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, reinforced potshell design. This is achieved in particular by redesigning the bottom 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 (Al ₂ 0 ₃ ) is dissolved in molten cryolite (Na ₃ AIF ₆ ), and the resulting mixture (typically at a temperature comprised between 940 °C and 970 °C) acts as a liquid electrolyte in an electrolytic cell. An electrolytic cell (also called“pot”) used for the Hall-Heroult process typically comprises a steel shell (so-called “pot shell”), a lining (comprising refractory bricks protecting said steel shell against heat, and cathode blocks usually made from graphite, anthracite or a mixture of both), 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 ^th century much effort has been undertaken to improve the energy efficiency (expressed in kW/h per kg or ton of aluminium), and there has also been a trend to increase the size of the pots and the current intensity at which they are operated in order to increase the plant productivity and bring down the capital cost per unit of aluminium produced in the plant.

Industrial electrolytic cells used for the Hall-Heroult process are generally rectangular in shape and connected electrically in series, the ends of the series being connected to the positive and negative poles of an electrical rectification and control substation. The general outline of these cells is known to a person skilled in the art and will not be repeated here in detail. They have a length usually comprised between 8 and 25 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 side- by-side cells (or ends for end-to end cells) of the cells by the terms“upstream” and “downstream” with reference to the current orientation in the series. The current enters the upstream and exits downstream of the cell.

The 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 / cm ² , 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 usually 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: potshells exhibit under conditions of normal use permanent thermal gradients. Furthermore, they support a permanent load at elevated temperature. As a consequence, 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.

Furthermore, during their lifetime, cathode blocks will undergo irreversible dimensional changes due to the combined action of temperature, chemical environment and abrasion due to movement of the liquid metal sheet induced by electromagnetic fields. As an example, sodium swelling of carbon (anthracite) blocks is known to generate permanent stress upon potshell walls and bottom, which will tend to be released by creep of the potshell.

Due to these various causes, 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. Graphite incorporates sodium less readily than carbon, but even for modern graphitized cathode blocks sodium swelling remains a problem that needs to be taken into account when designing potshells.

One of the goals of the present invention is to provide a new potshell design that decreases the deformation of the potshell under mechanical stress.

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 less weight will also contribute to minimize the capital cost. And at last but not at least, the subsequent cost of maintenance and repair of the potshell should be low. The present invention provides improvements of the potshell design that contribute to meet 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. As mentioned above, 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. While all potshells basically have a“shoebox” shape, prior are proposes various designs for obtaining high strength

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. This is intended to provide a lighter potshell structure that provides a mechanical resistance comparable to that of conventional potshells. However, 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 fine-tuning 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, low maintenance and repair cost. Any known solution that would meet several of these goals would result in a potshell structure that is lighter than conventional potshell designs, and that would tend to exhibit stronger deformation than potshells according to conventional design.

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 providing a potshell for an electrolytic cell suitable for the Hall-Heroult electrolysis process with a novel cradle, said potshell being intended to receive 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, 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, said novel cradle comprising a bottom part, or beam, intended to extend parallel to the bottom wall of potshell, in particular in a horizontal way, as well as two lateral parts, or poles, each of which being intended to extend parallel to one respective peripheral wall of potshell, in particular in a vertical way, said cradle being characterized in that said bottom beam comprises a reinforcing median region, as well as two end regions, adjacent said lateral parts, the cross section (H50) of reinforcement median region being superior to that (H60, H70) of said end regions.

Said cradle forms a first object of the invention. In a specific embodiment said median region and said end regions can have substantially the same width and the height (H50) of said median region is superior to height (H60, H70) of said end regions. It can comprise two transition regions, each extending between said median region and one respective end region. Each transition region defines two junctions, respectively with said median region and with said end region, the cross section, in particular the height, of said transition region increasing, in particular continuously, from its junction with said end region, to its junction with said median region. Another object of the invention is a potshell for an electrolytic cell 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, 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, 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 as well as a plurality of cradles, each said cradle comprising a bottom part, or beam, intended to extend parallel to the bottom wall of potshell, in particular in a horizontal way, as well as two lateral parts, or poles, each of which being intended to extend parallel to one respective peripheral wall of potshell, in particular in a vertical way, said potshell being characterized in that at least some cradles, in particular substantially all the cradles, are cradles according to the present invention.

Yet another object of the 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 present invention.

Yet another object of the 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 %, and most preferably substantially all of the electrolysis cells in at least one of said line, are electrolysis cell according to the present invention.

A last object 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.

Brief description of figures Figures 1 to 3, as well as 7, represent prior art, figures 4 to 6, as well as 8 and 9 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 perspective view, illustrating part of a potshell according to the invention.

Figure 5 is an end view along arrow V of figure 4, showing the potshell illustrated on said figure 4. Figure 6 is a front view at a greater scale, illustrating the cradle part of potshell of figures 4 and 5.

Figure 7 is an end view of prior art potshell of figure 3, showing the forces exerted on said potshell in use.

Figure 8 is an end view analogous to figure 5, schematically showing a variant of a cradle according to the invention.

The following reference numbers are used in the figures:

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, which aims at decreasing deformation of the end walls 33,34.

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 x _1t y- _t and z respectively.

Figures 4 and 5 show 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 46. Potshell 40 differs from that 30 of prior art, in particular in that it is equipped with cradles 47, one of which will be explained in greater detail hereafter. Said cradles extend the one beside the other, each being adjacent bottom 45 and side walls 41 , 42 of the shoebox. On figure 4, the three end cradles 47a, 47b and 47c are illustrated. Moreover, vertical end wall stiffeners 100, which are not part of the invention and will not be described more in detail, are provided on its end walls 43, 44. Referring in particular to figure 6, each cradle 47 comprises first a central part 48, also called beam, which is an essential feature of the invention. It also comprises two lateral parts 49 and 49’, also called poles, which do not belong to the invention. In this respect, said lateral poles may be of any appropriate type, known as such. Central part 48 extends parallel to the bottom wall 45 of potshell, in particular in a horizontal way, whereas lateral parts 49, 49’ extend parallel to said side walls 41 , 42, in particular in a vertical way. On right part of figure 4, the beams 48a, 48b and 48c of the three end cradles 47a, 47b and 47c are illustrated, as well as the right poles 49a, 49b and 49c thereof, whereas left part of said figure 4 shows the left pole of very end cradle 47a. The present invention is directed to central beam 48, in particular to the specific geometry thereof. As shown on figure 6, said beam essentially comprises a reinforcing median region 50, two end regions 60 and 70, as well as transition regions 80 and 90, each extending between median region and a respective end region. Let us note:

- 51 , 61 , 71 , 81 and 91 the upper walls of respective regions 50, 60, 70, 80, 90

- 52, 62, 72, 82 and 92 the lower walls of respective regions 50, 60, 70, 80, 90

- 53, 63, 73, 83 and 93 the front side walls of respective regions 50, 60, 70, 80, 90.

On figure 6, rear side walls are not illustrated, bearing in mind that they are parallel to above mentioned front side walls. Viewed from above, said beam has a substantially constant width. In other words, side walls of the respective regions are mutually aligned.

Let us note A48 the main axis of beam 48, which is typically horizontal. Upper walls 51 to 91 are mutually flush and parallel to said axis A48.

Lower walls 62 and 72 of end regions are mutually flush and parallel to said axis A48. Lower wall 52 is also parallel to A48, while its altitude is inferior to that of 61 and 71. In other words, height H50 of median region is superior to respective height H60 and H70 of end regions. Advantageously ratio ( H50/H60 ), which is equal to ( H50IH70 ), is superior to and typically between 1.1 and 3..

Moreover lower walls 82 and 92 of transition regions are preferably sloped with respect to main axis A48. Typically angles a82 and a92, between said walls 82 and 92 and said axis, are mutually equal and between 20 and 90.

Cradle 47 is attached to the shoebox by any appropriate means, in particular by welding.

According to the invention, a bottom beam provided with a median thick region and two thinner end regions is advantageous. Indeed, during operation, the center of the potshell bottom 35 tends to bend upwards due to temperature gradient mostly in side walls and forces from lining expansion acting to side walls. This is shown on figure 7 showing prior art potshell 30, by arrow F35. When the potshell bottom bends upwards this will transfer force to the cathode block 8 through the bottom lining, so that the latter is also pushed up. However collector bars 9, which are locked inside lining and collector bar windows, exert a resistance strength which tends to block said bending phenomenon. This is also shown on figure 7, by arrow F9. Due to the above described opposite strengthes F35 and F9, stresses appear at the top of blocks and if these stresses exceed the strength of cathode blocks they can crack, and as a consequence molten aluminium can reach collector bars and bottom lining. This may lead to pot cut-out and shorter service life.

In these conditions, providing a thicker bottom beam tends to decrease bending of potshell bottom, which reduces stresses in cathode blocks. However, increasing height of the whole bottom cradle would also increase overall cell height, leading to a higher overall building and to higher potroom building cost. That is why, according to the invention, the bottom cradle height is increased only in its mid region, i.e. at area free out cell support where it is most important. It should be noted that reduction of height close to extremities, i.e. in end regions, does not impact the stress level in cathode blocks (or, in other words: adding thickness (height) to the bottom cradle close to its extremities will not add much strength to the potshell bottom) 82.

In this respect, the following ratios are advantageous (these lengths correspond to the dimensions along axis A48) ratio ( L50/L48 ) between length ( L50 ) of said median region 50 and total length

( L48 ) of said beam is between 0.4 and 0.9

ratio ( L60/L48 ) or ( L70/L48 ) between length (L60) or (L70) of each end region 60 or 70 and total length ( L48 ) of said beam is between 0.05 and 0.3.

According to a variant shown on figure 8, beam 48 may not include any transition region. In this respect, median region 50 joins each end region 60 and 70, via vertical shoulders 55 and 55’. However, providing a sloped transition region is advantageous, because the edge between the vertical shoulders 55 and the horizontal end region 60 may become a weak point of the structure..

Advantageously, the whole beam 48 is symmetric with respect to vertical axis T48, which is perpendicular to main axis A48. In this respect, lengths L60 and L70 are equal, as well as lengths L80 and L90. This permits a convenient manufacture, as well as a good balance. Reinforced bottom beams according to the invention can be used for all bottom cradles of a potshell; this is a preferred embodiment. More generally, at least one of the bottom cradles is a bottom cradle with a reinforced bottom beam according to the invention. In cells having an underneath busbar it may not be possible to use bottom cradles with a reinforced bottom beam according to the invention without increasing the height of the potroom. However, the use of straight bottom cradles according to prior art for one or two bottom cradles in some region of the cell bottom will not lead to significant weakening of the potshell, compared to a potshell in which all bottom cradles are according to the invention.