EP0126700B1 - Sub-cathode shield comprising deformable areas for Hall-Héroult electrolytic pots - Google Patents

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 in the sub-space. cathodic.

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, it can form, through the play of differential expansions, cracks through which the molten electrolyte - consisting essentially of 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 for the whole of a series comprising many tanks of different ages.

To limit the effects of these infiltrations and impregnations, it has been 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 sodo-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 modifie son régime thermique de façon inacceptable, et dégrade les consommations énergétiques.
• - cet écart de température engendre les 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 very harmful deformations for the lining and for the cathode.

The object of the present invention is to remedy the aforementioned drawbacks.

The object of the invention is an electrolytic cell comprising at least one metallic steel screen, placed under the base of the carbon blocks constituting the cathode of the electrolytic cell, and extending at least over the entire space located directly above the cathode, characterized in that said screen consists of at least one continuous sheet of steel, at least half of the surface of which consists of a thick part having at least 5 mm, and preferably , 8 to 12 mm thick, and which comprises at least one deformable zone constituted by a steel sheet of thickness less than the thickness of the thick part, arranged at the periphery of said thick part and, s' extending in the zone situated substantially outside the vertical alignment of the cathode blocks, connected to the thick part by a continuous weld, and having at room temperature an elongation before rupture greater than 2%.

The screen may include, in the zone located directly above the cathode blocks, a deformable zone constituted by at least one open or preferably closed profile with a wall thickness less than - and preferably equal to approximately half of - l thickness of the thick part of the screen.

The tank may also include a complementary screen made up of a plurality of steel sheets of individual thickness less than 5 mm, in superimposed relationship, and disposed between the base of the carbonaceous blocks and the thick part of the screen, on the plumb with the cathode.

Finally, the tank can also be provided with a complementary screen 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 the zone located directly above the cathode blocks, in which the screen is provided with profiles absorbing the stresses of expansion, and in which a complementary screen appears consisting of the superposition of thin sheets.

Figure 2 shows the detail of the profiles.

FIG. 3 represents in diagrammatic section an electrolytic cell according to the invention, on which appears the deformable zone constituted by a thin sheet welded to the periphery of the screen in thick sheet.

FIG. 4 represents an additional screen device, welded to the base of 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 via 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 sheet metal - screen (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 sheet). 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 alternative embodiments of the screen in the area located directly above the cathode blocks appear in FIG. 2: the element absorbing the expansions can be a square tube (11) whose wall thickness is also about half the thickness of the screen, or an open profile such as a half square tube (12), which offers more flexibility, but can be a point of weakness due to the thickness reduced, and the resulting risk of faster breakthrough. The arrangement (13) of FIG. 2B is also very favorable from the point of view of flexibility, but has the same drawback.

Figure 3 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 are sealed the cathode bars (3 ) in steel, the liquid aluminum sheet (16), the electrolyte (17), the anode system (18), the installation bed (9) of the screen (19) and the heat-insulating brickwork at the bottom of the box . The screen (19) consists of a thick steel sheet (with a thickness greater than 5 nm and preferably between 8 and 12 mm) throughout the part where the thermal gradient is low, that is to say - say substantially in line with 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 draining a flow of heat. reduced 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 at room temperature a limit of elongation at break greater than 2%. In all cases, it is preferable that the thick part of the screen represents more than 50% of the total surface of the cathode blocks. The thinned, deformable peripheral part is situated substantially 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 2,546,184 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% of the surface of the base of the carbonaceous block (1) either directly or by means of a bonding layer (24), elastic and conductive of the common 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. Electrochemical corrosion of the screen (6) is thus avoided, and chemical corrosion (by 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 (4)

1. Electrolysis cell for the production of aluminium by the HALL-HEROULT process, the cathode of which consists of carbonaceous blocks in which the cathode bars are embedded, this cell comprising a. screen of steel placed under the cathode blocks and extending at least under the whole space situated vertically underneath the cathode, characterised in that the said screen consists of at least one continuous sheet of steel having at least half its surface formed by a thick part (6) which has a thickness of at least 5 mm, preferably 8 to 12 mm, and comprising at least one deformable zone consisting of a steel sheet (21) which is less thick than the thick part and which is situated at the periphery of the said thick part and extends into the zone substantially external to the area vertically underneath the cathode blocks (1) and is connected to the thick part by a continuous join (22) and has an elongation before rupture greater than 2% at the ambient temperature.

2. Electrolysis cell according to claim 1, characterised in that the screen comprises, in the zone situated vertically underneath the cathode blocks, a deformable zone formed by at least one open (12) or preferably closed (11) profile, the wall thickness of which is approximately equal to half the thickness of the thick part.

3. Electrolysis cell according to claim 1 or 2, characterised in that it comprises an additional screen (10) consisting of a plurality of steel sheets which each individually have a thickness of 5 mm and which are arranged one above the other and situated between the base of the carbonaceous blocks (1) and the thick part (6) of the screen vertically underneath the cathode.

4. Electrolysis cell according to any one of claims 1 to 3, characterised in that it comprises, in addition, an additional screen formed by a steel bedplate (23) connected to each cathode bar (3) by a join and in electrical contact with at least 50% of the surface of the lower base of the corresponding carbonaceous block (1).

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