CH649102A5 - ELECTROLYTIC CELL FOR THE ELECTROLYTIC PRODUCTION OF ALUMINUM. - Google Patents

Description

The present invention relates to an electrolytic cell for the electrolytic production of aluminum. The cell according to the invention is defined by claim 1. The invention will be described in more detail in embodiments which serve as examples and with reference to figures.

Fig. 1 is a schematic sectional view of the electrolytic cell of the present invention containing a chloride bath and illustrating the closed top of the cell as well as the relative positioning of the electrodes.

Fig. 2 is a partially broken diagrammatic view of an electrode used as an anode and being coated or coated with the mixture of aluminous materials and reducing agent.

Fig. 2A is a schematic perspective view of an alternative embodiment of the electrode of FIG. 2 illustrating a plurality of conductive cores in a matrix of aluminous materials and of reducing agent.

Fig. 2B is a schematic perspective view of a variation of the electrode illustrated in FIG. 2A.

Fig. 3 is a schematic view of an embodiment of the present invention which illustrates the use of a porous membrane for retaining the various anode materials comprising a material containing aluminum and a reducing agent.

Fig. 4 is a schematic sectional view of a combination of electrolytic cells with electrodes with inclined side walls and the composite anode of complementary shape.

Fig. 5 is a schematic sectional view of a modification of the combination of a single electrolytic cell and a composite anode.

Fig. 6 is a perspective illustration of the anode of FIG. 5.

Fig. 7 is a schematic sectional view of another embodiment of the combination of FIG. 5.

Fig. 8 is a perspective illustration of the anode of FIG. 7.

Fig. 9 is a schematic perspective view of another embodiment of the composite anode of the present invention illustrating a laminar construction.

Fig. 10 is also a schematic perspective view of the anode of FIG. 9.

The electrolytic system of the present invention uses an electrolytic cell C described in one of the figures for the single continuous production of aluminum.

In fig. 1 to 3, a shape of the structure of the electrolytic cell is illustrated generally in 10 and it consists of an external steel shell having a refractory lining 14 which can serve only as thermal insulator or at the same time as insulator and electrode. The refractory lining can be made of any material resistant to the action of the electrolytic bath 16. The refractory lining having conventional vertical sides 15 and a bottom 17 is designed to maintain the desired thermal balance in the operation of the cell and therefore may have a very thin section so as to obtain a small thermal gradient from which there results both a thin layer of a frozen salt on the surface of the refractory lining and a hot outer wall on the surface of the steel shell 12. The refractory lining can also be very thick to obtain a frozen layer of salt in the refractory lining resulting in a cold surface on the steel shell, although this is not necessary in the cell with vertical side walls of figs. 1 to 3. In contrast, in the cell with inclined walls of FIG. 4 the cathode 19 is a conductive lining formed on both sides of the anode. A thermal and electrical insulating liner may be disposed between the cathode 19 and the shell 12 if desired. The freezing line should be within

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limits of this conductive lining or cathode 19 so as to avoid the recovery of a solid layer of salt from the side of the electrode bath. Such a layer of salt would act as an electrical insulator and prevent efficient flow of current.

The cover 18 is provided on the top of the cell to obtain an airtight seal and is only necessary in a bath containing chloride. This cover thus prevents air and moisture from infiltrating the cell or vapors of the salt composition 16 from leaking out to react with the ambient medium. The cover 18 can be lined with the refractory material 20 which can be the same as the refractory lining 14 or any other refractory material which can maintain a temperature balance in the cell while being chemically inert to the salt composition 16. Joints sealing 22 are supported on the cover 18 and are fixed against the electrodes 24, 25 and 26 to prevent atmospheric air and humidity from infiltrating into the cell or the vapors of the cell from leaving towards the ambient medium . The sealing of the cover 18 and around the electrodes can be carried out by any means which prevent steam leaks and which can be a fur and a standard or conventional seal capable of withstanding the operating temperature, while resisting the vapors of the 'electrolyte. Acceptable materials for such a seal are, for example, asbestos, fibrous ceramics, Teflon, Vitron, silicon, liquid metal seals such as mercury, liquid solder, tin, lead, etc.

The electrodes 24, 25 and 26 can be anodes, cathodes or bipolar electrodes. They may include solid or coated conductors to transport electrical current for the operation or the operation of the cell. These conductors can be made of any material which can withstand the temperature in the cell which is of the order of 150 to 1060 ° C., be stable to the composition of halides 16 and be good electrical conductors. Materials useful for this purpose are carbon, graphite, carbides, nitrides or borides of titanium and aluminum metal suitably sized for the balance of heat transfer. The preferred materials for these conductors have been found to be graphite and titanium diboride in operation in the bipolar mode.

The aluminum chloride cycle cell also includes a down pipe or outlet tube 28 having a valve 30 for controlling the flow of any gaseous elements from the down pipe and for maintaining pressure in the cell for operation in continued. The gas vapors emanating from the cell are those of the oxidized reducing agent and in particular there is no chlorine gas detected from a salt containing aluminum chloride. If chlorine were to occur, it would react on anode 26 and be recycled as aluminum chloride. The molten aluminum 32 is drained through a conventional tap hole 34 or otherwise is sucked outward by traditional siphoning techniques well known in the art.

Instead of changing the anodes periodically for the supply of fresh aluminous materials, the present invention can adapt a feeding mechanism for continuous operation, as illustrated in FIG. 3.

An anode electrode 52 projecting through cell C in FIG. 3 penetrates deep into the fusion 16, but remains above the basin of molten aluminum 32 or of the cathode block 46. Around the anode 52 are anode raw materials, generally illustrated in R, including the aluminous material and the reducing agent. This anodic mixture can be formed into small particles of dimension between 25 µm and more than 2.5 cm and can have been formed by extrusion, casting or other and are fed into the cell by the hutch 54. The particles of raw materials of materials aluminous and reducing agent are specifically identified at 58 and are in close contact with anode 52 to provide the necessary source of aluminum and the reducing agent.

These anode raw materials are kept in close contact with each other and with the anode 52 while being retained in a container with a porous membrane 60 which surrounds the anode 52. As the anode materials 58 are used and their levels drop appreciably below the level of the molten bath 16, the supply 54 is operated to add additional anode materials 58 to the container with porous membranes 60.

In fig. 4 to 8, an electrolytic cell with inclined wall electrodes is illustrated which, in combination with the anode composition of the present invention, gives substantial savings in the electrodeposition of aluminum.

In the processes with characteristic Hall cells, the aluminum reduction cells have an anode-cathode space which must take into account the effect of the magnetic field and the feedback reaction due to the undulations of the aluminum basin. Such considerations prevent having a space smaller than 1.5 to 2.0 in. between the bottom surface of the anode where all erosion occurs and the top of the aluminum basin or cathode electrode. Another equally important reason for the need for more space between the cathode and the anode, whether in the construction of Hall cells using vertical walls or in any other construction using an electrode with inclined walls, is the serious difficulty in maintaining dimensional stability, difficulty due to the high temperatures required and the aggressive salts which are necessarily included to maintain a high temperature for the dissolution of alumina. By combining cells with inclined electrodes and the anode using aluminum oxide and a reducing agent to obtain the only source of aluminum, the use of low temperatures, just above the melting temperature of aluminum minimizes all problems with dimensional instability and therefore allows the cells of the present invention to be structured with a smaller anode-to-method spacing than that achieved in the past. Thus, it is the particular combination of the anode and the inclined walls for the construction of the cell which allows a lower IR power drop in the salt, due to the close admissible spacing between the inclined walls and the reduction in anodic current density.

The cells of fig. 4 to 8 are essentially identical to those described previously except for the inclined surfaces forming the electrodes. With this cell structure, the anode 74 is provided with sloping sides 76 which, as illustrated, are external and directed downward and inwardly, although the direction of the angle is not at all critical. The slope of the sides can be in any direction or any angle from the level of the bath B. The angle can even vary from 10 to 80 ° or more from the level of the bath. Via the anode base of sloping side electrodes and this portion of the sloping anode side which is immersed in the bath 16, the anode will undergo erosion over a larger area and supply the aluminum for its deposition. final on the cathode.

The cathode 78 has surfaces 79 of complementary shape on the sloping sides 76 of the anode to establish a spacing of electrodes on the sides as illustrated by the spacing Y. This spacing can be between 6 and 65 mm. Greater spacing results in greater energy consumption. The spacing between the bottom 80 of the anode 74 in FIG. 4 and the surface of the aluminum layer 82 forming part of the aluminum basin 84 is illustrated in X and can be between 6 and 65 mm. Preferably, the X and Y spacings should be between 6 and 26 mm.

The spacing between the anode and the cathode above the solidified bath layer 86 is not important to the utility of the invention. However, the spacings X and Y between the anode and the cathode can be identical or different according to the current density and the anode erosion desired but, when they are determined in the vicinity of the specific values given above, it this results in substantial energy savings.

The coating 78 forming the cathode of the cell can be made with a characteristic material used for electrolytic cells such as carbon, titanium diboride or the like, and it is

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formed as described above to adapt to the external shape of the anode 74. In addition, the base of the coating has an inclined floor 90 for the aluminum basin leading into a cap-tage well 92 for aluminum. As can be seen, the sloping ground 90 is such that it retains only a limited depth of a layer of aluminum which can be adjusted by means of purging (not illustrated) of the aluminum from from the catchment well. The purpose of the thin layer of aluminum under the base 80 of the anode is to substantially eliminate the ripples in the form of waves or waves from the layer of molten aluminum due to the magnetic effects in the cell.

In other aspects, the cell of FIG. 4 is identical to that of FIG. 1, in this sense that a cover 18 is provided with a discharge orifice 28 forming part of the shell 12. A refractory insulating material of any suitable shape as illustrated in 14 may also be included.

The combination of the use of the anode of the present invention with inclined sides to accommodate the inclined cathodes allows for substantial variation in the configurations of the cell and the anode, as illustrated in Figs. 5 to 8.

In fig. 5 and 6, the shape of the anode 74 varies and has divergent inclined sides located at the center 94 which form an apex 96 in the anode. The carbon or other coating material such as EiB2, etc., serving as a cathode, projects upward to complement the internal shape of the anode as best illustrated in FIG. 5. The operation of such a cell illustrated in FIGS. 5 and 6 is essentially the same as that described in FIG. 4, particularly with regard to the increased erosion areas 94.

In fig. 7 and 8 of the double anodes 100 and 102 with opposite inclined sides 104 and 106 respectively are positioned in a cell with a cathode 98 whose shape is essentially identical to that described in FIG. 5.

The use of the inclined cathodes of electrolytic cells illustrated in FIGS. 4 to 8 has been found not to allow the formation of a layer of frozen salt on the surfaces of the inclined cathode wall immersed in the bath and facing a portion of the anode surface. Otherwise, the desired spacing between the cathode and the anode can no longer be maintained. In addition, the frozen salt which would adhere to the wall of the cathode is a good electrical insulator and would prevent the passage of current from the anode to the inclined cathode side wall. In the previous use of such inclined wall electrodes, the problem of freezing salts on the walls as well as the dimensional instability of the coating prevented any extensive use of these cells. However, with the anode composition of the present invention and the lower bath temperatures, a variety of low melting salt compositions which do not freeze on the side wall can advantageously be used. Ideally, the melting point of the salt and the thermal equilibrium of the cell are adjusted so that the freezing line of the salt is in the coating or on the steel shell rather than on the cathode lining or interface. bath. It is not important where the freezing line is located as long as it is within the limits of the coating and the salt is maintained in a liquid state on the surface of the cathode lining immersed in the bath. In such a case, the appropriate cathode-anode spacing is maintained without difficulty.

The process

at. Chloride-containing bath

The electrolytic process of the present invention for the single continuous production of aluminum ions on the anode uses the electrolytic cell with closed top described in fig. 1 or any other cell described below, if the upper part is closed or action is taken to prevent:

a) moisture coming into contact with the chloride electrolyte,

or b) the oxidation of aluminum chloride, while retaining the salts of the vaporized bath.

The advantages of the present invention in using the chloride-containing bath are derived not only from the continuous in situ production of aluminum ions on the anode, but also from the significantly reduced energy requirements for producing high-grade aluminum. quality in the total absence of chlorine gas leaving the cell.

The continuous production of aluminum ions at the anode is caused by the formation of the anode from an aluminous material containing aluminum oxide and a reducing agent. This anode is immersed in a molten bath containing an alkali metal and / or alkali earth metal halide salts of any composition, provided that aluminum chloride is present in the bath. During electrolysis, the ionized aluminum in the bath is deposited as aluminum metal on the cathode while the reaction at the anode also forms CO 2 in addition to the aluminum ion. The aluminum is recovered in the form of molten aluminum and it is drawn off, but it is the reaction at the anode to reform the aluminum ions which constitutes an important part of the present invention.

It is possible that halogenated chlorine, whether it is the chloride ion, atomic chlorine or chlorine gas, takes part in the reaction of the chloride with the aluminum oxide of the aluminous material and the reducing agent of l anode to produce aluminum ions plus oxide reducing agents. Aluminum from the anode is ionized in the molten bath for the continuation of the cycle and the anions which may be chloride, oxide or the like, maintain the charge balance with the aluminum ions.

The aluminum produced at the cathode is generally as pure as the aluminous material forming the anode. It is possible to produce ultra pure aluminum according to the present invention using a very pure source of alumina or to produce slightly impure aluminum by direct use of aluminous ore materials such as bauxite or clays containing aluminum such as kaolin or mixtures of these minerals. In general, it is possible to obtain an aluminum purity of at least 99.5%.

It is known that in the reaction of Hall-Heroult cells, the carbon of the anode contributes to the overall reaction of aluminum gains by decreasing the breakdown voltage of A1203. For example, the decomposition of A1203 into cryolite on a platinum anode is approximately 2.2 V, but on a carbon electrode, considering approximately 50% by volume of CO produced and 50% of CO2, the decomposition voltage is 1.2 V approximately. Approximately, the same decomposition voltage is obtained with A1203 if methane is injected under the platinum anode to produce mainly

CO,

In the present invention, the use of the composite anode results in a decomposition voltage lower than that which would be obtained if A1C13 were decomposed with gas discharge Cl2 on the anode. In any electrochemical reaction, if the curve of the current voltage is extrapolated to a current 0, a number approaching the decomposition voltage is obtained. In an aluminum chloride electrolysis process, when using a graphite anode, a decomposition of 1.8 to 2.0 V can be obtained, a value which conforms to the values reported in the texts and conforms to the theoretical value calculated from thermodynamics.

It has been found that the decomposition voltage of the present invention varies slightly with the composition of the electrolyte. With pure NaAlCl4, the decomposition voltage is lowest, but as the A1C13 component of the electrolyte decreases, the decomposition voltage tends to increase slightly. The lowest decomposition voltage obtained was 0.5 V and the highest was 1.5 V. The average value was 1.2 V. Using the average value of 1.2 V as the decomposition voltage, it has been observed that in the present invention the decomposition voltage is 0.6 V lower than that for A1C13 when chlorine is

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discharged and the value obtained in this case approaches that of A1203 and carbon, suggesting that the same general reaction mechanism occurs both in the Hall-Heroult cell and in the present invention. This lower decomposition voltage results in considerable energy savings for the electrolytic production of aluminum not only when compared to the conventional aluminum chloride system where chlorine is discharged at the anode but also when considers the additional energy required to produce A1C13 from A1203, carbon and chlorine.

The process conditions for the electrolytic production of aluminum have not been found to be critical with respect to the applied voltage or current density. The temperature of the bath can vary considerably and it is necessary to maintain the molten bath which, depending on the composition of the halide salts present, can be obtained with a temperature between 150 and 1000 ° C, but generally with a temperature between the melting point of the aluminum and the boiling point of the cell components, preferably between 10 and 400 ° C and more preferably between 10 and 150 ° C until less 250 ° C above the melting point of aluminum. The conditions of the pressure in the enclosed cell are not particularly critical insofar as there is no chlorine gas escaping as in the processes with aluminum chloride salts of the prior art.

Although CO or CO 2 or both can be produced with the present process, these gases are not as corrosive as chlorine. Pressure conditions, being unimportant, can be between atmospheric pressure and 10 psig or more.

b. Bath containing entirely fluoride

The Hall cell functions chemically based on the fact that the alumina dissolves in the cryolite-fluoride salt bath at a temperature of 950-1000 ° C. Bayer alumina is soluble in the bath containing cryolite at a minimum temperature of at least 900 ° C or above. Any fluorine-containing bath at a temperature below about 900 ° C will not dissolve the commonly treated Bayer alumina and therefore alumina, as a source of aluminum, cannot enter the reduction reaction and it it is not possible for aluminum to be deposited at the cathode. Without this general solubility of alumina in the fluoride salt bath, it is not possible to electrogagnate aluminum.

It has been discovered, and this is one aspect of the present invention, that in all fluoride baths the temperatures can be between the melting point of aluminum and the boiling point of the components of the cell, preferably between 10 and 400 ° C and more preferably between 10 and 150 ° C up to less than 250 ° C above the melting point of aluminum. To electrogagnate aluminum from its corresponding oxide or other oxygen-containing compound, the bath temperature range would generally be 670-800 ° C and preferably 700-750 ° C.

The important aspect of this discovery which differentiates it from the traditional Hall-Heroult cell processes is that the composite anode containing the mixture of aluminum oxide and reducing agent transforms the aluminum oxide. and produces ionic aluminum in the low temperature fluoride bath. The general reaction, however, is assumed to be essentially the same as the Hall cell reaction as previously reported. Aluminum is produced in liquid form on the basin of liquid metal serving as cathode. It is assumed that a reaction occurs on the surface of the anode in a unique way which results in the reaction of aluminum oxide to produce aluminum ions similar to the mechanism that occurs in the Hall cell itself if the temperature is only slightly above the melting point of aluminum.

The importance of the use of the composite anode in the present invention should be made very clear because, under the same conditions as those of the present invention but using a carbon anode or other non-consumable anode, the addition of aluminum oxide in the bath will neither produce the dissolution of aluminum oxide nor the electrolytic deposition of aluminum. The composition of the bath, the current densities and other process parameters are not critical to the operation of the cell containing the chloride bath or the fluoride bath.

The anode

The main support for enjoying the advantages of the present invention consists in the use of a single composite anode consisting of an aluminous compound containing oxygen, generally aluminum oxide, and a reducing agent.

The anode consists of the only source of aluminum ions for the electrolytic reduction of aluminum at the cathode as well as, with a carbon reduction agent, in the means of conduction of the electric current through aluminum oxide, said electric to the reaction site for aluminum oxide in contact with the electrolyte and immersed in the latter. The anode also preferably at least in part provides a necessary source of a reducing agent which allows aluminum oxide to react in the anode environment to produce the deposition aluminum at the cathode as a metal. aluminum.

The reducing agent is preferably, at least in part, mixed with the aluminum oxide to obtain intimate contact between the reducing agent and the aluminum oxide. The reducing agent, if properly chosen, is conductive and when mixed with aluminum oxide may also have the function of an electric current conductor to the reaction site for the oxide 'aluminum. Following the reaction of each particle of aluminum oxide at a particular site in contact with the electrolyte and in the presence of an electric current, another particle at the same site is now discovered and can react. This occurs over the entire surface of the anode and continues until there is no more aluminum oxide to react. If the reducing agent is not conductive and is not mixed with aluminum oxide, the electrical conduction function must be performed otherwise by conductive rods to maintain the anodic aluminum oxide at the site of reaction.

In an aluminum chloride salt bath, the anode has the function of providing a reducing agent which helps in the theoretical reaction of the aluminous source, with chloride or oxygen or both, to maintain a constant concentration aluminum chloride. Maintaining a constant concentration of aluminum chloride is an important part of the chloride cycle of the present invention because it eliminates the need for an external replenishment or recovery of electro-lysed aluminum chloride or the discharge of chlorine. on the anode.

In the all fluoride bath process, the anode of this invention, as in the case of the chloride cycle, provides aluminum oxide which reacts in the fluoride bath to form aluminum ions at a single low temperature of 1 'order of 670-800 ° C. The cell can also be opened as in fig. 3 or 4.

The aluminum source is alumina A203, but it could also be any material containing aluminum oxide such as bauxite or a clay such as kaolin or other material which would react at the anode to produce aluminum ions which are reduced to molten metal at the cathode as in fluoride or chloride cycle processes.

When the mixture forms the anode, the proportion is of the order of at least 1.5 and more, with acceptable upper limits of the order of 7.5, 20.0 or even 50.0 or more. parts by weight of aluminum oxide in the aluminous material per part by weight of reducing agent. Preferably, the amount of aluminum oxide in the mixture of aluminous materials can be of the order of 2.0 to 6.5 and more preferably 2.5 to 6.0 parts by weight of aluminum oxide per part of reducing agent.

The reducing agent which can be used according to the present invention is not limited to a particular material, but could be one of those materials known to be effective in reacting with

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aluminum oxide. The reaction in the fluoride and chloride baths is not clearly defined, but the reducing agent may react with A1203 to produce aluminum ions which eventually settle on the cathode and CO2 at the anode. The reaction mechanism can be the same in the salt electrolytes any chloride, any fluoride or chloride / fluoride mixture.

Among the reducing agents which are more particularly useful for alumina and other oxides is carbon or a reducing carbon compound used in the mixture. Sulfur, phosphorus or arsenic can also be used independently or in combination with carbon. Carbon is particularly preferred because it is characterized both because it is able to transport the current to the reaction site of the aluminum oxide and to maintain a reduction function and to give a gaseous product to the 'anode.

The source of carbon in the mixture can be any organic material, particularly those having a fossil origin such as pitch, bitumen, coal and carbonaceous products, reduction gases, for example carbon monoxide, and can also include synthetic and natural resinous materials such as waxes, gums, phenolics, epoxies, vinyls, etc., and the like which may, if desired, be coked even in the presence of the aluminous material. The coking of the carbon source mixed with the aluminum oxide compound can be carried out by known techniques of the art such as those used in the prebaked anodes which are used in the Hall-Heroult cell. This is accomplished by casting, molding, extruding, etc., of a composite anode such as Al203-pitch in a desired ratio of, for example, 6.5 parts of aluminum oxide per part of carbon in the coked state. , and by slow heating of the anode formed in a carbon monoxide atmosphere to a coking temperature of 700 to 1200 ° C. After coking, the composite anode is then ready for use.

It is also considered, for example, in the context of the present invention, to produce carbon as a reducing agent in the mixture with aluminum oxide by coking the carbon source in the electrolytic bath both before and during electrolysis. Bath temperatures in the characteristic range of 670 to 850 ° C are adequate for coking the carbon source to produce the necessary carbon. The time to carry out such a coking is not critical, but it may be necessary from several minutes to several hours depending on the temperature of the molten bath and the mass of the mixture of the alumina source and the reduction carbon source. .

The raw composite anode material is gradually coked with the heat of the cell so that the end of the anode in the salt is always fully coked at the operating temperature of the cell. Soderberg style coking in the cell at 670-850 ° C produces an anode of lower conductivity when compared to composite anodes pre-baked at much higher temperatures.

The total source of reducing agent, as previously indicated, need not, if desired, be mixed with the source of aluminum oxide to form the anode. It has been found, for example, that the only conditions required for the reducing agent are: that it is in contact with anodic aluminum oxide and present in sufficient quantity to produce aluminum metal at the cathode. It is clear, however, that electrical current must be passed to the reaction site to allow the reaction to continue.

If the reducing agent such as carbon is not mixed to transport the current, it is conceivable that another conductor, compatible with the cell and its contents, is used. For example, combinations of aluminum or noble metals and high-melting conductive oxides such as silver-tin oxide or TIB2, alone or composite with carbon or graphite, can be mixed with aluminum oxide in quantities only sufficient to transport the electric current to the reaction site. Such an amount is not critical provided the aluminum oxide is anodic at the reaction site. Amounts as low as 0.001 up to at least 0.75 parts of conductive material per part of aluminum oxide can be used. Larger amounts increase the conductivity at the expense of the availability of reactive materials, but they are possible without an actual upper limit. Obviously, there must always be a reducing agent present to carry out the necessary reaction.

In the case of alumina as an aluminous material, hydrated or calcined alumina can be used. Anodes formed with hydrated alumina may have improved conductivity compared to calcined alumina, but hydrated alumina, A1203 x 3H20 or Al (OH) 3, tends to crack during coking of the precooked type and when placed in the hot bath, because of the water draining during the coking operation. In a salt containing aluminum chloride using a hydrated alumina bath coking, the water removed could undesirably hydrate A1C13.

Any crack or breakage of the anode due to the humidity exited does not cause any difficulty provided that the membrane illustrated in fig. 3 surrounds the anode. All of the particles of the anode which detach are retained in the membrane for a continuous reaction. The anode can also consist of any proportion of hydrated and calcined oxide to minimize cracking or cracks. The maximum amount of hydrated oxide that can be used affects energy savings in calcination.

The size and surface of the particles constituting the anode containing the aluminum oxide did not show any sensitivity with regard to the reaction speed of the anode.

This characteristic of the present invention is in contrast to the experience of the prior art in the reaction of A1203 and carbon with chlorine in the form of a gas-solid reaction in an oven. In the past, it has been found that the reaction temperature and speed are very sensitive to the size and surface of the particles.

In general, it is desired to use in the prior art alumina having a surface of the order of 10 to 125 m2 / g in the reaction of A1C13. However, in the present invention, no sensitivity was detected with respect to the reaction rate of the anode based on the size or surface of the particles. That is, A1203 with an area of 0.5 m2 / g or less apparently reacted as easily as AI203 with an area of 100 m2 / g. These results are based on experiments which have been made with anodes containing alumina having particles with different surfaces and dimensions. Anode current densities between 0.3 and 6.5 A / cm2 were passed through the cells with the output line connected to a starch-iodine indicator for chlorine detection. No chlorine gas was detected regardless of the current density or the surface of the alumina. This suggests that if chlorine is produced at the anode it reacts entirely to reform aluminum chloride or that only aluminum ions are formed at the anode from A1203, while oxygen from A1203 combines with the carbon producing C02. It is believed that to produce chlorine at the anode it would be necessary to raise the potential enough to exceed the decomposition potential of A1C13, but even then the chlorine produced would likely react with A1203 and carbon to produce more A1C13 rather than releasing chlorine at the anode.

Anodes used in electrolysis cells can be produced in a variety of forms and by a variety of manufacturing processes. A mixture of the aluminum oxide and the reducing agent can form the anode in a suitable manner. For example, a mixture can be bonded to a characteristic electrode to form a coating surrounding all or one side of the electrode as illustrated in FIG. 2 of the drawings. It is also possible to satisfy the conditions of the present invention for forming the anode in a manner other than by direct physical bonding to the electrode. It is desirable, however, that the aluminous material

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either in intimate physical contact with the carbonaceous material or other reducing agent. The latter concept can be applied if the mixtures of the aluminous material and the reducing agent are in the form of a homogeneous mixture of powders, small grains and mixed powders, or larger composite briquettes of these mixed materials which may have been formed by molding or extrusion in various dimensions, from 25 (jm up to 2.6 cm or more. It has been found that uniform distribution of carbon and aluminum oxide is desirable to achieve maximum efficiency of the anode during its dissolution or reaction under electrolysis.

To hold the aluminous material and the reducing agent forming the anode materials in the region of the electrode and thus in combination forming the anode, a container in the form of a porous membrane can be used.

For successful commercial use, the anode should be as conductive as possible. Since the anode of the present invention is not solid or pure carbon as is traditionally used in the Hall cell, it will be less conductive due to the presence of the aluminous compound. If the anode were allowed to become as resistant as the salt electrolyte, then the thermal equilibrium would be affected due to overheating which may occur resulting from the passage of the same current through the more resistant anode. It is therefore desirable to construct an anode having as high a conductivity as possible. Obviously, the more the anode material is conductive, the less energy is consumed to gain metal, but in all cases the conductivity of the anode should be higher than the conductivity of the salt for optimal operation. In particular, when it is desired to achieve maximum production of aluminum with a minimum of energy consumption, the resistance of the anode becomes important.

It has been found that the conductivity of the anode varies considerably depending on the manufacturing process. The parameters which have been found to affect the conductivity are: the rate of binder carbon materials such as pitch, carbon or coke particles included in the composite anode as the source of the reducing agent and the type of oxide aluminum. The higher the carbon content of the anode, in the previously specified rate of aluminum oxide relative to the reducing agent, the higher the conductivity. It is possible, for example, when using a rate of the order of 4/1 to 6/1 of aluminum oxide relative to carbon, to construct a solid composite anode which has at least a conductivity equal to one tenth of the conductivity of a standard Hall-Heroult anode.

To obtain greater conductivity and reduce energy losses through the composite prebaked anode, another embodiment uses one or more conductive cores 36 or 37 positioned in the anode as illustrated in FIGS. 2, 2A and 2B.

The composite anode 26A illustrated in FIG. 2 has a central conductive core 36 which may be carbon or graphite molded in the composite anode or the composition of composite anode materials 38 molded or coated in the form of a preformed conductive core. The central core 36 can also be a metal such as the same deposited metal, for example aluminum. The exterior of the conductor 36 is coated on one side for bipolar use or surrounded on both sides for monopolar use by a matrix 38 of a composite anode material comprising the mixture of aluminum oxide and reduction, as previously described. When coating is done on one side only, bipolar operation is considered. The term oxides should be interpreted to include silicates which often are a combination of metal oxide and silicon oxide or any other oxygen-containing compound from the aluminum to be deposited.

For large anodes, another alternative of an embodiment is illustrated in FIGS. 2A and 2B. To improve the conductivity, it is preferable to use aluminum rods of first grade purity 36 and 37 as an electrical conduction bus in a matrix of the composite anode composition 38

which can be of the precooked or Soderberg type. Since first grade aluminum is used to form the conductive rods, it will melt as the anode is consumed and join the metal of the cathode for a continuous cycle. The rods are spaced such that the voltage drop is minimized relative to the conductivity of the composite anode. In fig. 2B the conductive rods 36 are illustrated connected to a plate 40 supported by a central conductor 41.

The number and size of the conductors 36 and 37 are chosen based on the size of the anode, the current density of the anode, the size of the cell, the operating temperature and the heat transfer, such so that the aluminum conductors 36 and 37 melt at the same speed as the matrix 38 of the anode is consumed. The unique advantage of the anode embodiments illustrated in Figs. 2A and 2B is that large voltage drops are avoided in the relatively high resistivity anode, which makes it possible to operate with significantly reduced energy consumption. The size of the aluminum rods can be reduced to a diameter of the order of 1.5 to 75 mm, preferably 3 to 50 mm, more preferably 6 to 25 mm.

To obtain a desirable conductivity in the anode, the spacing between the outer surface of the composite anode 38 and the surface of any aluminum rod as shown in FIG. 2, 2A or 2B, and the mutual spacing between the external surfaces of these aluminum rods of FIGS. 2A and 2B are not critical and can be between 6 and 600 mm, preferably 25 to 150 mm, and more preferably 37 to 100 mm. As an example, if the conductivity of the composite anode is approximately 0.1 of an anode of a standard precooked Hall cell, the spacing of the aluminum rods of approximately 75 mm will give an acceptable voltage drop.

Since the cell operating temperature is usually between 700 and 750 ° C, the aluminum rods can be dimensioned so that they melt at approximately the same speed as the anode is consumed, so that they conduct energy to the bottom of the anode. If the diameter of the aluminum rod is too large, it will not melt and salt will freeze on its surface where it results that the anode is consumed leaving an aluminum strain which will shorten on the cathode as as the anode advances. If the diameter of the rod is too small, it will melt too far back in the anode, which will cause a too large voltage drop due to the longer conductivity passage. It is desirable that the aluminum rods melt slightly recessed in the anode rather than remaining flush with the bottom surface of the anode. This is such that the anodic oxidation of the aluminum rods will be minimized. A desirable shrinkage distance is that which produces the minimum voltage drop coupled with the minimum anodic oxidation of the aluminum rods. If the rods were to remain flush with the bottom surface of the anode, the aluminum ions from the rods (as in a refining operation) as well as the composite anode material would tend to pass into the bath, thereby lowering the faradic efficiency of the cell. The heat can be balanced so that the conductance of the bath through the anode and the energy produced through the conductors are balanced to obtain the desired amount of fusion of the conductive aluminum rods.

The embodiments of FIGS. 9 and 10 illustrate a variation in the conduction of electrical energy at the working surface of the anode. The block 94 of the composite anode A is interleaved with aluminum sheets 108. These sheets act precisely like the aluminum rods in FIGS. 2A and 2B. The shape and number of layers is not critical. The blocks can extend in continuous form and be brought into the cell through the fastener 47 to which, and with the aluminum sheets 108, the electrical connection is established.

The number, spacing and thickness of the aluminum sheets 108 are determined by the same factors as those described with respect to the conductors 36 and 37. Generally, the thickness of the

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aluminum sheets will be between 25 (im and 13 mm, preferably between 0.25 and 10 mm, and more preferably between 0.25 and 6 mm thick. The aluminum sheets may have a thickness sufficient to lead the current necessary to avoid a large voltage drop and also to melt in the cathode basin as the anode is consumed. The spacing between the aluminum sheets 108 is such that it avoids a voltage drop excessive through the composite block with respect to the conductors 36 and 37. Generally, the spacing will be between 3 and 600 mm, preferably between 25 and 150 mm, and more preferably between 35 and 100 mm.

The membrane

The membrane illustrated in fig. 3 of the drawings is designed to have a tripartite function or capacity.

First, the membrane acts as a separator or barrier at rest between the molten cathode metallic phase and the source of anode materials to be electrolyzed. After the use of the membrane of this invention, the spacing can be reduced significantly to achieve significant increases in conductivity and yield without turbulent effect which could otherwise produce a reduction in yield or a decrease in the quality of the product. aluminum.

Second, in the present invention, the membrane physically retains the materials of the composite anode which, for example, may include the aluminous raw material and the reducing agent. This constraint or restraint keeps these materials close to the electrode to form an anode for the production of aluminum ions in the most efficient way. The membrane also prevents the mixing of raw materials with molten aluminum at the bottom of the cell. If a hydrated metal oxide, such as hydrated alumina, were used as one of the anode materials, the membrane would retain any pieces of the anode which could detach and break due to the evolution of the humidity of the alumina during the coking of the bath. These pieces continue to be a source of aluminum during the reduction reaction as long as they remain in the anode circuit in the membrane.

Third, the membrane allows the free passage of ionic substances and dissolved solids in the electrolyte, but will not let pass and will reject the molten aluminum and the undissolved solid materials which constitute the usual impurities present in the aluminous source, preventing thus contamination of the cathodic deposit.

The external shape of the membrane is not important and it may have the shape of a cylinder, a prism, etc., or a portion of these. For example, the membrane may have a three- or four-sided shape with a bottom and thus form an enclosed container. This container is designed to contain the anode raw materials for the reaction in the salt bath.

Due to the corrosive nature of the molten salt bath, the selection of materials to form the membrane is important for the life of the cell and the success of the process. If the electrolyte to be used is an all chloride bath, the choice for the membrane is a little greater given the reduced corrosive nature of such a bath if it is compared to a bath containing fluorides. Baths containing certain fluorides are preferred, however, because of their lower volatility. The all fluoride bath has other advantages as indicated above. Appropriate materials in a fluoride bath would obviously be useful in a less corrosive chloride bath.

Among the materials which have proved useful are vitreous carbon foam, carbon or graphite as a porous solid or porous solids of refractory hard metals such as boron, aluminum, silicon nitrides (including oxynitride), titanium, hafnium, zirconium and tantalum; molybdenum, tantalum and tungsten silicides; carbides of hafnium, tantalum, columbium, zirconium, titanium, silicon, boron and tungsten; and the borides of hafnium, tantalum, zirconium, columbium, titanium and silicon. Other refractory hard metals known in the art may prove useful in forming the membrane provided they are resistant to the bath of molten salt.

The refractory hard metals forming the membrane of the present invention can be in the form of a fabric, a carpet, a felt, a foam, a porous sintered solid base or simply a coating on a such basis, all of which are known in the art for other purposes. The membrane must also meet the specific standards of porosity of passage and pore size.

These two characteristics can be defined as follows:

Porosity of passage - The percentage of the total volume of the membrane which is made up of passages which pass through the membrane from one side to the other;

Associated pore size - The smallest diameter of a passage through the membrane.

The porosity of passage varies with the nature of the membrane material, the temperature of the molten bath and the composition of the salt, but the common characteristic of useful membranes is that the porosity must be sufficient for all the metal ions to pass such that the aluminum and all electrolytic salts without undissolved impurities passing through. It has been found that the greater the porosity, the greater the current flow and therefore the greater the electrical efficiency of the cell. The porosity can vary from 1 to 97% or more, but generally it is between 30 and 70%. The preferred porosity for obtaining the highest yield is between 90 and 97%. A glassy carbon foam, for example, is capable of giving such a high porosity and of maintaining sufficient mechanical strength.

The associated size of the pores must be small enough to reject solid impurities that have not been dissolved, but it must be large enough to allow the ionic and dissolved particles to pass through. Generally, the acceptable size of the pores is between 1 H and 1 cm.

The thickness of the membrane material is a function of its porosity, the size of the pores and its ability to retain impure undissolved solids and molten metal. Obviously, the thicker the membrane, the greater the electrical resistance. It is therefore desirable to use a membrane as thin as it practically conforms to standard standards of porosity and pore size and to the mechanical strength of the membrane in position in the cell. The preferable thickness is between 3 and 13 mm but it can be as thick as 50 mm or more.

Typical membrane materials which have been found to be useful include, but are not limited to, glassy carbon foam, carbon or graphite in the form of porous solids, felt or fabric, aluminum nitride, silicon nitride , silicon carbide, silicon oxynitride, boron nitride and titanium nitride in the form of porous solids, in the form of a canvas or in the form of a coating on the surface of a glassy carbon foam or porous graphite. Aluminum nitride appears to be the most suitable material. It has been found that aluminum nitride can be appropriately formed into a porous structure by first producing a porous alumina structure and then impregnating with carbon followed by heating to 1750 ° C in an atmosphere nitrogen to convert the alumina to aluminum nitride. Such a process results in a porous solid structure which is chemically compatible with the corrosive salt medium and molten aluminum.

The composition of the molten bath

The electrolytic bath of the present invention can vary considerably in comparison to the salt composition of the hall characteristic cell. In the present invention, the composition of the bath can comprise any halide salt, in particular chlorides and fluorides are preferred. Any alkaline earth metal such as, in particular, sodium, potassium, lithium, calcium, magnesium, barium and the like can be used to form the halide salts. There are no critical compositions or desired proportion ranges required. It has been observed that

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aluminum salt does not need to be present initially in the electrolyte to produce aluminum under electrolysis using the composite anode. For example, a salt electrolyte containing only alkali and / or alkaline earth halides will produce aluminum metal at the cathode using the composite anode and without the anode effect.

It is generally preferred that the salt bath initially contains an aluminum halide, although this is not necessary for the application of the invention. In the case of baths containing A1C13 which may contain only chloride anions or both chloride and fluoride anions, the concentration of aluminum chloride can be from 2 to 60% but can also be in the range included between 1 and 95% by weight of A1C13. The all fluoride bath may include the same fluoride salts as previously indicated and may as well contain aluminum fluoride in any desired proportion.

Among the advantages and disadvantages of the different types of electrolyte, it can be noted that the all-chloride bath has a very low tolerance to contamination by oxides, but has a very high conductivity and is the least corrosive to refractory elements and components. the cell. In electro-lytes containing fluorides, aluminum is deposited in the form of droplets which agglomerate and pool easily, but the corrosivity of the electrolyte on refractory materials and components of the cell increases sharply.

A lithium component of any electrolyte will increase the conductivity but it is expensive and increases the cost of the electrolyte. It is therefore necessary in any operation to take into account and find a compromise between the cost of the electrolyte, the conductivity of the electrolyte and the resulting energy consumption for the production of aluminum.

The preferred electrolyte is a balance between the high cost of the salt components, the conductivity, the corrosivity to refractory materials and to the components of the cell, the tolerance to contamination by oxides and the agglomeration of the aluminum deposited in a basin. for easy recovery.

Example 1:

In flg. 1, the anodes are graphite plates and the cathode is a titanium diboride plate. The anodes were prepared with a coating 38 as illustrated in FIG. 2 which consisted of A1203 purified by the Bayer process calcined at 1000 ° C. and mixed in a proportion by weight of five parts of A1203 for one part of carbon in the coked phase. The carbon was obtained by mixing A1203 with a phenolic resin and gradually heating up to 1000 ° C in an inert atmosphere for the coking of the phenolic resin into carbon. The electrode coating was prepared by mixing A1203 and the phenolic resin by applying the mixture to the electrode with a trowel or otherwise, and heating to the coking temperature.

The electrolyte consisted of an equimolar mixture of sodium chloride and aluminum chloride forming the double salt NaAlCl4 at approximately 150 C. The cell temperature was raised to 700 ° C and the electrolysis of A1203 took place during several hours, which produced a layer of molten aluminum on the bottom of the cell. Examination of the anode revealed that the coating had dissolved and that the aluminum had deposited on the cathode. This aluminum deposition was equivalent to the aluminum content of A1203 dissolved at the anode. The general control reaction is assumed to be the ionization of A1203 in the anode with the carbon reacting to form essentially CO2. During electrolysis there was no evidence of chloride gas given off at the anodes and in the outlet tube. The outlet gas was analyzed and determined to be essentially CO2.

Example 2:

The salt composition of the electrolyte consisted of 63% NaCl, 17% LiCl, 10% LiF, 10% A1C13 and the electrode coating of fig. 2 was prepared from standard A1203 bauxite and petroleum bitumen which was coked to produce a 5.7 to 1 ratio of A1203 to carbon (coked). The electrolyte was led into the cell in fig. 1 at a temperature of 750 ° C. The spacing between the anode and the cathode was Vi in., Which produced an electrode current density of 15 A / in. 2 at an imposed voltage of 2.5 V. No chlorine gas was detected emerging from the anode. The aluminum settled. The harvested aluminum was produced with a faradic yield of 92% with an energy consumption of 3.67 kWh / lb.

Example 3:

The electrolyte salt composition consisted of 10% NaCl, 50% CaCl2, 20% CaF2, 20% A1C13. The coating of the electrode of fig. 2 was prepared as in Example 2, but only on one side of the electrode. The electrical connections were such that the anode adjacent to the outlet tube was connected to the positive terminal, and the negative terminal to the electrode furthest from the outlet tube. The coated sides of the electrodes 25 and 26 looked on the side opposite the outlet tube and toward the cathode. The electrode 24 and the cathode were not coated. As a result, the electrode 25 is not physically connected to the DC power supply. This electrode then becomes bipolar. The side coated with the mixture of A1203-C is thus positively charged. The side of the bipolar electrode 25 closest to the outlet tube becomes negatively charged on which the aluminum is deposited and flows into the melting tank. The aluminum is also deposited on the negatively charged electrode 24 and flows into the melting tank. The cell operating temperature was 800 ° C and the imposed voltage was 3 V with respect to each electrode or a total of 6 V on the terminals. This imposed voltage with an electrode spacing of 19 mm resulted in an electrode current density of 2 A / cm2.

Example 4:

The anode electrodes were composed of titanium diboride rods and the cathode electrode was also titanium diboride. The anodes were coated with bauxite as in fig. 2 which had been calcined at 600 ° C, mixed with phenolic resin, and coked at 800 ° C. The ratio of aluminum oxide in bauxite to carbon after coking was 6 to 1. The salt composition of electrolyte was 20% NaCl, 30% CaCl2, 10% CaF2, 4% NaF, 36% A1C13, and the operating temperature was 750 ° C with an electrode current density of 15 A / in2. This resulted in a voltage of 4 V between the electrodes for a spacing of% in. about. No chlorine gas was observed. The composition of the aluminum deposited in the melting tank was 97% pure aluminum, 0.5% Si, 1.5% Fe and 0.9% Ti with other minor constituents.

Example 5:

In the cell of fig. 3, a porous membrane was used of an aluminum nitride material having a thickness of 47 mm having a porosity of 50% with a pore size of the order of 12 to 24 [t. Aluminum nitride was obtained by impregnating a porous body of alumina with carbon and then heating to 1750 ° C in a nitrogen atmosphere. The anode conductor was a graphite rod and the aluminous anode material was a mixed powder of Bayer A1203 and carbon in a ratio of 6 to 1. The salt composition of the electrolyte was 20% NaCl , 25% LiCl, 30% LiF, 25% A1C13 and the electrolysis took place at 720 ° C. The spacing between the membrane and the aluminum basin was approximately 12 mm and the electrolysis was carried out with an anode current density of 2.35 A / cm 2. This gave a voltage of 2.8 V. The aluminum was produced with a yield of 92% and had a purity of 99.5%.

Example 6:

A salt composition consisting of 12% NaF, 25% LiF,

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28% of LaCl, 15% of LiCl, 10% of A1F3 and 10% of A1C13 was placed in fusion in a cell with straight side walls. A 50 mm thick aluminum block was melted on the bottom of the cell and the operating temperature was set at 700 ° C. Using an anode as shown in fig. 2A, a spacing between the bottom of the anode and the aluminum block was adjusted between 25 and 18 mm. With an anode current density of 1 A / cm2, the cell potential was 3.5 V.

After 8 h of electrolysis, the anode was removed from the cell and placed in a cell with side walls at 45 ° such as that illustrated in fig. 7. After a few hours of electrolysis, the anode had undergone an erosion such that its sides were parallel to the side walls of the cathode. The depth of immersion of the anode in the salt was 75 mm. Using the same total current as in the straight-walled cell, the potential was 2.45 V. This lower potential, due to the erosion of the lateral anode at 90% current efficiency and at a speed of constant production, reduces power consumption from 11.6 to 8.12 kWh / kg, which is a reduction of 3.48 kWh / kg.

Example 7;

A salt composition consisting of 20% NaCl, 25% LiCl, 25% LiF, 10% NaF, 10% A1F3, 10% A1C13 was melted in a cell with straight side walls. A 2 in aluminum block. thick was melted on the bottom of the cell, and the operating temperature adjusted to 700 ° C. Using a composite anode 30 cm long with a copper bus bar adjusted at one end in a conventional manner, the anode-cathode spacing was set to 18-25 mm With an anode current density of 1 A / cm2 the cell potential was 5.75 V once equilibrium was reached.

An identical anode, but with a slope of 45 ° on the end opposite to the busbar, was introduced into a cell with side walls at 45 ° like that illustrated in fig. 4. The depth of immersion of the anode in the salt was 75 mm. After a few hours of electrolysis, to ensure that the angle on the anode was the same as that of the side walls of the cell, the potential required to obtain the same total current as that which had been used in the cell to straight side walls was 4.4 V. This lower potential, due to erosion of the side anode at 90% of the current efficiency and a constant production speed between the two types of cells, reduces the consumption of power from 19 to 14.5 kWh / kg, which is a reduction of 4.5 kWh / kg.

The reduction in power consumption with a constant current between that used in the traditional cell where the bottom of the anode only undergoes erosion and that used in a tilted cathode cell is evident with this example.

Example 8:

An anode was made using rAl203 (Alcoa A-1) mixed with pitch and phenolic material and molded into a closed matrix by applying heat to cure the phe-nolic component. The content of the components was such that after coking the composite anode contained 17% carbon and 83% A1203. The electrolyte consisted of 20% NaCl, 20% LiCl, 30% LiF and 25% NaF, at an operating temperature of 700 ° C. A cell like the one illustrated in fig. 4 was used, but without adding an aluminum block to it. After several hours of electrolysis at approximately 700 ° C, the aluminum recovered in the well showing the composite anode will produce aluminum under electrolysis without the initial use of an aluminum salt in the electrolyte.

Example 9:

A cell with straight side walls such as that illustrated in FIG. 3 was used, but without the membrane 68 and the anode rod 52. Instead, a carbon cylinder was mounted just above the salt electrolyte layer into which a short section of anode was introduced such as than that illustrated in fig. 2A. A mixture of A1203, petroleum coke powder and a mixture of bitumen / pitch was added to the top of this short prebaked anode and around the conductor rods 37. The mixtures of A1203, coke powder and bitumen / pitches were such that they produced 18% carbon and 82% A1203 when coked at the salt electrolyte temperature of 740 ° C. A salt electrolyte consisting of 10% NaF, 20% LiF, 20% NaCl, 15% LiCl, 20% A1F3 and 10% A1C13 was used. The electrical connection on the conductive rods 37 was made with lugs in the cold region above the level of the Soderberg type anode composition in the carbon cylinder. The electrolysis took place with an anode current density of 10 A / in.2 with a spacing of 1 to Vi in. between the aluminum basin and the anode. The electrolysis continued with additions of A1203, carbon, bitumen / pitch in the carbon cylinder and the continuous supply of the anode which hardened and coked as it entered the salt electrolyte. The aluminum conductive rods melted as the anode was consumed and joined the cathode basin 32 of aluminum.

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The same experiment was made using a carbon rectangle, pre-baked rectangular blocks and aluminum foil as shown in fig. 10. The aluminum sheets were 1.5 mm thick and the precooked blocks were 50 mm thick. The electrical connection was made using rollers on each sheet of aluminum. As the anode advanced, additional precooked blocks were introduced between the aluminum sheets.

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Claims (15)

649102 1. Electrolysis cell for the electrolytic production of aluminum in a molten electrolyte, comprising at least one anode and a cathode, characterized in that the anode contains an oxygenated aluminum compound in sufficient quantity to produce l aluminum during electrolysis, a reducing agent mixed and in contact with the oxygenated aluminum compound and present in sufficient quantity to react at the anode, then to form metallic aluminum at the cathode, the weight ratio of the oxygenated compound expressed in A1203 to the reducing agent being at least 1.5, the anode comprising at least one internal conductor (36, 37, 52.108) of electrical conductivity greater than that of the mixture of reducing agent and of oxygenated compound of the aluminum and in contact, on the one hand, with a main electrical conductor and, on the other hand, with said mixture for conducting the electric current through said mixture, the internal conductor extending to the end of the anode at the interface between the anode and the electrolyte, and by means (34, 92) keeping the spacing between the anode and the cathode substantially constant while the anode is eroding while producing aluminum. 2. Cell according to claim 1, intended to operate at a temperature of the electrolyte higher than the melting temperature of aluminum, characterized in that the internal conductor of the anode is made of aluminum and is arranged in the mixture of so as to melt in the electrolyte at a speed substantially proportional to the speed at which the anode erodes. 2 3. Cell according to claim 1, characterized in that the mixture of reducing agent and oxygenated aluminum compound is in the form of at least two adjacent layers (94) and that the internal conductor comprises a metal sheet (108) disposed between the layers and forming with them a unitary structure. 4. Cell according to claim 1, characterized in that the means for keeping the spacing between the anode and the cathode substantially constant are arranged to collect the molten aluminum, produced at the cathode, in a compartment (92) so that the accumulation of molten aluminum does not modify said separation. 5. Cell according to claim 1, characterized in that the electrolyte is entirely composed of fluorides. 6. Cell according to claim 1, characterized in that the anode and the cathode have active surfaces (76, 79) which are oblique to the axis of the electrodes. 7. Cell according to claim 1, characterized in that the spacing between the anode and the cathode is less than 12.5 mm. 8. Cell according to claim 1, characterized in that the anode is placed in a porous membrane (60) containing said aluminum compound and said reducing agent. 9. Cell according to claim 8, characterized by supply means (54) capable of continuously filling the membrane with said compound and said reducing agent. 10. Cell according to claim 1, characterized in that said internal conductor is in the form of a central longitudinal core surrounded by said mixture. 11. Cell according to claim 1, characterized in that said internal conductor is in the form of a metal core (36) disposed longitudinally in the anode. 12. Cell according to claim 1, characterized in that said internal conductor comprises several conductor branches (37, 108) spaced from each other and extending through said mixture. 13. Use of the cell according to claim 1 for the electrolytic production of aluminum. 14. Use according to claim 13, characterized in that the temperature of the electrolyte is 10 to 150 ° C higher than the melting point of aluminum. 15. Use according to claim 13 of the cell according to claim 5, the reducing agent being capable of reacting by producing metal ions intended to be reduced to metal on the cathode.