EP0069681A1 - Cell for the electrolytic production of a metal from its halide - Google Patents

Abstract

Cell for the electrolytic production of a metal by electrolysis of its anhydrous halide in a bath of molten salts which comprises an outer shell of substantially parallelepiped shape having cooling means, inlet and outlet openings for liquid and gaseous fluids, as well that means for supplying electrical energy, envelope inside which there is, in its lower part, a receptacle zone for collecting the metal produced, in its central part at least one series of electrodes arranged in a stack, each stack comprising in the vertical direction and from top to bottom a current supply electrode, intermediate multipolar elements and a current output electrode defining regular interpolar spaces, and, in its upper part, a gas collection zone, said cell characterized by the fact that the multipolar elements are assembled in a vertical stack and that the interpolar spaces are sen so vertical. The cell according to the invention is particularly well suited to the production of aluminum by electrolysis of the corresponding chloride.

Description

The invention relates to a cell for the electrolytic production of metals by electrolysis, in a bath of molten salts, of their anhydrous halide, and, more particularly, of the electrolytic production of aluminum from the corresponding anhydrous chloride.

A person skilled in the art has long been inspired by the process of igneous electrolysis of alumina in a molten mixture of sodium and aluminum fluorides, in order to design devices intended for the igneous electrolysis of anhydrous aluminum chloride in a bath of molten salts.

The large number of documents published by the literature specializing in this field is, first of all, the consequence of certain observations by those skilled in the art on the advantages that such a process could have with respect to the Hall process -Héroult, such as, for example, that of an operation of the electrolysis at a lower temperature, of a reduced consumption of the electrodes by oxidation of the graphite which composes them, due to the oxygen released during the alumina electrolysis.

However, major drawbacks very quickly became apparent, pushing back the deadline for the industrial exploitation of such a process.

Indeed, a person skilled in the art has been confronted, as the French patent 2,158,238 expresses, with particularly troublesome phenomena since the most significant drawbacks arise from the presence of dissolved or not dissolved metal oxides, such as alumina, silica, titanium oxide, iron oxide in the electrolytic bath.

First of all, the undissolved metal oxides are at the origin of a gradual accumulation, on the graphite cathodes, of a viscous layer of finely divided solids, of liquid components of the bath and of droplets of molten aluminum, that hinder access to the cathodes of the electrolysis bath and which can lead to disturbances of the normal cathode mechanism, that is to say the reduction of the cations containing the metal to be produced at various degrees of oxidation. Thus, the aluminum chloride present in the viscous layer, then consumed by electrolysis, is increasingly difficult to renew, and, therefore, the other chlorides constituting the molten salt bath can be electrolysed, resulting in loss efficiency of the electrical energy used and pollution of the metal.

Then, because the chlorides constituting the bath of molten salts, such as alkali metal chlorides (e.g., sodium and / or po- _'tassium and / or lithium) and alkaline earth (e.g., magnesium and / or calcium and / or barium), are partially electrolyzed by lack of renewal of the aluminum chloride near the cathode, giving the corresponding metals which are inserted, under cathodic potential, in the graphite of the electrodes and cause their disintegration and their crumbling . This premature wear of the cathodes causes the introduction of carbon particles into the bath, which contribute to the formation of sludge at the cathode, and, moreover, cause a reduction in yield.

Finally, another drawback, also major, linked to the presence of metallic oxides dissolved in the bath, such as alumina, is that of the release of oxygen at the anode, which consumes carbon, consumption which disturbs the operation of electrolysis since it changes the geometrical characteristics of the anodes, in particular the anode-cathode distance.

Because the methods made available to it were at the origin of the aforementioned drawbacks, those skilled in the art focused their research efforts on equipment intended for the igneous electrolysis of anhydrous aluminum chloride in a bath of molten salts. , because, in addition to the aforementioned drawbacks, it was necessary to examine and resolve the obtaining of a high electroenergetic efficiency, for example, by means of a low voltage and a high current efficiency by limiting any reverse reaction of the chlorine-aluminum type.

Thus, a person skilled in the art has proposed the creation and use of cells with bipolar electrodes in order to obtain at least some of the desired improvements, and previously mentioned. Such cells have been made allowing the use of such electrodes, either in a horizontal position or in an inclined position, so that the metal formed on each cathode surface is deposited by gravity in the bottom of the cell and that the chlorine produced on each anodic surface moves in a direction opposite to that of the metal, that is to say migrates freely up the cell without making contact with the liquid metal.

A cell of the aforementioned type with bipolar electrodes is described in French patent 2,152,814 which comprises, stacked horizontally, and from top to bottom, an anode, at least one intermediate bipolar electrode and a cathode, superimposed and regularly spaced by means of insulating refractory spacers, thus creating substantially horizontal spaces between electrodes with the aim of carrying out the electrolysis of aluminum chloride in a bath of molten salts in each interpolar space, which leads to the release of chlorine on each anode surface and to the deposition of aluminum on each cathode surface.

To allow good circulation of the bath in each interpolar space and favor the entrainment of the metal formed outside these spaces, the chlorine produced plays the role of a pressure pump which, by suitable passages, draws the surface to the most light, and promotes decantation towards the bottom of the cell of the aluminum obtained.

For this purpose, each bipolar electrode is provided with an absolutely flat cathode surface and an anode surface hollowed out with transverse channels.

Thus, each anode surface has several of these channels which extend transversely to the lateral edge of each electrode on the side of the passage reserved for the return of the bath and for the gas flow. These channels are intended to remove chlorine, released from the interpolar space, from the aluminum deposited on the surface. cathodic to limit the rechlorination of the metal produced.

Another cell of the aforementioned type with bipolar electrodes is also described in French patent 2,301,443. Improvement to the cell described in French patent 2,152,814, this cell comprises from top to bottom, and placed horizontally, first an upper anode, then intermediate bipolar electrodes stacked one on top of the other spaced apart and held in place at equal distances by insulating refractory spacers creating regular, substantially horizontal inter-electrode spaces, each space being limited, above, by a lower electrode surface which acts as an anode surface and, below by an upper electrode surface which acts as a cathode surface, finally a lower cathode.

As in the aforementioned patent, the anddic surfaces can comprise transverse channels which favor the flow of chlorine out of the inter-electrode space towards a zone of gas rise formed in the middle part of the cell between the stacks d 'electrodes, this area widening from the bottom to the top of the cell. Thus, the existence of channels on the anode surface leading to a zone of rising gases, formed in the middle part of the cell between the stacks of electrodes, is intended to quickly remove the chlorine released from the interpolar space, but above all, to keep it away from the aluminum deposited on the cathode surface to limit its rechlorination.

Although such technologies can bring substantial and remarkable improvements in the field of aluminum chloride electrolysis, it is clear that the proposed devices still have drawbacks which are sufficiently significant to hinder their optimal industrial exploitation.

In addition to the fact of providing on the anode surface channels for evacuating the gases released to prevent their accumulation in the interpolar space, making it more particularly expensive to produce these types of electrodes on an industrial scale, such First of all, electrolysis cells are the site of numerous disturbances linked to the existence of stray bypass currents due to the excessive proximity of non-consecutive electrodes.

Such electrolysis cells are also the seat of poor thermal balances due to the disproportion between the energy power dissipated at the center of said cells and the radiating external surface.

Finally, such cells are the seat of prolonged maintenance of the aluminum droplets produced at a small distance from the anode with the appearance of a non-negligible risk of reoxidation of a fraction of the aluminum produced, this reoxidation disturbing the thermal balance due to its exothermic nature.

Aware of the interest that a new cell, well suited to the electrolysis of metal halides and, more specifically, of aluminum chloride in a bath of molten salts, can offer to those skilled in the art, but also aware of the drawbacks attached to the technologies described previously in this field, the applicant, continuing its research, has designed and developed an improved cell for the electrolysis of these halides practically free from the drawbacks previously listed.

According to the invention, the cell for the electrolytic production of a metal by electrolysis of its anhydrous halide in a bath of molten salts comprises an outer envelope of substantially parallel shape. piped system having cooling means, inlet and outlet openings for liquid and gaseous fluids, as well as means for supplying electrical energy, envelope inside which there is, in its lower part, a zone receptacle for collecting the metal produced, in its middle part, at least one series of electrodes arranged in stacks, each stack comprising in the vertical direction and from top to bottom a current supply electrode, intermediate multipolar elements and an electrode current output defining spaces between them regular interpolar, and, in its upper part, a gas collection zone, said cell being characterized in that the multipolar elements are assembled in a vertical stack and that the interpolar spaces are substantially vertical.

Intermediate multipolar elements are composed of stacked prismatic elements whose cross section generally has a shape resembling the letter Y.

Each prismatic multipolar element has an upper trough-shaped part playing the role of cathode surface, defined by the two upper branches of the Y, the walls of which have a constant thickness, and a lower part playing the role of anodic surface, ventral vertical edge. or oblique, defined by the lower branch of the Y, the thickness of which is at least equal to the thickness of the walls of the trough, but which is preferably equal to twice the thickness of each of the upper branches. Thus, by this means, the most homogeneous distribution of the current lines in the interpolar spaces is ensured.

The ends of each upper branch of the straight section of the trough, that is to say the Y-shaped section, can deviate from the axis of symmetry of the two upper branches, so as to avoid disturbances that could occur in this area between the multipole elements.

The thickness of the walls of the trough of each multipole element is generally between 10 and 100 millimeters, and preferably between 25 and 50 millimeters.

The bottom of the trough, formed by the upper branches of the Y-shaped section, can be provided with a longitudinal channel formed by a groove which promotes the collection and evacuation of the metal obtained during electrolysis.

The multipolar element is generally obtained by spinning a carbonaceous paste, followed by cooking, and finally, by graphitization according to well known methods.

In addition, the cathode part of the multipolar elements can be coated with a layer based on zirconium diboride or titanium diboride.

The height of each multipole element is generally at least equal to 200 millimeters and may preferably be between 300 and 500 millimeters. This height is neither limited nor critical with regard to the electrolysis operation. It is generally defined by the user for each particular case and does not have, by its structure, any limitation.

Likewise, the length of each multipole element is defined by the dimensional characteristics of the cell itself.

To constitute the electrodes implanted in the cell according to the invention, the prismatic multipole elements are stacked one above the other, and wedged together by means of insulating refractory pieces which are resistant to the aggressive action of the medium. The upper element, brought from the current, is constituted by a prismatic piece preferably deprived of a trough, the cross section of which can be cruciform, in T or in I, or even practically formed of the only lower branch of the section in Y. The lower element, current output, is a prismatic piece whose cross section is close to the letter H, the letter M or the letter N.

The various prismatic elements can be stacked horizontally or even very slightly inclined depending on the slope of the element resting on the bottom of the cell. In this last case; the flow of liquid metal is then favored.

According to a particularly interesting variant, it is possible to provide a longitudinal offset on each stack of the multipole elements, so that the liquid metal threads escaping from the troughs of the superimposed elements cannot be in contact, thus preventing the creation of short -circuits between the various elements of the same stack.

According to another variant, which can be combined with the previous one, the lower end of the ventral edge of the multipolar element can be provided with a device for guiding the liquid metal net, for example of the "pouring spout" type. best channeling its flow.

The stacking of the multipolar elements makes it possible, by the interposition of the wedging pieces, to ensure a regular distance between the elements and to create homogeneous interpolar zones ensuring good recirculation of the electrolysis bath.

The lower element is immersed in at least one liquid metal net in contact with the current outlet device.

Likewise, the upper element is connected to the electrical conductors by means of known means such as, for example, the sealing of graphite parts, of copper or steel bars.

In the case where the supply of electric current to the anode consists of hollow cylindrical pieces of graphite, these can act as discharge orifices for the gaseous effluents produced during electrolysis.

Thus, as soon as the multipole elements are regularly stacked, several of these stacks are placed parallel to each other and connected to the aforementioned electrical energy sources. Therefore, the liquid metal located in the receptacle area of the lower part of the cell, can be used as equipotential to l ° ensem ~ ble stacks placed in parallel.

The adjacent stacks thus created are regularly positioned, both with respect to each other and with respect to the walls of the cell, thanks to the pieces of wedging shapes, and, possibly, to other shaped pieces of refractory, insulating materials. , as well as through horizontal or sloping grooves made in the bottom of the cell.

The electrolysis bath, previously enriched in metallic chloride, then purified, is introduced into the cell through orifices located in its lower part, while the excess bath depleted by the electrolysis operation is evacuated by overflow in its upper part or by siphon.

The recirculation of the bath in the interpolar spaces is ensured by the mechanical drive caused by the release of chlorine gas, mainly along the side walls.

• La figure 1 est une vue en élévation, en coupe et à l'extrémité, d'une cellule d'électrolyse selon l'invention.
• La figure 2 est une coupe horizontale de la cellule d'électrolyse, permettant de voir la disposition des empilements d'électrodes.
• La figure 3 est une coupe d'un empilement d'électrodes.
• La figure 4 est une coupe agrandie d'un élément intermédiaire.
• La figure 5 est une vue en perspective déchiquetée de l'intérieur de la cellule selon l'invention.

The invention will be better understood thanks to the numerical description of the figures illustrating the invention:

• Figure 1 is an elevational view, in section and at the end, of an electrolysis cell according to the invention.
• Figure 2 is a horizontal section of the electrolysis cell, to see the arrangement of the electrode stacks.
• Figure 3 is a section through a stack of electrodes.
• Figure 4 is an enlarged section of an intermediate element.
• Figure 5 is a jagged perspective view of the interior of the cell according to the invention.

According to Figure 1, the electrolysis cell of anhydrous metal chloride in molten salt baths, comprises a casing (1) of refractory steel; provided with cooling fins (2) and internally lined with a coating (3) resistant to the action of chlorine and the bath of molten salts, such as, for example, silicon nitride, silicon oxy-nitride, carbonitride silicon, or boron nitride. A cover (4) provided with a rim (5) and closing the cell in its upper part thanks to the presence of a sealed means (6), is provided with orifices allowing the passage of the current leads (7), the introduction of the bath enriched in metallic chloride (8), the evacuation of the chloride-depleted bath (9), the evacuation of the liquid metal (10), as well as other orifices (11) intended for evacuation of gaseous effluents.

The internal surface of the cover (4), which is directly exposed to the aggressive vapors of the molten salt bath and to the gaseous effluents resulting from the electrolysis, is made of a suitable resistant material such as alloys containing nickel, chromium, iron, copper, molybdenum, and, optionally, coated with protective ceramics and / or provided with cooling means.

The interior of the electrolysis cell comprises, in its lower part, a zone (12) collecting the liquid metal produced, in its middle part, an electrolysis zone (13) filled with the bath of molten salts enriched in metal chloride , finally, in its upper part, a zone (14) making it possible to collect the gaseous effluents with a view to their evacuation by (11).

The various aforementioned holes, necessary for the proper functioning of the cell and located on the cover (4) of said cell, provide, for each, a particular function. A first orifice (10) extending through the cover in the upper (14), middle (13) and lower (12) zones allows the insertion of a liquid metal evacuation tube. Another opening (8) constitutes the means of entry of the bath enriched in metallic chloride, while the opening (9) is used for the evacuation of the depleted bath, and the opening (11) constitutes a means of exit of the effluents gaseous.

Inside the cell of the electrolysis cell according to the invention, vertical stacks (15) of electrodes are placed in parallel and at equal distance. Each stack (15) comprises a current supply electrode (16) provided with a supply bar (17) embedded in said electrode and connected to the current supply (7) passing through the cover (4), multipolar intermediate elements (18), a current output electrode (19) fitting, by grooves (20) in the bottom (21) of the tank, in which current output bars (22) can be embedded .

The intermediate multipole elements (18) create between them interpolar spaces (23), regular and substantially vertical.

According to Figure 2, which is a horizontal section of the cell according to the invention, said cell comprises the casing (1) of refractory steel, provided with cooling fins (2), internally lined with the resistant coating (3) to the action of the bath of molten salts and of chlorine, as well as the orifices (7) for the current leads, (8) for the introduction of the bath enriched in metallic chloride, (9) for the evacuation of the bath depleted by electrolysis in this chloride, (10) for the evacuation of the liquid metal and (11) for the exit of the gaseous effluents.

Said cell also comprises ten vertical stacks (15) comprising the aforementioned multipolar electrodes.

According to Figures 3 and 4, sectional views of a stack of electrodes, said stack consists of a current supply electrode (16), intermediate multipolar elements (18) and an output electrode current (19).

The current supply electrode (16), consisting of a prismatic piece of graphite, the cross section of which is formed from the lower branch of the letter Y, is also provided with a current supply bar (17 ) embedded in the mass and connected to the power supply (7) not shown. '

The intermediate multipole elements (18), also consisting of a prismatic piece of graphite, offer a cross section recalling the letter Y, and whose plane of symmetry is vertical.

Each intermediate multipole element (18) has a trough-shaped upper part (24) defined by the two upper branches (25) and (26) of the Y, and a lower part (27) called the ventral edge, defined by the lower branch of the Y whose thickness is at least equal to that of the walls (25) and (26). The bottom of the trough (24) is provided with a longitudinal channel (28), constituted by a groove favoring the collection and the evacuation of the metal obtained by electrolysis.

The current output electrode (19) is also a prismatic part, the cross section of which recalls the shape of the letter H, the lower branches (29) and (30) of which are nested in the grooves (20) of the sole ( 21), in which is embedded the current outlet bar (22).

Thus, the various prismatic elements constituting the stack (15) provide; thanks to the interposition of insulating refractory wedges (31), a regular distance between these elements by creating interpolar zones (23) also called interpolar spaces, ensuring good recirculation of the electrolysis bath, favorable recovery of the metal molten, and excellent evacuation of gaseous effluents between the walls (25), (26) of the trough and (27) of the ventral edge.

Thanks to this new technology, the multipolar elements are assembled in a vertical stack and vertical interpolar spaces, thus preventing the encounter between the molten metal flowing towards the bottom of the tank and the gaseous effluents migrating towards the upper part of said cell.

According to FIG. 5, a jagged perspective view of the interior of the cell according to the invention, the stacks (15) of electrodes are placed in parallel and at equal distance, as has already been expressed. Each stack comprises an electrode (16) for supplying the current then intermediate multipolar elements (18) and an electrode (19) which allows the current to be output.

The current supply electrode (16), consisting of a prismatic graphite piece, is provided with a current supply bar (17) connected to the current supply (not shown).

Each intermediate multipole element (18), formed of a prismatic piece of graphite, has an upper part (24) in the shape of a trough, defined by the walls (25) and (26) and a lower part (27), edge ventral. The bottom of the trough (24) is provided with a longitudinal channel (28) formed by a groove promoting collection and evacuation of the metal obtained during the electrolysis of metal chloride.

The electrode allowing the current output (19), consisting of a prismatic piece of graphite, has two lower walls (29) and (30) which fit into inclined grooves (20) of the sole (21) of the cell.

The electrode (19) is connected to the output terminal (34) of the current via the liquid metal located in the collector (35) at the bottom of the tank, in which the lower ends of the evacuation tube are immersed. (10) of the molten metal and of the current output terminal (34), protected by their sheath (36) and (37) in insulating refractories.

The various prismatic elements constituting a stack are kept at regular distance from each other by the interposition of wedging pieces (31) in insulating refractory, creating the interpolar spaces (23).

The various prismatic elements (16), (18) and (19) have a slight slope promoting the flow of the metal through the longitudinal channels (28).

In addition, these various prismatic elements (16), (18) and (19) of a stack are offset longitudinally with respect to each other as shown by the intermediate elements (38), (39) and (40) for example , so that the liquid metal threads escaping from the trough (24) of each of the prismatic elements via the longitudinal channel (28) cannot be in contact with each other, thus preventing the creation of short circuits between the various prismatic elements of the same stack.

Likewise, the ventral edge (27) is provided with a device (33) for guiding the thread of liquid metal, of the pouring spout type, best channeling the flow of said metal.

The bath of molten salts has not been shown in the case of FIG. 5, to allow a better perception and understanding of the internal structure of the cell according to the invention.

The level of the electrolysis bath in said cell can vary during the operation, but must bathe all the interpolar spaces.

During the electrolysis of metal chloride, in a bath of molten salts, a preferential passage for the rise of gaseous effluents is established in the interpolar spaces (23) delimited by the upper walls (25) and (26) of an intermediate prismatic element. and the ventral edge (27) of another intermediate prismatic element fitted in the previous one. The passage, thus reserved for the rise of the gaseous effluents on either side of each intermediate element forming a stack, allows the circulation of the bath of molten salts in the interpolar spaces, the flow of said bath being created by the effect of rise of gaseous effluents produced by electrolysis in the interpolar spaces (23).

Thus, when the gaseous effluents leave each interpolar space (23), they emerge and collect in an inter-stacking space (42) and flow in the desired direction, that is to say from the bottom up. of the cell and exit therefrom via the orifice (11) passing through the cover (4).

During this same electrolysis of metal chloride in a bath of molten salts, the molten metal placed on the cathode surfaces flows into the trough (24) of each interpolar space (23) by the longitudinal evacuation channel (28), falls in the liquid metal zone (12) and is collected in the liquid metal collector (35) from which the metal is discharged via the sump (10).

The current output can also be done through the output terminal (34) which plunges into the liquid metal located in the collector (35).

Consequently, thanks to the interpolar spaces (23), substantially vertical, which guide the rise of the gaseous effluents, and thanks to the longitudinal channels (28) for evacuation of liquid metal lying at the bottom. trough (24), at the slight slope of the intermediate multipole elements (18) and at the offset of each multipole element, such as those illustrated by (38), (39) and (40), which conduct the liquid metal of each multipolar intermediate element towards the bottom of the cell separating the path of the gaseous effluents and of the liquid metal, on the one hand, a rechlorination of the electrolysed metal by means of the gaseous effluents cannot occur and, short circuits between the intermediate multipole elements.

Finally, the level of the bath of molten salts inside the cell being kept practically constant, the bath spent in electrolysed metal chloride is discharged through the orifice (9), while the bath enriched in metal chloride to be electrolyzed is introduced. via the power supply (8) (not visible).

Example 1 (according to Figures 1 and 5):

An anhydrous aluminum chloride electrolysis cell was produced according to the invention, consisting of a casing (1) of refractory steel, provided with cooling fins (2) and internally lined with a coating (3). resistant to the action of chlorine and molten salt baths based on alkaline chloroaluminates. This coating consisted of a stack of bricks made of silicon carbonitrides with crossed joints, maintained by a grout based on silicon nitride.

Inside the cell were two vertical stacks (15) constituting five interpolar spaces, made with intermediate elements of section having substantially the shape of the letter Y, a height of 35 centimeters, a length of 50 centimeters, and the greatest width of which was 14 centimeters.

These multipolar elements were made of graphite and their upper branches (25) and (26) were 3 cm thick, while the lower branch (27), called the ventral edge, was 6 cm thick.

A very slight slope (5% relative to the horizontal) was maintained between each multipolar intermediate element (18) of each stack (15) in order to accelerate the evacuation of the effluents from the interpolar space (23).

The separation between each intermediate multipolar element was ensured by shims made of silicon nitride, a material resistant to corrosion of the medium, thus ensuring a distance between each multipolar intermediate element equal to 1 cm in the substantially vertical part.

The bottom of the trough (24), a little further from the ventral edge (27), defined by the walls (25) and (26) was provided with a longitudinal channel (28) 2 cm wide and 3 centimeters high.

The current supply electrode (16) was itself connected to the electrical supply circuit via a current supply bar (17). The electrode allowing the current output (19) was in contact with the liquid metal. The current outlet was effected by a steel bar sealed in the carbon sole.

The aluminum chloride electrolysis bath was composed, at the inlet of the tank, of 18.8% LiCl, 28.2% NaCl and 53% AlCl ₃ by weight.

The bath was maintained at the temperature of 720 ° C ± 10 ° C. The supply of AlCl _{3 was} effected by the supply orifice (8), while the depleted bath was evacuated by overflow using the orifice (9).

The feed rate in the enriched bath was 62 kg / h and was adjusted by measuring the conductivity of the bath using a ductimetric cell and a level detector (not shown).

The operating conditions of said cell were as follows:

The aluminum produced was extracted by suction inside the sump (10) formed in an insulating refractory tube.

The chlorine was discharged with the other gaseous effluents via the tube (11).

Thus, the Applicant has observed regular outflows of chlorine and aluminum without significant phenomena of metal chlorination or short-circuiting between the intermediate multipolar elements well known to those skilled in the art.

Example 2:

An aluminum chloride electrolysis cell according to the invention was produced, comprising, as in Example 1, the same multipolar intermediate elements, but of which the cathode part (internal wall of the trough) had been covered with '' a mixture composed of 60% by weight of zirconium diboride and 40% by weight of high temperature coal tar, then calcined at 1200 ° C.

Inside the cell, five pairs of adjacent stacks comprising five interpolar spaces were placed at a distance of 5 centimeters, the current supply electrode of each stack being linked by a graphite equipotential. The stacks were symmetrical with respect to the collecting channel.

et était maintenu à la température de 720°C - 10°C. Le débit d'alimentation en bain enrichi était de 248 kg/h. Son introduction était réglée en fonction de la réponse d'une cellule conduc- timétrique et d'un détecteur de niveau (non représenté).The aluminum chloride electrolysis bath had the following composition by weight, at the entrance to the cell:

and was maintained at the temperature of 720 ° C - 10 ° C. The feed rate in the enriched bath was 248 kg / h. Its introduction was regulated according to the response of a conduction cell and a level detector (not shown).

The operating conditions of said cell were as follows:

In the case of this example, the current output was carried out using a steel bar sealed in the sole.

Thus, it was possible to observe, compared with example 1, a gain of 1100 millivolts at the terminals of the tank. This improvement was the consequence, on the one hand, of a significant reduction in the current density, a phenomenon well known to those skilled in the art and, on the other hand, of the efficiency due to a weaker rediffusion towards the he anode of the aluminum produced, thanks to the coating based on zirconium boride.

Example 3:

An aluminum chloride electrolysis cell was produced according to the invention, retaining the same type of stack as in Example 2, but the intermediate cathode elements and the current output electrodes were coated with diboride. titanium.

The aluminum chloride electrolysis bath had the same composition as above and was maintained at the temperature of 720 ° C. ± 10 ° C. The feed rate in the enriched bath was 248 kg / h and was adjusted from measurement of the conductivity of the bath and of a level detector.

The operating conditions of said cell were as follows:

Thus, it has been observed, compared with example 1, a reduction in the cathodic drop linked to the presence of the titanium diboride coating.

The substitution of the current output steel bars, embedded in the hearth and emerging from the bottom of the cell, of the graphite terminal (34) covered with titanium diboride, led to a slight increase in the voltage drop across the terminals of the cell, but allowed a better sealing of the cell and a reduction of the risks of infiltration.

Claims (19)

1. Cell for the electrolytic production of metal by electrolysis of its halide in a bath of molten salts, which comprises an external envelope of substantially parallelepipedal shape, having means of cooling, inlet and outlet openings for liquid and gaseous fluids, as well as means for supplying electrical energy, envelope inside which there is, in its lower part, a receptacle zone for collecting the metal produced, in its middle part, at least one series of electrodes arranged in a stack , each stack comprising in the vertical direction and from top to bottom, a current supply electrode, intermediate multipolar elements and a current output electrode defining regular interpolar spaces and, in its upper part, a zone for collecting the gas, characterized in that the multipole elements are assembled in a vertical stack and that the interpolar spaces are substantially vertical. 2. Electrolytic production cell according to claim l, characterized in that the intermediate multipole elements are composed of prismatic carbon elements. 3. Electrolytic production cell according to claims 1 and 2, characterized in that the intermediate multipolar elements comprise an upper part in the form of a trough and a lower part constituted by a ventral edge, the cross section of said elements having a shape recalling the letter Y. 4. electrolytic production cell according to claim 3, characterized in that the upper trough-shaped part, defined by the two upper branches of the Y, has a constant wall thickness while the lower part, constituted by the edge ventral, has a wall thickness at least equal to that of the trough, but preferably equal to twice that of the trough. 5. Electrolytic production cell according to claim 3, characterized in that the upper ends of the two branches of the section in Y deviate from the axis of these two branches by widening. 6. electrolytic production cell according to claim 4, characterized in that the thickness of the walls of the trough is between 10 and 100 millimeters, and preferably between 25 and 50 millimeters. 7. electrolytic production cell according to claim 3, characterized in that the height of each multipole element is at least equal to 200 millimeters and is preferably between 300 and 500 millimeters. 8. Electrolytic production cell according to claim 3, characterized in that the bottom of the trough formed by the upper branches of the Y-shaped section is provided with a longitudinal groove promoting the collection of the metal. 9. Electrolytic production cell according to claim 3, characterized in that the end of the ventral edge of the multipolar intermediate element is provided with a device for guiding the thread of liquid metal. 10. Electrolytic production cell according to claim 1, characterized in that the vertical stacking of the intermediate multipole elements comprises an upper element for supplying the current which is constituted by a prismatic carbon part of cruciform cross section, in T or in I. 11. electrolytic production cell according to claim 1, characterized in that the vertical stacking of the intermediate multipolar elements comprises a lower current output element which is constituted by a prismatic carbon part of cross section recalling the shape of the H, the M or N. 12. Electrolytic production cell according to claim 1, characterized in that the intermediate multipole elements are stacked regularly by interposition between two elements, shims in insulating refractory. 13. electrolytic production cell according to claim 1, characterized in that the stacked intermediate multipole elements are horizontal. 14. electrolytic production cell according to claim 1, characterized in that the stacked intermediate multipole elements are inclined relative to the horizontal plane. 15. Electrolytic production cell according to claim 1, characterized in that the stacked intermediate multipole elements are offset longitudinally with respect to each other to avoid the creation of short circuits between the various elements of the same stack by flow of the metal. 16. Electrolytic production cell according to claim 1, characterized in that the cathodic surface of each graphite multipole element is covered with zirconium diboride. 17. Electrolytic production cell according to claim 1, characterized in that the cathode surface of each multipolar graphite element is covered with titanium diboride. 18. Electrolytic production cell according to claim I, characterized in that the general current output is produced by a bar, made of steel, copper or graphite, sealed to the conductive floor of the cell. 19. Electrolytic production cell according to claim 1, characterized in that the general output of the current is carried out by means of a vertical terminal, isolated from the electrolysis bath immersed in the sheet of liquid metal.

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