WO2017158501A1

WO2017158501A1 - Device for holding anode assemblies during electrical preheating of hall-héroult cells, and process for preheating hall-héroult cells using such device

Info

Publication number: WO2017158501A1
Application number: PCT/IB2017/051459
Authority: WO
Inventors: Mark Jordan; Mahmood ABDULMALIK; Syed Ahmed
Original assignee: Dubai Aluminium Pjsc
Priority date: 2016-03-15
Filing date: 2017-03-14
Publication date: 2017-09-21
Also published as: GB201604347D0; GB2548359A

Abstract

This holding device (101) is suitable for at least one anode assembly (14), said anode assembly comprising an anode rod (141) fixed to at least one anode (144), said anode assembly (14) being part of an electrolytic cell (1), suitable for the Hall-Héroult electrolysis process. This holding device (101) comprises: - a body (2) provided with handling means (22), adapted to cooperate with a lifting device, - at least one flexible mechanical holder (3), each being adapted to cooperate with a respective anode assembly, said flexible mechanical holder comprising an upper end (30') permanently attached to said body, a flexible zone (3C) and a lower end (30''), - means (5) for removable fixation of said anode rod (141) on said holder (3).

Description

Device for holding anode assemblies during electrical preheating of Hall-Heroult cells, and process for preheating Hall-Heroult cells using such device

Technical field of the invention

The invention relates to the field of fused salt electrolysis, and more precisely to electrolysis cells using the Hall-Heroult process for making aluminium by fused salt electrolysis. The invention relates to electrical preheating of such cells during their startup, and focuses on a device for mechanically holding and electrically connecting anode assemblies during the preheating of Hall-Heroult cells, and on a process for shunted resistance preheating of Hall-Heroult cells using said device.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (Al₂0₃) is dissolved in molten cryolite (Na₃AIF₆), 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 potshell), a lining (comprising refractory bricks protecting said steel potshell against heat, and cathode blocks usually made from graphite, anthracite or a mixture of both), a superstructure 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 by the electrolyte into aluminium and oxygen ions, 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 from where it needs to be removed from time to time, usually by suction into a crucible.

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 intensity at which they are operated in order to increase the plant productivity and bring down the capital cost per unit of aluminium produced in the plant.

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 metres and a width usually comprised between 3 and 5 metres. 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 current orientation in the series. The current enters the upstream and exits downstream of the cell. 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.

The superstructure of the cell comprises a fixed frame and a mobile metallic anode busbar, also called an "anode beam", which extends at the outer periphery of the fixed frame. Each anode assembly comprises at least one anode and is provided with a metallic rod, for mechanical attachment and electrical connection of said anode assembly to said anode beam. Said mechanical attachment and electrical connection to the anode beam is usually achieved using a mechanical locking or clamping mechanism; such mechanisms are described for instance in WO 2006/11 1649 and WO 2012/032234 (ECL).

Hall-Heroult pots operate normally at a temperature in the order of 950°C to 1000°C as a continuous fused salt electrolytic process: the metallic, liquid aluminium produced by the process is removed from time to time, and the alumina dissolved in the electrolytic bath that is consumed during the process is replenished from time to time. When the cell is put into service for the first time after its construction or after relining (typically after five to seven years of operation), it needs to be started up using a specific start-up procedure. During start-up of the cell, liquid electrolyte (also called "liquid bath") is poured into the cell until the electrolysis process can be started in the cell. Specific preheating procedures are needed in order to avoid thermal shock of the cell components in contact with the liquid bath or liquid metal; thermal shock may induce cracks.

The start-up of a pot, after its construction, refurbishment or repair, is a critical operation. This has been recognized in the book "Cathodes in Aluminium Electrolysis" by M. S0rlie and H.A. 0ye (3^rd edition, Dusseldorf 2010), pages 1 1 1 - 158). The start-up of a pot takes time (typically several days) during which the pot is not producing any metal, it requires manual operations inside the pot as well as intense supervision by qualified operators, and in pathological cases it may lead to the degradation of certain carbonaceous components (cathodes, lining) that may reduce the lifetime of the pot (typically five to seven years). Before start-up, the pot is preheated. Preheating the pot to temperatures above 800°C or 900°C is a critical step. The criticality of preheating for pot life is mainly due to the fact that preheating a cold pot will generate strong thermal gradients that may lead to abnormal thermal stresses, in particular on the sole (upper surface) of the cathode; these stresses may damage the cathode elements, in particular by inducing cracks. The start-up process generally includes a preheating step and a step during which liquid electrolyte is introduced into the preheated pot. The preheating step can be carried out by open heat (such as provided by gas burners). The use of electrical heating elements put in contact with the cathode and the anode has been proposed in WO 95/06145. Preheating is nowadays often carried out by resistance heating (also called "shunted resistance preheat" in the book by M. S0rlie and H.A. 0ye (2010) cited above, p. 124-125).

Such a method seems to have been described for the first time in 1923 (US 1 ,572,253). It involves lowering the carbon anodes into direct contact with the floor (also called "sole") formed by the cathode, and then passing a flow of current through the cathode connector terminal: due to the high resistance at the points of contact, heat is generated at such points. Instead of a direct contact between anode and cathode, a conductive layer of carbonaceous particles can be interposed between the anode and the cathode; such a method has been described for the first time in GB 1 046 705, and forms the basis of methods that are still in use. Numerous improvements and variants of this method have been proposed. As an example, WO 2004/027119 describes a preheating process in which a layer of granular, graphite-based conductive material is deposited on part of the cathode surface and then crushed between the anodes and the cathodes, and in which the periphery of the pot is filled with crushed electrolyte bath material and sodium carbonate. Sheets of rock wool are applied on the surface in order to minimize thermal losses. Liquid bath is added to the cell once the preheating temperature has been reached. This supposes that liquid bath is available from donor cells, which may not be the case for starting new plants. WO 2012/174641 describes a similar method using crushed bath only, then adding alumina powder and molten aluminium. In WO 2007/146492 compressed particles of exfoliated graphite are used as carbonaceous material. US 4, 146,444 describes a method in which the carbonaceous aggregate is ignited.

The present invention relates to shunted resistance preheating. As mentioned above, during this process the carbon anodes are lowered to be put into direct contact with the floor formed by the cathode, or they are put in direct contact with a layer of granules of carbonaceous electrically conductive material. The overall reproducibility of this process is not fully satisfactory. As modern Hall-Heroult cells may comprise more than over 30 identical anodes, any lack of reproducibility will lead to thermal inhomogeneity of the pot during preheating; as a rule, uncontrolled thermal gradients during preheating are not desirable.

The present invention aims at proposing an improved process for shunted resistance preheating of Hall-Heroult cells, which improves the thermal homogeneity of the cell during preheating. Such an improved process should however not complicate the electrical and mechanical connection of the anode assemblies to the anode beam, should remain compatible with standard pot tending equipment, and should not be more labor intensive than prior art processes.

Object of the invention

According to the invention, the problem is solved by a holding device for at least one anode assembly, said anode assembly comprising an anode rod fixed to at least one anode, said anode assembly being part of an electrolytic cell, suitable for the Hall-Heroult electrolysis process,

said holding device comprising:

- a body provided with handling means, adapted to cooperate with a lifting device, at least one flexible mechanical holder, each being adapted to cooperate with a respective anode assembly, said flexible mechanical holder comprising an upper end permanently attached to said body, a flexible zone and a lower end, means for removable fixation of said anode rod on said holder.

Advantageously, each flexible holder is adapted to bend downward due to the weight of said anode assembly.

In a typical embodiment, said fixation means are provided in an intermediate region of the holder, located between flexible zone and lower end of said holder. Said fixation means are in particular provided at lower end of said holder. Advantageously, said lower end of said holder is firmly attached to a mounting assembly for mounting said fixation means on said holder. Said holding device is typically provided with several mechanical holders, in particular between two and six mechanical holders. In an advantageous embodiment, said mechanical holder is made of a plurality of parallel aluminium sheets. Two adjacent parallel aluminium sheets may be separated by a gap. In a typical embodiment, said mechanical holder comprises a substantially horizontal part, as well as a substantially vertical part forming said intermediate fixation region, said flexible zone being shaped as a circle arc between said parts. Said upper end of mechanical holder is in particular welded on said body. In an advantageous embodiment, said means for removable fixation of said anode assembly comprise clamping means. Said clamping means may comprise a first clamping member firmly attached to flexible holder, as well as a second clamping member movable with respect to said first clamping member, said first and second clamping members defining a clamping space adapted to receive anode rod, the dimensions of this clamping space being variable as a function of the displacement of movable second clamping member with respect to first clamping member.

In still another advantageous embodiment, the holding device of the invention further comprises at least one electrical connector, adapted to collect current from one anode beam of said electrolytic cell. The holding device of the invention may further comprise blocking means, adapted to block said electrical connector against said anode beam. Said blocking means are in particular mobile between a released position, wherein they allow respective motion of connector and anode beam, and a blocking position, wherein they prevent said motion. In a typical embodiment, said blocking means comprise opposite front and rear walls, as well as moving means adapted to modify distance separating said walls, between said released position and said blocking position.

Another object of the invention is an electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising:

at least one cathode forming the bottom of said electrolytic cell,

a lateral lining defining together with the cathode, a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, said cathode and lateral lining being contained in an outer metallic shell, a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and at least one metallic anode rod, said electrolytic cell being characterized in that it further comprises at least one holding device as defined above.

In an advantageous embodiment, electrolytic cell according to the invention comprises between two and six holding devices as defined above. Still another object of the invention is a method of preheating an electrolytic cell as defined above, comprising:

fixing each anode rod on a respective flexible holder,

making each anode rest under a controlled weight (preferably under its own weight, which includes the weight of the anode assembly) onto a respective cathode, either directly or via an intermediate layer of granulated carbonaceous material,

heating said anode, in particular electrically,

lifting said anode above said cathode after heating is achieved. In an advantageous embodiment, the method of preheating according to the invention further comprises moving downwards said anode, releasing said anode rod from the anode beam and engaging said anode rod with said flexible holder, so as to make each anode rest under a controlled weight (preferably under its own weight, which includes the weight of the anode assembly) onto a respective cathode. In a typical embodiment, lifting anode above said cathode is carried out by engaging each anode rod onto said anode beam and lifting said anode beam. Still another object of the invention is an aluminium electrolysis plant comprising at least one line of electrolysis cells of substantially rectangular shape, said cells being arranged side by side, and said plant further comprising means for electrically connecting said cells in series and for connecting the cathodic busbar of a cell to the anode beam of a downstream cell, characterized in that more than 80% of the electrolysis cells in at least one of said line, and preferably each electrolysis cell of said line, is an electrolysis cell as defined above. Still another object of the invention is a method for making aluminium by the Hall-Heroult electrolysis process using electrolytic cells of substantially rectangular shape, characterized in that said method is carried out in an aluminium electrolysis plant as defined above. The inventors have discovered that the electrical resistance of the shunt depends on the force applied by the anode assembly onto the contact surface between the lower surface of the anode and the cathode or the layer of carbonaceous particles.

According to prior art, during the preheat procedure the anodes remain mechanically and electrically connected to the anode beam, but their mechanical positioning is done at the beginning of the process, at room temperature. This means that in practice the force exerted by the anode assembly onto the surface on which it rests during preheating is not well controlled, and may change during the preheating process. The inventors have found that this may limit the reproducibility of the preheat process.

According to one aspect of the invention, during preheating the anode assemblies do not have a rigid mechanical connection to the anode beam but they rest on a controlled weight (and preferably on their own weight, i.e. the weight of the anode assembly, which is the simplest way to achieve a controlled force exerted on the contact surface). This achieves a constant force exerted on the contact surface throughout the whole preheating process. A connecting device is provided for anode assembly that comprises a mechanically flexible electrical connector which ensures sufficient mechanical rigidity in order to avoid uncontrolled or accidental displacement of the anode assembly during the process. According to another aspect of the invention said flexible electrical conductor is made from a pack of parallel aluminium sheets. According to an essential aspect of the invention a plurality of connecting devices are grouped in a rigid frame or body. Figures

Figures 1 to 18 represent various embodiments of the present invention.

Figure 1 is a section view, showing the global arrangement of an electrolytic Hall-Heroult electrolysis cell, or pot, equipped with a holding device according to the invention.

Figure 2 is a top view of the electrolytic cell of figure 1.

Figure 3 is a side view, at a larger scale than figure 1 , showing detail of figure 1.

Figure 4 is a side view, analogous to figure 3, showing at still a larger scale the holding device according to the invention.

Figures 5 and 6 are respectively front and top views of the holding device according to the invention.

Figure 7 is a side view, showing in particular a flexible holder which belongs to the holding device according to the invention.

Figure 8 is a side view, showing the lower part of this flexible holder, as well as a clamping assembly which is attached to this holder.

Figures 9 and 10 are respectively perspective, side and top views of this clamping assembly.

Figure 1 1 is a side view, showing in particular an electrical connector which belongs to the holding device according to the invention.

Figure 12 is a longitudinal cross section of a blocking cap, which cooperates with electrical connector of figure 11.

Figure 13 is a front view, showing the lower part of this electrical connector and this blocking cap mounted thereon.

Figures 14 to 16 are schematic front views, illustrating three stages of a preheating process of an electrolytic cell, carried out with the holding device of the invention.

Figure 17 presents a variant of figure 15, in which the anode rests upon a layer of granules of a carbonaceous material.

Figure 18 is a perspective view, showing a holding device according to the invention in operation, with several anode rods mounted thereon. The following reference numbers and letters are used on the figures: 1 Potshell (cell) 1 1 Metal shell

12 Refractory materials 13 Cathodes

14 Anode assembly 141 Anode rod

142 Hooks 143 Anode yoke

144 Anode 15 Anode riser

16 Superstructure 161 Fixed frame of 16

162 Anode beam 163 Jacks

101 Holding device

2 Body of 101 2U Upper face of 2

21 Coupling 22 Bail

23 Cover 24 Stem

25 Chain

3 Flexible holder 30 Sheets of 3

31 Gap 3A Horizontal part of 3

3B Vertical part of 3 3C Transition part of 3

30' Upper end of 30 30" Lower end of 30

32 Block 33 Plate

34,35 Spacers

4 Electrical connector 4A Upper part of 4

4B Lower part of 4 4C Shoulder

41 Mounting block 42 Blocking cap

43,43' Flanges of 42 44 Core of 42

45 Rear wall of 42 46 Pin

47,47' Apertures in 43,43' 44' Hole in 44

48 Blocking screw

5 Clamp assembly 6,7 Clamp members

61 ,71 Flanges 62,72 Core

63,73 Clamp face or jaw S Clamp space between 63,73

64,74 Free ends 81 Pivot pin

82,83 Nuts 82', 83' Holes in 82,83

84 Clamping screw L Layer of granules

Detailed description

An aluminium electrolysis plant (also called "smelting plant" or "aluminium smelter") comprises a plurality of electrolytic cells arranged the one behind the other (and side by side), typically along two parallel lines. These cells are electrically connected in series by means of conductors, so that electrolysis current passes from one cell to the next. The number of cells in a series is typically comprised between 50 and over 400, but this figure is not substantial for the present invention. The cells are arranged transversally in reference of main direction of the line they constitute. 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.

The Hall-Heroult process as such, 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".

The subject matter of the invention is a holding device for at least one anode assembly, adapted to be used in a Hall-Heroult electrolysis cell. Figure 1 shows an electrolytic cell 1 (also called "pot") for the production of aluminium by electrolysis. In a way known as such, this pot 1 comprises a metal shell 11 (so-called "potshell") internally lined with refractory materials 12, a cathode 13 formed by rectangular and parallel blocks (so-called "cathode blocks") of carbonaceous material, as well as a plurality of anode assemblies 14. Each anode assembly 14 comprises one or more (usually at most two) anodes 144 and a vertical anode rod 141 that is, when the cell is in normal operation, in electrical contact with the anode busbar (anode beam, shown on figure 3).

Figure 1 also shows an anode riser 15 that carries electrical current from the cathodic busbar of the neighboring downstream cell to the anode busbar (called "anode beam"), a superstructure 16 supporting anode assemblies, as well as a cathodic busbar 17 surrounding the potshell 11. The holding device of the invention, designated as a whole as reference 101 , is also illustrated on figure 1. It is intended to cooperate with a plurality of anode assemblies, as will be described hereafter.

Figure 3 is a view at a larger scale of detail III of figure 1 , which shows more clearly the structure of anode assembly 14, superstructure 16, as well as of holding device 101 according to the invention. Said superstructure 16 comprises a fixed frame 161 and a mobile metallic anode frame 162, also called "anode beam", which extends to the outer periphery of the fixed frame. Vertical motion of beam 162 with respect to frame 161 is ensured by a set of jacks 163; such motion allows the adjustment of the anode height with respect to the cathode, and thus the anode-cathode distance (ACD). In a way known as such, each anode assembly 14 comprises a metallic rod 141 ("anode rod") for mechanical attachment and electrical connection to the anode beam. This beam is provided with hooks 142, adapted to facilitate the attachment of the anode rods in the usual way. The lower end of the anode rod 141 usually has the shape of a yoke 143 (so-called "anode yoke") that is sealed into the anode 144 using cast iron (see figure 1).

The general structure of a Hall-Heroult electrolysis pot is known per se and will not be explained here. It is sufficient to explain that, in use, the current is fed into the anode beam, flows from the anode beam to the plurality of anode rods and to the anodes in contact with the liquid electrolyte where the electrolytic reaction takes place. Then the current crosses the liquid metal pad resulting from the process and eventually will be collected at the cathode block.

The holding device 101 according to the invention substantially comprises a body 2, a plurality of flexible mechanical holders 3, a plurality of electrical connectors 4, and a plurality of clamp assemblies 5, each of said clamp assemblies being intended for removable fixation of a respective anode rod on a respective holder.

Body 2, which is shaped as a beam, is made for example of aluminium. Its longitudinal dimension is sufficient so as to face several anodes (see in particular figure 5). Upper face 2U of said body 2 is provided with a coupling 21 , on which a pivoting bail 22 is mounted (see in particular figure 4). The latter is adapted to be cooperative with a lifting equipment (not shown on the figures), such as a crane. Body 2 is also provided with protective covers 23, as well as horizontal stems 24, the end of which supports retaining chains 25. The function of both covers 23 and chains 25 will be explained later. Each holding device, such as reference number 101 , is provided with a plurality of flexible holders 3, the number of which is equal to the number of anodes to be handled by said holding device 101. In the illustrated example, six flexible holders are provided for each holding device (see in particular figure 5), the structures of these flexible holders being advantageously identical. As shown on figure 2, a typical electrolytic cell of the Hall- Heroult type comprises two rows of eighteen anode assemblies referenced as 14A to 14R, as well as 14Ά to 14'R. In an advantageous way, three holding devices 101,102,103 are provided for the first row, whereas three further holding devices 104,105,106 are provided for the second row. Holding devices 101 ,102,103 are adapted to handle respective set of anode assemblies 14A to 14F, 14G to 14L, and 14M to 14R, whereas holding devices 104,105,106 are adapted to handle respective set of anode assemblies 14Ά to 14'F, 14'G to 14'L, and 14'M to 14'R.

As more clearly shown on figure 4 and in particular on figure 7, each flexible holder 3 is formed by packs of parallel metal sheets 30, preferably made from aluminium. In the illustrated example, five packs 30A to 30E are provided. The thickness of said sheets should be such as to allow flexibility that is required for allowing movement of the anode assemblies. Two adjacent packs define an intermediate gap 31.

Viewed from the side, the holder 3 defines two rectilinear parts i.e., in the shown example, a horizontal part 3A and a vertical part 3B. The latter are separated by a transition rounded part 3C, shaped as a circle arc, in particular a quarter of circle. By way of example, the main dimension of horizontal part 3A is between 100 mm and 400 mm, the main dimension of vertical part 3B is between 200 mm and 800 mm, whereas the radius of rounded part of inner pack 30A is between 30 mm and 120 mm. These dimensions are given, out of external mechanical strain exerted on this flexible holder.

First upper end 30' of each pack is welded to the body 2, in a way known as such. Moreover, second lower end 30" of each pack is welded to a block 32. Said body 2 and block 32 are made of aluminium. Block 32 is also attached to a plate 33, which protrudes horizontally (see in particular figure 8). Both ends 30' and 30" of each sheet 30 are chamfered, so as to facilitate welding. A respective set of spacers 34 and 35 is provided at the junction between a respective end 30' or 30" and the body 2 or the block 32 (see in particular figures 4 and 8). These spacers, which are known as such, confer a precise dimension to the different gaps between the adjacent packs of the flexible holder.

Device 101 is further provided with a plurality of electrical connectors 4. Contrary to above holders 3, the number of connectors 4 may be different from that of anode assemblies. In the illustrated example, three connectors are provided, having the same structure (see in particular figure 5), bearing in mind that this number may be comprised between one and eight, preferably between two and five.

Each connector 4 is formed by a massive body of a conductive material, such as aluminium, the thickness of which is typically between 75 mm and 300 mm. This connector, which extends substantially vertically, comprises an upper part 4A, a lower part 4B as well as a transition part 4C forming a shoulder (see in particular figure 11). Upper part 4A is attached on body 2 via a mounting block 41. Due to the presence of the shoulder 4C, the lower part 4B protrudes forwards, so as to be placed against front wall of anode beam, viewed from the side. Lower part 4B defines a free end of the connector, since it is not attached to another component of the device. In the vicinity of this free end, lower part 4B is provided with a mobile blocking cap 42. As shown on figure 12 and figure 13, this cap comprises two flanges 43 and 43', extending in the vicinity of lateral walls of connector 4. This cap is also provided with a front core 44, as well as with a rear blocking wall 45. A pin 46 extends through the flanges 43 and 43', as well as through the connector 4. This pin has substantially no motion freedom with respect to the connector. On the other hand, it has a possibility of translation motion with respect to cap 42, along arrow T46, since it penetrates into elongated apertures, or slots, 47 and 47' drilled into said flanges. Core 44 is provided with an axial hole 44', which is threaded for cooperation with a blocking screw 48.

In use, before the connector 4 is mounted against the anode beam, the blocking wall 45 is in a retracted position with regard to the front core 44. In other words, the blocking wall defines with the connector an intermediary space, the dimension of which is sufficient to permit the engagement on the anode beam, along a vertical dimension. In this retracted position, pin 46 abuts against first end 47A of aperture 47 (see figure 12, illustrated in solid lines). Then, once the connector is placed in its desired position against the anode beam, an operator may tighten the blocking screw 48 so that core and connector move forwards, in the direction of the rear wall. The size of above defined intermediary space is therefore reduced, which induces the clamping of anode beam and connector, between core and rear wall of the pin. In this blocking position, pin 46 abuts against opposite end 47B of aperture 47 (see figure 12, illustrated in dotted lines).

Device 101 is provided with a plurality of clamp assemblies 5, the number of which is equal to the number of anode assemblies to be handled and of holders 3. In the illustrated example, six clamp assemblies 5 are therefore provided, having the same structure (see in particular figure 5). Each assembly 5 comprises two members, hereafter an inner member 6 as well as an outer member 7 (see in particular figure 9 and figure 10). They are respectively called "inner" and "outer" members since inner member 6 partly extends inside outer member 7. Viewed from the side, each member 6, 7 has two lateral walls or flanges 61 , 71 , as well as a junction wall or core 62, 72. Viewed from front, each member has substantially the same elongated shape, being provided with a clamp face or jaw 63, 73. These two jaws face each other, so as to define a clamp space S. Above described plate 33 is sandwiched between flanges 61 of inner member 6 (see in particular figure 8). Therefore, the latter is firmly attached to the holder 3, without any motion freedom. Moreover, above described chain 25 engages the outer member 7 (see in particular figure 8). Since this chain 25 is a flexible link, it ensures a maintaining function of member 7 while allowing a possibility of deflection.

The free end 64 of inner member 6 is connected by a hinge to the outer member, at the vicinity of jaw 73 thereof. A pivot pin 81 extends through successive flanges 61 and 71. Moreover, free end 74 of outer member 7 is provided with a nut 82, which extends between its flanges 71. Finally inner member 6 is provided with a further nut 83, which extends between its flanges 61 at the vicinity of its jaw 63. Both nuts 82 and 83 are provided with through holes 82' and 83', which extend parallel to flanges 61 and 71 , so as to permit the engagement of a clamping screw 84. It should be noted that only one of these holes 82' and 83' is threaded. In the present example, only hole 83' is threaded. Therefore, when screw is rotated, this induces the rotation of mobile outer member towards fixed inner member and, as a consequence, the movement of clamping jaw of this outer member in order to modify the size of clamping space S. On figure 10, anode rod 141 is shown in phantom.

Figures 14 to 16 show some stages of a preheating process of the electrolytic cell above described. On these figures, only one anode rod 141 is illustrated, bearing in mind that all anode rods are submitted to the same motions. On these figures, one clamping assembly 5 of the holding device, as well as one pair of hooks 142 of the anode beam 162, are schematically shown. When either the clamping assembly or hook is illustrated in solid lines, it means that it engages anode rod 141. On the other hand, when illustrated in dotted lines, it means that this anode rod is released from this clamping assembly or from this hook.

In a first step, illustrated on figure 14, each anode rod 141 is engaged on the anode beam 162, using hooks 142 in a known manner. The anode beam is then moved downwards, along arrow F1 , until the bottom end of the anode 144 comes in the vicinity of top wall of cathode 13. However, according to an essential feature of the invention, anode 144 does not come into contact with cathode 13. The distance D between facing walls of anode and cathode will be set to an appropriate value by those skilled in the art. Then, each anode rod is engaged with a respective flexible holder 3; using clamping means 5 as explained above. During this engagement operation itself, by way of safety, anode rods are still maintained by the anode beam. Once the engagement is achieved (see clamping means 5 in solid lines on figure 15), the hooks are released (see corresponding dotted lines on this figure 15), so that anode rods are maintained only by holders 3.

Anode rods are fixed onto holder 3 due to clamping means 5, which are provided at the free lower end 30" of holder 3 in the shown example. By way of variant, clamping means may be provided at another region of vertical part 3B of the holder, i.e. between flexible transition zone 3C and lower end 30". A perpendicular positioning of clamping means with respect to holder 3 is advantageous, so as to avoid uneven forces on screw 84 and plates 63 and 73.

As explained above, each holder 3 is flexible, in particular due to the fact that it is constituted from thin flexible sheets 30A to 30E. This means that, when anode assembly is attached, the resistance of this holder is inferior to gravity strength, along vertical direction. In other words, even though anode and holder are attached, the latter will not prevent anode to drop onto cathode and rest on its own weight on this cathode, as shown by arrow G on figure 15. The downward motion of anode 144, until it rests again cathode 13, also induces the downward bending of transition part 3C (see arrow B3 on figure 15, as well as on figure 7). In this respect, this bending possibility is made easier by the gaps 31 separating two adjacent sheets of the holder. Modifying the thickness of these gaps makes it possible to monitor the magnitude of this bending motion.

However, mechanical resistance of each holder is sufficient, so as to prevent any avoid uncontrolled or accidental displacement of the anode assembly during the process, in particular along a horizontal direction. In order to achieve both these functions, those skilled in the art will make a compromise between the different parameters of said holder, in particular: number of packs, dimensions of each pack (lengths of rectilinear parts, radii of rounded parts, thickness) and thickness of intercalary gap.

Once an anode rests onto the cathode (preferably under its own weight, i.e. the weight of the anode assembly 14 (the latter comprising the carbon anode 144 and the anode rod 141 with its anode yoke 143), a preheating stage is carried out in a way known as such, for example using electrical current. During this stage, current connector 4 is firmly pressed against anode beam, using blocking cap 42 as explained above. After this preheating stage has been processed, all the anode rods 141 are once again engaged with the hooks 142 of anode beam (see corresponding solid lines on this figure 16), while the clamping means 5 are released (see corresponding dotted lines on this figure 16). The anode beam is then moved upwards, along arrow F2, so that anodes 144 are lifted above the surface of cathode 13. Beam 162 is placed at the relevant elevation, in order to initiate the aluminium production. In a preferred variant of this embodiment, the cathode is covered at least in part with a layer L of granules of a carbonaceous material, in a way known as such. In step 2 the anode 144 rests on said layer L of granules, instead of on top surface of cathode 13; this is schematically shown in figure 17. The process according to the invention ensures that the pressure exerted by the anodes 144 upon the surface of the cathode or upon the layer of granules of a carbonaceous material is constant for all anode assemblies of the pot during the whole preheating process, said pressure being determined by the surface area of the anode 144 (knowing that all anodes are alike) and the mass of the anode assembly (which is nominally alike, too). Said pressure has an influence on the electrical resistance of the anode-cathode shunt. Keeping this pressure reproducible for all anode assemblies improves the thermal homogeneity of the preheating process. Another advantage of the process according to the invention is that for each anode said pressure is constant during the preheating process, since each anode rests upon its own weight.

In fact, if the anode assembly does not rest upon a controlled mass (preferably its own mass), i.e. if the anode rod is attached to the anode beam, the pressure exerted by the anode assembly upon the cathode or upon the layer of carbonaceous granules can decrease during the preheating process, as the cathode surface may change due to partial baking, and the bed of carbonaceous granules may change, too. In particular, if the anode assembly cannot freely move downwards under a controlled mass (such as its own mass) it cannot react to possible changes in the thickness of the layer of carbonaceous granules due to partial burn-off. Keeping the pressure constant is the best that can be done to ensure reproducible and controlled operating conditions during the preheating process. According to prior art, this required individual handling of each anode assembly is labour-intensive. Using the holding device according to the invention, in particular for a plurality of anode assemblies, simplifies the preheating process. In an advantageous embodiment of the invention, said holding device can handle at least two, preferably at least three, more preferably at least four, still more preferably at least five, and most preferably at least six anode assemblies. Another advantage of the device and method according to the invention is to avoid individual handling of the anode assemblies by the crane after preheating. The only step to be carried out on each single anode assembly after preheating is to attach each anode assembly to the anode beam and to remove the removable fixation means 5 by which the anode rod is clamped to the lower end of the flexible mechanical holder 3. No crane is necessary for this. In particular, no lateral movement of the anode assembly needs to be carried out in order to connect it to the anode beam. Once all the anode assemblies have been disconnected mechanically and electrically from the holding device 101 according to the invention said holding device 101 is removed by the crane. The process according to then invention therefore saves crane operation time, which allows the crane to be used elsewhere in the potline.

Claims

1. A holding device (101) for at least one anode assembly (14),

said anode assembly comprising an anode rod (141) fixed to at least one anode (144), said anode assembly (14) being part of an electrolytic cell (1), suitable for the Hall-Heroult electrolysis process,

said holding device (101) comprising:

a body (2) provided with handling means (22), adapted to cooperate with a lifting device,

- at least one flexible mechanical holder (3), each being adapted to cooperate with a respective anode assembly, said flexible mechanical holder comprising an upper end (30') permanently attached to said body, a flexible zone (3C) and a lower end

(30"),

means (5) for removable fixation of said anode rod (141) on said holder (3).

2. A holding device according to claim 1 , characterized in that each flexible holder (3) is adapted to bend downward due to the weight of said anode assembly (14).

3. A holding device according to claim 1 , characterized in that said fixation means (5) are provided in an intermediate region (3B) of the holder, located between flexible zone (3C) and lower end (30") of said holder.

4. A holding device according to claim 3, characterized in that said fixation means (5) are provided at lower end (30") of said holder.

5. A holding device according to claim 4, characterized in that said lower end (30") of said holder is firmly attached to a mounting assembly (32, 33) for mounting said fixation means (5) on said holder (3).

6. A holding device according to claim 1 , characterized in that said holding device (101) is provided with several mechanical holders (3), in particular between two and six mechanical holders.

7. A holding device according to claim 1 , characterized in that said mechanical holder (3) is made of a plurality of parallel aluminium sheets (30A to 30E).

8. A holding device according to claim 1 , characterized in that two adjacent parallel aluminium sheets are separated by a gap (31).

9. A holding device according to claim 1 , characterized in that said mechanical holder (3) comprises a substantially horizontal part (3A), as well as a substantially vertical part (3B) forming said intermediate fixation region, said flexible zone (3C) being shaped as a circle arc between said parts.

10. A holding device according to claim 1 , characterized in that said upper end (30') of mechanical holder (3) is welded on said body.

1 1. A holding device according to claim 1 , characterized in that said means for removable fixation of said anode assembly comprise clamping means (5).

12. A holding device according to claim 1 , characterized in that said clamping means comprise a first clamping member (6) firmly attached to flexible holder (3), as well as a second clamping member (7) movable with respect to said first clamping member (6), said first and second clamping members defining a clamping space (S) adapted to receive anode rod, the dimensions of this clamping space being variable as a function of the displacement of movable second clamping member (7) with respect to first clamping member (6).

13. A holding device according to claim 1 , characterized in that it further comprises at least one electrical connector (4), adapted to collect current from one anode beam (162) of said electrolytic cell (1).

14. A holding device according to claim 13, characterized in that it further comprises blocking means (42), adapted to block said electrical connector against said anode beam (162).

15. A holding device according to claim 14, characterized in that said blocking means (42) are mobile between a released position, wherein they allow respective motion of connector and anode beam, and a blocking position, wherein they prevent said motion.

16. A holding device according to claim 15, characterized in that said blocking means (42) comprise opposite front (44) and rear (45) walls, as well as moving means (48) adapted to modify distance separating said walls, between said released position and said blocking position.

17. Electrolytic cell (1) suitable for the Hall-Heroult electrolysis process, comprising:

at least one cathode (13) forming the bottom of said electrolytic cell,

a lateral lining defining together with the cathode, a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, said cathode and lateral lining being contained in an outer metallic shell (1 1), a plurality of anode assemblies (14) suspended above the cathode, each anode assembly comprising at least one anode (144) and at least one metallic anode rod

(141),

said electrolytic cell being characterized in that it further comprises at least one holding device (101) according to any of above claims.

18. Electrolytic cell according to claim 17, characterized in that it comprises between two and six holding devices (101-106) according to any of claims 1 to 16.

19. A method of preheating an electrolytic cell according to claim 17 or 18, comprising: fixing each anode rod (141) on a respective flexible holder (3),

making each anode (144) rest under a controlled weight (and preferably under its own weight) onto a respective cathode (13), either directly or via an intermediate layer (L) of granulated carbonaceous material,

heating said anode, in particular electrically,

lifting said anode above said cathode after heating is achieved.

20. A method of preheating according to claim 19 further comprising, moving downwards said anode (144), releasing said anode rod (141) from the anode beam (162) and engaging said anode rod (141) with said flexible holder (3), so as to make each anode (144) rest under a controlled weight (and preferably under its own weight) onto a respective cathode (13).

21. A method of preheating according to claim 20, wherein lifting anode (144) above said cathode is carried out by engaging each anode rod (141) onto said anode beam (162), and lifting said anode beam.

22. An aluminium electrolysis plant comprising at least one line of electrolysis cells of substantially rectangular shape, said cells being arranged side by side, and said plant further comprising means for electrically connecting said cells in series and for connecting the cathodic busbar of a cell to the anode beam of a downstream cell,

characterized in that more than 80% of the electrolysis cells in at least one of said line, and preferably each electrolysis cell of said line, is an electrolysis cell according to claim 17 or 18.

23. A method for making aluminium by the Hall-Heroult electrolysis process using electrolytic cells of substantially rectangular shape, characterized in that said method is carried out in an aluminium electrolysis plant according to claim 22.