EP0053564B1 - Process for monitoring the diaphragm permeability during the electrolytic preparation of polyvalent metals, and electrolysis cell for carrying out this process - Google Patents

Abstract

The process according to the invention concerns the production of polyvalent metals such as titanium by electrolysis of molten halides. It comprises controlling the permeability of the diaphragm which separates the anolyte from the catholyte, by causing growth or re-dissolution of a deposit of the metal to be produced. The process is applied in particular to the production of titanium by electrolysis from TiCl4.

Description

The process which is the subject of the invention relates to the preparation of polyvalent metals such as titanium, zirconium, hafnium, vanadium, niobium or tantalum by electrolysis in molten salt baths of their halides dissolved in one or more alkali or alkaline earth halides.

This process applies more particularly to the preparation of titanium by electrolysis of a bath of molten halides.

It is known that, in the preparation by electrolysis of such metals, it is necessary to separate in the electrolyser the cathodic and anodic zones of the electrolysis bath by means of a diaphragm.

This must oppose the migration towards the anode of the metal ions that we intend to deposit at the cathode. Indeed, these ions, which are present, at least for a part of them, at degrees of ionization lower than the maximum degree, would be oxidized at the higher level by the action of the halogen which is formed in contact of the anode and which is also present in the atmosphere above the anolyte. Such a mechanism would cause a very significant drop in the efficiency of electrolysis.

This diaphragm must, on the other hand, allow the alkaline or alkaline-earth ions to pass, as well as the halogen ions which transport most of the current.

In general, the permeability of this diaphragm must be sufficient to allow the circulation of the electrolyte in order to balance the pressures in the two compartments, while making as much as possible an obstacle to the passage of the metal to be deposited, in ionic form. or not towards the anode compartment.

In the US patent. 2 789 943, a process for the production of titanium by electrolysis of a molten halide bath is described in which a perforated metal structure is interposed between the anode and the cathode so as to separate the anolyte and the catholyte .

This structure is preferably made up of a grid or a perforated screen made of nickel or a nickel-based alloy. In order to give it a sufficiently low permeability for it to act as a diaphragm, it is covered with an electrolytic deposit of titanium in the electrolysis cell itself. For this, this structure is connected to the electrical supply circuit of the cell, so as to make it play the role of a cathode. The titanium deposit which then forms partially plugs the holes.

When the permeability of the structure has been sufficiently reduced, normal conditions of electrolysis are established between anode and cathode, and the structure thus coated with titanium, acts as an effective diaphragm.

The FR patent. 2423 555 describes another embodiment of a diaphragm for cells used for the electrolytic preparation of polyvalent metals. These diaphragms are preferably formed by a metallic nickel cloth on which an electrolytic or non-electrolytic deposition of cobalt has been carried out.

The use of these two types of diaphragm has shown, however, that they do not entirely solve the problems that arise.

Indeed, it is found that, whatever the nature of the material constituting the diaphragm, it forms thereon during electrolysis, a deposit of the polyvalent metal present in the form of halide in the bath. This filing is favored in cases where, as taught in the US patent. 2 789 943, the diaphragm is electrically connected to the cell supply circuit so as to give it a negative polarity with respect to the anode. However, even when the diaphragm is isolated, at least in certain areas thereof, the formation of deposits of the polyvalent metal is observed.

Experience has shown that these deposits are formed in particular in the vicinity or inside of the holes or channels which put the two faces of the diaphragm into communication.

Also, we find, most often, during the operation of the electrolysis cell, a gradual decrease in the permeability of the diaphragm.

To such a reduction corresponds first of all an improvement in the efficiency of the electrolysis current which can reach 80%, and even more, as the patent FR shows. 2,423,555.

Unfortunately, this high efficiency is not maintained for a prolonged period and, little by little, as the electrolysis continues, there is, on the one hand, an increase in the voltage across the terminals of the cell and , on the other hand, a drop in current efficiency. This phenomenon results from the almost total clogging of the diaphragm pores by the polyvalent metal.

The connection between anolyte and catholyte is gradually cut and the diaphragm starts to function like a bipolar electrode. This most often results in destruction of the diaphragm, either by corrosion or by crushing.

A process has therefore been sought which makes it possible to avoid such drawbacks and, in particular, to considerably extend the life of the diaphragms used for the preparation of polyvalent metals by electrolysis.

Such a method must also make it possible to maintain a high current yield, both with regard to the deposition of the polyvalent metal at the cathode and with regard to the release of halogen at the anode. Finally, it must make it possible to maintain the voltage at the terminals of the electrolysis cell within the determined limits corresponding to operating conditions close to the optimum.

The process which is the subject of the invention consists in controlling the permeability of the diaphragm of an electrolysis cell for the preparation of polyvalent metals such as Ti, Zr, Hf, V, Nb and Ta from a electrolyte based on molten metal halides, thanks to the formation of a deposit of the metal to be obtained on this diaphragm, said diaphragm being able to be positively or negatively polarized; this process is characterized in that the drop in potential in the electrolyte bath permeating the diaphragm is measured continuously and without interrupting the operation of the electrolysis, and in that a direct electric current is sent into the said diaphragm whose intensity and direction are slaved to the said drop in potential so as to maintain the permeability within determined limits.

It is therefore possible to maintain this permeability at an optimal value by slaving the growth or partial redissolution of the deposit of the polyvalent metal on the diaphragm to the voltage drop in the electrolyte impregnating the diaphragm.

This partial growth or redissolution of a deposit of a polyvalent metal is carried out without interrupting the operation of the electrolysis, continuously or discontinuously at constant or variable speed.

• La figure 1 est un schéma d'une cellule d'électrolyse à diaphragme pour la préparation d'un métal polyvalent tel que le titane.
• La figure 2 est un diaphragme de la répartition des potentiels dans une cellule à diaphragme du type de la figure 1, utilisée pour la préparation électrolytique du titane à partir d'un électrolyte à base de chlorures fondus.
• La figure 3 est un schéma d'un mode de mise en œuvre du procédé suivant l'invention appliqué à une cellule d'électrolyse à diaphragme du type de la figure 1.

The examples and figures below describe in more detail the characteristics of the process according to the invention and the main modes of implementing it.

• Figure 1 is a diagram of a diaphragm electrolysis cell for the preparation of a polyvalent metal such as titanium.
• Figure 2 is a diaphragm of the distribution of potentials in a diaphragm cell of the type of Figure 1, used for the electrolytic preparation of titanium from an electrolyte based on molten chlorides.
• FIG. 3 is a diagram of an embodiment of the method according to the invention applied to a diaphragm electrolysis cell of the type of FIG. 1.

Although the example below relates particularly to the preparation of titanium, the process also applies to the preparation of other polyvalent metals such as, in particular, Zr, Hf, V, Nb and Ta ...

FIG. 1 is the diagram of a diaphragm electrolysis cell, particularly suitable for the preparation by electrolysis of titanium, the general arrangement of which is similar to that described in the USBM report No. 764S-1972 entitled “Use of composite diaphragm in the Electrowinning of titanium (fig. 1, p. 3).

This cell comprises a receptacle (1) made of refractory steel heated from the outside by known and not described means which make it possible to bring the electrolyte (2) to a temperature of approximately 550 ° C. This consists of a LiCIKCI eutectic mixture containing titanium in solution, in the form of chlorides at a concentration of approximately 1 to 3% by weight of Ti.

A graphite anode (3) is immersed in the electrolyte and is surrounded by a diaphragm (4). This anode is connected by a rod (5) to the positive pole of a current source, not shown. A supply cathode consists of a mild steel tube (6) connected to the negative pole of a current source, not shown. This cathode is supplied with TiCI ₄ by the connection tube (7) from an injection system not shown. The end of the tube (6) has a perforated zone (8) also made of mild steel immersed in the electrolyte.

Finally, a mild steel deposition cathode (9) is also connected to the negative pole of the current source. A current distributor device, not shown, makes it possible to fix the ratio between the currents I ₁ and 1 ₂ which pass respectively through the supply (6) and deposition (9) cathodes. The current flowing through the anode has an intensity 1 equal to I ₁ + I ₂ .

As the diaphragm (4), an Ni grid is used which has been coated with a Ti deposit by a suitable method, such as that described in the US patent. No. 2,789,943, so as to reduce the permeability to the desired level.

The amount of TiCl ₄ injected through the supply cathode is such that the concentration of Ti dissolved in the electrolyte is preferably maintained in the concentration range of 1 to 3% by weight of Ti.

For a cathodic deposition yield of 100%, the quantity of TiC1 ₄ introduced in grams per hour for an electrolysis current of 1 (amps) is 1.772 × I.

Although we do not know exactly the nature of the titanium ions present in the catolyte, everything happens as if, under these conditions, the titanium was present in majority in the form of bivalent ions.

As explained above, it can be seen, in the case where the diaphragm is isolated from the electrical supply circuit of the cell, that, as the electrolysis takes place, this diaphragm becomes clogged, it that is, a deposit of titanium is formed which clogs the pores.

The diaphragm in Figure 2 shows the distribution of potentials in the electrolysis cell during its operation. In this figure, the potential (P) which exists in the electrolysis cell in operation in the interval between cathode and anode is represented on the ordinate, represented on the abscissa. Cathode C, diaphragm D and anode A are shown schematically.

The straight sections a, b and c respectively represent the potential variations which occur in the catholyte, in the electrolyte which impregnates the diaphragm and in the anolyte. Finally, the vertical vectors P _i , P ₂ . P ₃ and P ₄ respectively represent the potential differences between the cathode, the cathode and anode faces of the diaphragm and the anode with respect to the electrolyte in contact.

Through the diaphragm, the variation of the potential “b” is equal to the product of 1 (electrolysis current) by R _D (resistance of the electrolyte which permeates the diaphragm). 1 x R _D varies in the opposite direction to the permeability.

Dans cette formule :

représente le potentiel normal correspondant à la réaction chimique =

« aTi²⁺ cathodique représente l'activité des ions Ti2+ dans le catholyte.The studies carried out by the applicant have shown that the diaphragm, which is coated on its major part with titanium, tends to behave towards the electrolyte, like a titanium electrode and that it is constantly in equilibrium with potential with respect to this electrolyte. Cathodic side, this P ₂ equilibrium potential is defined by the formula well known to electrochemists:

In this formula:

represents the normal potential corresponding to the chemical reaction =

“ATi ²⁺ cathodic represents the activity of Ti2 + ions in the catholyte.

Dans cette formule, les conventions sont les mêmes que dans la précédente et « a^Ti2- anodique représente l'activité des ions Ti2+ dans l'anolyte.On the anode side, the equilibrium potential of the diaphragm P ₃ with respect to the anolyte is defined by the formula:

In this formula, the conventions are the same as in the previous one and “a ^Ti2- anodic represents the activity of Ti2 + ions in the anolyte.

In normal operation and, at a given instant, due to the high electronic conductivity of the material constituting the diaphragm, we have:

Any variation of a ^Ti2- cathodic automatically leads to a restoration of equilibrium, therefore a variation of a ^Ti2- anodic by dissolution or deposit of metallic titanium in the pores of the diaphragm.

IR_D est donc une mesure de l'efficacité du diaphragme en tant que moyen de faire obstacle à la diffusion des ions titane vers l'anolyte.By simplifying relation 1, we obtain;

IR _D is therefore a measure of the efficiency of the diaphragm as a means of preventing the diffusion of titanium ions towards the anolyte.

où eox"x" représente le potentiel de dépôt du métal alcalin ou alcalino-terreux « X de l'électrolyte, le plus facile à déposer dans les conditions opératoires.During electrolysis, there is generally a decrease in the porosity of the diaphragm and, therefore, a gradual increase in IR _o , which is first of all favorable to the yield. But, there is a limit defined by equality:

where eox "x" represents the deposition potential of the alkali or alkaline earth metal "X of the electrolyte, the easiest to deposit under the operating conditions.

Beyond this limit, the diaphragm becomes bipolar and an alkali or alkaline earth deposit appears on the face of the diaphragm opposite the anode. The current yield on the anode side then collapses quickly by recombination of the chlorine released at the anode with the alkalis formed. Furthermore, on the other side of the diaphragm, chlorine ions are discharged which cause a rapid attack on this diaphragm.

Likewise, too high permeability of the diaphragm is undesirable since the diffusion of Ti ions from the catholyte to the anolyte in too large a quantity would lead to too great a reduction in yield.

To avoid these drawbacks, the method for controlling the permeability of the diaphragm according to the invention consists in controlling the deposition of titanium which takes place there more or less naturally either with a view to increasing it, or with a view to partially redissolving it, this growth or this redissolution being subject to the variation of the voltage drop across the electrolyte which permeates the diaphragm. It is thus possible, in an extremely simple manner, to choose in advance, according to the characteristics of the cell, a special value of this drop in potential and to keep it within a determined range.

In practice, the potential drop across the diaphragm should be kept below a upper limit, of the order of a volt, which corresponds to the difference between the deposition potential of Ti2 + and that of the alkali or alkaline earth metal. Experience has shown that the efficiency is all the better as the drop in potential through the diaphragm is closer to this upper limit, provided however that the permeability remains sufficient to ensure the pressure balance between the two compartments.

It is possible to measure a value very close to this drop in potential by having on either side of the diaphragm, and in its immediate vicinity, but without contact with it, two reference electrodes (for example electrodes sensitive to CI ions such as AgIAgCI electrodes), immersed one in the catholyte, the other in the anolyte and connected to a voltmeter with high internal resistance. The operation of the diaphragm permeability control device will, as is well known to those skilled in the art, be subject to variations in the potential drop indicated by the voltmeter, so as to keep it within the desired limits. . It will thus be possible, in a much simpler way, to continuously measure the potential difference existing between the diaphragm and the anode by means of a voltmeter of the same type and compare this potential difference to a reference potential which will be, most often, that which will have been measured in the period following the entry into service of the diaphragm where its permeability was considered to have reached a satisfactory level. It will then suffice to control the growth or redissolution of the titanium deposit in the pores of the diaphragm to variations in the difference between the measured voltage and the reference voltage.

In any case, this difference should not exceed the value which corresponds to the discharge of alkaline ions on the diaphragm.

Other means of measurement, either directly of the drop in potential through the electrolyte which impregnates the diaphragm, or of a variable depending on this drop in potential, can be envisaged.

A particularly advantageous device represented in FIG. 3 consists in connecting the diaphragm to a current source capable of ensuring the passage of this current in both directions, the other pole of this source being connected to the cathode.

As can be seen, FIG. 3 schematically represents an electrolysis cell whose design derives from that of the cell in FIG. 1.

The metal electrolysis cell (10) contains the electrolyte (11) whose composition is similar to that described above for the preparation of titanium.

The anode (12) is surrounded by a diaphragm (15).

The anode is connected, as usual, to the positive pole of a first current source not shown, whose negative pole is connected to the cathodes. The diaphragm is connected either to the positive pole or to the negative pole of a second source of current not shown, the other pole of which is connected to the cathode, and which is capable of ensuring the passage through this diaphragm of a current 1 ₃ in the desired direction.

The TiCI ₄ supply cathode (14) and the deposition cathode (15) are similar to those already described in FIG. 1.

It is thus possible to inject through the diaphragm a current 1 ₃ which crosses the catholyte by cutting off or adding to the current 1 coming from the anode. This current 1 ₃ causes, depending on its direction of passage, the deposition or re-solution of the titanium on the diaphragm and thus makes it possible to ensure and maintain the permeability of the diaphragm at its optimal value.

A device known to a person skilled in the art enables the direction and intensity 1 ₃ of the current injected into the diaphragm to be controlled by the variation in the voltage drop IR _D , through the electrolyte permeating that this, which is detected by one of the means described above and, for example, by the continuous measurement of the potential difference between the diaphragm and the anode. The injection of current 1 ₃ through the diaphragm is started as soon as the voltage drop across the electrolyte permeating it deviates in one direction or the other from the reference voltage. The servo-control makes it possible to inject a current 1 ₃ in the desired direction the more intense the greater the difference between the voltage drop across the electrolyte impregnating the diaphragm and the reference voltage. We arrange for the process to be self-regulating, that is to say so that the increase in the intensity of the current as a function of the voltage difference is greater than the value strictly necessary, in order to accelerate the deposit or dissolution and thus promote, as far as possible, a return to normal conditions of permeability of the diaphragm.

Current 1 ₃ can, if necessary, be taken in parallel from the current source supplying the cell, independent regulation and inversion means making it possible to control this current in direction and in intensity to the variations in voltage drop in the electrolyte permeating the diaphragm, as explained above.

The means for controlling the permeability of the diaphragm according to the invention which has just been described, can be applied not only to the case of titanium, but also to that of the preparation by electrolysis of other polyvalent metals such as zirconium. , hafnium, vanadium, niobium or tantalum.

Many variants can be made to the embodiment of the method for controlling the permeability of the diaphragm without departing from the scope of the invention.

Claims (3)

1. A process for controlling the permeability of the diaphragm of an electrolysis cell for the preparation of polyvalent metals such as Ti, Zr, Hf, V, Nb or Ta, from an electrolyte based on molten metal halides, the said diaphragm is coated with a deposit of the metal to be produced and may be positively or negatively polarized, characterised in that the voltage drop in the electrolyte impregnating the diaphragm is measured without interrupting the electrolysis operation and a continuous current is injected into the said diaphragm, the intensity and the direction of the said current being controlled in dependance on the voltage drop so as to maintain the permeability within given limits.

2. A process according to claim 1 characterised in that, the voltage drop is measured between two electrodes immersed in the electrolyte on respective sides of the diaphragm.

3. A process according to claim 1 characterised in that the potential drop is measured between the anode of the cell and the diaphragm.

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