EP0210111B1 - Superstructure with intermediate portal for an electrolysis vat used in the production of aluminium - Google Patents

Abstract

The invention concerns a superstructure for a tank for the production of aluminum using the Hall-Heroult process by the electrolysis of alumina in molten cryolite, the tank being formed by a rigid metal heat-insulated casing of elongate parallelepipedic shape, the two ends of which are referred to as heads, and a superstructure formed by at least one rigid beam disposed along the long length of the casing, supporting in particular the anodic bus and the anodes and resting at its two ends on supports disposed at the two head ends of the tank, said superstructure being characterized in that each rigid beam is supported on at least one intermediate gantry in the central part thereof. The gantry may comprise four legs so as to constitute an inherently stable structure or it may be formed by two half-elements each provided with two legs. Preferably, each rigid beam is divided into two equal portions, the central ends of which are supported on the gantry.

Description

1. Technical field of the invention

The present invention relates to a device of the gantry type, intended to support in its middle, or at one or more intermediate points, the superstructure of a Hall-Héroult electrolysis tank for the production of aluminum.

2. State of the art

The superstructure of a modern electrolysis tank is made up of one or more horizontal steel beams supported on feet at their ends and supporting on the one hand the anodic current ascents and the anodic frame, formed by bars of aluminum from which the anodes are supplied, and on the other hand the alumina supply systems (alumina tank, crust breaker, metering distributor), the anode up-down control systems, and very often , the effluent, gas and dust collection pipes emitted by the tank.

The feet are supported at the ends of the metal box which constitutes the actual electrolysis tank. This arrangement has the advantage of freeing up space on the two large sides of the tanks, through which the anodes are changed, and of not transmitting to the superstructure the transverse thermal expansions which appear in the current part of the box. after starting the tank, during start-up.

3. Problems posed by the evolution of the technique

The current trend is to constantly increase the unit power of the tanks, which results in an elongation of the box, the length of which can now be between 15 and 20 meters for tanks operating under 250 KA and beyond.

In this case, the construction of the superstructure poses a difficult problem: the process of regulating the tanks indeed requires extremely precise positioning of the anode plane with respect to the cathode plane formed by the sheet of liquid aluminim which remains perfectly horizontal (except weak local deviations from magnetic effects).

• - supporter son propre poids,
• - supporter l'effort nécessaire pour rompre les croûtes d'électrolyte solidifié qui s'opposent aux déplacements verticaux des anodes, et qui sont particulièrement dures dans les cuves modernes utilisant des électrolytes teneur élevée en AIF₃,
• - assurer la constance de la distance anode- cathode sur toute la longueur de la cuve (cette distance est de l'ordre de 40 mm).

The anodes being supported by the superstructure, this must have sufficient rigidity to:

• - bear its own weight,
• - withstand the effort necessary to break the crusts of solidified electrolyte which oppose the vertical displacements of the anodes, and which are particularly hard in modern tanks using electrolytes high in AIF _{3 content} ,
• - ensure consistency of the anode-cathode distance over the entire length of the tank (this distance is of the order of 40 mm).

To give the superstructure satisfactory rigidity according to the preceding criteria, it is necessary to increase the mass (thickness of the beams) and the height. The increase in height affects that of the building, and therefore its cost. We are therefore quickly limited in this direction.

4. Subject of the invention

The invention aims to solve the problem of the rigidity of the superstructure of modern high-power tanks, for the production of aluminum by the Hall-Héroult process, working at intensities ranging from more than 200,000 to 500 000 amperes and even beyond, without interfering with the normal operation of the tanks which includes, in particular, the periodic changes of spent anodes.

It relates to a tank for the production of aluminum according to the Hall-Héroult process by electrolysis of alumina in molten cryolite, the tank being constituted by a rigid metal box, internally lined with heat insulation and provided with a carbon cathode, elongated parallelepiped shape, the two ends of which are called heads, and by a superstructure constituted by at least one rigid beam arranged along the long length of the box, in particular supporting the anode frame supplied with current by positive rises, and on which the anodes, this beam resting, at its ends, on supports arranged at the two heads of the tank, this superstructure being characterized in that each rigid beam rests on at least one intermediate gantry. In addition, the support leg (s) of this gantry are preferably placed in line with conductors bringing the current coming from the previous tank, and the supports include means for expanding independently of the box. The beam can be, in its length, constituted by a single element, or by several distinct elements.

5. Description of the figures

Figure 1 recalls the structure, in cross section of a modern electrolysis tank, PECHINEY type.

Figures 2 to 5 relate to the invention; to respect the proportions (width / length of the box), part of the length of the superstructures has been truncated in these 2 figures. Furthermore, the fixing devices have not been shown anodes, and the height adjustment devices of the anode assemblies, which are not part of the invention and are well known to those skilled in the art.

In FIG. 1, the essential organs of the electrolysis cell have been underlined the metal box (1), the internal lining (2), the cathode (3) and the cathode bar (4), the sheet of liquid AI ( 5), the molten cryolith bath (6) covered with a solidified crust (7), the anodes (8) suspended by the rods (9) and fixed to the anode frame (10) and the superstructure formed by the two rigid beams (11), which support, in particular, the anode frame (10), the set of anodes (8), as well as the alumina metering distributor (12), whose local storage silo (13) is often placed between the two beams (11), as well as the effluent collection pipes not shown.

In Figure 2 which shows an example of a superstructure according to the invention, the outline of the upper rim (15) of the box (1) is shown diagrammatically. The rigid beams (11) which form the superstructure are separated into two parts (11A) and (11B), the outer ends of which rest on the feet (16A) and (16B), arranged at each head of the tank and the central ends. are based on the gantry (17).

It would not however be contrary to the invention to constitute the rigid beams ((11) in a single length; regardless of transport and placement issues, the problem of their linear expansion should then be taken into account in the design of the superstructure.

In Figure 2 this gantry itself comprises 4 feet (18) which are supported on the upper edge (15) of the box in its central part. This support system must take account of the fact that the box expands during service, in particular in the transverse direction, and it therefore does not really constitute a fixed support point. It is therefore necessary to provide for a freedom of movement of this support in the direction of expansion of the box, that is to say in the general direction of the current flowing through the line of tanks arranged transversely relative to the axis of the line, for example by means of roller bearings suitably protected against abrasive alumina dust and other constituents of the electrolyte. To improve this protection, it is entirely consistent with the general principles governing the invention to place sliding or rolling supports allowing freedom of relative movement of support, relative to the box at the level of the flat edge or several centimeters or decimeters above. of the level of the flat edge (15). It is also necessary to take into account the space due to the presence of positive rises such as (29) on the side of the tank (current arrivals from the previous tank in the series and which connect to the anode frame of the tank; only one of these climbs is shown). Finally, to take into account the anode change operation which is also carried out on the side (only one of this anode is shown diagrammatically, but there are at least twenty on each side in the high-power tanks) , we will preferably choose to place the support leg (s) of the superstructure in the same overall size as the current rises from the upstream tank.

When the superstructure is divided into two independent parts (11 A) (11B), arranged on either side of the intermediate gantry, there are no longer any bending stresses at the gantry; in addition, the manufacture, transport and installation of the two elements are simplified, thanks to the reduction in length and weight.

The support of the ends of the beams (11A) and (11B) on the intermediate gantry (17) is preferably done by a means making it possible to absorb the slight relative movements of the supports of the beams on the gantry. A simple solution consists in letting the central ends of the beams (11A) and (11B) rest freely on the upper part (19) of the gantry (17).

The anode frame (10) is preferably also made up of two parts, so as to distribute the expansions on either side of the center of the superstructure. There is then between the two parts of said frame an expansion joint ensuring electrical contact, for example a loop of aluminum strips, or any other equivalent means.

The solution of the central gantry (17) with four legs offers the advantage of making it a stable element in itself, which can thus be used as a support and connection member for a superstructure in two independent sections, and even serve supporting the motorization assemblies of the up-down movements of the anodes; the central position favoring the distribution of the forces on the two half-frames supported by the two half-beams.

The support of the feet (18) of the gantry can be carried out, as said above, on the upper edge (15) of the box, but also outside, on specialized elements, for example a solid or reinforced concrete pillars. This arrangement - which overcomes the problems of transverse expansion of the box - however requires to increase the space between the tanks, therefore to lengthen or widen the building, which affects the cost price.

Another embodiment of the invention (fig. 3) consists in supporting each half-section of the beams (11A) (11B) by two independent gantries (20) with two feet (23A) (23B), one for the ends (24A) of (11A) and one for the ends (24B) of (11B). In this case, and to ensure stability, each gantry (20A, 20B) must be welded respectively to the beams (11A) (11B), at the supports (21A, 21 B).

In the various cases, it is possible to provide for the passage of the anode frame (s) (10) through openings (22) formed in the upper horizontal part of the intermediate gantry. However, these openings are not necessary if the gantry (18) is placed below, or above the anode frame (10).

In FIG. 4, another alternative embodiment of the invention has been shown, in which the intermediate gantry has a shape reminiscent of that of a capital A (25), the support of the beam (11) is making on the horizontal bar (26) of A. A particularly advantageous embodiment consists in using, as gantry, two elements in A, braced, so as to form a rigid gantry, the two elements being spaced apart by a distance such that the one can accommodate, between them, a current rise (29) which is connected to the frame (10). In some cases, it may be necessary to slightly separate the anodes such as (8A) located on either side of a gantry in at (25), so as to facilitate the operations of exchanging the used anodes.

The positioning of the gantry at at least one of the current rises (29) coming from the previous tank is, moreover, entirely compatible with the two and four foot gantry structures of FIGS. 2 and 3.

Finally, FIG. 5 represents another form of implementation of the invention, particularly simplified, since it is reduced to a horizontal support beam (27) resting on two feet (28) - (the second is not not visible in the figure). The horizontal support beam (27) is shown in tubular, square form, but it can also be formed by any conventional profile, for example in 1, the dimensions of which are based on well known calculations of resistance of materials.

In the case of such an embodiment, it is also preferable to have one of the climbs (29) in line with the intermediate support (27) (28), that is to say substantially in the same vertical plane as this support, the flexible foils (30) which provide the electrical connection between the rise (29) and the anode frame (10) being connected to this frame (10) in the hatched area (31), so as to free up the maximum space for anode change operations such as (8) or (8A) worn.

More precisely (see Figure 4) we can characterize this arrangement by considering that the vertical transverse plane P, (perpendicular to the major axis AA 'of the tank), and passing through the axis X of the rise and the vertical plane P 'passing through the Y axis of the corresponding intermediate gantry must be substantially coincident, or, at least, located a short distance from each other, and substantially parallel to each other so that the positive mounted leg assembly has a minimal footprint.

Although the invention has been described in the particular case where the rigid beams (11) are divided into two parts and rest on a central gantry, it is obvious, for the person skilled in the art, that it also applies to the case where, for a larger tank, the rigid beams (11) are divided into more than two parts, for example three parts, equal or not, supported by two intermediate gantries two or four feet . These tank superstructures can therefore be made in a modular fashion, so that their maximum length is no longer limited by the weight of the beams and the transport and installation difficulties, which added to the bending problems.

Finally, the invention makes it possible to take account of the effects of deflection of the beam, due to differential thermal expansions. Indeed, the horizontal support beam such as (26) or (27) of the gantry is subjected to temperature variations which are a function of the alumina covering of the crust. The highest temperature will be reached during a change of anode near this beam, this change causing a rupture of the solidified electrolyte crust, and direct radiation of the electrolysis, at about 930 - 960 °, on the superstructure.

The thermal gradient between the upper part and the lower part of the beam introduces an arrow. If this deflection is incompatible with the regulation of the tank, we are led to reduce this thermal gradient. A good expansion control simplifies the support of the gantry on the box if the expansions are similar.

• a) Matériau: il est possible d'utiliser pour constituer la poutre, des aciers au nickel qui présentent une dilatation inférieure de moitié à celle de l'acier ordinaire.
• b) Evacuation des calories par circulation d'air: les calories peuvent être évacuées par une circulation d'air à l'intérieur et/ou autour de la poutre.
• c) Evacuation des calories par Caloduc: des tubes fermés contenant un fluide à la limite de la température de vaporisation sont placés au contact de la partie inférieure de la poutre à une extrémité et à l'extérieur de la cuve à l'autre extrémité. La chaleur de la partie exposée de la poutre vaporise le liquide, ce gaz monte dans le tube et se condense dans la partie extérieure en libérant des calories.
• d) Equilibrage des températures: un pont thermique peut être installé entre la partie inférieure de la poutre et sa partie supérieure. Il doit être constitué en matériau bon conducteur thermique tel que l'aluminium.
• e) Ecran thermique: un écran réfléchissant et/ou isolant thermique, installé sous la poutre, la protège du rayonnement thermique occasionnel lors d'un changement d'anode.

For this, one or more of the following solutions can be implemented, which act on the different factors causing the deflection:

• a) Material: it is possible to use to form the beam, nickel steels which have an expansion lower by half than that of ordinary steel.
• b) Evacuation of calories by air circulation: calories can be evacuated by air circulation inside and / or around the beam.
• c) Evacuation of calories by heat pipe: closed tubes containing a fluid at the limit of the vaporization temperature are placed in contact with the lower part of the beam at one end and outside the tank at the other end. The heat of the exposed part of the beam vaporizes the liquid, this gas rises in the tube and condenses in the external part by releasing calories.
• d) Temperature balancing: a thermal bridge can be installed between the lower part of the beam and its upper part. It must be made of a material that is a good thermal conductor such as aluminum.
• e) Thermal screen: a reflective and / or thermal insulating screen, installed under the beam, protects it from the occasional thermal radiation during an anode change.

The invention, in the various modes of implementation which have been described, removes one of the most serious obstacles to the production of tanks at 500,000 amperes (and even beyond), whose technical and economic advantages are very attractive.

Claims (13)

1. A tank for the production of aluminium using the Hall-Heroult process by the electrolysis of alumina in molten cryolite, the tank being formed by a rigid metal heat-insulated casing of elongate parallelepipedic shape, the ends of which are referred to as heads, and a superstructure formed by at least one rigid beam disposed along the long length of the casing, supporting in particular the anodic frame structure and the current input members coming from the preceding tank in the series and the anodes and resting at its two ends on supports disposed at the two heads of the tank, said superstructure being characterised in that each rigid beam (11) is supported on at least one intermediate gantry (17) provided with legs.

2. A tank according to claim 1 characterised in that the gantry (17) comprises four legs (18) so as to provide a structure which is stable in itself.

3. A tank according to claim 1 characterised in that the gantry (17) is formed by two half-elements (20A, 20B) each provided with two legs (23A and 23B).

4. A tank according to claim 1 characterised in that the gantry is formed by a single horizontal support beam (27) which is supported on two legs (28).

5. A tank according to claim 1 characterised in that each rigid beam (11) is divided into at least two portions (11A, 118), the central ends (24A, 24B) of which are supported on the gantry (17) or (20A, 20B).

6. A tank according to claim 3 and claim 5 characterised in that the central ends (24A, 24B) of the rigid hearts (11A, 11 B) are fixed for example by welding to the half-elements (20A, 20B) which support them.

7. A tank according to any one of claim 1 to 4 characterised in that the legs (18) or (23A, 23B) or (28) of the gantry (17) are supported freely on the rim (15) of the casing.

8. A tank according to any one of claims 1 to 4 characterised in that the legs (18) or (23A, 23B) or (28) of the gantry (17) are supported on the rim (15) of the casing by a means permitting relative displacement such as roller hearing assemblies.

9. A tank according to any one of claims 1 to 4 characterised in that the legs (18) or (23A, 23B) or (28) of the gantry (17) are supported on the rim (15) of the casing by a means petting relative displacement, the legs at the upstream or downstream side being pivotally mounted on the casing edge, the pivot mounting being disposed along an axis which is parallel to the major axis of the tank, and that the opposite legs on the downstream or upstream side are supported directly on the casing.

10. A tank according to any one of claims 1 to 4 characterised in that the vertical transverse plane P which is perpendicular to the major axis AA' of the tank and which passes through the axis Y of the input member (29) and the vertical plane P' which passes through the axis Y of the intermediate gantry (17, 25) are substantially coincident.

11. A tank according to any one of claims 1 to 4 characterised in that the horizontal support beam (26, 27) of the intermediate gantry comprises at least one means for affording protection from thermal radiation from the electrolysis bath.

12. A tank according to claim 11 characterised in that the means for affording protection from radiation from the bath is foreed by a thermal screen disposed beneath the horizontal support beam.

13. A tank according to claim 11 characterised in that the means for affording protection from radiation from the bath is formed by a means for rapid remowal of the heat absorbed by the horizontal support beam, said means being selected from circulation of air in and/or round said beam, the use of caloducs, and thermal bridges of a metal which is a good conductor (aluminium or copper) between the lower part and the upper part of said beam.

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