AU2008327757B2

AU2008327757B2 - Grooved anode for an electrolysis tank

Info

Publication number: AU2008327757B2
Application number: AU2008327757A
Authority: AU
Inventors: Claude Ritter
Original assignee: Alcan International Ltd Canada
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 2007-09-14
Filing date: 2008-09-08
Publication date: 2011-12-22
Anticipated expiration: 2028-09-08
Also published as: WO2009066025A2; AU2008327757A1; FR2921074B1; CA2698894A1; WO2009066025A3; FR2921074A1

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 Z of at least 60% of the length Lr of said groove and is greater than a second value Z, which is at least equal to Z + 10% of the height H, over 3 to 40% of the length Lr of the groove.

Description

SLOTTED ANODE FOR AN ELECTROLYTIC POT Field of the invention 5 The invention concerns the production of aluminum by igneous electrolysis according to the Hall-Hdroult process and more specifically the anodes including a slotted carbon anode block used in aluminum manufacturing plants. 10 Background of related art Aluminum metal is produced industrially by igneous electrolysis, namely by electrolysis of alumina in a molten cryolite bath, referred to as the electrolytic bath, using the well-known Hall-Hdroult process. The electrolytic bath is contained in pots 15 comprising a steel shell, the inside of which is coated with refractory and/or insulating materials, and cathode elements located at the bottom of the pot. Anode blocks made of carbonaceous material are partially immersed in the electrolytic bath. Each pot and the corresponding anodes form what is frequently called an electrolytic cell. The electrolysis current that circulates in the electrolytic bath and possibly a 20 layer of liquid aluminum through anodes and cathode elements, causes alumina reduction reactions to take place and also makes it possible to keep the electrolytic bath at a temperature in the order of 950'C owing to the Joule effect. French patemt application FR 2 806 742 (corresponding to American patent 25 US 6 409 894) describes installations of an electrolysis plant intended to produce aluminum. According to the most widespread technology, the electrolytic cells comprise a plurality of prebaked anodes made of carbonaceous materials that are consumed 30 during reactions that cause the electrolytic reduction of aluminum.

2 Gases, and particularly carbon dioxide, are generated during electrolytic reactions and naturally accumulate in the form of gas bubbles under the lower face of the anode, essentially generally flat or horizontal, which influences the overall stability of the pot. 5 The following results from the accumulation of these gas bubbles: - electrical instabilities and variations, - a high frequency and significant duration of anode effects, - the increased possibility of inverse reaction and thus a loss of yield due to the small 10 distance between the layer of aluminum produced and the CO 2 bubbles, - increased consumption of carbon and the formation of noxious gases resulting from the transformation of the CO 2 into CO upon contact with the carbon. The use is known of anodes with carbonaceous anode blocks including one or more 15 slots in the lower part to facilitate the drainage of gas bubbles and prevent their accumulation in order to solve the problems cited above and to reduce energy consumption as shown in Light Metals 2005 "Energy saving in Hindalco's Aluminum Smelter", S.C. Tandon & R.N. Prasad. The slots make it possible to decrease the mean free path of the gas bubbles under the anode enabling them to exit 20 the space between the electrodes and thus to reduce the size of the bubbles forming under the anode. The interest in using slots has already been studied and proven, for example in Light Metals 2002 p. 305-310 "The impact of slots on reduction cell individual anode current variation", Geoff Bearne, Dereck Gadd, Simon Lix or Light Metals 2007 25 p. 299-304 "Development and deployment of slotted anode technology at Alcoa", Xiangwen Wang et al. . It is also known in the following documents: - WO 2006/137739 using slots (in the order of 2 to 8 mm) that are narrower than those commonly used (in the order of 8 to 20 mm) so as to optimise the useful 30 carbonaceous mass and the exchange area; 3 - US 7 179 353 using an anode including slots opening on only one side or side face of the anode block, and particularly toward the centre of the electrolytic cell so as to improve the dissolving of the alumina; - WO 2006/137739, US 7 179 353 and Light Metals 2007 p. 283-285 "Slot cutting in 5 anodes" by Jean-Jacques Grunspan, inclining the bottom of the slots at an angle up to 100 in order to accelerate the discharge of gases trapped in the slots and to control the orientation of the flow of gas. A well-known limit to the use of these slots results from the fact that the depth of the 10 slots is limited so as not to disrupt the mechanical and physical integrity of the carbonaceous anode blocks. The anode blocks are consumed progressively during the electrolysis reaction over a height greater than the depth of the slots such that the duration period of an anode's slots is less than the overall service life of the anode. As a result, c-ver a certain time during the service life of the anodes, slots no longer 15 exist on the lower part of the anode blocks. The problems mentioned above for anodes without slots thus appear. Furthermore, as mentioned in Light Metals 2007 p. 299-304 "Development and deployment of slotted anode technology at Alcoa", the depth of the slots is limited 20 for integrity reasons mainly in the case of slots molded on unbaked anode blocks so that the beneficial effects resulting from the presence of slots can be observed only over part of the anodes' lifespan. The slots weaken the unbaked anode block that crack during transport, storage or baking. 25 In practice, it is also impossible to reliably obtain, by sawing baked anode blocks, anodes with slots as deep as the height of the anode block intended to be consumed. The mechar ical stresses and vibrations exerted by the saw blades cause crumbling, crazing then rupture of the carbon blocks. 30 The dimensions of the anode blocks of anodes commonly used are in the order of 1,200 to 1,700 mm long, 500 to 1,000 mm wide and 550 to 700 mm high, with one to three slots generally between 150 and 350 mm deep.

4 Also, for an anode block measuring 600 mm in height with a consumable carbon height of 400 mm and a slot 250 mm deep, the slot has a beneficial effect for only 62.5% of the anode's service life. Object of the Invention 5 It is the object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages or to provide a useful alternative. Summary of the Invention According to a first aspect of the present invention there is disclosed herein an anode block mode of carbon for use in an electrolysis cell intended for the production of io metal, said block having a height H between an upper face and a lower face and including on the lower face at least one slot of depth p(x) and length Lr, the slot extending along a direction x and said depth p(x) being variable along the length of said direction x, wherein said depth p(x) varies in a non linear manner along said direction x and said depth p(x) is less than a first value Z, over at least 60% of the length Lr of said slot and is greater than is a second value Z 2 at least equal to Z, + 10% of the height H over 3 to 40% of the length Lr of the slot. Preferably, said depth p(x) is less than first value Zi over at least 70% of the length Lr of said slot and said second value Z 2 is at least qual to Zi + 15% of the height H 20 over 3 to 30% of the length Lr of the slot and preferably over 5 to 20% of the length of the slot. The upper face includes at least one mounting recess and the lower face is intended to be immersed into an electrolytic bath. 25 The non-linear variation of said depth p(x) along said direction x is understood as a variation that cannot be described using a single straight line or unique plane. The variation can nevertheless comprise one or more linear portions. 30 The shape of the slot according to preferred embodiments of the invention gives it an enhanced service life while maintaining a high structural integrity of the anode block. Once level Z, of consumed carbon is reached, at least a portion of the slot remains to drain the gases that accumulate under the lower face of the anode block. The 5 remaining portion of the slot, although having reduced length, makes it possible to limit the problems associated with the accumulation of gases under the anode. In one embodiment of the invention, the slot comprises at least one end opening 5 on a side of the anode block and the depth of the slot is greater than said second value Z 2 on the opening end. In a variant of this embodiment, the slot comprises two ends that each open on one side of the anode block and the depth of the slot is greater than the second value Z 2 on each of the opening ends. The depth of the slot is typically maximal on at least one opening end. 10 In a specific embodiment of the invention, the slot includes a central portion, with a bottom that is flat or slightly inclined less than 100 with respect to the horizontal, surrounded on each side by an end portion with a steeply inclined bottom from 20 to 800 with respect to the horizontal and extending in the direction of the upper face of the anode 15 block while nearing the sides of the anode block, the ends of the slot opening on opposite sides of the anode block. In other words, the bottom or depth p(x) of the slot follows a profile in the shape of a cross-section of a plate with inclined edges. In another specific embodiment of the invention, the slot includes a central 20 portion, with a bottom that is flat or inclined less than 10* with respect to the horizontal, surrounded on each side by an end portion with a bottom that is flat or inclined less than 100 with respect to the horizontal, said end portions being recessed vertically above said central portion, the ends of the slot opening on opposite sides of the anode block. In other words, the bottom or depth p(x) of the slot follows a profile in the shape of a cross-section 25 of an upside-down hat. According to these embodiments of the invention, the core of the anode block is not weakened by the slot over the major length of the slot corresponding to the central portion. Furthermore, the gaseous releases preferably occur at a high height on the sides 30 of the anode block in the electrolytic cell with the result that the agitation of the electrolytic bath is more regular and conductive to the cell's electrical, chemical and thermal equilibrium. According to yet other specific embodiments of the invention, the slot includes a 35 first portion, with a bottom that is flat or inclined less than 100 with respect to the 6 horizontal, and a second flat bottom portion recessed vertically above said first portion or with the bottom steeply inclined 20 to 800 with respect to the horizontal extending in the direction of the upper face of the anode block and nearing the sides of the anode block. 5 Preferably, the slot 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 slot over essentially the entire duration of the block's service life in an electrolytic cell. The presence of the slots reduces the turbulence of the electrolytic bath and the 10 kinetic energy of turbulence for the volume located below the lower surface of the anode block. The applicant considers that this effect is amplified even more when the slot is not entirely immersed as the path that the gas bubbles must travel for evacuation is shorter. The reduction of the turbulence is especially preferred in the region below the anode block as it reduces reoxidation of the dissolved metal in the electrolytic bath. 15 With the anode blocks according to preferred embodiments of the invention, part of the slots is not immersed in the bath over an extended period of time compared to the slots of prior art, and more particularly at the level of the opening ends of the slots. Gases are thus released above the bath and high in the bath, in the slot or on the sides of the 20 blocks, which reduces the turbulence of the bath between the electrodes and allows the distance between the electrodes to be reduced and to increase the energy efficiency. Also, compared to an anode block of prior art which passes from an effective slot to the absence of a slot through carbon consumption or wear, a progressive or step by 25 strep decrease of the length of slots, and thus of their efficiency, can be observed with the anode blocks according to embodiments of the invention, which prevents disturbances and sudden changes in the kinematics of the fluids with associated problems of electrical equilibrium and, for example, each adaptive adjustments. 30 Furthermore, slots can be oriented so that when gaseous releases occur in the electrolytic bath (after a certain amount of wear or consumption of the anode block), they are directed toward the alumina feeding points in order to facilitate the agitation and dissolving of the alumina, particularly toward the central corridor in the electrolytic cell.

7 According to a second aspect of the invention there is disclosed herein an anode including at least one anode block according to the first aspect. According to another aspect of the invention there is disclosed herein an igneous 5 electrolysis cell for the production of aluminium including a plurality of anodes, wherein at least one of the anodes is an anode according to the second aspect. According to a further aspect of the present invention there is disclosed herein an aluminium manufacturing processing comprising the steps of: io - providing at least one anode according to the first aspect; - installing the anode in an aluminium electrolytic cell; - passing current in the electrolytic cell through the anode; and - recovering the aluminium obtained by electrolysis in the bottom of the pot of the electrolytic cell. 1s Brief description of figures Preferred embodiments of the present invention will now be described, by way of an examples only, with reference to the accompanying drawings wherein: Figure 1 is a cross-sectional view of a typical electrolytic cell designed to produce aluminium.

8 Figures 2 to 9 represent a side view of various embodiments of an anode block according to the invention. 5 Figure 10 schematically represents the bottom of a slot curved upwards at an opening end. Figure 1 I shows the anode block of Figure 2 according to another side view. 10 Detailed description of the invention Aluminum works designed to product aluminum comprise a liquid aluminum production zone that includes one or more electrolysis rooms containing electrolytic cells. The electrolytic cells are normally placed in rows or columns, each row or 15 column typically including more than a hundred cells, and electrically connected in series by means of connecting conductors. As shown in Figure 1, an electrolytic cell 1 includes a pot 2, a support structure 3, referred to as the "superstructure", holding a plurality of anodes 4, means 5 to supply 20 the pot with alumina and/or AIF 3 and means 12 to recover the effluents released by the pot in operation. The pot 2 typically includes a metal shell 6 equipped on the inside with refractory materials 7, 8, a cathode assembly which includes blocks made of carbonaceous 25 material 9, called "cathode blocks", and metal connecting bars 10 to which the electrical ccnductors 11, which deliver the electrolysis current, are fastened. The anodes 4 each include at least one anode block 13 made of prebaked carbonaceous material and a metal rod 14. The anode blocks 13 typically have an essentially parallelepiped shape. The rods 14 are typically secured to the anode blocks 13 via 30 mounting clements 15, generally called "multipodes", including pins that are generally anchored in the anode blocks 13 by means of recesses and cast iron. The anodes 4 ar-2 fastened in a removable manner to a mobile metal frame 16, called the 9 "anode fram ", by mechanical fastening means, which typically include removable connectors and supports fastened to the anode frame. The anode frame 16 is supported by the superstructure 3 and fastened to electrical conductors (not illustrated) serving to route the electrolysis current. 5 The refractory materials 7, 8 and cathode blocks 9 form, inside the pot 2, a crucible able to contain an electrolytic bath 17 and a layer of molten metal 18 when the cell 1 is operating. The pot 2 has a bottom that is typically essentially flat and on which the layer of molten metal 18 forms. Usually, a crust 19 of alumina and solidified bath 10 covers the electrolyte bath 17 and all or part of the anode blocks 13. The means 5 to supply the pot with alumina and/or AIF 3 are typically selected among hoppers, dosing apparatus, troughs and crust breakers. The hoppers contain a reserve of alumina and/or AIF 3 in powder form. The dosers supply controlled quantities of 15 alumina and/or AIF 3 in powder form. The troughs are designed to guide the flow of alumina anc/or AIF 3 in the direction of the electrolyte bath 17. The crust breakers generally include a chisel and an actuator (such as a cylinder) to move the perforator in order to create an opening in the crust 19 and allow alumina and/or AIF 3 to be introduced into the electrolyte bath 17. 20 The means 12 to recover the effluents released by the pot in operation generally include a hooding equipped with removable hoods and suction ducts at one end of the cell. 25 The anodes 4, and more precisely the anode blocks 13, are partially immersed in the electrolyte bath 17, which contains dissolved alumina. Initially, each anode block 13 has a lower face typically essentially planar and parallel to the upper surface of the cathode blocks 9, which is generally horizontal. The distance between the lower face of the anode blocks 13 and the upper surface of the cathode blocks 9, called the 30 "interpolar distance", is an important parameter in regulating the electrolysis cells 1. The interpolar distance is generally controlled with high precision.

10 The anode carbonaceous blocks are progressively consumed during use. In order to compensate for this wear, it is standard practice to progressively lower the anodes by regularly lowering the anode frame. In addition, as shown in Figure 1, the anode blocks are generally at different degrees of wear, advantageously to avoid having to 5 replace all ancdes at the same time. Figures 2 to ) present various embodiments of slotted anode blocks 13a-13h with anodes according to the invention. The anode blocks 13a-13h are seen from the side, typically the long side, and include slots 20a to 20h respectively, the bottoms of 10 which, located at the core of the anode blocks, are represented by dashed lines, the portion below these dashed lines being hollowed out over a width varying 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 sides 21 and 22 typically vertical and of height H between a typically horizontal lower face 23 and a typically horizontal 15 upper face 24. According to other embodiments of the anode blocks, the upper edges can be bevelled to limit carbon losses. The anode blocks are intended to be consumed to a maximum wear height indicated by the arrows 25. The slots 20a to 20h extend along a direction x, typically parallel to the length L of 20 the anode, and have a depth p(x) which varies along this direction x, as a mathematical function as illustrated in Figure 2. It should be noted that the length Lr of a slot in this patent document refers to the length along direction x. The slot 20a of Figure 2 extends the entire length L of the anode block and has, as a 25 result, a length Lr equal to length L. It includes a central portion 30 with a flat horizontal bottom and two end portions 31, 32 with a steeply inclined bottom (angle +cc or -a) wi:h respect to the horizontal while moving away from the lower face 23 as it approaches sides 21, 22 and opening on sides 21, 22, respectively, of the anode block 13a. In the example shown in Figure 2, the central portion 30 extends along 30 70% of the length Lr of the slot 20a while the end portions extend over 15% of the length Lr of the slot 20a and the bottom of the slot 20a is inclined 45' with respect to the horizontal on these end portions 3 1, 32.

l1 With an anode block of height H equal to 600 mm, of maximum wear height equal to 400 mm, of length L equal to 1,500 mm and a slot 200 mm deep in the central part 30, the bottom of the slot opens on the sides of the anode block 25 mm above the 5 maximum wear height 25 such that a portion of the slot shall be present on the lower surface 23 throughout the entire service life of the anode. In comparison with an anode block with a flat bottom 200 mm effective depth on only half the anode's service life, the slot 20a shall be present on the lower surface 23 10 over 30% of the length Lr when the anode is half-worn, on 22% of the length Lr when the anode is 65% worn, on 14% of the length Lr when the anode is 80% worn and over more than 3% of the length Lr at the end of the anode's service life. This remaining slo, length contributes to reduce the mean free path of the gas bubbles to escape from below the lower surface. Also, the gaseous release occurs for a longer 15 period of time above the electrolytic bath or higher in the electrolytic bath on sides 21, 22 of the anode, which improves the stability of the flows in the electrolytic bath and particularly in the region between the electrodes. As the slots enter the anode block deeply at the end portions only, the core of the 20 anode block .s not touched and the overall mechanical strength of the block is not affected. The embodiment of Figure 2 with steeply inclined end portions presents the advantage of facilitating and accelerating the drainage of gases by gravitational effect 25 and to limit the formation of gas bubbles of large size which take metal along with them and cause its re-oxidation and thus losses of current efficiency. The embodiraent illustrated in Figure 2 can be broken down according to different ranges of values for different parameters, the angle of inclination a varying from 200 30 to 800 and preferably from 30' to 700 and still preferably from 350 to 550, and the central part can extend over 60 to 95% of the length Lr and preferably over 70 to 90% of the length Lr and preferably from 70 to 80% of the length Lr. The 12 relationships between the length of the central part and the angle of inclination a and end portions are interdependent so as not to weaken the anode block and to extend the efficiency of the slots. 5 More generally, it is preferable to maintain a shallow slot depth over at least 60% of the length Lr Af the slot and more preferably over more than 70% of the length Lr of the slot to endure the anode block's high level of strength. A difference of more than 10% of the height H of the anode block, and more preferably a difference of more than 15% of the height H of the anode block, between this shallow slot depth and the 10 depth of the raised portions of the slot which extend over at least 3%, and preferably over 5 to 20%, of length Lr of the slot allow slots to be obtained that are significantly efficient over an extended service life, and more particularly to facilitate the drainage of gases whet anode block wear exceeds this shallow slot depth. 15 Figure I1 represents a side view, along the short side 21, of anode block 13a. The anode block 13a particularly includes two slots 20a each typically positioned at one forth the width of the anode block 13a with respect to the long sides so as to obtain a minimal mean free path of the gas bubbles under the anode block. The slots have a width Wr that can typically vary from 2 to 35 mm, and preferably from 5 to 25 mm, 20 the proportions not being respected on the figure for clarity reasons. The change in inclination can thus be observed with the dotted line between the central portion 30 and the end portion 3 1. In the example shown in Figure 11, the upper edges 50 of the anode block are bevelled. Figure 1 1 also shows recesses 5 1, in dotted lines, forming locations inside which can be mounted "multipode" pins. In this example, the anode 25 block 13a more particularly includes six recesses arranged on two rows. In addition, these recesses are very shallow and thus have little impact on the integrity of the anode block's structure. In addition, the invention is not limited to an anode block with a symmetrical slot 30 configuration with respect to a plane as shown in Figure 2 where the end portions 3 1, 32 have the same inclination and same length and where the central part 30 is horizontal although can also extend to an anode block with a slot where both end 13 portions can have different inclinations and/or lengths and where the central portion can be inclined by an angle less than 100 with respect to the horizontal. An example of this is illustrated in Figure 3 showing the anode block 13b with a slot 5 20b including a slightly inclined central portion 30' and end portions 31', 32' of different lengths and different inclinations. According to another embodiment of the invention illustrated in Figure 4, the anode block 13c includes a slot 20c extending along the entire length L of the anode block, 10 as a result length Lr equals L, and the depth of which varies by steps. The slot 20c includes a central portion 33 with a flat horizontal bottom and two end portions 34, 35 with a horizontal flat bottom, the slot 20c being deeper on the end portions 34, 35 than on the central portion 33. According to the special embodiment presented in Figure 4, the end portions 34, 35 have open ends opening on sides 21 and 22 15 typically approximately at the level of the anode's maximum wear height 25 and the central portion extends over 70% of the length Lr of the slot at mid-depth between the maximurn wear height 25 and the lower 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 slot until the anode is replaced without unnecessarily 20 weakening the anode block. By approximately, this means more or less within 10 centimetres, and preferably more or less within 5 centimetres, the maximum wear height itself not being a strictly set value for all anode blocks although corresponding to an average value. 25 According to the invention, the central portion 33 extends over at least 60% of length Lr, and preferably over more than 70%, and the end portions 34, 35 are deeper than the central portion 33 by at least 10% the height H of the anode block, and preferably by at least 15% the height H of the anode block. 30 With such a configuration, when the anode block is consumed to the height of the bottom of th: central portion 33, the slot 20c is present on the end portions 34, 35 for 14 an additional period of time. As the deeper end portions are lateral and of reduced length, the structure of the anode block is not weakened by such a slot. The bottom of the central and/or end portions can be slightly inclined with respect to 5 the horizontal by an angle less than 100 and the end portions can have different lengths or dephs. Also, the slot can have a greater number of steps. As shown in Figure 5, according to the invention, the anode block l3d includes a slot 20d extending over the entire length of the anode block and including a first portion 10 36 that is slightly inclined with respect to the horizontal, by an angle al generally less than 100 z.nd a second portion 37 that is more steeply inclined with respect to the horizontal, preferably at an angle a2 greater than 20'. The inclination of the second portion 37 is sufficient so that the slot 20d ensures partly for a significant period of time after the slot has been entirely consumed on the first portion 36. 15 As shown in Figure 6, according to the invention, the anode block 13e includes a slot 20e opening cn a single side 22 of the anode block 13e. The length Lr of the slot 20e is less than the length L of the anode block. The slot 20e particularly includes a first portion 38 that is shallow and a second portion 39 that is deep. As shown in Figure 6, 20 the bottom of the slots can follow a curvilinear trajectory. The side on which slot 20e opens or yet the inclination of the bottom of the slot 20d is advantageously configured so as to direct and control the gaseous flows. Also, the orientation of the slots, preferably longitudinal in the anode block, but also 25 transversal or diagonal, influences the overall kinetics of the fluids in the pot. Preferably, the slots are oriented and inclined in such a manner as to direct the gaseous releases toward a corridor loaded with alumina. Such a corridor is typically placed between two rows of anodes, as shown in Figure 1, between the two anode blocks 13 but can also be placed between a side of the pot and a row of anodes. The 30 agitation caused by the gaseous flow can also improve the dissolution and distribution of the alumina in the electrolytic bath, as well as the thermal and chemical equilibrium of the bath.

15 According to another embodiment of the invention illustrated in Figure 7, the anode block 13f includes a slot 20f essentially similar to slot 20a although including a central non-linear V-shaped part 30, the two branches 40, 41 of the V being inclined 5 with respect to the horizontal at an angle 3 between +100 and -10*. According to yet another embodiment of the invention illustrated in Figure 8, the anode block :.3g includes two slots 20g' and 20g" of length Lr', Lr" along its length L, each opening on an opposite side 21, 22 of the anode block. Each of the slots 20g' 10 and 20g" consists of levels and forms a shallow first portion 42', 42" and a second deep section 43', 43" on the opening side. Between the two slots 20g' and 20g", the lower surface 23 of the anode block is not hollowed out so that such an anode block is particularly strong. According to the invention, the first portions 42', 42" extend over at least 60% of the length Lr', Lr" of the slot, and preferably over more than 15 70%, and thc second portions 43', 43" are deeper than the first portion 42', 42", respectively, by at least 10% the height H of the anode block, and preferably by at least 15% the height H of the anode block. According to yet another embodiment of the invention illustrated in Figure 9, the 20 anode block 13h includes a slot 20h of length Lr not opening on the sides 21, 22 of the anode block 13h. The slot 20h is in the shape of a hood with two side portions 44, 45 essentially horizontal extended in the centre by a conduit 46 that becomes narrower as it rises. The conduit 46 opens on the upper face 24 of the anode block where the gases are released. The conduit 46 narrows progressively such that it acts 25 as a slot for an extended period of time. Furthermore, the central position of the conduit contributes to decreasing the mean free path of the gas bubbles. The physical strength of such an anode block is attributed to the fact that the sides 21, 22 of the anode block 13h are not grooved. Also, the discharge of gases via the upper surface of the anode block improves the overall stability of the electrolytic bath. 30 Apart from :he embodiment of Figure 9, the bottom of each slot has a maximum depth at the: opening ends on the sides to allow the discharge of gasses by 16 gravitational effect. According to an advantageous embodiment of the invention, the bottom of the slots can have a rounded trajectory at the top at the opening ends as illustrated in Figure 10 so as to facilitate the continuous discharge of gases. 5 According to the invention, the slots have a depth that varies in a non-linear manner along the direction x, meaning that the depth of the slot does not vary in a constant manner from one end the slot to the other. The bottom of the slots includes a variation in inclination or a step, for example. 10 More generally, according to the invention, the slots can be divided into at least two parts, a first pirt counting for at least 60% and preferably 70% of the length Lr of the slot having a depth less than a threshold Z 1 , and a second part counting for at least 3%, preferably for more than 5%, and more preferably for more than 10%, of the length Lr of the slot, this second part having a depth greater than the threshold Z 2 15 equivalent to Z, + 10% of the height H and preferably equivalent to Z, + 15% of the height H. The first part, which is shallow, serves as a compact structure for the anode block to provide it significant strength. The second part is shifted vertically with respect to the first part in such a way that when the anode block is consumed up to threshold Z 1 , a portion of the slot, at least as long as the second part, remains and 20 facilitates the removal of gases. The length of this second part is restricted so as not to weaken thc structure of the anode block. These values and percentages are the result of a compromise enabling, in practice, to prolong the efficiency of the slot while maintaining overall satisfactory physical 25 strength. Advantageously, carbon losses from the anode are limited while maintaining a solid and efficient anode. The carbonaceous anode blocks used in the production of aluminum typically include 1 to 4 slots. Preferably, the width Wr of each slot is constant although it may also 30 include variations in width along the direction x or as a function of the depth. In addition, two slots may have different widths Wr. The cross-section of the bottom of each slot is preferably horizontal but can also include an inclination or special curve.

17 The slots can be produced either during the moulding of the unbaked blocks, or by sawing baked blocks. 5 This invention is particularly advantageous for the case where the slots are obtained by moulding due to the fact that such slotted anode blocks are more subject to damage during mould removal, storage, transport or baking. Vertical shapes are introduced in:o the moulds to shape the slots. The blocks are pushed from the moulds in a single di::ection, so that, depending on the embodiments of the invention, it may 10 be necessary to remove the shapes before removal or to push the anode block by the lower surface. 15

Claims

1. An anode block made of carbon 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 including on the lower face at least one slot of depth p(x) and length Lr, the slot extending along a direction x and said depth (px) being variable along the length of said direction x, wherein said depth p(x) varies in a non-linear manner along said direction x and said depth p(x) is less than a first value Z, over at least 60% of the length Lr of said slot and is greater than a second value Z 2 at least equal to Z, to 10% of the height H over 3 to 40% of the length Lr of the slot.

2. The anode blocking according to claim 1, in which said depth p(x) is less than said first value Z, over at least 70% of the length Lr of said slot and is greater than said second value Z 2 at least equal to Z, to 15% of the height H over 3 to 30% of the length Lr of the slot.

3. The anode block according to any one of the previous claims, in which said slot comprises at least one end opening on a side of said anode block and in which the depth of the slot is greater than said second value Z 2 at said opening end.

4. The anode block according to any one of the previous claims, in which said slot comprises two ends, each end opening on one side of said anode block, and in which the depth of the slot is greater than second value Z 2 at each of the opening ends.

5. The anode block according to either one of claims 3 or 4, in which the depth of the slot is maximal at the level of at least one opening end.

6. The anode block according to any one of previous claims, in which the slot includes a first portion with a bottom that is flat or inclined at an angle less than 100.

7. The anode block according to claim 6, in which the slot further includes an end portion with a bottom inclined from 20 to 800.

8. The anode block according to claim 6, in which the slot further includes two end portions with a bottom inclined from 20 to 80*. 19

9. The anode block according to any one of previous claims, in which the depth p(x) of the slot varies substantially by steps along said direction x.

10. The anode block according to any one of previous claims, in which the slot has a maximum depth corresponding to, within + 10 cm, a maximum wear height of the anode block.

11. An anode including at least one anode block according to any one of the previous claims.

12. An igneous electrolysis cell for the production of aluminium including a plurality of anodes, wherein at least one of the anodes is an anode according to claim 11.

13. The cell according to claim 12, in which the slots open into a corridor where alumina is introduced.

14. An aluminium manufacturing process including the steps of: - providing at least one anode according to claim 11; - installing the anode in an aluminium electrolytic cell; - passing current in the electrolytic cell through the anode; and - recovering the aluminium obtained by electrolysis in the bottom of the pot of the electrolytic cell.

15. An anode block substantially as hereinbefore described with reference to the accompanying drawings.

16. An anode substantially as hereinbefore described with reference to the accompanying drawings.

17. An igneous electrolysis cell for the production of aluminium, the cell being substantially as hereinbefore described with reference to the accompanying drawings. 20

18. An aluminium manufacturing process substantially as hereinbefore described with reference to the accompanying drawings. Dated 5 December, 2011 Alcan International Limited Patent Attorneys for the Applicant/Nominated Person SPRUSON & FERGUSON