductors 18 designed to collect the electrolysis current IE at cathode 16 to route it to other electrical conductors called cathode outputs 20 outside the pot shell 12, rising electrical conductors 22 for connecting to the anode assemblies 14 to route the electrolysis current IE to anode assemblies 14 and electrical routing conductors 24 connected to cathodic outputs 20 designed to route the electrolysis current IE from the cathode outputs 20 to the rising and connecting electrical conductors 22 of the next electrolytic cell 10.

(25) Pot shell 12 includes an inner lining 26 made of refractory materials. As illustrated in FIGS. 6 and 7, the pot shell 12 preferably includes reinforcement cradles 28. Pot shell 12 may be metallic, for example made of steel.

(26) Anode assemblies 14 comprise a support 30 and at least one anode 32. The anode or anodes 32 are particularly made of carbonaceous material, and more particularly of the pre-baked type. Support 30 includes a first electrically conductive portion 34, such as a crossbeam extending substantially along a transverse direction X of the electrolytic cells 10, and a second electrically conductive portion 36 formed of a plurality of electrically conductive elements that may be called stubs, the stubs having a distal end electrically connected to the first portion 34 of the support 30 and a proximal end electrically connected to the anode or anodes 32 to route the electrolysis current IE from the first portion 34 of the support 30 to this/these anode(s) 32. The anode assemblies 14 are designed to be removed and replaced periodically when the anode or anodes 32 are spent.

(27) Cathode 16 may be formed from several cathode blocks made of carbonaceous material. Cathode 16 is traversed by the cathode conductors 18 designed to collect the electrolysis current IE at the cathode 16 to route it to the cathode outputs 20 preferably exiting from the bottom of pot shell 12, as shown in FIG. 6.

(28) The rising and connecting electrical conductors 22 extend upwardly along two opposite longitudinal edges 38 of each electrolytic cell 10, to route the electrolysis current IE to the anode assemblies 14. It is specified that the longitudinal edges 38 of electrolytic cells 10 correspond to the edges with the largest dimension, i.e. the edges of the electrolytic cells 10 which are substantially parallel to the longitudinal direction Y. For example, an electrolytic cell 10 operating with a current intensity of 400 to 1,000 k Amperes may for example preferably comprise 4 to 40 rising and connecting conductors 22 regularly spaced over the entire length of each of its two longitudinal edges 38. The longitudinal rising and connecting electrical conductors 22 comprise upstream rising and connecting electrical conductors 22A, i.e. adjacent to the longitudinal edge 38 upstream of the electrolytic cell 10, and downstream rising and connecting electrical conductors 22B, i.e. adjacent to the longitudinal edge 38 downstream of the electrolytic cell 10. The upstream rising and connecting electrical conductors 22A are electrically connected to an upstream end of the first portion 34 of support 30, and the downstream rising and connecting electrical conductors 22B are electrically connected to a downstream end of this first portion 34 of support 30.

(29) Routing electrical conductors 24 are connected to the cathode outputs 20 and are designed to route the electrolysis current IE from these cathode outputs 20 to the rising and connecting electrical conductors 22 of the next electrolytic cell 10 in the series.

(30) The cathode conductors 18, the cathode outputs 20 and/or the routing conductors 24 may be metal bars, possibly composites, such as aluminum, copper and/or steel.

(31) A layer of liquid aluminum 40 is formed during the electrolysis reaction.

(32) Note that the electrolytic cells 10 of the aluminum smelter 1 according to the invention are preferably electrolytic cells 10 for which anode replacement is by vertical upward traction of the anode assemblies 14 above the electrolytic cell 10, as shown via the electrolytic cell 10 on the right in FIG. 6. The rising and connecting conductors 22 extend on either side of the cabinet 12 without extending in line with anodes 32, i.e. without extending within a volume obtained by vertical projection of the projected area of anodes 32 in a horizontal plane. Besides the advantage that this represents for allowing anode 32 to be changed by ascending vertical traction, it also reduces the length of the rising and connecting conductors 22 as compared with the use of conventional rising and connecting conductors 130, which can be seen in FIG. 2, which typically extend above electrolytic cell 110 into the longitudinal central portion of electrolytic cell 110. This helps to reduce manufacturing costs. It is also noted that the horizontal part 34 of support 30 is supported and connected at each of the two longitudinal edges 38 of each electrolytic cell 10.

(33) In this way, the anode assembly is no longer supported and electrically connected above the pot shell and the anodes by means of a superstructure 132, as is the case with electrolytic cells of prior art illustrated in FIG. 2. The electrolytic cells 10 of the aluminum smelter 1 according to this embodiment of the invention are therefore free from superstructure. The absence of a superstructure widens and/or elongates the electrolytic cells 10 in order to benefit from large electrolytic cells 10, as mentioned previously. Such an enlargement or elongation of electrolytic cells 110 of prior art is not possible because of the superstructure 132, since this widening and/or elongation would result in a widening and/or elongation of the superstructure 132 itself, and therefore of the span of the beam between the legs supporting the beam and the weight to be supported by the superstructure 132. There are superstructures comprising one or more intermediate arches supporting the beam, but such intermediate arches extending transversely above the pot shell 112 and the anodes 118 are bulky and render operations on the cells, in particular the changing of anodes, complex.

(34) The ability to increase the dimensions of the electrolytic cells, combined with an increase in the intensity of the electrolysis current IE, without creating MHD instability due to the particular magnetic configuration of the aluminum smelter 1 according to the invention described in more detail below, makes it possible to substantially improve the performance of the aluminum smelter 1 in comparison with prior art.

(35) The electrical conductors of the aluminum smelter 1 (in particular rising and connecting electrical conductors 22, support 30, cathodic outputs 20, routing conductors 24, electrical conductors of the first and second electrical compensating 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 MHD instability, and therefore improved efficiency.

(36) More particularly, the distribution of the electrolytic current IE flowing through rising and connecting electrical conductors 22 is asymmetric between the upstream 22A and downstream 22B rising and connecting electrical conductors. The electrolysis current IE is divided into an upstream electrolysis current IEA, which runs through the set of upstream rising and connecting electrical conductors 22A of the electrolytic cells 10, and a downstream electrolysis current IEB, which runs through all the downstream rising and connecting electrical conductors 22B of electrolytic cells 10. The intensity of upstream electrolysis current IEA is]50-100[% of the intensity of electrolysis current IE, while the intensity of the downstream electrolysis current IEB is]0-50[% of the intensity of electrolysis current IE, noting that the upstream IEA and downstream IEB electrolysis currents are complementary, i.e. the total intensities of the upstream IEA and downstream IEB electrolysis currents is equal to the intensity of the electrolysis current IE.

(37) This asymmetrical distribution with predominance of the upstream relative to the downstream is particularly advantageous when the electrolytic cells 10 of the aluminum smelter are electrolytic cells of large dimensions. The upstream/downstream asymmetry of electrolysis current IE avoids having to resort to an excessive increase in section of the routing conductors 24 under electrolytic cell 10, so that material savings and space are made, without prejudice to the magnetic stability of the electrolytic cell 10.

(38) The choice of distribution between upstream and downstream electrolysis current intensities IEA, IEB is made by means of an economic study. This choice depends mainly on the distance between two cells and the height of the cells. This distribution is carried out by adjusting the sections of the electrical conductors of the upstream and downstream electrical circuits, taking into account their length.

(39) Rising and connecting conductors 22 extend substantially vertically, preferably only vertically, so that the path of the electrolysis current IE through rising and connecting conductors 22 generates a magnetic field with only horizontal components, but no vertical component.

(40) Similarly, the second portion 36 of support 30 of anode assembly 14, and/or cathodic outputs 20 advantageously extend in a vertical direction, and preferably only vertically, so that the path of the electrolysis current IE through this second portion 36 and/or through the cathodic outputs 20 generates a magnetic field with only horizontal components, but no vertical component.

(41) It will be noted that the cathode outputs 20 advantageously pass through the bottom of pot shell 12. The fact of having cathode outputs 20 through the bottom, instead of cathode outputs on the sides of the electrolytic cell, as in prior art (FIG. 2) reduces the length of the routing conductors 24. The reduction in length of the routing conductors 24 allows a further saving of raw materials, a substantial reduction of horizontal currents in the liquids, and thereby improved MHD stability.

(42) Furthermore, also in order to effectively compensate for the magnetic field created by the flow of the electrolysis current IE, the first portion 34 of support 30 of anode assembly 14 extends, preferably only, substantially horizontally and parallel to the transverse direction X of electrolytic cells 10.

(43) Similarly, routing conductors 24 advantageously extend substantially straight and parallel to the transverse direction X of electrolytic cells 10, as far as rising and connecting conductors 22 of the next electrolytic cell 10. This limits the cost of routing electrical conductors 24, by minimizing their length. It also limits the magnetic fields generated by these electrical routing conductors 24 with respect to prior art, and particularly with respect to self-compensated electrolytic cells of prior art.

(44) The rising and connecting electrical 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 rising and connecting electrical conductors 22 in the longitudinal direction Y. This improves the equilibrium of the longitudinal horizontal component of the magnetic field (i.e. that parallel to the length of the electrolytic cell 10).

(45) The upstream rising and connecting electrical conductors 22A and the downstream rising and connecting electrical conductors 22B may be located equidistant from a longitudinal median YZ plane of each electrolytic cell 10, i.e. 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 rising and connecting conductors 22A are at the same distance from this longitudinal median plane YZ as the downstream electrical conductors 22B. In addition, the upstream rising and connecting conductors 22A are advantageously arranged substantially symmetrically to the downstream rising and connecting conductors 22B, with respect to this longitudinal median plane YZ. The advantageous substantially antisymmetrical characteristic of the distribution of the horizontal magnetic field in the liquids is therefore further improved.

(46) To limit the magnetic field generated by the flow of the electrolysis current through the rising and connecting electrical conductors 22, these rising and connecting electrical conductors advantageously extend above the liquid (electrolytic bath) at a height h between 0 and 1.5 meters. The length of the rising and connecting conductors 22 is in this way greatly decreased relative to the rising and connecting conductors 130 of conventional type which extend to heights greater than two meters for electrolytic cells 130 of prior art.

(47) To improve the compactness of the aluminum smelter 1 and limit the costs of raw materials, the upstream rising and connecting conductors 22A of electrolytic cells 10 may be in a staggered arrangement relative to the downstream rising and connecting conductors 22B of the previous electrolytic cell 10 in row 2. This makes it possible to bring the electrolytic cells 10 as close as possible to each other, either to have more electrolytic cells 10 in series over a same distance, which increases performance, or to reduce the length of a row 2 of electrolytic cells 10, thereby gaining space and making even more structural savings.

(48) For effective compensation of the horizontal components of the magnetic field generated by the flow of the electrolysis current IE, i.e. to have horizontal antisymmetric components, the first portion 34 of support 30 of anode assembly 14 and the second portion 36 of support 30 of anode assembly 14 are configured so that the intensity of the fraction of electrolysis current running through an upstream half of this second portion 36 is substantially equal to the intensity of the fraction of electrolysis current running through a downstream half of this second portion 36. In other words, and as shown in FIG. 8, the intensity of the portion of the electrolysis current running through all the stubs located upstream of a longitudinal center plane YZ of electrolytic cell 10 is substantially equal to the intensity of the fraction of the electrolysis current passing through all the stubs located downstream of this longitudinal center plane YZ. In particular, as is apparent from segment S9 of FIG. 8 read in conjunction with the table in FIG. 9, a portion of the upstream electrolysis current IEA goes as far as the stubs located downstream of the median plane YZ of electrolytic cell 10. This is achieved through global electrical balancing of the various sections of conductors.

(49) The principle of magnetic compensating or balancing of aluminum smelter 1 according to the invention makes it possible to obtain for aluminum smelter 1 a circuit of conductors that can be made in modular fashion, as shown in FIG. 7. Each module M may for example comprise an electrical conductor of the first compensating electrical circuit 4 and a particular number of routing conductors 24 and rising and connecting conductors 22 associated with each electrolytic cell 10. The fact is that the electrical conductors included in each module M (rising and connecting conductors 22, anode assembly 14, cathode 16, cathode conductors 18, cathode outputs 20, routing conductors 24, electrical conductors of the first compensating 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.

(50) The circuit of conductors, and therefore each electrolytic cell 10, may be composed of a number N of modules M, determining the length of the electrolytic cells 10 and the current flowing through the electrolytic cells 10 (the intensity of the electrolysis current IE flowing through the series of electrolytic cells being equal to the intensity of the portion of the electrolysis current running through each module M multiplied by the number N of modules M).

(51) It is important to note that, in view of the magnetic configuration of each module M, the choice of the number N of modules M per electrolytic cell 10, compensated for by the secondary compensating circuit 6 on the cell ends, only slightly disturbs the magnetic balance of electrolytic cells 10. This makes it possible to obtain an optimal magnetic configuration, for amperages above 1000 kA or even 2000 kA when designing or extending the length of electrolytic cells 10 by the addition of such modules. In contrast, the elongation of electrolytic cells of the self-compensated type or compensated for by compensating magnetic circuits arranged on the sides of cells known from prior art makes it necessary to completely redesign the conductor circuits. The ratio of the quantity of material forming the circuit of conductors to the production surface area of electrolytic cells 10 is not worsened when the electrolytic cells 10 are elongated; it increases in proportion to the number N of modules M and the current intensity passing through the electrolytic cells 10. The electrolytic cells 10 can therefore be elongated simply in relation to need, and the intensity of the current passing through them is not restricted. The modular design of the electrical conductors of electrolytic cells 10 therefore offers an advantage in terms of scalability, since the modular design, combined with a simple adjustment of the amperage of the secondary compensation circuit, means that electrolytic cells 10 can be changed without affecting their magnetic and electrical balancing.

(52) The table in FIG. 9 in conjunction with FIG. 8, shows a module for the intensities flowing through the different electrically conductive elements of the electrolytic cells 10, the conductive elements being symbolized by segments: S1 for upstream rising and connecting conductors 22A; S2, S5 and S8 for the first portion 34 of support 30; S3 and S9 for the second portion 36 of support 30, the anode(s) 32, the electrolytic bath, the aluminum layer 40, the cathode 16, the cathode conductors 18 and the cathode outputs 20; S4, S6 and S10 for routing conductors 24; S7 for the downstream rising and connecting conductors 22B.

(53) The sum of the currents i and i is as shown in the table in FIGS. 9, 13 and 14 is equal to the intensity of the upstream electrolysis current IEA divided by the number N of modules of electrolytic cell 10; intensity ib equals the intensity of the downstream electrolysis current IEB divided by the number N of modules of electrolytic cell 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 electrolytic cells is equal to the sum of the intensity of the upstream electrolysis current IEA crossing the entire upstream portion of the electrolytic cell and the intensity of the downstream electrolysis current IEB crossing the entire downstream portion of the electrolytic cell, i.e. the product of 2i and the number N of electrolytic cell modules.

(54) FIGS. 10 to 12 are schematic wiring diagrams of the electric circuit traversed by the electrolysis current in a module of an electrolytic 10 cell of aluminum smelter 1, and showing for this electrolytic cell 10 the three main zones P1, P2, P3 generating a significantly interfering magnetic field: an upstream zone P1, a central intermediate zone P2, and a downstream zone P3 symmetrical with the upstream zone P1 along a longitudinal median plane YZ of electrolytic cells 10.

(55) The table in FIG. 13, in conjunction with FIGS. 10, 11 and 12 schematically shows the vertical component of the magnetic field generated by the electrical conductors (schematically represented by segments) of the electrolytic cell 10, in the three zones P1, P2, P3 respectively of electrolytic cells 10, by the first and second compensating circuits 4, 6. By summing the contributions of each of the electrical conductors, and that of the first and second compensating circuit 4, 6, it is noted that the vertical component Bz of the magnetic field generated by the flow of the electrolysis current is zero, i.e. perfectly compensated for. In this way MHD instabilities are minimized; this makes it possible to substantially improve performance.

(56) In addition, the table in FIG. 14, also in conjunction with FIGS. 10, 11 and 12 schematically shows the horizontal longitudinal component of the magnetic field generated by the flow of the electrolysis current through the electrical conductors (symbolized by segments) of electrolytic cell 10, zone by zone, and through the first and second compensating circuits 4, 6. The transverse horizontal component of the magnetic field is itself antisymmetric because the conductors are symmetrical along plane XZ. By summing the contributions of each segment, and those of the first and second compensating circuits 4, 6, it can be seen that the longitudinal horizontal component By of the magnetic field is antisymmetric (opposite in zones P1, P3 upstream and downstream, and zero in the central zone P2). This antisymmetry eliminates deleterious effects due to the horizontal components of the magnetic field.

(57) The first electric compensating circuit 4 is described in more detail below.

(58) The first compensating circuit 4 extends under electrolytic cells 10. This first electrical compensating circuit 4 is designed to be traversed by a first compensating current IC1 in the opposite direction to the direction of overall flow of the electrolysis current IE, as can be seen in FIGS. 5 and 7. It will be recalled that the direction of overall flow of the electrolysis current IE means the direction of flow of electrolysis current IE across the aluminum smelter 1 or the row(s) 2 of electrolytic cells 10.

(59) The first electric compensating circuit 4 comprises electric conductors which may be metal bars, for example made of aluminum, copper or steel, or, advantageously, electrical conductors made of superconducting material, the latter helping to reduce energy consumption and, because of their mass which is lower than the equivalent metal conductors, to reduce the costs of structures to support or protect them from any molten metal using metal deflectors 42 (FIG. 7) or by burying them. Advantageously, these electrical conductors made of superconducting material may be arranged so as to make several turns in series beneath the row or rows of cells as described in patent application WO2013007893 in the name of the applicant.

(60) Aluminum smelter 1 comprises a power station 44 configured to cause to flow through the first compensating electrical circuit 4 a current of intensity IC1 equal to twice the intensity of the downstream electrolysis current IEB to the nearest 20%, and preferably to the nearest 10%.

(61) This power station 44 may be a separate electrical supply station, i.e. separate from the power station 8 powering electrolytic cells 10 with electrolysis current IE. The power station 44 of the first compensating circuit 4 is exclusively dedicated to powering the first compensating circuit 4.

(62) The first compensating circuit 4 is in this way also independent of the main electrical circuit traversed by electrolysis current IE notably including the row(s) 2 of electrolytic cells 10. If the first electric compensating circuit 4 sustains damage, for example one of the electrolytic cells 10 being pierced by the liquid contained in the electrolytic cells, the temperature of which is close to 1,000 C., the electrolysis reaction can continue, but with a lower yield because the magnetic compensation has been impacted. In addition, the intensity of the first compensating current IC1 is can be modified independently of the electrolysis current IE. This is of essential importance in terms of scalability and adaptability. Partly because if the intensity of the electrolysis current is increased during the lifetime of the aluminum smelter 1, the magnetic compensation can be adjusted to this change by varying the intensity of the first compensating current IC1 as necessary. Also because the intensity of the first compensating current IC1 can be adjusted to the characteristics and quality of the alumina available. In this way the velocities of MHD flows can be controlled to encourage or reduce stirring of the liquids and dissolution of alumina in the bath on the basis of the characteristics of the alumina available, which ultimately helps to provide the best possible performance in the light of alumina supplies.

(63) The electrical conductors of the first electric compensating circuit 4 extend under the electrolytic cells together forming a layer of parallel electrical conductors, advantageously two to twelve, preferably three to ten parallel electrical conductors. In other words, in the longitudinal section of an electrolytic cell 10, i.e. in a longitudinal plane YZ of the electrolytic cell 10, as shown in FIG. 7, the first electrical compensating circuit 4 extends under several parts of the electrolytic cell 10. It will be noted that the first compensating current IC1 flows in the opposite direction to the overall direction of flow of the electrolysis current IE, through all of the electrical conductors forming the layer. The layer may be formed by the same electric circuit forming several turns or loops in series under the electrolytic cells 10, each loop corresponding to an electrical conductor of the layer. Alternatively, the layer may be formed by division into a bundle of parallel electrical conductors of the first electric compensating circuit 4, the latter may optionally form a single loop under the electrolytic cells 10.

(64) The intensity of the first compensating current IC1 is equal to the sum of the compensating current intensities flowing through each electrical conductor of the layer. Preferably, the intensity of the first compensating current IC1 in each electrical conductor of the layer is equal to the intensity of the first compensating current IC1 divided by the number of electrical conductors of this layer.

(65) The electrical conductors of the layer are preferably equidistant from each other. The same distance therefore separates two adjacent electrical conductors of the layer. Compensation for the unfavorable magnetic field is therefore further improved.

(66) The electrical conductors of the layer may extend in parallel with each other. They preferably extend parallel to the transverse direction X of electrolytic cells 10 Furthermore, the electrical conductors forming the layer may be all be arranged in the same horizontal plane XY. This also improves compensation of the magnetic field generated by the flow of the electrolysis current.

(67) In addition, the electrical conductors of the layer may extend substantially symmetrically relative to the transverse median plane XZ of the electrolytic cells, i.e. with respect to the plane perpendicular to the longitudinal direction Y, this plane separating the electrolytic cells 10 into two substantially equal halves.

(68) According to the example in FIG. 7, the first electrical compensation circuit 4 forms a layer of three substantially equidistant conductors arranged in the same substantially horizontal plane XY. This layer includes as many electrical conductors as the electrolytic cell 10 has modules M.

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

(70) The second electric compensating circuit 6 is described in more detail below.

(71) The second electric compensating circuit 6 extends over at least one transverse side of electrolytic cells 10, substantially parallel to the transverse direction X of electrolytic cells 10, i.e. parallel to the row(s) 2 of electrolytic cells 10. The second electric compensating circuit 6 is designed to be traversed by a second compensating current IC2 in the same direction as the direction of overall flow of the electrolysis current IE.

(72) Preferably, the second electric compensating circuit 6 extends along both transverse sides of the electrolytic cells 10, as illustrated in FIG. 5. In this case, inner loop 61 denotes the electrical conductors of the second electrical compensating circuit 6 which are situated between the first two rows 2 of adjacent electrolytic cells 10, and outer loop 62 denotes the electrical conductors of the second electrical compensating circuit 6 which are situated outside of the rows 2 of electrolytic cells 10, i.e. which are on the other side of the electrolytic cells 10 with respect to the electrical conductors forming the inner loop 61. The inner loop 61 is traversed by a second compensating current IC21 and the outer loop 62 is traversed by a second compensating current IC22. The second compensating currents IC21 and IC22 flow in the same direction. The sum of currents IC21 and IC22 flowing in the inner loop 61 and outer loop 62 respectively is equal to the compensating current IC2. The inner loop 61 and/or the outer loop 62 may possibly make several turns in series; where appropriate, the intensity of the current IC21, IC22 respectively, is the product of the number of turns in series by the current flowing in each turn in series.

(73) The aluminum plant 1 comprises a power station 46 which is preferably configured to flow through the second electrical compensating circuit 6 (inner loop 61 and/or outer loop 62) a total intensity (as appropriate inner loop 61 plus outer loop 62) of compensating current IC2 between 50% and 100% of the intensity difference between the upstream and downstream electrolysis currents, and preferably between 80% and 100% of the intensity difference between the upstream and downstream electrolysis currents. This intensity value, determined according to the unsymmetrical distribution of the electrolytic current IE in each electrolytic cell 10, provides, in synergy with the choice of the asymmetrical distribution value IEA, IEB and the intensity of the first compensating current IC1, the best magnetic field compensation results, effectively applicable to large electrolytic cells 10.

(74) Preferably, the intensity of current IC21 flowing in the inner loop 61 differs from the current intensity IC22 flowing in the outer loop 62. More specifically, the intensity of the current IC21 flowing through the inner loop 61 is advantageously greater than the intensity of current IC22 flowing in the outer loop 62.

(75) The current flowing through the inner loop 61 may be increased to compensate for the impact of the neighboring row on the vertical magnetic field. This increase will have a typical value close (to the nearest 50%) to IE2D61/DP2 where IE2=IEIC1+IC2=IE+IEA3 IEB and DP2 is the distance from the neighboring row to the center of the cell and D61 is the distance from the inner loop 61 to the center of the cell. For a conventional electrolysis series IE2 is greater than or equal to IE. It may be noted that IE+IEA3 IEB is much lower than IE. This is a gain provided by this design that makes it possible to bring the neighboring row closer since the creation of the magnetic field by the neighboring row is much lower without any additional cost compared to what is known by those skilled in the art.

(76) The power station 46 powering the second compensating circuit 6 may be a separate power supply station, i.e. separate from power station 8 powering electrolytic cells 10 with electrolysis current IE and distinct from power station 44 supplying the first electric compensating circuit 4. The power station 46 of the second compensating circuit 6 is therefore exclusively dedicated to powering the second compensating circuit 6. The second compensating circuit 6 is in this way also independent of the main electrical circuit traversed by electrolysis current IE. The intensity of the second compensating current IC2 is modifiable independently of electrolysis current IE, offering substantial advantages in terms of scalability and adaptability of the aluminum smelter 1, as explained above concerning the first compensating electrical circuit 4. Advantageously, the second compensating circuit 6 can also be separate from the first compensating circuit 4.

(77) When the second compensating electrical circuit 6 extends on both sides of electrolytic cells 10, the electrical conductors forming the second electrical compensating circuit 6 may advantageously be symmetrical with respect to a median transverse plane XZ of electrolytic cells 10. This improves compensation of the adverse magnetic field.

(78) Moreover, still with a view to effectively compensating for this magnetic field, created by the circulation of electrolysis current IE, the electrical conductors of the second compensating circuit 6 advantageously extend in the same horizontal plane XY. Preferably, this XY horizontal plane is located at the height of the layer of liquid aluminum 40 formed within electrolytic cells 10 during the electrolysis reaction.

(79) It will be noted that the electrical conductors forming the second electrical compensating circuit 6 may advantageously be configured to limit end-of-row effects, as shown in FIG. 5.

(80) The electrical conductors forming the second electric compensating circuit 6 may be metal bars, for example made of aluminum, copper or steel, or, advantageously, electrical conductors made of a superconducting material, the latter to reduce energy consumption and, because of their lower mass than equivalent conductors made of metal, to reduce the costs of structures to support them. Advantageously, these electrical conductors made of superconducting material may be arranged so as to make several turns in series on the side(s) of rows 2 of electrolytic cells 10 as described in patent application WO2013007893 in the name of the applicant.

(81) The invention also relates to a method of compensating for a magnetic field created by the flow of an electrolysis current IE in the electrolytic cells 10 of an aluminum smelter 1 having the above characteristics. This method comprises: the fact of having the first compensating current IC1 flow through the first compensating circuit 4 in the opposite direction to the overall flow direction of electrolysis current IE, the fact of having the second compensating current IC2 flow through the second compensating circuit 6 in the same direction as the overall flow direction of electrolysis current IE.

(82) The method also advantageously includes the fact of asymmetrically dividing the electrolysis current IE between the upstream rising and connecting electrical conductors 22A and the downstream rising and connecting electrical conductors 22B.

(83) This step of asymmetrical distribution of the electrolysis current between the upstream and the downstream of the electrolytic cells 10 includes separating the electrolysis current

(84) IE into an upstream electrolysis current IEA, flowing through all the upstream electrical rising and connecting conductors 22A of each electrolytic cell 10, so that the intensity of the upstream electrolysis current IEA is between]50-100[% of the intensity of the electrolysis current IE, and a downstream electrolysis current IEB, flowing through all of the downstream electrical rising and connecting conductors 22B of each electrolytic cell 10, so that the intensity of the downstream electrolysis current IEB is 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.

(85) The step involving circulating the first compensating current IC1 is advantageously such that 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%.

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

(87) For these intensity values of the upstream electrolysis current IEA, of the downstream electrolysis current IEB, the first compensating current IC1 and the second compensating current IC2, the applicant found that the magnetic field generated by the flow of the electrolysis current is most effectively compensated.

(88) Furthermore, the intensity of current IC21 flowing in the inner loop 61 may differ from the current intensity IC22 flowing in the outer loop 62. More specifically, the intensity of the current IC21 flowing through the inner loop 61 is advantageously greater than the intensity of current IC22 flowing in the outer loop 62.

(89) Furthermore, the method may advantageously comprise a step of analyzing at least one characteristic of the alumina in at least one of the electrolytic cells 10 in the aluminum smelter 1 described above, and determining a distribution of current intensity values for the upstream and downstream electrolysis currents IEA, IEB to be made to flow based on this analyzed characteristic, which also defines as appropriate the intensity values of the first and second compensating currents IC1, IC2 and as appropriate the upstream and downstream electrolysis current IEA, IEB. The intensity values of the first and second compensating currents IC1, IC2, and as appropriate the upstream and downstream electrolysis currents IEA, IEB, can then be modified to the values previously determined if the intensity values of the first and second compensating currents IC1, IC2 and the upstream and downstream electrolysis currents IEA, IEB differ from the initial values determined. In this way, the process makes it possible to change the magnetic compensation in order to increase or decrease the mixing of liquids while controlling MHD instabilities. Generally, the greater the mixing (or flow) of liquids, the more effective the dissolution of alumina, but the more unstable the bath/metal interface (=MHD instability), which can be detrimental to the performance of the cells. Such a process is particularly advantageous with the configuration of the electrical conductors described above because it makes the electrolytic cells 10 magnetically very stable and therefore provides greater range for modulating/optimizing mixing depending on the quality of the alumina. The analyzed characteristics of the alumina may in particular be the ability of the alumina to dissolve in the bath, the fluidity of the alumina, its solubility, its fluorine content, its moisture content, etc.

(90) Determining a distribution of intensity values of the upstream and downstream compensating currents IEA, IEB and/or the intensity values of the first and second compensating currents IC1, IC2 based on the analyzed characteristics of alumina may be carried out using a nomograph, for example made by a person skilled in the art, by calculation, experimentation and documentation of the best correspondences between intensities of the upstream and downstream electrolysis currents IEA, IEB and the characteristics of the alumina. This is a question of quantifying the intensity of the desired mixing of liquids in relation to the level of MHD instabilities.

(91) It may happen that the alumina available for continuous operation of the aluminum smelter is of different quality, in particular more or less pasty, and thus has different abilities to dissolve in the electrolysis bath. In this case, movement of the liquids in the electrolytic cells 10 is an advantage because it can be used to stir this alumina to encourage it to dissolve. Now in the case of self-compensation in particular (used in prior art) the magnetic field giving rise to movement of the liquids is directly compensated for by the electrolysis current itself, with a distribution of the magnetic field imposed and fixed by the path of the linking conductors. In aluminum smelters where there is self-compensation it is therefore not possible to introduce a deliberate and temporary imbalance in the compensation for the magnetic field to increase the intensity with which the alumina is stirred in the cells with a view to increasing the efficiency of dissolution. So when the only alumina available is alumina which has greater than normal difficulty in dissolving, the performance of aluminum smelters with self-compensation may be substantially affected.

(92) Naturally, the invention is in no way limited to the embodiment described above, as this embodiment is provided only as an example. Modifications are possible, in particular from the point of view of the constitution of the various components, or the substitution of equivalent techniques, without thereby going beyond the scope of protection of the invention. In this way, the present invention is for example compatible with the use of anodes of the inert type at the level of which oxygen is formed during the electrolysis reaction.