EP0156744A1 - Process for winning a metal by electrolysis of molten halogenides whereby a simultaneous and continuous double disposition is taking place, and apparatus therefor - Google Patents

Abstract

This invention relates to an electrolysis process and cell for producing a metal by electrolyzing halides in a molten salt bath. It consists of using one cathode in the form of a basket, simultaneously passing a current I1 between this cathode and the anode in such a manner as to carry out a first deposit of crude metal in the basket and a current I2 between the said basket and the complimentary cathodes for depositing the final metal thereon. The cell consists of a central anodic system, a tubular cathodic basket surrounding the anode and a series of cathodes surrounding this basket, a positioning which can be reversed in relation to the basket. The invention finds its application in the continuous production of good quality refractory metal with a suitable material and electric yield.

Description

The present invention relates to a process for obtaining a metal by electrolysis of halides in a bath of molten salts comprising a simultaneous and continuous double deposition and to devices for applying said process.

Among the metals concerned, there are all those which have a melting point above 1000 ° C. and, preferably, several valence states such as in particular titanium, zirconium, hafnium, thorium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, uranium, plutonium as well as rare earth metals.

Those skilled in the art know that a metal can be deposited by introducing one of its derivatives such as a halide, for example, into a bath of molten salts and by subjecting it in its simplest principle. by the action of two electrodes connected to the poles of a direct current source: at the anode the halogen is released and at the cathode the metal is deposited. This so-called igneous electrolysis technique has been the subject of numerous studies which have led to the design of various processes which are distinguished between them by the composition of the bath, the physical and chemical state of the halide, the modulation of the applied current and the production of multiple devices in terms of their structure and shape, in particular at the level of the electrodes, halide injection systems and recovery of the deposited metal.

There is, however, one point common to all of these cells, it is the presence of a porous diaphragm which separates the anode from the cathode and divides the bath into two distinct volumes: the anolyte and the catholyte. This diaphragm has the particular effect of preventing the halogen released at the anode from reoxidizing the reduced halides dissolved in the electrolyte when the metal has several valences.

This diaphragm can be made of ceramic or by depositing a refractory metal on a suitably polarized metal mesh.

The present invention uses a metal cathode in the shape of a basket which, while being able to serve as a diaphragm, serves mainly as the first receiver of the metal to be deposited. British patents 781,311 and US 3,282,822 describe methods which could be compared to our invention.

The first of them describes a cell of parallelepiped shape containing a bath of chlorides; it is divided by a vertical partition into two compartments which communicate with each other by a space located between the bottom of the partition and the bottom of the cell. In one of the compartments, there is a metal log constituting the anode and in the other a cathode assembly, in perforated sheet, composed of a tubular cathode and a concentric cathode basket supplied with the halide of the metal to be deposited. According to the inventor, when this cell operates with gaseous titanium tetrachloride, the distribution of current on the cathode surfaces is such that dichloride is formed which is rapidly reduced to metal, not only near the peripheral walls of the basket, but also near the peripheral walls of the central tube, while trichloride is deposited between these two regions. As the trichloride is reduced less quickly in metal than the dichloride, this results in the maintenance of a large difference in concentration of reduced chlorides within the bath, which has the effect of giving metallic crystals of relatively large size on the walls. basket cathodics. These crystals are then recovered, after stopping the cell, by removing the basket from the bath and by peeling them from the wall. The basket is immersed again to undertake a new electrolysis operation.

USP 3,282,822 first recalls the difficulties encountered with processes such as that which has just been described, in particular with regard to the formation of both very fine and therefore highly oxidizable crystals and of clumps of crystals larger, but spongy and trapping large amounts of bath between them. To counter this, it teaches the need for better control of current densities and uniformity of current on the surface of the electrodes. He then recommends as a means a cathode in the shape of a cubic box having perforated side walls, a top wall and a solid bottom. In the center of the upper wall of this box emerges a tubing for supplying tetrachloride of titanium and inside hang vertical cathode bars. Around this box are symmetrically distributed in the bath other vertical bars acting as anodes. A vent placed on the cell cover allows the chlorine produced by electrolysis to be discharged to the outside.

This cell operates according to two chloride supply regimes. In a first stage, it is quickly fed so as to have a mole TiC1 ₄ for 10 to 20 Faradays. This results in the formation on the inner walls of the box of fine titanium crystals which develop to form a more or less spongy deposit. Then, the quantity of TiCl ₄ is increased so as to have one mole of TiCl ₄ for 4.5 to 6.5 Faradays. Under these conditions, a concentration of TiCl ₂ occurs and titanium metal is preferably deposited on the cathode bars in the form of branched crystals. When the box is full of metal, the supply of TiCl ₄ is interrupted, the current prolonged for a few moments to exhaust the soluble titanium, then the cathode box is taken out of the bath and cooled away from the atmosphere. The metal is then separated from the box, then washed with a dilute acid solution and dried. The product obtained contains between 50 and 80% of metal having a Brinell hardness of around 120.

• - son fonctionnement est discontinu du fait de la nécessité pour chaque opération de production de métal, d'utiliser successivement deux régimes de fonctionnement différents et de sortir le panier du bain pour récupérer le métal ;
• - lorsqu'on sort le bac cathodique du bain, il y a décrochage de fines particules de titane. Ces fines s'accumulent sous forme de boue au fond de la cellule et diminuent le rendement métal. De plus, du fait de leur bonne conductibilité électrique, elles provoquent le by passage d'une partie du courant destiné à l'électrolyse, réduisant ainsi le rendement électrique de la cellule. C'est pourquoi, il s'avère nécessaire de procéder périodiquement à l'enlèvement de ces boues. Cet enlèvement se fait soit de façon mécanique, opération très insalubre, soit par introduction de chlore comme dans l'USP 3 607 011 (qui se réfère d'ailleurs à l'_USP 3 282 822 cité plus haut), de manière à transformer le métal en TiCl₄, halogénure qui s'échappe alors du bain par vaporisation et que l'on condense en vue d'un recyclage ultérieur ;
• - le produit obtenu est hétérogène puisque le dépôt initial de métal sur les parois de la boite, au moment où la quantité de courant admise est grande, est de nature spongieuse, alors que le dépôt ultérieur sur les barres cathodiques, à quantité de courant réduite,est à plus gros cristaux ;
• - du fait de la présence de produits spongieux, le contenu du panier retient une quantité notable de bain dont il faut se débarrasser par un traitement en milieu acide ;
• - il n'est pas facile de saisir le moment où le pouvoir d'assimilation de cette cellule diminue, de sorte que du TiC12 et du TiCl₃ apparaissent dans l'anolyte et se recombinent avec le chlore libéré à l'anode, ce qui diminue le rendement matière de l'opération.

If this cell brings progress compared to the previous one, as the inventor claims, it nevertheless has the following disadvantages:

• - Its operation is discontinuous due to the need for each metal production operation, to successively use two different operating regimes and to remove the basket from the bath to recover the metal;
• - when the cathode bin is removed from the bath, there is a drop of fine titanium particles. These fines accumulate in the form of mud at the bottom of the cell and reduce the metal yield. In addition, due to their good electrical conductivity, they cause the bypassing of part of the current intended for electrolysis, thus reducing the electrical efficiency of the cell. This is why it is necessary to periodically remove this sludge. This removal is done either mechanically, a very unsanitary operation, or by the introduction of chlorine as in USP 3,607,011 (which also refers to _U SP 3 282 822 cited above), so as to transform the metal into TiCl ₄ , a halide which then escapes from the bath by vaporization and which is condensed for subsequent recycling;
• - the product obtained is heterogeneous since the initial deposit of metal on the walls of the box, at the moment when the quantity of current admitted is large, is spongy in nature, while the subsequent deposit on the cathode bars, at reduced quantity of current , has larger crystals;
• - due to the presence of spongy products, the contents of the basket retain a significant amount of bath which must be disposed of by treatment in an acid medium;
• - it is not easy to grasp the moment when the assimilation power of this cell decreases, so that TiC12 and TiCl ₃ appear in the anolyte and recombine with the chlorine released at the anode, which decreases the material yield of the operation.

This is why, the object of the invention is to propose a process in which, on the one hand, the halides are fed and reduced continuously and simultaneously while constantly maintaining the basket in the bath so as to avoid the discontinuity in walking and sludge formation; on the other hand, an essentially crystallized, homogeneous metal is obtained, which does not trap a bath; all of which contribute to improving both production capacity, material and electrical yields, working conditions and the quality of the products produced.

The invention essentially consists of a process for obtaining a metal by electrolysis of halides in a molten halide bath comprising a simultaneous and continuous double deposit in which a first deposit is formed by circulating a direct electric current II from an anode assembly to a basket-shaped deposition cathode, both being immersed in the bath, and the basket is supplied with a halide of the metal to be deposited. This process is characterized in that a current 1 ₂ is circulated simultaneously from the basket to at least one complementary cathode so as to form on it a second deposit, which is periodically extracted from the bath said complementary cathode to recover the metal obtained and simultaneously replaces it with another cathode.

Thus, unlike the process of the prior art, one can accept to deposit under the influence of the current I ₁ the metal in a more or less heterogeneous form. Then, under the action of the current I ₂ , this metal is ionized and transported to the complementary cathode (s) where it forms an essentially homogeneous crystalline deposit, having no porosity and practically retaining no bath. It then suffices to remove the cathode from the bath and allow it to cool in the absence of air to easily recover a well crystallized metal, capable, without intermediate treatment, of being agglomerated by fusion. This process therefore has a considerable advantage over that of USP 3,282,822 in which a metal was obtained distributed both on the perforated walls of a box and on the internal cathode bars whose recovery proved to be difficult and whose the quality differed depending on where it was taken; and this, due to the formation of two kinds of deposits corresponding to the two successive halide supply regimes, not to mention the presence of unreduced halides.

In the method according to the invention, it is possible to act easily and at any time on the ratio between the currents Il and I ₂ , that is to say increase the quantity of metal deposited in the basket by reducing the transfer to the cathode or instead decrease the amount of metal deposited in the basket by increasing the transfer to the cathode. If, in regime, one arrives at a certain equilibrium and therefore at a constant volume in time of metal deposited in the basket, it can happen that due to irregularities in the flow of halides or other disturbances, a beginning of clogging of the basket walls occurs; in this case, it suffices to increase I ₂ to dissolve the troublesome deposits and possibly the sludge, and restore correct operation. At the limit, it is possible for a given time to interrupt either the current Il or the current I ₂ so that the ratio Il / I ₂ can vary from 0 to infinity and this without any repercussions on the quality of the metal produced. This constitutes a notable advantage over the process of the prior art where in the event of clogging, there was no other resource than to stop the electrolysis and to remove the basket from the bath to recover only a highly polluted metal. by halides which could not be reduced.

The process as described above corresponds to the running conditions at cruising speed. But, it is advisable, when starting the operation, to follow a particular process consisting first of all in building up a metal reserve in the basket. For this, only a current Il is circulated between the anode and the basket, while several successive charges of halides are introduced into the cell. When the quantity of electricity delivered corresponds to the filling of approximately 1/2 to 2/3 of the basket volume with metal, the halide of the basket is introduced into the compartment delimited by the interior walls of the cell and the exterior walls of the basket. metal to be deposited which is reduced to valence 0 at the cathode in a single step, so as to have a combined metal content of between 1 and 7% by weight; this is to avoid the formation of powdery products. At this time, a current Il and a current I ₂ are passed simultaneously while continuing to supply the basket with halide. The cell thus acquires its cruising speed. It then only remains to regularly remove the cathode (s) from the bath to recover the metal and to replace them with blank cathodes. In the case of n cathodes, the procedure is as follows: a single cathode having been placed in the bath during start-up, the n-1 others are successively immersed at regularly spaced times so that at the time of immersion of the n ^th , the first is covered by the quantity of refined metal desired.

The above description describes a halide only feed. However, the process also accommodates a mixture of halide and bath, the latter circulating between the electrodes and being continuously withdrawn and recycled after recharging with halide the metal which one wishes to deposit.

The process according to the invention allows the supply of halide in a form which is not only liquid but also solid, which is different from the process of USP 3,282,822 which only admitted in the case of titanium tetrachloride and preferably gas. This enlargement to the three physical states of matter results from the production of a deposit in two stages which frees the process from the quality constraints of the metal present in the basket since the latter constitutes only a transitory phase of its obtaining.

The process is applicable to most halides of type M (halogen) _x , where M is a metal such as titanium, zirconium, hafnium, thorium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, uranium, plutonium, rare earth metals and X one of the valences of the metal considered. It can also be applied to metals which are difficult to obtain directly in a correct crystalline state.

Thus, in the case of titanium, all the halides of the TiCl _x type in which X has a value equal to or between 2 and 4, can be used. Thus, unlike the USP mentioned above, it is possible to use, in addition to TiCl ₄ , TiCl ₃ or TiCl ₂ or a mixture of the latter or a partially reduced product in which, for example, X is close to 2.3.

Subchlorides have the advantage over TiCl ₄ of being soluble in the bath of molten salts and therefore of giving a more homogeneous bath. They also allow better control of the quantity of halides introduced and therefore of being able to limit the content of combined titanium in the basket and thus reduce the diffusion of titanium ions towards the anode and, consequently, the losses. due to anodic reoxidation.

The possibility of using subchlorides therefore brings significant progress to the prior art, which could only be envisaged insofar as one was not forced to impose on the metallic deposit of the basket particular metallurgical characteristics, which is the case of the present process and which makes it all the interest.

The molten salt bath used is chosen according to the metal to be deposited. Thus for titanium, a mixture of calcium, barium and sodium chlorides is preferably taken, in suitable proportions. This bath has the advantage of having a composition similar to that of the bath used for the electrolytic production of sodium: it is thus possible to design a loop comprising the pre-reduction of TiCl ₄ to TiCl _x by sodium and the recirculation of the bath from the anode compartment to the sodium electrolysis cell.

As for the current intensities applied, they mainly depend on the metals to be deposited, on the type of halide and on the feed rate and are therefore subject to specific adaptations.

The present invention also relates to two devices for obtaining metals. They consist of a metal halide electrolysis cell to be deposited contained in a molten halide bath comprising a cylindrical tank designed to contain said bath sheltered from the atmosphere, an anode assembly comprising at least one cylindrical anode immersed in the bath, a cathode in the form of a basket in a mechanical lattice suspended in the bath, constituted by two vertical coaxial cylinders integral with a perforated or non-perforated bottom extending to the axis, a halide supply system placed at the upper part of the cathode and between the two cylinders, means for aspirating the released halogen, means for connecting the upper parts of the anode assembly and the cathode respectively to the positive and negative poles of a direct current source, characterized in that the anode assembly is placed in the center of the cell and symmetrically surrounded by the basket, that at least one complementary cathode, and preferably two, connected by their part high at a source of potential current which is more strongly negative than that of the basket, are placed in the compartment delimited by the internal walls of the cell and the external walls of the basket in a symmetrical manner and equipped with means for leaving the bath at the air shelter.

This cell therefore comprises, as in USP 3,282,822 and GB 781,311, a basket-shaped cathode, an anode assembly, means for supplying halides, injecting inert gas and evacuating halogen, but it also includes other means.

First of all, the anode assembly occupies a central position in the electrolysis cell and is symmetrically surrounded by the cathode basket, so that the whole of the inner side wall and the whole of the side wall outside of said basket are each equidistant from the anode. Such an arrangement creates in the anolyte a regularly distributed electric field which avoids local diffusions of titanium ions towards the anode and promotes the channeling of the released halogen.

Note that the presence of a perforated bottom or not at the base of the basket and which extends to the axis of the cell, contributes to forming a compartment where the anolyte is electrically separated from the rest of the bath and reinforces the advantages due to basket-anode symmetry.

But we can further accentuate the isolation of the anolyte by creating a double bottom, the interior of which may or may not communicate with the basket itself.

The basket is made up of a nickel wire mesh preferably, the meshes of which are such that they prevent a pass. metal particles are too easy without being a cause of clogging.

The upper part of the basket emerges from the bath and can be opened or closed. In both cases, it is in relation, on the one hand with the halide supply system, on the other hand with the negative pole of a current source by means of sealed passages made in the cover of the cell.

As for the central anode assembly, it preferably consists of a cylindrical piece of graphite, sculpted longitudinally in order to promote the release of gas, connected to the positive pole of a current source and placed inside d 'A bell tightly attached to the cell cover and through which the halide is sent to an external capture.

Around the basket are symmetrically placed one or more cathodes preferably of iron, nickel, titanium or metal to be deposited, which serve to receive the final metal. This arrangement allows a regular distribution of the second deposition current and obtaining a uniform deposition of the metal. It also contributes to facilitating the recovery of the metal due to good accessibility of said cathodes. By using a sufficient number of cathodes, the cathode current density is reduced and the quality of the deposit is improved. Such an arrangement results in the cell being divided into three separate compartments: an anode compartment where the titanium content of the bath is practically zero, a compartment constituted by the basket where the bath is very rich in titanium ions and a cathode compartment delimited by the inner walls of the cell and the outer walls of the basket, where the titanium content of the bath is relatively low and of the order of a few percent by weight. Hence an original structure which is perfectly suited to the application of the method according to the invention.

However, this is only a non-limiting preferential device and any other variant such as that, for example, consisting in reversing the arrangement of the anode assembly and the cathode assembly relative to the basket is part of the invention. The latter device has the advantage of grouping the cathodes in the center of the cell, which has the result, in particular, of reducing the number of devices for extracting said cathodes.

In the case where it is desired to supply the cell with a mixture of bath and halide of the metal to be deposited, the bottom of the cell is equipped with a nozzle for extracting the bath which opens into the anode compartment. The bath is then continuously recycled to the basket after having been properly recharged with the metal halide to be deposited.

The invention will be better understood with the aid of the attached drawings, which represent in FIG. 1 a section along an axial plane of an example cell according to the invention and in FIG. 2 a section of the same cell according to a horizontal plane of layout XX 'in FIG. 1.

• - une anode centrale (10) avec son prolongement extérieur (11) destiné à être raccordé au pôle positif d'une source de courant continu ;
• - une cloche (12) entourant l'anode et son prolongement extérieur (11) par l'intermédiaire d'une tubulure (13) raccordée à la tubulure (8) d'injection de gaz inerte de manière à faciliter l'entrainement de l'halogène hors de la cellule par la tubulure (9) ;
• - un panier cylindrique tubulaire (14) en treillis métallique comportant une paroi extérieure (15) et une paroi intérieure (16), un fond (17) qui s'étend jusqu'à l'axe de la cellule, une voûte (18) de fermeture se trouvant au-dessus du niveau du bain et équipée d'une tuyauterie (19) d'alimentation en bain et halogénure et d'un prolongement extérieur (20) destiné à être raccordé au pôle négatif d'une source de courant continu; le panier contient un certain volume de métal (21) ;
• - deux cathodes (22) et (23) placées dans le compartiment délimité par les parois intérieures de la cellule et par les parois extérieures du panier recouvertes d'un dépôt de métal avec leur prolongement extérieur (24) et (25) destinés à être raccordées au pôle négatif d'une source de courant continu à un potentiel plus négatif que (20) ;

In FIG. 1, there is a cylindrical tank (1) externally equipped with heating circuits (2), with thermal insulation (3) and internally lined with a bath (4) of molten salts up to level (5) and provided on its lower wall with a bath outlet orifice (6). On the cover (7) of the cell opens an inert gas injection pipe (8). Crossing said cover and, plunging into the bath, we see:

• - a central anode (10) with its external extension (11) intended to be connected to the positive pole of a direct current source;
• - A bell (12) surrounding the anode and its external extension (11) by means of a tube (13) connected to the tube (8) of injection of inert gas so as to facilitate the training of the halogen out of the cell through the tubing (9);
• - a cylindrical tubular basket (14) made of wire mesh comprising an outer wall (15) and an inner wall (16), a bottom (17) which extends to the axis of the cell, a vault (18) closure located above the level of the bath and equipped with a bath and halide supply pipe (19) and an external extension (20) intended to be connected to the negative pole of a direct current source ; the basket contains a certain volume of metal (21);
• - two cathodes (22) and (23) placed in the compartment delimited by the interior walls of the cell and by the exterior walls of the basket covered with a metal deposit with their exterior extension (24) and (25) intended to be connected to the negative pole of a direct current source at a more negative potential than (20);

In Figure 2, we find some of the elements of Figure 1, namely: the cell (1), the heating system (2); the insulation (3), the central anode (10) and the bell (12), the basket and its outer (15) and inner (16) walls containing the metal (21) and eight cathodes, two of which are marked (22) and (23), coated with a deposit of metal of decreasing thickness when one turns clockwise from (22), cathode which has experienced the longest residence time in the bath.

• Dans une cellule en Hastelloy C, de diamètre intérieur 235 mm et hauteur 700 mm, équipée d'un système de chauffage, et à l'intérieur de laquelle est placée une cuve cylindrique en nickel de diamètre 220 mm et hauteur 300 mm, on a chargé 13,7 kg d'un eutectique BaC1₂-caC1₂-NaCl (16,3 - 46,9 - 36,7 % en moles). Après mise en place d'un couvercle étanche muni d'une anode en graphite et d'une cathode en nickel et de différentes tubulures obturables, on a procédé à une mise sous vide de la cellule vers 550° C pour éliminer l'humidité, puis à un traitement par l'HCl pour éliminer les oxydes, hydroxydes, et autres impuretés contenues dans le bain, puis à une électrolyse sous une tension continue de 2,2 V jusqu'à stabilisation du courant résiduel.

The invention can be illustrated using the following example of application:

• In a Hastelloy C cell, with an internal diameter of 235 mm and a height of 700 mm, equipped with a heating system, and inside which is placed a cylindrical nickel tank with a diameter of 220 mm and a height of 300 mm, we have loaded with 13.7 kg of eutectic BaC1 ₂ -caC1 ₂ -NaCl (16.3 - 46.9 - 36.7% in moles). After fitting a sealed cover fitted with a graphite anode and a nickel cathode and various closable pipes, the cell was placed under vacuum at around 550 ° C. to remove the humidity, then a treatment with HCl to remove the oxides, hydroxides, and other impurities contained in the bath, then electrolysis under a DC voltage of 2.2 V until the residual current stabilizes.

The cover was then removed and an annular cylindrical basket made by means of a 3 mm diameter nickel wire frame covered with a metallic fabric made with 0.28 mm diameter nickel wire braided according to the bath was introduced. 35 mesh. Then, the cover was replaced with a central anode and its bell, its complementary cathodes, the various gas inlet and outlet pipes and the halide supply system.

The atmosphere of the cell was purged under vacuum, then under argon, then reheated to 550 ° C. The bath being liquid, the various electrodes were immersed there. The height of the bath in the cell was then 150 mm.

Furthermore, a solution of TiCl _x was prepared in a bath of molten salts, where X was equal to 2.3 by oxidation of titanium powder with titanium tetrachloride. This solution contained about 10% by weight of titanium in the form of halide.

This TiCl _x was introduced at the rate of 6 g / hour into the cell and a voltage of 3.5 V was applied. The current I ₁ then had the value 2 A.

After introduction into the basket of 1500 g of salt bath containing 10% of titanium in the form of TiCl _X , the metal deposited in the basket occupied approximately half of its volume. A bath sample taken near the complementary cathode showed the presence of a perfectly white salt, that is to say containing no trace of subchloride, a result confirmed by analysis.

The compartment delimited by the interior walls of the cell and the exterior walls of the basket was then supplied with TiCl _x until a composition of between 1.5 and 2% by weight of titanium was reached and a voltage of - 0 was imposed. , 9 V between cathode and basket. A current 1 ₂ with an average value of 1.5 A has been established.

After eight hours of walking, a deposit of well crystallized metal was formed on the cathode, resulting in only 1 to 2% by weight of molten salts. The weight collected was 2.3 g.

Le métal obtenu avait pour analyse en ppm :

et sa dureté Brinell était en moyenne de 80.The operation was carried out over a period of 66 hours. The voltages, intensities and weight of metal obtained are shown in the following table:

The metal obtained had for analysis in ppm:

and its Brinell hardness was on average 80.

Claims (12)

1. A process for obtaining a metal by electrolysis of halides in a molten halide bath comprising a simultaneous and continuous double deposition, characterized in that a continuous electric current Il is circulated first of anodic assembly towards a basket-shaped cathode, both being immersed: in the bath, by supplying the basket with a halide of the metal to be deposited, until the quantity of electricity delivered corresponds to the filling of 1/3 to 2/3 of the volume of the basket with the metal, but the halide of the metal is introduced into the compartment delimited by the interior walls of the cell and the exterior walls of the basket so as to have a combined metal content of between 1 and 7% by weight, then a current 1 ₂ is then circulated simultaneously with current I ₁ from the basket to at least one complementary cathode while continuing to supply the basket with halides, and said complementary cathode is extracted periodically to recover r the metal obtained and replaces it with another cathode. 2. Method according to claim 1, characterized in that the ratio I ₁ / I ₂ is modified at any time. 3. Method according to claim 1, characterized in that the halide supply of the metal to be deposited is in the form of a mixture with the bath which is circulated between the electrodes, continuously withdrawn and recycled after recharging in halides. 4. Method according to claim 1, characterized in that one supplies..in halides in the solid state. 5. Method according to claim 1, characterized in that at the start of the steady state operation, if n cathodes are used, a single electrode is immersed in the bath, then the n-1 others successively and periodically so that when plunges the n ^th , the first is covered with a desired quantity of refined metal. 6. Method according to claim 1, characterized in that one implements a halide M (Halog) _X of a metal having a point of melting generally higher than 1000 ° C and preferably several valence states such as in particular titanium, zirconium, hafnium, thorium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, uranium, plutonium and rare earth metals. 7. Method according to claim 4, characterized in that in the case of titanium, the halide is of the TiCl _X type where X is equal to or between 2 and 4. 8. Method according to claim 7, characterized in that X is between 2 and 3. 9. Method according to claim 8, characterized in that X is close to 2.3. 10. Device for the electrolysis of halides of the metal to be deposited contained in a bath of molten halides comprising a cylindrical tank designed to contain said bath sheltered from the atmosphere, an anode assembly comprising at least one anode (10) of cylindrical shape immersed in the bath (4), a cathode (14) in the shape of a wire mesh basket suspended in the bath, constituted by two vertical coaxial cylinders integral with a bottom (17) perforated or not, extending up to 'at the axis, a halide supply system (19) placed at the top of the cathode and between the two cylinders of the suction means (9) of the released halogen, connecting means (11) and (20) upper parts of the anode assembly and of the cathode respectively at the positive and negative poles of a direct current source, characterized in that the anode is placed in the center of the cell and symmetrically surrounded by the basket , that at least one complementary cathode (22) and preferably two connected by their upper part to a current source of potential more strongly negative than that of the basket are placed in the compartment delimited by the internal walls of the cell and the external walls of the basket and in a symmetrical manner and equipped with means of outlet from the air-protected bath. 11. Device for the electrolysis of halides of the metal to be deposited contained in a molten halide bath comprising a cylindrical tank designed to contain said bath sheltered from the atmosphere, an anode assembly comprising at least one anode (10) of cylindrical shape immersed in the bath (4), a cathode (14) in the form of a wire mesh basket suspended in the bath constituted by two vertical coaxial cylinders integral with a bottom (17), ribbed or not, extending to the axis, a supply system (19) in halides placed at the top of the cathode and between the two cylinders, suction means (9) of the released halogen, connecting means (11) and (20) of the top parts of the anode assembly and of the cathode respectively at the positive and negative poles of a direct current source, characterized in that one or two complementary cathode (s) (22) connected (s) by their upper part to a current source of potential more strongly negative than that of the basket are placed towards the center of the cell and surrounding symmetrically through the basket, the anode assembly is placed in the compartment delimited by the interior walls of the cell and by the exterior walls of the basket. 12. Device according to claim 10 or 11, characterized in that the bottom of the basket is double and that the interior communicates or not with the basket itself.

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