Aluminium smelter comprising a compensating electric circuit

Abstract

This aluminum smelter comprises a row of cells (50) arranged transversely in relation to the length of the row, the cells (50) individually comprising an anode (52), rising and connecting electrical conductors (54) running upwards along the two opposite longitudinal edges of the cell (50) to route the electrolysis current towards the anode (52), and a cathode (56) through which pass cathode conductors (55) connected to cathode outputs connected to linking conductors to route the electrolysis current to the rising and connecting electrical conductors of the next cell (50). Furthermore the aluminum smelter comprises a compensating electrical circuit separate from the electrical circuit through which the electrolysis current flows, running beneath the cells (50), through which a compensating current may flow beneath the cells (50) in a direction opposite to the overall direction of flow of the electrolysis current.

Claims

1. An aluminum smelter comprising at least one row of electrolytic cells arranged transversely in relation to a length of the at least one row of electrolytic cells, each electrolytic cell of the at least one row of electrolytic cells comprising a pot shell, anode assemblies each comprising a support and at least one anode, and a cathode through which pass cathode conductors intended to collect an electrolysis current at the cathode to route the electrolysis current to cathode outputs outside the pot shell, characterized in that each electrolytic cell comprises rising and connecting electrical conductors to the anode assemblies running upwards along two opposite longitudinal edges of the electrolytic cell to conduct the electrolysis current to the anode assemblies of the respective electrolytic cell, and linking conductors connected to the cathode outputs designed to route the electrolysis current from the cathode outputs to the rising and connecting electrical conductors of a next electrolytic cell of the at least one row of electrolytic cells, and in that the aluminum smelter comprises at least one electrical compensating circuit running beneath the electrolytic cells, through which the at least one compensating circuit may flow a compensating current flowing beneath the electrolytic cells in a direction opposite to an overall direction of flow of the electrolysis current passing through the electrolytic cells located above.

2. Aluminum smelter according to claim 1, in which the at least one compensating electrical circuit is a secondary compensating electrical circuit separate from the electrical circuit through which the electrolysis current flows.

3. Aluminum smelter according to claim 1, characterized in that the at least one row of electrolytic cells comprises two rows of electrolytic cells arranged parallel to each other, supplied from a single station and electrically connected in series in such a way that the electrolysis current flowing in a first of the two rows of cells then flows in a second of the two rows of cells in a direction which is overall opposite to that in which the electrolysis current flows in the first of the two rows, and in that the compensating electrical circuit forms a loop beneath the two rows of electrolytic cells.

4. Aluminum smelter according to claim 1, characterized in that each electrolytic cell comprises a plurality of the rising and connecting electrical conductors distributed at predetermined intervals over substantially an entire length of the corresponding longitudinal edge along each of two longitudinal sides of each electrolytic cell.

5. Aluminum smelter according to claim 1, characterized in that the rising and connecting electrical conductors are arranged in a substantially symmetrical way in relation to a longitudinal median plane of each electrolytic cell.

6. Aluminum smelter according to claim 1, characterized in that the linking conductors run substantially straight beneath each electrolytic cell in a transverse direction in relation to each electrolytic cell.

7. Aluminum smelter according to claim 1, characterized in that the at least one compensating electrical circuit comprises electrical conductors forming a plurality of secondary compensating electrical sub-circuits which are independent of each other.

8. Aluminum smelter according to claim 7, in which the electrical conductors are substantially equally spaced and are arranged substantially symmetrically in relation to a transverse median axis of the electrolytic cells.

9. Aluminum smelter according to claim 1, characterized in that the at least one compensating electrical circuit comprises electrical conductors running in parallel beneath the electrolytic cells.

10. Aluminum smelter according to claim 1, characterized in that electrical conductors forming the at least one compensating electrical circuit run beneath the electrolytic cells, together forming a layer of between two and twelve parallel electrical conductors.

11. Aluminum smelter according to claim 1, characterized in that the rising and connecting electrical conductors running along one of the two opposite longitudinal edges of each electrolytic cell are in a staggered arrangement in relation to the rising and connecting electrical conductors located on an adjacent longitudinal edge of a separate preceding or following electrolytic cell.

12. Aluminum smelter according to claim 1, characterized in that each of the cathode outputs leaves the pot shell only in a vertical plane perpendicular to a longitudinal direction of each electrolytic cell.

13. Aluminum smelter according to claim 1, characterized in that the support for each anode assembly comprises a cross-member extending transversely in relation to the electrolytic cell, being supported and electrically connected at each of the two opposite longitudinal edges on either side of each electrolytic cell.

14. Aluminum smelter according to claim 1, characterized in that the rising and connecting electrical conductors run on either side of the pot shell, without running above the at least one anode.

15. Aluminum smelter according to claim 1, characterized in that the rising and connecting electrical conductors run at a height of between 0 and 1.5 meters above a substantially horizontal plane, including a surface of liquids present in each electrolytic cell.

16. Aluminum smelter according to claim 1, characterized in that the at least one compensating electrical circuit comprises electrical conductors running beneath the electrolytic cells, and wherein the compensating current flows through all of the electrical conductors of the at least one compensating electrical circuit running beneath the electrolytic cells in the direction opposite to the overall direction of flow of the electrolysis current passing through the electrolytic cells located above.

17. Method for using an aluminum smelter according to claim 1, comprising passing the compensating current through the at least one compensating electrical circuit beneath the electrolytic cells in the direction opposite to the overall direction of flow of the electrolysis current flowing through the electrolytic cells located above.

18. Method according to claim 17, characterized in that an intensity of the compensating current is of the order of 50% to 150% of an intensity of the electrolysis current.

19. Method according to claim 18, characterized in that the intensity of the compensating current is of the order of 70% to 130% of the intensity of the electrolysis current.

20. Method according to claim 17, characterized in that a distribution of current between the rising and connecting electrical conductors located upstream of each electrolytic cell and the rising and connecting electrical conductors located downstream of each electrolytic cell is of the order of 30-70% upstream and 30-70% downstream respectively.

21. Method according to claim 20, characterized in that the distribution of current between the rising and connecting electrical conductors located upstream of each electrolytic cell and the rising and connecting electrical conductors located downstream of each electrolytic cell is of the order of 40-60% upstream and 40-60% downstream respectively.

22. Method according to claim 21, characterized in that the distribution of current between the rising and connecting electrical conductors located upstream of each electrolytic cell and the rising and connecting electrical conductors located downstream of each electrolytic cell is of the order of 45-55% upstream and 45-55% downstream respectively.

23. Process for stirring alumina present in the electrolytic cells of an aluminum smelter according to claim 1, the process comprising: analyzing of at least one characteristic of the alumina, determining an intensity value for an intensity of the compensating current which has to flow in the at least one compensating electrical circuit as a function of the at least one characteristic analyzed, changing the intensity of the compensating current to the intensity value determined, if the intensity of the compensating current differs from the intensity value.

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 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 line diagram of the electrical circuit through which the electrolysis current flows in two successive cells in an aluminum smelter according to the invention,

(8) FIG. 7 is a view in cross-section along a vertical longitudinal plane of an electrolytic cell in an aluminum smelter according to one embodiment of the invention,

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

(10) FIG. 9 is a line diagram of the electrical circuit through which the electrolysis current flows in two successive cells in an aluminum smelter according to the invention,

(11) FIG. 1 shows an aluminum smelter 100 according to the state of the art. Aluminum smelter 100 comprises electrolytic cells arranged transversely in relation to the length of the row which they form. Here the cells are aligned in two parallel rows 101, 102, and an electrolysis current I.sub.100 passes through them. Two secondary 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 cell to another and in the adjacent row. Currents I.sub.104, I.sub.106 flowing in the same direction as the electrolysis current I.sub.100 flow through circuits 104, 106 respectively. Power stations 108 provide power to the series of electrolytic cells and secondary electrical circuits 104, 106. According to this example, for an electrolysis current of intensity 500 kA, 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 secondary 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.

(12) It is 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.

(13) FIG. 2 shows two consecutive conventional electrolytic cells 200 in one row of cells. As shown in FIG. 2, electrolytic cell 200 comprises a pot shell 201 lined internally with refractory materials 202, a cathode 204 and anodes 206 immersed in an electrolyte bath 208, at the bottom of which a layer 210 of aluminum forms. Cathode 204 is electrically connected to cathode conductors 205 which pass through the sides of pot shell 201 at the level of cathode outputs 212. Cathode outputs 212 are connected to linking conductors 214 which route the electrolysis current to the rising and connecting conductors 213 of the next electrolytic cell. As shown in FIG. 2, these rising and connecting conductors 213 extend along a single side, the upstream side, of electrolytic cell 200 and then above anodes 206 as far as the central longitudinal part of the cell.

(14) FIG. 3 schematically illustrates the path travelled by electrolysis current I.sub.100 in each of cells 200 and between two adjacent cells, as shown in FIG. 2. It will in particular be noted that the electrolysis current I.sub.100 rises up to the anode assembly of a cell asymmetrically because this rise takes place only upstream of the cells 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).

(15) FIG. 4 shows a cross-sectional view of a conventional cell 200, in which it will be seen that electrical conductors forming secondary electrical circuits 104, 106, to compensate for the magnetic field generated by the flow of electrolysis current I.sub.100 from one cell 200 to another and in the adjacent row are located on the sides of cell 200.

(16) FIG. 5 shows an aluminum smelter 1 according to one embodiment of the invention. Aluminum smelter 1 comprises a plurality of electrolytic cells 50, which are substantially rectangular and are intended to produce aluminum by electrolysis, which can be aligned in one or more rows, in the case in point, two substantially parallel rows, connected in series, supplied with an electrolysis current I.sub.1.

(17) It is important to note that electrolytic cells 50 are arranged transversely in relation to the row which they form. It will be noted that by a transversely arranged electrolytic cell 50, is meant an electrolytic cell 50 whose largest dimension, its length, is substantially perpendicular to the overall direction in which electrolysis current I.sub.1 flows, that is the direction in which electrolysis current I.sub.1 flows in the scale of the row in electrolytic cells 50.

(18) Aluminum smelter 1 also comprises a compensating electrical circuit 6 through which a compensating current I.sub.2 flows. Unlike circuits 104, 106, illustrated in FIG. 1, it is important to note that compensating electrical circuit 6 runs beneath electrolytic cells 50. It will also be noted that compensating current I.sub.2 flows in the opposite direction to electrolysis current I.sub.1. Compensating electrical circuit 6 in FIG. 5 more particularly forms a loop beneath the rows of electrolytic cells 50.

(19) Advantageously a set of power stations 8 independently powers electrolytic cells 50 and compensating electrical circuit 6. In other words, compensating electrical circuit 6 is a secondary compensating electrical circuit which is separate from main electrical circuit 7 through which electrolysis current I.sub.1 flows.

(20) The intensity of compensating current I.sub.2 varies independently of electrolysis current I.sub.1. The intensity of compensating current I.sub.2 can therefore be changed without the intensity of electrolysis current I.sub.1 necessarily being changed.

(21) FIG. 8 shows three consecutive electrolytic cells 50 in aluminum smelter 1. Conventionally electrolytic cells 50 comprise a pot shell 60, fitted with reinforcing cradles 61, which may be of metal, for example steel, and an inner lining 62 of refractory materials.

(22) Electrolytic cells 50 comprise a plurality of anode assemblies comprising a support 53 (here a transverse horizontal bar) and at least one anode 52, in particular made of carbon material, and more particularly of the pre-baked type, rising and connecting conductors 54 which, unlike electrolytic cell 200, run on either side of each of electrolytic cells 50 to route electrolysis current I.sub.1 towards anodes 52 and a cathode 56, which may be formed of several cathode blocks made of carbon material, through which cathode conductors 55 pass in order to collect electrolysis current I.sub.1 to route it to cathode outputs 58 which pass through the base of pot shell 60 and are connected to linking conductors 57, which in turn carry the electrolysis current to rising and connecting conductors 54 of the next electrolytic cell 50. The anode assemblies are designed to be removed and replaced periodically as the anodes wear out.

(23) Cathode conductors 55, cathode outputs 58 and linking conductors 57, may take the form of metal bars, made, for example of aluminum, copper and/or steel.

(24) FIG. 6 schematically shows the path of electrolysis current I.sub.1 in two successive electrolytic cells 50 in aluminum smelter 1 according to the invention. By comparison with FIG. 3 it will easily be seen that here electrolysis current I.sub.1 advantageously rises along the two longitudinal sides of electrolytic cell 50. The presence of compensating circuit 6 beneath electrolytic cells 50, through which compensating current I.sub.2 flows in a direction opposite to the overall direction of electrolysis current I.sub.1 from one cell 50 to the next, will also be noted.

(25) FIG. 9 shows schematically the path of electrolysis current I.sub.1 in two successive electrolytic cells 50 of aluminum smelter 1 according to the invention, and differs from FIG. 6 in that cathode outputs 58 leave pot shell 60 in a more conventional way at the sides of pot shell 60.

(26) FIG. 7 shows a cross-sectional view of an electrolytic cell 50 in aluminum smelter 1. The presence of compensating circuit 6 beneath electrolytic cells 50, through which compensating current I.sub.2 flows in a direction opposite to the overall direction of electrolysis current I.sub.1 from one cell 50 to the next, will also be noted.

(27) It will also be noted that according to the example in FIG. 7 compensating circuit 6 forms a layer of three substantially equally spaced conductors located in the same substantially horizontal plane XY; furthermore the conductors in this layer may run substantially symmetrically in relation to a transverse median plane XZ.

(28) The circuit of electrical conductors for the cell and the aluminum smelter may advantageously be constructed in a modular fashion. FIG. 7 in particular shows a cell formed of three identical modules M. In this example each module comprises linking conductors 57 located between three adjacent cradles 61 of the pot shell and a conductor for compensating circuit 6 substantially located beneath central cradle 61 of the module. A current of the order of 50% to 150% of the intensity of the electrolysis current corresponding to the module passes through the conductor of compensating circuit 6 of the module. As the magnetic stability of the cell is provided on a module by module basis, the stability of the cell does not depend on the number of modules forming the circuit of electrical conductors for the cell and the aluminum smelter. The length and current intensity of the cells may therefore be adjusted simply by adding modules in order to satisfy the desired conditions for construction of the aluminum smelter.

(29) As shown in FIG. 8, rising and connecting conductors 54 run upwards, for example substantially vertically, along each longitudinal edge of electrolytic cells 50. The longitudinal edges of electrolytic cells 50 correspond to the edges having the largest dimension, substantially perpendicular to the transverse direction X.

(30) Upstream rising and connecting conductors 54 and those downstream may also be arranged equidistantly from a median plane YZ of electrolytic cell 50.

(31) Upstream rising and connecting conductors 54 may be substantially symmetrical to downstream linking electrical conductors 54 in relation to the median plane YZ of electrolytic cells 50.

(32) Although not shown, upstream rising and connecting conductors 54 of one of electrolytic cells 50 may be in a staggered arrangement in relation to downstream rising and connecting conductors 54 of the preceding electrolytic cell 50 in the row.

(33) FIG. 8 also shows that rising and connecting conductors 54 run on either side of pot shell 60 without running above anodes 52, i.e. without running within a volume projected vertically from the surface area of the anodes in a horizontal plane.

(34) It will be also noted that rising and connecting electrical conductors 54 run above liquids 63 at a height h of between 0 and 1.5 meters.

(35) Furthermore support 53 for the anode assembly comprises a cross-member extending transversely in relation to electrolytic cell 50, supported and electrically connected at each of the two longitudinal edges on either side of electrolytic cell 50.

(36) It will be noted that the distribution of electrolysis current I.sub.1 between upstream rising and connecting conductors 54 of electrolytic cells 50 and downstream rising and connecting conductors 54 of electrolytic cells 50 may for example be of the order of 30% to 70% upstream, and 70% to 30% downstream respectively. Advantageously, this current distribution is 40% to 60% upstream and 60% to 40% downstream respectively, and preferably 45% to 55% upstream and 55% to 45% downstream respectively. In other words it is of the order of 50% plus or minus 20% upstream and the remainder downstream, preferably of the order of 50% plus or minus 10%, and even more preferably of the order of 50% plus or minus 5%.

(37) As shown in FIG. 8, cathode outputs 58 and linking conductors 57 may run only in a vertical plane XZ perpendicular to the longitudinal direction Y of electrolytic cells 50. In particular cathode outputs 58 may extend only substantially vertically.

(38) Cathode outputs 58 may pass through the base of pot shell 60 of electrolytic cells 50 and linking conductors 57 may run between electrolytic cells 50 advantageously in a straight line substantially parallel to a transverse direction X of electrolytic cells 50 towards rising and connecting conductors 54 of the next electrolytic cell 50.

(39) The association of a compensating electrical circuit 6 passing beneath electrolytic cells 50, whose compensating current I.sub.2 flows in a direction opposite to the electrical current I.sub.1, and rising and connecting conductors 54 extending on the two opposite longitudinal edges of electrolytic cells 50 makes it possible to stabilize the liquids present in electrolytic cells 50 and to limit disturbances in electrolytic cells 50 at the end of a row because the magnetic fields generated by the electrical current conductors passing beneath the cells and the compensating electrical circuit conductors cancel each other out.

(40) The intensity of the compensating current flowing through the compensating circuit is advantageously of the order of 50% to 150% of the intensity of electrical current I.sub.1, preferably of the order of 70% to 130% of the intensity of electrolysis current I.sub.1, and even more preferably of the order of 80% to 120% of the intensity of electrolysis current I.sub.1, in order to ensure that the magnetic fields are appropriately cancelled out and stability of the cells is ensured.

(41) As a consequence, the distances between rows and the lengths of the electrolysis and compensating electrical circuit 6 may be reduced. Also, referring again to FIG. 5, distance D.sub.1 between electrolytic cells 50 closest to power stations 8 and/or distance D.sub.3 over which compensating electrical circuit 6 extends beyond the ends of a row is less than or equal to 30 m, for example less than or equal to 20 m, and preferably less than or equal to 10 m; the distance D.sub.2 between two rows is less than or equal to 40 m; for example less than or equal to 30 m, and preferably less than or equal to 25 m. As shown in FIG. 5, the two rows in aluminum smelter 1 according to the invention may therefore be located in the same building 12, which makes very major structural savings possible.

(42) Preferably, compensating electrical circuit 6 extends beneath cells 50 forming a layer of between two and twelve, and preferably between three and ten parallel electrical conductors which are substantially equally spaced and distributed substantially symmetrically in relation to a transverse median axis X of cells 50. Compensating current I.sub.2 passing for example in an equally distributed manner through the conductors of this layer of parallel conductors is therefore better distributed beneath the entire length of cell 50. The magnetic fields generated by linking conductors 57 through which electrolysis current I.sub.1 passes, which themselves are distributed beneath cell 50 over its entire length, are also better compensated for.

(43) The electrical conductor or conductors forming compensating electrical circuit 6 run beneath rows of cells 50 in a manner substantially parallel to a transverse axis X of electrolytic cells 50.

(44) It will be noted that compensating circuit 6 may be formed by electrical conductors forming a plurality of secondary compensating electrical sub-circuits which are independent of each other, through each of which there flows a compensating current flowing in a direction contrary to electrolysis current I.sub.1. The secondary compensating electrical sub-circuits may form parallel loops beneath electrolytic cells 50, for example two in the case of FIG. 5. So if an electrolytic cell 50 should be breached, and if one of its sub-circuits is affected, the or the other secondary compensating electrical sub-circuits may continue to compensate for the magnetic field.

(45) Furthermore, the electrical conductors of compensating circuit 6, or if applicable of one of the secondary compensating electrical sub-circuits, may make several turns in parallel and/or in series beneath the electrolytic cells, particularly when these electrical conductors are made of superconducting material.

(46) Electrical conductors forming compensating circuit 6 may take the form of metal bars, of for example aluminum, copper or steel, or advantageously electrical conductors of superconducting material, the latter making it possible to reduce energy consumption, and because of their smaller mass than that of the equivalent metal conductors reducing the structural costs for supporting them or protecting them from any flows of metal by means of metal deflectors. Advantageously, these electrical conductors of superconducting material may be arranged so as to make several turns in series beneath the row or rows of cells.

(47) The sum of the current intensities passing through the conductors of the compensating electrical circuit passing beneath the cell is advantageously of the order of 50% to 150% of the intensity of electrolysis current I.sub.1, preferably of the order of 70% to 130% of the intensity of electrolysis current I.sub.1, and even more preferably of the order of 80% to 120% of the intensity of electrolysis current I.sub.1.

(48) So if aluminum smelter 1 comprises a secondary compensating electrical circuit 6 forming a single turn beneath electrolytic cells 50, the intensity of the compensating current flowing through this compensating electrical circuit 6 may be of the order of 50% to 150% of the intensity of electrolysis current I.sub.1. If this secondary compensating electrical sub-circuit 6 forms N turns beneath electrolytic cells 50, then the sum of the N current intensities passing through each of these turns will be of the order of 50% to 150% of the intensity of the electrolysis current. So according to the example in FIG. 5 the intensity of current I.sub.2 corresponding to the sum of the intensities of currents I.sub.20 and I.sub.21 passing through each of the two turns may be of the order of 50% to 150% of the intensity of electrolysis current I.sub.1.

(49) The invention also relates to a process for stirring alumina in the electrolytic cells 50 of aluminum smelter 1. This process comprises a stage of modulating the intensity of the compensating current flowing in compensating electrical circuit 6, or, if applicable, the compensating currents flowing through the sub-circuits forming it. This modulation may more particularly be a function of the characteristics of the alumina, changes in the intensity of the electrolysis current or structural changes in the aluminum smelter.

(50) This process of stirring the alumina comprises the stages of: analyzing at least one characteristic of the alumina (for example, the ability of the alumina to dissolve in the bath, the fluidity of the alumina, its solubility, its fluorine content, its moisture content, etc.), determining a value of the intensity of the compensating current which has to pass through the compensating circuit on the basis of the said at least one analyzed characteristic (this determination stage being performed using a nomograph obtained by experiment providing a relationship between the value of the current intensity and the characteristic analyzed) in order to produce a velocity threshold for MHD flows which is appropriate for effective stirring of the alumina while having the least possible effect on performance, changing the intensity of compensating current I.sub.2 in accordance with the current intensity value determined in the previous stage.

(51) Of course the invention is not in any way limited to the embodiment described above, this embodiment only being provided by way of 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. This invention is for example compatible with the use of anodes of the inert type at which oxygen forms in the course of the electrolysis reaction.