Technical field of the invention

The invention relates to the field of fused salt electrolysis and more precisely to the Hall- Heroult process for making aluminium by fused salt electrolysis. In particular, the invention relates to a particular design of the busbar system in an electrolysis plant in which electrolytic cells are arranged side-by-side, which facilitates the start-up of the plant as well as the total or partial shut-down of the plant.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (Al ₂ 0 ₃ ) is dissolved in molten cryolite (Na ₃ AIF ₆ ), and the resulting mixture (typically at a temperature comprised between 940 °C and 970 °C) acts as a liquid electrolyte in an electrolytic cell. An electrolytic cell (also called "pot") used for the Hall-Heroult process typically comprises a steel shell (so-called pot shell), a lining (comprising refractory bricks protecting said steel shell against heat, and cathode blocks usually made from graphite, anthracite or a mixture of both), and a plurality of anodes (usually made from carbon) that plunge into the liquid electrolyte contained in the volume defined by the cathode bottom and a side lining made from carbonaceous material. Anodes and cathodes are connected to external busbars. An electrical current is passed through the cell (typically at a voltage between 3.5 V and 5 V) which electrochemically reduces the aluminium oxide, split in the electrolyte into aluminium and oxygen ions, then into aluminium at the cathode and oxygen at the anode; said oxygen reacting with the carbon of the anode to form carbon dioxide. The resulting metallic aluminium is not miscible with the liquid electrolyte, has a higher density than the liquid electrolyte and will thus accumulate as a liquid metal pad on the cathode surface below the electrolyte from where it needs to be removed from time to time, usually by suction into a crucible.

Industrial electrolytic cells used for the Hall-Heroult process are generally rectangular in shape and connected electrically in series, the ends of the series being connected to the positive and negative poles of an electrical rectification and control substation. The general outline of these cells is known to a person skilled in the art and will not be repeated here in detail. They have a length usually comprised between 8 and 25 meters and a width usually comprised between 3 and 5 meters. The cells (also called "pots") are always operated in series of several tens (up to several hundreds) of pots (such a series being also called a "potline"); within each series DC currents flow from one cell to the neighbouring cell. For protection the cells are arranged in a building, with the cells arranged in rows either side-by-side, that is to say that the long side of each cell is perpendicular to the axis of the series, or end-to-end, that is to say that the long side of each cell is parallel to the axis of the series. It is customary to designate the sides for side- by-side cells (or ends for end-to end cells) of the cells by the terms "upstream" and "downstream" with reference to the orientation of current flow in the series. The current enters the upstream and exits downstream of the cell: busbars collect current from the cathodic parts of a cell and feed it to the anodic part of the next downstream cell connected in series. Busbars are usually made from aluminium or aluminium alloys. Being traversed by high current densities, busbars develop considerable heat under normal operation conditions due to Joule effect, and they redistribute heat through thermal conduction. The electrical currents in most modern electrolytic cells using the Hall-Heroult process exceed 200 kA and can reach 400 kA, 450 kA or even more; in these potlines the pots are arranged side by side. Most newly installed pots operate at a current comprised between about 350 kA and 600 kA, and more often in the order of 400 kA to 500 kA.

These enormous electrical DC currents flow through various conductors, such as electrolyte, liquid metal, anodes, cathode, connecting conductors, where they generate heat with ohmic voltage drops and where they generate significant magnetic fields. As mentioned above, electrolysis according to the Hall-Heroult process is a continuous process driven by the flow of electric current across the electrolyte, whereby said electric current reduces the aluminium atoms that are bounded in the alumina added to the molten electrolyte. Conditions of electrical equilibrium of the cell are attained when the current distribution is as uniform as possible throughout the electrolyte; the thickness of electrolyte between the anode and the liquid metal pad which acts as cathode (inter- electrode spacing or anode-cathode distance) in a typical Hall-Heroult cell is of the order of about two to five centimeters.

The electrical energy is a major operational cost in the Hall-Heroult process. Capital cost is an important issue, too. Ever since the invention of the process at the end of the 19 ^th century much effort has been undertaken to improve the energy efficiency (expressed in kW/h per kg or ton of aluminium), and there has also been a trend to increase the size of the pots and the current at which they are operated in order to increase the plant productivity and bring down the capital cost per unit mass of aluminium produced in the plant. Moreover, it is desirable to shorten the downtime of pots and potlines for maintenance, and to speed up their construction, from ground breaking until production at full capacity. Electrolytic pots operate continuously, the consumed alumina being replenished regularly several times per day, and the spent anodes being replaced regularly and individually (typically each 25 to 35 days). Eventually, in the absence of technical problems, they need to be shut down typically each five to eight years for relining, i.e. replacement of the cathode bottom and side lining which suffer from ageing. Ageing is mainly due to wear (erosion) of the cathode surface by the moving metal pad, and due to penetration of liquid metal into the carbon materials used for the cathode bottom and the side lining.

Besides the fact that electrolytic pots must be designed for an individual shut-down for maintenance, a smelting plant should also be prepared for partial or even total shutdown of the plant without endangering the electrolysis cells: such a partial or total shutdown can be planned or at least foreseeable (in case of strategic capacity reduction, power shortage, raw material shortage, labour conflicts) or totally unforeseeable (in case of severe technical incidents, natural disasters and the like, see for instance V. Buzunov et al., "The quick shut down and restarting of 291 kA pre-baked potline at JSC Rusal Sayanogork from May to August 2011" , Light Metals 2013, p. 647—652).

It should be reminded here that a series of pots can comprise several hundreds of pots. This raises several practical problems relating to the construction, the testing, the start-up and the maintenance of a potline, and to the possible shutdown of pots for occasional maintenance or for temporarily decreasing the production capacity of the plant. These practical problems are of different kinds. The construction of a new plant (so-called greenfield project) or the addition of a new potline to an existing plant (so-called brownfield project) takes several years, from the ground breaking ceremony until the first day of full operation of the potline; it implies heavy civil engineering and construction work, followed by the installation of the electrical supply system and rectifier substations, the installation of the pots, the construction of the cathode busbar system, the connection of the anode beams. Then the cathodes are put in place, and the pot is, in principle, ready to be connected and to be started up.

Starting up a potline means energizing its busbar system. Individual pots in a potline are usually started up successively or in small groups (typically several per day), as this is a critical operation requiring experienced workforce and specific supervision. Any pot that has not been started up, that requires heavy maintenance or that needs to be stopped for whatever reason has to be "taken out" (or "cut out") of the series: its needs to be bypassed. In general, 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. Figure 3b is a very simplified representation of the busbar system, and does not account for the equilibrium of the current flow, as care must be taken that cutting out a pot does not create perturbation to the equilibrium of current flow to the individual anodic risers of the downstream pot.

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 8th 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. Bypassing is usually achieved by inserting wedges between two parallel conductors of the busbar system, thereby creating a new conductive path between them. The principle of this procedure will be explained below in relation with figure X. The insertion of such wedges allows cutting out a pot of the potline without interrupting the current flow for the downstream pots.

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 system of 230 kA aluminum reduction cells" by Ortega et al., Light Metals 2007, p.333-337. The insertion of wedges is usually done manually. The withdrawal of wedges when cutting-in the pot is carried out either using lifting chains or slings with a crane, or preferably using mechanically operated wedge extraction systems, also called "wedge pullers". These are movable pieces of equipment that are commercially available; detailed descriptions can be found in patent documents such as US 2013/0098755, EP 2 585 624 and EP 2 585 625 (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. For this reason fuses are put in place prior to withdrawing the wedges. The principle of using start-up fuses is well known and has also been applied 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 documents the fuses were used across shunt blocks on anode risers. WO 2015/121 796 describes fuses that can be inserted in the air-gap between the conductors of the cathode busbar system. The need to use fuses further complicates the start-up procedure of an electrolytic cell that had been shut down or cut out for whatever reason. According to the state of art, 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 energized. 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 shut down, 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. For taking a pot out of the potline a plurality of wedges is needed, such as five or ten, depending on the busbar design. As a consequence, a smelting plant with three hundred pots would need to have more than a thousand wedges. This number of wedges will actually be needed when a newly built potline is put into service, because each single pot must be taken out of the series prior to its start-up: when no pots are operating, all pots must be taken out of the series prior to energizing the potline, each pot using the appropriate number of wedges. These wedges contribute to the capital cost of the plant. As an example, the applicant uses electrolysis cells operating at 450 kA (so-called DX+™ technology) which require twelve wedges per cell. For a potline comprising (as an example) 400 cells, this amounts to almost 5 000 wedges.

Wedges are also needed for testing the whole busbar, as well of individual parts thereof, system prior to its first energization.

Wedges are also needed when the operating production capacity of the plant is to be reduced. Indeed, it may be desirable to shut down part of the plurality of pots in a potline to avoid overproduction, or in case of shortage of raw materials or energy, or in case of severe labour conflicts, or for any other reason. According to the state of the art this can be done only by taking each single pot out of series using an appropriate number of wedges, and these wedges must then be kept in place as long as the pot is not energized. However, current will continue to flow around the inactivated pots, thereby creating ohmic losses. It would be desirable to have a potline design and operating procedures that allow to decrease the number of wedges and the frequency of their use at different stages of the life of a potline, from its initial construction to its terminal shut down. It would also be desirable to have a potline design that decreases ohmic losses in the vicinity of inactivated pots.

These problems are addressed, and eventually solved, by the present invention, as will be described below. Brief description of the figures

Figures 4 to 9 illustrate embodiments of the invention.

Figure 1 schematically shows a vertical cross section of a typical Hall-Heroult electrolysis cell. It illustrates the current flow from the anode through the electrolyte to the cathode. Figure 3b shows a vertical cross sections of three neighboring electrolytic Hall-Heroult cells connected in series, showing in particular the series connection of the cells that allow to feed the cathodic current of a cell into the anode frame of the neighboring downstream cell through anodic risers.

Figures 1 and 3a show schematic views of a Hall-Heroult cell.

Figure 4 schematically shows a simplified electrical diagram of a so-called "series" of pots or '"potline" according to the invention. It shows in particular the so-called Temporarily

Used Crossover Busbars (TUCBs) that are an essential feature of the present invention.

Figures 5 and 6 illustrate the use of these Temporarily Used Crossover Busbars.

Figure 7 schematically shows the shape of a Temporarily Used Crossover Busbar: figure

7a shows a lateral view, figure 7b a view from above.

Figure 8 shows a view similar to that of figures 4 to 6, in a somewhat different configuration.

Figure 9a and Figure 9b schematically shows a simplified electrical diagram of the four pots of a sector adjacent to a sector of a potline being energized according to the invention, with a particular emphasis on the busbars and wedges.

The following reference signs are used on the figures:

1 Electrolysis cell (pot) 18 Anode riser

2 Potshell 19 Superstructure carcass

3 Cathode blocks 20 Anode yoke

4 Side lining 21 Wedge

5 Liquid electrolyte 22 Temporarily used switch busbar

6 Liquid metal pad 23 Contact zone

7 Anode rod 24 Ends of the opened TUCB

8 Carbon anode 25 Welded conductor 9 Cathode collector bar C Electrolysis cell

10, Downstream (10) and Upstream (1 1) D1.D2 Main direction of current flow (also: 1 1 longitudinal part of cathode busbar potroom)

12 Cathode flexible L1.L2 Parallel lines of cell alignment

13 Cathode bus bar conductor S Sector

14 Alumina feeding system G Group of cells within a sector

15 Hood RS Rectifier substation

16 Anode beam TUCB Temporarily Used Crossover Busbar

17 Anode Clamp PUCB Permanently Used Crossover Busbar

100-108; 110-118; 120-128; 130-138; 140-148; 150-158; 160-168 : Conductors

200-203; 210-213; 220-223 : Temporarily Used Crossover Busbar

Objects of the invention

According to the invention, the problem is solved by an electrolysis plant, comprising a power supply and a plurality of electrolytic cells arranged along a first and a second line that are preferably parallel and straight,

said cells being connected in series by means of conductors, the current collected at the cathode of a cell being fed into the anode of the neighbouring downstream cell, said power supply and said plurality of electrolytic cells forming together an electric loop, in which the current can flow from said power supply successively through the cells arranged in series along said first line, through the cells arranged in series along said second line, and then back to said power supply,

wherein

said series of electrolytic cells is arranged in at least two successive sectors, - each sector comprises a first group of electrolytic cells arranged along said first line and a second group of electrolytic cells arranged along said second line, the current flows from said power supply successively through the cells of the first groups of cells of the first sector, then through the first group of cells of the second sector (and possibly any further sector, until the last sector), is then, when leaving the last sector, derived by one or more conductors (so-called permanently used crossover busbars) and passes then through the second group of cells of the last sector into the second group of cells of the second last sector (and possibly into the second group of cells of any further sector, until the first sector), and enters eventually, when leaving the second group of the first sector, the power supply to complete said electric loop,

said electrolysis plant being characterized in that it comprises between two successive sectors namely a first sector and a second sector, one or more conductors (so-called temporarily used crossover busbars) capable of creating an electrical connection between the last cell of the first group of said first sector and the first cell of the second group of said first sector.

In an advantageous embodiment, said temporarily used crossover busbars are capable of reversibly creating an electrical connection between the last cell of the first group of said first sector and the first cell of the second group of said first sector.

The number of sector can be greater than two, namely three, four, or more than four. The number of cells in each sector can be the same or different, and can be comprised between several tens and more than one hundred, and advantageously between 50 and 250.

Advantageously, said temporarily used crossover bars are capable of creating an electrical connection and of disconnecting said electrical connection in a reversible manner. This facilitates a partial or total shutdown of the plant. To this end, in an advantageous embodiment said temporarily used crossover busbars comprise a temporarily used switch busbar that can be taken out in order to disconnect said electrical connection; when connected, said temporarily used switch busbar is advantageously bolted to the two end sections of the remaining temporarily used crossover busbar. In addition, welded connectors (flexible or not) can be used. Such an electrolysis plant according to the invention can comprise in particular three sectors, or four sectors, or even more than that.

Each sector typically comprises a number of cells comprised between ten and three hundred, and advantageously between 50 and 250. Another object of the present invention is a process for starting up an electrolysis plant according to the invention wherein successively:

(i) the temporarily used crossover busbars between the first sector and adjacent second sector are closed (connected),

(ii) the cells of the first sector are started (energized),

(iii) if said electrolysis plant comprises a third sector adjacent to the second sector, then the temporarily used crossover busbars between the second sector and the third sector are closed,

(iv) said temporarily used crossover busbars between the first sector and adjacent second sector are opened (disconnected),

(v) the cells of the second sector are started,

(vi) if said electrolysis plant comprises further sectors, then steps (i) to (v) are repeated for each subsequent sector adjacent to a sector that has been started.

In an embodiment of this process, prior to starting the electrolysis plant, the busbar system of each sector is tested for at least one of the following: electrical continuity, wedge voltage drops, earth leakage, voltage drops at cathode busbar flexibles, voltage drops across the temporarily used crossover busbars, temperatures and thermal expansions.

Another object of the invention is a process for at least partially shutting down an electrolysis plant according to the invention, comprising the following successive steps:

(i) Shutting down all cells of the last sector of the potline, said step including inserting wedges in the cathode busbar system of each individual cell,

(ii) Connecting the temporarily used crossover busbar between said shut down sector and its adjacent sector.

In a variant, if said adjacent sector is to be shut down, too, this process comprises a third step:

(iii) repeating steps (i) and (ii) for the still operating sector adjacent to the sector that has been shut down by carrying out steps (i) and (ii).

Still another object of the present invention is a process for producing aluminium from alumina, using an electrolysis plant according to any of the embodiments and variants of the present invention. Detailed description and advantageous embodiments

The present invention relates to the Hall-Heroult electrolysis process, which is carried out in electrolysis cells called "pots" of substantial rectangular shape. The Hall-Heroult process as such, the general structure of a Hall-Heroult electrolysis pot, the way to operate the latter, as well as the cell arrangement are known to a person skilled in the art and will not be described here in more detail. In the present description, the terms "upper" and "lower" refer to mechanical elements in use, with respect to a horizontal ground surface. Moreover, unless otherwise specifically mentioned, "conductive" means "electrically conductive". As the Hall-Heroult process is known as such, it is sufficient to explain (as done here in particular in relation with Figures 1 and 3), that the pot 1 typically comprises a potshell 2 usually made from steel, and a lining comprising a carbonaceous cathode formed from individual, parallel cathode blocks 3 and a side lining 4, said lining defining a volume for the liquid electrolyte 5 and the liquid metal pad 6 produced by the electrolysis. Current is fed into the anode busbar 16 (also called "anode beam"), flows from the anode beam 16 to the anode rod 7 (said anode rod 7 being connected to the carbon anode 8 by means of anode yoke 20) and to the carbon anode 8 in contact with the liquid electrolyte 5 where the electrolytic reaction takes place, crosses the liquid metal pad 6 resulting from the electrolysis process and eventually will be collected at the cathode block 3. As cathode blocks are symmetric and have collector bar 9 ends coming out on each side, in side by side arrangements of electrolytic cells half of the current collected by the collector bars 9 of the cathode blocks 3 will flow directly to the downstream longitudinal part 10 of the cathode busbar system, while the other half flows to the upstream longitudinal part 11. Flexible connectors 12 are used to connect the ends of the cathode collector bars 9 to the cathode busbar 10, 11. Said collector bars 9 can be full bars, as in figure 1 , or half bars 9a, 9b, as in figure 3b. Conductor 13 carries the current collected at the upstream part 11 of the cathodic busbar system to the anode risers 18 of the downstream pot. The current collected at the downstream part 10 of the cathodic busbar system directly feeds the anode risers 18. A Hall-Heroult cell further comprises an alumina feeding system 14 (usually located inside the carcass of the superstructure 19) through which alumina powder is dumped from time to time into the cell volume (see arrow on figure 3b). The air space above the cell is closed by a set of covers or hoods 15 that can be removed for maintenance and anode change; the anode rods 7 are adjustably fixed to the anode beam 16 using anode clamps 17 that allow to adjust the anode heights in order to keep the inter- electrode spacing constant as the anode is consumed.

The present invention is directed to the global arrangement of a plant, or aluminium smelter, using the Hall-Heroult process. As schematically shown on Figure 2, an aluminium smelter comprises a plurality of electrolytic cells Ci, C ₂ , ... , C _n- i, C _n , arranged the one behind the other (side by side or end to end) along two parallel lines L1 and L2, each of which comprises n/2, i.e. m cells. These cells are electrically connected in series by means of conductors, which are not shown on Figure 1. The number of cells in a series is typically comprised between 50 and over 500. The electrolysis current therefore passes from one cell to the next, along arrow DC. The cells are arranged transversally in reference of main direction D1 or D2 (axis of the row) of the line L1 or L2. In other words the main dimension, or length, of each cell is substantially orthogonal to the main direction of a respective line, i.e. the circulation direction of current. Figure 2 depicts a typical "clockwise" current orientation; the present invention applies also to counter-clockwise arrangements. This arrangement of n cells connected in series along two parallel lines is called a "series" of pots or "potline". The cells depicted on figure 2 are arranged side-by- side; end-to-end arrangements are generally not used in newly built plant, but the present invention could be applied to end-to-end arrangements, too.

As can be seen on Figures 3a and 3b, anode risers 18 are provided to carry the current collected at the downstream part 10 and at the upstream part 1 1 of the cathode busbar system of the upstream pot (noted here C _n- i) to the anode beam 16 of the neighbouring downstream cell (noted here C _n ).

Figure 4 schematically shows a simplified electrical diagram of a so-called "series" of pots or '"potline" according to the invention. The electrolytic cells are arranged side-by-side in two parallel lines L1 , L2. The cells are denoted by the letter C, as in figure 2. According to the invention they are grouped into at least two sectors S, as will be explained below. Fiqure 4 shows four such sectors Si, SII, Sin, Siv- Each sector can comprise an equal number or a different number of pots. The grouping into sectors is according to the order of the pots in the potline: the first sector Si is formed by the first C|,i, C|, ₂ , ... and last ... C|, _n- i, C|, _n pots in the potline (only four pots per sector are shown on figure 4); while the last sector (S _!V on figure 4) is formed by pots which follow each other in the potline. In the example of figure 4, when the potline is in normal operation, electrical current is supplied from a rectifier substation RS to the first pot C|,i of the first sector Si of the first potroom D1 of the potline. This current flows through the conductors of pots C|,i, C|, ₂ of the first sector Si and flows then through the conductors of pots C _M ,i, C _M , ₂ of the second sector S _M , and flows then through the conductors of pots C| _M ,i, C| _M , ₂ of the third sector S _m , and flows then through the conductors of pots C _!V ,i, C _!V , ₂ of the last sector S _!V .

As mentioned in relation with figure 1 , the current crosses a pot by entering from the anode busbar (anode beam), to which it is fed, through the anode rod 7 (shown on figure 1) into the anode 8, crosses the molten electrolyte 5, where alumina is reduced into aluminium and oxygen, crosses the underlying molten aluminium pad 6 and enters into the cathode 3 where it is collected by the cathode collector bar 9 and carried to the cathode busbar system 10,11. From there it is fed into the anode risers of the neighbouring downstream cell (not shown in figure 1). The conductors through which the current is fed from the cathode busbar of the upstream pot into the anode busbar of the downstream pot are referenced in figure 4 with the reference numbers 100 to 108, 110 to 118, 120 to 128, 130 to 138, 140 to 148, 150 to 158, and 160 to 168. They comprise so- called anodic risers 18 (shown on figure 3). In the example of figure 4 four such conductors are schematically represented; their number is of no importance in relation with the present invention. When leaving the last pot C _!V ,2 of the last sector S _N of the first potroom, the current is collected by a busbar system called here "Permanently Used Crossover Busbar" PUCB 164. The PUCB 164 conducts the current back to the second potroom D2 where it enters the last sector through the first pot Civ,n-i. Then the current flows from the cathode busbars of pot C| _V ,n-i to the anode risers of pot C| _V , _n , and flows then though the conductors of pots CiM,n-i Cin,n-i of the second potroom sector S _m and so on, and eventually leaves the potline by the last pot C|, _n of the first sector of the second potroom Si where it is collected by the rectifier substation RS. According to the invention each sector (except the last one) is defined by the presence of at least one downstream busbar system called here "Temporarily Used Crossover Busbars" TUCB. These are conductors located between two adjacent sectors and connecting the conductors of line L1 (potroom D1) and line L2 (potroom D2), thereby forming a low resistance electrical conduction path between the current output of the first half of cells in potroom D1 and the current input of the second half of cells in potroom D2. According to an essential feature of the invention, said connection achieved by said Temporarily Used Crossover Busbars is a reversible connection, and said Temporarily Used Crossover Busbars include appropriate means that allow either to close the circuit (thereby forming said low resistance path between the current output of the first half of cells of a sector and the current input of the second half of cells of said sector) or to open the circuit.

In particular, closing the TUCB allow to separate adjacent sectors. As an example, said TUCB allow to separate the first sector from the second sector, and (if present) the second sector from the third sector and so on.

In the embodiment of figure 4 the TUCB that separate the first sector Si from the second sector SII are referenced by the reference numbers 200 to 203, those that separate the second sector S _M from the third sector S _m are referenced by the reference numbers 210 to 213, and those that separate the third sector Sm from the fourth sector S| _V by the reference numbers 220 to 223. The number of pots of each sector can be identical or different. Advantageously each sector comprises at least 20, preferably at least 30 pots; it can comprise more than 100 pots.

Figure 5 shows the same simplified electrical diagram as figure 4, but according to a specific embodiment of the invention the first Temporarily Used Crossover Busbars TUCBi are used here to separate the second sector S _M from the first sector S|. To this end, the Temporarily Used Crossover Busbars TUCBi are used to create a shunt between the conductors 110-113 that connect the last cell C|, ₂ of the first half of cells of first sector Si to the first cell C _M ,i of the first half of cells of second sector S _M , and the conductors 115-118 that connect the last cell C _M , _n of the second half of cells of second sector S _M to the first cell C|, _n- i of the second half of cells of first sector Si . More precisely, as shown on figure 5, conductor 200 creates a shunt between conductors 110 and 115, and conductor 201 creates a shunt between conductors 111 and 116, and conductor 202 creates a shunt between conductors 112 and 117, and conductor 203 creates a shunt between conductors 113 and 118. It is well within the scope of the invention to apply this principle to any number of conductors within group 110-113, group 115-118 and group 200-203, the number of conductors within each of these three groups being preferably the same. Separating the second sector S _M from the first sector Si has the consequence that the first sector Si remains connected to the rectifier substation RS and can operate normally, whereas the second sector S _M and any subsequent sectors (in the example of figure 4 these are the third sector S _m and the fourth sector S| _V ) are separated (disconnected) from the rectifier substation RS and do not operate.

In general, according to the invention, wedges 21 only need to short-circuit pots located in sectors connected to the rectifier substation, inside the loop of current flowing from the rectifier substation RS to the energized TUCB and back to the rectifier substation RS. Before preparing to energize a following sector, the wedges 21 of pots included in the new loop of current must be inserted. Then the wedges are removed gradually when starting up each pot; these start-up procedures of individual pots are well known to a person skilled in the art and will not be described here.

According to prior art, the same result could be obtained by cutting out each individual cell of each of the second and any subsequent sector using wedges; this implies the use of a large number of wedges that need to be inserted (and then pulled out) manually by a specifically qualified workforce using specific equipment (usually a mobile wedge-puller).

It should be noted that the function of wedges 21 in the electrical circuit of a potline will be explained later in relation with figures 9a and 9b, in a peculiar context.

Figure 6 shows the same simplified electrical diagram as figure 5, but according to another specific embodiment of the invention the second Temporarily Used Crossover Busbars TUCB _M are used here to separate the second sector S _M from the third sector Sm. To this end, the Temporarily Used Crossover Busbars TUCB _M are used to create a shunt between the conductors 130-133 that connect the last cell C _M , ₂ of the first half of cells of the second sector S _M to the first cell C| _M ,i of the first half of cells of third sector Sm, and the conductors 135-138 that connect the last cell Cin, _n of the second half of cells of the third sector SIM to the first cell Cn, _n- i of the second half of cells of second sector S _M . Conductor 210 creates a shunt between conductors 130 and 135, and conductor 211 creates a shunt between conductors 131 and 136, and conductor 212 creates a shunt between conductors 132 and 137, and conductor 213 creates a shunt between conductors 133 and 138. It is well within the scope of the invention to apply this principle to any number of conductors within group 110-118, group 130-138 and group 150-158, the number of conductors within each of these three groups being preferably the same.

Separating the third sector S _m from the second sector S _M has the consequence that the first sector Si and the second sector S _M remain connected to the rectifier substation RS and can operate normally, whereas the third sector Sm and any subsequent sectors (in the example of figure 4 this is the fourth sector S| _V ) are separated from the rectifier substation RS and do not operate. According to prior art, the same result could be obtained by cutting out each individual cell of each of the third and any subsequent sector using wedges; this implies the use of a large number of wedges that need to be inserted manually by a specifically qualified workforce.

The plant design according to the invention makes it possible to cut-out one or more sections of pots, starting at the last section, i.e. the one next to the PUCB. This may serve several different purposes: to temporarily decrease the capacity of the plant (most frequently for commercial reasons or in case of power shortage or raw material shortage or labour conflicts) by cutting out one or more section, to allow maintenance or refurbishment of the cells or their busbar system in the cut-out section(s), or to shut down the whole plant (for instance prior to its demolition) section by section while continuing to produce metal in the sections that are still connected.

The plant design according to the invention makes it possible to start a plant section by section, the section next to the rectifier substation RS being started first, then the second section and so on. This is a particularly interesting use of the present invention, as it allows to start the use of the first sector(s) while the subsequent ones are being built or completed.

According to an advantageous embodiment of the invention, said temporarily used crossover busbars TUCB are capable of reversibly creating an electrical connection between the last cell of the first group G1 of a sector and the first cell of the second group G2 of said sector. Said reversibility can be achieved by providing bolted temporarily used switch busbars 22 that close the gap between a first branch of the TUCB extending from line L1 towards line L2, and a second branch extending from line L2 towards line LlThis is shown schematically on figure 7a. Welded strips 25a and 25b or flexibles can be used in addition to bolts. This is shown schematically on figure 7b (the depicted TUCB is arbitrarily labelled 200, knowing that all TUCBSs 200,201 ,202,203 should be identical). The contact zones 23 between the switch busbar 22 and the TUCB should be specifically prepared, by polishing for instance, in order to minimize contact resistances.

The gap G bridged by the temporarily used switch busbar 22 should not be too small for safety reasons, and when disconnected the ends 24a and 24b of the opened TUCB remaining in place should be protected by an appropriate insulating cover (such as a cover made from wood panels) in order to avoid any accidental bridging of the gap G by tools, workpieces or vehicles. If the temporarily used switch bars 22 are too long they will be too heavy for easy handling. The temporarily used switch busbar 22 can be stored on a trolley of appropriate height in order to facilitate its installation and uninstallation. It is advantageous to decrease the current when removing the strips 25a and 25b between busbar 22 and 24a and 22 and 24b which were welded to improve the contacts between these busbars. Then the potline current must be temporarily taken off in order to disconnect and uninstall the temporarily used switch busbar 22 from 24a and 24b. The same method must be applied when installing this temporarily used switch busbar 22. In another advantageous embodiment of the invention the permanently used crossover busbars PUCB are of similar construction as the temporarily used crossover busbars TUCB.

We will describe here now a method to start up an electrolysis plant according to the invention, in relation with figures 4 to 9; this plant has two parallel lines L1 , L2 of pots C grouped in four sectors S. The person skilled in the art will have no difficulty to adapt this method to a plant with less than four sectors (at least two sectors) or more than four sectors.

We suppose here that this is the first start-up of the plant, which means that normally the construction of sector Si, including all the pots C|, has been completed.

In a first step each pot C| of sector Si is bypassed by inserting wedges; this is known as such.

In a second step the TUCBi between sector Si and sector S _M are connected. In the example of figure 5, conductor 200 creates a shunt between conductors 110 and 115, and conductor 201 creates a shunt between conductors 111 and 116, and conductor 202 creates a shunt between conductors 112 and 117, and conductor 203 creates a shunt between conductors 113 and 118. This is the situation shown on figure 5. At this stage no current is flowing in the potline circuit.

Before energizing Sector Si busbar circuit, it is necessary to make sure that Sector Si busbar circuit is totally electrically insulated from Sector S _M . This is achieved by not having wedges installed and having anode risers 18 disconnected and insulated (using wooden strips or other insulation material) in the pots of Sector S _M adjacent to Sector Si, at least in the first pot (C _M , i) and in the last pot (C _{M> n} ) of Sector S _M in both lines L1 and L2, but preferably in more than one pot of each line.

Also before energizing Sector Si with the potline current, it is advantageous but not compulsory to carry out a continuity test through the busbar circuit of Sector Si. This is done using special equipment allowing to inject from and to the electrical rectifier substation RS, a low electrical current, e.g. below 1000 amperes and measuring the voltage and resistance of across the busbar circuit to verify the integrity and continuity of the busbar circuit before injecting the potline current into the potline busbar circuit.

In a third step, the potline current is fed to the potline circuit of sector Si and the pots C| of sector Si can be started in a manner known as such, usually one after the other, which requires energizing the pots C| by withdrawing the wedges from the pots C| to start the preheating process before pot start-up usually one after the other.

At this stage, sector S _M and subsequent sectors (if planned) can still be under construction. This is one of the advantages of the invention: it facilitates a progressive start-up of the plant as the construction work is proceeding. When the construction of sector SII, including all the pots C _M , has been achieved, the process can go on with step four.

In a fourth step, each pot C _M of sector S _M is bypassed by inserting wedges. These wedges can be those previously used for the pots of sector Si. This is one of the advantages of the invention: it greatly reduces the total number of wedges that are necessary for the plant.

Before energizing Sector S _M busbar circuit, it is necessary to make sure that Sector S _M busbar circuit is totally electrically insulated from Sector S _m . This is achieved by not having wedges installed and having anode risers 18 disconnected and insulated (using wooden strips or other insulation material) in the pots of Sector Sm adjacent to Sector S _M , at least in the first pot (Cm, i) and in the last pot (C _m , _n ) of Sector Sm in both lines L1 and L2, but preferably in more than one pot of each line. Before energizing Sector S _M with the potline current, the continuity and integrity of the busbar circuit in Sector S _M is verified by installing TUCB _M (fifth step) before uninstalling TUCBi (sixth step) which allows the potline current to flow in parallel through TUCBi and TUCBi,.

In a fifth step the TUCB _M between sector S _M and sector S _m are connected. In the example of figures 4 to 6, conductor 210 creates a shunt between conductors 130 and 135, and conductor 211 creates a shunt between conductors 131 and 136, and conductor 212 creates a shunt between conductors 132 and 137, and conductor 213 creates a shunt between conductors 133 and 138. If there is no sector Sm being installed or planned the TUCBII can be a PUCB ; the present invention requires at least two sectors of pots.

In a sixth step the TUCBi between sector Si and sector S _M are disconnected. This is the situation shown on figure 6.

In a seventh step the pots C _M of sector S _M are started, usually one after the other, which requires to withdraw the wedges.

At this stage, sector Sm and subsequent sectors (if planned) can still be under construction. When the construction of sector Sm, including all the pots C _m , has been completed, the process can go on with step eight.

In an eighth step each pot C _m of sector Sm is bypassed by inserting wedges.

Before energizing Sector Sm busbar circuit, it is necessary to make sure that Sector Sm busbar circuit is totally electrically insulated from Sector S| _V . This is achieved by not having wedges installed and having anode risers 18 disconnected and insulated (using wooden strips or other insulation material) in the pots of Sector S _!V adjacent to Sector Sm, at least in the first pot (C _!V , i) and in the last pot (C _!V , _n ) of Sector S _!V in both lines L1 and L2, but preferably in more than one pot of each line.

Before energizing Sector Sm with the potline current, the continuity and integrity of the busbar circuit in Sector Sm is verified by installing TUCBm (ninth step) before uninstalling TUCBII (tenth step) which allows the potline current to flow in parallel through TUCB _M and TUCBm.

In a ninth step the TUCB _m between sector S _m and sector S _!V are connected. In the example of figures 4 to 6, conductor 220 creates a shunt between conductors 150 and 155, and conductor 221 creates a shunt between conductors 151 and 156, and conductor 222 creates a shunt between conductors 152 and 157, and conductor 223 creates a shunt between conductors 153 and 158. If there is no sector S| _V the TUCB| _M can be a PUCB.

In a tenth step the TUCB _M between sector S _M and sector S _m are disconnected. This is the situation shown on figure 8.

In a eleventh step the pots Cm of sector Sm are started, usually one after the other, which requires to withdraw the wedges. At this stage, sector S| _V and subsequent sectors (if planned) can still be under construction. When the construction of sector S| _V , including all the pots C _!V , has been completed, the process can go on with step twelve.

Before energizing Sector S _!V with the potline current, the continuity and integrity of the busbar circuit in Sector S _!V is verified by installing PUCB (twelfth step) before uninstalling TUCBIII (thirteenth step) which allows the potline current to flow in parallel through TUCBm and PUCB.

In a twelfth step the PUCB after sector S _!V are connected. This is the situation shown on figure 4. If there are subsequent sectors planned, a TUCB _!V (not shown on the figures) can be connected instead.

In a thirteenth step the TUCB _m between sector Sm and sector S _!V are disconnected. In a Fourteenth step the pots C _!V of sector S _!V are started, usually one after the other, which requires to withdraw the wedges. The whole plant is now in operation. This is the situation shown on figure 4.

We will now describe a method to shut down an electrolysis plant, such as the plant schematically shown on figure 4.

In a first step all the pots C _!V of sector S _!V are shut down one after the other, in a manner known as such, and each pot is bypassed by inserting wedges in order to disconnect it from the power lines. When all pots pots C _!V of sector S _!V have been cut-out with wedges, in a second step the TUCB _m between sector S _!V and sector Sm are connected, as described above in relation with the start-up procedure. Sector S _!V is now disconnected; the wedges used for cutting out the pots can be withdrawn if they are needed elsewhere in the plant. It may be desirable to shut down just the last sector of the plant (for example as a reaction to power shortage or raw material shortage or labour conflicts or sluggish demand for aluminium). If it is planned to shut down also sector S _m then the process can go on with step three.

In a third step all the pots Cm of sector Sm are shut down one after the other, and each pot is bypassed by inserting wedges. When all pots C _m of sector Sm have been cut-out with wedges, in a fourth step the TUCB _M between sector S _M and sector Sm are connected, as described above in relation with the start-up procedure.

If the next sector S _M is to be shut down, in a fifth step all the pots C _M of sector S _M are shut down one after the other, and each pot is bypassed by inserting wedges. When all pots C _M of sector S _M have been cut out with wedges, in a fifth step the TUCBi between sector Si and sector S _M are connected, as described above in relation with the start-up procedure. Sector S _M is now disconnected.

If now the first sector Si is to be shut down, in a sixth step all the pots C| of sector Si are shut down one after the other, and each pot is bypassed by inserting wedges. When all pots C| of sector Si have been cut out with wedges, in a seventh step the potline can be disconnected from the rectifier station RS, or the rectifier station RS can be shut down.

As mentioned above, this method can be adapted to shut down only the last sector (in the present example S| _V ), or the two last sectors (in the present example S _!V and Sm) or the three last sectors of the electrolysis plant, as may be desirable for whatever reason.

Another use of the invention is related to testing of the electrical conductor system of a plant prior to its start-up, or to testing of the electrical conductor system of an added sector prior to its start-up).

As a rule, it is desirable to test the electrical conductor system of a plan prior to energizing it. This allows to identify potential problems that would be difficult or impossible to resolve once the potline or plant is running at full capacity. Such problems can be related to defective electrical continuity in the electrical conductor system (and in particular at connection points), to unwanted electrical resistances that may lead to hot spots, to accidental short-circuits, or to insufficient or defective fixation of conductors that may lead to their displacement under the influence of thermal expansion or induced magnetic fields.

Before energizing the potline or its sector, tests are carried out on the first sector and on each subsequently added sector. Indeed, the busbar system according to the invention allows to subdivide the potline into sections that can be tested separately prior to starting up the potline. A procedure for testing a potline will be presented below. The procedure is carried out sector by sector, i.e. for a plant with four sectors as in figures 4 to 6, successively on sectors Si, S _M , S _m , and S| _V , and the procedure of adding a sector is similar to the start-up procedure presented above, except that the pots are not actually started.

To this end, all cut out wedges are installed on each pot of the circuit to be tested (with the exception of the pots adjacent to connected TUCB or PUCB, as will be explained below), and all cathode collector bars are connected to the cathode busbar system. In an embodiment of the invention, the circuit is a sector as defined above.

As an example, Figure 9a schematically shows the first four pots of the first half of sector SII in potroom L1 in a configuration for testing sector S|. Figure 9b schematically shows the last four pots of the second half of sector S _M in potroom L2 in a configuration for testing sector Si.The direction D1 and D2 of the current flow is indicated by an horizontal arrow in Figures 9a and 9b. First sector Si will be energized. Each of the n cells of sector Si is short-circuited using wedges 21. Depending on the progression of the construction in sector SII, the cells of sector S _N can be short-circuited using wedges, except at least the first cell C _M ,i of the first half of cells of sector S _M and the last cell C _M , _n of the second half of cells of sector S _M that are adjacent to the TUCBi; this will be explained below. In the example of figure 9a and 9b eight wedges 21 are used for each short-circuited cell; this number can be different for different types of cells.

It can be seen that the busbar configuration of figures 9a and 9b correspond to that of figure 5, except for two essential features: the cells of sector Si are short-circuited by wedges 21 , and in the busbar system of the first cell C _M ,i of the first half of cells of sector SM that is adjacent to the TUCBi and in the busbar system of the last cell C _M , _N of the second half of cells of sector S _M that is also adjacent to the TUCBi, the (positive) anode risers are disconnected from the cathode busbars of their adjacent upstream cell: the anode risers are physically interrupted, having downstream parts 18b (welded to the anode beam 16) that are disconnected from the respective upstream parts 18a (connected to the cathode busbar of the upstream adjacent cell). The TUCBs 200,201 ,202,203 connecting the first half of pots of sector Si to the second half of pots of sector Si (shown on the figures) are installed, that is to say: connected, so that the sector Si (not shown on figures 9a and 9b) to be energized is totally separated (electrically insulated) the from next sector S _M . According to the invention, prior to testing a sector of the potline, four conditions must be fulfilled regarding the busbar structure:

(i) At least the first pot (and preferably the two first pots, and still more preferably the three first pots) of the first half of pots of the next sector, and at least the last pot (and preferably the two last pots, and still more preferably the three last pots) of the second half of pots of the next sector, must have all anode riser disconnected from the upstream cathode busbar system, and

(ii) At least the first pot (and preferably the two first pots, and still more preferably the three first pots) of the first half of pots of the next sector, and the last pot (and preferably the two last pots, and still more preferably the three last pots) of the second half of the next sector must have no wedges, and

(iii) All pots of the sector to be tested must have wedges, and

(iv) The downstream TUCB adjacent to the sector to be tested must be closed (i.e. connected).

The tests are carried out in as many stages as there are sectors, each stage comprising one or more steps.

In a first stage discrete steps of increasing amperage are applied to the first sector Si (i.e. the sector adjacent to the rectifier substation RS).

In an exemplary embodiment of an electrolysis plant designed to operate at a nominal current of 450 kA, a first step is about 50 kA, a second step is about 100 kA, a third step is of about 200 kA, a fourth step is about 450 kA, and possibly there is a fifth step at a maximum design amperage of about 500 kA.

The fourth step (and possibly also the third one) should be chosen in accordance with the maximum amperage: the values given here as examples are adequate for a maximum amperage of about 460 kA to 500 kA, knowing that it is advantageous to carry out testing at a current somewhat higher than the nominal operating current.

The first step comprises at least visual inspection (presence of sparking or arcing, preferably from the potroom floor and the basement) and pot voltage measurement.

The second step comprises electrical measurements that are preferably carried out from the potroom floor and from the basement; these measurements include one or more of the following:

earth leakage of potroom floors, concrete columns, pot control panels, air pipes, fixture brackets;

voltage drops at each of the wedges;

voltage drop on the cathode collector bar flexibles;

voltage drop across joints and flexes of the TUCB. In addition, thermal and mechanical measurements as well as visual inspection are carried out when the busbars have reached thermal equilibrium. These thermal and mechanical measurements (whereby the thermal measurement typically use contacts probes and/or infrared probes, as appropriate) and inspections comprise one or more of the following:

temperature of busbar head joints and flexes;

busbar thermal expansion by measuring the thermal displacement from previously marked areas at the busbar supports;

- visual inspection of all cell-to-cell busbar supports in order to detect the supports that may have been damaged due to thermal expansion of the busbar.

Tests at the third step and further steps comprise the same tests as in the first step.

In a variant of this testing process of sector Si, before energization of the first sector close to the rectifier substation (i.e. before the first step), it is possible to inject a small current in the circuit to make a simple continuity test without taking any risk.

Stage 2 and the following stages are different from stage 1 but all similar: the risk is reduced by having first the two TUCBs (or at the last stage the last TUCB and the PUCB) in parallel circuit before removal of the first TUCB.

In a first step during stage 2, short circuit test the continuity of the busbar electrical circuit of Sector S _M is verified by connecting TUCB _M , therefore energizing the Sector S _M before disconnecting the TUCBi between line L1 and line L2; in this situation the current flows through the TUCBi and in parallel through Sector S _M and TUCB _M , allowing to detect defects in the busbars of Sector S _M .

The voltage drops of the TUCB located between the operating sector and the tested sector as well as the voltage drops of the TUCB located after the tested sector and operating in parallel during the test (in the example: TUCBi and TUCB _M if Sector S _M is tested) is measured.

Although Sector Si operates normally during this stage 2 test, the amperage through TUCBi is reduced since part of the current is flowing through Sector S _M and TUCB _M .

In a second step during stage 2, the voltage drop across the TUCB located between tested sector and the next sector (can be a PUCB if the tested section is the last one) (in the example: TUCB _M if section S _M is tested) is measured after opening (disconnecting) the TUCB located between operating sector and the tested sector (in the example: TUCBi). The connection and disconnection of TUCBs requires a short period of power reduction and power outage in order to safely carry out the connections and disconnections.

The second step of stage 2 comprises electrical measurements that are preferably carried out from the potroom floor and from the basement; these measurements include one or more of the following:

earth leakage of potroom floors, concrete columns, pot control panels, air pipes, fixture brackets;

voltage drops at each of the wedges;

- voltage drop on the cathode collector bar flexibles;

voltage drop across joints and flexes of the TUCBs and PUCB,

In addition, thermal and mechanical measurements as well as visual inspection are carried out when the busbars have reached thermal equilibrium. These thermal and mechanical measurements (whereby the thermal measurement typically use contacts probes and/or infrared probes, as appropriate) and inspections comprise one or more of the following:

temperature of busbar head joints and flexes;

busbar thermal expansion by measuring the thermal displacement from previously marked areas at the busbar supports;

visual inspection of all cell-to-cell busbar supports in order to detect the supports that may have been damaged due to thermal expansion of the busbar.

As a rule during all these tests, once the pots are in the current loop but not yet started they need to have the wedges and wedges will only be removed as and when starting each individual pot.

We will now summarize the numerous advantages of the invention.

The invention allows to decrease the number of wedges to be available in the plant; this saving will be the higher the more sectors there are. As an example, for a potline with four sectors each comprising the same number of cells, the invention allows to divide the number of required wedges by four.

The invention allows to separate the part of potline that is ready for operation from the rest of the potline that is still under construction, thereby permitting early production which is important for the profitability of the project. The temporarily used crossover busbars are used during the start-up phase of the plant, but can stay in place (after disconnection) and can be used later if required for various reasons, such as any kind of emergency situation (labour conflict, serious damage to the busbar system in one or more pots, prolonged power outage, prolonged disruption in raw material supply) or any kind of strategic decision (partial shutdown of the plant to reduce its capacity for economic reasons, complete shutdown of the plant for whatever reason, restart of the plant after its partial or complete shutdown).

The invention allows to save energy during the start-up phase of the plant by having the current flow only through a limited number of wedged pots (that is to say: the pots belonging to the same sector), instead of having all pots of the potline wedged, knowing that the voltage drop through a wedged pot is typically of the order of 0.15 to 0.30 V, depending upon the busbar design and the amperage.

Another advantage of the invention is related to the internal organization of the plant: the start-up and early operation stage of a pot being highly critical for its lifetime, it may be advantageous to concentrate the experienced operation workforce in a smaller part of the potline when starting up the plant, namely in a sector separated from the plant still under construction where the construction work force is concentrated.