EP0126700A1 - Sub-cathode shield comprising deformable areas for Hall-Héroult electrolytic pots - Google Patents

Abstract

The invention relates to a metallic steel screen, intended to prevent the infiltration of the metal and of the constituents of the electrolysis bath into the refractory and heat-insulating lining of the casing of a tank for the production of aluminum by the Hall-Heroult process. , this screen being placed under the base of the carbon blocks constituting the cathode of the electrolytic cell and in which the cathode bars are sealed, and extending at least over the entire space located below the cathode. It is constituted by at least one continuous steel sheet such as (6) or (19) of which at least half of the surface consists of a part having at least 5 mm, and preferably from 8 to 12 mm thickness, and which comprises at least one deformable zone absorbing the stresses due to temperature differences between the central part, located directly above the cathode, and the peripheral part. The deformable zone can be constituted either by a closed (11) or open profile, with a wall thickness less than the thickness of the thick steel sheet (6) or (9), or by a steel sheet ( 21) of thickness less than the thickness of the thick screen (19), disposed at the periphery of said screen, extending in the zone situated substantially outside the vertical alignment of the cathode blocks (1), connected with a thick screen such as (19) by a continuous weld (22), and having a cold (20 ° C) elongation before rupture greater than 2%.

Description

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 screen intended to prevent the infiltration of the constituents of the electrolyte into the sub-cathodic space.

The tanks for the production of aluminum by the Hall-Héroult process consist universally of a metal box whose bottom is lined with refractory and insulating material, which supports the carbonaceous blocks forming the cathode on which the liquid aluminum is deposited. The seal between the cathode blocks and between the cathode blocks and the walls of the box is generally ensured by a carbonaceous paste based on pitch and coke or anthracite.

From the first heating, cracks can form through the play of differential expansions, through which the molten electrolyte - essentially constituted by cryolite - begins to infiltrate. This infiltration of cryolite tends to degrade the insulating properties of the underlying refractory material. Liquid aluminum can also infiltrate through the same routes, and therefore attack the insulating refractories located between the carbon cathode and the metal box. In fact, these refractories generally comprise silica or silicates, reducible by liquid aluminum.

In addition, in particular during the first months of operation of the electrolysis tank, there occurs a gradual impregnation of the carbon lining of the tank with elements constituting the electrolysis bath, and in particular sodium and fluorine. After passing through the carbon lining, these sodo-fluorinated impregnations can attack the underlying insulating lining.

As a result of this degradation of the thermal insulators by the impregnations, the thermal insulation of the tank decreases, and the thermal losses increase. This is directly harmful for energy consumption per tonne of aluminum produced, but also has the consequence that it is difficult to find a satisfactory thermal equilibrium. To limit the effects of these infiltrations and impregnations, it was pro ~ proposed to place, above the insulating material, a protective layer of steel (patent FR -A- 2,388,901 = US -A- 4,175,022). But, for such a screen to be effective, the Applicant has found that it must be given a relatively large thickness (more than 5 mm); in addition, it must be continuous, and its periphery must be kept at a sufficiently low temperature (500 to 600 ° C) to prevent soda-fluorinated infiltrations (cryolite) from bypassing it.

• - l'écart de température entre le centre (env. 900°C) et la périphérie (env. 500°C) de l'écran provoque un flux thermique important vers la périphérie de la cuve, ce qui ùodifie son régime thermique de façon inacceptable, et dégrade les consommations énergétiques.
• - cet écart de température engendre des dilatations thermiques différentielles importantes entre le centre et la périphérie de l'écran, ce qui provoque des déformations très néfastes pour le garnissage et pour la cathode.

Under these conditions, a thick monobloc screen has two major drawbacks:

• - the temperature difference between the center (approx. 900 ° C) and the periphery (approx. 500 ° C) of the screen causes a significant heat flux towards the periphery of the tank, which modifies its thermal regime so unacceptable, and degrades energy consumption.
• - This temperature difference generates significant differential thermal expansions between the center and the periphery of the screen, which causes deformations very harmful for the lining and for the cathode.

The object of the invention is a metal steel screen, placed under the base of the carbon blocks constituting the cathode of the electrolysis cell, and extending at least over the entire space located below the cathode. , characterized in that it is constituted by at least one continuous steel sheet of which at least half of the surface consists of a part having at least 5 mm, and preferably from 8 to 12 mm thick, and which comprises at least one deformable zone absorbing the stresses due to temperature differences between the central part, located directly above the cathode, and the peripheral part.

The deformable zone can be constituted by at least one closed profile, with a wall thickness less than the thickness of the thick steel sheet or by at least one open profile with equal or less wall thickness to the thickness of the thick steel sheet, or else by a steel sheet of thickness less than the thickness of the thick screen, disposed at the periphery of said screen, extending in the zone situated substantially at the outside of the vertical alignment of the cathode blocks, connected to the thick screen by a continuous weld.

The screen can also be made up of two distinct parts, one, thick, at least 5 mm thick, provided with means for absorbing the stresses of expansion, the other, made up of a plurality of thin sheets in superimposed relationship, each less than 5 mm thick, arranged between the base of the carbon blocks and the thick part, directly above the cathode.

• Les figures 1 à 4 illustrent la mise en oeuvre de l'invention.
• La figure 1 représente un premier mode de mise en oeuvre de l'invention, selon lequel les contraintes thermiques sont absorbées par la déformation d'un tube.
• La figure 2 représente des variantes de mise en oeuvre de ce premier mode de réalisation (2A et 2B).
• La figure 3 représente un deuxième mode de mise en oeuvre, selon lequel la zone déformable est constituée par une tôle mince soudée à la périphérie de l'écran en tôle épaisse.
• La figure 4 représente un dispositif d'écran complémentaire, lié aux barres cathodiques.

The screen may, finally, further comprise an upper part formed by a steel soleplate connected to each cathode bar by a weld, and in electrical contact with at least 50% of the surface of the lower base of the corresponding carbon block.

• Figures 1 to 4 illustrate the implementation of the invention.
• FIG. 1 represents a first embodiment of the invention, according to which the thermal stresses are absorbed by the deformation of a tube.
• FIG. 2 represents variants of implementation of this first embodiment (2A and 2B).
• FIG. 3 represents a second embodiment, according to which the deformable zone consists of a thin sheet welded to the periphery of the screen of thick sheet.
• FIG. 4 represents a complementary screen device, linked to the cathode bars.

The cathode of the electrolytic cell consists of carbon blocks (1), assembled by seals (2) made of carbon paste.

The steel cathode bar (3) is sealed with cast iron in a housing (4), at the base of the carbon block (1). Separated by a powdery laying bed (5), the screen (6) made of sheet steel with a thickness at least equal to 5 mm, (and preferably between 8 and 12 mm), is constituted by a certain number of sections (6A, 6B) connected by means of a hollow profile such as a steel tube (7) on which they are welded by a continuous watertight bead (8). The thickness of the walls of the tube (7) is less than the thickness of the screen sheet (6) so that the tubes constitute a deformation zone which absorbs the expansions of the screen: it can for example be equal to half (3 mm for a 6 mm screen plate). The screen rests on the lining (9) at the bottom of the box.

It is possible to increase the efficiency of the screen (6) and its lifespan by placing between the base of the cathode blocks (1) and the screen (6) one or more steel sheets (10) of thin (1 to 3 mm for example) which act, as it were, as a sacrificial barrier, vis-à-vis the sodo-fluorinated infiltrations which occur predominantly during the first months of operation of the electrolysis tank.

Other variant embodiments appear in FIG. 2: the element absorbing the expansions can be a square tube (11) whose wall thickness is also of the order of half the thickness of the screen, or an open profile such as a square half-tube (12), which offers more flexibility, but can constitute a point of weakness due to the reduced thickness, and the risk of faster breakthrough which results therefrom. The arrangement (13) of FIG. 2B is also very favorable from the point of view of flexibility, but has the same drawback.

FIG. 3 illustrates the second embodiment of the invention. It shows, very schematically, an electrolysis tank in cross section, with the metal box (14), the side lining (15) in carbonaceous paste, the cathode blocks (1) in which the cathode bars (3) are sealed. steel, the sheet of liquid aluminum (16), the electrolyte (17), the anode system (18), the laying bed (9) of the screen (19) and the heat-insulating brickwork of the bottom of the box.

The screen (19) consists of a thick steel sheet (>, 5, and preferably about 8 to 12 mm) throughout the part where the thermal gradient is weak, that is to say substantially perpendicular to the cathode blocks (1). The temperature of the different parts of the screen is indicated in the lower part of Figure 3.

In the peripheral zone of the screen, where there is a significant thermal gradient (800 to 500 ° C.), it has been extended by a peripheral part made of thin sheet metal (21), for example 2 to 5 mm, therefore less conductive of heat and more easily deformable, especially in traction. The thin sheet is connected to the thick part by a sealed continuous weld (22). Preferably, this thin sheet will present when cold (20 ° C) an elongation limit at break greater than 2%.

In all cases, it is preferable that the thick part of the screen represents more than 50 ° o of the total surface of the cathode blocks. The thinned, deformable peripheral part is preferably situated outside the vertical line of the cathode, that is to say in the region with a high thermal gradient.

As in the case of FIG. 1, the screen can either be placed directly on the thermal insulating brickwork (20), or on an intermediate laying bed (9), and it can be separated from the cathode blocks by the laying bed powdery (5).

Another means of improving the efficiency and the lifespan of the screen consists in simultaneously using the device which is the subject of our French patent application 8308334 which consists of a thick sheet steel plate (23) connected to each cathode bar (3) by welding and in electrical contact with at least 50 ° o of the surface of the base of the carbonaceous block (1) either directly or via a bonding layer (24), elastic and conductive current, for example graphite or carbon felt.

This sole, in addition to constituting a first barrier to the penetration of sodo-fluorinated impregnation products, offers the advantage of bringing together, on either side of the laying bed (5), two identical materials (steel ), and thus eliminate the risk of forming an electrochemical cell in the case where the bedding has -or acquires- an ionic conductivity. The electrochemical corrosion of the screen (6) is thus avoided, and chemical corrosion (rar impregnation products), greatly delayed.

The implementation of the invention makes it possible to significantly increase the life of the electrolytic cells, and to maintain until the end, the heat losses as low as possible.

Claims (6)

1. Steel metallic screen, intended to prevent the infiltration of the metal and the constituents of electrolysis bath in the refractory lining and insulation of the box of a tank for the production of aluminum by the HALL-HEROULT process, this screen being placed under the base of the carbon blocks constituting the cathode of the electrolytic cell and in which the cathode bars are sealed, and extending at least over the entire space situated directly above the cathode, characterized in that '' it is constituted by at least one continuous steel sheet such as (6) or (19) of which at least half of the surface consists of a part having at least 5 mm, and preferably from 8 to 12 mm d thickness, and which comprises at least one deformable zone absorbing the stresses due to the temperature differences between the central part, located at the base of the cathode, and the peripheral part. 2. Metal screen, according to claim 1, characterized in that the deformable zone consists of at least one closed profile (11), of wall thickness less than the thickness of the thick steel sheet. 3. Metal screen, according to claim 1, characterized in that the deformable zone consists of at least one open profile (12) with a wall thickness equal to or less than the thickness of the thick steel sheet. 4. Metal screen, according to claim 1, characterized in that the deformable zone is constituted by a steel sheet (21) of thickness less than the thickness of the thick screen, disposed at the periphery of said screen, s' extending in the zone situated substantially outside the vertical alignment of the cathode blocks (1), connected to the thick screen such as (19) by a continuous weld (22), and having a cold (20 ^° C.) a elongation before rupture greater than 2 Ü. 5. Metal screen, according to claim 7, characterized in that it consists of two separate parts, one thick, at least 5 mm thick, provided with means to absorb the stresses of expansion, the other consisting of a plurality of thin sheets (10) in super relationship placed, each less than 5 mm thick, arranged between the base of the carbon blocks (1) and the thick part (19), directly above the cathode. 6. Metal screen, according to claim 1 or 5, characterized in that it further comprises an upper part formed by a steel base (23) connected to each cathode bar (3) by a weld, and in electrical contact with at least 50% of the surface of the lower base of the corresponding carbon block (1).

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