Title of Invention

ELECTROLYTIC CELL LEAK LIMITER`

Abstract

ABSTRACT 596/CHENP/2005 ELECTROLYTIC CELL LEAK LIMITER The present invention relates to a leak limiter of an electrolytic cell for the production of aluminium provided with confinement means comprising passage openings for the insertion of anode stems, characterized in that it comprises at least one support, capable of surrounding all or part of an anode stem, and at least one flexible sealing body arranged around all or part of the periphery of the support and designed to close off all or some of the free space between the inside edge of an opening and an anode stem. Figure 3

Full Text

Domain of the Invention The invention relates to the production of aluminium by igneous electrolysis. More particularly, it relates to means of confining gaseous effluents produced during the electrolysis. State of the art Metal aluminium is produced industrially by igneous electrolysis, namely by electrolysis of alumina in solution in a molten cryolite bath called an electrolyte bath, using the well-known Hall-Heroult process. The electrolyte bath is contained in pots called « electrolytic pots », comprising a steel pot shell lined on the inside with refractory and/or insulating materials, and a cathode assembly located at the bottom of the pot. Anodes made of a carbonaceous material are partially immersed in the electrolyte bath. The assembly consisting of an electrolytic pot, its anodes and the electrolyte bath is called an electrolytic cell. The electrolysis reaction, secondary reactions and high operating temperatures cause the production of gaseous effluents that in particular contain carbon dioxide and products containing fluorine. Release of these effluents into the atmosphere is severely controlled and governed by strict regulations, firstly for the ambient atmosphere in the electrolysis room in consideration of the working conditions of personnel close to the cells, and secondly for atmospheric pollution. Regulations in several States impose pollution limits on effluent quantities released into the atmosphere. At the moment there are solutions for extracting, recuperating and treating these effluents in a reliable and satisfactory manner. One widespread solution consists of providing the electrolytic cells with an effluent collection device. This device covers the electrolytic pots and comprises confinement means that in particular include a covering device, and means of suction and chemical treatment of the effluents. Known processes for the treatment of effluents include particularly recuperation of gases containing fluorine by reaction with alumina. The covering device comprises access means such as hoods, usually removable, and a tapping door used to work on the pot. The covering device delimits a confined suction area at a negative pressure relative to the ambient atmosphere, so that effluents can be recovered efficiently. The result is that collection efficiencies under steady state conditions exceed 97% in the most modern industrial installations, such that releases of gaseous products containing fluorine into the atmosphere are significantly lower than regulatory limits. In general, the anodes are connected to an electric power supply bar located outside the collection device through metallic stems that pass through the device by openings formed in it. The free -space (or « clearance ») left by stems in these openings is not sealed, to enable vertical and horizontal displacements of the metallic stems. Vertical displacements are frequent, and in particular compensate for wear of anodes during electrolysis. Horizontal displacements generally occur during operations to replace used anodes. Free spaces between anode stems and the inside edge of passage openings form a confinement break that is small for each anode stem, but which becomes significant when considering all anodes in a pot, and even more significant for a series of several hundred cells. Description of the invention An object of this invention is a leak limiter capable of reducing the confinement break originating from openings to allow anode stems to pass through. More precisely, the leak limiter according to the invention is designed to limit air and gas passages between the inside and outside of the collection device in an igneous electrolysis aluminium production cell through anode stem passage openings. The leak limiter in an electrolytic cell according to the invention is characterised in that it comprises at least one support, capable of surrounding all or part of an anode stem, and at least one flexible sealing body arranged around all or part of the periphery of the support and designed to close off all or some of the free space between the inside edge of the openings and the anode stem. The flexible body provides a certain degree of leak tightness around the anode stem and this leak tightness can be maintained due to the flexibility of the body, despite inevitable variations in the position of the stem. In particular, the invention significantly limits gaseous exchanges through the said free space. The support is advantageously in the form of a notch to simplify construction of the leak limiter and to enable lateral insertion of an anode stem through the headwall opening. Another object of the Invention is an electrolytic cell comprising at least one leak limiter according to the invention. The Invention will be better understood after reading the detailed description of a preferred embodiment of it as described below, illustrated with reference to the attached figures. Figure 1 shows a cross-section of a typical electrolytic cell intended for production of aluminium. Figure 2 shows a simplified perspective view of part of a typical electrolytic cell designed for production of aluminium (a) without and (b) with a leak limiter according to the invention. Figures 3 to 5 illustrate leak limiters according to the invention. Figure 6 illustrates the U-shaped brush of a leak limiter according to one variant of the invention. Figure 7 illustrates a cross-section along the I-axis of the U-brush of the leak limiter illustrated in Figure 5. Figure 8 illustrates sections I' to C of the leak limiter illustrated in Figure 5. Figures 9 and 10 illustrate methods of inserting an anode stem in the leak limiCers according to the invention. As illustrated in Figure 1, an electrolytic cell (1) for production of aluminium by the Hall-Heroult process typically comprises a pot (10), anodes (2) supported by attachment means typically including a stem (3) and a multipode (4) , and mechanically and electrically connected to an anode frame (5) using connection means (6) . Typically, the anode stem (3) has a substantially rectangular or square cross-section. The pot {10) comprises a steel pot shell (7), inside lining elements [8} and a cathode assembly (9). The lining elements (8) and the cathode assembly (9) form a crucible inside the pot (10) that contains the electrolyte bath (11) and a liquid metal pad (12). The electrolytic cell (1) also comprises a metallic structure (13) that in particular supports the anode frame (5) free to move, and a device for collecting effluents comprising confinement means (14, 15} and delimiting an internal confined space (16). Typically, the confinement means comprise removable hoods (14) and a fixed hood (15). As illustrated in Figure 2 (a), the collection device comprises openings (17) through which an anode stem (3) can pass freely. This opening is usually in the form of a crenel so that an anode stem can be inserted. The anodes (2) are usually inserted or removed from an electrolytic cell by lateral insertion after removal of one . or several hoods (14). Consequently, the opening (17) is such that it enables lateral insertion of the stem (3) of the anode (2), with or without longitudinal displacement of the anode, in other words with or without the anode moving along the main axis of the cell. Figure 2 (b) diagrammatically illustrates the position of the leak limiter (20) according to the invention in Che anode passage opening (17). The Leak limiter (20) of an electrolytic cell (1) for the production of aluminium provided with confinement means (14, 15) comprising passage openings (17) for the insertion of anode stems (3), is characterized in that it comprises at least one support (21) capable of surrounding all or part of an anode stem, and at least one flexible sealing body (30, 30a, 30b, 30c) arranged around all or part of the periphery (23) of the support (21) and designed to close off all or some of the free space between the inside edge (18) of an opeaing (17) and an anode stem (3) , The support (21) may be in different shapes, such as substantially straight, curved or other shapes. The support (21) may also be made from different elements. In one advantageous embodiment of the invention, the support (s) (21) form an opening, or « notch », (26), through which an, anode stem (3) can be inserted laterally. Typically, the opening (26) is U-shaped or forms a frame with three sides. The sealing body(ies) (30, 30a, 30b, 30c) are arranged around Che internal periphery (23) of the opening (26). In this embodiment, the iealc limiter (20) surrounds at least three sides of the anode stem (3) . The shape of the sealing body (30) may be such that it also covers the fourth side of the stem. The leak limiter (20) may possibly comprise a complementary closing element [20') that may be movable or removable, and is capable of limiting leaks through the fourth side after the stem has been inserted. This complementary closing element (20') may include a support (21') provided with a flexible sealing body (30' ) . Optionally, this complementary element may be fixed to the fixed hood (15) or to the mobile hood (14) close to the anode stem. Figure 3 illustrates the case in which the sealing body is formed from a single element {30). Figure 4 illustrates the case in which the sealing body is made from three separate adjacent elements (30a, 30b, 30c). As illustrated in Figure 5, the sealing body is adjacent to the anode stem, but is not necessarily in contact with it, It may be separated by a few millimetres, typically 2 or 3 mm, without significantly reducing the improved leak tightness achieved with the device according to the invention. The flexible sealing body may be made of any flexible element capable of efficiently closing off all or some of the said free space. For example, it may be formed from wires, strips, spongy substances or flexible tubes, or any combination of these possibilities. It may be metallic or non-metallic. The flexible sealing body (30) is preferably capable of resisting the atmosphere in the internal space {16) in the electrolytic cell and maintaining its mechanical properties at the temperature reached in this environment. Advantageously, the flexible sealing body (30) is formed from h bundle of metallic and/or non-metallic wires. The applicant found that the bundle of wires is a means of providing leak tightness around the anode stem due to the density of the wires, and that this leak tightness is maintained due to the flexibility of the wires, despite inevitable variations in the position of the stem. The wires also help to maintain good leak: tightness despite surface defects on the anode stem. The use of stainless steel wires has been found to be very satisfactory. Sealing bodies made from this type of wire resist mechanical loads applied by the anode stem during its movements efficiently and enable sufficient flexibility. The wires of the bundle (30) are sufficiently tight to cause a significant loss of pressure between the outside and Inside of the collection device. It has been found that a linear density of 100 to 1000 wires per centimetre around the periphery is sufficient. The bundle thickness is typically more than 0.5 cm. The wire diameter is typically between 0.1 and 1 mm. The aperture angle a of the bundle of metal wires is typically between 0 and 45°, and more typically between 0 and 30°. The length L of metallic wires output from the support is -typically between 1 and 10 cm. According to one advantageous variant of the invention, at least one flexible sealing body {30, 30a, 30b, 30c] is fixed to a second support or « frame » (32) free to move with respect to the support (21} . In this variant, the support (21) is typically provided with an elongated opening (22) around its inside periphery (23), and the frame (32) is inserted free to move in this opening. The frame (32) and the flexible sealing body (30, 30a, 30b, 30c) then form a mobile assembly or « drawer », (31) that improves self-positioning of the sealing means during movements of the anode stem. The frame/sealing body assembly (31) is typically free to move substantially perpendicularly to the anode stem (3). In this embodiment of the invention, the flexible sealing body (30, 30a, 30b, 30c) and the frame (32) are preferably made of non-magnetic materials to avoid developing a magnetic force in the presence of the intense magnetic field present in the environment of the cell, preventing this magnetic field from stopping the movement. For example, the frame (32) is advantageously made of aluminium or aluminium alloy, and the wires are preferably made of non-magnetic stainless steel. The fact that the elements (31) are free to move in the support (21) can facilitate maintenance and replacement of these elements following wear or damage. The leak limiter (20) preferably also comprises at least one connecting element (25) between the support (21) and each frame (32), to control the displacement of the sealing body(ies) (30, 30a, 30b, 30c) with respect to the support (21). The connecting element is typically fixed to the frame (32). At least one connecting element is advantageously an elastic element such as a spring or elastic blade, in order to facilitate self-positioning of the brush(es) with respect to the anode stem (3), It may also be possible to use connecting rods and/or guidance means, possibly combined with one or several elastic element(s). Figures 5 to 8 illustrate a preferred embodiment of the invention in which the sealing body 130, 30a, 30b, 30c) is made of wires fixed to a single mobile frame {32) capable of displacing with respect to the frame (21). Figure 5(b) shows a longitudinal sectional view of the limiter in Figure 5(a) that reveals the frame/wire assembly (31) called a « brush », located partly inside the support (21). The profile of the anode stem (3) is seen in dashed lines. Figure 6 shows the brush (31) alone, seen in its main plane (a) and on its edge (b). An anode stem (3) is normally inserted laterally along the I-I' axis illustrated in Figures 5 and 6. Figures 9 and 10 illustrate two methods of inserting an anode stem. Figure 9 shows the case of a single-directional insertion. Figure 10 shows the case of a two-directional insertion with movement of the leak limiter with respect to the electrolytic cell. The support (21) and the frame (32) are typically made of metal, to provide sufficient mechanical strength. Aluminium and aluminium alloys are non¬magnetic and may be used advantageously. The support (21) is sufficiently stiff so that the leak limiter can resist the force applied by an operator's foot, without damage. The leak limiter (20) may be fixed to the electrolytic cell, and more precisely to a structural element of the electrolytic cell or to the collection device, either rigidly or free to move, For that purpose, the support (21) advantageously comprises means (24) of fixing it to the electrolytic cell, preferably removable. A removable attachment, for example using bolts and nuts (2 9) , makes it easy to remove the leak limiter without removing the anode. Although a rigid attachment is sufficient in many cases, a mobile attachment gives the leak limiter an additional degree of freedom so that it is easier to adapt its position with respect to the anode stem. This additional degree of freedom is particularly useful when the anode stem passage opening (17) is large compared with the stem cross-section and enables large stem movements when the stem is being put into place and/or used- This degree of freedom is also useful when the shape of the opening (17) is more complicated than a simple crenel or notch and the insertion of the anode stem (3) in the opening (17) involves displacements in two directions, in other words in the longitudinal and transverse directions with respect to the main axis o£ the cell as illustrated in Figure 10. In this case, the leak limiter typically has an open position (Figure 10(a)) and a closed position (Figure 10(b)). The leak limiter (20) then advantageously comprises one or more complementary closing elements (33, 34) such as a plate, to maintain the leak tightness of the limiter during its displacements. These complementary elements may be fixed or mobile. The rnobile leak limiter t20) may possibly cooperate with one or more fixed closing elements (20'} to keep the device leak tight during its displacements. Displacements of the leak limiter may be guided by a guide element (35) such as a rail. When the leak limiter (20) contains metallic elements such as a metallic support or metallic wires, particularly if they are close to the anode stem, it is preferable to electrically insulate the leak limiter from the electrolytic cell to,avoid short circuits when the anode is being manipulated. This insulation may be obtained by inserting an electrical insulator (27, 28, 28') between the leak limiter and the electrolytic cell. For example, in the case illustrated in Figure 8, the leak limiter (20) is insulated from the cell (1) using an insulating plate (27) inserted between the support (21) and the confinement means (15), and using a tube (28) and a washer (23') inserted between the attachment means (29) and the confinement means (15). The simplicity of the leak tightness mechanism of the leak limiter according to the invention means that it has sufficient resistance to ambient conditions, and particularly the presence of alumina or crushed bath dust that could block or jam mechanisms containing pivot or rotation pins. Another advantage of the leak limiter according to the invention is that it can easily be contained in a small volume. The total thickness of the limiter according to the invention is typically not more than 3 to 4 cm, so that it can be easily positioned between the anode frame (5) and the hood (15). Another advantage of the invention is that it does not require any manual intervention or special actuator, which simplifies its use and increases its reliability. List of reference numerals 1 Electrolytic cell 2 Anodes 3 Attachment and current input means (stem) 4 Attachment and current input means (multipode) 5 Anode frame 6 Means for connecting the stem onto the anode frame 7 Pot shell 8 Inside lining 9 Cathode assembly 10 Pot 11 Electrolyte bath 12 Liquid aluminium 13 Metallic structure 14 Confinement means (removable hood) 15 Confinement means (fixed hood) 16 Confined internal space 17 Anode stem passage opening 18 Inside edge of the anode stem passage opening 20 Leak limiter 20' Complementary closing element 21, 21' Leak limiter support 22 Elongated opening 23 Support inside periphery 24, Attachment means 25 Connecting element 26 Limiter opening 27 Electrical insulator (plate) 2a Electrical insulator (tube) 28' Electrical insulator (washerl 29 Bolt and nut 30, 30a, 30b, 30c, 30' Flexible sealing bod 31 Mobile frame/sealing body assembly 32 Frame 33, 34 Complementary closing element 35 Guide means WE CLAIM : 1. A leak limiter (20) of an electrolytic cell {1) for the production of aluminium provided with confinement means (14, 15) comprising passage openings (17) for the insertion of anode stems (3), characterized in that it comprises at least one support (21), capable of surrounding all or part of an anode stem, and at least on flexible sealing body (30, 30a, 30b, 30c) arranged around ail or part of the periphery (23) of the support (21) and designed to close off all or some of the free space between the inside edge (13) of an opening (17) and an anode stem (3). 2. The leak limiter (20) as claimed in claim 1, wherein the support(s) (21) form an opening or notch (26), through which an anode stem (3) can be inserted laterally. 3. The leak limiter (20) as claimed in claim 1 or 2, wherein the flexible sealing body {30, 30a, 30b, 30c) is made of at least one element selected among wires, strips, spongy substances or flexible tubes, or any combination thereof. 4. The leak limiter (20) as claimed in claim 1 or 2, wherein the or each flexible sealing body (30, 30a, 30b, 30c) is formed from a bundle of metallic and / or non-metallic wires. 5. The leak limiter (20) as claimed in claim 4, wherein the bundle is composed of stainless steel wires. 6. The leak limiter (20) as claimed in any one of claims 1 to 5, wherein at least one flexible sealing body (30, 30a, 30b, 30c) is fixed to a second support or frame (32) free to move with respect to the support {21). 7. The leak limiter (20) as claimed in claim 6, wherein the said flexible sealing body (30, 30a, 30b, 30c) and the frame (32) are made of non-magnetic materials. 8. The leak limiter (20) as claimed in claims 6 or 7, wherein it also comprises at least one connecting element (25) between the support (21) and the or each frame (32) to control the displacement of the sealing bodies) (30, 30a, 30b, 30c) with respect to the support (21). 9. The leak limiter (20) as claimed in claim 8, wherein at least one connecting element (25) is an elastic element. 10. An electrolytic cell comprising at least one leak limiter as claimed in any one of the preceding claims.

Documents:

0596-chenp-2005 abstract-duplicate.pdf

0596-chenp-2005 abstract.jpg

0596-chenp-2005 claims-duplicate.pdf

0596-chenp-2005 correspondence-po.pdf

0596-chenp-2005 drawings-duplicate.pdf

0596-chenp-2005 form-26.pdf

0596-chenp-2005 petition.pdf

596-CHENP-2005 ABSTRACT.pdf

596-CHENP-2005 CLAIMS.pdf

596-CHENP-2005 CORRESPONDENCE OTHERS.pdf

596-CHENP-2005 POWER OF ATTORNEY.pdf

596-chenp-2005-abstract.pdf

596-chenp-2005-abstract.pdf-image.jpg

596-chenp-2005-claims.pdf

596-chenp-2005-correspondnece-others.pdf

596-chenp-2005-description(complete).pdf

596-chenp-2005-drawings.pdf

596-chenp-2005-form 1.pdf

596-chenp-2005-form 3.pdf

596-chenp-2005-form 5.pdf

596-chenp-2005-form18.pdf

596-chenp-2005-pct.pdf