WO2017046677A1

WO2017046677A1 - Removable cover and flap for easy access to the intercalary space in a series of electrolytic hall-héroult cells

Info

Publication number: WO2017046677A1
Application number: PCT/IB2016/055348
Authority: WO
Inventors: Mark Jordan; Mahmood ABDULMALIK; Abdalla ALZAROONI; Syed SYED
Original assignee: Dubai Aluminium Pjsc
Priority date: 2015-09-16
Filing date: 2016-09-08
Publication date: 2017-03-23
Also published as: GB2542555A; GB201516354D0

Abstract

The electrolysis plant of the invention comprises at least one line of adjacent electrolysis cells ranged side by side, means for supplying electrical current to these cells, and means for electrically connecting these cells in series in order to drive this current,each electrolysis cell comprising a plurality of anode assemblies, a cathode, a plurality of mounting electric conductors ("risers"), a system of electric conductors. Two adjacent cells define an intercalary space provided with a top structure (S) comprising a top cover (T), said top cover (T) defining a main working surface for the walk of an operator (OP), extending over at least part of the surface of this intercalary space, viewed from above. According to the invention top cover (T) comprises at least one slab body (4A, 4B, 5A, 5B), said slab body comprising a plurality of adjacent flaps (70) movably connected to said slab body, each of said flaps being movable between an opening position wherein it defines a passage (P) towards part of the inner volume of the intercalary space (H), and a closing position, wherein it closes said passage.

Description

Removable cover and flap for easy access to the intercalary space in a series of electrolytic Hall-Heroult cells

Technical field of the invention

The invention relates to an industrial plant comprising a plurality of electrolysis cells, in view of producing aluminium by fused salt electrolysis using the Hall-Heroult-process. In particular, the invention relates to a particular arrangement of the top cover of the intercalary space between the potshells of two adjacent electrolysis cells arranged side by side in a series of electrolysis cells.

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 potshell, a lining (comprising refractory bricks protecting said steel shell against heat, and cathode blocks forming a cathode covering the whole bottom of the potshell, said cathode blocks being usually made from graphite, anthracite or a mixture of both), and a plurality of anodes made from carbon that plunge into the liquid electrolyte. Anodes and cathodes are connected to external busbars. An electrical current is passed through the cell (typically at a voltage between 3.8 V to 5 V) which splits the aluminium oxide in aluminium ions and oxygen ions. The oxygen ions are reduced to molecular oxygen at the anode, said oxygen reacting with the carbon of the anode. The aluminium ions move to the cathode where they accept electrons supplied by the cathode; 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. 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 tonne 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 reduce the capital cost per unit of aluminium produced in the plant.

Electrolytic cells presently used for the Hall-Heroult process are rectangular and have a length usually comprised between 8 and 20 metres and a width usually comprised between 3 and 5 metres. Most newly installed pots operate at a current intensity comprised between approximately 400 kA and 600 kA. The outer rectangular shape of a pot is defined by its outer potshell. Industrial Hall-Heroult electrolysis cells are always operated in series of several tens (usually up to more than a hundred) of pots (such a series being called a "potline"); within each series DC current flows from one cell to the neighbouring cell. Much effort is still being made within the industry to optimise the process in order to increase its energy efficiency. In particular, the passage of the enormous current intensities through the electrolytic cell leads to ohmic losses at various locations of the pot.

The present invention is more particularly directed to an installation or plant, wherein a plurality of such rectangular electrolysis cells (pots) are arranged along at least one straight line, the large dimension of said rectangular pot being orthogonal to the main direction of said straight line. Therefore, the facing walls of the potshells of two adjacent cells define an intercalary space, wherein some functional accessories and elements of the plant are accommodated. By way of example, part of the cathodic busbar system extends into this space.

Typically this intercalary space is limited by the facing walls (potshells) of two adjacent pots. In order to avoid uncontrolled heat loss as well as for practical reasons the intercalary space needs to be closed by a top structure, which is at least partly formed by a top cover (so-called "slab"), allowing operators to walk thereon. In existing plant, this top cover is formed by one or more slab(s), in particular made of concrete. Each slab is heavy and needs to be handled by a pot tending machine or by crane, in particular when it is to be laid down onto a support and to be removed from the latter in case of necessity. The construction of the top covers needs to take into account the fact that the electrical potential of the elements located in the intercalary space (and of the cover itself) must be either at the potential of one or the other pot, or must be floating. The difference of the potential between two adjacent pots is approximately 4 V under normal conditions of operation, but can increase up to about 100 V in case of anode effects (a phenomenon occurring exceptionally when the alumina content in the electrolyte is very low and the automatic alumina addition did not increase alumina content rapidly enough). Therefore the parts at the two different potentials must be electrically insulated from each other by means of insulating plates.

In general, this top cover does not extend over the whole surface of the intercalary space, viewed from above. While its length is usually close to the length of the potshell, the top cover is typically located in a median position, i.e. its longitudinal edges are remote from facing walls of the potshells. Therefore, two longitudinal apertures are defined between these edges and these walls, which may be covered by grids if they are so large as to present a safety risk for workers. These apertures, possibly covered by grids, allow air heated by thermal contact with the pot shell to move upwards, thereby cooling the potshell by convection. These apertures form part of the whole top structure, mentioned above.

Said top covers are heavy and are usually installed and removed using pot tending machines. From time to time it may be necessary to remove them, because during the normal operation of the plant, it may be necessary to proceed with inspection and maintenance of certain functional elements covered by the top cover. In particular, operators may need to carry out visual inspections and cleaning operations, as well as measurement of certain physical parameters, such as temperatures or voltage drops.

Furthermore, each access to the busbar systems in the intercalary space requires removal of the top cover. Such access is necessary for separating a pot electrically from the potline by inserting a so-called short-circuiting wedge in a pocket provided between two facing busbars. Such wedge insertion is necessary to cut out the pot from the series, for example when the lining must be replaced, but also during certain emergency situations when a pot needs to be shut down immediately. Removing the wedges is necessary to start up the pot, initially or after having cut it out from the series. In emergency situations, an easy access to the intercalary space covered by the top structure and, in particular, by the top cover is particularly desirable. In practice the swiftness of such access is governed by the availability of the pot tending machine which may be carrying out other operations on the potline such as change of anode or metal tapping, which cannot be interrupted or speeded up easily.

According to the state of the art, any access to the intercalary space requires the removal of the top cover by mechanical means, such as a pot tending machine or a crane. The above described known arrangement has several drawbacks, in relation with the requirements of normal and emergency situations of the plant. Indeed, the structure of the top cover and the grids does not allow a convenient access to the inner volume of the intercalary space, in normal use. Moreover, in case of emergencies, interventions of the operators can not be carried out as quickly as required. And finally, the pot tending machine might not be able to move in case of auxiliary power failure.

US 2005/0230265 and US 2006/0124471 describe arrangements of a cell, the cover of which comprises removable portions, which enable access to the electrolyte. Both these prior documents are fully unconcerned with the technical problem to which the present invention relates, i.e. the arrangement of the intercalary space defined by the facing walls of the potshells.

Hence, it is the objective of the present invention to come up with a new design of a plant of the above type, which facilities quick and convenient intervention in the intercalary space, while meeting all the requirements of electrical insulation and worker safety.

It is another objective to provide such a design, which is simple and economical to manufacture.

It is another objective to provide a process to access to the intercalary space for various purposes, and in particular for the insertion of wedges into the busbar system underneath the top cover.

Object of the invention

According to the invention, the problem is solved by an electrolysis plant using the Hall- Heroult process for making aluminium, comprising

- at least one line of adjacent electrolysis cells ranged side by side, each cell having a rectangular shape and being contained in its potshell,

- means for supplying electrical current to these cells, and

- means for electrically connecting these cells in series in order to drive this current,

- and each electrolysis cell comprising

o a plurality of anode assemblies comprising at least one anode, o a cathode comprising cathode collector bars intended to collect the cathode current at the cathode in order to carry it via cathode connections to a cathodic busbar system outside of said potshell,

o a plurality of mounting electric conductors ("risers") placed parallel to a longitudinal rim of said electrolysis cell, intended to carry the electrolysis current to the anode assemblies,

o a system of electric conductors ("cathodic busbar system") connected to said cathode connections, intended to carry the electrolysis current from said cathode connections to cathodic busbars and to risers of the adjacent downstream cell, said system of electric conductors comprising at least one means to cut a cell reversibly out of its potline,

- the main dimension of these adjacent cells being substantially orthogonal to the main direction of the line, two adjacent cells defining an intercalary space,

- said intercalary space being provided with a top structure (S) comprising a top cover (T), said top cover defining a main working surface for the walk of an operator, extending over at least part of the surface of this intercalary space, viewed from above, said electrolysis plant being characterized in that said top cover comprises at least one slab body, said slab body comprising a plurality of adjacent flaps movably connected to said slab body, each of said flaps being movable between an opening position wherein it defines a passage towards part of the inner volume of the intercalary space, and a closing position, wherein it closes said passage.

Each flap can be pivotably mounted on said body preferably around a rotation axis parallel to the facing wall of the cell. Said slab body may comprise a metallic frame, as well as a band made of an insulating structural material, such as concrete, said band extending adjacent to at least one flap, parallel to the lateral direction of the top cover. Said band may be located opposite the hinge of said flap.

Said passage is located above at least some of said functional elements of the potline, such as cathodic busbars and/or flexible current collector joints. This allows operators to access such to functional means through said passage.

In a specific embodiment of the invention, said passage is located above said means to cut a cell reversibly out of its potline. Advantageously, said means to cut a cell reversibly out of its potline comprises a wedge pocket. Easy access to this means is important in case of emergency shut down of a pot.

In an embodiment, said top cover comprises at least two slabs, located one behind the other, according to longitudinal direction of the intercalary space. At least one first slab can be provided with at least one flap, whereas at least another slab is not provided with a flap. Typically said slab not provided with a flap is located in a central longitudinal position, in particular between two central risers of the cell. On each side of said central longitudinal slab not provided with flap, a top cover comprises an intermediate slab and an end slab, both provided with at least one flap. In an embodiment that can be combined with any other embodiment of the invention, said slab body rests on at least one support member attached to a facing cell; optional insulation means can be interposed between said slab body and said support member.

In a typical embodiment, said top cover extends over only a first portion of the surface covering the intercalary space, viewed from above, and this top cover defines at least an aperture of said top structure, said aperture extending over a second portion of the surface covering this intercalary space, distinct from said first portion. The top cover can be arranged in a median lateral position of the surface covering the intercalary space, so that the top structure also comprises two lateral apertures on either side of the top cover.

Said intercalary space can also be provided with at least one cover member, preferably a grid or grating, moveable between a closing position, wherein it closes at least one part of said at least one aperture and defines a secondary working surface for the walk of an operator, and an opening position wherein it permits access to said at least one part of said aperture. Each cover member may be movably mounted on a respective support member, which is attached to the potshell. Each cover member can be pivotably mounted on said support member, preferably around a rotation axis parallel to the facing wall of the cell.

In an advantageous embodiment of the invention, said support member (which carries said slab bodies and, if present, said cover members) is fixed on a cradle surrounding the potshell. In a particularly advantageous embodiment, said support member comprises brackets hooked over said cradle.

Another object of the invention is a method of operating a plant according to any of its inventive embodiments, comprising:

- moving at least one flap from its closing position to its opening position, such that a passage located above said means to cut a cell reversibly out of its potline becomes accessible;

- carrying out in the intercalary space through said passage at least one operation of maintenance and/or repair and/or connecting or disconnecting a cell.

Said operation of connecting or disconnecting a cell may be using said means to cut a cell reversibly out of its potline in order to cut out and/or to reconnect said cell to said potline. Said means to cut a cell reversibly out of its potline may comprise a wedge pocket, and using said means may comprise inserting a wedge into and/or withdrawing a wedge from said wedge pocket. This method is particularly advantageous in case of emergency shut down of a pot in the potline. Said method may further comprise installing and using a wedge puller above said passage to remove said wedge and to cut in the cell.

Said operation of maintenance and/or repair may comprise tightening of loose cathode connections between collector bar and flexible aluminium current collector members, which are located in the intercalary space. Visual inspection of the inner volume of said intercalary space may be carried out, too. The method further comprises moving back said at least one cover member from its opening position to its closing position.

Figures

Figures 1 to 10 represent an embodiment of the present invention; they do not limit the scope of the invention.

Figure 1 is a schematic view, showing the global arrangement of a series of cells in an electrolysis plant according to the invention.

Figure 2 is a top view, showing from above the intercalary space formed between two electrolysis cells which belong to the potline of figure 1.

Figure 3 is a cross section along line Ill-Ill of figure 2, showing two gratings and a slab which extend throughout the space of figure 2.

Figure 4 is a cross section along line IV-IV of figure 2, showing another detail of the gratings and the slab of figure 3.

Figure 5 is a cross section analogous to figure 4, showing the tilting operation of the gratings of figure 3 and the insertion of a probe by the operator.

Figure 6 is a cross section analogous to figure 4, showing the tilting operation of the flap that is part of the slab of figure 3.

Figure 7 is a top view, showing at a greater scale a flap that is part of the slab of the figure 3.

Figure 8 is a top view, showing at a greater scale a grating of the figure 3.

Figure 9 is a perspective view of an opened grating showing the potshell of a potline of figure 1 ; this figure also shows a probe introduced through the open grating, similar to that of figure 5.

Figure 10 is a schematic view of the metallic reinforcement structure around which concrete is cast to form certain slabs used in the present invention.

The following reference signs are used on the figures:

C1 -Cn Electrolysis cells (pots) DH External height of potshell

L1 .L2 Lines of cells (pots) in a potline X-X Longitudinal axis of intercalary space

DC Current direction Y-Y Lateral axis of intercalary space

H1 -Hn-1 Intercalary space between two cells T Top cover of intercalary space

S1 .S2 Shells of cells (potshell) A1 .A2 Apertures

LH Length of intercalary space WT Width of top cover

WH Width of intercalary space

1A-1 E Support members 1 Ά-1 Έ Support members

1 1 Bracket 12 Body of 1 1

13 Flange 16 Bores

14 Hole 17 Stiffening Rod 15 Projection

20 Recess 21 Protrusion

3 Central Slab 31 Handling slots

4A.4B Intermediate Slabs 5A.5B End Slabs

41 ,51 Recess 42 Frame

43 Band of concrete 45 Profile of 42

46 Insulating block 47 Flange

48 Seat 49 Insulating member

6A-6E Cover members (gratings or grids) 6Ά-6Έ Cover members (gratings or grids)

A6 Rotation axis of 6

61 Recess 62 Hinge

L6 Length of 6 W6 Width of 6

63 Openings 70 Flap

A70 Rotation axis of 70 71 Slot

72 Hinge L70 Length of 70

W70 Width of 70 LF Length of flap portion

81 ,82 Cathode current collectors 83,84 Flexible current collector

85-88 Risers 89,90 Busbars

91 Wedge pocket 92 Transverse rods

94,95 Alley slabs 96,96' Fin

97,97' Cradle of C1 .C2 98 Flange

OP Operator OPEN Opening of 70

W Wedge PR Probe

P Passage through 70

100,101 Cathode collector bar 102,103 Connection

104,105 Alleys 106,107 Gap between potshell and alley slab

108,109 Slab end 1 10,1 1 1 Reinforcement bars

Detailed description

The present invention is directed to the arrangement of a plant, also called aluminium smelting plant or aluminium smelter, using the Hall-Heroult process. This plant comprises a plurality of electrolysis cells (potline) connected in series. The Hall-Heroult process as such, the way to operate the latter, as well as the general structure of above electrolysis cells are known to a person skilled in the art and will not be described here. In the present description, the terms "upper" and "lower" refer to mechanical elements in use, with respect to a horizontal working surface. Moreover, unless otherwise specifically mentioned, "conductive" means "electrically conductive".

As schematically shown on figure 1 , the aluminium smelter of the invention comprises a plurality of electrolytic cells C1 , C2, ... , Cn-1 , Cn, typically arranged 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 electrolysis current therefore passes in a cascade fashion from one cell Ci to the next cell Ci+1 , along arrow DC. The number of cells in a series is typically comprised between 50 and over 100, but this figure is not substantial for the present invention. The cells are rectangular shaped and are arranged transversally (side by side), in reference of the line they constitute. In other words the main dimension, or length, of each cell is substantially orthogonal to the main direction of the line, i.e. the circulation direction of current. The large sides of two adjacent cells are parallel.

The electrolytic cells, or pots, can implement various technological variants that do not form a part of the present invention; such pots are known to a person skilled in the art. On the figures, only the external metal (steel) shell, or "potsheH", of the cells is shown. The present invention is more particularly directed to the top structure S of the intercalary space H1 , Hn-1 , defined by two adjacent rectangular cells (C1.C2), (C2, C3), (Cn- 2, Cn-1), (Cn-1 ,Cn) of one given line. However, the invention is not directed to the structure of the intercalary space, between adjacent cells of two different lines, such as C1 and Cn on the drawings. Hereafter, the arrangement of the top structure of the intercalary space H1 between the potshells S1 and S2 of cells C1 and C2 will be described in connection to figures 2 and following. As will be described more in detail hereafter, this top structure is formed by a top cover T, as well as by two apertures A1 , A2 which may be selectively closed by moveable cover members (see figure 5). The arrangement of a majority of the other top structures of the intercalary spaces and, preferably, of all the top structures of intercalary spaces of the plant, is similar to that of the top structure of intercalary space H1.

Turning now to figure 2, the top structure S of intercalary space H1 has a rectangular shape, the length of which substantially corresponds to the length of each cell C1 , C2, whereas the width of which substantially corresponds to the distance between the potshells S1 , S2 of the two adjacent cells. By way of example, length LH is between 8 metres and 20 metres, whereas width WH is between 1.5 and 3 metres, and preferably between 1.5 and 2.2 metres. The axis X-X' defines a longitudinal direction of the intercalary space, whereas axis Y-Y' defines a transversal, or lateral direction of the intercalary space and its top structure.

The top structure of this intercalary space H1 is formed by several parts or mechanical elements, which will be described hereafter more in detail, and which form part of present invention. This top structure comprises first a top cover T, the upper face of which forms a main working surface, on which operators can freely walk. By way of example, the external height of the potshell as defined on figure 3 is typically between 1.5 and 2.0 metres. This top cover T defines two distinct first and second portions of the intercalary space H1 , viewed from above. It extends in a median lateral location, over the first portion of this intercalary space H1 which contains parts of the cathodic busbar system. Moreover this top cover T defines, with facing walls of respective cells C1 , C2, two lateral and globally symmetric apertures A1 , A2 (see in particular figure 5).

Therefore the first portion of whole top structure is formed by top cover T, whereas second portion of this top structure is formed by these two apertures, which may be covered as will be explained hereafter. In alternative embodiments (not shown on the figures), the top cover T may define one single aperture, between one cell and facing lateral edges of this top cover. However, it is preferred that top cover T is in a median lateral position with respect to the width of the intercalary space, as shown on the figures.

Some mechanical elements of the plant, which are known per se and will not be described in detail, are accommodated in the intercalary space, i.e. underneath its top cover. Referring to figure 3, two first cathode current collectors 81 and 82, or so-called cathodic busbars, extend along respective upstream C1 and downstream C2 cells. Each cathodic busbar is electrically linked through flexible current collectors 83, 84 with the respective cathode collector bars of a pot. More precisely, cathodic busbars 81 , 82 are connected through flexible current collectors 83, 84 to cathode collector bars 100, 101 that cross the potshell using connections 102, 103 that can be, for example, clamped or welded. Several risers 85, 86, 87, 88 (shown on figure 2) connect one of these cathodic busbars to the anode beam of the adjacent downstream cell. Turning back to figure 3, two further busbars 89, 90 extend between the above first busbars, parallel to the latter. Facing walls of these busbars are parallel and vertical.

However, at different locations, the wall of one of these busbars 89 and 90 is tapered towards the bottom, which makes it possible to define so called "wedge pockets", the width of which decreases towards the bottom. One 91 of these pockets is more clearly shown on figures 4 and 6. Transverse rods 92, electrically insulated from the busbars, extend through said busbars and cooperate with end bolts in order to laterally stabilize the busbars in the neighbourhood of the wedge pocket (see figure 4); this busbar system comprising wedge pockets is known as such. The function of these wedge pockets will be described hereafter, in relation with the way to carry out emergency operations on busbars, which is facilitated by the removable hinged flaps according to the invention.

As will be described more in detail hereafter, the top cover T of the intercalary space is formed by several slabs 3, 4A, 4B, 5A, 5B, which are mounted on support members 1A- 1 E, as well as 1Ά-1Έ (see figure 2). As can be seen on figure 5 in relation with figure 2, the two lateral apertures A1 , A2 may be selectively closed thanks to moveable cover members 6A-6E, as well as 6Ά-6Έ, which are also mounted on above mentioned support members. At its opposite longitudinal ends, the top cover leads to alley slabs 94 and 95, which are typically made of concrete and which run beside the line of cells, in a known way, parallel to the alleys 104, 105 made of concrete which are fit for circulation of heavy vehicles.

Support members 1A-1 E, 1Ά-1Έ extend over substantially the whole length of each cell as shown on figure 2. In the illustrated example, five support members, the length of which is similar, are provided for each cell. Each support member, such as reference number 1A of figure 9, comprises several brackets 1 1 arranged one behind the other. Each bracket 11 is attached to the adjacent potshell (see in particular figure 4). The cradle 97, 97' of the potshell is provided with flanges 98 which project towards the facing potshell. Each bracket 11 comprises a body, the upper surface of which supports the cover member 6A-6E. Each bracket 1 1 further comprises a recess 20 and a protrusion 21 and is hooked over said flange of the cradle, by a vertical downward movement, such that said flange 98 fits into said recess 20, cooperating with said protrusion 21 to lock it in its position.

At their upper end, at substantially the same height as the top cover, two adjacent brackets define a flange 13 (see figure 9) provided with a hole 14 (see figure 4) for the passage of an articulation pin (not shown on the figure). Each bracket is also provided with a projection 15 (see figure 4), which extends horizontally towards the opposite potshell. This projection is adapted to form a seat for the slabs of the top cover, as will be explained hereafter. Finally bores 16 in each bracket define passages for a stiffening rod 17 (see figure 9), which extends along the whole support member; said stiffening rod 17 can be welded to the bracket 11.

Turning to figure 2, top cover T first comprises a central slab 3, which globally extends between central risers 86 and 87. This central single slab, typically made of concrete, can be handled by a crane or analogous thanks to handling slots 31. It may rest on the projections 15 of the support brackets of the support means. This central slab has no moveable member and its main function is to allow operators to walk on it.

On each side of central slab 3, the top cover also comprises two further removable slabs, i.e. a respective intermediate slab 4A and 4B, as well as a respective end slab 5A and 5B. Each intermediate slab is adjacent to a respective central riser 86 and 87, whereas each end slab is adjacent to a respective end riser 85 and 88. The structure of intermediate and end slabs is however different from that of central slab. As shown first on figure 2, each intermediate or end slab is first provided with a recess 41 or 51 , to allow the passage of a corresponding riser.

Figure 3 shows slab 4B more in detail, bearing in mind that other slabs 4A, 5A and 5B have a similar structure. This slab 4B comprises a body, formed by a metallic frame 42 as well as a band 43 of structure material, typically concrete. This band, which extends laterally over only part of the slab, is embedded in the metallic frame. As can be seen on this figure 3, metallic frame comprises a profile 45, which rests on the seat 15 of the facing bracket, with interposition of an insulating block 46. At its upper end, said profile defines a flange 47 for a flap 70, which can rotate relative to the frame around a longitudinal axis A70 (see figure 7). At its free end, opposite the flange, the flap may abut against a seat 48 of the frame, adjacent the band of concrete 43.

In the shown example, each intermediate slab 4A and 4B is provided with four flaps, whereas each end slab 5A and 5B is provided with three flaps. Each flap 70 has a slot 71 (see figure 7), which allows a single operator to lift the flap using an appropriate grasping tool, or by introducing his hand (protected against heat by a suitable glove) into the slot 71. In a preferred manner, each flap is plain (i.e. is not a grid) which renders easier the walking of operators and avoids hot air coming out of the intercalary space in the area where operators will walk. In the shown example, each flap has two hinges 72 (see figure 7), each of which is adapted to cooperate with a respective flange 47 of the frame.

The flap 70 is typically made from steel. Its mass is typically sufficiently low so that, in spite of the high magnetic field, is can be opened by a single operator using his hand and arm, and possibly a hand tool (such as a rod) to loosen it. In the shown example, each flap is rectangular (in the example it is in fact quadratic) and, in a preferred manner, its width W70 (see figure 7) is greater than 0.5 m, in particular greater than 0.6 m. Therefore, the flap may define a passage which is large enough to allow introducing tools (such as a voltage probe) or accessories, and in particular a short-circuiting wedge, as described hereafter. Preferably flaps 70 are arranged in a way such that there is a flap just above a wedge pocket 91. It is obviously possible, if necessary, to open two adjacent flaps in order to access more readily to the intercalary space between two adjacent pots, and according to the invention this can carried out by a single operator, possibly using a hand tool or rod. The above described figure 3 refers to the part of the slab 4B, which is remote from a riser. Let us turn now to figure 4, which shows the part of this slab which is adjacent to facing riser. This latter part differs, from that shown on figure 3, in that metallic frame 42 comprises an insulating member 49, of any known and appropriate type (such as a plate of epoxy resin), which projects towards the facing riser. Therefore, any accidental contact between metallic frame and riser (which are at a different electric potential) is avoided, so as to prevent arcing. In the illustrated drawings (in particular figure 2), slabs 4A, 4B, 5A and 5B, provided with flaps 70, extend between a central riser 86 or 87 and an end riser 85 or 88, whereas another slab 3, not equipped with flaps (because not located above wedge pockets), extend between the two central risers 86 and 87. According to variants not shown on the figure (and which are less preferred), slabs provided with flaps may form the whole length of the top cover, including the portion between central risers. The length LF of the so called "flap portion" of the top cover, i.e. portion provided with flaps, corresponds to the sum (LF1 + LF2), as shown on figure 2. In a preferred way, the ratio (LF / LH) between the length of this flap portion and the whole length of the intercalary space H, is superior to 0.5, advantageously superior to 0.6.

Still referring to figure 2, cover means comprise several gratings 6A to 6E, as well as 6Ά to 6Έ, which are arranged the one beside the other. These gratings are optional in the framework of the present invention. In a very advantageous variant of this embodiment, said gratings are pivotably connected to a support member. In the shown example, there are as many gratings as support members, i.e. five gratings for each cell. As shown on figure 8, each grating is provided with recesses 61 , the walls of which form hinges 62 which are intended to cooperate with the flanges 13 of the brackets 1 1. Therefore, the above mentioned articulation pin may cross both these hinges and these flanges, to rotate the grating on the bracket around a longitudinal rotation axis A6 (see figure 8) which is parallel to the facing wall of the potshell.

Viewed from above, as also shown on figure 8, each grating (such as 6A) is substantially rectangular, with a main dimension parallel to that of potshell. By way of example, its length L6 is between 1.5 m and 3.0 m, whereas its width W6 is between 0.1 m and 0.4 m. The grating is typically made from steel. Its mass is typically sufficiently low so that, in spite of the high magnetic field, it can be opened by a single operator using his hand (protected against heat by a suitable glove) and arm, and possibly a hand tool (such as a rod) to loosen it. The grid or grating is provided with a net of bars, which define elementary openings 63, the typical size of which is from 10 to 40 millimetres.

In the illustrated example, the two rows of gratings cover substantially the whole length of each aperture A1 and A2; they are located above cooling fins 96 that protrude away from the potshell S1 to the centre of the intercalary space. However, at least one row of these gratings may not extend along a whole aperture A1 or A2. In a preferred way, each row of gratings extend over at least 80%, advantageously at least 90 %, and typically about 100%, of the length of a corresponding aperture. In case part of the length of an aperture is not provided with gratings, it may be permanently covered by an integral slab, such as that marked as reference number 3 on figure 2.

In normal operation of the plant, as shown on figure 3, both the flaps 70 and the gratings (in particular those 6D and 6'D of this figure 3) are in their closed position. On the one hand, operators may freely walk on the plain upper surface of the closed flaps without any risk of falling into the intercalary space. Therefore, these flaps form part of the top cover of the intercalary space. In a variant (not shown on the figures), at least some of the flaps may be provided with small openings, similar to those marked as reference number 63 on the gratings, although this is not a preferred option. On the other hand, an operator may freely walk on the upper surface of the closed gratings without any risk of injury, due to the small size of the openings 63. Therefore these gratings form part of the top structure S of the intercalary space. As an alternative (not shown on the figures), at least some of the gratings may be plain (possibly replaced by a sheet flap, similar to the flaps of the figures) or plugged; this allows fine tuning of convective heat losses and can be useful if cooling of the potshell needs to be limited. In the illustrated embodiment, the openings of the gratings are wide enough as to enable the access of some specific tools or accessories into the intercalary space. By way of example, pipes of small section may be laid along the potshell, in order to cool the latter and to avoid its deformation, in particular during start-up and early operation of the pot. Such an operation does not require the tilting (opening) of the gratings; indeed, the pivotable gratings or grids should not remain open unnecessarily for safety reasons.

Providing insulator blocks 46, between the seats 15 of the brackets 1 1 and the metallic frame 42 of the slabs, is advantageous. Indeed, each bracket is at the same potential as that of a respective cell. Moreover, in normal use, there is a difference of potential between two adjacent cells, such as C1 and C2 of the drawings, which is typically between 4.5 and 5 Volts, but may reach up to 50 or even 100 Volts in case of so-called anode effects. Hence, insulation makes it possible to keep each bracket at the potential of the closest cell.

As mentioned above, electrical potentials require some consideration in the framework of the present invention. Starting with the alleys 104, 105, they are supported by a structure that is insulated from the earth: the alleys 104, 105, made from concrete, are at floating potential. Alley slabs 94, 95 are removable by a crane; they have their external protective (but discontinuous) steel frame connected to reinforcement steel bars at three different potentials: at the potentials of each of the two pots on one side, and on the other side at the floating potential of alleys 104, 105. Each of the cover members 6A-6E, 6Ά-6Έ is at the potential of the adjacent pot. Said central slab 3, intermediate slabs 4A, 4B and end slabs 5A, 5B are each connected to cover members 6A-6E, 6Ά-6Έ. The extremity of the outer cover members 6A, 6Ά, 6E, 6Έ must be insulated with respect to the adjacent alley slabs 94, 95. Likewise, the slab ends 108, 109 (which may be made in concrete or in steel) need to be insulated from end risers 85, 88, preferably by using vertical insulation pads.

Alley grids (not shown on the figures) may be provided parallel to the alley slabs 94, 95 to cover the gap 106, 107 between the alley slabs 94, 95 and the potshell: they are typically welded to the poshell. Risers 85, 86, 87, 88 are at the cathode potential of the neighboring upstream pot, they need to be insulated from the top cover T, preferably by vertical insulating pads.

In relation with the management of electrical potentials, the following embodiment of the invention is particularly preferred: it applies to any slabs or parts of slabs that are made of concrete, such as the concrete part of central slab 3 or the concrete part of end slabs 5A, 5B or the concrete part of intermediate slabs 4A, 4B, and will be explained here with reference to figure 10 for the band of concrete 43 of intermediate slab 4B as shown on figure 4. These slabs each have a core made of concrete, which is partly surrounded by an external frame. This frame is formed of a first and a second frame member, each comprising a steel band 42A and 42B and reinforcement bars 110, 1 11.

First 42A and second steel 42B bands extend on a respective longitudinal rim; they form a (discontinuous) frame capable of protecting the concrete externally. Said first steel band 42A is connected to first plurality of parallel concrete reinforcement bars 1 10, and said second steel band 42B is connected to second plurality of parallel steel bands 1 11. Said reinforcement bars 110, 11 1 are typically made of steel, and said connection to the steel bands 42A, 42B is typically made by welding; this leads for each of the first steel band 42A and second steel band 42B to a comb-like structure. These first and second comblike structures are acting as reinforcing structures, as schematically shown in figure 10. Knowing that each of the adjacent gratings 6A-6E, 6A'-6E' are each at the potential of the adjacent cell, it is essential that said first and second reinforcing structures do not come into electrical contact. When pouring the concrete into the mold, it is therefore useful to use spacers made from an insulating material in order to ensure that the reinforcement bars 110 of said first reinforcing structure will never come in contact with the reinforcement bars 11 1 of said second reinforcing structure.

Let us suppose now that an operator needs to access the portion of the inner volume of the intercalary space, which is located under the top cover, in particular to the busbars 89 and 90. He may stand on the concrete band 43 and tilt at least one flap 70, from its closing position of figure 3 to its opening position of figure 6 (see arrow OPEN). Once the flap 70 has been tilted to its opened (raised) position, it clears the passage P, through which wedges or voltage probes may be introduced. As shown on figure 6, to locate the concrete band 43 opposite the hinge 47 is advantageous, since the operator may stand on this band and access to the inner volume, without being hindered by the raised flap 70. Of course the operator may also open two or more adjacent flaps if he need to access to a larger portion of the intercalary space, for example for cleaning of elements of the cathodic busbar system. In the illustrated embodiment of figure 6, the operator OP drops a wedge W in the pocket 91 , along arrow AW. This permits, in a known manner, to electrically separate the adjacent pot from the potline; as explained above, this operation can be carried out as a scheduled operation or as an emergency procedure. According to other variants (not shown on the figures), once passage P is cleared upon tilting of the flap, the operator may carry out other usual operations, such as tightening of loose cathodic connection elements, or installing a wedge puller to restart the pot.

Flaps 70 according to the invention are advantageous, since their handling is convenient and quick. In particular, they can be tilted easily within a very short period of time, in case of an emergency, and by a single operator without the use of tools. By comparison, prior art teaches top covers comprise integral slabs which are not provided with mobile opening elements. Therefore, to access the inner volume of the intercalary space, according to prior art, the whole slab of prior art must be removed by a pot tending machine or crane. This is a rather time-consuming operation, and requires the pot tending machine or crane to be immediately available, which may not be consistent with a swift response to emergency situations, and in particular for the management of certain abnormal situations (such as auxiliary power outage).

Let us suppose now that an operator needs to monitor at least one parameter in connection with the operation of the cell, typically the potshell temperature or the cathode connection voltage drops. The operator may tilt at least one grid or grating, from its closed position of figure 3 to its opened position of figure 5 (see arrow OPEN). He may then have access to the inner volume of the intercalary space, via aperture A1 or A2 which has much larger dimensions than those of the openings 63 provided in the gratings. As illustrated on figure 5, the operator OP, standing on the concrete band 43, or on the closed flap 70, may introduce any appropriate tool, such as a probe PR, in order to carry out a relevant operation, amongst which: electrical voltage drop measurements, potshell side temperature measurement, and/or cathode current distribution measurements. In particular, electrical voltage drop measurements may require to establish an electrical contact between probe PR and flexible current collectors 82, 83 or connections 102, 103, which are located deep below the top cover level in the intercalary space: knowing that said probe PR is a rather heavy rod and said intercalary space is not illuminated, this is much easier if the grid or grating is lifted, according to the invention, than across the grating closed grating, according to prior art.

Temperature measurements can also be carried out contactless by an infrared probe, but will, too, be far easier (and possibly more reliable) if the grating is open.

Gratings according to the invention are optional but advantageous, since their handling is convenient and fast; they can be opened by a single operator, without using a pot tending machine or crane. They facilitate access to the apertures.

In normal operation of the plant, the body of each slab 4A, 4B, 5A and 5B, i.e. its metallic frame and its concrete band, is fixed in relation with the potshells. It means that this body is stationary, whereas only the flaps (and possibly the gratings) are adapted to be moved between their above described closing and opening positions. It is however to be noted that these slabs 4A, 4B, 5A and 5B may be removed from support members, in case of situations out of normal use, such as repair of the cathodic busbar system. These slabs may then be lifted and put aside by a pot tending machine, in the same way as that described for central slab 3.

Claims

1. An electrolysis plant using the Hall-Heroult process for making aluminium, comprising

- at least one line (L1 , L2) of adjacent electrolysis cells (C1 , Cn) ranged side by side, each cell having a rectangular shape and being contained in its potshell (S1 , ... , Sn),

- means for supplying electrical current to these cells, and

- and each electrolysis cell comprising

- the main dimension of these adjacent cells being substantially orthogonal to the main direction (AC) of the line, two adjacent cells defining an intercalary space (H1 , Hn-1),

- said intercalary space being provided with a top structure (S) comprising a top cover (T), said top cover defining a main working surface for the walk of an operator (OP), extending over at least part of the surface of this intercalary space, viewed from above,

said electrolysis plant being characterized in that said top cover (T) comprises at least one slab body (4A, 4B, 5A, 5B), said slab body comprising a plurality of adjacent flaps (70) movably connected to said slab body, each of said flaps being movable between an opening position wherein it defines a passage (P) towards part of the inner volume of the intercalary space (H), and a closing position, wherein it closes said passage.

2. A plant according to claim 1 , characterized in that each flap is pivotably mounted on said body preferably around a rotation axis (A70) parallel to the facing wall of the cell.

3. A plant according to any of claims 1 to 2, characterized in that said slab body comprises a metallic frame (42), as well as a band (43) made of an insulating structural material, such as concrete, said band extending adjacent to at least one flap (70), parallel to the lateral direction (Y-Y) of the top cover (T).

4. A plant according to claim 3, characterized in that said band (43) is located opposite a hinge (72) of said flap.

5. A plant according to any claims 1 to 4, characterized in that said passage (P) is located above at least some functional elements of the potline, such as cathodic busbars and/or flexible current collector joints.

6. A plant according to any of claims 1 to 5, characterized in that said passage (P) is located above said means to cut a cell reversibly out of its potline.

7. A plant according to any of claims 1 to 6, characterized in that said means to cut a cell reversibly out of its potline comprises a wedge pocket.

8. A plant according to any of claims 1 to 7, characterized in that said top cover (T) comprises at least two slabs (3, 4A, 4B, 5A, 5B), located one behind the other, according to longitudinal direction (X-X) of the intercalary space.

9. A plant according to claim 8, characterized in that at least one first slab (4A, 4B, 5A, 5B), is provided with at least one flap (70), whereas at least another slab (3) is not provided with flap.

10. A plant according to claim 9, characterized in that said slab (3) not provided with flap is located in a central longitudinal position, in particular between two central risers (86, 87) of the cell.

1 1. A plant according to claim 10, characterized in that, on each side of said central longitudinal slab (3) not provided with a flap, top cover comprises an intermediate slab (4A, 4B) and an end slab (5A, 5B), both provided with at least one flap.

12. A plant according to any of claims 1 to 1 1 , characterized in that said slab body rests on at least one support member (1 A - 1 E, 1 Ά - 1 Έ) fixedly attached to a facing cell.

13. A plant according to any of claims 1 to 12, characterized in that said top cover (T) extends over only a first portion of the surface covering the intercalary space, viewed from above, and this top cover defines at least an aperture (A1 , A2) of said top structure (S), said aperture extending over a second portion of the surface covering this intercalary space, distinct from said first portion.

14. A plant according to claim 13, characterized in that the top cover is in a median lateral position of the intercalary space, so that the top structure also comprises two lateral apertures (A1 , A2), on either side of the top cover.

15. A plant according to any of claims 1 to 14, characterized in that said intercalary space is also provided with at least one moveable cover member (6A - 6E, 6Ά - 6Έ), preferably a grid or grating, moveable between a closing position, wherein it closes at least one part of said aperture (A1 , A2) and defines a secondary ground for the walk of an operator, and an opening position wherein it permits access to said at least one part of said aperture.

16. A plant according to claim 15, characterized in that each moveable cover member (6A - 6E, 6Ά - 6Έ) is movably mounted on said respective support member (1A - 1 E, 1 Ά - 1 Έ).

17. A plant according to claim 16, characterized in that each moveable cover member is pivotably mounted on said support member, preferably around a rotation axis (A6) parallel to the facing wall of the cell.

18. A plant according to any of claims 12 to 17, characterized in that said support member (1A - 1 E, 1 Ά - 1Έ) is fixed on a cradle (97, 97') surrounding the potshell.

19. A plant according to claim 18, characterized in that said support member comprises brackets (1 1) hooked over said cradle (97, 97').

20. A plant according to any of claims 1 to 19, characterized in that at least one slab comprises a core and a frame, said frame comprising two frame members (42A, 110; 42B, 1 11), a first frame member being at an electrical potential close to that of a first cell, and a second frame member being at an electrical potential close to that of a second cell, said first and second frame members being mutually electrically isolated.

21. A plant according to claim 20, characterized in that each of said frame members comprises a longitudinal band (42A, 42B) and a series of reinforcement lateral bars (1 10, 11 1), each series of lateral bars (110, 11 1) forming a comb-like structure.

22. A method of operating a plant according to any of claims 1 to 21 , comprising:

- carrying out in the intercalary space, through said passage, at least one operation of maintenance and/or repair and/or connecting or disconnecting a cell.

23. A method according to claim 22, wherein said operation of connecting or disconnecting a cell is using said means to cut a cell reversibly out of its potline in order to cut out and/or to reconnect said cell to said potline.

24. A method according to claim 23, wherein said means to cut a cell reversibly out of its potline comprises a wedge pocket, and wherein said using of said means comprises inserting a wedge into and/or withdrawing a wedge from said wedge pocket.

25. A method according to claim 24, further comprising installing and using a wedge puller above said passage to remove said wedge and to cut in the cell.

26. A method according to claim 24, wherein said operation of maintenance and/or repair comprising tightening of loose cathode connections between collector bar and flexible aluminium current collector members.

27. A method according to any one claims 22 to 26, further comprising visual inspection of the inner volume of said intercalary space.

28. A method according to any of claims 22 to 27, further comprising moving back said at least one cover member from its opening position to its closing position.