WO2015121796A1

WO2015121796A1 - Start-up fuse for aluminium reduction electrolysis cell

Info

Publication number: WO2015121796A1
Application number: PCT/IB2015/051008
Authority: WO
Inventors: Amal ALJASMI; Wael ABOU SIDOU
Original assignee: Dubai Aluminium Pjsc
Priority date: 2014-02-14
Filing date: 2015-02-11
Publication date: 2015-08-20
Also published as: GB2525156A; GB201402659D0; GB2525156B

Abstract

Start-up fuse for aluminium reduction electrolysis cell use (3) for bridging the gap (8) between an upstream cathode busbar (1) and a downstream cathode busbar (2) in an electrolysis plant, said fuse (3) comprises a metallic band-shaped body comprising a. two contact surfaces (19a,19b) for contacting respectively a surface of said upstream cathode busbar (1) and a surface of said downstream cathode busbar (2), b. two main sections (4a,4b) respectively adjacent to said contact surfaces (19a, 19b), c. a central section (5) adjacent to said main sections (4a,4b), wherein said central section (5) has a transverse cross section having a surface area that is smaller than the surface area of any transverse cross section of said main section (4).

Description

Start-up fuse for aluminium reduction electrolysis cell

Technical field of the invention

The invention relates to the field of aluminium production using igneous electrolysis by means of the Hall-Heroult process. Alumina present in a fused bath is reduced to aluminium in aluminium reduction cells. These reduction cells are electrically connected in series operated at high amperage, through a system of busbars. Each cell is provided with an anode system and a cathode assembly. Within a series, the current is entering a pot through the anode busbar system and leaves it through the cathode busbar system; the current leaving an upstream cell though the cathode busbar system is passed to the anode assembly of the next downstream pot. The invention relates more precisely to electrical fuses used to short-circuit the cathode busbars before the start-up of a cell.

State of the art

An aluminium electrolysis plant comprises a plurality of electrolysis cells connected in series. Under normal operation each cell is traversed by a very high current, nowadays typically of the order of 300 to 500 kA, at a low voltage of the order of 4 to 4.5 V. Busbars collect current from the cathodic parts of a cell and feed it to the anodic part of the next cell connected in series. Busbars are usually made from aluminium or aluminium alloys. Being traversed by very high current densities, busbars develop considerable heat under normal operation conditions due to Joule effect, and they redistribute heat through thermal conduction.

Bypassing a cell is necessary in two situations: during the start-up procedure of a potline, and for certain heavy maintenance operations of individual cells such as cathode replacement.

Busbar systems of typical reduction cells are described in the paper "Evolution of busbar design for aluminium reduction cells" by Kjar, Keniry and Severo, published at the 8^th Australiasian Aluminium Reduction Technology Conference, 3-8 October 2004. Among other design criteria identified by these authors, they state that the busbar system of a reduction cell should allow the cells to be bypassed individually without the need to take the potline off load; this operation must be safe, not labour intensive and able to be deployed rapidly, and it should neither damage busbars that carry extra-current nor disturb the magneto-hydrodynamic stability of neighboring cells.

Before start-up, aluminium reduction cells are connected together by short circuiting the cathode busbars of two adjacent cells. It is known to use short-circuiting bypass joints, and in particular metallic wedges which are inserted in the air-gap between the conductors of the cathode busbar system. Such wedges are described in the paper "Numerical simulation of electrical joints in the by-pass sytem of 230 kA aluminum reduction cells" by Ortega et al., Light Metals 2007, p. 333-337. At cell start-up these wedges are withdrawn either manually or using mechanically operated wedge extraction systems, also called wedge pullers; these are commercially available pieces of equipment. A detailed description of a mechanically operated wedge puller can be found in US 2013/0098755 (Rio Tinto Alcan International Ltd).

When an electrical circuit is opened, such as during wedge pulling, an electrical arc develops between two separating surfaces, if the voltage drop between open contact surfaces is large enough to sustain the arc. Arcing could damage the wedges and the wedge pockets and could have potential safety implications on people working in the pot vicinity. It should be noted that even at continuous operation conditions, the close vicinity of reduction cells is a rather dangerous working environment due to high temperatures, enormous current densities, magnetic fields, differences in potentials; this is the more so for discontinuous operations like start-up and shutdown of a cell.

The principle of using start-up fuses is well known and has been applied also to pot technologies that do not use wedges for pot bypass. US patent US 8,048,286 and the publication "Balco Fuse Technology" by Ramaswamy et al. (Light Metals 2007 Vol 2, TMS, p. 461-464) disclose method for their use. In these prior art documents the fuses were used across shunt blocks on anode risers. However, the fuse design depends largely on busbar technology and has to be developed for every new technology. The required time to melt the fuse (fusing time) is a design parameter that depends on the operation practices and safety considerations. Operation practices that involve manual work or people staying close to the pot when the wedges are removed require longer fusing time on the order of minutes. In case of the prior art documents referenced above, the required fusing time was 10 minutes because the shunt blocks were disconnected manually.

The pot goes through different stages before cut-in. For pots in a new potline, the wedges must be installed in a full section before the busbars can be energised. Then, the cathode flexes are connected, the superstructure is placed, the preheat resistor bed is prepared and anodes are installed. The pot is ready to be energized by pulling the wedges. When a pot of an operating potline is shutdown, the first step is to install the wedges. Then, the superstructure is removed and the cathode flexes are disconnected. After the new pot is in place, the same steps are followed as in a new potline. At each of these stages, the current distribution in the wedges and busbars is different and all possibilities have to be studied in order to determine the highest load in the busbars and wedges.

When designing fuses, an additional problem is that once the busbars are energized, it is virtually impossible to carry out welding operations on the busbars. While it is possible to use welded fuses for the first start-up of a potline, only clamped fuses can be used at a later stage (for example when changing cathodes).

The fuses, being in parallel with the wedges, carry some current while the wedges are connected. The cross-section of the fuses must be designed large enough to handle the loading while the wedges are connected and small enough to break the circuit quickly when the wedges are disconnected. Another important aspect of fuse design is the provision for cases when one or more of the wedges are stuck and fail to be removed. As a rule, reliability, safety, predictable behaviour and operational ease are the main design criteria for pot start-up fuses. Their construction, shape and location must ensure safe mounting, safe operation and safe removal operations. In particular their presence and use should not interfere with the installation and removal of wedges. Practical considerations should be also taken into account when designing a fuse. One of these is that it is possible to weld the fuse on the busbars when there is no current in the potline (situation in potline start-up) and that it is not possible to do so when the busbars carry current (start-up of a pot in an operating potline). For pots in a new potline, the option to weld simplifies the design of the fuse. In an operating potline, the fuse has to be bolted or clamped to the busbars. Also for pot start-up in an operating potline, it is preferable to use a fuse that can be carried by operators.

The purpose of the present invention is to provide a new fuse design that allows safe and reliable operation and that can be used at any stage of pot operation, and that can be applied to both welding and clamping.

Brief description of the figures

Figures 1 to 7 illustrate embodiments of the invention: Figures 1 and 2 show an embodiment for welded fuses, while Figures 3 to 7 show an embodiment for clamped fuses.

Figure 1 a is the cross section across a busbar arrangement showing a welded fuse according to the invention, according to the cut line A-A shown on Figure 2. Figure 1 b shows a variant of the embodiment of Figure 1 a with the fuse welded at the bottm surfaces of the busbars.

Figure 2 is a view from the top of the welded fuse shown in Figure 1a.

Figure 3 shows the cross section across a clamped fuse according to the invention.

Figure 4 is a view from the top of the same clamped fuse as in Figure 3.

Figure 5 is a cross section across a busbar arrangement showing a clamped fuse assembly according to the invention.

Figure 6 shows the same clamped fuse assembly according to Figure 5 in its location between two electrolysis cells (pots),

Figure 7 shows the same fuse from the same perspective as Figure 4, after the fuse has melted under service conditions (see corresponding photograph on figure 12).

Figures 8 to 11 show examples for the use of fuses according to the invention.

Figure 8 shows the temperature evolution in welded and clamped fuses according to the invention for a DX+ pot operated at 440 kA (curve A : Clamped fuse; curve B : Welded fuse).

Figure 9 shows the temperature evolution in fuses according to the invention and in a stuck wedge for a DX+ pot operated at 440 kA (curve A : Wedge 1 ; curve B : Fuse 1 ; curve C : Fuse 2).

Figure 10 shows the measured temperature and current in two clamped fuses according to the invention in a DX pot operated at 385 kA (curve A : Tap end fuse temperature; curve B : duct end fuse temperature; curve C : tap end fuse current; curve D : duct end fuse current).

Figure 11 shows the measured temperature and current in two welded fuses in a DX+ pot operated at 440 kA (curve A : Tap end fuse temperature; curve B : duct end fuse temperature; curve C : tap end fuse current; curve D : duct end fuse current).

Figure 12 schematically shows a clamped fuse that has melted under service conditions. Figure 13 shows the corresponding photograph.

The following reference numbers are used in the figures: 1 Upstream cathode busbar 19a,b Contact surface

2 Downstream cathode busbar 20 Spanner

3 Fuse 21 Clamp top

4a,b Main section 22 Upper insulation piece

5 Central section 23,24 Upper and lower insulation guide

6 Hole 25 Lower insulation piece

7 Welding seam 26 Clamp bottom

8 Gap between busbars 1 ,2 27 Handle

10, 1 1 Potshell 28 Lower end of spanner 20

12, 13 Cathode collector bar 29 Crack in the central section 5

14, 15 Negative flexes 30 Hand tool

16 Clamped fuse assembly 31 Tool head

17 Upper section (arched or domed) 32a,b Contact section

18 Lower section (flat)

Objects of the invention

According to the invention the problem is solved by a fuse for bridging the gap between an upstream cathode busbar and a downstream cathode busbar in an electrolysis plant, said fuse comprising a metallic band-shaped body comprising

o two contact surfaces for contacting respectively a surface of said upstream cathode busbar and a surface of said downstream cathode busbar, o two main sections respectively adjacent to said contact surfaces,

o a central section adjacent to said main sections,

wherein said central section has a transverse cross section having a surface area that is smaller than the surface area of any transverse cross section of said main section.

In an embodiment, said contact surfaces, said main sections and said central section are formed from a single metallic sheet or plate, and more generally said band-shaped body can be formed from a single metallic sheet or plate.

Said central section can comprise a hole or aperture that decreases the surface area of the transverse cross section of said central section compared to the surface area of the transverse cross section of the main sections, while ensuring electrical continuity between the main sections. The diameter of said hole in the direction of the width of said band- shaped body is preferably at least 8% of the width, and preferably comprised between 10% and 30% of the width, and most preferably between 15% and 25% of the width. In one embodiment the band-shaped body is substantially flat. In another embodiment said central section is bent. In another embodiment said main sections and said central section are bent along the length of said band-shaped body. For example, said main sections and said central section can be bent along the length of said band-shaped body so as to present an arched or domed shape.

In an advantageous embodiment, said band-shaped body comprises an upper section that has an arched or domed shape, and a lower section comprising said contact surfaces, said central section being located at the top of said upper section. Said contact surfaces can represent the lower surface of contact sections that are essentially flat and respectively folded under said main sections.

In another embodiment said contact surfaces represent the lower face of said main sections. Another object of the invention is a fuse assembly comprising

• a clamp top section and a clamp bottom section linked together by a spanner, said clamp top section and clamp bottom section being configured to be clamped on an upstream cathode busbar and a downstream cathode busbar of an electrolysis plant, said busbars being configured to let a gap between each other,

· a fuse according as described above for bridging the gap between said upstream and said downstream cathode busbar,

• an upper insulation piece intended to electrically insulate said upstream and downstream cathode busbars from the clamp top section,

• a lower insulation piece intended to electrically insulate said upstream and downstream cathode busbars from said clamp bottom section,

wherein said contact surfaces form the lower surface of contact sections that are intended to be respectively inserted between said upper insulation piece and said upstream cathode busbar and between said upper insulation piece and said downstream cathode busbar.

Said fuse assembly can further comprise at least one insulation guide capable of being fitted in the gap between said upstream and downstream cathode busbars, wherein said spanner goes through said insulation guide. In particular, said fuse assembly may comprise an upper insulation guide and lower insulation guide placed, respectively, below the fuse and above the clamp bottom section, respectively in an upper section of said gap between said upstream and downstream cathode busbar and in a lower section of the gap between said upstream and downstream cathode busbar. Another subject-matter of the invention is a method for placing a fuse according to the invention on a gap between an upstream cathode busbar and a downstream cathode busbar in an electrolysis plant, said method comprising the steps of

o providing a fuse according to the invention or providing a fuse assembly according to the invention, said fuse assembly comprising a fuse,

o positioning the contact surfaces of the fuse in contact with respectively said upstream cathode busbar and said downstream cathode busbar,

o securing said fuse or fuse assembly onto said busbars by welding said fuse onto said upstream and downstream busbars or by clamping said fuse assembly onto said busbars.

This method can further comprise the following steps:

o Orienting the clamp bottom and the lower insulation piece of said fuse assembly in such a way that they will go through the gap between the upstream cathode busbar and the downstream cathode busbar;

o Placing said fuse assembly into said gap;

o Rotating said clamp bottom by about 90° to lock the fuse assembly,

o Fastening said spanner.

Another subject matter of the invention is a method to start up a pot in a series of at least two pots in an electrolysis plant, one of the two pots being electrically downstream to the other, each pot having a cathode busbar assembly comprising an upstream cathode busbar and a downstream cathode busbar, said upstream cathode busbar and said downstream cathode busbar being separated by a gap,

said method comprising the steps of

(a) inserting at least one electrically conductive wedge into said gap, such that current can flow from the upstream cathode busbar to the downstream cathode busbar across said wedge or wedges;

(b) installing at least one fuse according to the invention across said gap, such that current can flow from the upstream cathode busbar to the downstream cathode busbar through the fuse,

(c) withdrawing said wedge or wedges from said gap.

In this methodwherein said steps (a) and (b) can be carried in any order.

The fuse can be installed prior to withdrawing at least one wedge from said gap. It can installed prior to withdrawing the last wedge from said gap.

The fuse has been designed for being used with electrolysis cells of a Hall-Heroult smelting plant. It can also be used in other types of electrolysis plants, where electrolysis cells connected in series and operated at high current densities need to be protected during start-up, i.e. both during initial start-up of the potline and during start-up after maintenance or repair of individual pots.

Detailed description of advantageous embodiments

As can be seen on Figure 6, the fuse 3 according to the invention can be installed on the top of the upstream cathode busbar 1 and on the top of a neighboring downstream cathode busbar 2 of a busbar assembly linking two electrolysis pots 10, 11 connected in series, in a way that the fuse forms an electrical link between both busbars 1 ,2 across the gap 8 separating them. In the example of Figure 6 the upstream cathode busbar 1 is electrically linked to the cathode collector bar 13 of the upstream pot 11 through an uspstream negative flex assembly 14, and the downstream cathode busbar 2 is electrically connected to the cathode collector bar 12 of the downstream pot 10 through a downstream negative flex assembly 15. The top of the cathode busbars 1,2 is typically close to the circulation aisle level of the potline. As a consequence the cathode busbars 1 ,2 are readily accessible and the fuse 3 can easily be installed by an operator.

The invention is not limited to fuses that can be installed on the top of the upstream 1 and downstream 2 cathode busbars; the fuses 3 according to the invention can also be installed at other locations allowing to bridge the gap 8 separating them. As an example the fuses can be installed at the bottom surfaces, as shown on Figure 1 b for the example of a welded fuse; the same applies to a clamped fuse according to the invention (this embodiment is shown on the figures).

While Figure 6 displays a clamped fuse 3, the location is the same for a welded fuse. This can be seen on Figure 1 which shows the same view as Figure 6, but limited to the upstream 1 and downstream 2 busbars and to the fuse 3. According to the invention, the fuse 3 is made from a single metal sheet or plate by cutting and/or sawing. More generally, the fuse 3 comprises a metallic band-shaped body. The welded fuse advantageously has a substantially rectangular shape, while the clamped fuse advantageously has an arched or domed shape, which can be obtained by bending a metallic band-shaped body, such as a rectangular sheet or plate section.

In one embodiment said band-shaped body comprises an upper section 17 that has an arched or domed shape, said central section 5 being advantageously located at the top of said upper section 17, and a lower section 18 comprising said contact surfaces 19a, 19b. The lower section can also comprise contact sections 32a, 32b ; said contact surfaces 19a, 19b can then be represented by the lower surface of said contact sections 32a, 32b that are essentially flat. Said contact sections 32a, 32b can be respectively folded under said main sections 4a, 4b. Such an embodiment is shown on Figure 3; it will be discussed in more detail below.

According to the invention, and as can be seen on Figures 1 and 2 for the welded fuse and on Figures 3 and 4 for the clamped fuse, the fuse 3 comprises a central section 5 and on each side a main section 4a, 4b. The main section 4a, 4b comprises on each bottom side a substantially flat contact surface 19a, 19b through which an electrical contact is established with the substantially flat upper surfaces of the upstream cathode busbar 1 and the downstream cathode busbar 2. Advantageously the surfaces to be put in contact are cleaned prior to establishing the electrical contact, for instance by buffing. According to an essential aspect of the invention, the central section 5 of the fuse 3 is electrically, mechanically and thermally weaker than the main section 4. This can be achieved by a design in which said central section 5 has a transverse cross section having a surface area that is smaller than the surface area of any transverse cross section of said main section 4. In an advantageous embodiment the width of the central section 5 is smaller than the width of the gap 8 between the cathode busbars 1 ,2.

In an advantageous embodiment of the invention, the central section 5 of the fuse can be weakened by drilling a hole 6, preferably in the center of the central section 5. This simplifies the manufacturing process of the fuse, as it allows using plate or sheet of constant thickness for manufacturing the fuse 3, and avoids machining. This hole 6 can be circular or elongated (and in this case with its long axis parallel to the gap between the busbars 1 ,2). The hole 6 serves as a weak point for breaking the fuse 3. The hole size and/or diameter can be adapted easily to the specific needs of the fuse in it operating environment, as will be explained below in more detail.

According to an embodiment of the invention, and as can be seen best on Figures 1 and 2, the fuse 3 can be welded on a surface of the upstream 1 and downstream 2 cathode busbars, across their gap 8.

According to another embodiment of the invention, and as can be seen best on figure 5, the clamped fuse 3 can be a part of a clamped fuse assembly 16. Said clamped fuse assembly 16 comprises a fuse 3 and at least two clamp sections, namely a clamp top 21 and a clamp bottom 26, which interact through a spanner 20. The spanner 20 is typically a metallic rod, which is threaded at least on its lower end 28. The spanner 20 may be provided with a head 31 comprising means (such as a nut, a bolt or a shaped tip) allowing to turn it readily by using a corresponding tool 30 ; said tool 30 preferably is a hand tool 30 carrying a handle 27 on its upper end, allowing to seize it and to turn it when screwing and unscrewing the clamped fuse 3. The clamp top 21 is placed inside the clamped fuse 3. It can be made from a conductive material, such as steel. To force the current through the fuse alone and not through the clamp top 21 and clamp bottom 26, an insulation piece 22 is placed between the clamp top 21 and the fuse 3. The spanner 20 runs through the clamp top 21 to the clamp bottom 26, where it can be screwed in a threaded counter piece 29 that keeps the clamp top 21 and clamp bottom 26 together. The clamp bottom 26 has also an insulation piece 25 on top of it to prevent current flowing directly from the busbar 1 to the clamp bottom 26. In addition, upper 23 and lower 24 insulation guides are placed, the first below the fuse 3 and the second above the clamp bottom 26. These insulation guides 23,24 will create an air gap between the spanner and the busbars 1 ,2 to avoid any electrically conducting contact between them.

As can be seen best on Figure 3, in one embodiment the clamped fuse 3 comprises an upper section 17 which is arched (domed), and a lower section 18 which is flat. The flat lower section 18 comprises at least two flat contact surfaces 19 designed to establish a good electrical contact with flat surfaces of the upstream 1 and downstream 2 busbars. Said flat surfaces of said busbars 1 ,2 can be upper surfaces, but can also be lower surfaces or other surfaces, as may be convenient in a specific environment. (The terms "upper" and "lower" as well as "top" and "bottom" used in the designation of the various components of the clamped fuse assembly 16 refer to the case in which said fuse assembly is mounted on the upper surface of the busbars 1 ,2, as shown on Figures 5 and 6). The fuse's upper section 17 is designed with a weaker central section 5. In an advantageous embodiment this weaker central section 5 comprises a hole 6, preferable in its center (i.e. in the arched section 17). This hole 6 can be circular or elongated (and in this case with its long axis parallel to the gap between the busbars 1 ,2). The hole 6 serves two purposes: it is a weak point for breaking the fuse 3 and it allows tightening of the spanner 20 through the hole 6. This can be seen from Figure 7 which shows that a neat and continuous crack 29 has formed on both sides of the hole 6, ensuring electrical discontinuity of the fuse 3 after its use (this figure relates to a clamped fuse, but the crack is essentially the same for the welded fuse). Figure 12 shows a three dimensional drawing of the cracked, clamped fuse. Figure 13 shows a corresponding photograph. The upper part of the clamped fuse assembly 16 (and preferably the whole fuse assembly 16) is assembled beforehand and then installed on the busbars 1 ,2 when required. When ready to install, the upper part of the fuse assembly 16 (comprising the fuse 3, the clamp top 21 and the upper insulation piece 22) is placed in parallel with the wedges after cleaning (for instance by buffing) the busbars' contact surfaces to ensure good electrical contact with the fuse's contact surfaces 19a, 19b. The clamp bottom 26 is oriented in such a way that it will go through the gap 8 between the upstream cathode busbar 1 and the downstream cathode busbar 2. After placement into the gap 8, the clamp bottom 26 is rotated by 90 degrees to lock the fuse assembly 16. The spanner rod 20 goes through the hole 6 in the fuse 3 and is used to tighten the clamp as to ensure good electrical contact between the fuse 3 and the busbars 1 ,2. The spanner's upper part is advantageously formed as a handle 27 for easier handling and clamping of the clamped fuse assembly 16. The welded or clamped fuse 3 according to the invention is typically formed from a metallic sheet or plate. In an advantageous embodiment an aluminium or aluminum alloys material is used. As an example, an alloys of the 1xxx series (according to the definition of The Aluminum Association), such as AA 1350 or AA 1370, can be used. The thickness of the metal sheet is typically 10 mm to 20 mm. Both the welded and the clamped fuses 3 according to the invention can be installed and removed easily. In particular, the clamped fuse assembly 16 according to the invention can be installed and removed easily. Its total mass allows easy carrying, installation and demounting by one single operator. The fuse 3 can be replaced easily if necessary. The fact that the clamped fuse assembly 16 can be preassembled avoids any loose pieces to fall down during the installation and dismounting of the fuse assembly 16.

The length and width of the fuse 3 according to the invention take into account practical considerations; in particular the fuse must be sufficiently long in order to fit across the gap between the cathode busbars 1 ,2, and must be sufficiently wide in order to be easy to be manipulated and welded using standardized operating conditions. As a consequence, the main adjustable parameter in order to adapt the lifetime of the fuse 3 to the specific needs of the operating environment are the plate thickness and the surface ratio of the hole with respect to the geometric surface of the central section 5. The fuses 3 according to the invention have many advantages. Their use leads to a decrease in the voltage drop between the opening contact surfaces and minimizes electrical arcing during disconnection of the wedges. They are easy to manufacture from aluminum plate or sheet. For a given plate thickness, their cross section can easily be adapted to the specific operating conditions of a potline (amperage, melting time) by changing the diameter of the hole. They are quick and easy to use (i.e. to carry, to mount, to dismount) by a single operator. They are reliable and safe in operation, leading to a neat crack 29 ensuring reliable electrical discontinuity with no molten material falling down. If necessary a wood wedge can be introduced into the crack 29. Due to the arched or domes design of the clamped fuse, its central section 5 is put under some mechanical stress, especially due to thermal expansion upon heating, which ensures in many cases that it will break at elevated temperature prior to melting, therefore avoiding uncontrolled spillage of molten metal particles in the vicinity of the busbars.

The fuses 3 according to the invention can then be welded on the cathode busbars 1 ,2 for pot-line start-up procedures, or they can be clamped on the cathode busbars 1 ,2 for cell re-starts (for instance due to pot replacement) in an operating potline. Clamped fuses can also be used for pot-line start-up, but welded ones are preferred for pot-line start-up. Used in parallel with wedges, the fuses 3 according to the invention minimize electrical arcing during disconnection of the wedges. The fuses 3, being in parallel with the wedges, carry some current while the wedges are connected. The cross-section of the fuses 3 must be designed large enough such as to stay connected while the wedges are connected and small enough to break or melt quickly when the wedges are disconnected. When wedge extraction is automated, this usually allows for a shorter fusing time, in the order of seconds.

Another important aspect of fuse design is the provision for cases when one or more of the wedges are stuck and fail to be removed. In such cases, it should be possible to put back the wedges in order to reduce the load on the stuck wedge. The fuse 3, in this case, should preferably stay connected for the duration required to re-install the remaining wedges, otherwise the broken fuse has to be re-installed before cut-in.

Fuses 3 according to the invention have been used successfully on the applicant's potlines based on DX and DX+ cell technologies.

Examples

The following examples describe the use of the fuses 3 according to the invention in the applicant's plant based on the applicant's DX and DX+ technology which use wedges to stop a pot. The fuse connects the downstream of an operating pot to the upstream of the stopped pot in parallel with the wedges. Current load in wedges is different along the pot due to specifics of busbar design. This is one reason why the tendency to arc is not equally distributed among the wedges. The fuse is most effective if it is installed near the wedge with highest current loading. However, there is also a random component in wedge pulling because there may be a slight delay between one wedge exit and another, potentially causing more arcing in the wedges that get removed last due to higher current loading. When all wedges are removed, full pot current flows through the fuses leading them to rapid breaking and/or melting, depending on their current density.

To design the fuse, ANSYS finite element modelling was used. Two kinds of models were created for this purpose: a global and a local model. The global model represents all the busbars as well as anodes and cathodes. The model starts in the metal pad of the upstream pot and ends in the metal pad of the downstream pot in a similar way as in the publication "Impact of Amperage Creep on Potroom Busbars and Electrical Insulation: Thermal-Electrical Aspects" by Schneider et al., Light Metals (2011), p. 525-530. Different stages in the cut-out were simulated:

- Wedges are in place before connecting the cathode flexes,

- When the cathode flexes are connected and

- When anodes are installed.

For the global model, ANSYS 1 D link elements were used. The models were developed and validated for DX and DX+ pot technology. This model helps determine the best location and the size of the fuse.

Local models of the wedges and fuses were also developed in ANSYS. For that, 3D solid elements were used. The wedge models were similar to the ones in the publication "Numerical Simulation of Electrical Joints in the By-Pass Section of 230 kA Aluminium Reduction Cells" by Ortega et al., Light Metals (2007), p. 333-337. Boundary conditions for those models were obtained from the global model. The results at steady state conditions were obtained first and used later on as initial condition for transient model runs. Transient models were used to study the impact of increasing the current loading in the fuses and wedges with time.

Contact resistances of the wedges were derived from the measured voltage drops in the wedges in DUBAL DX+ Eagle prototype pots during the start-up.

Based on the physical limitation and the space available only a few locations were taken into consideration for the fuse. These locations were compared, using the same fuse size in each location and evaluating the current distribution and the temperature. The fuse location and size satisfied the criterion that the fuse does not melt when the wedges are connected. Different fuse cross sections were tested. A fuse with bigger cross sectional area will have less current density and, thus, will have lower temperature. For DX+ pot operated at 440 kA, an arrangement with two fuses and ten wedges was chosen. Table 1 shows current distribution and temperature in the wedges for welded (W) fuses in a DX+ pot (DUBAL proprietary technology) operated at 440 kA. Results are shown for half of the pot due to assumed symmetry in the model.

Table 1

Similarly, the clamped (C) fuse is designed for the start-up of DX+ pots in an operating potline. Current and temperature in one half of the bolted fuses and wedges are shown in Table 2; the other half is identical due to assumed symmetry in the model.

Table 2

Based on these results, local models of the wedges and fuses were run. The currents entering and exiting the adjacent busbars, on which the fuses are attached, are obtained from the global model. Also, the temperature in one busbar on the model boundary is specified from the global model. These boundary conditions enable 3D modelling of the local conditions in any of the wedges or the fuse. Using ANSYS transient analysis, the local model of the fuse can be used to estimate the fusing time. For the fuse transient model, the initial temperatures are read from the steady state model. Also, the currents in the local model are taken from the global model. To calculate the maximum current in the fuse, which occurs when the last wedge is removed, the global model was run with all the wedges removed and only the two fuses left in place. Results show that 84% of the total current (about 370 kA out of 440 kA) flows through the two welded fuses. For the clamped fuse case, only 73% of the current passes through the fuses (about 320 kA out of 440). These values were then used in the local model to calculate the temperature evolution in the fuse and the fusing time. Figure 8 shows the temperature in the fuse from the local ANSYS transient model. The melting temperature of the fuse was assumed to be 650 °C. As can be seen from figure 8, the fusing time is about 5 seconds for the welded fuse and about 6 seconds for the clamped (bolted) fuse.

An additional aspect of the fuse design is to consider the condition when one or more of the wedges fail to be removed by the extraction unit for whatever reason. This will redistribute the current in the wedges and increase the loading in the fuse. Different scenarios with different wedges stuck were modelled with the global model to get the current and temperature distribution in the fuse and in the remaining wedges. The worst case is when only one of the 10 wedges gets stuck. Table 3 below shows the current in a stuck wedge as well as the two clamped (bolted) fuses (DX+ pot operated at 440 kA).

Table 3

The transient model of the fuse shows that fuse C2, which is far away from the stuck wedge, will melt in about 3 minutes. The current will redistribute again and the wedge will take the additional 27 kA which was taken by fuse 2. Fuse 1 , which is adjacent to the wedge, will melt in about 20 minutes. The second fuse melts much later than the first one because the wedge near the second fuse takes off the load from the fuse. The temperatures in the two fuses and the stuck wedge are shown in Figure 9. The wedge temperature reaches around 500°C in 25 minutes and would melt much later than the fuses. However, from a practical point of view, the increase in wedge temperature will cause expansion of the wedge making it more difficult to be removed and possibly fusing the wedge to the busbars before it would melt. The modelling of various scenarios helps in setting some guidelines for pot operation in case of any abnormalities. When a wedge gets stuck, the common practice is to re-install the remaining wedges. Based on these results, the pot start-up team can be informed that if a wedge gets stuck, the fuse furthest from the wedge will melt in approximately 3 minutes and the fuse neighbouring the wedge in 20 minutes. In such cases, the melted fuses would need to be replaced and safety precautions should be taken around the fuses.

To validate the model results, clamped fuses were tested in DUBAL DX pots at 385 kA. The fuse thickness was decreased proportional to the amperage. A thermocouple and two voltage probes were installed on the fuses in order to monitor the temperature and the current through the fuse continuously. (The thermocouple was placed on the rim of the fuse, close to the limit between the central section 5 and the main section 4), and the voltage probes were placed close to the limit between the main section 4 and the contact surfaces 19).

Figure 10 shows the increase in temperature and current as wedges are pulled out. Extraction of each wedge can be seen from the step changes of the current in the fuses. The steps in the fuse current vary depending on the number of wedges pulled and their distance from the fuse. The delay between the first and last wedge pulled was about 30 seconds. The fusing time is considered to be the time between the last wedge pulled and the breaking of the fuse. The measured fusing time is thus approximately 9 seconds. The maximum measured current through both fuses was 328 kA. This means that 57 kA was going through the anode risers to the pot at that time. Figure 10 also shows that the currents in the two fuses are not equal, which indicates some randomness in the fuse and wedge behaviour.

As can be also seen from Figure 10, the clamped fuse did not reach the melting temperature of 650°C. In fact, the fuse broke due to mechanical stresses at around 500°C (this temperature value is somewhat uncertain as the probe was about 60 mm away from the breaking line). Figure 7 shows a drawing of the bolted fuse after it broke, Figure 12 shows a photograph (arrows D and E show the location of the voltage probes, while arrow F shows the location of the thermocouple). The welded fuses according to the invention, on the other hand, usually break by melting. Figure 1 1 shows the temperature and current in the two fuses over a time frame of 60 seconds, from just before the wedged started to be pulled to a few seconds after the fuses melted. The maximum temperature of the fuses was 700°C to 770°C which is higher than the melting point of the aluminium material used for the fuse, indicating the presences of an electrical arc and not just simple melting. The current in the fuses shows the delay in wedge removal between the first and last wedge of approximately 45 seconds. The fusing time, counted from the moment of last wedge removal, was about 5 seconds which is exactly as predicted by the model (figure 8). Maximum current in both fuses was 405 kA just after the removal of all wedges, which means that 35 mA was flowing thought the pot at that moment. Later the total current in the fuses decreased to 378 kA at the moment of fuse melting, because the fuse resistance increased with temperature. However, the two fuses did not react identically, which shows some randomness in the fusing process. The fuse design according to the invention leads to a neat breaking line in all cases. After the fuse breaks it is important to ensure that no future short circuit will take place across the fuse, which might occur particularly during anode effects in cases when a fuse does not burn out completely and with a neat crack. The clamped fuse is easily removed right after the cut-in. For the welded fuse, it is advantageous to insert a piece of wood immediately in between the two parts of the melted fuse to avoid any short circuit between them; later-on the fuses can be cut out.

In a specific embodiment of welded fuses 3 according to the invention, AA1370 alloy was used, and the plate thickness of the band-shaped body and the hole 6 diameter were chosen according to the desired melting time at a given current density. As an example, melting time was about 225 seconds at a current density of 25 000 kA/m², about 65 seconds at 28 170 kA/m², about 17.5 seconds at 41 670 kA/m² and about 6.5 seconds at 62500 kA/m². Whichever the embodiment of the fuse according to the invention (in particular whether welded or clamped), once the thickness of the band-shaped body and the dimensions (length and width) of the fuse 3 have been defined for a given potline or range of current densities, the fine tuning of the melting time or breaking time of the fuse 3 can be done most conveniently by varying the diameter of the hole 6.

Claims

1. Fuse (3) for bridging the gap (8) between an upstream cathode busbar (1) and a downstream cathode busbar (2) in an electrolysis plant,

said fuse (3) comprising a metallic band-shaped body comprising

o two contact surfaces (19a, 19b) for contacting respectively a surface of said upstream cathode busbar (1) and a surface of said downstream cathode busbar (2),

o two main sections (4a, 4b) respectively adjacent to said contact surfaces (19a,19b),

o a central section (5) adjacent to said main sections (4a, 4b),

wherein said central section (5) has a transverse cross section having a surface area that is smaller than the surface area of any transverse cross section of said main section (4).

2. Fuse (3) according to claim 1 , wherein said contact surfaces (19a, 19b), said main sections (4a, 4b) and said central (5) section are formed from a single metallic sheet or plate.

3. Fuse (3) according to any of claims 1 to 2, wherein said central section (5) comprises a hole or aperture (6) that decreases the surface area of the transverse cross section of said central section (5) compared to the surface area of the transverse cross section of the main sections (4a, 4b), while ensuring electrical continuity between the main sections (4a, 4b).

4. Fuse (3) according to claim 3, wherein the diameter of said hole (6) in the direction of the width of said band-shaped body is at least 8% of the width, and preferably comprised between 10% and 30% of the width, and most preferably between 15% and 25% of the width.

5. Fuse (3) according to any of claims 1 to 4, wherein the central section (5) is bent.

6. Fuse (3) according to any of claims 1 to 5, wherein said main sections (4a, 4b) and said central section (5) are bent along the length of said band-shaped body.

7. Fuse (3) according to any of claims 1 to 6, wherein said main sections (4a, 4b) and said central section (5) are bent along the length of said band-shaped body so as to present an arched or domed shape.

8. Fuse (3) according to any of claims 1 to 7, wherein said band-shaped body comprises

o an upper section (17) that has an arched or domed shape, and

o a lower section (18) comprising said contact surfaces (19a, 19b),

said central section (5) being located at the top of said upper section (17).

9. Fuse (3) according to any of claims 1 to 8, wherein said contact surfaces (19a, 19b) represent the lower surface of contact sections (32a, 32b) that are essentially flat and respectively folded under said main sections (4a, 4b).

10. Fuse (3) according to any of claims 1 to 9, wherein said contact surfaces (19a, 19b) represent the lower face of said main sections (4a, 4b).

1 1. Fuse (3) according to any of claims 1 to 4 or 10, wherein the band-shaped body is substantially flat.

12. Fuse assembly (16) comprising

o a clamp top section (21) and a clamp bottom section (26) linked together by a spanner (20), said clamp top section (21) and clamp bottom section (26) being configured to be clamped on an upstream cathode busbar (1) and a downstream cathode busbar (2) of an electrolysis plant, said busbars (1 ,2) being configured to let a gap (8) between each other,

o a fuse (3) according to any of claims 1 to 10 for bridging the gap (8) between said upstream (1) and said downstream (2) cathode busbar,

o an upper insulation piece (22) intended to electrically insulate said upstream and downstream cathode busbars (1 ,2) from the clamp top section (21), o a lower insulation piece (25) intended to electrically insulate said upstream and downstream cathode busbars (1 ,2) from said clamp bottom section (26), wherein said contact surfaces (19a, 19b) form the lower surface of contact sections (32a, 32b) that are intended to be respectively inserted between said upper insulation piece (22) and said upstream cathode busbar (1) and between said upper insulation piece (22) and said downstream cathode busbar (2).

13. Fuse assembly (16) according to claim 12, further comprising at least one insulation guide (23,24) located in the gap (8) between said upstream and downstream cathode busbars (1 ,2), wherein said spanner (20) goes through said insulation guide (23,24).

14. Fuse assembly (16) according to claim 12 or 13, comprising an upper insulation guide (23) and lower insulation guide (24) placed, respectively, below the fuse (3) and above the clamp bottom section (25), respectively in an upper section of said gap (8) between said upstream and downstream cathode busbar (1 ,2) and in a lower section of the gap (8) between said upstream and downstream cathode busbar (1 ,2).

15. Method for placing a fuse (3) on a gap between an upstream cathode busbar (1) and a downstream cathode busbar (2) in an electrolysis plant, comprising the steps of

o Providing a fuse (3) according to any of claims 1 to 1 1 or a fuse assembly (16) according to any of claims 12 to 16 comprising a fuse (3);

o Positioning the contact surfaces (19a, 19b) of the fuse (3) in contact with respectively said upstream cathode busbar (1) and said downstream cathode busbar (2);

o Securing said fuse (3) or fuse assembly (16) onto said busbars (1 ,2) by welding said fuse (3) onto said upstream and downstream busbars (1 ,2) or by clamping said fuse assembly (16) according to any of claims 12 to 16 onto said busbars (1 ,2).

16. Method according to claim 15, further comprising:

o Orienting said clamp bottom (26) and said lower insulation piece (25) of said fuse assembly (16) in such a way that they will go through the gap (8) between the upstream cathode busbar (1) and the downstream cathode busbar (2); o Placing said fuse assembly (16) into said gap (8);

o Rotating said clamp bottom (26) by about 90° to lock the fuse assembly (16), o Fastening said spanner (20).

17. Method to start up a pot in a series of at least two pots (10, 11) in an electrolysis plant, one (10) of the two pots being electrically downstream to the other (1 1), each pot having a cathode busbar assembly comprising an upstream cathode busbar (1) and a downstream cathode busbar (2), said upstream cathode busbar (1) and said downstream cathode busbar (2) being separated by a gap (8), said method comprising the steps of

(a) Inserting at least one electrically conductive wedge into said gap (8), such that current can flow from the upstream cathode busbar (1) to the downstream cathode busbar (2) across said wedge or wedges; (b) Installing at least one fuse (3) according to any of claims 1 to 6 across said gap (8), such that current can flow from the upstream cathode busbar (1) to the downstream cathode busbar (2) through the fuse (3);

(c) Withdrawing said wedge or wedges from said gap (8);

wherein said steps (a) and (b) can be carried in any order.

18. Method according to claim 17, wherein said fuse (3) is installed prior to withdrawing at least one wedge from said gap (8).

19. Method according to claim 17 or 18, wherein said fuse is installed prior to withdrawing the last wedge from said gap (8).

20. Method according to any of claims 15 to 19 wherein said electrolysis plant is an aluminium smelting plant using the Hall-Heroult process.