GB2572564A

GB2572564A - Potshell for electrolytic cell to be used with the Hall-Heroult process

Info

Publication number: GB2572564A
Application number: GB1805436.1A
Authority: GB
Inventors: Kumar Mishra Lait; Nazeer Ahamad Basha Kasim
Original assignee: Dubai Aluminium PJSC
Current assignee: Dubai Aluminium PJSC
Priority date: 2018-04-03
Filing date: 2018-04-03
Publication date: 2019-10-09
Also published as: GB201805436D0; WO2019193451A1

Abstract

A potshell (40) for an electrolytic cell (figure 1, 1) for Hall-Heroult electrolysis process is detailed. The potshell has a plurality of parallel cathode blocks (figure 1, 8), where each cathode block comprises a metallic cathode collector bar (figure 1, 9) which protrudes out of each of the two ends of the cathode block (figure 1, 8). A lateral lining (figure 1, 5) which in combination with the cathode defines a volume containing the liquid electrolyte (figure 1, 11) and the liquid metal (figure 1, 12) resulting from the Hall-Heroult electrolysis process. The potshell (40) itself comprises: a bottom wall (45) and peripheral walls (41-44) which extend upwards from said bottom wall (45) The peripheral walls comprise side walls (41, 42) and end walls (43, 44), with a plurality of reinforcement members or stiffeners (50, 60). The stiffeners also comprise an attachment wall (51) with a facing wall (42) of potshell. The potshell being attached by fixation means. The attachment may be permanent and achieved via welding.

Description

Technical field of the invention

The invention relates to the technical field of electrolysis in molten salts for making aluminium using the Hall-Heroult process. More precisely, the invention relates to an improved potshell design that maximizes the usable cathode surface area for a given total outside dimension. This is achieved in particular by redesigning the cradles of the potshell.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (AI₂O₃) 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 “pot shell”), a lining (comprising refractory bricks protecting said steel shell against heat, and cathode blocks usually made from graphite, anthracite or a mixture of both), a superstructure and a plurality of anodes (usually made from carbon) wherein part of anodes is submerged into the liquid electrolyte. Anodes and cathodes are connected to external aluminium busbars. An electrical current is passed through the cell (typically at a voltage between 3.7 V and 5 V) which electrochemically reduces the aluminium oxide, split in 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 tapped 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^thcentury 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 meters and a width usually comprised between 3 and 5 meters. The cells (also called “pots”) are always operated in series of several tens (up to more than four hundred) 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 sideby-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 production of aluminium in an electrolytic cell is proportional to the current supplied to 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. It is generally accepted that modern electrolysis cells using the Hall-Heroult process are limited to an electrode current density of the order of 1 A / cm², and that the productivity of the cell is proportional to the area of the electrodes, which can be represented as the area of the cathodes or anodes in a horizontal plane.

The present invention is more particularly related to the potshell of such electrolysis cells. Potshells are made of low carbon structural steel. Their interior cavity is defined by sheet steel and has a cuboid shape, the potshell bottom being horizontal and the potshell sidewalls being arranged so as to lie approximately vertically. The sheeting is stiffened by means of an external carrying structure. The external carrying structure includes an upper rim (also called “deckplate”) that forms the circumference of the rectangular structures, and lateral structural elements (so-called “cradles”) arranged at a right angle with respect to the direction of the sidewall, at a regular spacing.

As mentioned above, the interior cavity contains the lining, i.e., refractory bricks to protect the sheeting, and the cathodes and the side lining intended to be in contact with the liquid electrolyte and liquid metal (in operation, the side lining is protected from the liquid electrolyte by a layer of frozen electrolyte). An array of regularly spaced windows, arranged on a straight line parallel to the upper rim, is provided in both long sidewalls, to allow the cathode bars to cross the sidewall; these windows are then tightly closed with carbonaceous or refractory material. Pots need to be relined regularly; the typical lifetime of a lining is five to eight years. During relining the old lining is removed, the potshell is cleaned and then a new lining is installed. The lifetime of a potshell can be of the order of two decades, or even longer.

Potshells represent a significant cost. Their lifetime is limited by deformation: as they support a permanent load at elevated temperature, potshells are subject to creep. Also, transient abnormally high temperatures of the potshell (for example during start-up and during so-called prolonged anode effects or when the pot is “sick”) may lead to transient deformation of the steel potshell due to thermal expansion and softening; overheating can also lead to permanent deformation of the potshell. Over the years of use, the potshell tends to “open”, that is to say permanently deformed; the deformation is quite significant in the longitudinal, in the transverse as well as in the vertical directions. As the length of the newly designed pots tends to increase, the forces acting on the end walls of the potshell tend to increase, too due to accumulation of the expansion of the individual cathode blocks towards the end walls. These forces need to be taken into account when designing very long electrolytic cells.

Furthermore, in order to bring down the capital cost per unit of aluminium produced in the plant, it is desirable to maximise the surface area of the inner cavity for a given footprint (i.e. outside area of the potshell), without compromising on mechanical integrity, strength and lifetime. As an example, it would be desirable to replace an end-of-life potshell in a potline by a new potshell that fits into the available space (i.e. having the same footprint) but offers a greater cathode surface area; this will lead to a direct increase of both the production and the productivity of the pot (provided that the additional electrical power is actually available along with the availability of the longer anodes). Providing a potshell design that is easy to fabricate and has lesser weight will also contribute to minimize the capital cost, as well as subsequent maintenance cost. The present invention provides improvements of the potshell design that meets these targets.

It should be noted that most modern high amperage cells use graphitized or graphitic cathodes that absorb much less sodium from the bath than anthracitic cathodes. Sodium absorption by anthracite leads to swelling, which needs to be counteracted by very large confining forces. As a consequence, potshells used in modern high amperage cells do not need to withstand swelling forces that are as high as in older cells; this may be counterbalanced at least in part by their larger size which increases deformation.

US 2,861,036 (Pechiney), US 4 322 282 (Swiss Aluminium Ltd), and US 4,421,625 (Swiss Aluminium Ltd) are representative for a potshell design that needs to withstand enormous swelling forces; the first patent uses springs to exert a counterforce against opening forces, in second and third the potshell is stiffened by a set of horizontal expansion rails, with additional springs in the third one. US 3,702,815 uses horizontal rails only for the short sides of the potshell.

WO 2011/028132 (Norsk Hydro) discloses a potshell with vertical stiffeners and horizontal webs on its outside, said web having openings for efficient air cooling.

WO 2016/077932 (Hatch Ltd) presents a potshell design wherein the sidewalls are stiffened using vertical structural elements with free upper ends acting as cantilever springs that can be loaded and adjusted using wedges. These wedges need to be adjusted manually throughout the life of the potshell, using a hammer, a portable hydraulic jack or a wrench. While this design seems indeed to be able to provide a larger cathode surface for a given footprint compared to prior art cells, the perspective of manual finetuning of several dozens of wedges per cell during the lifetime of the pot, carried out in the hot working environment of a potroom and at a location of the pot that is particularly hot and not readily accessible is not especially appealing for the workers involved in this operation, and for their supervisors.

Moreover, all these potshell designs are complex and do not meet all of the multiple goals mentioned above: low capital cost, sufficient strength, long lifetime, small footprint. The present invention aims at providing an improved potshell design that meets at least several of these goals.

Objects of the invention

According to the invention, the problem is solved by a redesign of the potshell that uses modified cradles, thereby increasing their strength and decreasing their dimensions, and more precisely, both their height (i.e. their dimension parallel to the height of the potshell) and their width (i.e. the dimension in the direction protruding out of the sidewall to which they are attached, parallel to the upper rim of the adjacent sidewall).

Compared to the approach described in WO 2016/077932 the side stiffeners are not spring-loaded structural elements but rigid profiles. Said profiles can be welded profiles.

According to an essential feature of the invention, these stiffeners are fixedly connected to the shoebox along substantially their whole length. According to a first variant of the invention, fixation means between stiffeners and shoebox are distinct from both said stiffeners and shoebox. In other words, said fixation means are external or inserted fixation means. In an advantageous embodiment, said fixation means are welding means, so that stiffeners are welded against the shoebox; said welding connection can comprise point welding spaced over substantially the whole length of the stiffener, and/or can comprise one or more welding seams, such as one continuous welding seam extending over at least three quarter (and preferably over at least 90%) of the length of the stiffener, or two or more sections of welding seams spaced over substantially the whole length of the stiffener.

According to a second variant of the invention, said fixation means are form-fitting or interlock fixation means. In other words, this second variant does not require external fixation means, since the latter are defined by the facing shapes of stiffeners and shoebox themselves. In this respect, said stiffeners and shoebox are adapted to directly contact one another in view of said form-fitting fixation. When starting the pot (i.e. starting the electrolysis process), stiffeners and shoebox have been mutually mounted, potentially providing a slight clearance there between; this will help to release stresses. Typically, such a clearance is inferior to 5 mm, preferably inferior to 3 mm, and still more preferably inferior to 2 mm. Then, during operation of the pot, said shoebox progressively undergoes a functional deformation, in particular by creep, so that said shoebox and said stiffeners come into close contact and create said form-fitting fixation.

The stiffeners are arranged in a way such that between each two neighboring stiffeners there is a cathode collector bar protruding out of the potshell; this is known as such and applies both in the case where each cathode block has one collector bar protruding out of the cathode block on each end, and in the case where each cathode block has two cathode collector bars protruding out of the cathode block on each end.

A first object of the present invention is a potshell for an electrolytic cell suitable for the Hall-Heroult electrolysis process, said potshell being intended to receive firstly a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block comprising at least one metallic cathode collector bar protruding out of each of the two ends of the cathode block, and secondly 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 potshell comprising a bottom wall and peripheral walls extending upwards from said bottom wall, so as to define an inner reception volume, said peripheral walls comprising side walls and end walls, and said potshell further comprising a plurality of reinforcement members or stiffeners, provided side by side along at least part of said peripheral walls, said potshell being characterized in that at least part of said stiffeners comprise at least one attachment wall, said attachment wall being provided with an attachment region, said potshell further comprising fixation means, said fixation means allowing for attachment of said attachment region with facing wall of potshell.

In an embodiment said fixation means are permanent fixation means such a welding means. In any case they can be distinct from both said attachment region and said facing wall of potshell.

In another embodiment said fixation means are defined by both said attachment region and said facing wall of potshell, said attachment region and said facing wall being adapted to directly contact one another in view of a form-fitting fixation.

Advantageously, viewed from above, said attachment region defines an attachment line, the main dimension of attachment line being superior to 30 millimeters, preferably to 60 millimeters, more preferably to 100 millimeters.

Said stiffeners provided with said attachment wall can be located on side walls of said potshell. They can be formed by a hollow body, which is closed by peripheral walls, in cross section, said peripheral walls comprising said attachment wall. Said peripheral walls can for example be polygonal, in particular rectangular. At least the top of said hollow body can be open.

According to another advantageous feature of the invention the height of the stiffeners is reduced with respect to stiffeners used in prior art pot shells. More precisely, their height does not reach the upper rim of the potshell. Therefore, in another embodiment which can be combined with any other embodiment of the invention, at least part of said stiffeners extend only over a lower region of said side wall of said potshell, so as to define a socalled free upper region of said side wall of said potshell. In particular, the ratio between the height of the stiffener and the height of the side wall of the potshell is between 0.4 and 0.7, in particular between 0.45 and 0.55.

In an advantageous embodiment the height of the stiffeners does not exceed the level of the top of the cathode blocks by more than 15 cm, preferably not by more than 10 cm, still more preferably not by more than 5 cm, and still more preferably not by more than 2 cm. It can be at the same level. There is no advantage not to reach this level, and a loss of mechanical stiffness would result from this option.

The design according to the invention has several advantages. First of all, it uses less steel while providing an equivalent stiffness compared to prior art. Secondly, it leaves free a significant portion of the shoebox, that is to say a substantially planar portion of the potshell, for the action of heat exchanger and/or cooling devices. In particular, it leaves free the hottest portion of the potshell, that is to say the zone corresponding to the position and thickness of the liquid metal pad and the molten bath. This zone typically extends until about 30 cm to 50 cm above the bottom of the cathode blocks.

According to a particularly advantageous embodiment of the invention, said stiffeners extend only over part of said side wall, and in particular only over a lower region of side wall, so as to define a free upper region, as mentioned above. Said free upper region can be equipped with heat exchange means. Said heat exchange means can be fixed against a planar and smooth region of the potshell (shoebox). Providing said heat exchange means on the upper region of side walls can be advantageous, and in particular if mounted close to a height of the potshell that corresponds to the level of the liquid bath, as this is the hottest region of the potshell, in use.

Said heat exchange means may be of any appropriate type. In particular, they may be similar to those described in patent documents WO 2012/039642, WO 2013/055228, WO 2013/055229 and WO 2014/182176. According to the invention, said heat exchange means extend substantially horizontally, along above cited free upper region of the potshell.

Another advantage of the invention is that the total depth of the sidewall stiffeners (measured from the shell outside to the outermost wall of the stiffener) is reduced with respect to prior art; it can be less than 180 mm, preferably less than 165 mm, and still more preferably less than 155 mm, whereas in the design disclosed in WO 2016/077932 this value is given as about 200 mm.

Another object of the present invention is an electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks (each cathode block comprising at least one metallic cathode collector bar protruding out of each of the two ends of the cathode block), 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, an outer metallic potshell containing said cathode and lateral lining, a plurality of anode assemblies suspended above the cathode (each anode assembly comprising at least one carbon anode and at least one metallic anode rod connected to an anode beam), a cathodic bus bar surrounding said potshell, a plurality of connectors (each connecting one end of a cathode collector bar of a cathode block to said cathodic bus bar), said electrolytic cell being characterized in that said outer metallic potshell is a potshell according to the invention.

Yet another object of the present invention is an aluminium electrolysis plant comprising at least one line of electrolysis cells of substantially rectangular shape, 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 60 % (preferably 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 the invention.

Yet another object of the invention is a method for making aluminium by the Hall-Heroult electrolysis process, characterized in that said method is carried out in an aluminium electrolysis plant according to the invention.

Yet another object of the invention is a method for adding heat exchange means to a potshell, wherein said cooling fins are removed from said upper free region, and said heat exchange means are mounted in said upper free region.

A last object is a method of operating a potshell according to the invention, characterized in that it comprises mutually mounting said stiffeners and said potshell upon starting the potshell, potentially providing a slight clearance in between, and then operating the pot so that said shoebox undergoes a functional deformation, in particular by creep, and so that said shoebox and said stiffeners come into close contact and create said form-fitting fixation.

Brief description of figures

Figures 1 to 3 represent prior art, figures 4 to 11 illustrate several embodiments of the invention.

Figure 1 shows a schematic transverse cross-sectional view of a prior art electrolytic cell for aluminium production according to the Hall-Heroult process.

Figure 2 is a schematic cross section along a transversal plane across a Hall-Heroult electrolytic cell. The arrows represent the current flow across the cell.

Figure 3 is a schematic perspective view of a prior art potshell.

Figure 4 is a schematic perspective view of a potshell according to the invention.

Figure 5 is a transverse cross section along line V-V of figure 4, analogous to figure 1 but at a greater scale, showing a stiffener and the end of a cathode bar, provided on the side wall of the potshell illustrated on figure 4.

Figure 6 is a front view along arrow VI of figure 4, showing two adjacent stiffeners as well as the end of a cathode bar provided between said stiffeners.

Figure 7 is a perspective view, showing at a much greater scale the upper end of a stiffener illustrated on figures 5 and 6.

Figure 8 is a perspective view, analogous to figure 4 but at a greater scale, showing some stiffeners provided on potshell of figure 4, the walls of said potshell being not illustrated on said figure 8.

Figure 9 is a top view, showing at a much greater scale a stiffener provided on the end wall of the potshell illustrated on figure 4.

Figure 10 is a perspective view, showing at a greater scale the end wall of a potshell, which is provided with stiffeners according to a variant of the invention.

Figure 11 is a perspective view, analogous to figure 10 but along a different angle, showing some stiffeners of figure 10, the walls of the potshell being not illustrated on said figure 11.

The following reference numbers are used in the figures:

1	Electrolytic cell (prior art)	41,42	Sidewalls of potshell 40
2,3	Sidewalls of potshell	43,44	Endwall of potshell 49
4	Bottom of potshell	45	Bottom of potshell 40
5	Lining	46	Stiffener (cradle) of potshell 40
6	Anode rod	47	Deckplate of potshell 40
7	Anode	48	Window for cathode collector bar
8	Cathode block	49	Bottom beam
9	Cathode collector bar	50	Side wall stiffener
10	Anode frame	51,52	Front walls of stiffener 50
11	Electrolytic bath	53,54	Lateral walls of stiffener 50
12	Liquid aluminium pad	55	Attachment line
13	Hood	60	End wall stiffener
20,21	Cathodic busbar	61,62	Front and rear walls of stiffener 60
22,23	Flexible connector	63	Median wall of stiffener 60
30	Potshell (prior art)	65	Attachment line
31,32	Sidewalls of potshell 30	68	Base beam
33,34	Endwalls of potshell 30	69	Base plate
35	Bottom of potshell 30	70	End wall stiffener
36	Stiffener (cradle) of potshell 30	76	Intermediate panel of potshell
37	Deckplate of potshell 30	77	Cooling fins
38	Window for cathode collector bar	78	Base beam
39	Belt-like stiffener	79	Base plate
40	Potshell (invention)	42’	Free upper region of 42
T50	Thickness of 50	x50	Inner front dimension of 50
y50	Inner lateral dimension of 50	S50	Spacing between 50 and 50’
S59	Spacing between 9 and 50	L55	Length of 55

Z50	Height of 50	Z42	Height of 42
Z42’	Height of 42’	9’	Free end of 9
P50	Distance between 9’ and 50	Z58	Distance between 8 and 58
T61T63	Thickness of 61-63	W61W63	Width of 61-63

Detailed description of the invention

The general structure of a Hall-Heroult electrolysis pot is known per se and will not be explained here in detail. It is sufficient to explain, in particular in relation with Figure 1, that a typical cell 1 includes a potshell comprising a first longitudinal sidewall 2, a second longitudinal sidewall 3, first and second transversal end walls (not visible on figure 1) and a bottom 4. The potshell walls define a space lined on its bottom and sides with refractory materials 5 (protecting the potshell against heat) along with the cathode blocks 8, thereby defining a volume containing the molten metal and electrolyte. The side lining 5 comprises a layer of carbonaceous material (not shown on the figures) protected in steady state operation by solid electrolyte in contact with molten liquid material. Said cathode blocks 8 comprise one or more cathode collector bars 9. They protrude out of the potshell. Electrical current enters the cell through anodes 7 (suspended above the cell by anode rods 6 attached to an aluminium frame called anode beam 10), passes through the molten electrolytic bath 11 and the molten aluminium pad 12, and then enters the carbon cathode block 8. The current is carried out of the cell by the cathode collector bar 9 connected to the cathode busbar 20, 21 (shown on figure 2). The cell 1 is closed by a set of hoods 13.

Figure 2 explains in more detail the typical current flow in a Hall-Heroult electrolysis cell. The current is fed into the anode frame 10 (called anode beam, shown on figure 1), flows from the anode beam 10 to the anode rod 6 and to the anode 7 in contact with the liquid electrolyte 11 where the electrolytic reaction takes place, crosses the liquid metal pad 12 resulting from the process and eventually will be collected at the cathode block 8. Each collector bar end 24, 25 is connected through a flexible connector 22, 23 to the closest cathode busbar 20, 21 extending parallel to each of the longitudinal sidewalls of the potshell.

As shown on figure 3 representing a typical prior art embodiment, the potshell 30 of an electrolytic cell for the Hall-Heroult process can be represented as a “shoe box” of external length x, width y and depth z, made from steel sheet, comprising two parallel upright sidewalls 31, 32, two parallel upright endwalls 33, 34, and a bottom 35. Said sidewalls 31, 32 and endwalls 33, 34 are connected to a reinforcement structure. Said reinforcement structure comprise stiffeners 36 (so-called “cradles”). They are usually regularly spaced and extend over all the length and width of the potshell. These stiffeners are often T-profiles, as on figure 3, the width of the section parallel to the potshell being larger at the bottom than at the top.

A deckplate 37 can be provided over the whole rim of the potshell. In some embodiments of prior art potshells 30 there is an additional belt-like stiffener 39 at each of the small sides of the potshell.

Figure 3 does not show the cathode blocks which are positioned on the bottom 35 of the potshell 30, but shows the windows 38 provided in both sidewalls 31, 32 for allowing the cathode bars to protrude out of the potshell; electrical connectors (usually flexible ones) are used to connect the collector bars to the cathode busbars (not shown on the figure) that extend parallel to both sidewalls 31, 32. The internal length, width and depth of the potshell are designated by x₇, y-ι and z_1t respectively.

Figure 4 shows an embodiment of a potshell 40 according to the invention. It has the same kind of shoebox structure as the prior art potshell 30, with two parallel upright sidewalls 41, 42, two parallel upright endwalls 43, 44, a bottom 45 and a deckplate 47. Potshell 40 differs from that 30 of prior art, in particular for what concerns the structure of stiffeners. Potshell 40 is equipped with cradles 46, shown on figure 8, which are provided on its side walls 41,42, as well as with end wall stiffeners 60, provided on its end walls 43, 44.

Figure 8 shows the structure of some cradles 46, as well as some windows 38 there between, but does not illustrate the walls of potshell for sake of clarity. Each cradle 46 comprises a central part 49, as well as two terminal parts 50 and 50’. Central part 49 extends parallel to the bottom wall of potshell, in particular in a horizontal way, whereas terminal parts extend perpendicular to said bottom wall, in particular in a vertical way.

Central part 49 is attached, in particular is welded on the lower face of the bottom wall of potshell. Central part 49 is called bottom beam, whereas terminal parts 50 and 50’ are called side wall stiffeners. Stiffeners 50 are provided on side wall 42, whereas stiffeners 50’ are provided on side wall 41. The structure of stiffeners 50 will now be described, bearing in mind that structure of stiffeners 50’ is analogous.

Figures 5 to 7 show more in detail some stiffeners 50, provided on side wall 42. Said stiffeners 50 are formed by a hollow body, which is defined and closed by peripheral walls, in cross section. Said walls comprise two so called opposite front walls 51 and 52, as well as two so called opposite lateral walls 53 and 54 (see in particular figure 7). By way of example, the following typical numeric values are given, with reference to this figure 7:

- thickness T50 (figure 7) of peripheral walls is between 1 mm and 15 mm, and preferably between 2 mm and 10 mm;

- inner front dimension x50 (figure 7) between inner faces of front walls 51 and 52 is between 100 mm and 180 mm;

- inner lateral dimension y50 (figure 7) between inner faces of lateral walls 53 and 54 is between 120 mm and 220 mm;

- spacing S50 (figure 8) between 150 mm to 300 mm facing faces of adjacent stiffeners; in particular, distance (S50) between facing faces (53’, 54) of adjacent stiffeners (50, 50’) can be between 250 mm and 300 mm, and preferably between 275 mm and 300 mm.

- distance S59 (figure 6) between one cathode bar 9 and facing face of lateral wall 54 of adjacent stiffener 50 is between 30 mm and 70 mm, in particular between 40 mm and 50 mm. This distance S59 substantially corresponds to distance window 38 and facing face of lateral wall 54. Front wall 51, which is adjacent to side wall 42 of the potshell, forms an attachment wall, i.e. it is suitable for a fixation with respect to said potshell. Said attachment wall defines an attachment region, which contacts side wall 42 and is attached thereto by fixation means. Said contact may be a direct contact, or an indirect contact, i.e., said fixation means are interposed between said attachment region and said facing wall. Advantageously, said attachment or contact region extends over a substantial part of the attachment wall, in particular over the entire height of the vertical cradle, preferably tack welded at the edges. The contact between the vertical cradle and the sidewall can be left even free to allow free sliding of the potshell wall against the cradle and would help to release mechanical stresses between the said surfaces.

Viewed from above (see in particular figure 5), said attachment region defines an attachment line 55, also called a contact line, the length of which is noted L55 (see figure 7). The tack weld achieved on the entire length L55 is superior to any weld carried out on part of the cradle height. This allows a reliable fixation of the whole stiffener 50 on the wall 42 of the potshell. Let us note that L55 is slightly superior to x50, since it takes into account twice the thickness T50 of the walls.

In an advantageous embodiment, attachment wall 51 is welded against the wall 42. Said welding connection can comprise point welding spaced over substantially the whole length of the line 55, and/or can comprise one or more welding seams, such as one continuous welding seam extending over at least three quarter (and preferably over at least 90%) of the length of the line 55, or two or more sections of welding seams spaced over substantially the whole length of the line 55.

The hollow stiffeners 50 have a rectangular shape, in cross section. As an alternative, this shape may be different. However, providing a polygonal shape is advantageous: a side of this polygon is adapted to be in contact with the side wall of the potshell and, therefore, to create a reliable attachment wall. As shown in particular on figure 7, the top of said hollow body preferably is closed to avoid accumulation of loose alumina and other foreign material.

As shown by figures 5 and 6, stiffeners 50 extend advantageously over only part of the height of side wall 42 of potshell. Let us note z50 and z42 the respective heights of said stiffener 50 and said side wall 42 (see figure 6). Height z42 is considered from the bottom of deckplate 47. Advantageously, the ratio (z50!z42) is between 0.4 and 0.7, in particular between 0.45 and 0.55.

This makes it possible to define a so called free, or smooth, upper region 42’ of said side wall 42 of potshell. Advantageously, said free upper region is equipped with not shown heat exchange means. Advantageously, height Z42’ (see figure 6) of upper region 42’ is between 500 mm to 600 mm, in particular between 530 mm to 560 mm. This allows an easy positioning and a reliable fixation of above mentioned heat exchange means.

Turning back to figure 5, let us note the so called distance of protrusion P50. This distance corresponds to the distance, along main longitudinal axis of cathode bar 9, between free end 9’ of said cathode bar and outer face of front wall 52 of stiffener 50. Said distance P50 is between 80 mm and 150 mm, in particular between 90 mm and 110 mm. By way of convention, if this distance is positive, it means that stiffener 50 extends beyond cathode bar 9. Figure 5 shows, in solid lines, a first possibility according to which stiffener extends beyond cathode bar and, in dotted lines, a second possibility according to which cathode bar extends beyond stiffener.

Referring again to figure 5, let us note Z58 the distance, along vertical axis, between top of stiffener 50 and top of cathode blocks 8. Advantageously, Z58 is inferior to 100 millimeters, preferably inferior to 50 millimeters, and still more preferably inferior to 20 millimeters.

Figures 4 and 9 show more particularly the structure of a first embodiment of stiffeners 60, provided on both end walls 43 and 44. Stiffeners 60 differ from those 50, such as described above, essentially in that they are not hollow. On the other hand, they are substantially H or l-shaped. Each stiffener is formed by three walls, i.e. two parallel opposite walls 61 and 62, and a medium wall 63 which links said walls 61 and 62.

Wall 61, adjacent potshell end wall 43, is called front wall, whereas wall 62, opposite potshell end wall 43, is called rear wall. Both walls 61 and 62 extend parallel to potshell end wall 43, whereas medium wall 63 extends perpendicular to said potshell end wall. Stiffeners 60 rest each on a base beam 68, said beams being attached to a base plate 69 which extends also under the central parts 49a and 49b of last and last but one cradles 46a and 46b (see also figures 10 and 11 for details).

By way of example, the following typical numeric values are given, with reference to figure 9:

- thickness T61 of wall 61 and thickness T62 of wall 62 are between 8 mm and 12 mm;

- thickness T63 of wall 63 is between 5 mm and 7 mm;

- width W61 of wall 61 and width W62 of wall 62 are between 120 mm and 140 mm;

- length L63 of wall 63 is between 160 mm and 185 mm;

- spacing S60 (figure 4) between facing faces of adjacent stiffeners 60 is between 300 mm and 500 mm.

By analogy with above described front wall 51, front wall 61 forms an attachment wall. The contacting face thereof defines an attachment line 65 (figure 9), the length of which is noted L65, which is equal to W61. The same way as for line 55, the entire length L65 is to be welded on its edges. The same way as for line 55, attachment wall 61 is advantageously welded against the wall 43.

In the first embodiment of figures 4 and 9, the stiffeners extend only over the lower part of potshell end wall 43. By analogy with potshell side wall 42, this makes it possible to define a so called free, or smooth, upper region 43’ of said end wall 43 of potshell. Advantageously, said free upper region is equipped with heat exchange means (not shown on the figures), typically similar to those described here above. Advantageously, height z43’ (see figure 4) of upper region 43’ is between 450 mm to 600 mm, in particular between 470 mm to 520 mm. This allows an easy positioning and a reliable fixation of above mentioned heat exchange means.

Figures 10 and 11 show another embodiment of end wall stiffeners of potshell end walls 43, which are referenced as 70. In this first variant, said stiffeners 70 extend over the whole height of end wall 43. The same way as above stiffeners 60 of first embodiment, each stiffener 70 rests on a base beam 78, said beams being attached to a base plate 79 which extends also under the central parts 49a and 49b of last and last but one cradles 46a and 46b.

Let us note 76 the so called intermediate panels, between two adjacent stiffeners. Said intermediate panels 76 are part of the potshell end wall 43. According to an advantageous embodiment, the upper region of panels 76 is equipped with cooling fins 77. These fins are vertical plate-like elements. In a typical way, between 3 and 8 fins are provided on each panel 76. The height H77 of each fin is between 40 to 60 % of the whole height of said end wall 43.

The purpose of said cooling fins 77 in the upper free region (42’) is to provide sufficient cooling of the potshell when there are no heat exchangers. That is to say that the potshell according to the invention, equipped with said cooling fins 77 if required in order to ensure a convenient thermal equilibrium, can be mounted in an electrolysis cell and operated without heat exchangers; if the potshell is to undergo a retrofit with heat exchangers, said cooling fins 77 can be removed, totally or in part, in order to ensure thermal equilibrium. If required, heat exchangers can be put in place at the section of the end walls 43 that were previously covered by said cooling fins 77.

Said cooling fins 77 can be removed for instance by simple cutting operations. This can be useful because the installation of heat exchangers in a potline will require various installations for the circulation of heat transfer fluids that will transport the heat from the heat exchanger to the point where said heat is needed; these installation may not be ready when a potshell according to the invention is mounted and the pot is put into operation. As a consequence, the pot will have to operate without heat exchangers for a certain period of time, and the potshell should therefore be designed such as to reach a convenient thermal equilibrium with natural cooling only. The use of removable cooling fins 77 meets this target. A method for adding heat exchange means to a potshell according to the invention equipped with cooling fins 77 in the upper free region 42’ comprises therefore the steps of removing said cooling fins 77 from said upper free region 42’, and mounting said heat exchange means in said upper free region 42’.

Likewise and for the same purpose, cooling fins 77 can be provided at the side walls of the potshell, and during a retrofit operation they can be replaced, totally or in part, by heat exchangers; this variant is not shown on the figures.

The invention has many advantages. The side wall and end wall stiffeners 50, 60 are wider (i.e. their main dimension L55 is greater) than the cradles used in prior art potshells, but their height z50 is less than that of the cradles used in prior art potshells. It both increases the available surface of the cavity (i.e. the surface that is available for the cathodes forming the bottom of the electrolytic cell) of the potshell (at a given “footprint” (i.e. at a given total outside dimension) and decreases the height of the cradles (fins) below the deckplate.

Due to the higher surface of the cavity the potshell according to the invention allows to increase the cathode surface area by about 10 % to about 21% depending on the cell technology. It should be noted that potshells according to the invention can be used for virtually any cell technology using the Hall-Heroult process. The increase in cathode surface area allows using longer anodes, thereby reducing the voltage drop in the electrolyte region and reducing the anodic current density at any given amperage. As a consequence, end-of-life electrolytic cells can be replaced by new electrolytic cells, using pothshells according to the invention, that fit into the footprint of the old ones but having a higher production capacity.

Furthermore, the new design leads to decreased heat loss because of the smaller surface area of the cradles that unavoidably act not only as stiffeners but also as cooling fins. This is due to their smaller height and (optionally) smaller width.

Another advantage of the potshell according to the invention is that the upper part of the sidewalls and/or endwalls is fully accessible over a height designated as z42’. This allows providing the potshell with heat exchangers in contact with the upper part of the sidewalls and/or endwalls of the potshell.

Example

A test cell was based on so-called DX+ technology that is used and offered for sale by Emirates Global Aluminium for many years. Concerning the potshell, two major changes were made in side and end wall cradle design in accordance to the present invention: narrowing down the portion of the cradle width to bare minimum, and reducing the cradle height below the deckplate up to the top of the cathode blocks. Additionally the potshell height was reduced by about 150 mm from the top of the deckplate to the bottom of the side cradles at the location of concrete support where potshell rests.

The structural features of the new potshell design are the following: narrower side cradles, shorter side cradles, side cradles at every cathode block, increased inner cavity width, increased inner cavity length, maintaining similar mass of the potshell, reduced shell height by 165 mm.

This innovative potshell design also enables to install heat recovery system in the upper portion of the potshell facing the meal and bath, utilizing the full perimeter on the potshell. This thermal energy can be utilized further for other purposes and potentially can be helpful to reduce energy consumption of the cell.

Another aspect of the wider cavity and narrower cradle potshell design is to increase cell productivity by increasing cell current or reducing the specific energy consumption. This increase in cell current can be easily calculated by the % gain in anode panel surface area due to longer anodes. Similarly reduction in specific energy consumption can be 5 estimated by taking into the account of reduction in bath voltage for a given anode to cathode distance and anode size.

Claims

1. A potshell (40) for an electrolytic cell (1) suitable for the Hall-Heroult electrolysis process, said potshell being intended to receive a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks (8), each cathode block comprising at least one metallic cathode collector bar (9) protruding out of each of the two ends of the cathode block (8), a lateral lining (5) defining together with the cathode a volume containing the liquid electrolyte (11) and the liquid metal (12) resulting from the Hall-Heroult electrolysis process, said potshell (40) comprising :

a bottom wall (45) and peripheral walls (41-44) extending upwards from said bottom wall (45), so as to define an inner reception volume, said peripheral walls comprising side walls (41, 42) and end walls (43, 44), a plurality of reinforcement members or stiffeners (50, 60), provided side by side along at least part of said peripheral walls, said potshell being characterized in that at least part of said stiffeners comprise at least one attachment wall (51), said attachment wall being provided with an attachment region (51), said potshell further comprising fixation means, said fixation means allowing for attachment of said attachment region (51) with facing wall (42) of potshell. ,

2. Potshell according to claim 1, characterized in that said fixation means are permanent fixation means.

3. Potshell according to claim 1 or 2, characterized in that said fixation means are distinct from both said attachment region (51) and said facing wall (42) of potshell, said fixation means being in particular welding means.

4. Potshell according to claim 1 or 2, characterized in that said fixation means are defined by both said attachment region (51) and said facing wall (42) of potshell, said attachment region (51) and said facing wall (42) being adapted to directly contact one another in view of a form-fitting fixation.

5. Potshell according to any of above claims, characterized in that, viewed from above, said attachment region defines an attachment line, the main dimension (Z.55) of attachment line being superior to 30 millimeters, preferably to 60 millimeters, more preferably to 100 millimeters.

6. Potshell according to any of above claims, characterized in that said stiffeners (50) provided with said attachment wall (51) are located on side walls (41, 42) of said potshell.

7. Potshell according to any of above claims, characterized in that said stiffeners (50) provided with said attachment wall (51) are formed by a hollow body, which is closed by peripheral walls (51-54), in cross section, said peripheral walls comprising said attachment wall (51).

8. Potshell according to claim 7, characterized in that said peripheral walls are polygonal, in particular rectangular.

9. Potshell according to any of above claims, characterized in that at least part of said stiffeners (50) extend only over a lower region of said side wall (41, 42) of said potshell, so as to define a so called free upper region (42’) of said side wall (41, 42) of said potshell.

10. Potshell according to claim 9, characterized in that ratio (z50/z42) between height (z50) of stiffener (50) and height (z42) of side wall of potshell is between 0.4 and 0.7, in particular between 0.45 and 0.55.

11. Potshell according to any of above claims 9 or 10, characterized in that free upper region (42’) is equipped with heat exchange means.

12. Potshell according to any of above claims, characterized in that distance (S50) between facing faces (53’, 54) of adjacent stiffeners (50, 50’) is between 275 mm and 300 mm.

13. Potshell according to any of claims 11 or 12, characterized in that distance (S59) between one window (38) and facing face of adjacent stiffener is between 30 mm and 70 mm, in particular between 40 mm and 50 mm

14. Potshell according to any of claims 9 to 13, characterized in that it is provided with cooling fins (77) in said upper free region (42’).

15. An electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks (8), each cathode block (8) comprising at least one metallic cathode collector bar (9) protruding out of each of the two ends of the cathode block (8), a lateral lining (5) defining together with the cathode a volume containing the liquid electrolyte (11) and the liquid metal (12) resulting from the Hall-Heroult electrolysis process, an outer metallic potshell (40) containing said cathode and lateral lining (5), a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one carbon anode (7) and at least one metallic anode rod (6) connected to an anode beam (10), a cathodic bus bar surrounding said potshell (40), a plurality of connectors, each connecting one end of a cathode collector bar (9) of a cathode block (8) to said cathodic bus bar (20, 21), said electrolytic cell being characterized in that said outer metallic potshell is a potshell according to any of claims 1 to 14.

16. An electrolytic cell according to claim 15, characterized in that the distance of protrusion (P50) between said cathode collector bar (9) and said stiffener is between 80 mm and 150 mm.

17. An electrolytic cell according to claim 15 or 16, characterized in that the distance (Z58), along vertical axis, between top of stiffener (50) and top of cathode block (8) is inferior to 100 millimeters, preferably inferior to 50 millimeters, and still more preferably inferior to 20 millimeters.

18. An aluminium electrolysis plant comprising at least one line of electrolysis cells of substantially rectangular shape, and said plant further comprising means for electrically connecting said cells in series and for connecting the cathodic busbar (20, 21) of a cell to the anode beam (10) 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 any of claims 15 to 17.

19. A method for making aluminium by the Hall-Heroult electrolysis process, characterized in that said method is carried out in an aluminium electrolysis plant according to claim 18.

20. A method for adding heat exchange means to a potshell according to claim 14, wherein said cooling fins (77) are removed from said upper free region (42’), and said heat exchange means are mounted in said upper free region (42’).

21. A method of operating a potshell according to any of claims 4 to 14, characterized in that it comprises mutually mounting said stiffeners and said potshell upon starting the potshell, potentially providing a slight clearance therebetween, and then operating the pot so that said shoebox undergoes a functional deformation, in particular by creep, and so that said shoebox and said stiffeners come into close contact and create said form-fitting fixation.