DK201670543A1

DK201670543A1 - Device for drilling a crust of a cryolite bath that can be positioned on the periphery of an electrolytic cell

Info

Publication number: DK201670543A1
Application number: DKPA201670543A
Authority: DK
Inventors: Steeve Renaudier; Yves Rochet; Frédéric Brun; Benoit BARDET; Olivier Martin; Christian Duval
Original assignee: Rio Tinto Alcan Int Ltd
Priority date: 2014-01-27
Filing date: 2016-07-19
Publication date: 2016-09-05
Also published as: CN105940148A; RU2016134748A3; WO2015110907A1; DK179337B1; FR3016898A1; EP3099846A1; FR3016898B1; RU2016134748A; CA2935486A1; EP3099846A4; AU2015208861A1

Abstract

The present invention relates to a piercing device (6) comprising a jack (61) consisting of a body (611) and a rod (612)extending along a longitudinal axis (A-A') and a piercing component (62) connected to the rod (611), the piercing device being used to form a hole in a crust of alumina and solidified bath of an electrolytic cell by a to-and-fro movement of the piercing component (62) along a translational axis (T), characterized in that the longitudinal axis (A-A') is parallel to and separate from the translational axis (T).

Description

DEVICE FOR DRILLING A CRUST OF A CRYOLITE BATH THAT CAN BE POSITIONED ON THE PERIPHERY OF AN ELECTROLYTIC CELL

Technical field

The present invention relates to the general technical field of the production of aluminum by electrolysis in an electrolytic cell containing a bath of cryolite.

It relates more particularly to a device for piercing a solidified crust extending over the cryolite-based bath (hereinafter called "cryolite bath") of said cell.

The piercing device can be mounted on an electrolytic cell with pre-baked anodes or on a continuous anode electrolytic cell known as a Soderberg cell.

Presentation of prior art

Aluminum is mostly produced by electrolysis of alumina dissolved in a cryolite bath. Currently, the production of aluminum on an industrial scale is carried out in an electrolytic cell composed of a steel pot shell open in its upper part, and whose inside is covered with refractory material, and a cathode surmounted by one or more anodes, the anode being immersed in the cryolite bath at a temperature between 930 and 980°C.

The application of an electric current between the anode and the cathode is used to initiate the electrolysis reaction. The anode is gradually consumed during the electrolysis reaction. Once the anode is spent, it is replaced by a new anode.

In the production of aluminum by electrolysis, a solidified crust of alumina and solidified electrolyte forms on the surface of the cryolite bath. The formation of this crust has the advantage of thermally insulating the cryolite bath and confining some of the polluting gases generated by the electrolysis reaction.

However, the production of aluminum by electrolysis causes permanent changes in the composition of the cryolite bath. Firstly, alumina is consumed by the electrolysis reaction and, secondly, the amount and composition of cryolite bath are gradually modified by secondary mechanisms such as absorption of cryolite constituents by the walls of the cell or decomposition of fluorinated constituents through anode effects.

It is therefore necessary to periodically add compounds such as alumina, cryolite (Na3AIF6) or aluminum fluoride (AIF3), in order to stabilize and control the operating parameters of the electrolytic cell.

This is why an electrolytic cell is generally equipped with piercing devices to make holes by piercing the crust, and feeders to add the compounds in the form of powder through said holes.

Each piercing device usually comprises a jack and a piercing component (known by the names of "plunger" or "chisel") attached to the end of a rod of the jack. The piercing component, positioned generally vertically, is lowered by activating the jack to break the crust extending over the solidified bath.

Document FR 2,262,700 discloses an electrolytic cell having a rectangular parallelepiped shaped pot shell, a superstructure carried by the pot shell, anode assemblies suspended in pairs along the superstructure, cathode blocks in the bottom of the pot shell, and a cryolite bath in which the anode assemblies are immersed. The electrolytic cell also includes a plurality of piercing devices positioned above the pot shell and extending in a median plane of the pot shell between the pairs of anode assemblies, over the entire length of the pot shell.

However, the presence of piercing devices arranged in line with the pot shell makes some maintenance operations difficult, such as maintenance of the piercing device itself but also the replacement of worn anode assemblies with new anode assemblies. This is because the piercing devices extending between pairs of anode assemblies prevent the vertical extraction of spent anode assemblies.

In addition, the fact of having the piercing devices on the edge of the pot shell would lead to the presence of obstacles that would not allow lateral movement of the anode assemblies once they have been extracted from the pot shell.

Such obstacles, because of their great height, would also involve making significant structural changes to the building in which the electrolytic cell is installed. In particular, the building height would have to be equal to the sum of the heights of the pot shell, an anode assembly and a piercing device to allow the anode assemblies to pass above the piercing devices.

An object of the present invention is therefore to provide a piercing device whose configuration makes it possible to improve the ergonomics of an electrolytic cell, and in particular to minimize the height of the electrolytic cell.

Summary of the invention

To this end, the invention provides a piercing device comprising a jack consisting of a body and a rod extending along a longitudinal axis and a piercing component connected to the rod, the piercing device being used to form a hole in a crust of alumina and solidified bath of an electrolytic cell by a to-and-fro movement of the piercing component along a translational axis T, the longitudinal axis being parallel to and separate from the translational axis T.

Within the context of the present invention, "parallel and separate axes" is understood to mean two parallel and non-coincident axes, i.e. spaced apart by a nonzero distance "d".

The electrolysis cell may comprise a pot shell covered with a liner designed to receive a cryolite bath and including a base, transverse side walls, and longitudinal side walls, and a confinement chamber including side walls offset outwardly relative to the side walls of the pot shell so that said side walls of the confinement chamber extend around and above the side walls of the pot shell. In this way, the planes in which the side walls of the confinement chamber extend surround the side walls of the pot shell.

The fact that the longitudinal axis is parallel to and distinct from the translational axis T allows the piercing component to be offset relative to the jack so as to give a piercing device designed to be positioned at the edge of the electrolytic cell and whose height is minimized, especially the height of the piercing device above the crust.

Such piercing on the edge of the electrolytic cell allows various compounds (alumina, AIF3, etc.) to be supplied around the edge of the cell and therefore makers it possible to maintain cold spots in the cryolite bath near the walls of the pot shell, thereby improving the resistance of the pot shell and the cell to the high temperatures of the cryolite bath.

Advantageously, the free end of the piercing component is opposite the jack. More specifically, at least one plane perpendicular to the longitudinal axis of the jack passes through the free end of the piercing component and the jack. This limits the height of the piercing device, and therefore the height of the electrolytic cell. In particular, this makes it possible to limit the height of a confinement chamber defining with the pot shell a volume for containment of the gases above the crust.

Advantageously, the end of the jack body through which the rod comes out is located at a height lower than the piercing component. The height of the piercing device above the crust is less than that of a piercing device from prior art wherein the height above the crust is at least the sum of the heights of the piercing component and the jack body.

To make the longitudinal axis and the translational axis parallel and non-coincident, the piercing device may comprise a U-shaped structure composed of two wings connected by a transverse core, a first wing being connected to the rod and a second wing being integral with the piercing component. This in particular makes it possible to position the piercing device on the edge of the electrolytic cell, and more specifically on one of its side walls.

In an alternative embodiment, the first wing is mounted integral with the jack rod. In another alternative embodiment, the first wing is connected to the rod via a cross beam mounted integral with one end of the rod, and mounted integral with one end of the first wing opposite the transverse core.

The piercing device may also comprise means for translational guidance of the piercing component along the translational axis T. The guide means are used to define the path of movement of the piercing component and limit the radial forces endured by the jack rod during perforation of the crust by the piercing component, as these radial forces can damage the jack.

The guide means may be coupled to the first wing and define a sliding guide path for the first wing. The guide means comprise for example a plate including a channel surrounding the first wing, the channel protruding from at least one face of the plate. The channel protrudes particularly from the face of the plate opposite to the transverse core in order to minimize the height of the transverse core above the plate. The channel preferably extends perpendicularly to the plane of the plate.

So during the to-and-fro movement of the piercing component, the first wing slides within the channel. The fact that the channel extends by protruding from the face of the plate opposite to the transverse core makes it possible to limit the number of parts of the piercing device extending inside the electrolytic cell.

As indicated above, the side walls of the confinement chamber extend around and above the side walls of the pot shell.

The top edges of the pot shell and/or the bottom edges of the confinement chamber can form a ledge to mechanically connect the side walls of the pot shell and the confinement chamber, so that the confinement chamber defines with the pot shell a containment volume above the cryolite bath.

The plate of each guide means may be part of the confinement chamber and/or the ledge or make it possible to close an opening in said ledge. This plate is in particular removable to facilitate disassembly of the piercing device.

A first portion of the piercing device composed of the jack and a portion of the first wing extends outside the confinement chamber, and a second part of the piercing device consisting of a portion of the first wing, the transverse core and the second wing extends inside the confinement chamber.

The jack is therefore located outside the confinement chamber while the piercing component is located within the confinement chamber. This reduces the risk of damage to the jack by limiting its exposure to gas and heat radiation.

Alternatively or in combination, the guide means may be coupled to the first wing and define a sliding guide path for the first wing. In this case, the guide means may comprise a plate including a channel surrounding the second wing and positioned in line with the alumina and solidified bath crust when the piercing device is fixed to the electrolytic cell. The channel preferably extends perpendicularly to the plane of the plate at one of the ends. The channel surrounding the second wing may protrude from the plate on one side of the plate opposite the transverse core, or protrude from the plate on one side of the plate opposite the piercing component, or protrude from the plate on both sides.

The jack may be secured to the electrolytic cell at a free end of the jack body opposite the rod.

This free end can be fixed to an upper edge of the cell. In this case, the free end of the body is further from the base of the cell than the end of the rod opposite to the body. In such a configuration, the jack body is positioned at an elevation higher than that of the bath contained in the cell, thereby limiting exposure of the jack body to the high temperatures.

Alternatively, the free end may be secured to the electrolytic cell so as to be closer to the base of the cell than the end of the rod opposite to the body. In such a configuration, the jack body is positioned at a height less than or equal to that of the bath contained in the cell, which makes it possible to limit the lateral space required between the cells by the piercing device.

The invention also relates to an electrolytic cell used for producing aluminum, comprising a pot shell covered with a liner and including a base, transverse side walls and longitudinal side walls, the pot shell being designed to receive a cryolite bath, the cell further comprising at least one piercing device as described above, each piercing device extending along at least one of the longitudinal side walls of the pot shell.

The fact of positioning the piercing devices on the edge of the electrolytic cell - and more specifically along its side walls - means that there is no obstruction to the vertical travel of the anode assemblies of the electrolytic cell. This allows anode assemblies to be replaced from the top of the electrolytic cell, without requiring the anode assemblies to undergo complex motion kinematics.

Each piercing device can be fixed to one of the side walls of the pot shell. Alternatively, each piercing device may be attached to one of the lateral walls of a confinement chamber, the enclosure including transverse and longitudinal side walls, the enclosure being designed to define with the pot shell a volume for confining gases above the cryolite bath. Each piercing device can in particular be fixed to an upper edge of the confinement chamber opposite to the base of the pot shell. This makes it possible to position the piercing device above the cryolite bath to limit exposure of the jack body to the high temperatures. The side walls of the pot shell are free so as to allow air to circulate at the periphery of the pot shell so as to cool it. In particular, the electrolytic cell may include a gas collection device including at least one gas capture sleeve having suction holes for sucking gas, each piercing device being fixed to said capture sleeve. In this case the capture sleeve provides, in addition to its function of sucking polluting gases, an attachment support function for the piercing device. The piercing device can be attached via the jack body and means for guiding the piercing component in translation. The guide means are advantageously fixed to the pot shell and/or the confinement chamber in order to be removable so as to facilitate disassembly and maintenance of the piercing device.

In some alternative embodiments, the confinement chamber may bear on the pot shell, the piercing device passing through the confinement chamber or the pot shell. The piercing device - and more specifically the first wing - can pass through the confinement chamber or in a direction parallel to the translational axis T and the longitudinal axis A-A'. This makes it possible to limit the size of the opening for the piercing component to pass through to the dimensions of the first wing so as to improve sealing of the cell. Of course, a ring-shaped dynamic seal may be arranged over the opening to allow the piercing device though.

In particular, a ring-shaped dynamic seal may be associated with the first wing; in this case, the first wing is able to move translationally through the ring-shaped dynamic seal. This ensures sealing of the electrolytic cell at the opening provided on the cell for passage of the first wing. This seal can be positioned at the edge of a bore or a channel through which the first wing passes.

Brief description of the figures

Other advantages and characteristics of the electrolytic cell and gas collection process will become apparent from the following description of several alternative embodiments, given by way of non-limiting examples, from the attached drawings, on which:

Figure 1 is a schematic perspective view of an example of an electrolytic cell,

Figure 2 is a schematic view of a piercing device,

Figures 3 to 5 are partial schematic sectional views of different variants of the piercing device and an electrolytic cell,

Figure 6 is a perspective view of an electrolytic cell comprising a plurality of piercing devices.

Detailed description

We will now describe an example of an electrolytic cell including one or more piercing devices for forming a hole in an alumina and solidified bath crust. It is obvious to those skilled in the art that the piercing device can be used with other types of cells.

In the different figures, equivalent elements bear the same reference numerals.

The expressions "side wall", "base", and "top opening" will be used later in the text in reference to a right-angled parallelepiped.

The reader will appreciate that in the context of the present invention: "base" means a horizontal wall of a right-angled parallelepiped located near the ground, "top opening" means an opening in a horizontal wall of a right-angled parallelepiped opposite the base, "side face/wall" means a vertical face/wall of a right-angled parallelepiped extending in a plane perpendicular to the base, "longitudinal faces/walls" means vertical faces/walls of a right-angled parallelepiped of which at least one dimension is greater than the dimensions of the other sides faces/walls, "transverse faces/walls" means vertical faces/walls extending perpendicularly to the longitudinal faces/walls.

Figure 1 illustrates an example of an electrolytic cell according to the invention.

The electrolytic cell is of rectangular parallelepiped shape and comprises a pot shell 1, a confinement chamber 2, one or more anode assemblies 3, a cathode 4, a gas collecting device 5 and one or more lifting devices 6.

This cell is used for the production of aluminum. It may be associated with a plurality of other electrolytic cells, possibly identical, the various cells being arranged one after the other, two successive electrolytic cells being adjacent at the level of one of their longitudinal side walls.

The pot shell 1 is of a generally parallelepiped shape. It has a base 11, two transverse side walls 12, and two longitudinal side walls 13. This pot shell 1 may be metallic, for example made of steel. The internal faces of the base 11 and sidewalls 12, 13 of the pot shelM are coated with a refractory material 14 to insulate the pot shell. This refractory material 14 is only partially shown so as not to overload figure 1.

Pot shell 1 is open in its upper part. It is designed to receive a cryolite bath 16 in which the anode assembly(ies) 3 is (are) immersed.

The confinement chamber 2 defines a closed volume above the cryolite bath 16 in which the anode assembly(ies) 3 is (are) moved.

It extends over pot shell 1. It more particularly bears on the upper edges 17 of pot shell 1. Confinement chamber 2 has four side walls 22, 23: two transverse side walls 22, and two longitudinal side walls 23. It is open at the bottom 24 and top 25 edges of the side walls 22, 23.

The side walls 22, 23 of confinement chamber 2 are offset outwardly relative to the side walls 12, 13 of pot shell 1 so that the side walls 22, 23 of confinement chamber 2 extend around and above the side walls of pot shell 1. In this way, the planes in which the side walls of confinement chamber 2 extend surround the side walls 12, 13 of pot shell 1.

The top edges 17 of pot shell 1 and/or the bottom edges 24 of confinement chamber 2 can form a ledge to mechanically connect the side walls 12, 13, 22, 23 of pot shell 1 and confinement chamber 2, so that confinement chamber 2 defines with pot shell 1 a gas containment volume above the cryolite bath.

The confinement chamber 2 also includes a removable cover means designed to cover the assembly composed by the pot shell 1 and the four side walls 22, 23 of confinement chamber 2. The cover means 26 may be composed of an assembly of panels or hoods extending generally in a horizontal plane, and may bear on the upper edges 25 of the side walls 22, 23 of the confinement chamber 2.

Each anode assembly 3 comprises an anode 31 and anode structure 32. The anode structure 32 firstly makes it possible to handle the anode 31, and secondly to supply it with electric power. During the electrolysis reaction, the anode 31 immersed in the cryolite bath 16 is consumed. The anode assemblies 3 need to be replaced periodically.

The anode 31 is preferably a block of carbon material of the pre-baked type.

The anode and the anode structure can be of any type known to those skilled in the art and will not be described in more detail later.

Cathode 4 is composed of one (or more) block(s) of carbonaceous material. Cathode 4 can be of any type known to those skilled in the art and will not be described in more detail later.

The gas collection device 5 retrieves the polluting gases generated during the electrolysis reaction.

The gas collection device 5 comprises one (or more) capture sleeve(s) 55 on which suction holes 53 for the suction of gas are distributed.

The capture sleeve(s) is (are) associated with one (or more) suction device(s) (not shown). They (it) extend(s) over the longitudinal side walls 23 of the confinement chamber 2, and possibly over the transverse side walls of the confinement chamber 2. The presence of suction holes 53 along the longitudinal walls 23 of the confinement chamber 2 makes it possible to improve the efficiency of the collection of gas pollutants 5.

The suction holes 53 formed in the different side walls 22, 23 of the confinement chamber 2 can be positioned vertically at different heights. Advantageously, the number of suction holes 53 of the collection device 5 can be equal to the number of piercing devices 6 attached to the electrolytic cell. In this case, each suction hole 53 may be associated with a respective piercing device 6 and be positioned close to it.

Within the context of the present invention "close" when speaking of two objects is understood to mean a distance between two objects of less than one meter.

Advantageously, each capture sleeve 55 may be of square or rectangular section, and be made of a material having a high mechanical strength, such as steel. This makes it possible to increase the rigidity and strength of the suction sleeve. A capture sleeve is thereby formed which, apart from its primary function of routing the gases, can be used in particular as: a strapping belt for the assembly consisting of the pot shell 1 and the confinement chamber 2, and as an attachment support for different components of the electrolytic cell such as the piercing devices.

Adding several functions to the capture sleeve makes it possible to limit the size of the cell, reduce its cost and make manufacturing it easier.

The piercing devices are used to make holes in an alumina and solidified bath crust forming on the surface of the cryolite bath during the electrolysis reaction. These holes are formed regularly to enable various compounds such as alumina, cryolite (Na3AIF6) or aluminum fluoride (AIF3) to be added in order to stabilize and control the operating parameters of the electrolytic cell.

Figure 2 shows an example of a piercing device according to the invention. The latter comprises a jack 61 and a piercing component 62.

The jack 61 includes a body 611 and a rod 612 extending along a longitudinal axis A- A'. Advantageously, the jack 61 may be pneumatic to withstand the high operating temperatures of the cell (approximately 500°C). Jack 61 drives the piercing component 62 with a vertical to-and-fro movement along a translational axis T.

The piercing component 62 is designed to be positioned on top of the crust to be pierced. The piercing component 62 may consist of a bar whose free end terminates in a tip adapted to penetrate the crust in order to form a hole.

Advantageously, the piercing component 62 is arranged so that the translational axis T- Τ' is separate (i.e., not the same) and parallel to a longitudinal axis B- B' of the jack 61. This allows the piercing component 62 to be offset relative to the jack 61 so as to limit the height of the piercing device 6 over the crust, especially in the confinement chamber 2. Such a piercing device 6 can then also be positioned at the edge of the electrolytic cell.

It offers the possibility of changing anode assemblies 3 via the top of the electrolytic cell without the piercing devices 6, and more specifically the jacks 61, hindering the vertical travel of the anode assembly 3 replacement operation, which means that major structural savings can be envisioned.

Different solutions may be considered to make the translational axis T not the same as, and parallel to the longitudinal axis A- A'. For example in the embodiment illustrated in figure 2, the piercing component 62 is coupled to the jack 61 via a U-shaped structure 63 composed of first and second wings 631, 632 connected to a transverse core 633.

The first wing 631 is interdependent with the rod 612 and extends as an extension thereof along the longitudinal axis A-A'. The second wing 632 is interdependent with the piercing component 62 and extends along the translational axis T.

A thermally insulating part may be fixed between the first wing 631 and the rod 612 to limit the risk of heat spreading to the body 611 of jack 61, since an excessive increase in temperature of the body 611 may cause damage to the jack 61.

As regards the transverse core 633 of the U-shaped structure, the latter preferably extends perpendicularly to the first and second wings 631, 632. It is mounted interdependently with the first and second wings 631,632.

The guide means 64 are used to guide the translational movement of the piercing component. In the embodiment illustrated in Figure 2, the guide means 64 include a plate 640 extending perpendicularly to the longitudinal axis A-A' and to the translational axis T. The plate 640 includes first and second traversing channels 641, 642 surrounding a portion of the first and second wings 631,632. These traversing channels 641,642 define sliding guide paths for the first and second wings 631, 632. The presence of at least one traversing channel 641, 642 serves to limit the radial forces F ' applied to the rod 612 which may damage the jack 61.

Preferably, the diameter of the channels is slightly greater than the diameter of the first and second wings 631, 632 to allow sliding and guiding of the first and second wings 631, 632 within the first and second channels 641, 642. The fact that the diameter of the first channel is complementary to the diameter of the first wing improves the sealing of the cell A ring-shaped dynamic seal may also be fitted to the channel 641; in this case, the first wing is able to move translationally through the ring-shaped dynamic seal.

In this way, the seal advantageously remains motionless at the edge of channel 641, while the first wing translates vertically within said seal. This solution has the advantage of being economical. Such a seal is additionally not exposed to impacts.

The piercing device is designed to be fixed onto the electrolytic cell at the level of the guide means 64 on the one hand, and the body 612 of the jack 61 on the other hand, in particular the free end 613 of the body 612 of the jack 61. Figures 3 to 5 illustrate various examples of mounting the piercing device on an electrolytic cell.

Referring to figure 3, we have illustrated an example of assembling the piercing device of figure 2 in an electrolytic cell. In this embodiment, jack 61 is housed in a free space between the reinforcing cradles 131 of pot shell 1.

Jack 61 is attached to the pot shell "upside down". More specifically, the body 611 of jack 61 is mounted on the pot shell 1 so that its free end 613 is closer to the base 11 of pot shell 1 than the rod 612.

In this configuration, when the body 611 of jack 61 applies a pushing force P on the rod 612 to cause it to move from a retracted position (where the rod extends partially inside the body) to a deployed position (wherein the rod extends outside the body), the piercing component 62 tends to move away from the surface of the cryolite bath 16. Conversely, when the body 611 applies a stretching force Tr on the rod 612 to cause it to move from the extended position to the retracted position, the piercing component 62 tends to approach the surface of the cryolite bath 16 to break the crust.

Having the jack 61 positioned in this way makes it possible to limit the height of the piercing device above the crust to the travel distance needed to break the crust, without it being necessary to add the length of the jack.

Guide means 64 comprise a plate 640. Plate 640 includes a through bore 643 through which the first wing 631 extends. Plate 640 also includes a single channel 642 through which the second wing 632 extends. The presence of a single guide channel 642 permits greater tolerance with regard to potential assembly play between the U-shaped structure 63, the jack 61 and the piercing component 62.

The single channel guide 642 defines a vertical guide path for the piercing component 62. The single channel 642 extends in line with the cryolite bath.

Preferably, the diameter of the through bore 643 is slightly greater than the diameter of the first wing 631 to improve sealing of the cell. As with the previous embodiment, a ring-shaped dynamic seal may be mounted on the through bore 643.

Referring to figure 4, another embodiment of the piercing device and its arrangement on the electrolytic cell is illustrated. In this embodiment, jack 61 is connected to the U-shaped structure 63 via a connecting cross beam 65. This connecting cross beam 65 preferably extends perpendicularly to the rod 612 and the first wing 631. The connecting cross beam 65 is mounted interdependently with the rod 612 of the jack 61 and on the first wing 631.

Jack 61 is attached to the cell "head-up ". More specifically, the body 611 of jack 61 is mounted so that its free end 613 is further from the base 11 of pot shell 1 than the rod 612. The free end 613 of body 611 of jack 61 can be fixed on the longitudinal side wall 23 of the confinement chamber 2, and preferably on the capture sleeve 55 of the confinement chamber 2, so that the body 611 extends against the longitudinal side wall 23 to a height greater than that of the cryolite bath 16. This limits the risk of damage to the jack 61 by exposure of the body 611 to excessive temperatures. The temperature of the side walls 12, 13 of the pot shell 1 is usually higher than the temperature of the side walls 22, 23 of the confinement chamber 2 due to the presence of the cryolite bath 16 whose temperature is of the order of 1000°C. Such positioning of the jack also, and advantageously, leaves the longitudinal side wall 13 of the pot shell 1 accessible to use, for example, a ventilation and cooling device for the walls of the pot shell as known from patent EP 1070158.

In this embodiment, the guide means 64 consists of a plate 640 including a single channel 641 designed to be associated with the first wing 631. This makes it possible to increase the tolerance of the piercing device with regard to any assembly play between the jack 61, the piercing component 62, and the U-shaped structure 63. In addition this makes it possible to improve the ergonomics of the guide means 64 and limit the number of cell parts covering the surface of the cryolite bath to make access easier.

The single channel 641 protrudes from one side of the plate. The guiding means 64 are fixed to the electrolytic cell so that the channel 641 extends downwardly from the cell (i.e. towards the base of the pot shell). This limits the number of parts of the piercing device projecting into the closed volume defined by the confinement chamber 2 above the cryolite bath 16. This also limits the height to which the transverse core 633 rises above the crust.

Referring to figure 5, another embodiment of the piercing device 6 is illustrated. This embodiment uses some features of the previous two embodiments in order to combine their advantages.

In particular, the body 611 of jack 61 is fixed ’’upside down" as in the embodiment illustrated in figure 3. It extends as a prolongation of the first wing 631, between the reinforcing cradles 131 of pot shell 1. The guide means 64 consist of a plate 640 including a single channel 641 surrounding the first wing 631 and projecting from the plate 640 toward the body 611 of the jack 61. No channel is associated with the second wing 632 so as to limit the size and complexity of the piercing device 6. Moreover, this avoids having parts of the guide means 64 in line with the cryolite bath 16.

Referring to figure 6, we have shown a perspective view of an electrolytic cell including a plurality of piercing devices 6 such as those described above.

Piercing devices 6 are arranged at the level of the longitudinal side walls 13, 23 of the cell (made up of the longitudinal side walls of the pot shell and of the confinement chamber).

Each piercing device 6 is associated with one or more metering unit(s) for adding compounds such as alumina, cryolite (Na3AIF6) or aluminum fluoride (AIF3), in order to stabilize the operating parameters of the electrolytic cell. More specifically, in the embodiment illustrated in figure 6, each piercing device 6 is associated with two metering units 66, the first for introducing alumina through the hole formed by the piercing component 62, the second for introducing cryolite and aluminum fluoride through the hole formed by the piercing component 62.

Having the drilling devices 6 and metering units 66 on the edge of the cell makes it possible to improve the dispersion and dissolution of compounds added to the cryolite bath. This is because fluid streams form at the edge of the cryolite bath. The introduction of compounds into these fluid streams facilitates their dispersion and dissolution in the cryolite bath.

The piercing device described above therefore presents numerous advantages, in particular with reference to the operation of an electrolytic cell used for producing aluminum.

Claims

1. Piercing device (6) comprising a jack (61) consisting of a body (611) and a rod (612) extending along a longitudinal axis (A-A’) and a piercing component (62) connected to the rod (612), the piercing device being used to form a hole in a crust of alumina and solidified bath of an electrolytic cell by a to-and-fro movement of the piercing component (62) driven by the jack along a translational axis (T), characterized in that the longitudinal axis (A-A’) is parallel to and separate from the translational axis (T).

2. Piercing device according to claim 1, wherein the free end of the piercing component (62) faces the jack (61).

3. Piercing device according to any one of claims 1 or 2, wherein the end of the body (611) of the jack (61) through which the rod (612) comes out is at a height lower than the piercing component (64).

4. Piercing device according to any one of claims 1 to 3, which comprises a U-shaped structure (63) comprising two wings (631, 632) connected to a transverse core (633), a first wing (631) being connected to the rod (612) and a second wing (632) being interdependent with the piercing component (62).

5. Piercing device according to claim 4, comprising a ring-shaped dynamic seal through which the first wing (631) is able to move in translation.

6. Device according to any one of claims 4 or 5, wherein the first wing (631) is interdependent with the rod (612).

7. Device according to any one of claims 4 or 5, wherein the first wing (631) is connected to the rod (612) via a cross beam (65) mounted interdependently with one end of the rod (612) on the one hand, and mounted interdependently with one end of the first wing (631) opposite the transverse core (633) on the other hand.

8. Piercing device according to one of claims 1 to 7, which further comprises translational guide means (64) for the piercing component (62) along the translational axis (T).

9. Electrolytic cell used for producing aluminum, comprising a pot shell (1) covered with a liner (14) and including a base (11), transverse side walls (12) and longitudinal side walls (13), the pot shell (1) being designed to receive a cryolite bath (16), the cell further comprising at least one piercing device (6) as described above, each piercing device (6) extending along at least one of the longitudinal side walls of the pot shell (1).

10. Electrolytic cell according to claim 9, wherein each piercing device (6) is attached to one of the side walls of the pot shell (1).

11. Electrolytic cell according to claim 9, which further comprises a confinement chamber (2), the confinement chamber (2) including transverse (22) and longitudinal (23) side walls, the confinement chamber (2) being designed to define with the pot shell (1) a confinement volume for gases above the cryolite bath (16), each piercing device (6) being fixed to a one of the longitudinal side walls of the confinement chamber (2).

12. Electrolytic cell according to claim 11, wherein each piercing device is attached to an upper edge (25) of the confinement chamber (2) opposite the base of the pot shell (1).

13. Electrolytic cell according to any one of claims 9 to 12, wherein the piercing device is fixed via the body (611) of the jack (61) and translation guide means (64) for the piercing member (62).

14. Electrolytic cell according to claim 9, wherein the confinement chamber (2) bears on the housing (1), and wherein the piercing device passes through the confinement chamber (2) or the pot shell (1).

15. Electrolytic cell according to claim 14, wherein the piercing device passes through the confinement chamber (2) or the pot shell (1) in a direction parallel to the translational axis (T) and the longitudinal axis (A - A') through a ring-shaped dynamic seal.