WO2009066025A2

WO2009066025A2 - Grooved anode for an electrolysis tank

Info

Publication number: WO2009066025A2
Application number: PCT/FR2008/001248
Authority: WO
Inventors: Claude Ritter
Original assignee: Alcan International Limited
Priority date: 2007-09-14
Filing date: 2008-09-08
Publication date: 2009-05-28
Also published as: AU2008327757A1; CA2698894A1; FR2921074B1; FR2921074A1; WO2009066025A3; AU2008327757B2

Abstract

The subject of the invention is a carbon anode block for use in a metal electrolysis cell, of height H between an upper face (24) and a lower face (23), said block having, on the lower face (23), at least one groove (20a) of depth p(x) and length Lr, the groove extending along a direction x and said depth p(x) varying along said direction x, characterized in that said depth p(x) varies non-linearly along said direction x and in that said depth p(x) is smaller than a first value Z1 of at least 60% of the length Lr of said groove and is greater than a second value Z2, which is at least equal to Z1 + 10% of the height H, over 3 to 40% of the length Lr of the groove.

Description

GROOVED ANODE OF ELECTROLYTIC TANK

Field of the invention

The invention relates to the production of aluminum by igneous electrolysis according to the Hall-Héroult process and more particularly to the anodes comprising a grooved carbon anode block used in aluminum production plants.

State of the art

Aluminum metal is produced industrially by igneous electrolysis, namely by electrolysis of alumina in solution in a bath of molten cryolite, called electrolysis bath, according to the well-known Hall-Héroult process. The electrolysis bath is contained in tanks comprising a steel box, which is lined internally with refractory and / or insulating materials, and cathode elements located at the bottom of the tank. Anode blocks of carbonaceous material are partially immersed in the electrolysis bath. Each vat and the corresponding anodes form what is often called an electrolysis cell. The electrolysis current, which circulates in the electrolysis bath and possibly a sheet of liquid aluminum through the anodes and cathode elements, operates the alumina reduction reactions and also allows to maintain the bath of electrolysis. electrolysis at a temperature of the order of 950 ° C by Joule effect.

French patent application FR 2 806 742 (corresponding to US Pat. No. 6,409,894) describes installations of an electrolysis plant intended for the production of aluminum.

According to the most widespread technology, the electrolysis cells comprise a plurality of so-called "precooked" anodes of carbonaceous material which are consumed during the electrolytic reduction reactions of aluminum. Gases, and more particularly carbon dioxide, are generated during the electrolysis reactions and naturally accumulate in the form of gas bubbles under the lower, generally substantially flat, horizontal face of the anode, which influences the overall stability of the tank.

It results indeed from the accumulation of these gas bubbles:

- electrical variations and instabilities,

a high frequency and a significant duration of anode effects,

an increased possibility of reverse reaction and therefore a loss of efficiency due to the small distance between the aluminum layer produced and the CO ₂ bubbles,

- increased carbon consumption and the formation of harmful gases due to the transformation of CO ₂ into CO in contact with carbon.

It is known to use anodes with carbonaceous anodic blocks having one or more grooves in the lower part so as to facilitate the evacuation of the gas bubbles and prevent their accumulation in order to solve the problems mentioned above and to reduce the consumption. of energy as shown in Light Metals 2005 "Energy Saving in Hindalco's Aluminum Smelter", SC Tandon & RN Prasad. The grooves make it possible to reduce the average free path of the gas bubbles under the anode to leave the space between the electrodes and thus reduce the size of the bubbles that form under the anode.

The interest of the use of grooves has already been studied and proven, for example in Light metals 2007 p.305-310 "The impact of slots on reducing cell individual anode current variation", Geoff Bearne, Dereck Gadd, Simon Lix or Light metals 2007 p.299-304 "Development and deployment of slotted anode technology at Alcoa", Xiangwen Wang et al. . It is also known from the following documents:

- WO 2006/137739 use thinner grooves (of the order of 2 to 8 mm) than those commonly used (of the order of 8 to 20 mm) so as to optimize the useful carbon mass and the surface area 'exchange; US 7 179 353 to use an anode having grooves opening on a single side or side face of the anode block, and more particularly towards the center of the electrolysis cell so as to improve the dissolution of the alumina;

- WO 2006/137739, US 7 179 353 and Light Metals 2007 p.283-285 "Slot cutting in anodes" by Jean-Jacques Grunspan, to incline the bottom of the grooves at an angle of up to 10 ° so to accelerate the evacuation of the trapped gases in the grooves and to control the orientation of the gas flows.

A well known limit to the use of these grooves results from the fact that the depth of the grooves is limited so as not to disturb the mechanical and physical integrity of the carbonaceous anode blocks. However, the carbonaceous anode blocks are progressively consumed during the electrolysis reaction over a height greater than the depth of the grooves so that the lifetime of the grooves of an anode is less than the lifetime of the anode. . Therefore, for a certain period of time during the life of the anodes, the lower part of the anode blocks no longer has a groove. The problems mentioned above for anodes without grooves are then felt.

Indeed, as mentioned in Light metals 2007 p.299-304 "Development and deployment of slotted anode technology at Alcoa", the depth of the grooves is limited for reasons of integrity mainly in the case of grooves formed by molding on blocks anodic raw so that the beneficial effects resulting from the presence of the grooves are observable only over part of the life of the anodes. The grooves create weaknesses in the raw anodic blocks which then crack during their transport, storage or cooking.

In practice, it also proves impossible to reliably obtain anodes with deep grooves such as the anodic block height to be consumed by sawing anode blocks. The mechanical stresses and the vibrations exerted by the saw blades provoke the crumbling, the cracking then the bursting of the blocks of carbons. The dimensions of the anode blocks of the commonly used anodes are of the order of 1200 to 1700 mm for the length, 500 to 1000 mm for the width and 550 to 700 mm in height, with one to three grooves of depth generally between 150 and 350 mm.

Also for an anodic block of 600 mm height with a consumable carbon height of 400 mm and a groove of 250 mm depth, the groove produces a beneficial effect for only 62.5% of the life of the anode.

An object of the invention is to provide anodes to overcome the disadvantages mentioned above, that is to say to provide anodes producing a beneficial effect for a longer time without compromising the integrity of the anode blocks during their manufacture, storage, transportation or use.

Description of the invention

For this purpose, the subject of the invention is an anode carbon block for use in an electrolysis cell intended for the production of metal, said block having a height H between an upper face and a lower face and comprising on the lower face at least one groove of depth p (x) and length Lr, the groove extending in a direction x and said depth p (x) varying along said direction x, characterized in that said depth p (x ) varies non-linearly along said x direction and in that said depth p (x) is less than a first value Z ₁ over at least 60% of the length Lr of said groove and is greater than a second value Z ₂ at least equal to Z ₁ + 10% of the height H over 3 to 40% of the length Lr of the groove. Preferably, said depth p (x) is less than said first value Z ₁ over at least 70% of the length Lr of said groove and said second value Z ₂ is at least equal to Z ₁ + 15% of the height H on 3 to 30% of the length Lr of the groove and more preferably 5 to 20% of the length of the groove. The upper face has at least one fixing recess and the lower face is intended to be immersed in an electrolysis bath.

The non-linear variation of said depth p (x) along said x direction means a variation that can not be described using a single line or a single plane. The variation may, however, include one or more linear portions.

The particular and innovative shape of the groove according to the invention gives it an increased lifetime while maintaining a high structural integrity of the anode block. Once the level Zi of consumed carbon reaches, at least a groove portion remains to evacuate the gases that accumulate under the underside of the anode block. The remaining groove portion, although of reduced length, makes it possible to limit the problems related to the accumulation of gases under the anode.

According to one embodiment, the groove comprises at least one end opening on one side of the anode block and the depth of the groove is greater than said second value Z? at the open end. According to a variant of this embodiment, the groove comprises two ends each opening on one side of the anode block and the depth of the groove is greater than the second value Z? at each of the open ends. The depth of the groove is typically maximum at at least one open end.

According to a particular embodiment of the invention, the groove comprises a central portion, with a flat bottom or slightly inclined at an angle less than 10 ° relative to the horizontal, surrounded on each side by an end portion , with a steep incline of 20 to 80 ° to the horizontal extending towards the upper face of the anode block towards the sides of the anode block, the ends of the groove opening on opposite sides of the block anodic. In other words, the bottom or depth p (x) of the groove follows a profile in the form of a cross-section of attitude with inclined edges. According to yet another particular embodiment of the invention, the groove comprises a central portion, with a flat bottom or inclined at an angle less than 10 ° relative to the horizontal, surrounded on each side by a portion of end, with a flat bottom or inclined at an angle of less than 10 ° to the horizontal, said end portions being vertically unhooked above said central portion, the ends of the groove opening on opposite sides of the anode block . In other words, the bottom or the depth p (x) of the groove follows a profile in the form of an inverted cap cross-section.

According to these embodiments, the core of the anode block is not weakened by the groove on the greater length of the groove corresponding to the central portion. Furthermore, the offgas advantageously occurs at a high height on the sides of the anode block in the electrolysis cell so that the agitation of the electrolysis bath is more regular and conducive to electrical, chemical and thermal equilibrium. of the cell.

According to still other particular embodiments of the invention, the groove comprises a first portion, with a flat or inclined bottom of an angle less than 10 ° with respect to the horizontal, and a second flat-bottomed portion unhooked. vertically above said first portion or at a steeply inclined bottom of 20 to 80 ° with respect to the horizontal extending in the direction of the upper face of the anode block while approaching the sides of the anode block.

Advantageously, the groove has a maximum depth corresponding, within ± 10 cm, to a maximum wear height of the anode block in order to maintain the effect of the groove on substantially the entire duration of use of the block in a cell. electrolysis.

The presence of grooves has the effect of reducing the turbulence of the electrolysis bath and the kinetic energy of turbulence for the volume located below the lower face of the anode block. The plaintiff considers that this effect is even more marked when the groove is not completely immersed because of the shorter path to travel through the gas bubbles to be evacuated. The reduction of turbulence is particularly beneficial in the region below the anode block because it reduces the reoxidation of the dissolved metal in the electrolysis bath.

With the anode blocks according to the invention, part of the grooves is not immersed in the bath for a prolonged period with respect to the grooves of the prior art, and more particularly at the open ends of the grooves. The gaseous releases are therefore above the bath or high in the bath, in the groove or on the sides of the blocks, which reduces the turbulence of the bath between the electrodes and reduces the distance between the electrodes and increase the energy yields.

Also, with respect to an anode block of the prior art for which, from carbon consumption or wear, an effective groove to an absence of a groove was used, the anode blocks according to the invention show a progressive or by step of the length of the grooves and therefore of their efficiency, which avoids disturbances and abrupt changes in the kinetics of the fluids with the problems of associated electrical balances and facilitates for example adaptive adjustments.

Furthermore, the grooves can be oriented so that when the gas emissions occur in the electrolysis bath (after some wear or consumption of the anode block), they are directed to the alumina loading points so to facilitate agitation and dissolution of the alumina, more particularly towards a central corridor in the electrolysis cell.

The invention extends to anodes having at least one anode block as described above and a fixing rod.

The invention also extends to an igneous electrolysis aluminum production cell comprising at least one anode as described above, as well as to a process for the manufacture of aluminum comprising the steps of: provide at least one anode as defined above;

- install the anode in an aluminum electrolysis cell;

- Pass current in the ewe electrolysis cell through the anode;

recovering the aluminum obtained by electrolysis in the bottom of the cell of the electrolysis cell.

The invention is described in more detail below with the aid of the appended figures.

Brief description of the figures

Figure 1 illustrates, in cross sectional view, a typical electrolysis cell for the production of aluminum.

Figures 2 to 9 show in side view different embodiments of anode anode block according to the invention.

Figure 10 schematically shows the bottom of a groove curved upwardly at a through end.

FIG. 11 shows according to another side view the anode block of FIG.

Detailed description of the invention

Electrolysis plants for aluminum production include a liquid aluminum production zone that includes one or more electrolysis rooms including electrolysis cells. The electrolysis cells are normally arranged in rows or rows, each row or line typically having more than one hundred cells, and electrically connected in series using connecting conductors.

As illustrated in FIG. 1, an electrolysis cell 1 comprises a tank 2, a support structure 3, called a "superstructure", carrying a plurality of anodes 4, means 5 for supplying the alumina and / or AlF ₃ tank and means 12 for recovering the effluents emitted by the tank in operation.

The tank 2 typically comprises a metal box 6 internally lined with refractory materials 7, 8, a cathode assembly which comprises carbon material blocks 9, called "cathode blocks", and metal connecting bars 10 to which electrical conductors are attached. used for the conveyance of the electrolysis current. The anodes 4 each comprise at least one anode block 13 of precured carbon material and a metal rod 14. The anode blocks 13 typically have a substantially parallelepiped shape. The rods 14 are typically attached to the anode blocks 13 by means of fasteners 15, generally called "multipodes", having studs which are anchored in the anode blocks 13 generally via recesses and cast iron. The anodes 4 are removably attached to a movable metal frame 16, referred to as the "anode frame", by mechanical fastening means, which typically include removable connectors and supports attached to the anode frame. The anode frame 16 is carried by the superstructure 3 and attached to electrical conductors (not shown) for the routing of the electrolysis current.

The refractory materials 7, 8 and the cathode blocks 9 form, inside the tank 2, a crucible adapted to contain an electrolyte bath 17 and a sheet of liquid metal 18 when the cell 1 is in operation. The tank 2 has a bottom, which is typically substantially flat and on which the sheet of liquid metal 18 is formed. In general, a cover 19 of alumina and solidified bath covers the electrolyte bath 17 and all or some of the blocks anodic 13.

The means 5 for feeding the alumina and / or AlF 2 tank are typically selected from hoppers, dosing units, chutes and piercers. The hoppers serve to contain a reserve of alumina and / or AlF 2 in powder form. The feeders are used to supply controlled amounts of alumina and / or AlF 2 in powder form. The chutes serve to guide the flow of alumina and / or A! F ₃ in the direction of the electrolyte bath 17. pointerolle and an actuator (such as a cylinder) for moving the chisel to form an opening in the cover 19 and allow the introduction of alumina and / or A! F ₃ into the electrolyte bath 17.

The means 12 for recovering the effluents emitted by the tank in operation generally include a cowling provided with removable hoods and suction ducts at one end of the cell.

The anodes 4, and more precisely the anode blocks 13, are partially immersed in the electrolyte bath 17, which contains dissolved alumina. The anode blocks 13 initially each have a bottom face that is typically substantially planar and parallel to the upper surface of the cathode blocks 9, which is generally horizontal. The distance between the underside of the anode blocks

13 and the upper surface of the cathodic blocks 9, called "interpolar distance", is an important parameter in the regulation of the electrolysis cells 1. The interpolar distance is generally controlled with great precision.

The anodic carbonaceous blocks are gradually consumed in use. In order to compensate for this wear, it is common practice to gradually lower the anodes by regularly moving the anode frame downwards. In addition, as illustrated in FIG. 1, the anode blocks are generally at different degrees of wear, advantageously to avoid having to change all the anodes at the same time.

Figures 2 to 9 show different embodiments of anode blocks 13a-13h anode grooves according to the invention. The anode blocks 13a-13h are seen from the side, typically on the long side, and respectively comprise grooves 20a to 20h, the bottoms of which, arranged in the heart of the anodic blocks, are represented by dotted lines, the portion below these lines. dotted being recessed over a width that can vary from 2 to 35 mm, and preferably from 5 to 25 mm. The anode blocks 13a-13h are typically rectangular parallelepipeds of length L between two sides 21 and 22 typically vertical and of height H between one face lower 23 and an upper face 24 typically horizontal. According to other embodiments of the anode blocks, the upper edges can be trimmed to limit carbon losses. The anode blocks are intended to be consumed up to a maximum wear height indicated by the arrows 25.

The grooves 20a to 20h extend in a direction x, typically parallel to the length L of the anode, and have a depth p (x) which varies along this direction x, in the manner of a mathematical function such as illustrated in Figure 2. It should be noted that the length Lr of a groove in this patent document means the length in the direction x.

The groove 20a of FIG. 2 extends over the entire length L of the anode block and therefore has a length Lr equal to the length L. It comprises a central portion 30 with a horizontal flat bottom and two end portions 31, 32 with a strongly inclined bottom (an angle + α or -α) relative to the horizontal away from the underside 23 as it approaches the sides 21, 22 and opening respectively on the sides 21, 22 of the anode block 13 a. In the example of FIG. 2, the central portion 30 extends over 70% of the length Lr of the groove 20a while the end portions each extend over 15% of the length Lr of the groove 20a and the bottom of the groove 20a is inclined 45 ° relative to the horizontal on these end portions 31, 32.

With an anodic block with a height H equal to 600 mm, a maximum wear height equal to 400 mm, a length L equal to 1500 mm and a deep groove of 200 mm at the central portion 30, the bottom of the groove opens on the sides of the anode block 25 mm above the maximum wear height 25 so that a groove portion will be present in the lower surface 23 throughout the life of the anode.

In comparison with an anodic block with a flat bottom groove of 200 mm effective depth over only half of the life of the anode, the groove 20a will be present in the bottom surface 23 over 30% of the length Lr mid-wear of the anode, on 22% of the length Lr at 65% of the wear of the anode, on 14% of the length Lr at 80% of the wear of the anode and on more than 3% of the length Lr at the end of the life of the anode. This remaining groove length helps to reduce the average free path of the gas bubbles to escape from beneath the bottom surface. Also, the gas evolution occurs longer above the electrolysis bath or higher in the electrolysis bath on the sides 21, 22 of the anode, which improves the stability of the flows in the electrolysis bath and more particularly in the region between the electrodes.

As the grooves sink deep into the anode block only at the end portions, the core of the anode block is not touched and the overall mechanical strength of the anode block is not affected.

The embodiment of FIG. 2 with steeply inclined end portions also has the advantage of facilitating and accelerating the evacuation of gases by gravitational effect and of limiting the formation of large gas bubbles which lead to with them metal and cause its re-oxidation and therefore losses of faraday yield.

The embodiment illustrated in FIG. 2 can be broken down into different ranges of values for different parameters, the angle of inclination α being able to vary from 20 ° to 80 ° and preferably from 30 ° to 70 ° and still more preferably from 35 ° to 80 °. At 55 °, and the central portion may extend over 60 to 95% of the length Lr and preferably over 70 to 90% of the length Lr and more preferably from 70 to 80% of the length Lr. The relationships between the length of the central portion and the angle of inclination α of the end portions are interdependent so as not to weaken the anode block and to obtain a prolonged effectiveness of the grooves.

In general, it is preferable to keep a small depth of groove on at least 60% of the length Lr of the groove and more preferably on more than 70% of the length Lr of the groove to guarantee a high resistance of the groove. anodic block. A deviation of more than 10% from the height H of anodic block, and more preferably a difference of more than 15% of the height H of the anode block, between this shallow groove depth and the depth of raised groove portions which extend over at least 3% , and preferably 5 to 20%, of the length Lr of the groove makes it possible to obtain grooves having a significant efficiency during a prolonged service life, and more particularly to facilitate the evacuation of gases when the wear of the block anodic exceeds this small groove depth.

Shown in Figure 11 in side view, along the short side 21, the anode block 13a. The anode block 13a more particularly comprises two grooves 20a typically disposed one quarter of the width of the anode block 13a with respect to the long sides so as to obtain a mean free path of the gas bubbles under the minimum anode block. The grooves have a width Wr which can vary typically from 2 to 35 mm, and preferably from 5 to 25 mm, the proportions not being respected in the figure for the sake of clarity. The change in inclination between the central portion 30 and the end portion 31 is thus dotted. In the example of FIG. 11, the upper edges 50 of the anodic block are trimmed. Dotted lines in FIG. 11 also show recesses 51 forming locations within which studs of "multipodes" can be fixed. In this example, the anode block 13a has more particularly six recesses arranged in two rows. These recesses are also very shallow and therefore have little impact on the integrity of the structure of the anode block.

In addition, the invention is not limited to an anodic block with a groove of symmetrical configuration with respect to a plane as can be seen in FIG. 2, where the end portions 31, 32 have an identical inclination and the same length and where the central portion 30 is horizontal but may also extend to an anode block with a groove where the two end portions may have different inclinations and / or lengths and where the central portion may be inclined relative to the horizontal at an angle less than 10 °. Such an example is illustrated in Figure 3 showing the anode block 13b with a groove 20b having a central portion 30 'slightly inclined and end portions 31', 32 'of different lengths and different inclinations.

According to another embodiment of the invention illustrated in FIG. 4, the anode block 13c has a groove 20c extending over the entire length L of the anode block, therefore of length Lr equal to L, the depth of which varies. in steps. The groove 20c has a central portion 33 with a horizontal flat bottom and two end portions 34, 35 with a horizontal flat bottom, the groove 20c being deeper on the end portions 34, 35 than on the central portion 33. According to the particular embodiment shown in FIG. 4, the end portions 34, 35 have open ends opening on the sides 21 and 22, typically approximately at the level of the maximum wear height 25 of the anode and the portion The center extends over 70% of the length Lr of the groove at mid-depth between the maximum wear height 25 and the bottom surface 23 of the anode block. The end portions advantageously open approximately at the level of the maximum wear height so as to maintain a groove until the change of anode without unnecessarily weakening the anode block. By approximately, one understands with approximately 10 centimeters, and preferably with plus or minus 5 centimeters, the height of maximum wear itself not being a strict fixed value for all the anode blocks but corresponding to a average value.

According to the invention, the central portion 33 extends over at least 60% of the length Lr, and preferably over more than 70%, and the end portions 34, 35 are deeper than the central portion 33 of at least 10% of the height H of the anode block, and preferably at least 15% of the height H of the anode block.

With such a configuration, when the anode block is consumed up to the bottom of the central portion 33, the groove 20c is present on the end portions 34, 35 for a further period of time. Portions With deeper ends, being lateral and of reduced length, the structure of the anode block is not weakened by such a groove.

The central and / or end portions may have a bottom slightly inclined to the horizontal by an angle of less than 10 ° and the end portions may have different lengths or depths. Also, the groove may have a greater number of bearings.

As can be seen in FIG. 5, according to the invention, the anode block 13d has a groove 20D extending over the entire length of the anode block and having a first portion 36 slightly inclined with respect to the horizontal angle α1 generally less than 10 ° and a second portion 37 more strongly inclined relative to the horizontal, preferably an angle α2 greater than 20 °. The inclination of the second portion 37 is sufficient for the groove 20D to persist for a substantial amount of time after the groove has been fully consumed on the first portion 36.

As shown in Figure 6, according to the invention, the anode block 13e has a groove 20e opening on a single side 22 of the anode block 13e. The length Lr of the groove 20e is less than the length L of the anode block. The groove 20e more particularly comprises a first portion 38 of shallow depth and a second portion 39 of deep depth. As can be seen in FIG. 6, the bottom of the grooves can describe a curvilinear trajectory.

The side on which the groove 20e or the inclination of the bottom of the groove 2Od opens is advantageously configured so as to direct and control the gas flows. Also, the orientation of the grooves, preferably longitudinally in the anode block, but also sometimes transversely or diagonally influences the overall kinetics of the fluids of the tank. Preferably, the grooves are oriented and inclined so as to direct the offgases to a corridor in which the alumina is charged. Such a corridor is typically arranged between two rows of anodes as can be seen in FIG. 1 between the two anode blocks 13 but can also be disposed between an edge of the tank and a row of anodes. The agitation caused by the gas flows can further improve the dissolution and distribution of alumina in the electrolysis bath, as well as the thermal and chemical equilibrium of the bath.

According to another embodiment of the invention illustrated in FIG. 7, the anode block 13f has a groove 20f substantially similar to the groove 20a but having a central nonlinear V-shaped portion 30, the two branches 40, 41 of the V being inclined relative to the horizontal of an angle β between + 10 ° and -10 °.

According to yet another embodiment of the invention illustrated in FIG. 8, the anode block 13g has along its length L two grooves 20g 'and 20g "of length Lr', Lr", each opening on an opposite side 21, 22 anodic block. Each of the grooves 20g 'and 20g "is constituted by steps and comprises a first portion 42', 42" of shallow depth and a second portion 43 ', 43 "of deep depth on the opening side. , the lower surface 23 of the anode block is not hollowed so that such anode block has a particularly high strength. According to the invention, the first portions 42 ', 42 "extend over at least 60% of the length Lr', Lr" of the groove, and preferably more than 70%, and the second portions 43 ', 43 are respectively deeper than the first portions 42 ', 42 "of at least 10% of the height H of the anode block, and preferably at least 15% of the height H of the anode block.

According to yet another embodiment of the invention illustrated in FIG. 9, the anode block 13h comprises a groove 20h of length Lr that does not open on the sides 21, 22 of the anode block 13h. The groove 20h is in the form of hood with two portions of sides 44, 45 substantially horizontal extended in the center by a duct 46 rising narrowing. The conduit 46 opens on the upper face 24 of the anode block where the gases are evacuated. The duct 46 tapers gradually so that it acts as a groove for a prolonged period. In addition, the central position of the duct contributes to reducing the average free path of the gas bubbles. The physical strength of such anode block comes from the fact that the sides 21, 22 of the anode block 13h are not notched. Also, the evacuation of gases through the upper surface of the anode block improves the overall stability of the electrolysis bath.

Apart from the embodiment of Figure 9, the bottom of each groove has a maximum depth at the ends opening on the sides to allow evacuation of gases by gravitational effect. According to an advantageous embodiment of the invention, the bottom of the grooves may have a rounded upward trajectory at the open ends as illustrated in FIG. 10 so as to facilitate the continuous evacuation of the gases.

According to the invention, the grooves have a depth that varies non-linearly in the x direction, that is to say that the depth of the groove does not vary constantly from one end to the other of the groove. The bottom of the grooves has for example a variation of inclination or a recess.

In general, according to the invention, the grooves can be divided into at least two parts, a first part accounting for at least 60% and preferably 70% of the length Lr of the groove having a depth less than a threshold Z ₁ , and a second portion accounting for at least 3%, preferably more than 5%, and more preferably more than 10%, of the length Lr of the groove, this second part having a depth greater than a threshold Z _{2 is} equivalent to Z ₁ + 10% of the height H and is preferably equivalent to Z ₁ + 15% of the height H. The shallow first part serves as a compact structure at the anode block in order to give it an important resistance. The second part is offset vertically relative to the first part so that when the anode block is consumed up to the threshold Z ₁ , a groove portion, at least as long as the second part, remains and facilitates the evacuation of gas. This second part has a restricted length so as not to weaken the structure of the anode block. These values and percentages result from compromises which in practice make it possible to obtain a prolonged efficiency of the groove while maintaining satisfactory overall physical strength. Advantageously, the carbon losses of the anode are limited while maintaining a solid and efficient anode.

The carbonaceous anode blocks used for the manufacture of aluminum typically comprise 1 to 4 grooves. Each groove preferably has a constant width W n but may also have variations in width along the x direction or functions of the depth. In addition, two grooves may have different widths Wr. The cross-sectional profile of the bottom of each groove is preferably horizontal but may also have a particular inclination or curvature.

The grooves can be made either during the molding of the green blocks or by sawing the fired blocks.

This invention is particularly advantageous in the case where the grooves are obtained by molding because such anodic groove blocks are more subject to degradation during demolding, storage, transport or cooking. Vertical scraps are introduced into the molds to conform the grooves. The blocks are demolded by pushing in a single direction so that according to the embodiments of the invention, it may be necessary to remove the remains before demolding or push the anode block by the lower surface.

Claims

Anode block (13) made of carbon for use in an electrolysis cell (1) for producing metal, said block having a height H between an upper face (24) and a lower face (23) and comprising on the lower face (23) at least one groove (20) of depth p (x) and length Lr, the groove extending in a direction x and said depth p (x) being variable along said direction x characterized in that said depth p (x) varies non-linearly along said x direction and in that said depth p (x) is less than a first value Z ₁ over at least 60% of the length Lr of said groove and is greater than a second value Z ₂ at least equal to Z ₁ + 10% of the height H over 3 to 40% of the length Lr of the groove.

Anode block according to claim 1, wherein said depth p (x) is less than said first value Z ₁ over at least 70% of the length Lr of said groove and is greater than said second value Z ₂ at least equal to Z ₁ + 15% of the height H over 3 to 30% of the length Lr of the groove.

3. Anode block according to any one of the preceding claims, wherein said groove comprises at least one end opening on one side (21, 22) of said anode block and wherein the depth of the groove is greater than said second value Z ₂ at said open end.

4. Anode block according to any one of the preceding claims, wherein said groove comprises two ends each opening on one side of said anode block and wherein the depth of the groove is greater than said second value Z ₂ at each of the ends. open out.

5. Anode block according to any one of claims 3 and 4, wherein the depth of the groove is maximum at at least one open end.

6. Anode block according to any one of the preceding claims, wherein the groove comprises a first portion with a flat bottom or inclined at an angle less than 10 °.

Anode block according to claim 6, wherein the groove further comprises an end portion with a bottom inclined from 20 to 80 °.

Anode block according to claim 6, wherein the groove further comprises two end portions with a bottom inclined from 20 to 80 °.

Anode block according to any one of the preceding claims, wherein the depth p (x) of the groove varies substantially stepwise along said x direction.

10. Anode block according to any one of the preceding claims, wherein the groove has a maximum depth corresponding, ± 10 cm, to a maximum wear height of the anode blocks of the anode block.

11. Anode comprising at least one anode block according to any one of the preceding claims.

12. An igneous electrolysis aluminum production cell comprising a plurality of anodes, characterized in that at least one of the anodes is an anode according to claim 11.

13. The cell of claim 12, wherein the grooves open into a corridor where is introduced alumina.

A process for producing aluminum comprising the steps of:

provide at least one anode according to claim 1 _. ;

- install the anode in a cell electrolytic aluminum cell;

- Pass current in the cell electrolysis cell through the anode;