EP0053565B1 - Apparatus and process for feeding ticl4 to electrolysis cells used for the manufacture of titanium - Google Patents

Abstract

The apparatus and the process according to the invention concern the preparation of titanium by electrolysis in a bath of molten halides. The apparatus comprises a porous diaphragm which is disposed around a feed cathode permitting partial reduction of the TiCl4 introduced into the electrolyte. The intensity of the current which passes through the feed cathode is so regulated as to maintain a low but non-zero voltage drop, across the diaphragm which is insulated with respect to the feed cathode.

Description

The device and the method which are the subject of the invention relate to the preparation of titanium by electrolysis in a bath of molten halides. They relate more particularly to the method of supplying the cell with TiCl ₄ .

Several publications have described electrolysis cells allowing the preparation of titanium and comprising devices for continuous supply of TiCl ₄ .

• Figure 1: cellule d'électrolyse du type connu pour la préparation du titane.
• Figure 2: dispositif d'alimentation en TiC1₄ suivant l'invention, d'une cellule d'électrolyse pour la préparation du titane.

The figures below will allow a better understanding of the characteristics of the known devices and those of the device and of the method which are the subject of the invention.

• Figure 1: electrolysis cell of the type known for the preparation of titanium.
• Figure 2: TiC1 ₄ supply device according to the invention, an electrolysis cell for the preparation of titanium.

In patent FR 2 423 555, an improved cell is described for the preparation by electrolysis of polyvalent metals and more particularly for the preparation of titanium. In this cell, a feeding device makes it possible to continuously introduce, as the electrolysis takes place, titanium tetrachloride into a bath of alkali or alkaline earth halides.

FIG. 1 represents such an electrolysis cell of the type described in FR 2 423 55 which comprises a metal envelope (1) containing the molten electrolysis (2). An anode (3) is placed inside an anode compartment (4). A diaphragm (5) separates this compartment from the rest of the cell in which the deposition (6) and supply (7) cathodes are located. As can be seen, the supply cathode (7) is tubular in shape. It is, for example, in wire mesh and connected to the negative pole of a current source. This metallic fabric can be, for example, carbon steel or another metal such as nickel or stainless steel optionally coated with cobalt. A supply of TiC1 ₄ is carried out inside the supply cathode (7) by means of the tube (8) placed in the vicinity of its axis. This tube is insulated from the cathode. Under these conditions, it can be seen that the TiC1 ₄ which leaves through the orifice (9) of the tube (8) is reduced at least partially to titanium subchloride which dissolves in the bath.

The reduction mechanism of TiC1 ₄ is not known with certainty. According to a generally accepted theory, a first reaction takes place between the TiC1 ₄ gas and the TiC1 ₄ dissolved in the bath:

Then, during a second reaction, the Ti ³⁺ ions formed by reaction (1) diffuse in the electrolyte and come to reduce at the supply cathode according to the reaction:

We see that, if we wish to obtain the transformation of all the TiC1 ₄ into TiCl ₄ , it is theoretically necessary, assuming an ampacity of 100%, that the current I ₁ which flows through the supply cathode, be equal at:

In this relation (3), q is the mass of TiC1 ₄ introduced into the electrolyte in g / h and I ₁ the intensity of the current in amperes.

At the same time, the deposition cathode (6) must be traversed by a current 1 ₂ = I ₁ , in order to collect at each instant an amount of titanium corresponding to that which has been dissolved. The total current I which flows through the anode is such that:

Under these conditions, no titanium deposit should be observed on the supply cathode. In fact, experience shows that this is not the case. In fact, metallic titanium is deposited in the form of a very fine powder in the vicinity of the cathode. This deposit, which forms with the electrolyte a kind of mud, disturbs the operation of the cathode; it is very difficult to extract from the cell and corresponds to a loss of yield which can be significant.

• - 1ère hypothèse: dans les conditions opératoires, TiCl₂ n'est stable qu'en présence d'une certaine quantité de TiCl₃, sinon il se dismute par déplacement vers la droite de la réaction équilibrée:
  

• - 2ème hypothèse: la réduction de Ti³⁺ en Ti2+ ne s'effectue pas directement à la cathode par la réaction (2), mais par l'intermédiaire des réactions suivantes:
  
  • Ti métal + 2 TiCI₃ dissous ⇄ 3 TiCl₂ dissous (5 bis)
  • c'est-à-dire par la réaction (4) déplacée vers la gauche.

Two hypotheses, practically equivalent, can account for this abnormal behavior:

• - 1st hypothesis: under the operating conditions, TiCl ₂ is only stable in the presence of a certain amount of TiCl ₃ , otherwise it disassembles by displacement to the right of the balanced reaction:
  

• - 2nd hypothesis: the reduction of Ti ³⁺ to Ti2 + is not carried out directly at the cathode by reaction (2), but by means of the following reactions:
  
  • Ti metal + 2 TiCI ₃ dissolved ⇄ 3 TiCl ₂ dissolved (5 bis)
  • that is to say by reaction (4) moved to the left.

ampères aboutit à un dépôt de titane, soit par dismutation d'une partie de TiC1₂ formé, soit par réduction incomplète du TiCl₃, par le titane métal déposé sur la cathode par la réaction (5) conformément à l'équilibre (4).In both cases, the end result is the same: passing an intensity

amperes leads to a deposition of titanium, either by disproportionation of a part of TiC1 ₂ formed, or by incomplete reduction of TiCl ₃ , by the titanium metal deposited on the cathode by the reaction (5) in accordance with equilibrium (4) .

The object of the invention is a device for supplying TiC1 ₄ to an electrolysis cell for the preparation of titanium in which the cover is traversed by a tube which allows the introduction of TiC1 ₄ and by a cathode of feed formed by two steel rods arranged on either side of the tube connected to the negative pole of a current source, electrically isolated from the cover, the tube and the rods immersed in the molten electrolyte characterized in that the tube and the cathode are surrounded by a diaphragm electrically isolated from the cathode.

Another object of the invention is a method for supplying TiC1 ₄ to an electrolysis cell equipped with the device described above and in which a current of intensity I ₁ is passed through the supply cathode the ratio of the activities of the Ti ³⁺ and Ti ³⁺ ions is the same on both sides of the diaphragm.

Under these conditions, a small but not zero potential drop is maintained in the electrolyte which permeates the diaphragm.

To adjust the intensity of this current l ₁ , the value of the potential measurement in the electrolyte impregnating the diaphragm or a variable linked to it is used.

This value can be given by equipping the supply device with two measurement electrodes placed on either side of the diaphragm.

Thanks to this device and to this process, the contents of TiCI ₃ and of TiC1 ₂ dissolved at the supply cathode are regulated in proportions in accordance with equilibrium (4).

FIG. 2 represents, by way of nonlimiting example, a feed cathode device according to the invention.

This device is placed inside a cell (10) for the preparation by electrolysis of titans from a molten electrolyte (11). This cell comprises, like that of FIG. 1, an anode (12) surrounded by a diaphragm (13) and, at least, a deposition cathode (14): Only the supply device according to the invention is described from in detail. It comprises a tube (15) which makes it possible to introduce the TiC1 ₄ which leaves the electrolyte through the orifice (16). The feed cathode consists of two steel rods (17) and (18) arranged on either side of the tube (15). They pass through the cover (19) of the cell through insulating joints and are connected to the negative pole of a current source not shown.

The diaphragm (20), which constitutes one of the objects of the invention, is made of a metal having sufficient resistance to the electrolyte under the temperature conditions in which it is operated. It is possible, for example, to use unalloyed nickel, or nickel-based alloys or else steels, preferably stainless. This diaphragm is connected to the upper part of the cover (19) of the electrolyser by means of a sealed metallic annular wall (21) which is fixed to the cover (19) by insulating and waterproof annular seals (22) and (23 ). The annular wall (21) penetrates through its lower part into the electrolyte and thus obstructs the circulation of the gases present above the level of the electrolyte on either side of the diaphragm (20). The essential role of the latter is to obstruct the diffusion of TiC1 ₃ formed according to reaction (2) by the action of TiC1 ₄ on TiCl ₂ , outside the space delimited by this diaphragm. In this way, the electrolyte zone located in the immediate vicinity of the rods (17) and (18), which constitute the supply cathode, is enriched in TiCl ₃ , which allows a very rapid redissolution of the titanium formed in the contact of these rods by discharge of bivalent titanium ions.

The structure of the diaphragm (20) can be produced, for example, in the form of a metallic cloth such as a nickel cloth, or of a sheet obtained perfitting a metallic powder, for example based on stainless steel , and having sufficient residual porosity.

Experience has shown that such a diaphragm, made for example of nickel cloth and insulated with respect to the supply cathode, behaves like an electrode indicative of the redox reaction:

The potential of this diaphragm (20) relative to the electrolyte in which it is immersed, can be calculated by one of the following two equations, considering either the potential of the internal face of this diaphragm relative to the electrolyte (24) which is inside this diaphragm, that is to say the potential of the external face of the same diaphragm with respect to the electrolyte (25) situated outside this diaphragm.

The potential of the internal face of the diaphragm is calculated by the formula well known to electrochemists:

In this formula, e ₀ ^{Ti3 + / Ti2 +} represents the normal potential for the reaction (6), and a ₁ ^{Ti3 +} and a ₁ ^{Ti2 +} represent the respective activities of the Ti2 + ions in the volume of electrolyte (24) contained in the space surrounded through the diaphragm (20).

The potential of the external face of the diaphragm (20) is calculated identically by the reaction:

In this formula, a ₂ ^{Ti3 +} and a ₂ ^{Ti2 +} represent the respective activities of the Ti ²⁺ ions in the electrolyte (25) with which the external face of the diaphragm (20) is in contact.

où e_o ^Ti2+/Ti0 représente le potentiel normal de la réduction des ions Ti2+ à l'état métallique, les autres paramètres de l'équation étant déjà définis plus haut. Cette relation montre que, pour une teneur donnée en titane dissous dans l'électrolyte (25) ce potentiel est déterminé.Due to the presence of Ti metal on the deposition cathode (14), the electrolyte (25) is in equilibrium with the titanium metal in accordance with equilibrium (4) and one can also write:

where e _o ^{Ti2 + / Ti0} represents the normal potential for the reduction of Ti2 + ions in the metallic state, the other parameters of the equation being already defined above. This relationship shows that, for a given content of titanium dissolved in the electrolyte (25), this potential is determined.

• a) l'intensité I₁ est choisie de manière à ce que
  
  reste constant et conforme à la valeur d'équilibre de la réaction (4) c'est-à-dire égal à
  
  ; on observe alors une simple réduction des ions Ti³⁺ à l'état Ti²⁺ à la cathode d'alimentation à l'exclusion de tout dépôt métallique, les potentiels de part et d'autre du diaphragme restent inchangés et égaux, et un courant électronique égal à l_i s'établit dans les portions métalliques du diaphragme entraînant la réduction des ions Ti²⁺ à l'état métallique sur la face externe du diaphragme et l'oxydation en quantité équivalente des ions Ti²⁺ à l'état Ti³⁺ sur la face interne; cependant, simultanément, l'alimentation en TiCl₄ entraîne l'apparition d'un flux d'électrolyte (24) à travers le diaphragme qui entraîne le TiCl₃ ainsi formé vers la face externe du diaphragme, où il réagit sur le Ti déposé en redonnant du TiCl₂.
• b) l'intensité I₁ est choisie à une valeur supérieure à la précédente; un excès de TiCl₂ est produit à la cathode d'alimentation, une partie de cet excès est réduite directement à l'état métallique sur cette cathode, et le reste se dismute dans l'électrolyte (24) conformément à l'équilibre (4) en donnant naissance à des particules de Ti très finnes qui forment avec l'électrolyte une sorte de boue; le rapport
  
  reste ainsi inchangé, ainsi que les potentiels de part et d'autre du dia- phragme. Le courant I₁ passe toujours par voie électronique avec, comme conséquence, le dépôt de titane métallique sur la face externe et formation en quantité équivalente d'ions Ti³⁺ sur la face interne; cependant, le TiCl₃ en excès ainsi formé est à son tour immédiatement réduit à l'état de TiC1₂ par les boues de titane métallique en suspension dans l'électrolyte (24), le flux d'électrolyte (24) traversant le diaphragme ne contient plus alors assez de TiCI₃ pour redissoudre le titane métallique sur la surface externe du diaphragme qui se colmate progressivement.
• c) l'intensité I₁ est choisie à une valeur inférieure à celle qui a été choisie en a): un excès de TiCl₃ non réduit à la cathode reste dissous dans l'électrolyte, et le terme
  
  diminue en valeur absolue, une chute de potentiel
  
  apparaît dans l'électrolyte, imprégnant le diaphragme qui est alors parcouru par un courant ionique, ce qui entraîne une diminution du courant électronique dans les parties métalliques du diaphragme; le dépôt de titane métallique sur la surface externe du diaphragme, qui résulte du passage de ce courant, diminue et ce dépôt est complètement redissous par le flux d'électrolyte (24) traversant le diaphragme par suite de l'alimentation en TiC1₄, du TiC1₃ en excès diffuse dans le bain (25). Cependant, étant donné l'importance du volume du bain (25) par rapport à celui du bain (24), la présence de ce TiCl₃ en excès ne modifie pas sensiblement le potentiel de la face externe du diaphragme, mais il vient se réduire au contact de la cathode de dépôt, ce qui diminue le rendement de la cellule.

The cell being with a current l = I ₁ + 1 ₂ , the supply cathode with a current I ₁ and a flow of TiC1 ₄ , q = 1.772 I, the deposition cathode with a current 1 ₂ , it is observed that, depending on the intensity of the current I ₁ , three situations may arise:

• a) the intensity I ₁ is chosen so that
  
  remains constant and conforms to the equilibrium value of reaction (4), i.e. equal to
  
  ; a simple reduction of the Ti ³⁺ ions in the Ti ²⁺ state is then observed at the feed cathode to the exclusion of any metallic deposit, the potentials on either side of the diaphragm remain unchanged and equal, and a electronic current equal to l _{i is} established in the metal portions of the diaphragm causing the reduction of Ti ²⁺ ions in the metallic state on the external face of the diaphragm and the oxidation in equivalent quantity of Ti ²⁺ ions in the state Ti ³⁺ on the internal face; however, simultaneously, the supply of TiCl ₄ leads to the appearance of an electrolyte flow (24) through the diaphragm which drives the TiCl ₃ thus formed towards the external face of the diaphragm, where it reacts on the Ti deposited in restoring TiCl ₂ .
• b) the intensity I ₁ is chosen at a value greater than the previous one; an excess of TiCl ₂ is produced at the feed cathode, part of this excess is reduced directly to the metallic state on this cathode, and the rest is disproportionated in the electrolyte (24) in accordance with the equilibrium (4 ) by giving birth to very fine Ti particles which form with the electrolyte a kind of mud; The report
  
  thus remains unchanged, as well as the potentials on either side of the diaphragm. The current I ₁ always passes electronically with, as a consequence, the deposition of metallic titanium on the external face and formation in equivalent quantity of Ti ³⁺ ions on the face internal; however, the excess TiCl ₃ thus formed is in turn immediately reduced to the state of TiC1 ₂ by the slurries of metallic titanium suspended in the electrolyte (24), the flow of electrolyte (24) passing through the diaphragm more then contains enough TiCl ₃ to redissolve the metallic titanium on the external surface of the diaphragm which gradually becomes clogged.
• c) the intensity I ₁ is chosen at a value lower than that which was chosen in a): an excess of TiCl ₃ not reduced at the cathode remains dissolved in the electrolyte, and the term
  
  decreases in absolute value, a drop in potential
  
  appears in the electrolyte, permeating the diaphragm which is then traversed by an ion current, which causes a decrease in the electronic current in the metal parts of the diaphragm; the deposition of metallic titanium on the external surface of the diaphragm, which results from the passage of this current, decreases and this deposit is completely redissolved by the flow of electrolyte (24) passing through the diaphragm as a result of the supply of TiC1 ₄ , of the TiC1 ₃ in diffuse excess in the bath (25). However, given the importance of the volume of the bath (25) relative to that of the bath (24), the presence of this TiCl ₃ in excess does not substantially modify the potential of the external face of the diaphragm, but it is reduced in contact with the deposition cathode, which reduces the efficiency of the cell.

Ultimately, this is case no. Which meets the best operating conditions of the cell, the solution, and this is the object of the invention, consists in regulating the current I ₁ so that there is a drop in potential as small as possible, but not zero, in the bath permeating the diaphragm. The porosity of it is not critical. It must be large enough not to overly brake the flow of electro-yoke which crosses the diaphragm. It should be low enough to allow easy detection of a potential drop in the electrolyte permeating the diaphragm.

The exact measurement of this drop in potential is relatively difficult to achieve. It is however possible to measure a value very close to this potential difference by having on either side of the diaphragm, but without contact with it, two reference electrodes, for example electrodes sensitive to chlorine ions (26 ) and (27) such as Ag / AgCI electrodes immersed in the electrolyte: the ends of these electrodes pass through the cell cover by insulating seals and are connected to a means of measuring the potential difference which will be used to control the current I ₁ or what amounts to the same, the ratio I ₂ / I ₁ .

Other methods for measuring the center of potential difference can be used for carrying out the device or method according to the invention, which are not outside the scope of the invention.

Claims (4)

1. Device for supplying an electrolytic cell (10) with TiC1₄ for the preparation of titanium in which the cover (19) is traversed by a tube (15) which allows introduction of the TiC1₄ and by a supply cathode formed by two steel rods (17) and (18) arranged on either side of the tube (15), the cathode being connected to the negative pole of a source of current and the steel rods being electrically insulated from the lid, the tube (15) and the rods (17) and (18) dipping into the molten electrolyte (11), characterised in that the tube and the cathode are surrounded by a diaphragm (20) which is electrically insulated from the cathode.

2. Process for supplying an electrolytic cell with TiC1₄ for the preparation of titanium, equipped with the device according to claim 1, characterised in that a current of intensity I₁ is passed into the supply cathode in such a way that the ratio of the activities of the ions Ti3+ and Tk²⁺ is the same on either side of the diaphragm (20).

3. Process according to claim 2, characterised in that the value of the potential measured in the electrolyte impregnating the diaphragm or of a variable connected thereto is used for regulating the intensity of the current passing into the cathode.

4. Process according to claim 2, in which the supply device is equipped with two measuring electrodes (26) and (27) placed on either side of the diaphragm, characterised in that the value of the potential difference measured between the said electrodes is used for regulating the intensity of the current passing into the cathode.

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