GB2585217A

GB2585217A - A maintenance device for a superstructure of an electrolytic cell, and maintenance method using said maintenance device

Info

Publication number: GB2585217A
Application number: GB1909549.6A
Authority: GB
Inventors: Ecuacion Leyson; Kunnathurampalayam Palanisamy Balakrishan; Mohanty Anurag; Bin Rahal Ali
Original assignee: Dubai Aluminium PJSC
Current assignee: Dubai Aluminium PJSC
Priority date: 2019-07-03
Filing date: 2019-07-03
Publication date: 2021-01-06
Also published as: GB201909549D0

Abstract

A maintenance device (1) for a superstructure (100) of an electrolytic cell suitable for the Hall-Heroult electrolysis (aluminium refining) process, said superstructure comprising a fixed frame (102) provided with a top (104). The device comprises a body (10), a working platform (40), displacement means adapted to move the working platform with respect to said body, as well as a protection system (60), provided with peripheral walls so as to avoid falling of an operator. The dimensions of the protection system are adjustable along one direction, the protection system being adapted to allow a user to directly stand on said top of the fixed frame. The device (1) may also comprise an intermediate platform (30): the working platform (40) can be adapted so that it may rotate with respect to the platform (30) around a vertical axis (note figure 4). Slewing means (90) may also be provided so that the working platform (40) may move relative to the body (10)

Description

A MAINTENANCE DEVICE FOR A SUPERSTRUCTURE OF AN

ELECTROLYTIC CELL, AND MAINTENANCE METHOD USING SAID

MAINTENANCE DEVICE

Technical field of the invention

The invention relates to the field of fused salt electrolysis, and more precisely to electrolytic cells for use in the Hall-Heroult process for making aluminium by fused salt electrolysis. The invention relates to a maintenance device for the superstructure which is part of such a cell. Said device allows easy and safe access to the superstructure. The invention also deals with a maintenance method of such a superstructure using said maintenance device.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (A1203) is dissolved in molten cryolite (Na3A1F6), and the resulting mixture (typically at a temperature comprised between 940 °C and 970 °C) acts as a liquid electrolyte in an electrolytic cell. An electrolytic cell (also called "pot") used for the Hall-Heroult process typically comprises a steel shell (so-called potshell), a lining (comprising refractory bricks protecting said steel potshell against heat, and cathode blocks usually made from graphite, anthracite or a mixture of both), a superstructure and a plurality of anodes (usually made from carbon) that plunge into the liquid electrolyte contained in the volume defined by the cathode bottom and a side lining made from carbonaceous material. Anodes and cathodes are connected to external busbars. An electrical current is passed through the cell (typically at a voltage between 3.5 V and 5 V) which electrochemically reduces the aluminium oxide, split by the electrolyte into aluminium and oxygen ions, into aluminium at the cathode and oxygen at the anode; said oxygen reacting with the carbon of the anode to form carbon dioxide. The resulting metallic aluminium is not miscible with the liquid electrolyte, has a higher density than the liquid electrolyte and will thus accumulate as a liquid metal pad on the cathode surface from where it needs to be removed from time to time, usually by suction into a crucible (so-called "tapping" procedure).

The electrical energy is a major operational cost in the Hall-Heroult process. Capital cost 35 is an important issue, too. Ever since the invention of the process at the end of the 191h century much effort has been undertaken to improve the energy efficiency (expressed in kW/h per kg or ton of aluminium), and there has also been a trend to increase the size of the pots and the current intensity at which they are operated in order to increase the plant productivity and bring down the capital cost per unit of aluminium produced in the plant.

Industrial electrolytic cells used for the Hall-Heroult process are generally rectangular in shape and connected electrically in series, the ends of the series being connected to the positive and negative poles of an electrical rectification and control substation. The general outline of these cells is known to a person skilled in the art and will not be repeated here in detail. They have a length usually comprised between 8 and 25 meters and a width usually comprised between 3 and 5 meters. The superstructure of the cell comprises a fixed frame and a mobile metallic anode busbar, also called "anode beam", which extends at the outer periphery of the fixed frame. The cells (also called "pots") are always operated in series of several tens (up to more than a hundred) pots (such a series being also called a "potline"); within each series DC currents flow from one cell to the neighbouring cell. The superstructure also comprises ducts for exhaust gas collection; those ducts lead to a gas treatment centre where exhaust gases are cleaned.

For protection the cells are arranged in a building, with the cells arranged in rows either side-by-side, that is to say that the long side of each cell is perpendicular to the axis of the series, or end-to-end, that is to say that the long side of each cell is parallel to the axis of the series. It is customary to designate the sides for side-by-side cells (or ends for end-to end cells) of the cells by the terms "upstream" and "downstream" with reference to the current orientation in the series. The current enters the upstream and exits downstream of the cell. The electrical currents in most modern electrolytic cells using the Hall-Heroult process exceed 200 kA and can reach 400 kA, 450 kA or even more; in these potlines the pots are arranged side by side. Most newly installed pots operate at a current comprised between about 350 kA and 600 kA, and more often in the order of 400 kA to 500 kA.

Anodes in the Hall-Heroult process are usually prebaked cuboids made from a carbonaceous material. The anodes axed fixedly connected to so-called anode hangers.

The latter serve two different purposes, namely to keep the carbon anodes at a predetermined distance from the cathode, and to carry the electrical current form an anode busbar (also called "anode beam") down to the carbon anodes. Anode hangers are fixed to the overhanging anode beam in a detachable manner using a system comprising hooks and clamps. Anode hangers comprise an upper part called "anode rod", which is connected to the anode beam, and a lower part, called "anode yoke". The anode yoke has a number of legs each of which terminates in a cylindrical stub that is embedded in preformed stubholes of the anodes and fixed with cast iron acting as temperature-resistant, electrically conductive glue; this process is called "rodding". The assembly formed by an anode fixed to its anode hanger is called an "anode assembly". The average lifetime of an anode is typically twenty to thirty days; the spent anode assembly is changed against a new one. The spent anode assembly is transported to the rodding shop, where the anode stub is dismounted, the anode yoke and rod are cleaned, and then a new anode is fixed.

Besides the periodic change of anode assemblies to replace spent anodes, it is necessary to carry out periodic, but less frequent maintenance operations on the above mentioned superstructure of the cell. This may require physical access by an operator to the superstructure. Such operations are, in particular, the inspection, maintenance and replacement of crust breakers, AIF3 feeders, alumina and AIF3 hopper level inspections, crust breaker and feeder solenoid replacements, electric jack motors, aeration conveying lines and aeration gaskets. Also, there may be certain emergency or otherwise unusual situations during which easy access by an operator to superstructure is desirable. To this end, mobile, wooden ladders or light wooden platforms are used, or movable elevating access platforms.

However, this kind of prior art implies a number of shortcomings. While removable ladders can be used virtually everywhere, they do not ensure a satisfactory safety for the operator, even if used properly. Moreover, while it is desirable that the operator can walk freely on the superstructure to access various breaker and feeder positions, he should be prevented from falling down. Furthermore, the work in a potroom implies electric hazards, due to the presence of unprotected metallic parts that may be at different electrical potentials. The superstructure of a pot is at the pot potential, but the floor is at floating potential. The difference of the potential between two adjacent pots is approximatively 4 V under normal conditions of operation, but can increase up to about 100 V in case of abnormal operating conditions (so-called anode effects). Certain parts of the wall of the potroom may be at ground potential, and the operator must not touch them while standing on the superstructure or while standing on the operating floor of the pot rooms.

Prior art devices, such as movable elevating platforms, do not adapt to different superstructures, in a convenient manner. They may be specifically adapted to one respective size of superstructure, but do not easily suit other superstructures, the size thereof is different. However, as mentioned above, there may be cells of different size and type in one potroom, having different superstructures. On the other hand, platforms supported by a telescopic and/or articulating boom, while allowing a certain degree of flexibility for the user, are undesirable in potrooms, as they may accidentally lead to unwanted electrical contact between components at a different potential.

Deployable mobile elevating access platforms are widely used for construction and maintenance, and many types of them are commercially available or have been described in the patent literature (see for instance US 2014/0041963 assigned to CV International, and DE 26 04 141 assigned to Mero Raumstruktur GmbH), but none of them is specifically designed for use in potrooms and meets all the specific requirements of safety and flexibility of use.

In view of the above, there is a need to provide a maintenance device, which permits to carry out reliable maintenance devices on a superstructure of an electrolytic cell.

There is also a need to provide such a device, which allows operators to work in a total safety, during maintenance operations.

There is also a need to provide such a device that can easily be carried to the pot for the maintenance of which it is needed.

There is also a need to provide such a device, which provides adequate electrical insulation to the maintenance personnel using said device, and that operates safely under high magnetic field during maintenance operations, without being pulled towards the pot 20 due to presence of strong magnetic field in the pot rooms.

There is also a need to provide such a device, which permits an easy walking of the operators on the top of said superstructure.

There is also a need to provide such a device, which can easily be adapted to different kinds of superstructures, in particular to superstructures of different length and width.

Object of the invention According to the invention, the problem is solved by a maintenance device which is provided with a protection system, the dimensions of which are adjustable. This protection system ensures the safety of operators, since it prevents them from falling out of superstructure top. Moreover adjustable dimensions of said protection system permit the maintenance device of the invention to cater different superstructures, in particular to suit different sizes of superstructures.

More precisely the invention first avoids the presence of potential so-called dead zones of 35 superstructure. in other words, thanks to adjustable protection system, operators may access any location of said superstructure, in order to carry out maintenance operations.

the invention also avoids the presence of potential so-called danger zones of superstructure. in other words, whatever the location of operators, the latter may adjust protection system so as to work in total safety.

In addition the maintenance device according to the invention allows a very convenient working for operators. In particular it makes it possible to directly stand and walk on superstructure top. In this respect, the protection system according to the invention is not provided with a floor.

Finally the maintenance device according to the invention is not cumbersome. in particular it may adopt a so-called folded configuration, wherein protection system as minimal dimensions. said maintenance device may therefore be easily displaced from one cell to another.

Figures Figures 1 to 24 represent several embodiments of the present invention; they do not limit the scope of the invention.

Figure 1 is a schematic view, showing the global arrangement of a series of cells in an electrolysis plant which is equipped with at least one maintenance device according to the 20 invention.

Figures 2 to 4 are perspective views, showing a maintenance device according to the invention in three different functional configurations.

Figures 5 and 6 are perspective views, showing at a greater scale a telescopic railing part of said maintenance device, in two different functional configurations.

Figures 7 and 8 are schematic views, showing an operator respectively reaching superstructure top and advancing along said superstructure top, while unfolding said telescopic railing.

Figure 9 and 10 are perspective views, showing the maintenance device according to the invention, as well as a cell superstructure, in two different working positions of said 30 telescopic railing with respect to the top of said superstructure.

Figure 11 is a schematic view analogous to figures 7 and 8, showing an operator walking back along said superstructure top, while folding back said telescopic railing.

Figure 12 is a perspective view, showing means for varying the width of telescopic railing part.

Figure 13 is a perspective view, showing varying means of figure 12 as well as an actuation brake adapted to cooperate with said means.

Figure 14 is a perspective view, showing at a greater scale locking pins adapted to cooperate with varying means of figures 12 and 13.Figure 15 is an end view, showing a ladder which belongs to maintenance device according to the invention, as well as means for locking said ladder.

Figure 16 is a perspective view, showing under another angle said locking means and said ladder.

Figures 17 and 18 are perspective views, showing at greater scale said locking means in their respective functional positions.

Figure 19 is a front view, showing maintenance device according to the invention as well 10 as slewing means which are pail of said device.

Figure 20 is a perspective view showing the constitutive parts of said slewing means. Figure 21 is a perspective view showing at greater scale said slewing means.

Figure 22 is a perspective view showing at greater scale a stationary bearing which is part of said slewing means.

Figure 23 is a perspective view showing at greater scale a braking mechanism which is part of said slewing means.

Figure 24 is a perspective view showing at still greater scale a latching pin which is part of said braking mechanism.

Detailed description

The following reference numbers are used in the figures: 1 Maintenance device Body 12 Wheel 14 Tool box 16 Towing bar 18 Access ladder 19 Jack Frame 22 Scissor lifting system Intermediate platform Main platform 42 Access edge of 40 44 Rear edge of 40 46,48 Lateral edges of 40 Telescopic ladder 52,54 Fixed railings Mobile railing 62 Safety space 70,72 Rigid railing 73 Slider Telescopic railing 82 Telescopic front member 84,86 Telescopic side member 88 Access space Superstructure 102 Fixed frame 104 Flat top 105 Facing edge of 104 106 Anode frame 108 Anode rod Hook 120 Running path 1026 End region of top 104 200 Operator C1-Cn * Cl-On; Ci; Ci; Cj; C'j Electrol tic cells L1,L2 Lines of electrolytic cells XX Main axis of cell line YY Transverse axis of cell line

Detailed description

The present invention is directed to a maintenance device, such as a mobile elevating work platform, intended to be used in a plant, also called aluminium smelting plant or aluminium smelter, using the Hall-Heroult process. This plant comprises a plurality of electrolysis cells (potline) connected in series. The Hall-Heroult process as such, the way to operate the latter, as well as the general structure of above electrolysis cells are known to a person skilled in the art and will not be described here. In the present description, the terms "upper" and "lower" refer to mechanical elements in use, with respect to a horizontal working surface. Moreover, unless otherwise specifically mentioned, "conductive" means "electrically conductive".

As schematically shown on figure 1, a typical aluminium smelter comprises a plurality of electrolytic cells, typically arranged along two parallel lines L1 and L2, each of which comprises n cells, i.e. C1 to Cm and Cm+1 to Cn. The electrolysis current therefore passes in a cascade fashion from one cell to the next cell, along arrow DC. The number of cells in a series is typically comprised between 50 and over 400 in the most recent smelters. The electrolytic cells are rectangular shaped and are arranged transversally (side by side), in reference of the line they constitute. In other words the main dimension, or length, of each cell is substantially orthogonal to the main direction of the line, i.e. the circulation direction of current. The large sides of two adjacent cells are parallel. The electrolytic cells, or pots, can implement various technological variants; in general the current is fed into the anode beam, flows from the anode beam to the plurality of anode rods and to the anodes in contact with the liquid electrolyte where the electrolytic reaction takes place. Then the current crosses the liquid metal pad resulting from the process and eventually will be collected on the cathode block surface.

Each electrolytic cell substantially comprises a potshell, a superstructure and a plurality of anodes, as shown on figures 9 and 10. This superstructure is referenced in whole as 100. It first comprises a fixed frame 102, which is provided with a flat top 104. It also comprises a mobile metallic anode frame 106, hereafter called "anode beam", which extends to the outer periphery of the fixed frame. Each anode A is provided with a metallic rod 108 for mechanical attachment and electrical connection to the anode beam. In a way known as such, beam 106 is provided with fixation means 110 such as hooks, adapted to facilitate the attachment of the anode rods to the anode beam. The general structure of said superstructure is known per se and will not be explained in further detail.

Often all the cells of a potline have the same structure, but there may be differences, and different potlines may use different types of cells. Two not shown sheds are also typically provided, each covering a respective line. Each cell is arranged so as to leave running paths 120 (see figure 1) extending between said cell and facing wall of said shed. These paths allow the displacement of a maintenance device according to the invention along main axis of each line, as will be explained here after. On figures 9 and 10, the main axis of superstructure 100 is noted A100, whereas main axis of path 120 is noted A120 on figure 1. Typically, said main axes 100, 120 are mutually perpendicular.

Figures 2 to 4 more particularly show a maintenance device according to the invention, which is referenced 1 as a whole. This device first comprises a body 10, which forms a chassis, mounted on transport wheels 12. Let us note Al the main axis of said device 1, which corresponds to the transport direction when wheels are straight. Said wheels 12 advantageously comprise an electrically insulating tread, such as a rubber thread, such as to ensure electrical insulation of the maintenance device 1 with respect to the ground and floating potential of the pot rooms.

Body 10 is provided with at least one tool box 14, preferably two of such boxes on either lateral side. Moreover said body is equipped with a towing bar 16, which enables the displacement of device 1 with any not shown auxiliary assembly, such as a motor vehicle. As an alternative, said device 1 may be self-propelled and comprise propelling means, of any appropriate type. Said body 10 is also provided with an access ladder 18, which cooperates with a telescopic ladder, described here after. Finally at least one jack 19 makes it possible to immobilize said body with respect to the ground. Said jack comprises means for electrically insulating the device 1 with respect to the ground, such as a rubber pad.

Maintenance device further comprises a frame 20 (see in particular figure 4), which has a rectangular shape. Said frame is movable with respect to body 10, along a vertical direction.in the illustrated example, said vertical movement is ensured by a scissor lifting system 22. However, any other displacement means may be provided, such as a vertical boom. An intermediate platform 30 is slidably mounted on said frame 20, along main axis Al. Motion of said platform 30 may be ensured, either manually, or via not shown motor means of any appropriate type.

A so-called main or working platform 40 is pivotably mounted on intermediate platform 30, 35 around a vertical axis. In the same way as for platform 30, said working platform may be moved manually or thanks to motor means. Working platform 40, which has a substantially rectangular shape, comprises a so called access edge 42, which allows an operator to reach the top of the superstructure, as will be described below. Let us note 44 is a so-called rear edge, opposite said access edge as well as 46 and 48 the two lateral edges of platform 40. Finally, let us also note A40 the main axis of said platform 40.

Figures 19 to 24 illustrate a specific embodiment of mechanical means, which make it possible to move platform 40 with respect to frame 20. As shown in particular on figure 19, reference 23 designates slewing means which cooperate with both said frame and said platform. Frame 20, which forms an upper platform of scissor lifting system 22, is provided with a guide channel 21. Moreover, as shown on figure 21, sliding intermediate platform 30 is equipped with rollers 31 that are adapted to roll along said channel. In an advantageous manner, said rollers are made of an insulating material, such as rubber.

In addition, platform 30 is provided with a central shaft 32, which is cylindrically shaped. A first so-called stationary bearing 34 is pushed around said shaft. Said bearing 34 is fixed on upper surface of platform 30 by any appropriate means, in particular by screwing. As shown on figure 22, said bearing 34 is provided with a top flange 36, which is drilled with a plurality of holes 37 extending over the periphery of said flange. Said bearing is also provided with an annular plate 35, advantageously made of an insulation material, in particular analogous to that of rollers 31.

Lower surface of main platform 40 is equipped with a second so-called rotary bearing 41, which is fixed on said platform by any appropriate means, in particular screwing. As shown on figure 20, said bearing 41 is provided with a central opening, at the periphery of which extend a plurality of insulated washers 43 and bushes 45. The latter are for example made of the same materials, as above mentioned rollers and plate.

Finally said slewing means comprise a so-called braking system 90, which is mounted on upper surface of bearing 41. As shown on figure 24, said braking system is in particular equipped with a latch pin 95. In operation said pin is adapted to selectively engage with one of the above mentioned latch holes 37. A telescopic ladder 50 is mounted on rear edge 44, by any appropriate fixing means. The top of this ladder is located above the surface of platform 40. Two fixed railings (also called guard-rails) 52 and 54 are also provided, on both sides of said ladder 50. Each railing 52, 54, which has a L shape, extends over part of rear edge and a respective lateral edge. Said platform is also provided with a mobile railing 60, which forms an essential feature of the invention. Said mobile railing constitutes a protection system, which avoids operators from falling out of the top of superstructure, as will be explained in further details.

Mobile railing 60 comprises first two rigid railings 70 and 72, which are parallel to main 5 axis A40. Each rigid railing slightly extends beyond access edge of platform 40. Said railings 70 and 72 are mounted on oblique sliders 73, which are provided at the upper surface of platform 40. As will be described in more details, this makes it possible to mutually move said railings 70 and 72. Said mobile railing 60 further comprises a telescopic railing 80, which is formed by a telescopic front member 82 and two opposite 10 telescopic side members 84, 86. Each side member 84, 86 is mechanically connected with a respective rigid railing 70, 72.

Said railing 60 defines a so-called safety space 62, the dimensions of which are adjustable. Said space 62 is surrounded by peripheral walls of said protection system 60, which comprise lateral walls and front wall. Lateral walls of said safety space are respectively formed, on the one hand by members 70 and 84 and, on the other hand, by members 72 and 86. Moreover front wall of said safety space 62 is formed by member 82. Beyond access edge 42 of platform 40, safety space 62 is not provided with a bottom, which permits a direct walking on the top of the superstructure. In the folded position of telescopic railing 80, access edge 42 and front member 82 are mutually remote, so as to define a so-called access space 88, the axial dimension L88 of which is typically between zero and 25 meters (see figure 3).

The structure of telescopic railing 80 is known as such. Those skilled in the art will choose the mechanical parameters of said railing, so that it may advantageously fulfil at least the two following conditions. First said railing may advantageously be subjected to an easy folding and unfolding by an operator, without any excessive human strength. Moreover, once said railing is positioned in a given position, it shall advantageously stand with a satisfactory stability in said position. To this end, any appropriate maintaining means may equip said telescopic railing 80.

Operation of maintenance device 1 will now be described. In a first configuration, so-called parked configuration shown on figure 2, jacks 19 are in their active position so as to immobilise device 1 with respect to the ground. Let us suppose now that a maintenance work has to be carried out on a given cell Ci of the plant. In this respect, jacks 19 are first placed in their inactive position so that device 1 may be displaced along path 120 as shown by arrow Fl on figure 1. In the example of figure 1, the device 1 is towed from a first position (which may be a storage position) to its use position next to pot Ci.

Once the maintenance device 1 is positioned in front of said cell Ci, jacks 19 are placed back in their active position in order to immobilise said device. Lifting system 22 provokes then the raising of frame 20, intermediate platform 30 and working platform 40. The latter is then rotated with respect to platform 30, so that axis A40 is perpendicular to axis Al and that access edge 42 faces superstructure top 104. A rotating working platform 40 is advantageous, since it may ensure access to superstructure 100 either from its left or right side. Adjustment of the positioning of said working platform 40 may be carried out, by sliding intermediate platform 30 with respect to frame 20. By rotating the working platform 40 with respect to the intermediate platform 30, the maintenance device 1 changes from a so-called traveling mode (in which it can be towed or pushed, provided that the jacks 19 are not in their active position) to a so-called maintenance mode, in which the maintenance device 1 can be used for accessing the superstructure top.

As illustrated on figure 7, access edge 42 of platform 40 and facing edge 105 of top 104 of superstructure are now in close vicinity. An operator 200 is then in a position to reach platform 40, thanks to telescopic ladder 50. As shown on this figure 7, said operator may then easily access superstructure top, from said working platform. In this respect, above mentioned access space 88 is advantageous, since said operator may access said top, without handling the telescopic railing 80. This possibility is both safe and convenient. In particular, said operator may lift heavy tools and place them on the superstructure top.

Thereafter, said operator walks along main axis of superstructure top, until he reaches a so-called target zone of said top, on which a maintenance operation has to be carried out. While walking on said top, operator pushes front member 82 so as to unfold telescopic railing 80 and so as to increase the length of protection system 60 (see arrow F82 on figure 8). If any, operator may also displace rigid railings along sliders 73 (see arrows F73 on figure 6), so as to adjust the width of said protection system. On figures 5 and 6, respective minimal 160 and maximal 1'60 widths are illustrated. By way of example, said widths may respectively be about 60 and 120 cm, which makes it possible to cater different sizes of superstructures.

Figure 9 shows a target zone of superstructure, which is close to working platform. In these conditions, front member 82 is fixed in the mid-region of said top. On the other hand, figure 10 shows a target zone of superstructure which is opposite said working platform. Front member 82 is then positioned close to end region 126 of said top. Let us note L60 the maximal length of protection system, taken from access edge 42. Said maximal length is typically between 10 meters and 20 meters, which makes it possible to suit both short and long superstructures. Fixing of said front member 82 may be ensured by above-mentioned maintaining means. Once maintenance operation has been fulfilled, operator 200 walks back along superstructure top, from its end region 126, so as to reach back working platform. While walking, he pulls front member 82 so as to fold back telescopic railing 80 and, as a consequence, the whole protection system (see arrow F'82 on figure 11).

The maintenance device according to the invention can also be used in its travelling mode (provided that the jacks 19 are in their active position), and in this case the access ladder 18 is used for climbing up to the working platform 40, as can be seen on figure 3.

The maintenance device can be made using different lifting mechanisms. In an advantageous embodiment it is of the scissor-type, known as such. In another embodiment it is of the boom-type, with a telescopic or an articulating mechanism, or a combination of both. Scissor-type devices are preferred because they offer a better stability and less risk when the platform is moved, knowing that there is a general hazard in potrooms related to accidental simultaneous electrical contact with two conductors at different electrical potentials; as a consequence, telescopic boom-type platforms are not preferred in close proximity to electrolytic cells.

The maintenance device according to the invention is advantageously of the non-self-propelled type (also called "push-around platform"). If it is of the self-propelled type it should be pedestrian controlled.

The raising mechanism comprises usually hydraulic cylinders. It can be powered by gasoline or propane engines, or by electric motors, either AC or DC, and the same type of engines can be used for the propelling mechanism, if present.

Figures 12 to 14 illustrate another embodiment of the invention. On these figures mechanical elements, which are analogous to those of previous figures, are given the same reference numbers added by 200.

Embodiment of figures 12 to 14 differs from first embodiment, in particular in that it is not equipped with a slider, such as that 73. On the other end, there are provided several pivot arms 273, typically four of such arms regularly provided close to the angles of mobile railing 260. Each pivot arm 273 has a first end 274 which is pivotably mounted on platform 240, whereas other end 275 is pivotably mounted on railing 260. Mechanical link between a respective end and platform or railing is ensured by any appropriate means, in particular by flanges 290 and 291. Respective rotation axes of said arm ends are noted A274 and A275.

Flange 290 is also provided with a curved wall 292, which is drilled with a plurality of holes 294. Moreover said arm 273 is provided with a pin 276, which is adapted to selectively engage with one of said holes. As shown on figure 14, maintenance device is also provided with an actuator, in the structural form of brakes 296 likely to be used by an operator standing on platform 240. The above mentioned brakes are adapted to cooperate with the first end of a not illustrated cable, whereas the other end of said cable cooperates with a finger 278 provided on said arm.

In operation, a person standing on the working platform 240 is in a position to push or pull mobile railing. This causes each arm 273 to pivot with respect to both axes A274 and A275, so that the width of said railing 260 increases or decreases (see arrow F260). During this motion, locking pin 276 may be engaged by the operator in one of holes 294, so as to secure mobile railing in a given position.

The embodiment of figures 12 to 14 has several specific advantages. First it makes it possible to preserve mechanical strength of upper surface of platform 240, while rendering easier walking of operators. In addition, providing locking pins avoids any unintentional motion of mobile railing, once it has been secured in its chosen position.

Figures 15 to 18 illustrate another embodiment of the invention. On these figures mechanical elements, which are analogous to those of previous figures, are given the same reference numbers added by 300.

Embodiment of figures 15 to 18 differs from first embodiment, in particular in that it further comprises means 351 for locking ladder 350 in its upper position, or so-called home position. Said locking means first comprise a runner 355 which is provided at the upper part of rigid railing 354. In addition a latch plate 356 is pivotably mounted on said rail, around an horizontal axis. Said plate is hinged on an intermediate crank 357, which is in turn hinged to an actuation rod 358. The latter, which extends vertically, is driven by guides 359. Moreover upper end of said ladder is provided with a slider 353, which is adapted to slide along runner 355.

The embodiment of said figures 15 to 18 has specific advantages. Indeed it enables operator to lock and unlock said ladder, either when standing on the floor or when standing on top of platform 340. As shown on figure 17, plate 356 may first be directly pulled by an operator 200 standing on top of platform, according to arrow F356. As further shown on figure 16, an operator 201 standing on the floor may pull lower end 358' of rod 358 according to arrow F358, which then causes motion of latch plate 356. In other words, this remote control makes it possible to act on latch plate in the same way as in case of a direct pulling.

In the locking position of figure 17, latch face 356' of plate 356 prevents telescopic ladder from undue down wards motion. Moreover, said ladder ensures not only an access function, but also acts as a safety barrier. In the unlocking position of figure 18, latch plate does not interfere with the slider 351. As a result, telescopic ladder may be unfolded, so as to enable operators standing on the floor to reach working platform.

Claims

CLAIMS1. A maintenance device (1) for a superstructure (100) of an electrolytic cell suitable for the Hall-Heroult electrolysis process, said superstructure comprising a fixed frame (102) provided with a top (104), said device comprising a body (10), a working platform (40), displacement means adapted to move said working platform with respect to said body, as well as a protection system (60), provided with peripheral walls so as to avoid falling of an operator, wherein the dimensions of said protection system are adjustable along at least one direction, said protection system being adapted to allow a user to directly stand on said top of said fixed frame.
2. A maintenance device according to claim 1, wherein said peripheral walls of said protection system defines a safety space (62), the dimensions of which are adjustable along at least one direction.
3. A maintenance device according to any of claims 1 or 2, wherein said protection system comprises a telescopic railing.
4. A maintenance device according to any of claims 1 to 3, wherein said protection system comprises rigid railings, said railings being movable with respect to each other along a transverse direction of said working platform (40).
5. A maintenance device according to any of claims 1 to 4 wherein said protection system has no bottom wall.
6. A maintenance device according to any of claims 1 to 5, wherein the front wall of said telescopic railing defines a so-called access space in folded position of said telescopic railing, said access space (88) having no bottom wall.
7. A maintenance device according to any of claims 1 to 6, wherein said displacement means comprise elevating means, adapted to vary the altitude of said working platform with respect to said body.
8. A maintenance device according to any of claims 1 to 7, wherein said device comprises an intermediate platform, said working platform being adapted to rotate with respect to said intermediate platform, in particular around a vertical axis.
9. A maintenance device according to any of claims 1 to 8, wherein said protection system is provided with maintaining means adapted to maintain said protection system in a given position on the top of the superstructure.
10. A maintenance device according to any of claims 1 to 9, wherein said body comprises rolling means (12).
11. A maintenance device according to claim 02, wherein said device comprises immobilisation means (19) adapted to immobilise said body with respect to the ground.
12. A maintenance device according to any of claims 1 to 11, wherein said body is provided with a towing bar (16) adapted to cooperate with an auxiliary assembly for moving said maintenance device.
13. A maintenance device according to any of claims 1 to 12, comprising adjusting means for adjusting the width of said protection system.
14. A maintenance device according to preceding claim, wherein said protection system is mounted on at least one transverse slider of working platform, so as to adjust the width of said protection system.
15. A maintenance device according to claim 13, wherein said adjusting means comprise at least one pivot arm, which is pivotably mounted with respect both to said protection system and said working platform.
16. A maintenance device according to any preceding claims, further comprising an access ladder which permits access to said working platform, said ladder being mobile between an upper folded position and an unfolded position, said device also comprising means for locking said ladder in said upper position.
17. A maintenance device according to above claim, further comprising remote control means adapted to control said locking means in particular from the ground.
18. A maintenance device according to claim 16 or 17, wherein said locking means comprise a runner mounted on said working platform, as well as a latch plate hinged on said runner, said latch plate being adapted to lock a slider, mounted on said ladder, into said runner.
19. A maintenance device according to claim 17 or 18, wherein said remote control means comprise a rod extending down wards from said locking means.
20. A maintenance device according to any preceding claims, further comprising slewing means adapted to permit motion of said working platform with respect to said body.
21. A maintenance device according to preceding claim, wherein said slewing means comprise a so-called stationary bearing mounted on said body, as well as a so-called rotary bearing mounted on said platform.
22. A maintenance device according to preceding claim, further comprising latch means adapted to latch said rotary bearing with respect to said stationary bearing in one given position.
23. A maintenance device according to preceding claim, wherein said latch means comprise a braking mechanism mounted on said rotary bearing, said braking mechanism comprising at least one latch pin adapted to selectively engage in a respective hole provided in said stationary bearing.
24. Use of a device according to any of claims 1 to 23, for the maintenance of a superstructure (S) of an electrolytic cell, suitable for the Hall-Heroult electrolysis process, said superstructure comprising a fixed frame (4) provided with a top, as well as an anode beam (6) adapted to support anode rods (9) of said cell.
25. A maintenance method of a superstructure (100) of an electrolytic cell, suitable for the Hall-Heroult electrolysis process, said superstructure comprising a fixed frame (102) provided with a top (104), said method being carried out with a maintenance device (1) according to any of claims 1 to 23, said method comprising: - moving said maintenance device(1) so that facing edges of working platform and top of superstructure are in close vicinity, - advancing along said top of superstructure, while increasing at least longitudinal dimension of said protection system, so as to reach a so-called target zone of said top, carrying out a maintenance operation on said target zone.
26. A method according to claim 25, further comprising moving back from said target zone to said working platform, while decreasing at least said longitudinal dimension of said protection system.
27. A method according to any of claims 25 or 26, comprising accessing said top of superstructure, from said working platform, without handling said protection system.