EP0053566A1 - Process for the electrolytic production of titanium - Google Patents

Abstract

The process which is the subject of the invention relates to the preparation of titanium by electrolysis from a bath of molten halides. It consists in carrying out a partial reduction of the TiCl4 used as starting material in a separate reactor and then in directly supplying (11) the electrolyzer (10) by means of titanium halides of valence less than 3. The partial reduction of the TiCl4 is carried out by divided titanium or by a metal other than Na.

Description

The process which is the subject of the invention relates to the preparation of titanium by electrolysis of molten halides. It relates more particularly to the electrolysis of titanium dissolved in an electrolyte based on chlorides using, as starting material, titanium tetrachloride.

Various works are already known which describe the preparation of titanium from TiCl ₄ by electrolysis of chlorides.

• Figure 1 : cellule d'électrolyse pour la mise en oeuvre du procédé décrit dans le rapport RI 7648 de l'USBM.
• Figure 2 : cellule d'électrolyse pour la mise en oeuvre du procédé suivant l'invention.

Thus the report RI 7648 from USBM, published in 1972, describes an electrolysis cell allowing the production of Ti by such a process. The figures below provide a better understanding of the process described in this report and the improved process which is the subject of the invention.

• Figure 1: electrolysis cell for the implementation of the process described in report RI 7648 from the USBM.
• Figure 2: electrolysis cell for implementing the method according to the invention.

The electrolysis cell shown in Figure 1 has a container (1) heated from the outside to 520 ° C which contains a molten electrolyte (2), based on a LiCl KCl mixture, in which 8 to 12 are dissolved. % of TiCl ₂ .

An anode (3) surrounded by a porous diaphragm (4) is connected to the positive pole and a deposition cathode (5) to the negative pole. To maintain the con _- centration of the electrolyte Ti ⁺⁺ to the desired level, it is necessary to introduce in the electrolyte, continuously or discontinuously, the TiCl ₄ to replace the titanium fixed to the cathode.

This introduction is carried out by means of a supply cathode (6) which comprises a supply tube of TiCl ₄ (7) whose perforated end (8) is immersed in the electrolyte. The electrolysis current 1, which passes through the electrolyte from the anode, is divided into two parts: a current I ₁ which passes through the feed cathode (6) and a current I ₂ which passes through the deposition cathode. In fact, for the electrolysis to take place under satisfactory conditions, the titanium must be present in the electrolyte in the bivalent form. It is therefore necessary to reduce to a valence close to 2 the titanium which is introduced at valence 4 in the form of TiCl ₄ . This result is achieved when the flow of TiCl ₄ and the intensity of the current I ₁ , which passes through the supply cathode, are adjusted properly. With an ampere efficiency of 100%, the TiCl ₄ flow rate in g / h should theoretically be equal to I (ampere) X1,772. The current I _I should then be equal to 1/2 of I. Experience has shown that the operation of the supply cathode is quite delicate. It is difficult to ensure a regular flow of TiCl ₄ and the risks of blockage of the inlet pipe by the electrolyte are not negligible.

Likewise, the bubbling of TiCl ₄ in the electrolyte causes an often violent agitation which disturbs the electrolysis.

Au niveau de la cathode d'alimentation, grâce au courant I₁, des ions Ti²⁺ se déchargent :

Theoretical studies have shown that the phenomenon of reduction of TiCl ₄ to TiCl ₂ , at the level of the supply cathode, is complex. It can be described in a simplified manner by the following reactions: first of all, the TiCl ₄ introduced reacts on the TiCl ₂ dissolved in the electrolyte according to the reaction:

At the feed cathode, thanks to the current I ₁ , Ti ²⁺ ions are discharged:

The Ti formed in turn reacts with the TiCl ₃ dissolved in the electrolyte according to the equilibrium reaction:

On voit que l'une des difficultés est dûe au fait que la réaction (1) et la réaction (3) peuvent très bien se produire en des points différents de l'électrolyte, en particulier si celui-ci est fortement agité par l'introduction du TiCl₄. On pourra alors observer, d'une part, en certains points de la cathode d'alimentation, des dépôts de Ti plus ou moins importants, en même temps que la teneur en TiCl₃ de l'électrolyte s'accroîtra,In total, the reaction can be represented globally as follows:

It can be seen that one of the difficulties is due to the fact that reaction (1) and reaction (3) can very well take place at different points of the electrolyte, in particular if the latter is strongly agitated by the introduction of TiCl ₄ . We can then observe, on the one hand, at certain points of the supply cathode, more or less significant deposits of Ti, at the same time as the TiCl ₃ content of the electrolyte will increase,

which could be a cause of redissolution of titanium at the deposition cathode.

conduit normalement à un dépôt de Ti.In addition, reaction (3) being an equilibrium reaction, the supply of the cathode with a current I ₁ =

normally leads to a deposition of Ti.

We therefore looked for the possibility of greatly simplifying the design of electrolysers for the preparation of titanium in order to allow stable operation thereof.

Research has been carried out, in particular, on the possibility of avoiding disturbances caused by the movements of the baths due to the introduction of liquid or gaseous TiCl ₄ . Attempts have also been made to avoid variations in the concentration and valence of the titanium dissolved in the bath.

The process according to the invention relates to the preparation by electrolysis of titanium dissolved in the form of halide in an electrolyte based on at least one alkali or alkaline earth halide. It is characterized by the use of a supply device which allows titanium to be introduced into the cathode zone of the electrolysis cell in the form of a halide or a mixture of halides of lower average valence to 3. According to a preferred embodiment of the invention, the titanium halides are titanium chlorides obtained by partial reduction of TiCl ₄ . An alkali or alkaline earth metal, or alloys of these metals, or titanium or a titanium alloy can be used as reducing agent.

This reduction of TiCl ₄ to the desired valence level is carried out by means of the reducing agent chosen in a separate installation. The titanium chloride (s) with an average valence of less than 3 are most often obtained in solution in a halide or mixture of molten alkali or alkaline earth halides. This molten mixture thus produced is gradually introduced into the cathode compartment of the electrolyser as and when required. Simultaneously a corresponding quantity of electrolyte depleted in titanium halides is extracted from the anode compartment.

FIG. 2 schematically represents an electrolysis cell (10) for the production of titanium, in which the method according to the invention is implemented. This cell is heated from the outside by means not shown. It can be seen that the complex relativizing feed cathode device described above is here replaced by a simple feed tube (11) which makes it possible to introduce the mixture of molten halides containing titanium, in the form of valence ions average less than 3, in the cathode compartment. This tube is connected to an installation, not shown, in which the partial reduction of TiCl ₄ is carried out. The cell also comprises a deposition cathode (12) on which the titanium is deposited. In the anode compartment (13) there is, next to the anode (14), a withdrawal tube (15) which makes it possible to extract from the cell amounts of electrolyte equivalent to those which are introduced by the tube ( 11). A tube (16) allows the release of chlorine formed at the anode.

Thanks to the diaphragm (17), the electrolyte thus withdrawn contains very little titanium in solution.

Many modes of implementing the method according to the invention can be envisaged. In particular, different methods can be used to reduce titanium tetrachloride.

The following examples describe, without limitation, two particularly advantageous modes of implementation.

EXAMPLE 1:

A first method of reducing titanium tetrachloride is to carry out this using metallic titanium. Such an operation is particularly justified in the case where there is waste of titanium or titanium-based alloys, in the divided state, such as turnings, falling sheets, etc. We can also use as a reducing agent for titanium sponge and, in particular, for sponge which does not have a sufficient degree of purity for its direct use. Finally, electrolytic titanium can also be used, to which its crystalline structure, generally quite loose, confers great reactivity.

puis

The following reactions are carried out:

then

We generally operate in a steel reactor in which the titanium waste is placed. After heating to the suitable temperature in a neutral atmosphere, the TiCl _{4 is} gradually introduced. It is generally desirable to introduce a certain quantity of electrolyte into the reactor, preferably coming from the anode compartment of the electrolyser, so as to dissolve the titanium subchlorides formed. Indeed, the introduction of these titanium subchlorides in the solid state into the cathode compartment of the electrolyser would present greater difficulties than their introduction in the form of a mixture of molten salts. In the case, for example, of electrolysis in LiCl-KCl media, a quantity of said electrolyte taken from the anode compartment of the electrolyser is introduced into the reactor such that, after reduction of TiCl ₄ by titanium, the content of the mixture of molten salts in titanium subchlorides, of the order of 8 to 12%. As reaction 6 is balanced, it is not possible to achieve a complete reduction of _TiCl3 to TiCl ₂ . Furthermore, it should also be noted that, during the introduction of TiCl ₄ into the reactor, there may be a direct reaction of TiCl4 on TiCl ₂ :

We will therefore generally obtain an average valence of titanium in solution of between 3 and 2. This valence will be all the closer to that corresponding to equilibrium 6 the greater the excess of titanium relative to the TiCl ₄ introduced. and that the specific surface of this titanium will be significant. For the latter reason, preference will be given to the use of titanium in the form of shavings or fine turnings, sponge or better still crystals of electrolytic titanium. The mixture of salts thus prepared is finally introduced into the electrolyser through the pipe (11).

EXAMPLE 2:

A second method consists in carrying out this reduction with sodium. It is known, in fact, that TiCl ₄ can be reduced by Na according to the following reactions: either in total

We can also observe, as in the case of the reduction of TiCl ₄ by Ti, reaction (7).

These reactions are carried out, for example, in a steel reactor in which the sodium is melted away from air, in the presence of an inert gas such as argon, and inside which is introduced. gradually TiCl ₄ in the desired proportion. The salt mixture obtained is then transferred in the liquid state to the electrolysis cell by means of the feed tube (11). It can be seen that, in the case of the use of sodium as a TiCl ₄ reducing agent, it is desirable to use as electrolyte only a mixture of sodium chloride and titanium subchlorides.

It is then possible to use the sodium chloride which is withdrawn from the anode compartment to prepare again sodiua by electrolysis according to the usual process.

It is also possible, in certain cases, to extract from the anode compartment an additional quantity of sodium chloride which will be used to dilute the mixture of salts prepared according to reaction (7), in order to obtain a mixture of salts in which the content of titanium subchlorides is closer to that of the catolyte into which this mixture will be transferred via the pipe (11).

It is advantageous to carry out in all cases, continuously or semi-continuously, the operations of introducing electrolyte into the cathode compartment and withdrawing electrolyte into the anode compartment to avoid jolts.

We can, for this, perform, also continuously, the reduction of TiCl ₄ by Ti or Na.

Furthermore, other reducers can be envisaged for the preparation of titanium subchlorides. In particular, use may be made of other alkali or alkaline earth metals. It is also possible, optionally, to use, as reducing agents, alloys of alkali or alkaline earth metals such as NaK or LiK or other alloys.

Claims (2)

1 ° / - Process for the production of titanium by electrolysis in which the titanium is dissolved in the form of halide in an electrolyte based on at least one alkali or alkaline earth halide, and in which a feed device makes it possible to add in the cathode zone of the electrolysis cell, titanium in the form of a halide with an average valence of less than 3, characterized in that the titanium halide with a valence of less than 3 is prepared by reduction of TiCl ₄ by one or more metals alkaline, alkaline earth, or their alloys. 2 ° / - Process for the production of titanium by electrolysis in which the titanium is dissolved in the form of halide in an electrolyte based on at least one alkali or alkaline earth halide, and in which a supply device makes it possible to introduce in the cathode zone of the electrolysis cell, titanium in the form of a halide with an average valence of less than 3, characterized in that the titanium halide with a valence of less than 3 is prepared by reduction of TiCl ₄ by one or more metals alkaline, alkaline earth, or their alloys, or by titanium or a titanium-based alloy and in that one withdraws from the anode zone a quantity of molten electrolyte equivalent to that introduced into the cathode zone.

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