EP1155167A1

EP1155167A1 - Electrolytic cell arrangement for production of aluminium

Info

Publication number: EP1155167A1
Application number: EP00901689A
Authority: EP
Inventors: Jean-Marie Gaillard; Jacques Colin De Verdiere; Pierre Homsi
Original assignee: Aluminium Pechiney SA
Current assignee: Rio Tinto France SAS
Priority date: 1999-02-05
Filing date: 2000-02-01
Publication date: 2001-11-21
Anticipated expiration: 2020-02-01
Also published as: NO20013714L; DE60000721T2; EP1155167B1; ZA200105654B; US6551473B1; FR2789407B1; DE60000721D1; NO20013714D0; EG21884A; FR2789407A1; GC0000125A; NZ512913A; AR022463A1; AU764224B2; RU2227179C2; CA2361671A1; WO2000046429A1; AU2301000A; BR0007986A

Abstract

The invention concerns an electrolytic cell arrangement (1), disposed transversely, for producing aluminium by fused-salt electrolysis in accordance with the Hall-Heroult process, comprising at least a first row of electrolytic cells, forming a first electric circuit, and at least a second electric circuit located at a specific mean distance from the first row. The invention is characterised in that at least one conductor (7), said to be axial, passes beneath each upstream cell, in the central zone, and at least a conductor (8), said to be lateral, passes beneath each upstream cell, in the inner lateral zone, and at least one conductor (11A, 11B), said to be bypassing, bypasses each upstream cell, and said or each lateral conductor is connected to a first set of said cathode bar ends located on the upstream side so as to transmit to said risers (6A, 6B, 6D, 6E) a first part (I1) of the current (Im), ranging between 20 and 20 % of said current (Im), and said or each axial conductor is connected to a second set of cathode bar ends located on the upstream side so as to transmit to said risers (6A, 6B. 6D, 6E) a second part (I2) of said current (Im), ranging between 10 and 20 % of said current Im, and said or each bypassing conductor is connected to a third set of cathode bar ends located on the upstream side so as to transmit a third part (I3) of the current (Im), corresponding to the remainder of the current Im, and said risers are connected to said cathode bar ends located on the downstream side of the corresponding upstream cell, to the conductors passing beneath said cell, and to said or each bypassing conductor of said cell, such that a fraction (Mc) of said current (Io) less than 15 % is transmitted to the risers located in the central zone of the row.

Description

ARRANGEMENT OF ELECTROLYSIS TANKS FOR PRODUCTION

ALUMINUM

Field of the invention

The invention relates to the production of aluminum by igneous electrolysis according to the Hall-Héroult process, and more particularly to the methods and means enabling it to be used industrially. The invention relates very particularly to the rows of electrolytic cells arranged crosswise, that is to say the long sides of which are perpendicular to the axis of the row.

State of the art

Aluminum metal is produced industrially by igneous electrolysis, namely by electrolysis of alumina in solution in a molten cryolite bath, called electrolysis bath, according to the well-known Hall-Héroult process. The electrolysis bath is contained in a tank comprising a steel box, which is coated internally with refractory and / or insulating materials, and a cathode assembly located at the bottom of the tank. Anodes made of carbonaceous material are partially immersed in the electrolysis bath. The cell and the anodes form what is often called an electrolysis cell. The electrolysis current, which circulates in the electrolysis bath and the sheet of liquid aluminum via the anodes and cathode elements, operates the alumina reduction reactions and also makes it possible to maintain the bath. electrolysis at a temperature of the order of 950 ° C by the Joule effect.

For reasons of profitability of a factory, one seeks, on the one hand, to reduce the investment and operating costs and, on the other hand, to simultaneously obtain the highest possible intensities and Faraday yields, all by preserving, or even improving, the operating conditions of the electrolysis cells. For this purpose, the most modern factories contain a large number of electrolysis cells arranged in line, in so-called electrolysis halls, and electrically connected in series using connecting conductors, so as to optimize the factory floor occupancy. The tanks, which almost always have a rectangular shape, are generally arranged side by side, that is to say that the long sides are perpendicular to the axis of the queue (we also say that they are oriented "crosswise"), but they can also be arranged head-to-head (we also say that they are oriented "lengthwise"). The tanks are generally arranged so as to form two or more parallel rows which are electrically linked together by end conductors. The electrolysis current thus cascades from one cell to the next. The length and mass of the conductors are as small as possible so as to limit the corresponding investment and operating costs, in particular by reducing losses by Joule effect in the conductors. In addition, the bringing together of the electrolysis cells and the increase in the intensities of the electrolysis current have led to the development of configurations of conductors capable of compensating for the effects of the magnetic fields produced by the electrolysis current.

For this same purpose, it is known to provide cells, or rows of cells, with sophisticated control means which allow great control of the electrolysis process. In particular, French application FR 2 753 727, in the name of the applicant, proposes a process for fine regulation of the temperature which makes it possible to achieve high values of the Faraday yield.

The electrolytic cells are generally controlled in such a way that they are in thermal equilibrium, that is to say that the heat dissipated by each electrolytic cell is generally compensated by the heat produced in it, which comes mainly from the electrolysis current. The conditions of thermal equilibrium depend on the physical parameters of the tank, such as the dimensions and the nature of the constituent materials, and on the operating conditions of the tank, such as the electrical resistance of the tank, the temperature of the bath or the intensity of the electrolysis current. The tank is often constructed and driven so as to cause the formation of a solidified embankment on the side walls of this tank, which in particular makes it possible to inhibit the attack of the coatings of said walls by the liquid cryolite. The point of thermal equilibrium is generally chosen so as to achieve the most favorable operating conditions from a point of view not only technical, but also economic.

French patent FR 2 552 782 (corresponding to American patent US 4 592 821), in the name of the applicant, describes a line of electrolysis cells which can operate industrially at intensities above 300 kA and with Faraday yields greater than 90%.

Problem

The continuous development of the performance of electrolysis factories, both technically and economically, has led the applicant to seek solutions to increase the profitability of factories in general, by providing in particular for the possibility of a range operating intensities of the tanks. Indeed, the possibility of carrying out voluntary variations in the operating conditions, which can be significant compared to the nominal conditions, is often useful in the management of an electrolysis plant. For example, we can try to vary the power of a series of electrolytic cells according to an electrical energy contract.

However, the Applicant has found that the electrolytic cells exhibit temperature heterogeneities, and more precisely a dispersion of the temperature values throughout the liquid mass, which, although relatively low, tend to be maintained over time. , that is to say that certain temperature deviations from the average value of the tank do not cancel out by an effect of average over time. These heterogeneities in particular have the drawback of limiting the finesse of the thermal regulation of the tanks. Known control methods certainly allow temperature fluctuations to be controlled over time, but do not do not directly limit the dispersion of temperature values over the entire tank. In addition, the temperature zones below the set point favor material deposits at the bottom of the tank and the formation of a running slope (that is to say that part of the slope partially covers the cathode), which increase cathodic drop and cause instability of the tank, and temperature zones above the set value tend to reduce the protective solidified bath slopes on the sides of the tank and can lead to irregular wear of the coatings.

The Applicant has therefore sought solutions to reduce the dispersion of temperatures and thermal fluctuations in the electrolytic cells which overcomes the drawbacks of the prior art while remaining satisfactory for the general design of the cells, in particular as regards the floor occupation and investment and operating costs, and for the operation of tanks.

Subject of the invention

The first object of the invention is an arrangement of electrolytic cells arranged across, for the production of aluminum by igneous electrolysis according to the Hall-Héroult process.

The invention also relates to an electrolysis plant comprising an arrangement of cells according to the first object of the invention.

Description of the invention

According to the invention, the arrangement of electrolysis cells, for the production of aluminum by igneous electrolysis according to the Hall-Héroult process with an electrolysis current of intensity lo, comprises at least a first row of cells electrolysis, forming a first electrical circuit, and at least a second electrical circuit located at a determined average distance from said first queue, said first queue comprising N cells arranged across and connecting conductors for transmitting said electrolysis current lo from a cell of said line, said upstream cell, to the next cell of said line, said downstream cell, each cell comprising a metal box, interior cladding elements, anodes and cathode elements, said cathode elements being provided with cathode connection outputs projecting from the upstream side and from the downstream side of the tank of each tank, a first part Im of the current lo exiting through the cathode outputs making protruding from the upstream side of each tank, a second part Iv of the current lo exiting through the cathode outlets projecting from the downstream side of each tank, said connecting conductors comprising ascending conductors, called "mounted", the current lo coming from the all the cathode elements of an upstream tank being transmitted to the anodes of the downstream tank by means of said assemblies, and e st characterized in that at least one conductor called "axial" passes under each upstream tank, in the central zone, in that at least one conductor said "lateral" passes under each upstream tank, in the interior lateral zone, c that is to say the zone of each tank located on the side of said second electrical circuit, in that at least one conductor called "bypass" bypasses each upstream tank, in that the or each lateral conductor is connected to a first set of said cathode outputs located on the upstream side so as to transmit to said mounted a first part II of the current Im, between 10 and 20% of said current Im, in that the or each axial conductor is connected to a second set of said cathode outputs located on the upstream side so as to transmit to said mounted a second part 12 of said current Im, between 10 and 20% of said current Im, in that the or each bypass conductor is connected to a third set of said cathode outputs located on the upstream side so as to transmit a third part 13 of the current Im, corresponding to the rest of the current Im, in that said assemblies are connected to the cathode outputs located on the downstream side of the corresponding upstream tank, to the conductors passing under said tank, and at, or at each, bypass conductor of said tank, so that a fraction Me of the current lo less than 15%, and preferably less than 10%, is transmitted by the climbs located in the central area of the queue. The lateral and central areas of the tank and the queue are delimited by two imaginary vertical planes parallel to the axis of the queue. Each of said planes intercepts the tanks so as to form three zones corresponding to three comparable volumes of liquid mass inside each tank in the queue. Preferably, the central volume is between 25 and 40% of the total volume, and more preferably between 30 and 35% of the total volume. The exact volume of each zone, as well as the exact distribution of the current under the tank, depends on the structure of the tank (in particular the number of cathode outlets) and the operating mode of the tank (in particular the thickness of the slopes solidified bath on the edges of the crucible of the tank, which changes the distribution of liquid masses).

Said second electrical circuit, also called "neighboring queue" in the following text, is generally substantially parallel to the queue and generally comprises at least one electrolysis tank. It most often includes a line of electrolytic cells, but it can optionally consist only of conductors. In operation, a current of intensity lo 'flows in said second circuit. The arrangement of the tanks is preferably such that the currents lo and lo 'have substantially equal intensities and flow in opposite directions from one another.

The sharing of the upstream current of the electrolytic cells between the conductors is a function of the intensity of the current of the line lo and that of the neighboring line lo ', as well as the distance between the two lines of cells.

Description of the figures

FIG. 1 shows the electrical connection between two successive tanks of a file according to the prior art (corresponding to French patent FR 2,552,782 and to American patent US 4,592,821). The direction of the neighboring queue is indicated by the arrow FN. The direction of the electrolysis current is indicated by the arrow lo. FIG. 2 illustrates the parameters for distributing the current in a row of electrolytic cells according to the invention. To simplify the figure, only two tanks are shown: an upstream tank of rank n and a downstream tank of rank n + 1. The upstream side of a tank is identified by the letters AM; the downstream side is identified by the letters AN. The lateral and central zones of the tank plane are delimited by two vertical planes PI and P2 parallel to the axis A of the queue and placed on either side of this axis. The inner, central and outer lateral zones are identified respectively by the letters F, C and E. The arrow indicates the direction of the electrolysis current.

Figure 3 shows the electrical connection between two successive tanks of an arrangement according to the invention. The direction of the neighboring queue is indicated by the arrow FN. The direction of the electrolysis current is indicated by the arrow lo.

Detailed description of the invention

In a tank arrangement according to the invention, each tank comprises a box (1), generally made of steel, lined internally with insulating refractory materials, anodes and cathode elements. The anodes and the cathode elements are not illustrated to simplify the figures. The cathode elements include carbon blocks and cathode bars sealed in said blocks; a cathode element generally comprises one or two cathode bars. The cathode bars protrude from each side of the tanks and form said upstream (3) and downstream (4) cathode outlets (the term "cathode outlet" designates all the cathode bars of the same element projecting on one side of the tank). In general, the cathode elements are arranged side by side in the transverse direction of the tanks. The anodes, generally consisting of precooked carbonaceous pastes and metal anode rods sealed in said pasta, are fixed to a movable spider (5). The means of electrical connection between the cathode outputs and the spider comprise ascending (or mounted) conductors (6 A, 6B, 6B ', 6C, 6D, 6D', 6E), axial conductors (7), lateral conductors ( 8) and bypass conductors (11 A and 11B). In order to allow mobility of the spider, the mounts are connected to the spider by means of flexible electrical conductors (10A, 10B, 10B ', 10C, 10D, 10D', 10E). The circuit can include intermediate conductors (12, 13, 14A, 14B, 15A, 15B, 16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B, 20A, 20B, 21) and equipotential bonding conductors (22 , 23 A, 23B) to distribute the electrolysis current in the climbs.

The intensity of the current II is preferably comparable to the intensity of the current 12, in the sense that they differ by less than 15% compared to the average of II and 12 (i.e. (II + 12) / 2).

The axial conductor is preferably single. Preferably also, the lateral conductor is unique. It is also advantageous that a single bypass conductor (said interior bypass conductor) bypasses the tank on the inside and / or a single bypass conductor (said outside bypass conductor) bypasses the tank on the outside. These measures make it possible to implement the invention effectively while maintaining a relatively simple electrical circuit.

According to a preferred variant of the invention, each tank comprises at least one internal bypass conductor and at least one external bypass conductor, and the intensity li of the current flowing in the, or all of, the conductor (s) of internal bypass is comparable to the intensity of the current flowing in the, or all, external bypass conductor (s). Preferably, the intensities li and le differ by less than 15% compared to the average of li and le (that is to say (li + Ie) / 2). In the preferred embodiment of the invention, the central climb 6C carries no current, and is preferably absent, the climbs (6A, 6B, 6B ', 6D, 6D', 6E) are placed symmetrically on either side other from the axial plane of the queue, outside said central zone C, each tank comprises a single axial conductor (7), a single lateral conductor (8), a first single bypass conductor (11B) on the side of the neighboring queue , or "inside side", and a second single bypass conductor (HA) on the side opposite to the neighboring line, or "outside side". No current flows under the box in zone E located on the outside of the tank.

The climbs are preferably located between the tanks, that is to say between the two adjacent sides of successive tanks. Preferably, the number of said climbs is even and an equal number of climbs is placed on each side of the axis of the queue.

Preferably, the intensity of the current flowing in the axial conductor (7) and the intensity of the current flowing in the lateral conductor (8) are comparable, that is to say that they differ by less than 15% by compared to the average of their values. Preferably, the bypass conductors (11 A, 11B) also carry a current of comparable intensity.

Preferably, the, or each, lateral conductor passing under the tank is located near the end of the tank, and more preferably near the last cathode outlet.

In practice, the N tanks in a row typically include two end tanks (namely the row 1 tank and the row N tank) which do not have an upstream or downstream tank, or whose upstream or downstream tank is not located at the same distance as the tanks in the queue (which are generally equidistant), or whose upstream or downstream tank is not located in the axis of the queue. In these cases, the supply conductors of the first tank in the queue and / or the connection conductors of the last tank in the queue to the electrical circuit or to the next queue may have a configuration different from that of the connecting conductors between the N tanks of the queue. In particular, said connection conductors of the last tank may not include any climbs.

Comparative tests

Temperature measurements were carried out on an arrangement of tanks according to the closest prior art (FIG. 1) and on an arrangement of prototype tanks according to the invention (FIG. 3). In these tests, each tank included 20 cathode outputs on each side, that is to say 20 outputs on the upstream side and 20 outputs on the downstream side. Each cathode output included two cathode bars. The electrolysis current lo was substantially the same in all these tests, namely 300 kA. The neighboring queues were located at the same distance in all cases, namely approximately 85 m center-to-center. The current lo 'flowing in the neighboring lines was substantially equal to the electrolysis current lo.

In the arrangement of electrolytic cells of the prior art (FIG. 1), the cathode current of the upstream outputs (Im) was distributed as follows in the transmission conductors: 15 kA in the conductor (9 A), 7, 5 kA in the conductor (9B), 22.5 kA in the conductor (9C), 52.5 kA in the conductor (HA) and 52.5 kA in the conductor (11B). The total cathodic current of the downstream tank was distributed as follows in the climbs: 60 kA in the climbs (6 A) and (6E), 15 kA in the climbs (6B) and (6D ¹ ), 45 kA in the climbs ( 6B ¹ ) and (6D), and 60 kA in the central climb (6C). Each cathode output carried a current of approximately the same intensity, that is to say approximately 7.5 kA.

The number of climbs was 7 arranged as in Figure 1. These climbs were arranged between the upstream and downstream tanks and symmetrically on either side of the axis of the queue of tanks. In the arrangement according to the invention, the electrical conductors had a configuration similar to that illustrated in FIG. 3. The three zones cut the plane of the tank into three surfaces substantially of the same dimensions, that is to say that the planes PI and P2 intercepted the plane of the tank so as to form a central zone (C) corresponding to 32% of the liquid mass and two lateral zones (a zone E on the outside side and a zone F on the side of the neighboring file) each corresponding to 34% of the liquid mass (taking into account the slopes). The central zone included 6 cathode outputs and each lateral zone included 7 cathode outputs. Each of the cathode outputs carried a current of approximately the same intensity, that is to say approximately 7.5 kA.

The current from the upstream cathode outputs (Im), or "upstream current", was distributed as follows in the transmission conductors: 20.0 kA in the axial conductor (7), 25.0 kA in the lateral conductor (8) , 52.5 kA in the bypass conductors (11 A) and (11B). This distribution corresponds to: 13.3% in the axial conductor, 16.7% in the lateral conductor, 35% in the bypass conductor on the side of the neighboring queue and 35% in the bypass conductor on the outer side.

The total cathode current of the downstream tank was distributed as follows in the climbs: 76.5 kA in the climbs (6 A) and (6E), 28.0 kA in the climbs (6B) and (6D ¹ ), and 45 , 5 kA in the climbs (6B ¹ ) and (6D). The updraft flowing in the central area was therefore zero.

The number of climbs was 6, 3 climbs in the outer side area and 3 climbs in the inner side area (and therefore no climb in the central area). These climbs were arranged between the upstream and downstream tanks and symmetrically on either side of the axis of the tank line.

The temperature measurements were carried out using thermocouples plugged into the vertical wall of the tank casing and arranged around the casing. In the case of tanks of the prior art, the measurements were carried out on 20 tanks of the same line. In the case of tanks according to the invention, the measurements were carried out on 3 tanks in a row.

These tests have shown that the arrangement according to the invention makes it possible to obtain a significant reduction in the temperature difference between the upstream and downstream sides of each tank. Typically, the difference between the temperature values measured in the central zone on the upstream side, at the interface between the electrolysis bath and the liquid metal, and those measured in the central zone on the downstream side, also at l the interface between the electrolysis bath and the liquid metal observed on the cells according to the invention was 25 ° C ± 10 ° C lower than that observed on the cells according to the prior art.

Advantages of the invention

The arrangement of tanks according to the invention makes it possible to advantageously modify the rows of tanks of existing factories without requiring a significant investment.

Claims

1. Arrangement of electrolysis cells, for the production of aluminum by igneous electrolysis according to the Hall-Héroult process using an electrolysis current of intensity lo, comprising at least a first row of cells electrolysis, forming a first electrical circuit, and at least a second electrical circuit located at a determined average distance from said first row, said first row comprising N cells arranged across and connecting conductors for transmitting said electrolysis current lo d ' a tank of said line, called upstream tank, to the next tank of said line, called downstream tank, each tank comprising a metal box, internal coating elements, anodes and cathode elements, said cathode elements being provided with cathode outputs of connection projecting from the upstream side and from the downstream side of the box of each tank, a first part Im of the current lo exiting through the cathod outputs ics projecting from the upstream side of each tank, a second part Iv of the current lo exiting through the cathode outputs projecting from the downstream side of each tank, said connecting conductors comprising ascending conductors, called "mounted", the current lo coming from all the cathode elements of an upstream tank being transmitted to the anodes of the downstream tank via said assemblies, said arrangement being characterized in that at least one conductor called "axial" passes under each upstream tank, in the central zone, in that at least one so-called "lateral" conductor passes under each upstream tank, in the interior lateral zone, that is to say the zone of each tank situated on the side of said second electrical circuit, in that 'at least one conductor called "bypass" bypasses each upstream tank, in that the or each lateral conductor is connected to a first set of said cathode outputs located on the upstream side to transmit to said mounted a first part II of the current Im, between 10 and 20% of said current Im, in that the or each axial conductor is connected to a second set of said cathode outputs located on the upstream side so as to transmit to said mounted a second part 12 of said current Im, between 10 and 20 % of said current Im, in that the or each bypass conductor is connected to a third set of said cathode outputs located on the upstream side so as to transmit a third part 13 of current Im, corresponding to the rest of current Im, in that said mounted are connected to the cathode outputs located on the downstream side of the corresponding upstream tank, to the conductors passing under said tank and to, or to each, bypass conductor of said tank, so that a fraction Me of the current lo less than 15% is transmitted by the climbs located in the central zone of the said queue.

2. Arrangement according to claim 1, characterized in that the fraction Me is less than 10%.

3. Arrangement according to claim 1 or 2, characterized in that the climbs are located between the two adjacent sides of successive tanks.

4. Arrangement according to one of claims 1 to 3, characterized in that the second circuit comprises at least one tank.

5. Arrangement according to one of claims 1 to 4, characterized in that the axial conductor is unique.

6. Arrangement according to one of claims 1 to 5, characterized in that the lateral conductor is unique.

7. Arrangement according to one of claims 1 to 6, characterized in that the intensity of current II and the intensity of current 12 differ by less than 15% compared to the average of II and 12.

8. Arrangement according to one of claims 1 to 7, characterized in that each tank comprises a single bypass conductor.

9. Arrangement according to one of claims 1 to 7, characterized in that each tank comprises at least one internal bypass conductor and at least one external bypass conductor, and in that the intensity of the current li flowing in the, or all of the internal bypass conductor (s) and the intensity of the current flowing in the or all of the external bypass conductor (s) differ by less than 15% from the average li and the.

10. Arrangement according to one of claims 1 to 7 and 9, characterized in that each tank comprises a single bypass conductor on the outside and a single bypass conductor on the inside.

11. Electrolysis plant comprising at least one arrangement of electrolytic cells according to claims 1 to 10.