EP0144371A1

EP0144371A1 - Cathode bar comprising a metal sole for hall-heroult electrolysis tanks.

Info

Publication number: EP0144371A1
Application number: EP84902015A
Authority: EP
Inventors: Michel Leroy
Original assignee: Aluminium Pechiney SA
Current assignee: Rio Tinto France SAS
Priority date: 1983-05-16
Filing date: 1984-05-14
Publication date: 1985-06-19
Also published as: WO1984004547A1; IT8420920A1; US4647356A; EP0144371B1; IT1173645B; FR2560613B2; AU2963884A; AU569524B2; NO850095L; CA1214752A; DE3466561D1; IS1305B6; NZ208161A; GR81586B; ES288259Y; YU84284A; FR2560613A2; ES288259U; IS2911A7; SU1349702A3

Abstract

Une barre cathodique assurant l'extraction du courant d'une cuve pour la production d'aluminium par électrolyse, selon le procédé Hall-Héroult, scellée dans au moins une rainure ouverte à la base de chacun des blocs carbonés (1) formant la cathode de la cuve d'électrolyse. Selon l'invention, la barre cathodique (2) se prolonge par une semelle métalique (5) en contact électrique avec la base des blocs carbonés (1) sur au moins 20 % de la surface totale de cette base. La semelle (5) est constituée par une tôle métallique d'une épaisseur au moins égale à 4 mm, et, de préférence, au moins égale à 10 mm, soudée à la barre cathodique (2) avant la mise en place du bloc carboné (1) dans la cuve. Pour empêcher les infiltrations d'électrolyte dans l'espace sous-cathodique, la face inférieure de la semelle (5) est placée en relation superposée et en contact électrique avec un écran métallique (26), épais, continu, disposé à la partie supérieure du garnissage isolant thermique.A cathode bar ensuring the extraction of current from a cell for the production of aluminum by electrolysis, according to the Hall-Héroult process, sealed in at least one open groove at the base of each of the carbonaceous blocks (1) forming the cathode of the electrolytic cell. According to the invention, the cathode bar (2) is extended by a metal sole (5) in electrical contact with the base of the carbonaceous blocks (1) over at least 20% of the total surface of this base. The sole (5) consists of a metal sheet with a thickness at least equal to 4 mm, and preferably at least equal to 10 mm, welded to the cathode bar (2) before the installation of the carbon block (1) in the tank. To prevent electrolyte infiltration into the sub-cathode space, the underside of the soleplate (5) is placed in superposed relation and in electrical contact with a metal screen (26), thick, continuous, disposed at the upper part thermal insulation padding.

Description

CATHODE ROD WITH A METAL SOLE, FOR TANKS

ELECTROLYSIS HALL-HEROULT

The invention relates to the construction of electrolytic cells for the production of aluminum by the Hall-Héroult process. It relates more particularly to a cathode bar comprising a metal sole, intended to increase the passage section and to standardize the distribution of the cathode current.

EXHIBITION OF PRIOR ART

The cathode of a Hall-Héroult electrolysis tank is formed by the juxtaposition of a set of carbon blocks, provided, at their lower base, with one (or sometimes two) open groove (s) in which are sealed, generally by casting, steel bars of square, rectangular or circular section, to which the connecting conductors between the successive tanks forming a series are connected.

The steel bars used for the extraction of the cathode current offer a limited contact surface with carbon, which causes a significant voltage drop at the carbon / cast iron interface.

To reduce this voltage drop, it is also known to increase the section of the steel bar, at least in the area sealed in the carbon, while keeping a normal or narrowed section at the crossing of the external part of the insulation. from the tank, so as to avoid excessive thermal leakage.

However, such an action is necessarily limited, because the carbon thickness of the wings of the groove must be sufficient to mechanically resist the stresses due to the thermal expansion of the cathode bar, and of its sealing, when the tank.

OBJECT OF THE INVENTION

The object of the present invention is to increase substantially (more than 10%) The section of steel available for the discharge of current cathodic, and the contact surface between carbon and cathodic conductors. It consists in providing each cathode bar with a metal sole, in electrical contact with the horizontal base of each carbon block, the sole being welded to the cathode bar to ensure the passage of electric current. In addition, a continuous steel screen can be placed under the soleplate, in electrical contact with the soleplate, which prevents infiltration of liquid aluminum and molten cryolite, which significantly increases the life of the electrolysis tank.

Figure 1 relates to the prior art.

Figures 2 to 15 illustrate the implementation of the invention.

All the figures are representations in vertical section.

DESCRIPTION OF THE FIGURES

Figure 1 shows the classic arrangement of a carbonaceous cathode block (1) in which the bar (2A) is sealed by cast iron (3).

In this case, the bar is flush with the base of the carbon block. On the right part of Figure 2, the bar (2B) in an alternative embodiment, can more or less exceed the base plane of the carbon block (3). The successive cathode blocks are most often assembled by a joint (4) made of carbonaceous paste.

FIG. 2 illustrates a first embodiment of the invention. On the cathode bar (2), two thick sheets of mild steel (5) were welded connected to the base of the carbonaceous block (1) by a layer of electrically conductive elastic material (6).

As a variant (FIG. 2A), the steel soleplate (5) can be constituted by a steel-copper laminate, the copper-colored face (5A) being in contact either directly with the carbonaceous block (1), or by means of the elastic conductive layer (6). Preferably, the thickness of the copper layer (5A) must be greater than a minimum value, which can be estimated at approximately 5% of the steel layer, corresponding to the solubility of copper in the steel to be 900-950 ° C, so that the entire copper layer does not not disappear by diffusion in the solid state in steel.

The malleability of hot copper facilitates the establishment of good contact with the cathode block and can, if necessary, partially compensate for deformations of the steel soleplate.

In addition, the electrical conductivity of copper being much higher than that of steel, it follows a significant lowering of the voltage drop in the cathode collectors.

Figure 3 shows the four stages 3a, 3b, 3c, 3d of the procedure used to carry out the assembly of Figure 2.

Figure 3a shows the first step:

The carbon block (1) having been turned over so that the groove (7) is upwards, the cathode bar (2) is sealed by casting iron (3).

Figure 3b shows the second step:

After sealing the cathode bar (2), an elastic, electrically conductive layer (6) is placed on the upper face of the inverted block: it is advantageous to use a carbon or graphite felt, or a laminated graphite sheet, or another complex formed by bonding a strip of carbon felt or graphite and a strip of laminated graphite.

By way of example, it is possible to use RVG graphite felts, or "PAPYEX" (R) (trademarks registered by the company "Le Carbone-Lorraine").

Figure 3c shows the third step:

The sole (5) consists of two thick steel sheets, which are applied strongly, by pressing, to the elastic link layer (6) on the elastic connection layer (6). It is then possible to produce weld beads (8), preferably continuous, to connect these thick sheets to the cathode bar: a steel soleplate is thus electrically connected to the cathode bar. The thickness of the sole is at least equal to 4 mm and, preferably, at less equal to 10 mm and generally of the order of 10 to 15 mm. The cross-section of the cathode bar can be, for example, 160 x 120 mm.

Figure 3d shows the cathode carbon block returned to normal position by inversion.

FIG. 4 shows a variant of the invention in which the steel base is located astride two carbonaceous cathode blocks, and in electrical contact with these two blocks.

Figure 5 shows that when mounting is carried out, it is preferable to provide a slight clearance between the flanges (5) of two adjacent blocks (1) and (1 ') so that when the normal operating temperature is reached, and due to the greater expansion of the steel base compared to the carbonaceous block, the edges of the two adjacent flanges (5) and (5 '), are in contact with a pressure just sufficient to ensure a weld to hot of these edges between them, without this pressure being excessive to the point of causing deformation of the soles, detrimental to the electrical contact between carbon blocks and steel soles.

The opposite ends of the two adjacent flanges (5) and (5 ') can be perpendicular to the plane of the flange - and parallel to each other as shown in the figure, or cut in a bevel (Figures 5a, 5b).

The blanks of the bevels of (5) and (5 ') may be parallel to each other (5A) or not (5B).

To prevent the penetration of powdery products from the laying bed (11) into the clearance (9) between the two sheets (5) and (5 '), provision may be made for the interposition of a strip of thin sheet (10) acting as a seal. This sheet also prevents the carbonaceous paste which fills the joint (4) from flowing, during the first heating, into the space (9).

The value of the clearance (9) necessary for mounting or installation depends on the exact nature of the carbonaceous block: based on anthracite, or semi-graphite or semi-graphitized or graphite, and on the exact size of the blocks and footings , as well as the nature and thickness of the joint between blocks carbonaceous: blocks glued together or separated by a small joint (4) of pot lining. Generally, this game will be defined by an e / L ratio of the order of 1 to 2%.

Figure 6 shows the mounting detail of the sealing strips (10). The upper strip (10A) is welded for example on the sheet (5) and the lower strip (10B) is welded on the sheet (5 ') so that, during the first heating, they can slide freely and take their definitive place.

Furthermore, and provided that adequate grooving is provided in the cathode blocks (1), a lower porosity graphite piece (12) can be placed at the lower part of the joint (4) which improves the sealing of the joint. (4) and reduces the risk of infiltration of molten cryolite when the electrolysis tank is started.

Figure 7 shows another alternative embodiment of the joint between the flanges (5) and (5 ') of adjacent cathode blocks (1) and (1'). We renounce to establish a direct weld between (5) and (5 ') and we have in the set (9) a flexible joint (14), preferably electrically conductive and compressible, such as graphite braid, or a tube metallic with thin wall (thickness less than half the thickness of the sole (5) or (5 '), resting freely between the strips 10A and 10B. It is also possible, during assembly, to provide a covering and bonding sheets of elastic carbonaceous material (13) which improves the sealing of the joint, for the purpose previously indicated.

FIG. 8 represents another variant, in which the flexible carbon joint (14) is replaced by a deformable tube (15), welded beforehand to at least one of the flanges (5) or (5 ′), which absorbs the effects of expansion and which can be filled with an inert pulverulent material (19), to limit internal hot oxidation.

Of course, such flexible or deformable connections can be used for the junction between the half-screens of the same block when the latter comprises a seal with the steel half-bars, separated in the center of the block by a space of dilation. Figures 9 and 10 illustrate the implementation of the invention in the case of carbonaceous blocks (1) provided with two parallel cathode bars (2C and 2D), an arrangement which is sometimes encountered in order to increase the surface contact with the carbon block.

In FIG. 9, it appears that the two bars 2C, 2D have been sealed simultaneously by casting iron (3), the thickness (e) of the iron plate between the two bars preferably being less than or equal to the difference in dimensions h ₀ and h ₁ (e <h ₀ - h ₁ ).

The sole (6) can be made of simple steel sheet or mixed steel-copper as described above.

In FIG. 10, the two cathode bars 2C, 2D have been individually sealed, then connected by welding with a sheet (21) preformed in a vault, so as to obtain, when hot, good electrical contact with the central part of the carbonaceous block. via the elastic conductive layer (6).

In all the cases shown (Figures 2 to 10), the metal soleplate is in contact with the base of the carbonaceous blocks - directly or through the elastic material (6) - over at least 20% of the surface of this base.

It is possible to improve the tightness of the sub-cathode space and to almost completely eliminate the infiltration of liquid aluminum and molten cryolite by placing a screen (26) under the base of the carbon blocks constituting the cathode of the electrolytic cell and in which the cathode bars are sealed, this screen extending at least over the entire space located below the cathode (fig. 11). It consists of at least one continuous sheet of steel, at least half of the surface of which consists of a part having at least 5 mm, and preferably 8 to 12 mm thick, and which comprises at least one zone. deformable absorbing stresses due to temperature differences between the central part, located directly above the cathode, and the less hot peripheral part. To avoid any risk of electrochemical corrosion, it is then preferable to put the metal soleplate (5) in contact electric with the screen (26).

The electrical connection between the sole (5) and the screen (26) can be ensured by welding, for example, by a continuous or discontinuous bead between at least one edge of the sole and the continuous screen. It can also be ensured by the fusion of a brazing alloy previously disposed between the sole and the continuous screen, the solidus and liquidus points of this alloy being suitably chosen.

FIG. 12 shows an embodiment of this principle, according to which the sole (5) of each cathode bar (2) is placed directly on the continuous screen (26) of thick steel, to which it is connected by solder (16) or by welding (17).

The advantage of this arrangement is that the continuous thick steel screen

(26) is, at all points, at the electrical potential of the cathode bars, which eliminates any effect of electrochemical cell generating rapid corrosion. The thickness of the screen is at least 5 mm, and preferably between 8 and 12 mm. The sole (5) has a thickness at least equal to 4 mm and, preferably, at least equal to 10 mm.

Figure 12 shows how this device can be implemented during the construction of the tank. The cathode bar (2) was put in place and sealed with cast iron (3) in each cathode block (1). Then welded in (18), a sole (5) whose length is at most equal and, in practice slightly less than the distance between two successive bars. The block is then placed on the screen (26), resting on the insulating layer (20), and welded at (9), preferably by a continuous cord, so as to ensure good electrical contact. The layers (24) and (25) are insulating and refractory bricks, arranged on the bottom (27) of the casing of the electrolysis tank.

For the installation of the first and last block, it may be necessary to modify the length and the arrangement of the soles to facilitate assembly.

It is possible to make the electrical connection between the sole (5) and the screen (26) by brazing (16) by means of an alloy, having a point of sol lidus and a suitably chosen liquidus point, interposed between the sole and the screen.

This brazing alloy must meet the following conditions:

1. Its solidus temperature (= finishing solidification temperature) must be greater than approximately 600 ° C and preferably 650 ° C, so that when the tank is put into service for the first time, it allows relative movements between the soles (5) and the screen (26), caused by the differential expansions between the blocks and cathode bars provided with their soles, and the continuous screen. If the solder is introduced in the form of a pulverulent bed, or in very fine grains, of alloy remaining solid up to around 650 ° C., this condition will be fulfilled.

2. This same solidus temperature should preferably not exceed the temperature reached by the soles in continuous running mode, that is to say about 850 to 920 ° C, so that the brazing alloy liquefies at least partially during the warm-up, when starting the tank. There then occurs a weld by metallic interdiffusion between the steel sheets of the flanges (5) and of the screen (26) during this at least partial melting of the intermediate alloy. This implies that:

. at least one of the alloying elements is sufficiently soluble in iron, in the solid state, in a temperature range corresponding to the operating temperatures of the soles and of the screen;

. the iron is at least partially soluble in the liquid intermediate alloy, so that the soldering is effective after the alloying elements of the solder have been absorbed by a diffusion in the solid steel: a surface fusion of steel by the alloy, this alloy then disappearing by diffusion in the steel, and leaving in place a solid weld.

3. The alloy, or one of its constituents, must not favor the oxidation of steel.

4. The alloy, or one of its constituents, must not favor a fra mechanical or chemical stabilization of steel.

5. Finally, for industrial practice, the cost of this alloy must be moderate.

The optimal compositions of the solder for the implementation of the invention comprise at least 50% of a first metal chosen from aluminum, copper, zinc, the rest being at least a second metal chosen from manganese, nickel, vanadium, beryllium, silicon, tin and titanium, as well as aluminum and copper if the first metal is not copper or aluminum.

Among these alloys, compositions no. 1, 3, 6 and 7 are particularly suitable in industrial practice. Some are fragile and can be ground to the desired fineness, others must be treated in a known manner, by spraying in the liquid state.

If the composition of the alloy allows, the solder can be used in the form of a thin, laminated sheet, introduced during assembly between the sole and screen. The presence, among the main or secondary constituents of the alloy, of a reducing metal with respect to iron oxide (calamine) which most often covers the steel plates used to constitute the sole or the screen ( metal such as Al and / or Si) dispenses with using any other stripper to promote spreading of the solder when it passes into the liquid state.

It is also possible as a variant of the invention to confuse the sole (5) and the screen (26) in a single part, constituted by a thick steel sheet (22), as shown in the figures. 13 and 14, which can be provided with seals or deformable zones, capable of withstanding thermal expansions, for example the tube (15) of FIG. 2. In such a case, the thickness of the screen can be between 10 and 20 mm.

For mounting, the bars (2) are positioned on the screen (22), then connected by a weld bead (23). After which, the cathode blocks (1) are put in place, the sealing being ensured by carbonaceous paste (13). The connection can also be made by a solder (16).

It is also possible to interpose, between the carbon blocks (1) and the thick screen (22) a layer of flexible material and good electrical conductor, for example "Papyex" (trademark registered by the Company "Le Carbone-Lorraine") which is a flexible graphite sheet or RVG graphite felt, from the same company.

FIG. 14 shows another method of assembly, according to which the cathode bases (2) are no longer placed directly above the axis of the block (1), but straddling two blocks adjacent to the base of the joint between these two blocks. The advantage of this arrangement is that the carbonaceous paste (4) ensuring the sealing between the blocks (1) and the bars (2) can be injected, hot, in the space separating two adjacent blocks.

Figure 15 shows very schematically, the partial cross section of an electrolytic cell according to the invention, with the external metal box (30), the side soldering (31) in carbonaceous paste, the cathode carbon block (1 ) surmounted by the sheet of liquid aluminum (32), the electrolyte (33) and the anode system (34), the cathode bar (2) in steel, sealed with cast iron (3), and the sole (5) in steel, object of the invention. It is noted that the cross section of the cathode bar (2) is reduced in the crossing of the external part of the lining (31) and of the box (30).

BENEFITS PROVIDED BY THE INVENTION

The implementation of the invention provides the following advantages:

1. The metal soleplate, on each cathode bar (a tank can have several tens) increases by at least 10% and up to 20 to 50% the cross-section of the cathode current and the steel-carbon contact surface with correlative reduction of the voltage drop on steel-carbon contact.

2. The metal sole associated with the screen ensures a very good distribution of the current over the entire surface of the cathode, hence the reduction of horizontal currents in the liquid aluminum, which have a harmful influence on the stability and the yield of the tank, due to the swirling effects in the resulting sheet of liquid aluminum.

3. The metal sole associated with the screen also ensures excellent temperature uniformity of the entire cathode, which reduces the risk of infiltration in hot areas and condensation in relatively cooler areas. .

4. In the event of a cathode rod breaking (by corrosion under the effect of infiltration of cryolite and liquid aluminum in the joints), the soleplate functions for the block considered as an emergency collector, which delays as much when to stop and disassemble the tank to redo the cathode. In addition, the electrical imbalance of the tank is limited, which is favorable for Faraday performance during the period between the breaking of a bar and the stopping of the tank.

The presence of the screen (26) provides the following additional advantages:

5. Blockage of all infiltration of sodo-fluorinated products and cryolite in the direction of the thermal insulation placed on the bottom of the box, which are the main cause of decommissioning of the electrolytic cells.

6. Greater ease of construction of the cathode which, in the case of FIG. 3 for example, requires, compared to the previous solutions, only a simple additional weld bead, which is made flat and in an accessible space , or an application of solder powder.

7. Elimination of the risk of electrochemical corrosion of the screen because it is, at all points, at the potential of the cathode bars. The implementation of the invention contributes to increasing the useful life of the electrolytic cells and to maintaining the good thermal insulation of the bottom during the service life.

All these advantages combine to significantly increase the life of an electrolytic cell.

Claims

1. Cathodic bar ensuring the extraction of the current from a tank for the production of aluminum by electrolysis, according to the Hall-Héroult process, sealed in at least one groove (7) open at the base of each of the carbonaceous blocks (1 ) forming the cathode of the electrolytic cell, characterized in that said cathode bar (2) is extended by a metallic sole (5) in electrical contact with the base of the carbon blocks (1) over at least 20% of the surface total of this base.

2. Cathode bar, according to claim 1, characterized in that the sole (5) is constituted by a metal sheet with a thickness at least equal to 4 mm, and, preferably, at least equal to 10 mm, welded to the cathode bar (2) before the carbon block (1) is placed in the tank.

3. Cathode bar, according to claim 2, characterized in that the metal sole is made of sheet steel.

4. Cathode bar, according to claim 2, characterized in that the metal sole is an iron-copper composite, the copper part of which faces upwards, facing the base of the carbon block.

5. Cathode bar, according to claim 4, characterized in that the thickness of the copper part is at least equal to 5% of the thickness of the steel part.

6. Cathode bar according to any one of claims I to 5, characterized in that the electrical contact between the sole (5) and the base of the carbon block (1) is ensured by at least one layer of elastic material (6) conductor of electricity.

7. cathode bar, according to claim 6, characterized in that the elastic material (6) is a carbon product chosen from carbon felt, graphite felt, laminated graphite sheet and laminated graphite sheet complexes bonded on a graphite or carbon felt.

8. Cathode bar, according to any one of claims 1 to 7, characterized in that the flanges (5) and (5 ') of two adjacent cathode blocks (l) and (1') are separated by a space (9 ) such that they come into contact when they have reached, in service, their equilibrium temperature, which is between about 800 and 900 ° C.

9. Cathode bar, according to any one of claims 1 to 7, characterized in that the flanges (5) and (5 ') of two adjacent cathode blocks (1) and (1') are separated by a space (9 ) fitted with a flexible seal (14).

10. Cathode bar, according to claim 8 or 9, characterized in that the space (9) is provided with sealing means such as (10A), (10B).

11. Cathode bar, according to claim 1, characterized in that the underside of the sole (5) is placed in superimposed relation and in electrical contact with a metallic screen (26), thick, continuous, disposed at the upper part of the lining. thermal insulator (10).

12. Cathode bar, according to claim 11, characterized in that the electrical connection between the sole (5) and the screen (26) is ensured by welding.

13. Cathode bar, according to claim 12, characterized in that the weld is obtained by a continuous or discontinuous weld bead between at least one edge of the sole (5) and the screen (26).

14. Cathode bar, according to claim 12, characterized in that the welding is ensured by the fusion of a brazing alloy previously placed between the sole and the continuous screen.

15. Cathode bar, according to claim 14, characterized in that the brazing alloy has a solidus point of between 600 and 920 ° C, and preferably between 650 and 850 ° C.

16. Cathode bar, according to claim 14, characterized in that the brazing alloy comprises at least 50% of a first chosen metal among aluminum, copper and zinc, the remainder being at least a second metal chosen from manganese, nickel, vanadium, beryllium, silicon, tin and titanium, as well as aluminum and copper if the first metal is not copper or aluminum.

17. Cathode bar, according to claims 11 or 12, characterized in that the screen (26) comprises means such as (7) for absorbing thermal expansion and stresses.

18. Cathode bar, according to claim 11, characterized in that the sole (5) and the screen (26) are combined into a single plate (22), in direct contact with each cathode bar, to which it is connected by welding. or brazing and provided with means such as (15) for absorbing thermal expansion and stresses.

19. Cathode bar, according to claim 18, characterized in that it is arranged directly above the axis of each cathode block (1).

20. Cathode bar, according to claim 18, characterized in that it. is positioned directly above the joint separating two adjacent cathode blocks.

21. Cathode bar, according to claim 18, characterized in that it is sealed in the block (1) by a carbonaceous paste (4) subsequently baked in place.