AU2015208857B2

AU2015208857B2 - Hooding system for an electrolytic cell

Info

Publication number: AU2015208857B2
Application number: AU2015208857A
Authority: AU
Inventors: Frederic Brun; Guillaume Girault; Steeve RENAUDIER
Original assignee: Rio Tinto Alcan International Ltd
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 2014-01-27
Filing date: 2015-01-23
Publication date: 2018-08-16
Anticipated expiration: 2035-01-23
Also published as: DK179126B1; EP3099841B1; DK201670545A1; CA2935478C; FR3016890A1; EP3099841A1; WO2015110903A1; CN105934537A; CN105934537B; EP3099841A4; FR3016890B1; RU2682498C2; AU2015208857A1; CA2935478A1; RU2016134372A3; BR112016015587A2; RU2016134372A; BR112016015587B1

Abstract

The invention relates to a system (1) comprising hoods (2), each hood (2) comprising two opposite edges designed to rest on two opposite edges of the electrolytic cell, such that each hood (2) extends from one side of the electrolytic cell to the other, above an opening (116). Furthermore, the system (1) is designed such that it has longitudinal maintenance windows (6) parallel to the hoods (2). The system (1) also comprises closing covers (8), each cover (8) being moveable in relation to the hoods (2), between a closing position wherein each cover (8) closes a window (6), and a maintenance position wherein each closing cover opens up a passage via a window (6). The covers (8) rest at least partially on the hoods (2) and are designed so as to be able to be moved from the closing position to the maintenance position, independently of each other, without moving the hoods (2) on which the closing covers (8) rest.

Description

HOODING SYSTEM FOR AN ELECTROLYTIC CELL

The present invention relates to a covering system for an electrolytic cell, an electrolytic cell comprising the covering system and a method for changing an anode assembly.

Aluminum is conventionally produced in aluminum smelters by electrolysis using the Hall-Heroult process.

An aluminum works traditionally includes hundreds of electrolytic cells connected in series and carrying an electrolysis current which may reach several hundreds of thousands of amperes. It is known that electrolytic cells can be arranged transversely to the flow direction of the electrolysis current across the series.

Electrolytic cells conventionally comprise a steel pot shell within which there is a lining of refractory materials, a cathode of carbon material, through which pass cathode conductors designed to collect the electrolysis current at the cathode to route it to the cathode outputs which pass through the bottom or sides of the pot shell, linking conductors extending substantially horizontally to the next cell from the cathode outputs, an electrolyte bath in which the alumina is dissolved, at least one anode assembly comprising at least one anode immersed in this electrolyte bath and an anode rod sealed in the anode, an anode frame on which the anode assembly is suspended via the anode rod, and risers for the electrolysis current running upwards connected to linking conductors from the preceding electrolytic cell to route the electrolysis current from the cathode outputs to the anode frame and the anode assembly and anode in the next cell. The anodes are more particularly of the pre-baked anode type with pre-baked carbon blocks, i.e. baked before they are placed in the electrolytic cell.

The anode assemblies are consumed in the course of the electrolysis reaction and have to be periodically replaced with new anode assemblies.

The sides of the electrolytic cell define an opening through which the anode assemblies are inserted into the electrolytic cell to be immersed in the electrolytic bath or extracted from the electrolytic cell to be replaced.

To limit heat loss and prevent the diffusion out of the electrolytic cell of gases generated during the electrolysis reaction, hereinafter called cell gases, provision is made to close the opening defined by the electrolytic cell with a covering system.

Known covering systems, such as those disclosed in patent documents US4043892 and W02007067061, comprise lateral, removable hoods, tilted relative to the horizontal. These hoods rest on one side of the electrolytic cell and also against a portion of superstructure, designed to support the anode assemblies, extending in a longitudinal direction of the electrolytic cell, above the opening defined by the sides of the cell, i.e. directly above the anode assemblies and the electrolytic bath.

The hoods thus form a confinement chamber that limits the diffusion of gases when the covering system is completely closed. This also reduces heat losses.

However, during an operation requiring the covering system to be opened, as is the case for replacing a spent anode assembly by a new anode assembly, conventional covering systems offer a limited response to the problem of gases diffusing out of the electrolytic cell and the preservation of the thermal equilibrium of the electrolytic cell.

During an operation such as changing an anode assembly, the hoods are removed to create an opening through the covering system. This opening - a necessary one - allows access to the inside of the cell, in particular to remove a spent anode assembly. However, the opening thus created allows cell gases to diffuse outside the confinement chamber. This opening may also disturb the thermal equilibrium of the cell.

The larger the opening thus created, the greater the amount of cell gases that can escape, and the greater the potential heat losses. The same is true for the time during which the covering system is open during an operation: the longer the covering system remains open, the greater the amount of cell gas that can escape, and the greater the disturbance to the thermal equilibrium of the cell.

Given the presence, above the pot shell, of a superstructure on which the hoods of conventional covering systems are supported, the hoods that have been removed to create the opening required for the operation are often placed next to the electrolytic cell, especially in a space between cells separating two adjacent cells. This can cause a problem of congestion, and this congestion problem can slow down the operation, i.e. increase the length of time the covering system remains open. It can also cause a safety problem, in that an operator may stumble.

In addition, the hoods of known covering systems are designed to have their adjacent edges overlapping each other. This overlap limits cell gas leaks and energy losses at the interface between two adjacent hoods.

However, conventional solutions with overlapping hoods have a disadvantage: the hoods are interlocked with each other, and the removal of one of them requires one or more adjacent hoods to be moved or removed. It is therefore clear that, for maintenance work theoretically requiring a single hood to be removed, several hoods must be moved or removed. The open area through the covering system is then larger than necessary. From patent documents US4043892 and W02007067061 we learn of the removal of hoods in groups of three.

Finally, some maintenance operations may require smaller open areas than for other maintenance operations. For example, to break the crusts generated in the electrolytic bath during the electrolysis reaction, it is sufficient to have an opening large enough to insert a suitable tool to break these crusts at the right place while to extract or set up an anode assembly requires a larger opening suited to the size of the anode assembly to be extracted or set up.

However, the hoods of conventional covering systems are similar, especially in terms of dimensions, so that the only way of selecting an opening area through the covering system is to select the number of hoods to be removed. This does not allow for fine adjustment of the opening area, i.e. the selection of a minimum but sufficient opening area for performing a given maintenance operation.

Furthermore, the presence of the superstructure and risers for the electrolysis current above the opening defined by the sides of the cell makes it difficult to break the crusts formed between the anode assemblies because access under the superstructure and the current risers is particularly cramped. It follows that crust breaking operation, conventionally performed with a chipping hammer mounted on an arm with angular inclination requires more time than if there were no such obstacles, which increases the time during which the covering system is open. In addition, because of this accessibility problem, breaking the crust is sometimes incomplete around the edges of the anode assembly and the anode assembly extracted has solid crust pieces that increase its section and its dimensions, and may damage the adjacent hoods still in place.

Finally, the lower parts of the hoods rest on top of the pot shell on which the anode covering material collapses, so that the supports of the hoods are unstable and their positioning inaccurate. They are also exposed at the bottom part to the flames and hot spots associated with discontinuities in the anode covering, causing them to deteriorate rapidly.

It is the object of the present invention to substantially overcome or ameliorate one or more of the above disadvantages, or at least provide a useful alternative.

In accordance with an aspect of the present invention, there is provided an electrolytic cell comprising a plurality of anode assemblies, sides defining an opening through which anode assemblies are designed to be set up or withdrawn with a respectively ascending or descending vertical translational movement, and a covering system extending above the anode assemblies in order to cover said opening, the covering system comprising a plurality of hoods, wherein: - each hood comprises two opposite bearing edges designed to rest on two opposite sides of the electrolytic cell from the sides of the electrolytic cell defining the opening, so that each hood extends from one side of the electrolytic cell to the other, above the opening, - the covering system is designed to have, substantially parallel to the hoods, longitudinal servicing windows to free a predetermined path through the plurality of hoods, - the covering system further comprises sealing lids, each sealing lid being movable relative to the hoods between a closed position, wherein each sealing lid closes one of the servicing windows, and a servicing position, wherein each sealing lid frees a passage through the covering system via one of the servicing windows, the sealing lids being designed to rest at least partly on the hoods, and - the sealing lids are designed to be moved from the closed position to the servicing position, independently of each other, without moving the hoods on which the sealing lids rest.

Preferably, the covering system offers the possibility of accessing the inside of the electrolytic cell by removing only one of the sealing lids, without moving or removing the hoods.

This makes it possible to form an opening of contained dimension through the covering system while leaving the hoods in place. For an operation such as an anode assembly change, this allows for certain preliminary operations, such as sawing through crusts formed around the anode assembly consumed during the electrolysis reaction, with a minimum open surface area through the covering system.

This limits the discharge of cell gas outside the electrolytic cell and prevents disturbance to the thermal equilibrium of the electrolytic cell.

Opposite sides of the electrolytic cell means the sides located on either side of a median plane especially a longitudinal median plane of the electrolytic cell. In this way, each hood is designed to extend on either side of this median plane to rest simultaneously on these two opposite sides.

The hoods and sealing lids have a vertical assembly travel, which has a significant advantage with a view to automating the installation of the hoods, because there are no complex angular movements to be made, in contrast to the state of the art.

According to a preferred embodiment, the sealing lids have longitudinal edges which are designed to rest on each one of the hoods. Sealing at the junction between the sealing lids and the hoods is provided over the entire length of the hoods, and the sealing lids respectively, by overlapping one edge of the sealing lid over one edge of the hood.

According to a preferred embodiment, the sealing lids have a T-shaped cross-section defining two longitudinal flanges, the hoods have a cross-section in the shape of an inverted T delimiting two longitudinal flanges, each flange of one of the sealing lids resting on one of the flanges of an adjacent hood, so that the covering system has an alternation of interlocking hoods and sealing lids.

This configuration provides a simple solution to enable removal of the sealing lids without interfering with the hoods on which they rest and the other sealing lids, and at the same time to improve the sealing of the covering system. This makes it possible to limit the leakage of cell gas and heat losses.

Advantageously, the flanges of the hoods and sealing lids have an L-shaped section, so that the interlocking of a hood and a sealing lid forms a sealing baffle.

This feature also provides the advantage of an improved seal, containing cell gas leakage and heat loss.

Advantageously, the covering system comprises sealing means interposed between the flanges of each sealing lid and the flanges of the adjacent hoods on which each sealing lid rests.

Sealing is thereby improved.

According to an advantageous embodiment, the hoods and lids extend horizontally and the longitudinal flanges of the hoods comprise channels containing a powder material and having a top opening, the longitudinal flanges of the lids having an L-shaped section, so that an end portion of the L-shaped section of the lid is pressed into the powder material through the top opening in the channel when the hood and the lid are interlocked. Making such a seal by means of a powder material is possible because the hoods and lids extend horizontally so that the powder material remains distributed with a uniform height throughout the length of the channel. The powder material forms a barrier preventing cell gas from escaping.

Advantageously, the powder material contains alumina. More particularly, the powder material may be formed of alumina or crushed electrolytic bath comprising alumina. These materials have the advantage of being available in an aluminum works and are additionally introduced into the electrolytic cells so they are not likely to pollute the electrolytic cell in the event of accidental spillage in the cell. In addition, alumina is a very good adsorbent for the HF and SO2 generated by the electrolytic cell so that any infiltration of cell gas through the powder material will have a lesser environmental impact.

Advantageously, the hoods and I or lids comprise a shutter arranged to close the opening of the channel when the hood and the lid are interlocked. The purpose of this shutter, which can be fixed or mobile, especially swiveling, is to retain the powder material in the channel.

According to an advantageous embodiment, the sealing means comprise elastic seals designed to compensate for a difference in relative deformation between two consecutive hoods of the covering system between which a sealing lid in closed position is designed to extend.

In other words, the space or clearance between hoods and sealing lids is sized so that flattening of the seals separating them, taking into account the bending of the hoods and sealing lids, falls within the elastic range of flattening of the seals. Sealing is thereby improved.

According to an advantageous embodiment, the hoods include a surface provided with at least one reinforcing rib designed to limit bending of the hoods.

This stiffens the hoods. In this way, flattening of seals is relatively uniform. Sealing is thereby improved.

According to a preferred embodiment, the hoods include a surface provided with thermal insulation means.

This helps reduce heat loss through the hood system.

Preferably, the insulating means are arranged on the underside of the hoods in order to limit distortion and thereby degradation of the hoods.

Advantageously, the hoods include a substantially longitudinal tubular body, the tubular body defining a cavity within which is arranged a thermally insulating material.

These features help protect the thermally insulating material and limit heat losses, by synergistic effect between the thermally insulating material, which slows the spreading of heat through the covering system and the improved rigidity of the hoods due to the tubular nature of the body, this rigidity allowing the hood to bear uniformly against the surface on which it rests and the sealing lids to bear uniformly on this hood.

According to a preferred embodiment, the hoods comprise a lower surface provided with deflection means for deflecting a flow of cell gas.

Cell gases can be diverted toward a capture system that the electrolytic cell can be equipped with, so that cell gas leaks are limited.

According to a preferred embodiment, the sealing lids comprise gripping means designed to allow substantially vertical lifting of each sealing lid without moving the hoods and independently of the other sealing lids.

Substantially vertical removal of the sealing lids limits the risk of moving adjacent hoods during withdrawal and is the easiest solution to implement a sealing system between the sealing lids and the hoods adjacent to them.

According to an advantageous embodiment, the sealing lids comprise a lower bearing surface designed to allow the sealing lids to rest stably on one of the hoods or on another sealing lid.

The sealing lids, when removed, may therefore be stacked on an adjacent hood or on another nearby sealing lid. Consequently, the trajectories described by the pot tending machine during an operation are minimal, so that the opening time of the servicing window is also minimal. This results in a reduction in cell gas leaks and heat losses that are likely to occur during an operation.

According to a preferred embodiment, the hoods comprise a bottom bearing surface designed to allow the hoods to rest stably on one of the sealing lids.

The hoods, when removed, may therefore be stacked on an adjacent hood or on another nearby sealing lid. This reduces the trajectories described by the pot tending machine, and therefore the opening time of the servicing window. Cell gas leakage and heat losses during the operation, in particular when changing an anode assembly, are lower.

According to a preferred embodiment, the hoods and the sealing lids extend in a substantially horizontal plane.

It is therefore easier to stack them quickly during an operation, which reduces the duration of the operation, and thereby the opening time of the covering system.

Advantageously, the servicing window has a width less than that of the hoods that the servicing window separates.

This small opening surface area of the cover means makes it possible to create, in combination with the conventional suction of cell gases, a suction effect of external air towards the inside of the cell, against the movement of the cell gases. Cell gas leaks are thereby limited.

Also, each sealing lid has a width less than the width of the hoods.

Preferably, the hoods have a bending stiffness greater than that of the sealing lids.

In other words, the hoods are more difficult to bend than the sealing lids, and the sealing lids are easier to bend than the hoods due to their weight, so that the sealing lids can bend to compensate for the lesser bending of the hoods on which they rest. This improves sealing

In accordance with another aspect of the present invention, there is provided an electrolytic cell comprising a plurality of anode assemblies, sides defining an opening through which anode assemblies are designed to be set up or withdrawn with a respectively ascending or descending vertical translational movement, and a covering system having the above characteristics, the covering system extending above the anode assemblies in order to cover said opening.

Preferably, this electrolytic cell has a stable thermal equilibrium and limits cell gas emissions, including during servicing work such as anode assembly replacements.

According to a preferred embodiment, the electrolytic cell comprises sealing means interposed between the bearing edges of the hoods and the sides of the electrolytic cell on which the bearing edges rest.

Sealing is thereby improved. Cell gas leaks are prevented and heat loss limited.

Advantageously, the sealing means, interposed between the bearing edges of the hoods and the sides of the cell upon which the bearing edges rest, comprise a seal, and the electrolytic cell comprises means for pinching the seal.

This makes it possible to correct possible defects in flatness of the hoods and optionally the sealing lids, in order to limit cell gas leakage and heat loss.

According to a preferred embodiment, each sealing lid extends above and along a subjacent space between anodes separating two adjacent anode assemblies of the electrolytic cell.

It is therefore possible to provide access in line with the space between anodes, so that an operation such as crust sawing can be achieved with a minimum open area. This operation, prior to changing an anode assembly is therefore performed with a minimum of cell gas leakage and heat loss.

Space between anodes means space between two adjacent anode assemblies.

According to a preferred embodiment, each hood extends above and along a subjacent anode assembly of the electrolytic cell.

It is therefore necessary to remove the covers only when an anode assembly has to be removed. The rest of the time, the hoods can remain in place to prevent cell gas leaks and to limit heat loss. This configuration also greatly minimizes the risk of operating personnel falling into the cell.

According to a preferred embodiment, the electrolytic cell comprises suitable indexing means to indicate a predetermined position of the hoods such that the hoods extend in line with the anode assemblies.

This feature allows for rapid, repeatable and accurate fitting of the hoods, to quickly close the opening and prevent them from moving.

Hoods extending in line with the anode assemblies means that under each head there extends a single anode assembly.

According to a preferred embodiment, the electrolytic cell comprises means for capturing cell gases, designed to capture and collect the cell gases emitted during the electrolysis reaction.

This therefore limits the amount of cell gas that may leak through an opening in the covering system.

Advantageously, the capture means comprise suction holes arranged in the hoods and the sealing lids.

This limits the air temperature close to the hoods and sealing lids, so as not to reduce the mechanical strength of the materials from which the hoods and the sealing lids are made. This limits bending of the hoods and sealing lids, such bending being able to cause cell gas leaks and heat loss.

Also, in conjunction with the deflection means arranged on the underside of the hoods for deflecting a flow of gas, this improves the efficiency of the cell capture system.

Advantageously, the capture means comprises a diaphragm for changing a section of the air passage with a view to changing the capture flow rate of the cell gases.

This feature offers the possibility of increasing the capture rate when a hood and / or a sealing lid is removed, for example when changing an anode assembly. The diffusion of cell gas during an operation is thereby significantly limited.

According to a preferred embodiment, the width of each hood is less than the width of an anode assembly of the electrolytic cell.

In accordance with another aspect of the present invention, there is provided a method for changing a spent anode assembly of an electrolytic cell for a new anode assembly, the method comprising: - a step involving moving a first sealing lid from among the sealing lids of a covering system having the aforementioned features, from the closed position to the servicing position, without moving the hoods of the covering system and the other sealing lids, in order to free a passage through the covering system via one of the servicing windows, and - a step involving breaking or sawing a crust formed on the surface of an electrolytic bath, by inserting a suitable tool for breaking or sawing the crust around the path freed in the previous step.

Preferably, this method offers the possibility of providing access to the interior of the electrolytic cell with a minimum opening surface, and therefore breaking or sawing the crust formed during the electrolysis reaction, with a minimum of gas leakage and heat loss. When changing the anode assembly, the diffusion of cell gas and heat losses are substantially limited.

According to a preferred embodiment, the method comprises a step involving placing the first sealing lid on one of the hoods adjacent to the first sealing lid.

This minimizes the journeys of the pot tending machine when it handles the first sealing lid. The duration of the operation is limited, so that the period during which the servicing window is open is also limited.

According to a preferred embodiment, the method comprises a step involving moving a second sealing lid, from the closed position to the servicing position, without moving the hoods from the covering system and the other sealing lids, the second sealing lid being arranged on the other side of one of the hoods next to which the first sealing lid was arranged, so as to free a second passage on the other side of this hood, and a step involving breaking or sawing a crust formed on the surface of an electrolytic bath, by inserting a suitable tool to break or saw the crust through this second passage.

In anticipation of changing an anode assembly, this allows the crust to be sawed, broken or chipped on both sides of the spent anode assembly to be replaced, i.e. both of sides of the hood initially arranged between the first and second sealing lids, without any open space above the spent anode assembly. Consequently, gas leaks and heat losses during an anode assembly change are limited.

According to a preferred embodiment, the method comprises a step involving placing the second sealing lid on the first sealing lid.

The stack of sealing lids limits the journeys made by the pot tending machine, and therefore the time during which the corresponding servicing windows are open.

According to a preferred embodiment, the method comprises a step involving withdrawing a hood initially adjacent to the first sealing lid.

In this way, it is not until the last moment, i.e. the moment when the spent anode assembly to be replaced is grasped and lifted, that one of the hoods is removed. During all previous steps required to change the anode assembly, this hood was in place. The method according to the invention therefore limits cell gas leakage and protects the thermal equilibrium of an electrolytic cell during an anode assembly change.

Advantageously, the method comprises a step involving stacking said hood on the first sealing lid or as appropriate on the second sealing lid.

This reduces the journey made by the pot tending machine having removed the hood, so that the opening resulting from withdrawing the hood, and the first and second sealing lids for the change of anode assembly, is formed for only a limited time.

The method may then include a step involving extraction of the spent anode assembly below the previously removed hood, and a step involving inserting the new anode assembly inside the electrolytic cell.

These steps can be performed by substantially upward or downward vertical translation, of the spent anode assembly and the new anode assembly respectively.

Finally, the method may comprise a step involving repositioning the previously removed hood, and then the first and second sealing lid.

The fact of beginning by fitting the hood makes it possible to close a greater portion of open area than that which would be closed with a sealing lid.

Other characteristics and advantages of this invention will be clearly apparent from the following description of a particular embodiment provided by way of a non-limiting example with reference to the appended drawings, in which: - figure 1 is a schematic sectional view of an electrolytic cell and a covering system according to one embodiment of the invention, in a substantially longitudinal plane of the electrolytic cell, - figure 2 is a schematic view from above of the inside of the electrolytic cell of figure 1, - figures 3 and 4 are schematic and sectional views of the electrolytic cell and the covering system of figure 1, in a substantially longitudinal plane of the electrolytic cell, and through which an access window is formed, - figures 5 to 7 are schematic and side views of a portion of a covering system according to one embodiment of the invention, - figure 7bis is a schematic and side view of a portion of a covering system according to one embodiment of the invention. - figure 8 is a schematic view from below of a hood of a covering system according to one embodiment of the invention, - figure 9 is a schematic sectional view of an electrolytic cell and a covering system according to one embodiment of the invention, in a substantially longitudinal plane of the electrolytic cell, - figure 10 is a schematic view from above of the electrolytic cell of figure 9, - figure 11 is a schematic sectional view of an electrolytic cell and a covering system according to one embodiment of the invention, in a substantially transverse plane of the electrolytic cell, - figures 12-14 are schematic and sectional views of a portion of an electrolytic cell and a covering system according to one embodiment of the invention, illustrating different steps of a method for changing an anode assembly according to one embodiment of the invention.

Figure 1 shows, according to one embodiment of the invention, an electrolytic cell 100, designed to produce aluminum by electrolysis, and a covering system 1, for closing an opening of the electrolytic cell.

The electrolytic cell 100 can equip an electrolysis plant such as an aluminum works. The electrolysis plant may include a plurality of electrolytic cells 100 aligned and electrically connected to each other to form a line or series of electrolytic cells. Electrolytic cells 100 are designed to be traversed by an electrolysis current of up to several hundred thousand amperes. Electrolytic cells 100 may be arranged transversely to the direction of the line or series, i.e. substantially perpendicular to the overall direction of flow of the electrolysis current across the line or series.

As shown in the figures, the electrolytic cell 100 may include two opposite longitudinal sides 101, which can be substantially parallel to one another and two opposite transverse sides 103 that can be substantially parallel to one another and perpendicular to the longitudinal sides 101 so that the electrolytic cell 100 may have a substantially rectangular shape.

Electrolytic cell 100 has a fixed structure. The fixed structure comprises a pot shell 102 and / or a side wall 105 of a gas confinement chamber.

The pot shell 102 may include a base 104 made of refractory materials, a plurality of cathode blocks 106 and conductors 108 for collecting an electrolysis current passing through the cathode blocks 106.

Electrolytic cell 100 also includes a plurality of anode assemblies 109 which are movable in a substantially vertical translation relative to the fixed structure of the electrolytic cell so that they can be immersed in an electrolytic bath 110 as they are consumed.

Anode assemblies 109 include here a plurality of carbonaceous blocks 112, supported by an electrically conductive anode cross member 114. The anode cross member 114 advantageously extends in a substantially transverse direction Y of the electrolytic cell, in a substantially horizontal plane. The ends of this cross member 114 are electrically connected to the connecting conductors (not shown) to route the electrolysis current there from a previous electrolytic cell.

The sides of the electrolytic cell 100 define an opening 116 which is designed for inserting or extracting the anode assemblies 109 inside or outside of the electrolytic cell 100 respectively.

Note that this opening 116 is designed to allow this insertion or extraction by substantially vertical upward or downward movement, respectively, of the anode assembly 109.

The electrolytic cell 100 here also comprises the covering system 1. Covering system 1 is designed to close the opening 116, to prevent the diffusion of cell gas generated during the electrolysis reaction, outside of the electrolytic cell 100. Covering system 1 also limits heat loss.

As can be seen in figures 1, 3 to 5, 9 and 11 to 14, covering system 1 extends above the anode assemblies 109 to fully cover the opening 116.

Covering system 1 comprises a plurality of hoods 2.

Hoods 2 are self-supporting. Each hood 2 comprises two opposite bearing edges 4, visible in particular in figure 11, designed to rest on two opposite sides of electrolytic cell 100, including an upper edge of the two longitudinal sides 101 of electrolytic cell 100.

Each hood 2 rests completely on the electrolytic cell 100, extending between the two longitudinal sides 101, above the opening 116.

The covering system 1 is also designed to have, substantially parallel to hoods 2, longitudinal servicing windows 6, as for example shown in figure 4. Each servicing window 6 can free a predetermined path through the plurality of hoods 2.

According to the embodiment of figure 4, the servicing windows 6 extend longitudinally between two adjacent hoods 2.

Covering system 1 further comprises sealing lids 8, each designed to close a servicing window 6.

Each sealing lid 8 is movable relative to the hoods 2 between a closed position, wherein each sealing lid 8 closes one of the servicing windows 6, and a servicing position in which each sealing lid 8 frees a passage through the covering system 1 via one of the servicing windows 6.

Sealing lids 8 are designed to rest at least partly on the hoods 2, as can be seen for example in figure 5.

Sealing lids 8 have longitudinal edges which are designed to rest on each one of the hoods 2 and have lateral edges which can rest on a top edge of the longitudinal sides 101 of the electrolytic cell 100, and.

In this way, sealing lids 8 can rest astride two adjacent hoods 2 and extend between these two adjacent hoods 2.

It is important to note that the sealing lids 8 are designed to be moved from the closed position to the servicing position without moving the hoods 2 on which the sealing lids 8 rest.

Sealing lids 8 may additionally be moved from the closed position to the servicing position independently of each other, i.e. without the movement of one of the sealing lids 8 implying that of another sealing lid 8.

As can be seen in the figures, hoods 2 and as appropriate sealing lids 8 advantageously extend in one piece from one longitudinal side of electrolytic cell 100 to the other.

Hoods 2 and optionally sealing lids 8 extend substantially parallel to a transverse direction Y of the electrolytic cell 100.

Note also that hoods 2 and sealing lids 8 extend in a substantially horizontal plane. Hoods 2 and sealing lids 8 preferably extend above the working floor, which minimizes the risk of operating personnel falling into the cell.

It will be noted that hoods 2 and sealing lids 8 are designed to extend above the electrolytic bath 110, whose temperature can reach about 1000°C. Hoods 2 and sealing lids 8 must be adapted to withstand a temperature of the order of several hundred degrees Celsius, without harm to their mechanical properties and, as appropriate, thermal insulation.

As can be seen in figures 6 and 7, sealing lids 8 have a T-shaped cross section defining two longitudinal flanges 10. Hoods 2 have a cross-section in the shape of an inverted T defining two longitudinal flanges 12.

Each flange 10 of one of the sealing lids 8 rests on one of the flanges 12 of an adjacent hood 2.

In this way, the covering system 1 has an alternation of upright Ts and inverted Ts formed by alternating interlocked hoods 2 and sealing lids 8.

In addition, as shown in figure 7, flanges 10, 12 of hoods 2 and sealing lids 8 have an L-shaped section.

In this way, the interlocking of a hood 2 and a sealing lid 8 forms a sealing baffle.

Covering system 1 advantageously further comprises sealing means interposed between the flanges 10 of each sealing lid 8 and the flanges 12 of the two adjacent hoods.

The sealing means comprise for example elastic seals 14. The elastic seals 14 are designed to compensate for a difference in relative bending between two consecutive hoods 2 between which a sealing lid 8 extends.

Also, as shown in figure 7bis, the flanges 12 of the hoods comprise channels having an upper opening and containing a powder material 31. The flanges 10 of the lids have an L-shaped section and an end portion of the L-section of the lid is pressed into the powder material 31 through the upper opening in the channel when the hood and the lid are interlocked. Making such a seal by means of a powder material is possible because the hoods and lids extend horizontally so that the powder material remains distributed with a uniform height throughout the length of the channel. The powder material forms a sealing means forming a barrier which prevents cell gas from escaping.

The powder material may in particular include alumina or crushed electrolytic bath comprising alumina. These materials have the advantage of being available in an aluminum works and are additionally introduced into the electrolytic cells so they are not likely to pollute the electrolytic cell in the event of accidental spillage in the electrolytic cell. In addition, alumina is a very good adsorbent for the HF and SO2 generated by the electrolytic cell so that any infiltration of cell gas through the powder material will have a lesser environmental impact.

In figure 7bis, the hood further comprises, on its upper side, pivoting shutters 32 arranged to close the opening of the channel when the hood and the lid are interlocked. These shutters are designed to hold the powder material in the channel. The shutters may alternatively be arranged on the lid and be fixed.

Hoods 2 have a lower surface 16, that can be seen in particular in figure 8.

The lower surface 16 may be provided with at least one reinforcement rib 18 to limit bending of the hoods 2. According to the example of figure 8, hoods 2 two may comprise two crossed ribs 18.

Moreover, the lower surface 16 may be provided with thermal insulation means. The thermal insulation means include, for example, rock wool, advantageously held and protected by a steel plate.

According to the embodiment illustrated in figures 6-8, hoods 2 and sealing lids 8 comprises a main body 20 in the form of substantially flat plate. This body 20 is designed to extend longitudinally in a transverse direction Y of the electrolytic cell 100.

Body 20 may be tubular. In this way, the body 20 advantageously defines a cavity within which a heat insulating material, such as rockwool be arranged.

Body 20 may alternatively be solid, so that the mass of the hoods 2 is greater. This allows compression or pinching of a seal 22, shown in figure 5 and in figure 11, extending between the bearing edges 4 of hoods 2 and the portion of the electrolytic cell 100 on which bearing edges 4 rest in order to improve sealing.

Although it is not shown, the lower surface 16 of the hoods 2 may be provided with deflection means for deflecting a flow of cell gas to the holes 120 of a cell gas capture system that can be fitted to the electrolytic cell 100, as will be described in more detail below.

Sealing lids 8 advantageously comprise gripping means, such as a handle 24. The gripping means are designed to allow each sealing lid 8 to be lifted substantially vertically by a pot tending machine, such as a handling crane, and without moving hoods 2 or other sealing lids 8.

The lower surface 16 can also be designed to enable the hoods 2 to stably rest on one of the sealing lids 8 to allow hoods 2 to be stacked on sealing lids 8.

For example, the lower surface 16 may have a housing (not shown) adapted to receive the means for gripping the sealing lids 8.

This is useful when one of the hoods 2 is removed from its location, since it is possible to place this hood 2 on one of the adjacent sealing lids 8.

Sealing lids 8 may also have a lower surface 26 designed to allow sealing lids 8 to rest stably on one of the hoods 2, or on another sealing lid 8.

Similarly, the lower surface 26 may have a housing (not shown) adapted to receive means for gripping the hoods 2, said gripping means being adapted to enable hoods 2 to be lifted by a pot tending machine.

In this way, when one of the sealing lids 8 is removed from its location, this sealing lid 8 can be placed on one of the adjacent hoods 2.

The lower surface 26 of sealing lids 8 may be also provided with thermal insulation means and I or means for deflecting cell gases.

It will be noted that the hoods 2 advantageously have a bending stiffness greater than that of the sealing lids 8. In other words, hoods 2 are stiffer than sealing lids 8.

As can be seen in figure 4, the servicing window 6 has a width less than that of the hoods 2 it separates.

The sealing lids 8 may also have a width less than the width of the hoods 2.

In this way, the opening made by removing a sealing lid 8 is small, the function of the sealing lids 8 being to be able to provide access to the interior of the electrolytic cell 100 with a minimum surface area opened.

The invention also relates to the electrolytic cell 100 comprising the covering system 1.

This electrolytic cell 100 advantageously includes sealing means interposed between the bearing edges 4 of the hoods 2 and the sides of the electrolytic cell 101 on which the bearing edges 4 rest.

These sealing means comprise, for example, the seal 22, which extends along the longitudinal sides 101, i.e. along a longitudinal direction X of the electrolytic cell.

In addition, the electrolytic cell 100 preferably comprises means for pinching this seal 22. The pinching means may comprise a screw pressing the edges of the bearing edges 4 of hoods 2 against the upper edge of the longitudinal sides 101, where the seal 22 is located. The pinching means may comprise ballast fitted to hoods 2 and I or sealing lids 8, the weight of the hoods 2 and I or sealing lids 8 compressing the seal 22.

As can be seen for example in figure 5, each sealing lid 8 preferably extends above and along a subjacent space 111 between anodes. This space between anodes separates two adjacent anode assemblies 109 of the 100 electrolytic cell.

Each hood 2 extends above and along a subjacent anode assembly 109 of electrolytic cell 100.

The electrolytic cell 100 may include indexing means adapted to indicate a predetermined position of the hoods 2, said predetermined position being such that the hoods 2 extend in line with the anode assemblies 109, i.e. above assemblies 109 and parallel to these anode assemblies 109. The indexing means include, for example, pins, slugs or slots.

According to the embodiment of figures 1 to 4 and 9 to 11 the electrolytic cell 100 includes means for capturing cell gases. These gas capture means are designed to capture and collect the cell gases emitted during the electrolysis reaction.

The capture means here comprise suction holes 120, represented in figures 10 and 11, arranged in the hoods 2 and sealing lids 8.

The suction holes 120 allow air to communicate between the inside of the electrolytic cell 100 and a capture sleeve 122 which is designed to lead the sucked cell gases to a manifold where the cell gases will be treated. As can be seen in figures 2 and 10, the sleeve 122 extends along the longitudinal sides 101 of the electrolytic cell 100. The sleeve 122 may also run along the transverse sides 103.

The capture means may also include a diaphragm (not shown), arranged for example at the connection between the capture sleeve and the manifold. The diaphragm is designed to change a section of the air passage with a view to changing the capture flow rate of the cell gases.

As shown particularly in figure 5, the width of the each hood 2 is less than the width of the underlying anode assembly 109.

For example, the servicing window 6 may have a width I" of the order of 350-480 mm, particularly 360 mm, and each hood has a width I less than a width I' between 700 mm and 2000mm.

The invention also relates to a method for changing a spent anode assembly 130 of an electrolytic cell, in particular the electrolytic cell 100 described above, for a new anode assembly.

The method includes a step involving moving a first sealing lid 8a from among sealing lids 8 of a covering system 1 as described above, from the closed position to the servicing position, as shown in figure 12.

This step is performed without moving hoods 2 and the other sealing lids 8. In this way, a passage is freed through the covering system 1 via one of the servicing windows 6.

The method advantageously includes a step involving fitting the first sealing lid 8a onto one of the hoods 2 adjacent to this first sealing lid 8a, as shown in figure 13.

The method also includes a step involving breaking or sawing a crust formed on the surface of an electrolytic bath 110, by inserting a suitable tool for breaking or sawing the crust around the path freed previously.

According to the embodiment of figures 12-14 the method comprises a step involving moving a second sealing lid 8b, from the closed position to the servicing position, without moving the hoods 2 and the other sealing lids 8

Still according to the embodiment of figures 12 to 14, the second sealing lid 8b is initially arranged on the other side of one of the hoods 2, particularly the hood 2 where the first sealing lid 8a, if appropriate, is not placed, next to which hoods the first sealing lid 8a was arranged so that a second passage is freed on the other side of this hood 2.

In addition, the method includes a step involving breaking or sawing a crust formed on the surface of the electrolytic bath 110, by inserting a suitable tool for breaking or sawing the crust around this second passage.

Still according to the embodiment of figures 12 to 14, the method may include a step involving placing the second sealing lid 8b on the first sealing lid 8a, as is particularly visible in figure 13.

As can be seen in figure 14 the method further comprises a step involving withdrawal of one of the hoods 2 initially arranged beside the first sealing lid 8a, particularly hood 2 where the first sealing lid 8a, if appropriate, is not placed.

The method may comprise an additional step involving stacking this hood on the second sealing lid 8b.

Note that the first and if appropriate the second sealing lid 8a, 8b and the hood 2 are stacked above an unchanged anode assembly 109.

The method may then include a step involving extraction of the spent anode assembly 130, below the previously removed hood 2, and a step involving inserting the new anode assembly inside the electrolytic cell.

These steps can be performed by substantially upward or downward vertical translation, of the spent anode assembly 130 and the new anode assembly respectively.

Finally, the method may comprise a step involving repositioning the previously removed hood 2, and then the first and second sealing lid 8a, 8b.

It will be noted that movement of the first sealing lid 8a and of the second sealing lid 8b and of the hood 2, is achieved by means of a pot tending machine, such as a handling crane, suitable for approaching these shutter covers 8 and the hood with their gripping means.

Of course the invention is not in any way limited to the embodiment described above, this embodiment only being provided by way of example. Modifications are possible, in particular from the point of view of the constitution of the various components, or through replacement by technical equivalents, without thereby going beyond the scope of protection of the invention.

Claims

1. Electrolytic cell comprising a plurality of anode assemblies, sides defining an opening through which anode assemblies are designed to be set up or withdrawn with a respectively ascending or descending vertical translational movement, and a covering system extending above the anode assemblies in order to cover said opening, the covering system comprising a plurality of hoods, wherein: each hood comprises two opposite bearing edges designed to rest on two opposite sides of the electrolytic cell from the sides of the electrolytic cell defining the opening, so that each hood extends from one side of the electrolytic cell to the other, above the opening, the covering system is designed to have, substantially parallel to the hoods, longitudinal servicing windows to free a predetermined path through the plurality of hoods, the covering system further comprises sealing lids, each sealing lid being movable relative to the hoods between a closed position, wherein each sealing lid closes one of the servicing windows, and a servicing position, wherein each sealing lid frees a passage through the covering system via one of the servicing windows, the sealing lids being designed to rest at least partly on the hoods, and the sealing lids are designed to be moved from the closed position to the servicing position, independently of each other, without moving the hoods on which the sealing lids rest.

2. Electrolytic cell according to claim 1, in which the sealing lids have longitudinal edges which are designed to rest on each one of the hoods.

3. Electrolytic cell according to claim 1 or claim 2, in which the sealing lids have a T-shaped cross-section defining two longitudinal flanges, the hoods have a cross-section in the shape of an inverted T delimiting two longitudinal flanges, each flange of one of the sealing lids resting on one of the flanges of an adjacent hood, so that the covering system has an alternation of interlocking hoods and sealing lids.

4. Electrolytic cell according to claim 3, in which the flanges of the hoods and sealing lids have an L-shaped section, so that the interlocking of a hood and a sealing lid forms a sealing baffle.

5. Electrolytic cell according to claim 3 or claim 4, in which the covering system comprises sealing means interposed between the flanges of each sealing lid and the flanges of the adjacent hoods on which each sealing lid rests.

6. Electrolytic cell according to any one of claims 3 to 5, in which, the hoods and lids extend horizontally and the longitudinal flanges of the hoods comprise channels containing a powder material and having a top opening, the longitudinal flanges of the lids having an L-shaped section, so that an end portion of the L-shaped section of the lid is pressed into the powder material through the top opening in the channel when the hood and the lid are interlocked.

7. Electrolytic cell according to any one of claims 1 to 6, in which the hoods include a surface provided with thermal insulation means.

8. Electrolytic cell according to any one of claims 1 to 7, in which the sealing lids comprise gripping means designed to allow substantially vertical lifting of each sealing lid without moving the hoods and independently of the other sealing lids.

9. Electrolytic cell according to any one of claims 1 to 8, in which the hoods and the sealing lids extend in a substantially horizontal plane.

10. Electrolytic cell according to any one of claims 1 to 9, in which the servicing window has a smaller width than that of the hoods that the servicing window separates.

11. Electrolytic cell according to any one of claims 1 to 10, in which each sealing lid has a smaller width than the width of the hoods.

12. Electrolytic cell according to any one of claims 1 to 11, in which the hoods have a bending stiffness greater than that of the sealing lids.

13. Electrolytic cell according to any one of claims 1 to 12 in which the electrolytic cell advantageously includes sealing means interposed between the bearing edges of the hoods and the sides of the electrolytic cell on which the bearing edges rest.

14. Electrolytic cell according to any one of claims 1 to 13, in which each sealing lid extends above and along a subjacent space between anodes separating two adjacent anode assemblies of the electrolytic cell.

15. Method for changing a spent anode assembly of an electrolytic cell according to any one of claims 1 to 14 for a new anode assembly, the method comprising: a step involving moving a first sealing lid from among the sealing lids of the covering system from the closed position to the servicing position, without moving the hoods of the covering system and the other sealing lids, in order to free a passage through the covering system via one of the servicing windows, and a step involving breaking or sawing a crust formed on the surface of an electrolytic bath, by inserting a suitable tool for breaking or sawing the crust around the path freed in the previous step. Rio Tinto Alcan International Limited Patent Attorneys for the Applicant/Nominated Person SPRUSON & FERGUSON