EP0099840A1 - Electrolytic pot for the production of aluminium having a conductive floating screen - Google Patents

Abstract

The invention relates to an electrolysis tank for the production of aluminum by electrolysis of alumina dissolved in a molten cryolite bath, according to the Hall-Héroult process, between at least one carbon anode (6) and an aluminum sheet. covering a carbonaceous cathode substrate (12). It comprises at the interface of the aluminum sheet and the molten cryolite bath a floating screen (2), conductor of the electric current, not linked to the carbonaceous cathode substrate and free to move at least in the vertical direction. The floating conductive screen can extend over the entire interface or be limited to the vertical line of each anode. The distance between each anode and the floating conductive screen can be reduced to approximately 20 mm.

Description

The present invention relates to a tank for the production of aluminum by electrolysis of alumina dissolved in the molten cryolite according to the Hall-Héroult process, comprising a floating conductive screen, between the anode and the cathode.

In the most efficient installations producing aluminum according to the Hall-Héroult process, the consumption of electrical energy is at least equal to 13,000 KWh per tonne of metal, and often exceeds 14,000. In a modern tank operating under a difference potential of 4 volts, the voltage drop in the electrolyte represents approximately 1.5 volts, it is therefore responsible for more than a third of the total energy consumption. It is due to the obligation to maintain a sufficient distance between the anode and the sheet of cathodic liquid aluminum (at least equal to 40 mm and, most often, of the order of 50 to 60 mm) to avoid the reoxidation of the aluminum entrained towards the anode by the movements of the sheet of liquid metal due to the magnetic effects and facilitated by the non-wettability of the cathode carbon substrate by liquid aluminum.

To reduce the interpolar distance, without causing the cathodic aluminum to drive towards the anode, it has been proposed to use cathodes based on electrically conductive refractories, such as titanium diboride TiB ₂ , which is perfectly wet. by liquid aluminum and undergoes practically no attack by this metal at the temperature of electrolysis. Such cathodes have been described, in particular, in English patents 784,695, 784,696, 784,697 to BRITISH ALUMINUM C °, and in the article by KB BILLEHAUG and HA OYE in "ALUMINUM", Oct. 1980, pages 642. -648 and Nov. 1980, pages 713 to 718.

One of the major problems posed by these titanium diboride cathodes is their gradual dissolution in liquid aluminum, a slow but significant phenomenon, which requires the periodic replacement of the worn elements and involves total stopping and dismantling. of the tank.

The present invention constitutes another solution to the problem of reducing the interpolar distance without the risk of entraining the cathode aluminum towards the anode.

It is characterized by the installation, between the anode and the cathode, at the interface of the sheet of liquid aluminum and of the electrolyte layer, of a floating screen conducting the electric current, and not linked to the carbonaceous cathode substrate. Since this screen must resist both the action of aluminum and the action of the molten cryolite bath, it must be made of a carbonaceous material such as graphite, or an electrically conductive refractory such as diboride. titanium.

il apparaît que l'écran flottant doit être constitué d'éléments dont la densité globale se situe entre environ 2,15 et 2,30 à 960°.If we consider the respective densities of the elements present at the average temperature of electrolysis (~ 960 ° C)

it appears that the floating screen must consist of elements whose overall density is between about 2.15 and 2.30 at 960 °.

• Sur la figure 1, l'écran conducteur flottant (1) est constitué par des billes (2) de TiB₂ poreuses, mais étanchéisées en surface, d'une densité moyenne de 2,25. Ces billes peuvent être fabriquées par exemple selon la technique décrite dans le brevet français 1 579 540 au nom d'ALUMINUM PECHINEY, et qui consiste à fritter un mélange de TiB₂ et d'une substance éliminable à la température de frittage. Le diamètre de ces billes est compris entre 5 et 50 mn et, de préférence, entre 10 et 40 mm. La limite inférieure de diamètre est liée aux coûts de fabrication et la limite supérieure correspond à environ deux fois la distance interpolaire prévue.

Figures 1 to 4 represent different modes of implementing the invention:

• In FIG. 1, the floating conductive screen (1) consists of porous TiB ₂ balls (2), but sealed on the surface, with an average density of 2.25. These beads can be manufactured, for example, according to the technique described in French patent 1,579,540 in the name of ALUMINUM PECHINEY, which consists of sintering a mixture of TiB ₂ and a substance which can be removed at the sintering temperature. The diameter of these beads is between 5 and 50 min and, preferably, between 10 and 40 mm. The lower diameter limit is related to manufacturing costs and the upper limit is approximately twice the planned interpolar distance.

(Such beads having a porosity of around 50% may be too fragile. In this case, a mixture of TiB ₂ and boron nitride is sintered (d = 2.20 to 2.25 to 960 °) or graphite (d = 1.7 to _1, 9), with the desired proportion of removable substance hot to obtain a final density substantially equal to 2.25 at 960 ° C.

It is essential to seal the balls with a surface coating to avoid their progressive impregnation by the electrolyte and / or the metal, which would destroy their buoyancy. This etarichisation is carried out by various known methods making it possible to carry out a conptact deposition of TiB ₂ , for example plasma spraying or chemical deposition. The thickness of this tight layer is sufficient for the dissolution by liquid aluminum to allow a lifetime of at least a few years, that is to say at least equal to 20 micrometers.

Sealing can be carried out in two stages: depositing a medium dense bonding layer with plasma, then a thin sealing layer by chemical deposition or else by a chemical vapor deposition carried out in two stages, the first being performed at lower pressure and temperature than the second.

Another solution, to obtain the average density of 2.25, consists in manufacturing composite balls with a graphite core and a compact TiB ₂ shell, the weight proportion of the two constituents being determined to obtain d = 2.25 (substantially 20 % of TiB ₂ and 80% of graphite), the quality of graphite then being chosen so that the coefficient of expansion of the graphite is substantially equal to that of TiB ₂ between 0 and 1000 ° C.

The TiB ₂ floating balls (2) form a substantially continuous layer at the interface (3) of the metal (4) and the electrolyte (5). It is this layer which forms the screen (1) between the anode (6) and the metal (4) and, at the same time, acts as a cathode on which the liquid aluminum droplets produced by electrolysis are formed. . These droplets wet the floating balls (2) and collect in the already formed layer (4). The risk of entrainment of droplets to the anode, where they would reoxidize, is therefore practically eliminated, which makes it possible to reduce the interpolar distance d to approximately 20 millimeters and to lower the voltage drop in the electrolyte to less than 1 volt. In FIGS. 1 and 2, the floating balls (2) have been drawn above the interface (3), but it is obvious that their exact position depends on their density ratio with the bath and the metal.

Although the invention has been described in the particular case where the floating screen is formed from TiB _2- based balls, this shape is not compulsory and any other shape may be suitable, for example cylindrical elements which, according to their length / diameter ratio, will float with the axis in vertical or horizontal position. Flat discs, for example, can be used. In such a case, (elements not linked to each other), it is desirable that the largest dimension of the elements used does not exceed 50 nm and, preferably, 40 mm, that is to say twice the target interpolar distance.

The solution of Figure 1 has the disadvantage that the entire interface of the metal (4) and the electrolyte (5) is covered by the screen of balls (2) while its presence is only necessary plumb with the anodes (6).

FIG. 2 represents a solution in which the floating conductive screen is confined to the plumb of the anodes (6) by means of barriers (7) made of dense refractory material. Openings (8) should preferably be made in these barriers to ensure the circulation of the liquid aluminum (4).

FIG. 3 represents another embodiment of the floating conductive screen; the screen is no longer made up of individual elements which are simply juxtaposed, but of a monolithic unit arranged at the base of the anode. This monolithic screen (8) can be produced in different variants, without departing from the scope of the invention, in so far as it meets the two basic criteria: density between that of the electrolyte and that of liquid aluminum, and sufficient electrical conductivity, i.e. less than that of the electrolyte (at least 10 times lower, for example).

The screen (8) can, moreover, be kept in line with the anode by the barriers (7) and it can, optionally, be provided with bosses (9) of refractory material resistant to the electrolyte and to liquid aluminum, and not very conductive of electricity such as boron nitride, aluminum nitride, or various carbides such as silicon carbide. The purpose of these bosses is to avoid any accidental contact between the anode (6) and the screen (8). The freedom of movement of the screen in the vertical direction is in fact almost total due to the absence of any anchoring means on the carbonaceous cathode substrate (12).

The screen (8) can be made of graphite or carbon felt or a carbon-carbon composite, covered with TiB ₂ on at least its upper face. If the proportion of TiB ₂ is not sufficient to obtain the required average density (2.25), the screen can be ballasted by means of dense refractory inserts, or else constitute it not by pure graphite, but by an agglomerated mixture of graphite and silicon carbide (d = 3 to 3.10) or titanium diboride (d = 4.5 to 4.6).

In the case where the screen is made of porous carbon composite, it is preferably subjected to an impregnation to the core with titanium diboride, in a proportion such that an apparent average density of the order of 2 is reached, 20, then a surface seal with a compact layer of titanium diboride 10 to 100 micrometers thick.

Another embodiment of the conductive floating screen is shown in FIG. 4. Graphite slabs (10) are provided with hooking means (11 A, 11 B), which cooperate to form assemblies provided with '' sufficient flexibility to adapt to any unevenness of the electrolyte metal interface (3).

As in the previous case, these tiles can be covered with TiB ₂ on the face opposite the anode, and the density necessary to the flotation is obtained by any of the means previously described.

The implementation of the invention, under the different variants, allows a significant reduction in the interpolar distance, up to around 20 mm, without loss of the electrolysis efficiency. The difference in potential across the terminals of the electrolysis cells thus modified is reduced from 4 volts to approximately 3.2 to 3.3 volts, with a proportional reduction in the energy consumption per tonne of aluminum produced.

Claims (7)

1 ° / - Electrolysis tank for the production of aluminum by electrolysis of alumina dissolved in a molten cryolite bath, according to the Hall-Héroult process, between at least one carbon anode and an aluminum sheet covering a cathode substrate carbonaceous, characterized in that it comprises at the interface of the aluminum sheet and the bath of molten cryolite a floating screen, conductor of the electric current, not linked to the carbonaceous cathode substrate and free of movement at least in the vertical direction . 2 ° / - electrolytic cell according to claim 1, characterized in that the floating conductive screen extends over the entire interface of the aluminum sheet and the cryolite bath. 3 ° / - electrolytic cell according to claim 1, characterized in that the floating conductive screen is limited to the vertical alignment of each anode. 4 ° / - electrolytic cell according to any one of claims 1 to 3, characterized in that the floating conductive screen is constituted by discrete elements juxtaposed. 5 ° / - electrolytic cell according to any one of claims 1 to 3, characterized in that the floating screen is constituted by discrete elements connected to each other by flexible hooking means. 6 ° / - electrolytic cell according to claim 5, characterized in that the floating conductive screen comprises stop means, little or not conductive of the electric current, directed towards the underside of the anode, and whose height is substantially equal to the minimum interpolar distance. 7 ° / - electrolytic cell according to any one of claims 1 to 6, characterized in that the distance between each anode and the floating conductive screen is less than 40 nm and, preferably, equal to about 20 mm.

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