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EP1733075A2 - Cathode element for an electrolysis cell for the production of aluminium - Google Patents

Cathode element for an electrolysis cell for the production of aluminium

Info

Publication number
EP1733075A2
EP1733075A2 EP05744310A EP05744310A EP1733075A2 EP 1733075 A2 EP1733075 A2 EP 1733075A2 EP 05744310 A EP05744310 A EP 05744310A EP 05744310 A EP05744310 A EP 05744310A EP 1733075 A2 EP1733075 A2 EP 1733075A2
Authority
EP
European Patent Office
Prior art keywords
bar
insert
block
cathode
cathode element
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP05744310A
Other languages
German (de)
French (fr)
Other versions
EP1733075B1 (en
Inventor
Delphine Bonnafous
Jean-Luc Basquin
Claude Vanvoren
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Rio Tinto France SAS
Original Assignee
Aluminium Pechiney SA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Aluminium Pechiney SA filed Critical Aluminium Pechiney SA
Priority to PL05744310T priority Critical patent/PL1733075T3/en
Priority to SI200532251T priority patent/SI1733075T1/en
Publication of EP1733075A2 publication Critical patent/EP1733075A2/en
Application granted granted Critical
Publication of EP1733075B1 publication Critical patent/EP1733075B1/en
Anticipated expiration legal-status Critical
Active legal-status Critical Current

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/16—Electric current supply devices, e.g. bus bars
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes

Definitions

  • the present invention relates to the production of aluminum by igneous electrolysis. It relates more particularly to the cathode elements used in the electrolysis cells intended for the production of aluminum.
  • the cost of energy is an important item in the operating costs of electrolysis plants. Consequently, reducing the specific consumption of electrolysis cells becomes a major challenge for these factories.
  • the specific consumption of a cell corresponds to the energy consumed by the cell to produce one tonne of aluminum. It is expressed in kWh / t and, at a constant Faraday yield, it is directly proportional to the electrical voltage across the terminals of the electrolysis cell.
  • the electrical voltage of an electrolysis cell can be subdivided into several voltage drops: the anode voltage drop, the voltage drop in the bath, the electrochemical voltage, the cathode voltage drop and the line losses.
  • the present invention relates to the reduction of the cathode voltage drop in order to reduce the specific consumption of the electrolysis cells.
  • the cathode voltage drop depends on the electrical resistance of the cathode element, which comprises a cathode block made of carbonaceous material and one or more metal connection bars.
  • the materials making up the cathode blocks have evolved over time to become less and less resistant to the flow of current. This made it possible to increase the intensities crossing the cells, while maintaining a constant cathode voltage drop.
  • the cathode blocks were made of anthracite (amorphous carbon). This material offered fairly strong electrical resistance. Faced with the needs of factories to increase their intensity in order to increase their production, these blocks were gradually replaced, from the 1980s, by so-called “semi-graphitic” blocks (containing quantities of graphite ranging from 30% to 50%) then by so-called “graphitic” blocks containing 100% of graphite grains but whose binder joining these grains remains amorphous. The graphite grains of these blocks being not very resistive, the blocks offer a lower resistance to the passage of the current and consequently, at constant intensity, the cathode voltage drop decreases. Finally, the latest generations of blocks are so-called “graphitized” blocks.
  • cathode blocks have led to the appearance of new problems such as, for example, the erosion of cathodes.
  • new problems such as, for example, the erosion of cathodes.
  • the more the cathode blocks contain graphite the more they are sensitive to erosion problems at the head of the block.
  • the current density is not distributed homogeneously over the entire width of the tank and there is, at the surface of the cathode, a peak of current density located at each end of the block. This peak in current density generates localized erosion of the cathode, erosion all the more marked as the block is rich in graphite. These areas of very strong erosion can limit the life of the tank, which is economically very penalizing for an electrolysis plant.
  • the subject of the invention is a cathode element, for equipping an electrolytic cell cell intended for the production of aluminum, comprising: - a cathode block made of carbonaceous material having at least one longitudinal groove on one of its side faces; - At least one steel racco rd bar, at least part of which is called “external section” is intended to be located outside the tank, which is housed in said groove so that part of the bar called “part outside the block” emerges from at least one end of the block called the "block head”, and which is sealed in the groove by interposition of a conductive sealing material, such as cast iron or conductive paste, between the bar and the block.
  • a conductive sealing material such as cast iron or conductive paste
  • the cathode element according to the invention is characterized in that, for each external section: - the connection bar comprises at least one metal insert, of length Le, the electrical conductivity of which is greater than that of said steel, which is disposed longitudinally inside the bar and which is located, at least in part, in said section; - The connection bar is not sealed to the cathode block in at least one so-called “non-sealing" area of determined surface S located at the end of the groove at the head of the block.
  • the insert is flush - with a determined tolerance - the surface of the end of said outer section.
  • the or each insert is made of copper or a copper-based alloy.
  • the Applicant had the idea of combining a non-sealing area near the head of the cathode block and at least one insert in each outer section of the connecting bar which preferably extends over substantially the entire length of the section. It has found that, unexpectedly, the combined effect of these characteristics makes it possible to very significantly reduce the density peak of the coura nt existing at the head of the block, that is to say near the ends of the block, while very significantly reducing the drop in cathode voltage. In particular, it noted that the non-sealing zone makes it possible to significantly reduce the impact of the slope foot on the peak of current density.
  • the invention is particularly advantageous when said carbonaceous material contains graphite.
  • a method of manufacturing a connecting bar which can be used in a cathode element according to the invention, advantageously comprises the formation of a longitudinal cavity - typically a blind hole - in a steel bar from from one end thereof, the manufacture of an insert made of a more conductive material than the steel constituting the bar, of length and section corresponding to those of the cavity, then the introduction of the insert into the cavity .
  • An intimate contact between the insert and the bar is generally obtained during the temperature rise of the tank, thanks to the differential thermal expansion between the insert and the bar (because the steel expands relatively little compared to d other metals).
  • the invention also relates to an electrolysis cell comprising at least one cathode element according to the invention. The invention is described in detail below with the aid of the appended figures.
  • Figure 1 is a cross-sectional view of a traditional half-tank.
  • Figure 2 is a view similar to Figure 1 in the case of a cell comprising a cathode element according to the invention.
  • Figure 3 is a bottom view of a cathode element according to an embodiment of the invention.
  • Figure 4 is a bottom view of a cathode element according to another embodiment of the invention.
  • Figure 5 is a perspective view of one end of the cathode block of Figures 3 or 4.
  • Figure 6 shows a connection bar section equipped with an insert of circular section.
  • FIG. 7 represents a section of connection bar equipped with an insert of circular section in a lateral groove.
  • FIG. 8 shows curves of distribution of the cathode current along a cathode block. As illustrated in FIG.
  • an electrolysis cell 1 comprises a cell 10 and at least one anode 4.
  • the cell 10 comprises a cisson 2 whose bottom and side walls are covered with elements of refractory material 3 and 3 '.
  • Cathode blocks 5 rest on the bottom refractory elements 3.
  • Connection bars 6, generally made of steel, are sealed in the lower part of the cathode blocks 5.
  • the sealing between the connection bar or bars 6 and the cathode block 5 is typically produced by means of cast iron or conductive paste 7.
  • the cathode blocks 5 have a substantially parallelepiped shape, of length Lo, one of the side faces 21 of which has one or more longitudinal grooves 15 intended to accommodate the connection bars 6.
  • the grooves 15 open at the head of the block and generally extend from one end to the other of the block.
  • the so-called "out of block” part 22 of the bar 6 which emerges from the cathode block 5 has a length E.
  • the cathode blocks 5 and the connection bars 6 form cathode elements 20 which are generally assembled outside the tank and added to the latter during the formation of its interior lining.
  • An electrolytic cell 10 typically comprises more than a dozen cathode elements 20 arranged side by side.
  • a cathode element 20 may include one or more connecting bars, which pass right through the block, or one or more pairs of half-bars, typically aligned, which extend only over part of the block.
  • connection bars 6 have the function of collecting the current having passed through each cathode block 5 and sending it back into the network of conductors located outside the tank. As illustrated in FIG. 1, the connection bars 6 pass through the tank 1 O and are typically connected to a connection conductor 13, generally made of aluminum, by a flexible aluminum connector 14 connected to the section (s) 19 of the bars coming out of the tank 10.
  • the tank 10 contains a sheet of liquid aluminum 8 and an electrolyte bath 9, above the cathode blocks 5, and the anodes 4 plunge into the bath 9
  • a solidified bath slope 12 is generally formed on the side coverings 3 ′.
  • a part 12 ′ of this slope 12, called “slope foot”, can encroach on the upper lateral surface 28 of the cathode block 5.
  • FIG. 2 represents an electrolysis cell 1 for manufacturing aluminum, in which the same elements are designated by the same references as above.
  • each end of the connection bar 6 is equipped with a metal insert 16, preferably made of copper or a copper alloy, which extends over a length Le, typically starting from substantially the or each outer end of the bar 6.
  • the insert 16 is located, at least in part, in the or each outer section 19 of the connecting bar 6 which is intended to be killed if outside the tank 10.
  • the or each insert 16 is preferably housed in a cavity forming a blind hole inside the bar 6. This variant makes it possible to avoid exposure of the insert to possible infiltration of bath or liquid metal.
  • the cavity may optionally be a groove on a lateral face of the bar, as illustrated in FIG. 7.
  • the insert preferably covers at least 90% of the length of the of the or each external section 19 of the bar ⁇ in which it is housed in order to op timiser the reduction in voltage drop obtained using the invention.
  • the end surface 24, which is intended to be outside the tank 10, is generally substantially vertical when the cathode element 20 is installed in a tank.
  • the or each insert 16 is substantially flush, that is to say with a determined tolerance, the surface 24 of the end of the outer section 19 of the bar 6. Said determined tolerance is preferably less than or equal to ⁇ 1 cm.
  • the outer e ⁇ -end of each insert 16 is set back, by a determined distance, relative to the surface 24 of the end of the outer section 13 of the bar 6. Said determined distance is preferably less than or equal to 4 cm.
  • the cavity formed by the withdrawal of the insert can advantageously contain a refractory material in order to avoid the loss of heat by radiation and / or convection.
  • the length Le of the insert 16 is typically between 10 and 300%, preferably between 20 and 300%, and more preferably between 1 10 and 270%, of the length E of the so-called "off-block" part 22 of the bar 6 which emerges from the cathode block 5 and in which the insert is housed. The longer the insert, the more the cathode voltage drop decreases.
  • At least one zone 17 situated between the bar 6 and the cathode block 5 does not contain any sealing material.
  • This area known as "non-sealing" is advantageously filled, in whole or in part, with an electrically insulating material, such as a refractory material, typically in the form of fibers or fabrics; this material is interposed between the bar 6 and the cathode block 5, in the non-sealing zone 17, as illustrated in FIG. 5.
  • the or each non-sealing zone 17 is located near the end 25 of the cathode block 5, called “block head", from which the bar emerges and covers a determined surface S.
  • the or each non-sealing area 17 is flush with the surface 27 of the block head 25 from which the bar 6.
  • Figures 3 and 4 illustrate two particular embodiments of the cathode element 20 according to the invention.
  • the cathode element comprises two parallel connection bars which cross the cathode block right through. Each bar then comprises two parts outside the block 22 and two external sections 19.
  • the cathode element comprises four connecting bars (also called "half-bars") which each open at one end of the block .
  • Each bar then has a single part outside the block 22 and a single outer section 19.
  • a conductive sealing material 7 is interposed between the block 5 and each bar 6, except in the areas located at the ends of the block 5 where there are non-sealing areas 17, which can be filled with refractory materials.
  • the total area A of the determined surface (s) S of the non-sealing zone (s) 17 of each connection bar 6 is typically between 0.5 and 25%, preferably between 2 and 20%, more preferably still between 3 and 15%, of the area Ao, the surface So of the bar 6 which is capable of being sealed, called "sealable surface".
  • the sealable surface So corresponds to the surfaces of the part 23 of the bar 6 which are opposite the internal surfaces of the groove 15 in the block 5.
  • the area Ao of the sealable surface So is typically equal to Lo x (2 H + W), where H is the height of the bar and W its width.
  • the total area A is equal to the sum of the areas of each determined surface S.
  • each connecting bar half 6 has a non-sealing area 17 at a single end 25, the total area A is equal to the area of the determined surface S of this non-sealing area.
  • the Applicant has noted, however, that when the discontinuity of the bar near the center of the block is relatively short, which is generally the case, it does little to modify the distribution of the current and the voltage drop, so that the area A can be determined as if the bars were continuous from one end to the other.
  • the determined surface S is typically of simple shape in order to facilitate the formation of the non-sealing area 17. In the case, illustrated in FIGS.
  • the non-sealing area 17 is formed by the absence of sealing over a length Ls, starting from the surface 27 of the block head 25, the area of the determined surface S is typically equal to Ls x (2 H + W).
  • the length Ls of each non-sealing zone 17 is preferably between 0.5 and 25%, preferably between 2 and 20%, more preferably between 3 and 15%, of the half length Lo / 2 of the block.
  • the section of the insert 16 also influences the reduction in the cathode voltage drop.
  • the cross section of each insert is between 1 and 50%, and preferably between 5 and 30%, of the cross section of the bar 6. In fact, beyond 30% of total section in insert, the additional quantity of conductor brings a significant additional cost for a small increase in performance.
  • the insert 16 typically takes the form of a bar.
  • the shape of the cross section of the insert 16 is free, this shape can be rectangular (as illustrated in FIG. 5), circular (as illustrated in FIG. 6 or 7), ovoid or polygonal ... It is however advantageously circular in order to facilitate the manufacture of the connection bar, in particular the production of the cavity intended to house the insert.
  • the Applicant has performed numerical calculations intended to evaluate the distribution of the cathode current at the surface 28 of the cathode block obtained with configurations according to the prior art and according to the invention.
  • Figure 8. presents the results of a calculation corresponding to connection bar dimensions and current intensity typical of existing electrolysis cells.
  • the curves correspond to the current density J at the upper surface 28 of the block, expressed in kA / m 2 , as a function of the distance D from the end of the block.
  • the cell comprises 20 cathode elements arranged side by side and each comprising two connection bars, as illustrated in FIG. 3.
  • the total intensity is 314 kA.
  • the connection bars have a length L equal to 4.3 m, a height H equal to 160 mm and a width W equal to 1 10 mm.
  • the length E of the connecting bars leaving the cathode blocks is 0.50 m.
  • Curve A relating to the prior art, corresponds to a connection bar made entirely of steel.
  • the cathode voltage drop is 283 mV (between the center of the liquid metal sheet and the anode frame of the downstream tank).
  • Curve B relating to the prior art, corresponds to a steel bar having the same dimensions as in case A, but comprising a cylindrical copper insert with a length equal to 1.53 m, the diameter of which is equal to 4.13 cm. The insert is placed along the longitudinal axis of symmetry of the bar and extends approximately from the center of the bar (i.e. approximately from the central plane P of the tank) to approximately half of the thickness of the coating on the 3 ′ side of the cell.
  • the cathode voltage drop is 229 mV.
  • Curve C corresponds to a steel bar having the same dimensions as in case A, but comprising a cylindrical copper insert with a length Le equal to 1.30 m, the diameter of which is equal to 4.5 cm (corresponding to a volume of copper identical to that of case B).
  • the insert is placed along the longitudinal axis of symmetry of the bar and extends, as in Figure 2, from the outer end of the bar to the inside of the cell.
  • the non-sealing area is 0.18 m long and concerns the three normally sealed faces of the bar.
  • the cathode voltage drop is 190 mV.
  • the reduction in cathodic drop is approximately 32% and the reduction in peak current density is approximately 37%.
  • the distribution of cathode current is much more homogeneous than in cases A and B.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electrolytic Production Of Metals (AREA)

Abstract

The invention relates to a cathode element for an electrolysis cell bath for the production of aluminium, comprising a cathode block (5), made from a carbon material with at least one longitudinal groove on one of the lateral faces thereof and a steel connector bar (6), fixed in said groove such that a part of the bar extends from one end of the block, sealed in the groove by means of the introduction of a conducting sealant material between the bar and the block and which contains at least one metal insert, the electrical conductivity of which is greater than said steel. According to the invention, the insert (16) is arranged longitudinally within the bar and is located, at least partly, in the section (19) of the connector bar located outside the bath and the connector bar (6) is not sealed to the cathode block in a non-sealing region (17) of given surface (S) located at the end of the groove at the head of the block. The presence of such an insert simultaneously provides a large reduction in the global cathode voltage drop and the current density at the head of the block.

Description

ELEMENT CATHODIQUE POUR L'EQUIPEMENT D UNE CELLULE D'ELECTROLYSE DESTINEE A LA PRODUCTION D'ALUMINIUM CATHODE ELEMENT FOR THE EQUIPMENT OF AN ELECTROLYSIS CELL INTENDED FOR THE PRODUCTION OF ALUMINUM
La présente invention concerne la production d'aluminium par électrolyse ignée. Elle concerne plus particulièrement les éléments cathodiques utilisés dans les cellules d'électrolyse destinées à la production d'aluminium. Le coût de l'énergie est un poste important dans les coûts de fonctionnement des usines d'électrolyse. Par conséquent, la réduction de la consommation spécifique des cellules d'électrolyse devient un enjeu majeur pour ces usines. La consommation spécifique d'une cellule correspond à l'énergie consommée par la cellule pour produire une tonne d'aluminium. Elle s'exprime en kWh/t et, à rendement Faraday constant, elle est directement proportionnelle à la tension électrique aux bornes de la cellule d'électrolyse. La tension électrique d'une cellule d'électrolyse peut se sous- diviser en plusieurs chutes de tension: la chute de tension anodique, la chute de tension dans le bain, la tension électrochimique, la chute de tension cathodique et les pertes en lignes. La présente invention se rapporte à la réduction de la chute de tension cathodique en vue de réduire la consommation spécifique des cellules d'électrolyse. La chute de tension cathodique dépend de la résistance électrique de l'élément cathodique, qui comporte un bloc cathodique en matériau carboné et une ou plusieurs barres de raccordement en métal. Les matériaux constituant les blocs cathodiques ont évolué dans le temps pour devenir de moins en moins résistifs au passage du courant. Ceci a permis d'augmenter les intensités traversant les cellules, tout en conservant une chute de tension cathodique constante. Dans les années 1970, les blocs cathodiques étaient en anthracite (carbone amorphe). Ce matériau offrait une résistance électrique assez forte. Devant les besoins des usines d'augmenter leur intensité afin d'augmenter leur production, ces blocs ont été progressivement remplacés, à partir des années 1980, par des blocs dits "semi-graphitiques" (contenant des quantités de graphite allant de 30 % à 50 %) puis par des blocs dits "graphitiques" contenant 100 % de grains de graphite mais dont le liant joignant ces grains reste amorphe. Les grains de graphite de ces blocs étant peu résistifs, les blocs offrent une plus faible résistance au passage du courant et en conséquence, à intensité constante, la chute de tension cathodique baisse. Enfin, les dernières générations de blocs sont des blocs dits "graphitisés". Ces blocs subissent un traitement thermique de graphitisation à haute température permettant d'augmenter la conductivité électrique du bloc par graphitisation du carbone. Parallèlement à ces avancées visant à réduire la résistance électrique des matériaux, les usines d'électrolyse pour la production d'aluminium ont augmenté leur intensité afin d'augmenter leur production (à rendement Faraday constant, le nombre de tonnes de métal produit par une cellule est proportionnel à l'intensité du courant qui la traverse). En conséquence, comme la chute de tension cathodique Uc est égale au produit de la résistance cathodique Rc et de l'intensité I du courant circulant dans la cathode (Uc = Rc x I), les chutes de tension cathodique restent de nos jours élevées, soit typiquement autour de 300 mV. En outre, l'évolution des propriétés des blocs cathodiques a conduit à l'apparition de nouveaux problèmes comme, par exemple, l'érosion des cathodes. On constate par exemple que plus les blocs cathodiques contiennent de graphite, plus ils sont sensibles à des problèmes d'érosion en tête de bloc. En effet, la densité de courant ne se répartit pas de façon homogène sur toute la largeur de la cuve et il existe, à la surface de la cathode, un pic de densité de courant situé à chaque extrémité du bloc. Ce pic de densité de courant engendre une érosion localisée de la cathode, érosion d'autant plus marquée que le bloc est riche en graphite. Ces zones de très forte érosion peuvent limiter la durée de vie de la cuve, ce qui est économiquement très pénalisant pour une usine d'électrolyse. Il est connu de réduire la chute de tension cathodique Uc par l'utilisation de barres de raccordement composites comprenant une partie en acier et une partie en un métal de conductivité électrique supérieure à celle de l'acier, généralement du cuivre. On peut citer, par exemple, la demande de brevet français FR 1 161 632 (Pechiney), les brevets américains US 2 846 388 (Pechiney) et US 3 551 319 (Kaiser) et la demande internationale WO 02/42525 (Servico). Il est par ailleurs connu des demandes internationales WO 01 /63014 (Comalco) et WO 01 /27353 (Alcoa) que l'utilisation d'inserts en cuivre permet de mieux répartir le courant le long du bloc cathodique. Ces documents enseignent d'enfermer un insert en cuivre dans la barre de raccordement en acier et de confiner l'insert à l'intérieur de la cellule afin de réduire la conduction thermique vers l'ex térieur de la cellule. Toutefois, d'un point de vue économique, ces solutions sont a priori onéreuses car le cuivre est plus cher que l'acier et les quantités de cuivre mises en œuvre peuvent être importantes. En effet, dans les technologies les plus courantes, le nombre de barres par cuve d'électrolyse est généralement compris entre 50 et 1 OO. Le surcoût global dû à la présence de composants en cuivre peut donc augmenter très rapidement. En outre, les configurations connues de l'art antérieur ne donnent pas entière satisfaction. En effet, ces configurations conduisent à des diminutions de la chute de tension cathodique globale (c'est-à-dire incluant la chute de tension dans la barre) de l'ordre de 50 mV, qui est une valeur trop faible pour que les surcoûts d' investissement soient rentables, et à des pics de densité de courant en tête de bloc qui restent relativement importants, à savoir plus de 12 kA/m2 envi ron. La demanderesse a donc recherché des solutions satisfaisantes aux inconvénients de l'art antérieur, et notamment au problème de la consommation spécifique.The present invention relates to the production of aluminum by igneous electrolysis. It relates more particularly to the cathode elements used in the electrolysis cells intended for the production of aluminum. The cost of energy is an important item in the operating costs of electrolysis plants. Consequently, reducing the specific consumption of electrolysis cells becomes a major challenge for these factories. The specific consumption of a cell corresponds to the energy consumed by the cell to produce one tonne of aluminum. It is expressed in kWh / t and, at a constant Faraday yield, it is directly proportional to the electrical voltage across the terminals of the electrolysis cell. The electrical voltage of an electrolysis cell can be subdivided into several voltage drops: the anode voltage drop, the voltage drop in the bath, the electrochemical voltage, the cathode voltage drop and the line losses. The present invention relates to the reduction of the cathode voltage drop in order to reduce the specific consumption of the electrolysis cells. The cathode voltage drop depends on the electrical resistance of the cathode element, which comprises a cathode block made of carbonaceous material and one or more metal connection bars. The materials making up the cathode blocks have evolved over time to become less and less resistant to the flow of current. This made it possible to increase the intensities crossing the cells, while maintaining a constant cathode voltage drop. In the 1970s, the cathode blocks were made of anthracite (amorphous carbon). This material offered fairly strong electrical resistance. Faced with the needs of factories to increase their intensity in order to increase their production, these blocks were gradually replaced, from the 1980s, by so-called "semi-graphitic" blocks (containing quantities of graphite ranging from 30% to 50%) then by so-called "graphitic" blocks containing 100% of graphite grains but whose binder joining these grains remains amorphous. The graphite grains of these blocks being not very resistive, the blocks offer a lower resistance to the passage of the current and consequently, at constant intensity, the cathode voltage drop decreases. Finally, the latest generations of blocks are so-called "graphitized" blocks. These blocks undergo a high temperature graphitization heat treatment making it possible to increase the electrical conductivity of the block by graphitization of the carbon. In parallel with these advances aimed at reducing the electrical resistance of materials, the electrolysis plants for aluminum production have increased their intensity in order to increase their production (at constant Faraday yield, the number of tonnes of metal produced by a cell is proportional to the intensity of the current flowing through it). Consequently, as the cathode voltage drop Uc is equal to the product of the cathode resistance Rc and the intensity I of the current flowing in the cathode (Uc = Rc x I), the cathode voltage drops remain high today, typically around 300 mV. In addition, the evolution of the properties of cathode blocks has led to the appearance of new problems such as, for example, the erosion of cathodes. We note, for example, that the more the cathode blocks contain graphite, the more they are sensitive to erosion problems at the head of the block. In fact, the current density is not distributed homogeneously over the entire width of the tank and there is, at the surface of the cathode, a peak of current density located at each end of the block. This peak in current density generates localized erosion of the cathode, erosion all the more marked as the block is rich in graphite. These areas of very strong erosion can limit the life of the tank, which is economically very penalizing for an electrolysis plant. It is known to reduce the cathode voltage drop Uc by the use of composite connection bars comprising a part made of steel and a part made of a metal with an electrical conductivity greater than that of steel, generally copper. Mention may be made, for example, of French patent application FR 1,161,632 (Pechiney), American patents US 2,846,388 (Pechiney) and US 3,551,319 (Kaiser) and international application WO 02/42525 (Servico). It is also known from international applications WO 01/63014 (Comalco) and WO 01/27353 (Alcoa) that the use of copper inserts makes it possible to better distribute the current along the cathode block. These documents teach to enclose a copper insert in the steel connection bar and to confine the insert inside the cell in order to reduce the thermal conduction towards the outside of the cell. However, from an economic point of view, these solutions are a priori expensive since copper is more expensive than steel and the quantities of copper used can be significant. In fact, in the most common technologies, the number of bars per electrolytic cell is generally between 50 and 1 OO. The overall additional cost due to the presence of copper components can therefore increase very quickly. In addition, the known configurations of the prior art are not entirely satisfactory. Indeed, these configurations lead to decreases in the overall cathodic voltage drop (that is to say including the voltage drop in the bar) of the order of 50 mV, which is a value too low for the additional investment costs are profitable, and at peaks in current density at the head of the block which remain relatively large, namely more than 12 kA / m 2 approximately. The Applicant has therefore sought satisfactory solutions to the drawbacks of the prior art, and in particular to the problem of specific consumption.
Description de l'inventionDescription of the invention
L'invention a pour objet un élément cathodique, pour l'équipement d'une cuve de cellule d'électrolyse destinée à la production d'aluminium, comportant : - un bloc cathodique en matériau carboné ayant au moins une rainure longitudinale sur une de ses faces latérales ; - au moins une barre de racco rdement en acier, dont au moins une partie dite "tronçon extérieur" est destinée à se situer à l'extérieur de la cuve, qui est logée dans ladite rainure de façon à ce qu'une partie de la barre dite "partie hors bloc" émerge d'au moins une extrémité du bloc dite "tête de bloc", et qui est scellée dans la rainure par interposition d'un matériau de scellement conducteur, tel que de la fonte ou de la pâte conductrice, entre la barre et le bloc. L'élément cathodique selon l'invention est caractérisé en ce que, pour chaque tronçon extérieur : - la barre de raccordement comprend au moins un insert métallique, de longueur Le, dont la conduc-tivité électrique est supérieure à celle dudit acier, qui est disposé longitudinalement à l'intérieur de la barre et qui se situe, au moins en partie, dans ledit tronçon ; - la barre de raccordement n'est pas scellée au bloc cathodique dans au moins une zone dite de "non-scellement" de surface déterminée S située à l'extrémité de la rainure en tête de bloc. De préférence, l 'insert affleure — avec une tolérance déterminée - la surface de l'extrémité dudit tronçon extérieur. Avantageusement, le ou chaque insert est réalisé en cuivre ou en alliage à base de cuivre. La présence d'un insert selon l'invention permet d'obtenir simultanément une très forte réduction de la chute de tension cathodique globale (par exemple 0,2 V pour une barre avec un insert en cuivre contreThe subject of the invention is a cathode element, for equipping an electrolytic cell cell intended for the production of aluminum, comprising: - a cathode block made of carbonaceous material having at least one longitudinal groove on one of its side faces; - At least one steel racco rd bar, at least part of which is called "external section" is intended to be located outside the tank, which is housed in said groove so that part of the bar called "part outside the block" emerges from at least one end of the block called the "block head", and which is sealed in the groove by interposition of a conductive sealing material, such as cast iron or conductive paste, between the bar and the block. The cathode element according to the invention is characterized in that, for each external section: - the connection bar comprises at least one metal insert, of length Le, the electrical conductivity of which is greater than that of said steel, which is disposed longitudinally inside the bar and which is located, at least in part, in said section; - The connection bar is not sealed to the cathode block in at least one so-called "non-sealing" area of determined surface S located at the end of the groove at the head of the block. Preferably, the insert is flush - with a determined tolerance - the surface of the end of said outer section. Advantageously, the or each insert is made of copper or a copper-based alloy. The presence of an insert according to the invention makes it possible to simultaneously obtain a very strong reduction in the overall cathode voltage drop (for example 0.2 V for a bar with a copper insert against
0,3 V pour une barre totalement en acier) et une très forte réduction de la densité de courant en tête de bloc (au moins de l'ordre de 20 %). ( Dans ses recherches, la demanderesse a noté qu'une partie importante de Ja chute de tension cathodique (environ un tiers) se situe dans la partie dite "hors bloc" de la barre qui sort du bloc. En effet, plus on se rapproche de la partie hors bloc de la barre, plus la densité de courant dans celle-ci augmente pour atteindre sa valeur maximale dans la partie hors bloc. Par conséquent, sur toute la partie hors bloc de la barre, une faible section assure la transmission d'une importante quantité de courant, ce qui engendre une forte chute de tension. La demanderesse a eu l'idée de combiner une zone de non- scellement à proximité de la tête du bloc cathodique et au moins un insert dans chaque tronçon extérieur de la barre de raccordement qui s'étend, de préférence, sur sensiblement toute la longueur du tronçon. Elle a constaté que, de manière inattendue, l'effet combiné de ces caractéristiques permet de réduire de manière très significative le pic de densité du courant existant en tête de bloc, c'est-à-dire près des extrémités du bloc, tout en réduisant de manière très significative la chute de tension cathodique. En particulier, elle a noté que la zone de non-scellement permet de diminuer sensiblement l'impact du pied de talus sur le pic de densité de courant. L'invention est particulièrement intéressante lorsque ledit matériau carboné contient du graphite. Un procédé de fabrication d'une barre de raccordement, qui est susceptible d'être utilisée dans un élément cathodiqu e selon l'invention, comprend avantageusement la formation d'une cavité longitudinale - typiquement un trou borgne - dans une barre en acier à partir d'une extrémité de celle-ci, la fabrication d'un insert en matériau plus conducteur que l'acier constituant la barre, de longueur et de section correspondant à celles de la cavité, puis l'introduction de l'insert dans la cavité. Un contact intime entre l'insert et la barre est généralement obtenu lors de la montée en température de la cuve, grâce à la dilatation thermique différentielle entre l'insert et la barre (car l'acier se dilate relativement peu par rapport à d'autres métaux). L'invention concerne également une cellule d'électrolyse comprenant au moins un élément cathodique selon l'invention. L'invention est décrite en détail ci-dessous à l'aide des figures annexées. La figure 1 est une vue en coupe transversale d'une demi-cuve traditionnelle. La figure 2 est une vue similaire à figure 1 dans le cas d'une cellule comprenant un élément cathodique selon l'invention. La figure 3 est une vue de dessous d'un élément cathodique selon un mode de réalisation de l'invention. La figure 4 est une vue de dessous d'un élément cathodique selon un autre mode de réalisation de l'invention. La figure 5 est une vue en perspective d'une extrémité du bloc cathodique des figures 3 ou 4. La figure 6 représente un tronçon de barre de raccordement équipée d'un insert de section circulaire. La figure 7 représente un tronçon de barre de raccordement équipée d'une insert de section circulaire dans une rainure latérale. La figure 8 présente des courbes de répartition du courant cathodique le long d'un bloc cathodique. Tel qu'illustré à la figure 1 , une cellule d'électrolyse 1 comporte une cuve 10 et au moins une anode 4. La cuve 10 comporte un c aisson 2 dont le fond et les parois latérales sont recouvertes d'éléments en matériau réfractaire 3 et 3'. Des blocs cathodiques 5 reposent sur les éléments réfractaires de fond 3. Des barres de raccordement 6, généralement en acier, sont scellées dans la partie inférieure des blocs cathodiques 5. Le scellement entre la ou les barres de raccordement 6 et le bloc cathodique 5 est typiquement réalisé par l'intermédiaire de fonte ou de pâte conductrice 7. Tel qu'illustré aux figures 3 à 5, les blocs cathodiques 5 ont une forme sensiblement parallélépipédique, de longueur Lo, dont une des faces latérales 21 possède une ou plusieurs rainures longitudinales 15 destinées à loger les barres de raccordement 6. Les rainures 15 débouchent en tête de bloc et s'étendent généralement d'une extrémité à l'autre du bloc. La partie dite "hors bloc" 22 de la barre 6 qui émerge du bloc cathodique 5 a une la longueur E. Les blocs cathodiques 5 et les barres de raccord ement 6 forment des éléments cathodiques 20 qui sont généralement assemblés hors de la cuve et ajoutés à celle-ci lors de la formation de son reΛ/êtement intérieur. Une cuve d'électrolyse 10 comporte typiquement plus d'une dizaine d'éléments cathodiques 20 disposés côte à côte. Un élément cathodique 20 peut comporter une ou plusieurs barres de raccordement, qui traversent le bloc de part en part, ou une ou plusieurs paires de demi- barres, typiquement alignées, qui ne s'étendent que sur une partie du bloc. Les barres de raccordement 6 ont pour fonction de collecter le courant ayant traversé chaque bloc cathodique 5 et le renvoyer dans le réseau de conducteurs se trouvant à l'extérieur de la cuve. Comme illustré à la figure 1 , les barres de raccordement 6 traversent la cuve 1 O et sont typiquement reliées à un conducteur de liaison 13, généralement en aluminium, par un raccord souple en aluminium 14 raccordé au(x) tronçon(s) 19 des barres qui sort(ent) de la cuve 10. En fonctionnement, la cuve 10 contient une nappe d'aluminium liquide 8 et un bain d'électrolyte 9, au-dessus des blocs cathodiques 5, et les anodes 4 plongent dans le bain 9. Un talus 12 de bain solidifié se forme généralement sur les revêtements de côté 3'. Une partie 12' de ce talus 12, appelée "pied de talus", peut empiéter sur la surface latérale supérieure 28 du bloc cathodique 5. Le pied de talus isole électriquement la ca~thode et augmente le pic de densité de courant en tête de bloc. La figure 2 représente une cellule d'électrolyse 1 pour fabrication d'aluminium, dans laquelle les mêmes éléments sont désignés par les mêmes références que précédemment. Tel qu'illustré à la figure 2, chaque extrémité de barre de raccordement 6 est équipée d'un insert métallique 16, de préféren ce en cuivre ou en alliage de cuivre, qui s'étend sur une longueur Le, typiquement à partir sensiblement de la ou chaque extrémité extérieure de la barre 6. L'insert 16 se situe, au moins en partie, dans le ou chaque tronçon extérieur 19 de la barre de raccordement 6 qui est destiné à se si"tuer à l'extérieur de la cuve 10. Le ou chaque insert 16 est de préférence logé dans une cavité formant un trou borgne à l'intérieur de la barre 6. Cette variante permet d'éviter l'exposition de l'insert aux infiltrations éventuelles de bain ou de métal liquides. La cavité peut éventuellement être une rainure sur un e face latérale de la barre, tel qu'illustré à la figure 7. L'insert couvre de préférence au moins 90 % de la longu eur Le du ou de chaque tronçon extérieur 19 de la barre de raccordement © dans lequel il est logé afin d'optimiser la diminution de chute de tension obtenue à l'aide de l'invention. La surface d'extrémité 24, qui est destinée à être à l'ex térieur de la cuve 10, est généralement sensiblement verticale lorsque l'élément cathodique 20 est installé dans une cuve. Selon une variante avantageuse de l'invention, le ou chaque insert 16 affleure sensiblement, c'est-à-dire avec une tolérance déterminée, la surface 24 de l'extrémité du tronçon extérieur 19 de la barre 6. Ladite tolérance déterminée est de préférence inférieure ou égale à ± 1 cm. Selon une autre variante avantageuse de l'invention, l'eχ-trémité extérieure de chaque insert 16 est en retrait, d'une distance déterminée, par rapport à la surface 24 de l'extrémité du tronçon extérieur 13 de la barre 6. Ladite distance déterminée est de préférence inférieure ou égale à 4 cm. La cavité formée par le retrait de l'insert peut avantageusement contenir un matériau réfractaire afin d'éviter la perte de chaleur par rayonnement et/ou convection. La longueur Le de l'insert 16 est typiquement comprise entre 10 et 300 %, de préférence entre 20 et 300 %, et de préférence encore entre 1 10 et 270 %, de la longueur E de la partie dite "hors bloc" 22 de la barre 6 qui émerge du bloc cathodique 5 et dans laquelle l'insert est logé. Plus l'insert est long, plus la chute de tension cathodique diminue. Toutefois, la demanderesse a constaté que, au-dessus d'une longueur d 'insert de 270 % de la partie hors bloc 22 de barre, l'augmentation n'intervient que faiblement sur la valeur de la chute de tension cathodique. Tel qu'illustré à la figure 2, au moins une zone 17 située entre la barre 6 et le bloc cathodique 5 ne contient pas de matériau de scellement. Cette zone, dite de "non-scellement", est avantageusement remplie, en tout ou partie, d'un matériau électriquement isolant, tel qu'un matériau réfractaire, typiquement sous forme de fibres ou de tissus ; ce matériau est interposé entre la barre 6 et le bloc cathodique 5, dans la zone de non- scellement 17, tel qu'illustré à la figure 5. La ou chaque zone de non- scellement 17 est située à proximité de l'extrémité 25 du bloc cathodique 5, appelée "tête de bloc", de laquelle émerge la barre et couvre une surface déterminée S. De préférence, la ou chaque zone de non-scellement 17 affleure la surface 27 de la tête de bloc 25 de laquelle émerge la barre 6. Les figures 3 et 4 illustrent deux modes de réalisation particuliers de l'élément cathodique 20 selon l'invention. Dans l'exemple de la figure 3, l'élément cathodique comporte deux barres de raccordement parallèles qui traversent le bloc cathodique de part en part. Chaque barre comporte alors deux parties hors bloc 22 et deux tronçons extérieurs 19. Dans l'exemple de la figure 4, l'élément cathodique comporte quatre barres de raccordement (également appelées "demi-barres") qui débouchent chacune à une extrémité du bloc. Chaque barre comporte alors une seule partie hors bloc 22 et un seul tronçon extérieur 19. Dans les deux exemples, un matériau de scellement conducteur 7 est interposé entre le bloc 5 et chaque barre 6, sauf dans les zones situées aux extrémités du bloc 5 où il existe des zones de non-scellement 17, qui peuvent être remplies de matériaux réfractaires. L'aire totale A de la ou des surface(s) déterminée(s) S de la ou des zone(s) de non-scellement 17 de chaque barre de raccordement 6 est typiquement comprise entre 0,5 et 25 %, de préférence entre 2 et 20 %, de préférence encore entre 3 et 15 %, de l'aire Ao la surface So de la barre 6 qui est susceptible d'être scellée, dite "surface scellable". La surface scellable So correspond aux surfaces de la partie 23 de la barre 6 qui sont en regard des surfaces internes de la rainure 15 dans le bloc 5. Lorsque la ou chaque barre de raccordement 6 traverse le bloc cathodique 5 de part en part, comme illustré à la figure 3, l'aire Ao de la surface scellable So est typiquement égale à Lo x (2 H + W), où H est la hauteur de la barre et W sa largeur. Dans ce cas, comme chaque barre de raccordement 6 possède une zone de non-scellement 17 à chaque extrémité 25, l'aire totale A est égale à la somme des aires de chaque surface déterminée S. Lorsque les barres de raccordement 6 s'interrompent vers le centre du bloc pour former deux demi-barres alignées, comme illustré à la figure 4, l'aire Ao de la surface scellable So de chaque demi-barre est typiquement égale à Li x (2 H + W), où H est la hauteur de la barre et W sa largeur. Dans ce cas, comme chaque demi-barre de raccordement 6 possède une zone de non-scellement 17 à une seule extrémité 25, l'aire totale A est égale à l'aire de la surface déterminée S de cette zone de non- scellement. La demanderesse a toutefois constaté que lorsque la discontinuité de la barre près du centre du bloc est relativement courte, ce qui est généralement le cas, elle modifiait peu la répartition du courant et la chute de tension, de sorte que l'aire A pouvait être déterminée comme si les barres étaient continues d'une extrémité à l'autre. La surface déterminée S est typiquement de forme simple afin de faciliter la formation de la zone de non-scellement 17. Dans le cas, illustré aux figures 2 à 4, où la zone de non-scellement 17 est formée par l'absence de scellement sur une longueur Ls, à partir de la surface 27 de la tête de bloc 25, l'aire de la surface déterminée S est typiquement égale à Ls x (2 H + W). Dans ce cas, la longueur Ls de chaque zone de non- scellement 17 est de préférence comprise entre 0,5 et 25 %, de préférence entre 2 et 20 %, de préférence encore entre 3 et 15 %, de la demi- longueur Lo/2 du bloc. La section de l'insert 16 influence également la réduction de la chute de tension cathodique. Avantageusement, la section transversale de chaque insert est comprise entre 1 et 50 %, et de préférence entre 5 et 30 %, de la section transversale de la barre 6. En effet, au-delà de 30 % de section totale en insert, la quantité supplémentaire de conducteur apporte un surcoût important pour une faible augmentation des performances. L'insert 16 prend typiquement la forme d'une barre. La forme de la section transversale de l'insert 16 est libre, cette forme pouvant être rectangulaire (tel qu'illustré à la figure 5), circulaire (tel qu'illustré à la figure 6 ou 7), ovoïde ou polygonale... Elle est toutefois avantageusement circulaire afin de faciliter la fabrication de la barre de raccordement, notamment la réalisation de la cavité destinée à loger l'insert. La demanderesse a effectué des calculs numériques destinés à évaluer la répartition du courant cathodique à la surface 28 du bloc cathodique obtenue avec des configurations selon l'art antérieur et selon l'invention. La figure 8 . présente les résultats d'un calcul correspondant à des dimensions de barre de raccordement et une intensité de courant typiques des cellules d'électrolyse existantes. Les courbes correspondent à la densité de courant J à la surface supérieure 28 du bloc, exprimée en kA/m2, en fonction de la distance D de l'extrémité du bloc. La cellule comporte 20 éléments cathodiques disposés côte à côte et comportant chacun deux barres de raccordement, tel qu'illustré à la figure 3. L'intensité totale est de 314 kA. Les barres de raccordement ont une longueur L égale à 4,3 m, une hauteur H égale à 160 mm et une largeur W égale à 1 10 mm. La longueur E des barres de raccordement sortant des blocs cathodiques est de 0,50 m. La courbe A, relative à l'art antérieur, correspond à une barre de raccordement entièrement en acier. La chute de tension cathodique est de 283 mV (entre le centre de la nappe de métal liquide et le cadre anodique de la cuve aval). La courbe B, relative à l'art antérieur, correspond à une barre en acier ayant les mêmes dimensions que dans le cas A, mais comportant un insert cylindrique en cuivre d'une longueur égale à 1 ,53 m dont le diamètre est égal à 4,13 cm. L'insert est placé le long de l'axe de symétrie longitudinal de la barre et s'étend approximativement du centre de la barre (c'est-à-dire approximativement du plan central P de la cuve) jusqu'à environ la moitié de l'épaisseur du revêtement de côté 3' de la cellule. La chute de tension cathodique est de 229 mV. Par rapport au cas A, la réduction de la chute cathodique est d'environ 19 % et la réduction du pic de densité de courant est d'environ 18 %. La courbe C, relative à l'invention, correspond à une barre en acier ayant les mêmes dimensions que dans le cas A, mais comportant un insert cylindrique en cuivre d'une longueur Le égale à 1 ,30 m dont le diamètre est égal à 4,5 cm (correspondant à un volume de cuivre identique à celui du cas B). L'insert est placé le long de l'axe de symétrie longitudinal de la barre et s'étend, comme dans la figure 2, de l'extrémité extérieure de la barre jusqu'à l'intérieur de la cellule. La zone de non-scellement a une longueur de 0,18 m et concerne les trois faces normalement scellées de la barre. La chute de tension cathodique est de 190 mV. Par rapport au cas A, la réduction de la chute cathodique est d'environ 32 % et la réduction du pic de densité de courant est d'environ 37 %. La répartition du courant cathodique est nettement plus homogène que dans les cas A et B. 0.3 V for a completely steel bar) and a very strong reduction in the current density at the head of the block (at least of the order of 20%). ( In her research, the Applicant has noted that a significant part of the cathodic voltage drop (approximately one third) is located in the so-called "out-of-block" part of the bar coming out of the block. Indeed, the closer one gets of the non-block part of the bar, the more the current density in it increases to reach its maximum value in the non-block part. Consequently, over the whole non-block part of the bar, a small section ensures the transmission of 'a large amount of current, which generates a sharp drop in voltage. The Applicant had the idea of combining a non-sealing area near the head of the cathode block and at least one insert in each outer section of the connecting bar which preferably extends over substantially the entire length of the section. It has found that, unexpectedly, the combined effect of these characteristics makes it possible to very significantly reduce the density peak of the coura nt existing at the head of the block, that is to say near the ends of the block, while very significantly reducing the drop in cathode voltage. In particular, it noted that the non-sealing zone makes it possible to significantly reduce the impact of the slope foot on the peak of current density. The invention is particularly advantageous when said carbonaceous material contains graphite. A method of manufacturing a connecting bar, which can be used in a cathode element according to the invention, advantageously comprises the formation of a longitudinal cavity - typically a blind hole - in a steel bar from from one end thereof, the manufacture of an insert made of a more conductive material than the steel constituting the bar, of length and section corresponding to those of the cavity, then the introduction of the insert into the cavity . An intimate contact between the insert and the bar is generally obtained during the temperature rise of the tank, thanks to the differential thermal expansion between the insert and the bar (because the steel expands relatively little compared to d other metals). The invention also relates to an electrolysis cell comprising at least one cathode element according to the invention. The invention is described in detail below with the aid of the appended figures. Figure 1 is a cross-sectional view of a traditional half-tank. Figure 2 is a view similar to Figure 1 in the case of a cell comprising a cathode element according to the invention. Figure 3 is a bottom view of a cathode element according to an embodiment of the invention. Figure 4 is a bottom view of a cathode element according to another embodiment of the invention. Figure 5 is a perspective view of one end of the cathode block of Figures 3 or 4. Figure 6 shows a connection bar section equipped with an insert of circular section. FIG. 7 represents a section of connection bar equipped with an insert of circular section in a lateral groove. FIG. 8 shows curves of distribution of the cathode current along a cathode block. As illustrated in FIG. 1, an electrolysis cell 1 comprises a cell 10 and at least one anode 4. The cell 10 comprises a cisson 2 whose bottom and side walls are covered with elements of refractory material 3 and 3 '. Cathode blocks 5 rest on the bottom refractory elements 3. Connection bars 6, generally made of steel, are sealed in the lower part of the cathode blocks 5. The sealing between the connection bar or bars 6 and the cathode block 5 is typically produced by means of cast iron or conductive paste 7. As illustrated in FIGS. 3 to 5, the cathode blocks 5 have a substantially parallelepiped shape, of length Lo, one of the side faces 21 of which has one or more longitudinal grooves 15 intended to accommodate the connection bars 6. The grooves 15 open at the head of the block and generally extend from one end to the other of the block. The so-called "out of block" part 22 of the bar 6 which emerges from the cathode block 5 has a length E. The cathode blocks 5 and the connection bars 6 form cathode elements 20 which are generally assembled outside the tank and added to the latter during the formation of its interior lining. An electrolytic cell 10 typically comprises more than a dozen cathode elements 20 arranged side by side. A cathode element 20 may include one or more connecting bars, which pass right through the block, or one or more pairs of half-bars, typically aligned, which extend only over part of the block. The connection bars 6 have the function of collecting the current having passed through each cathode block 5 and sending it back into the network of conductors located outside the tank. As illustrated in FIG. 1, the connection bars 6 pass through the tank 1 O and are typically connected to a connection conductor 13, generally made of aluminum, by a flexible aluminum connector 14 connected to the section (s) 19 of the bars coming out of the tank 10. In operation, the tank 10 contains a sheet of liquid aluminum 8 and an electrolyte bath 9, above the cathode blocks 5, and the anodes 4 plunge into the bath 9 A solidified bath slope 12 is generally formed on the side coverings 3 ′. A part 12 ′ of this slope 12, called “slope foot”, can encroach on the upper lateral surface 28 of the cathode block 5. The foot of the slope electrically isolates the ca ~ method and increases the peak of current density at the head of the block. FIG. 2 represents an electrolysis cell 1 for manufacturing aluminum, in which the same elements are designated by the same references as above. As illustrated in FIG. 2, each end of the connection bar 6 is equipped with a metal insert 16, preferably made of copper or a copper alloy, which extends over a length Le, typically starting from substantially the or each outer end of the bar 6. The insert 16 is located, at least in part, in the or each outer section 19 of the connecting bar 6 which is intended to be killed if outside the tank 10. The or each insert 16 is preferably housed in a cavity forming a blind hole inside the bar 6. This variant makes it possible to avoid exposure of the insert to possible infiltration of bath or liquid metal. The cavity may optionally be a groove on a lateral face of the bar, as illustrated in FIG. 7. The insert preferably covers at least 90% of the length of the of the or each external section 19 of the bar © in which it is housed in order to op timiser the reduction in voltage drop obtained using the invention. The end surface 24, which is intended to be outside the tank 10, is generally substantially vertical when the cathode element 20 is installed in a tank. According to an advantageous variant of the invention, the or each insert 16 is substantially flush, that is to say with a determined tolerance, the surface 24 of the end of the outer section 19 of the bar 6. Said determined tolerance is preferably less than or equal to ± 1 cm. According to another advantageous variant of the invention, the outer eχ-end of each insert 16 is set back, by a determined distance, relative to the surface 24 of the end of the outer section 13 of the bar 6. Said determined distance is preferably less than or equal to 4 cm. The cavity formed by the withdrawal of the insert can advantageously contain a refractory material in order to avoid the loss of heat by radiation and / or convection. The length Le of the insert 16 is typically between 10 and 300%, preferably between 20 and 300%, and more preferably between 1 10 and 270%, of the length E of the so-called "off-block" part 22 of the bar 6 which emerges from the cathode block 5 and in which the insert is housed. The longer the insert, the more the cathode voltage drop decreases. However, the Applicant has found that, above an insert length of 270% of the part outside the block 22 of the bar, the increase takes place only slightly on the value of the cathode voltage drop. As illustrated in FIG. 2, at least one zone 17 situated between the bar 6 and the cathode block 5 does not contain any sealing material. This area, known as "non-sealing", is advantageously filled, in whole or in part, with an electrically insulating material, such as a refractory material, typically in the form of fibers or fabrics; this material is interposed between the bar 6 and the cathode block 5, in the non-sealing zone 17, as illustrated in FIG. 5. The or each non-sealing zone 17 is located near the end 25 of the cathode block 5, called "block head", from which the bar emerges and covers a determined surface S. Preferably, the or each non-sealing area 17 is flush with the surface 27 of the block head 25 from which the bar 6. Figures 3 and 4 illustrate two particular embodiments of the cathode element 20 according to the invention. In the example of FIG. 3, the cathode element comprises two parallel connection bars which cross the cathode block right through. Each bar then comprises two parts outside the block 22 and two external sections 19. In the example of FIG. 4, the cathode element comprises four connecting bars (also called "half-bars") which each open at one end of the block . Each bar then has a single part outside the block 22 and a single outer section 19. In the two examples, a conductive sealing material 7 is interposed between the block 5 and each bar 6, except in the areas located at the ends of the block 5 where there are non-sealing areas 17, which can be filled with refractory materials. The total area A of the determined surface (s) S of the non-sealing zone (s) 17 of each connection bar 6 is typically between 0.5 and 25%, preferably between 2 and 20%, more preferably still between 3 and 15%, of the area Ao, the surface So of the bar 6 which is capable of being sealed, called "sealable surface". The sealable surface So corresponds to the surfaces of the part 23 of the bar 6 which are opposite the internal surfaces of the groove 15 in the block 5. When the or each connecting bar 6 crosses the cathode block 5 right through, as illustrated in Figure 3, the area Ao of the sealable surface So is typically equal to Lo x (2 H + W), where H is the height of the bar and W its width. In this case, since each connection bar 6 has a non-sealing area 17 at each end 25, the total area A is equal to the sum of the areas of each determined surface S. When the connection bars 6 are interrupted towards the center of the block to form two aligned half-bars, as illustrated in FIG. 4, the area Ao of the sealable surface So of each half-bar is typically equal to Li x (2 H + W), where H is the height of the bar and W its width. In this case, as each connecting bar half 6 has a non-sealing area 17 at a single end 25, the total area A is equal to the area of the determined surface S of this non-sealing area. The Applicant has noted, however, that when the discontinuity of the bar near the center of the block is relatively short, which is generally the case, it does little to modify the distribution of the current and the voltage drop, so that the area A can be determined as if the bars were continuous from one end to the other. The determined surface S is typically of simple shape in order to facilitate the formation of the non-sealing area 17. In the case, illustrated in FIGS. 2 to 4, where the non-sealing area 17 is formed by the absence of sealing over a length Ls, starting from the surface 27 of the block head 25, the area of the determined surface S is typically equal to Ls x (2 H + W). In this case, the length Ls of each non-sealing zone 17 is preferably between 0.5 and 25%, preferably between 2 and 20%, more preferably between 3 and 15%, of the half length Lo / 2 of the block. The section of the insert 16 also influences the reduction in the cathode voltage drop. Advantageously, the cross section of each insert is between 1 and 50%, and preferably between 5 and 30%, of the cross section of the bar 6. In fact, beyond 30% of total section in insert, the additional quantity of conductor brings a significant additional cost for a small increase in performance. The insert 16 typically takes the form of a bar. The shape of the cross section of the insert 16 is free, this shape can be rectangular (as illustrated in FIG. 5), circular (as illustrated in FIG. 6 or 7), ovoid or polygonal ... It is however advantageously circular in order to facilitate the manufacture of the connection bar, in particular the production of the cavity intended to house the insert. The Applicant has performed numerical calculations intended to evaluate the distribution of the cathode current at the surface 28 of the cathode block obtained with configurations according to the prior art and according to the invention. Figure 8. presents the results of a calculation corresponding to connection bar dimensions and current intensity typical of existing electrolysis cells. The curves correspond to the current density J at the upper surface 28 of the block, expressed in kA / m 2 , as a function of the distance D from the end of the block. The cell comprises 20 cathode elements arranged side by side and each comprising two connection bars, as illustrated in FIG. 3. The total intensity is 314 kA. The connection bars have a length L equal to 4.3 m, a height H equal to 160 mm and a width W equal to 1 10 mm. The length E of the connecting bars leaving the cathode blocks is 0.50 m. Curve A, relating to the prior art, corresponds to a connection bar made entirely of steel. The cathode voltage drop is 283 mV (between the center of the liquid metal sheet and the anode frame of the downstream tank). Curve B, relating to the prior art, corresponds to a steel bar having the same dimensions as in case A, but comprising a cylindrical copper insert with a length equal to 1.53 m, the diameter of which is equal to 4.13 cm. The insert is placed along the longitudinal axis of symmetry of the bar and extends approximately from the center of the bar (i.e. approximately from the central plane P of the tank) to approximately half of the thickness of the coating on the 3 ′ side of the cell. The cathode voltage drop is 229 mV. Compared to case A, the reduction in cathodic drop is approximately 19% and reduction in peak current density is approximately 18%. Curve C, relating to the invention, corresponds to a steel bar having the same dimensions as in case A, but comprising a cylindrical copper insert with a length Le equal to 1.30 m, the diameter of which is equal to 4.5 cm (corresponding to a volume of copper identical to that of case B). The insert is placed along the longitudinal axis of symmetry of the bar and extends, as in Figure 2, from the outer end of the bar to the inside of the cell. The non-sealing area is 0.18 m long and concerns the three normally sealed faces of the bar. The cathode voltage drop is 190 mV. Compared to case A, the reduction in cathodic drop is approximately 32% and the reduction in peak current density is approximately 37%. The distribution of cathode current is much more homogeneous than in cases A and B.

Claims

REVENDICATIONS
1 . Elément cathodique (20), pour l'équipement d'une cuve (10) de cellule d'électrolyse (1 ) destinée à la production d'aluminium, comportant : - un bloc cathodique (5) en matériau carboné ayant au moins une rainure longitudinale (15) sur une de ses faces latérales (21 ) ; - au moins une barre de raccordement (6) en acier, dont au moins une partie dite "tronçon extérieur" (19) est destinée à se situer à l'extérieur de la cuve (10), qui est logée dans ladite rainure (15) de façon à ce qu'une partie (22) de la barre dite "partie hors bloc" émerge d'au moins une extrémité (25) du bloc dite "tête de bloc", et qui est scellée dans la rainure (15) par interposition d'un matériau de scellement conducteur (7), tel que de la fonte ou de la pâte conductrice, entre la barre et le bloc, et caractérisée en ce que, pour chaque tronçon extérieur (19) : - la barre de raccordement (6) comprend au moins un insert métallique (1 6), de longueur Le, dont la conductivité électrique est supérieure à celle dudit acier, qui est disposé longitudinalement à l'intérieur de la barre et qui se situe, au moins en partie, dans ledit tronçon (19) ; - la barre de raccordement (6) n'est pas scellée au bloc cathodique (5) dans au moins une zone dite de "non-scellement" (17) de surface déterminée S située à l'extrémité de la rainure (1 5) en tête de bloc. 1. Cathode element (20), for equipping an electrolysis cell (1) cell (1) intended for the production of aluminum, comprising: - a cathode block (5) made of carbonaceous material having at least one groove longitudinal (15) on one of its lateral faces (21); - At least one connecting bar (6) made of steel, at least one part of which is called "external section" (19) is intended to be located outside of the tank (10), which is housed in said groove (15 ) so that a part (22) of the bar known as "part outside the block" emerges from at least one end (25) of the block called "block head", which is sealed in the groove (15) by interposing a conductive sealing material (7), such as cast iron or conductive paste, between the bar and the block, and characterized in that, for each external section (19): - the connection bar (6) comprises at least one metal insert (1 6), of length Le, whose electrical conductivity is greater than that of said steel, which is arranged longitudinally inside the bar and which is located, at least in part, in said section (19); - the connection bar (6) is not sealed to the cathode block (5) in at least one so-called "non-sealing" area (17) of determined surface S located at the end of the groove (1 5) at the head of the block.
2. Elément cathodique (20) selon la revendication 1 , caractérisé en ce que chaque insert (16) est en cuivre ou en alliage à base de cuivre. 2. Cathode element (20) according to claim 1, characterized in that each insert (16) is made of copper or a copper-based alloy.
3. Elément cathodique (20) selon l'une quelconque des revendications 1 et 2, caractérisé en ce que la longueur Le de chaque insert (16) est comprise entre 10 et 300 % de la longueur E de la partie hors bloc (22) de la barre (6) dans laquelle l'insert est logé. 3. cathode element (20) according to any one of claims 1 and 2, characterized in that the length Le of each insert (16) is between 10 and 300% of the length E of the part outside the block (22) of the bar (6) in which the insert is housed.
4. Elément cathodique (20) selon l'une quelconque des revendications 1 et 2, caractérisé en ce que la longueur Le de chaque insert (16) est comprise entre 20 et 300 % de la longueur E de la partie hors bloc (22) de la barre (6) dans laquelle l'insert est logé. 4. cathode element (20) according to any one of claims 1 and 2, characterized in that the length Le of each insert (16) is between 20 and 300% of the length E of the part outside the block (22) of the bar (6) in which the insert is housed.
5. Elément cathodique (20) selon l'une quelconque des revendications 1 et 2, caractérisé en ce que la longueur Le de chaque insert (16) est comprise entre 1 10 et 270 % de la longueur E de la partie hors bloc (22) de la barre (6) dans laquelle l'insert est logé. 5. cathode element (20) according to any one of claims 1 and 2, characterized in that the length Le of each insert (16) is between 1 10 and 270% of the length E of the non-block part (22) of the bar (6) in which the insert is housed.
6. Elément cathodique (20) selon l'une quelconque des revendications 1 à 5, caractérisé en ce que la section transversale de chaque insert (16) est comprise entre 1 et 50 % de la section transversale de la barre (6). 6. cathode element (20) according to any one of claims 1 to 5, characterized in that the cross section of each insert (16) is between 1 and 50% of the cross section of the bar (6).
7. Elément cathodique (20) selon l'une quelconque des revendications 1 à 5, caractérisé en ce que la section transversale de chaque insert (1 6) est comprise entre 5 et 30 % de la section transversale de la barre (6). 7. cathode element (20) according to any one of claims 1 to 5, characterized in that the cross section of each insert (1 6) is between 5 and 30% of the cross section of the bar (6).
8. Elément cathodique (20) selon l'une quelconque des revendications des revendications 1 à 7, caractérisé en ce l'aire totale A de la ou des surface(s) déterminée(s) S de la ou des zone(s) de non-scellement8. Cathodic element (20) according to any one of claims 1 to 7, characterized in that the total area A of the surface (s) determined S of the zone (s) of non-sealing
(17) de chaque barre de raccordement (6) est comprise entre 0,5 et 25 % de l'aire Ao de la surface So de la barre (6) qui est susceptible d'être scellée. (17) of each connecting bar (6) is between 0.5 and 25% of the area Ao of the surface So of the bar (6) which is capable of being sealed.
9. Elément cathodique (20) selon l'une quelconque des revendications des revendications 1 à 7, caractérisé en ce que l'aire totale A de la ou des surface(s) déterminée(s) S de la ou des zone(s) de non- scellement (17) de chaque barre de raccordement (6) est comprise entre 2 et 2O % de l'aire Ao de la surface So de la barre (6) qui est susceptible d'être scellée. 9. cathode element (20) according to any one of claims 1 to 7, characterized in that the total area A of the determined surface (s) S of the zone (s) non-sealing (17) of each connecting bar (6) is between 2 and 20% of the area Ao of the surface So of the bar (6) which is capable of being sealed.
10. Elément cathodique (20) selon l'une quelconque des revendications des revendications 1 à 7, caractérisé en ce que l'aire totale A de la ou des surface(s) déterminée(s) S de la ou des zone(s) de non- scellement (17) de chaque barre de raccordement (6) est comprise entre 3 et 1 5 % de l'aire Ao de la surface So de la barre (6) qui est susceptible d'être scellée. 10. Cathodic element (20) according to any one of claims 1 to 7, characterized in that the total area A of the determined surface (s) S of the zone (s) non-sealing (17) of each connecting bar (6) is between 3 and 15% of the area Ao of the surface So of the bar (6) which is capable of being sealed.
1 1 . Elément cathodique (20) selon l'une quelconque des revendications 1 à 10, caractérisé en ce qu'un matériau électriquement isolant est interposé entre la barre de raccordement (6) et le bloc cathodique (5) dans la ou chaque zone de non-scellement (17). 1 1. Cathode element (20) according to any one of Claims 1 to 10, characterized in that an electrically insulating material is interposed between the connection bar (6) and the cathode block (5) in the or each zone of non sealing (17).
12. Elément cathodique (20) selon l'une quelconque des revendications 1 à 1 1 , caractérisé en ce que chaque insert (1 6) affleure, avec une tolérance déterminée, la surface (24) de l'extrémité du tronçon extérieur (19) de la barre (6). 12. cathode element (20) according to any one of claims 1 to 1 1, characterized in that each insert (1 6) is flush with a determined tolerance, the surface (24) of the end of the outer section (19 ) of the bar (6).
13. Elément cathodique (20) selon la revendication 12, caractérisé en ce que ladite tolérance déterminée est inférieure ou égale à ± 1 cm . 13. cathode element (20) according to claim 12, characterized in that said determined tolerance is less than or equal to ± 1 cm.
14. Elément cathodique (20) selon l'une quelconque des revendications 1 à 1 1 , caractérisé en ce que l'extrémité extérieure de chaque insert (16) est en retrait, d'une distance déterminée, par rapport à la surface (24) de l'extrémité du tronçon extérieur (19) de la barre (6). 14. cathode element (20) according to any one of claims 1 to 1 1, characterized in that the outer end of each insert (16) is set back, by a determined distance, from the surface (24 ) from the end of the outer section (19) of the bar (6).
15. Elément cathodique (20) selon la revendication 14, caractérisé en ce que ladite distance déterminée est inférieure ou égale à 4 cm. 15. cathode element (20) according to claim 14, characterized in that said determined distance is less than or equal to 4 cm.
16. Elément cathodique (20) selon la revendication 15, caractérisé en ce que la cavité formée par le retrait de l'insert contient un matériau réfractaire. 16. Cathode element (20) according to claim 15, characterized in that the cavity formed by the withdrawal of the insert contains a refractory material.
17. Elément cathodique (20) selon l'une quelconque des revendications 1 à 16, caractérisé en ce que la section transversale de chaque insert (16) est circulaire. 17. Cathode element (20) according to any one of claims 1 to 16, characterized in that the cross section of each insert (16) is circular.
18. Elément cathodique (20) selon l'une quelconque des revendications 1 à 17, caractérisé en ce que chaque insert (16) est logé dans une cavité formant un trou borgne à l'intérieur de la barre (6). 18. cathode element (20) according to any one of claims 1 to 17, characterized in that each insert (16) is housed in a cavity forming a blind hole inside the bar (6).
19. Elément cathodique (20) selon l'une quelconque des revendications 1 à 18, caractérisé en ce que ledit matériau carboné contient du graphite. 19. Cathode element (20) according to any one of claims 1 to 18, characterized in that said carbonaceous material contains graphite.
20. Cellule d'électrolyse (1 ) destinée à la production d'aluminium, caractérisée en ce qu'elle comprend au moins un élément cathodique (20) selon l'une quelconque des revendications 1 à 19. 20. Electrolysis cell (1) intended for the production of aluminum, characterized in that it comprises at least one cathode element (20) according to any one of claims 1 to 19.
EP05744310.3A 2004-04-02 2005-03-30 Cathode element for an electrolysis cell for the production of aluminium Active EP1733075B1 (en)

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FR0403497A FR2868435B1 (en) 2004-04-02 2004-04-02 CATHODIC ELEMENT FOR THE EQUIPMENT OF AN ELECTROLYSIS CELL INTENDED FOR THE PRODUCTION OF ALUMINUM
PCT/FR2005/000757 WO2005098093A2 (en) 2004-04-02 2005-03-30 Cathode element for an electrolysis cell for the production of aluminium

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CN1938454B (en) 2011-12-28
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