A method for the electrolytic production of a polyvalent metal and equipment for carrying out the method.

Process for the electrolytic production of a multipurpose metal and equipment for carrying out the method ”

The present invention relates to a method for the electrolytic production of a polyvalent metal, such as titanium, zirconium or hafnium, by cathodically dissolving a metal halide in an alkali or alkaline earth metal halide electrolyte. molten state and electro-extraction of the metal from the electrolyte.

The process relates more particularly to the preparation of titanium by electrolysis of a fused halide electrolyte.

The electrolytic production of titanium in a molten salt bath differs in many respects from the production of other monovalent metals produced in the molten state, and those aspects are reflected in the particular operating problems.

With regard to the aspects of a true industrial installation, the problems arising from the cathodic deposition of the solid state metal and also from the extreme reactivity of the metal and its ions are well known. An important contribution to solving these problems is provided by the industrial installation described in European Patent Application No. EP-A-0210961, on behalf of the applicant, whose descriptive content is to be considered as part of this specification because of its quote. The industrial plant described therein allows the electrolysis process to operate continuously and to prevent air from oxidizing the metal produced, thereby producing a high production yield and a good quality metal product.

As regards the process, an important feature that differentiates titanium electrolysis from electrolysis from other metals normally produced in molten salts is the difference between the valence of titanium in the electrolyte and its valence in the raw material titanium tetrachloride, which is not very soluble in the electrolyte. To allow efficient electrolytic extraction it is necessary to reduce titanium tetrachloride to the divalent oxidation state which is soluble in the electrolyte.

Another important aspect of titanium electrolysis is associated with its multivalence in the electrolyte with the simultaneous presence of divalent and trivalent ions, the balance of which is affected by conditions such as temperature and the presence of impurities in the electrolyte. Since the efficiency of electrolytic production is higher the higher the percentage of divalent titanium, it is necessary to keep the average valence of titanium in the electrolyte very low, generally not exceeding 2.1.

Another important factor in titanium electrolysis is the high reactivity of titanium ions in the electrolyte with nascent chlorine, both dissolved atoms and dispersed gas, which necessitates maintaining the chlorine-releasing zone separate from the chlorine. rest of the electrolyte.

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Due to this reactivity, it is necessary to prevent diffusion of titanium ions from migration to the anode vicinity, in order to avoid their oxidation to the trivalent oxidation state, to avoid their reaction with nascent chlorine and to avoid the formation of TiCl3. which is volatile at operating temperature while maintaining ion transfer between the cathode and anode due to chlorine ions.

In order to increase the efficiency of titanium extraction, and in view of the difficulties associated with the factors described above, it has been proposed in US-A-2. 789 94-3 interpose a conductive diaphragm between the anode and cathode, surrounding the anode and having walls that were permeable to the electrolyte and adapted to support a deposit in the form of a panel (cover) of the metal to be produced, and connect this diaphragm to the cell power circuit to give it an anode negative potential, in order to avoid the formation of the cathodic deposit of the metal to be produced on the permeable walls of the diaphragm which has a permeability that allows ion transfer due to chlorine ions, but also to substantially prevent the migration of the ions. titanium by diffusion from cathode to anode.

EP-B-53564- describes a method for controlling the permeability of the diaphragm covered with the desired metal deposit, which is achieved by forcing the metal deposit to increase or dissolve as a result of the drop. of voltage in the electrolyte that pervades the illllií

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own diaphragm.

The first of the above processes does not allow continuous working conditions to be maintained industrially due to the continuous variation in the thickness of the deposited panel which in itself constitutes the mass of metal produced which must be periodically removed, so that the operator is obliged to repeat the startup procedure several times a day.

The process according to said European patent EP-B-53564 does not allow the oxidation of divalent titanium in the cathode compartment and cannot prevent the consequent increase in the average valence of titanium in the bath during the formation of the metal deposit on the diaphragm, resulting in inevitably poor extraction efficiency.

The two methods described in the aforementioned patents require complex initial procedures which are costly in time and electricity and very difficult to control. In such processes, starting with the open diaphragm, starting with an electrolyte mass containing no ions of the metal to be produced, requires a sequence of operations that is unacceptable in industrial production.

In order to avoid these problems, a first object of the present invention is a process of the type indicated in the introduction of this specification, wherein the electrode extraction step of the metal is carried out in a cell consisting of at least one anode and a cathode and by a conductive structure that acts as an intermediate electrode and surrounds the

A diode to define an anode compartment and a cathode compartment, and has walls which are permeable to the electrolyte and are capable of supporting a deposit of the metal to be produced in the form of a panel to permit ionic transfer between the cathode and anode compartments, but substantially limiting the ion transfer of the metal to be produced from the cathode compartment to the anode compartment, characterized by the following steps:

(a) provide the extraction cell with an ion-containing electrolyte of the metal to be produced in solution

(b) confining within the structure an alkali metal or alkaline earth metal halide bath which is substantially free of ions from the metal to be produced by electrolyte-shielding the permeable walls of the structure; with metal sections which are susceptible to anodic dissolution,

(c) injecting an electric current between the anode and the structure in such a way as to cause the cathodic deposition of alkali metal or alkaline earth metal on the permeable walls of the structure for a period of time sufficient to allow the metal to accumulate;

d) provide an electric current between the anode and the cathode in such a way as to cause the deposition of the metal to be produced on the cathode, with simultaneous anodic dissolution of the sections, in order to allow the diffusion of ions of the metal to be produced, from the cathode compartment towards the anode compartment with metal deposit formation

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which is intended to be produced on the permeable walls of the structure as a result of the reduction of the metal ions by alkali or alkaline earth metal,

e) maintain the power supply to the anode and cathode to promote the deposition of metal at the cathode and simultaneously

f) regulating the current between the anode and the structure to keep the deposit permeability essentially constant.

During step f) the intensity of the electric current between the anode and the structure constituting the intermediate electrode is maintained to such an extent as to cause the alkaline or alkaline earth metal to deposit on the interface of the structure facing the anode compartment with a velocity sufficient to reduce the ions of the metal to be produced (eg Ti) which diffuses from the cathode compartment into the metal state, thus establishing a substantial equilibrium state between the flux of these ions (Ti ^⁺) that are being deposited and the anodic dissolution flux of the metal (eg titanium) that is being deposited on the structure interface that faces the cathode compartment.

A further aspect of the present invention is a particularly suitable composite electrode for carrying out the process described above for the electrolytic production of a polyvalent metal in a fused halide electrolyte, including:

at least one anode provided with a terminal for its

Electrical connection means an anode-isolated electrical conductive structure provided with a terminal for its electrical connection and enclosing the anode in the form of a basket, such structure having anode-facing wall portions that are permeable to the electrolyte and which are adapted to support a cathodic metal deposit, characterized in that it has supporting means associated with the walls of the structure, designed to support sealing elements in the form of sections, adjacent to the parts of the electrolyte permeable walls and to confine within the structure an electrolytic bath which is free of the metal to be produced, and to prevent infiltration of the electrolyte within the structure through the permeable wall parts, the sections of the sealing elements consisting of a metal which is susceptible to anodic dissolution under the operating conditions of the electrode ·

Other features and advantages of the method and device according to the present invention will become more apparent from the following detailed description by reference to the accompanying drawings, which are presented by way of non-limiting example only, in which:

Figure 1 is a front section of a composite electrode according to the present invention.

Figure 2 represents a projection along line II-II of Figure 1,

Figures 3 to 5 are sectional projections of a detail of Figure 1 according to various aspects,

Figure 6 is a schematic representation showing the mechanism by which metal is extracted, and

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Figure 7 is a schematic representation of the industrial installation required to perform the process.

The electrode shown in Figures 1 and 2 is particularly suited for use in an industrial installation of the type described in European Patent Application No. EP-A-0210961, which describes electrodes for suspension in a molten salt bath supported on media. supportive, the electrical connecting means consisting of a pair of conducting members facing each other and supported respectively by the opposite walls of the crucible containing the molten salt bath.

Similarly, the electrode illustrated in Figures 1 and 2 is provided with a pair of brackets described in more detail below; However, it should be understood that the innovative principle of the electrode according to the present invention can be applied whatever the technical details of its electrical connection. In the present specification the composite electrode itself will also be referred to hereinafter by the abbreviation TA, since it consists essentially of a Bipolar Titanium Electrode (TEB or ETB) which forms in situ during the initiation step of the extraction process, and by an anode A.

Referring to the drawings, the electrode according to the present invention is comprised of an anodic graphite crossbar 1 which supports three anedicated graphite bars 2 through a notch joint. With ο N2 5 is indicated a generally parallelelipedic metal structure surrounding the anodic bars 2 in a shape similar to a

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basket. The frame 3 has flat side walls 4, 5, 6 and has a base wall 8. The upper portion of the frame 3 surrounds the anodic crossbar 1 and is electrically isolated therefrom by prismatic sleeves 9 of insulating refractory material. The side walls 6 and 7 θ and the base wall 8, as well as the upper portions of the side walls 4- and 5 ', are covered with insulating refractory material panels 10. A concave element 11 is mechanically and electrically bonded to frame 3 but is electrically isolated from the anodic crossbar and is intended to support and terminate the connection of the frame to an electromotive power supply (rectifier not shown) .

There is an identical, electrically insulated concave support member 12 of the structure 3, electrically connected to the anodic crossbar and which functions as a terminal for its electrical connection.

The front walls 4 and 5 of the structure each have an opening where a lattice 13 is formed formed by a variety of tile-shaped elements 14- arranged in horizontal rows and defining passages 15 between them, through which the electrolyte. Figures 3 to 5 illustrate three different configurations of each tile-shaped element which, as will be seen in more detail below, are particularly suitable for allowing the alkali metal or alkaline earth metal deposited by cathodic reduction to accumulate. during electrode operation. The configuration of the tile element of Figure 3 »with a

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V-shaped cross section is particularly preferred.

Adjacent to each reticulum 13 of the anodic bar-facing side is a refractory ceramic fiber panel 16 which is permeable to the electrolyte. On the opposite side of the reticulum a diversity of grids is mounted 17.

The metal sections designated 18 are engaged to form an electrolyte tight seal between two annular members 19 and 20 of the frame. Each section 18, preferably comprised of a sheet of the metal itself to be produced with the aid of the composite electrode, acts as a sealing member that closes the openings in the side walls 4 and 5, allowing the electrolytic bath of molten salts in which if the anode bars are immersed, they are confined within the cavity defined by structure 3, while preventing infiltration into this cavity of the production electrolyte outside the anode, during the start-up phase of the extraction process.

The electrode according to the present invention is also provided with deflectors 21 to reduce the spraying caused by the formation of chlorine bubbles that release at the anode and the consequent drag of the electrolyte towards the anode transverse element when the electrode is in operation.

The process for producing a polyvalent metal, hereinafter described with particular reference to the production of titanium, is preferably carried out in an industrial installation of the type described in European patent application no.

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EP-A-0210961, requested by the same applicant

As illustrated schematically in Pigura 7? A crucible 22 is used which is advantageously divided into a first cell 25 for dissolution of tetrachloride and a second extraction cell 24 for the deposition of metallic titanium on the cathode. Dissolution and extraction cells communicate with each other via a valve 25.

Referring to the principle of the metal extraction phase, an electrolyte is supplied to the extraction cell from the dissolution cell, which is comprised of an alkali metal halide or alkaline earth metal halide bath containing titanium. solution. Preferably, the electrolyte is sodium chloride. The use of sodium chloride has several advantages over other electrolytes due to the simple structure of the non-complex liquid that would interfere with the titanium deposition mechanism and which, by condensing on the crucible walls above bath level, provides solid adherent layer that provides good protection for materials against the corrosive action of chlorine gas.

At the start of the extraction operation, the bath preferably contains a titanium concentration of 5% to 10%, with an average valence not exceeding 2.1.

The extraction cell comprises at least one cathode 26 and at least one composite electrode (TA) of the type described above. During the initiation phase of electro-extraction, the composite electrode structure 3 is provided with titanium sheet sections 18 and the electrode bath 12 banho

Trolytic of molten salts of alkali or alkaline earth metal halides, preferably sodium chloride, substantially free of titanium ions, is confined within the structure.

The electrolyte temperature is adjusted to preferably between 800 ° and 880 ° C. The process develops at sub-atmospheric environmental pressure.

After placing the composite electrode in the electrolyte, an electrical voltage is applied through the rectifier 27 between the anode 2 and the metal frame 3, which assumes a negative potential relative to the anode, the intensity of the current being produced such that cause the cathodic deposition of alkali metal or alkaline earth metal, preferably sodium, · The tile-like structure of the lattices favors the accumulation of metallic sodium in the concavity towards the downward part of each tile-like element, since sodium, which is lighter than electrolyte, tends to rise and become trapped under the arched wall of each tile-shaped element.

Electrical voltage is applied between the anode and the structure until substantial sodium accumulation is achieved.

An electrical voltage is then applied between anode 2 and cathode 26 to cause deposition of the titanium and simultaneous anodic dissolution of the confining sections 18. As a result of the anodic dissolution of sections 18, a material transfer is established between the electrolyte outside the structure, which contains titanium ions, and the bath inside the structure. Ti ions migrate by diffusion towards

-U

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to the anode and are reduced to metallic titanium with the sodium axuyl which has accumulated within the crosslinked structure 13, thereby providing the formation of a microcrystalline deposit in the form of porous panels that function as permeable diaphragms for ionic ion transfer. chloride but

A »M which are substantially impermeable to the diffusion flux of +2 Ti ions toward the anode.

Pigura 6 schematically represents the mechanism that is established as a result of the formation of a porous microcrystalline titanium panel, designated 28.

It should be noted that simultaneously the panel becomes the seat of various processes so that it itself functions as an electrode with the following functions:

1) the anode facing panel surface acts as a monopolar cathode; There is limited production of metallic sodium on the panel with an independent power source;

2) the panel face opposite to the above acts as a monopolar cathode in which the reaction occurs:

Ti ^⁺ + and ~ —Ti²⁺ whereby the average valence of the electrolyte remains low;

3) The interior of the panel acts as a monopolar cathode in which half reaction occurs:

You²⁺ + 2e “-> Ti with formation of fine titanium crystals;

4) acts as a bipolar electrode for a fraction of the current supplied between the production cathodes and the anodes,

<img file="PT90299B_D0009.tif" />

with limited sodium production at the anode-oriented interface and with an oxidation that turns Τϊθ into Ti ^⁺ at the interface facing the cathode; and still

5) functions as a diaphragm allowing free passage of Cl ions carrying the ionic current between the cathodes and anodes, with substantially complete precipitation of the titanium ions at the cathode facing interface caused by reaction with the sodium released by the processes 1) and 4) described above.

The electrical voltage applied between the anode and cathode is then maintained to achieve deposition of titanium on the cathode, and at the same time the current between the anode and the panel is regulated to substantially maintain the permeability of the panel. To achieve this objective, the intensity of the current between the anode and the panel is preferably adjusted to such a value as to cause sodium deposition flux at the anode-facing interface to be sufficient to precipitate the flow of Ti1 ions.⁺ which reaches the cathode interface of the panel by diffusion from the catholyte and such that a substantial equilibrium state is achieved between the reduction of Ti ^ ions.⁺ and the anodic dissolution of Τΐθ at the interface facing the cathode.

Using an industrial installation of the type described in European patent application EP-A-0210961, it is particularly easy to replace a perfectly developed cathode with a new cathode without interrupting the production cycle.

Another innovative aspect of the process of the present invention is the steps for dissolving the raw material.

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to enrich the titanium concentration in the electrolyte to be supplied to the extraction cell. Dissolution is carried out with the aid of a dissolution cathode 28 connected to a rectifier 27 θ consisting of a metal structure having a large surface immersed in the electrolyte and into which liquid titanium tetrachloride is supplied by means of a mouthpiece 29. The operation may advantageously be carried out with the aid of a TA composite electrode of the type previously described, initially provided with confining titanium sections and including a substantially titanium ion-free sodium chloride bath within the structure.

Starting with, for example, an exhausted electrolyte having a titanium ion concentration of the order of 2%, with an average valence of approximately 2.1, a potential between the anode and the structure is applied to cause sodium deposition by the above-described mechanism as referred to in the extraction step, and then an electrical voltage is applied between the dissolution cathode and the anode to cause formation of the titanium panel.

Titanium tetrachloride is then supplied to the dissolution cathode at a rate that is essentially in a stoichiometric ratio with the electric current supplied to the dissolution cathode to enrich the electrolyte to provide the desired concentration of titanium ions in the solution, which generally it is about 10%.

Dissolution process can be represented by the reactions:

TiCl₄ -} tici₂ + ci₂ ç

that is, by the half catholic reaction /

TiCl₄ + 2e “Ti²⁺ + 4C1 ~ and by anodic half reaction:

2Cl “—Cl₂ + 2e ~

It should be noted that, in reality, the cathodic process en -5+ rotates the ion according to the reaction:

2Ti⁵⁺ 2e “—2Ti²⁺ ~ 3+ where Ti ia produced by the chemical reaction:

TiCl₄ + Ti²⁺ - $ 2Ti⁵⁺ + 4G1 “

After the first phase in which the concentration of dissolved titanium in the electrolyte is increased, it is preferable to provide a further reduction in the average valence of dissolved titanium by an impregnation operation ”by interrupting the supply of titanium tetrachloride by reducing the current supplied to the electrolyte. dissolving cathode and adjusting on the composite electrode the intensity of the current between the anode and the panel, to such a value as to maintain the production of metallic sodium at the anode interface of the panel and to further reduce the trivalent titanium to the divalent phase at the cathode interface of the intermediate electrode.

During this operation, chlorine released at the anode is sent outside and the sodium produced inside the ETB reacts with the high valence electrolyte according to the reaction:

TiCl₃ + Na -> TiCl₂ + NaCl or better:

TiCl₂ + 2Na —Ti + 2NaCl 2TiCl₅ + Ti> 5TiCl₂

<img file="PT90299B_D0011.tif" />

Alternatively, it can be assumed that the high reduction efficiency of the cathodic interface is due to the direct reaction of Ti4.⁺ with the electrons supplied to the intermediate ETB electrode described above, this reaction being more energy-favored than metallic sodium deposition, although the current path configuration has a higher resistance.

After the impregnation operation it is possible to achieve not only the chemical equilibrium of reaction 2) described above, with an average valence of 2.07 ^The 825 ° C but prolonging the electrochemical reaction Ti⁹ + and “-> Ti, it is also possible to achieve average valences between 2.00 and 2.07 without equilibrium.

After the dissolution phase has been completed and an appropriate average valence has been achieved in the bath, valve 25 is opened for a time sufficient to allow the electrolyte in the extraction cell and the dissolution cell to become homogeneous.

In one embodiment, the dissolution process can be carried out without supplying an electric current to the dissolution cathode, but using the aforementioned TA composite electrode, which provides between the anode and the intermediate ETB electrode a total current. which is the sum of two currents:

(a) a first stream corresponding to the stoichiometric proportion to the flow of tetrachloride supplied to the dissolution unit according to the reaction:

2Ti⁵⁺ + 2e ~ -> 2Ti²⁺

<img file="PT90299B_D0012.tif" />

(b) a second stream corresponding to the stream required to maintain a sufficient production of sodium metha. . ~ 2+ The logical for precipitation of Ti as Ti.

In this variant it is possible to eliminate the dissolution cathode by keeping only the injection site. As far as the titanium-confining composite electrode TA sections 18 are dissolved, in the absence of the dissolution cathode, a cathodic current may be supplied to the metal wall of the crucible to cause anodic dissolution of these sections.

In one embodiment, the impregnation operation may be performed to reduce the average valence of the dissolved titanium in the electrolyte allowing the TiCl4 and TiCl4-containing electrolyte to react spontaneously with the electrolyte. metallic titanium consisting of, for example, scrap of titanium or recycled titanium from the extraction cell in the absence of electric current, according to the reaction:

2TiCl₅ + Ti ---> 3TiCl₂

This operation may be carried out for a period of 12 to 16 hours.

In short, the preferred procedures are as follows:

1) Dissolution cell including metallic titanium added to the bath:

(a) injection of titanium tetrachloride for approximately 8 hours by keeping mechanical valves 25 closed between the extraction cell and the dissolution cell;

(b) soaking for approximately 16 hours without current, keeping mechanical valves 25 open for the last two hours;

2) Dissolution cell without containing added metallic titanium:

(a) injection of tetrachloride for approximately 16 hours while the mechanical valves are closed;

(b) Impregnation for approximately eight hours with a current limited to the ETB, keeping the mechanical valves open for the last two hours.

It is also possible to maintain the extraction phase and dissolution phase simultaneously by maintaining the circulation of electrolyte between the extraction cell and the dissolution cell through valve 25 and by regulating the operating parameters of the cathodic dissolution cell, in particular the supply. halide by regulating the current to the dissolution unit and regulating the current to the intermediate electrode structure, in order to maintain the concentration and average valence of the dissolved titanium ions at their operating values.

Example

The process for the production of titanium is carried out using the industrial plant described in European Patent Application No. ER-A-O21O961 in which the crucible is divided into an extraction cell and a dissolution cell. The extraction cell comprises 6 iron cathodes each having a surface of 2 m and 5 composite electrodes.

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TA are provided with titanium-abutting sections and include a sodium chloride bath within the structure as described above. The electrolytic bath is made up of sodium chloride and 5% by weight of titanium chloride Ti.

In the start-up phase, an electric current density of approximately 4000 A / m of cathodic surface is provided at the extraction cell Ta over a period of 1 hour, after which operating conditions are achieved by providing a density of 1500 A / mp electrical current

cathodes and an electrical current density of 500 A / m to the cathode surface of the BTB and giving the cells electrical voltages of the order cbs 6.5 V between anode and cathode * and of the order

5.5 V between 0 anode and 0 ETB.

In the dissolution cell, which consists of 3 dissolution cathodes, each having a surface of 2 m and two TA composite electrodes, is provided to the 2

AT a current density of 4000- A / m of the cathodic surface at start-up over a period of one hour, simultaneously with the start-up of the extraction cells, and then the dissolution cathodes are provided with a working electrical current density of 2500 A / m and the ETB is provided with an electrical current density of 500 A / m cathodic surface and electrical voltages are established in the cells in the chamber. 6 V between 0 anode and cathode and about 5 5 V between 0 anode and ETB, providing 33,5 kg / hour of TiO1.

Within 12 hours approximately 12 kg is collected ii ί

i of titanium per square meter of cathode after checking that it is of the quality given in Table 1.

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ANALYSIS OF ELECTROLYTIC DEPOSITION

LE IMPURITY CONCENTRATION (ppm) * -

<td>Element</td><td>Conventional Value</td><td>Process according to the invention</td>

<td>Oxygen</td><td> 650</td><td> 390</td>

<td>Nitrogen</td><td> 35</td><td> 25</td>

<td>Carbon</td><td> 85</td><td> 50</td>

<td>Chloride</td><td> 14-00</td><td> 160</td>

<td>Erer</td><td> 200</td><td> 50</td>

<td>Hydrogen</td><td> 325</td><td> 217</td>

<td>Aluminum</td><td> 100</td><td> 50</td>

<td>Vanadium</td><td> 100</td><td> 50</td>

<td>Manganese</td><td> 100</td><td> 50</td>

<td>Nickel</td><td> 100</td><td> 50</td>

<td>Chromium</td><td> 100</td><td> 50</td>

<td>Molybdenum</td><td> 100</td><td> 50</td>

<td>Tin</td><td> 100</td><td> 50</td>

<td>Copper</td><td> 100</td><td> 50</td>

<td>Silicon</td><td> 100</td><td> 50</td>

<td>Zirconium</td><td> 100</td><td> 50</td>

<td>Boron</td><td> 100</td><td> 30</td>

<td>yttrium</td><td> 100</td><td> 10</td>

<td>Magnesium</td><td> 100</td><td> 10</td>

<td>Sodium</td><td> 1100</td><td> 100</td>

<td>Phosphorus</td><td> 30</td><td> 30</td>

<td>Bhn</td><td> 90/100</td><td> 85/86</td>

^ BHN: Brinnel Hardness Coefficient

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