US2915441A - Baths used in igneous electrolysis - Google Patents

Description

Dec. 1, 1959 E. J. B. PRUVOT 2,915,441

BATHS USED IN IGNEOUS ELECTROLYSIS Filed March 22, 1956 INVENTOR EMILE JEAN BAPT/5T E PRUVOT ATTORNEY nited States Patent f; 2,915,441 BATHS USED INIGNEOUS ELECTROLYSIS Emile Jean Baptiste Pruvot,Saint-Jean-de-Maurienne, France, assignor to Pechiney, Compagnie de Produits Chimiques et Electrometallnrgiques, Paris, France, a corporation of France Application March 22, 1956, Serial No. 573,139 Claims priority, application France March 30, 1955 8 Claims. c1, 204-44 The present invention is directed to improvements in the composition of and method of use of chlorofluorinated baths employed inii'geneous electrolysis for producing and refining .metalsand :moreparticularly, aluminum, beryllium, titanium and zirconium. t A further "and-particular obje'ct'oi the present invention is' an improved method for refiningtitanium and zirconium'scrap'and an improved apparatus' for carrying out this .method.

2,915,441 Patented Dec. 1, .1959

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inally present or ,produced by a secondary reaction, is

out according to the three-layer method: A layer of aheavy alloy containing thesimpure metal to berefined and servingas anode, rests in the liquid state on the bottom of thecrucible of the electrolysis .cell. Superposed .thereon, is -a layer of molten electrolyte, ontop of which floats the refined metal constituting thecathode.

Theel'ectrolyte is, generally composed'of a mixture of chlorides and fluorides containing an appreciable proportion of a halogenated compound: of the metal which it is desired to refine in order. the'avoid the deposition,'at

the cathode, of alkali or alkaline-earth metals, the salts of which generallycons'titute the major portion of the electrolyte. I

For example, in refining aluminum, there is frequently used a bath containing 6O% .BaCl 'and 40% chiolithe (2AlF 3NaF), a small amount of sodium chloride being added to this mixture.

It has also been proposed to use a sodium-free bath containing bariumchloride,calcium fluoride and aluminum fluoride, the proportion of'the latter compound varying between 25% and In the course of the electrolysis,.additions oflsalts are frequently made so .as to". maintain the composition of the bath within'thosedimits' which make it possible to obtain the refined metal atthe cathode. '5 These additions contain aluminum fluoride having a certain content of free alumina which isalways present as a result of its method of production. The residual moisture in this salt causes hydrolysis reactions of the general type:

During the operation, this alumina is precipitated on the anode increasing thereby the ohmic resistance at the contact of the two layers; it islikewise" deposited all around the electrolysis crucible and forms crusts heavily loaded with alumina, since the alumina content may reach 50%,"thusleading to a contraction in the" crosssection of the crucible'and, hence,"to an increase in the current densityanda further cause for the'increas'e. in the'voltageat the terminals'of'the cell.

For all of'these 'reasons,'fre'e' alumina, whether o'rig highly undesirable.

The present invention, which is based on applicants investigations, relates to a process which makes it possible to avoid these drawbacks by causing the disappearance of the oxidized compounds from the electrolysis bath or the correcting bath. This process consists in treating the bath with a compound which is capable of exerting onthe oxidized compounds an energetic fiuorinatin It is action and of transforming them into fluorides. possible to use gaseous hydrofluoric acid; however, good results are obtained by the use of alkaline or alkalineearth bifluorides, as for example NaF.HF, KF.HF, CaF .2l-IF.

According to a preferred form of the invention, there is added to the constituents of the bath'ammonium bifluoride, which disappears completely during the course of the treatment.

The fluorinating action of ammonium bifluoride starts at 200 C., a temperature sufficiently high to dehydrate most of the halides of which the bath is constituted.

The action of the bifluoride may advantageously be extended up to 600 C.

The example set out below, which is solelygiven by way of illustration, and not limitation, will give a better understanding of the invention:

Example 1 In the electrolytic refiing of aluminum by means of a bath containing 60% barium chloride and 40% chiolithe (2AlF 3NaF), the correction of the bath is carried out most often by means of a mixture of salts containing NaCl, BaCl and AlF in suitable proportions, sometimes in equal parts, and sometimes in the proportion ofby way of example25% NaCl, 35% BaCl 40% AlFg. v i

a If there be heated together grams BaCl containing 1.5% water 8 0grams NaCl containing 1.4% water 80 grams All-' containing 9% water, and 6% free'Al O the. mixture melts at 600 C. but contains considerable quantities of insoluble alumina muds.

In contrast, if there be added to a mixture having the amount of hydrofluoric acid and NH F vapor is evolved,v

there is obtained a perfectly fluid and transparent bath, without alumina. This bath solidifies around 500 C., can be cooled and then remolten, which operation can be carried out four times in succession without formation of any'turbidity. The alumina, which is insoluble in this bath, has therefore been completely transformed into aluminum fluoride and the moisture of the starting materials, which would produce alumina by hydrolysis, has been eliminated.

It is advantageous to dispose a hood above the-furnace during the melting operation so as to recuperate the evolved vapors.

While the example set out above relates to the elimination of alumina from a correcting bath, the process can equally be applied to the refining bath itself before the cell is started.

The process which is the object of the present invention is not limited to the refining of aluminum, but can also be applied in all cases where the presence of an oxidized compound is undesirable, both in electrolytic production (elaboration) as well as in the refining of metals. I

As will be apparent from the foregoing, applicant has disclosed a process which enables the elimination of undesirable, oxidized compounds contained in the chlorofiuorinated baths used in electrolysis. To recapitulate: this process consists in treatingthe bath with an energetic fluorinating agent, for example, hydrofluoric acid or ammonium bifluoride, which is capable ofv transforming the oxidized compounds into fluorides.

However, in the practical application of the present invention, it has been observed that in the course of the treatment certain chlorides may be partially transformed into fluorides'which, occasionally, presents drawbacks.

Accordingly, a further feature of the present invention consists in excluding from the treatment with the fluorinating agent, those chlorides which are susceptible of being transformed into fluorides undesirable in the electrolysis. These chlorides may be separately dried, if necessary in a vacuum, before being mixed with the substances which are subjected to the fluorinating treatment. The following example, merely given by way of illustration and not limitation, will explain further this particular feature of applicants invention:

Example 2 In the continuous refining of aluminum by means of a bath containing 60% BaCl and 40% chiolithe (2AlF .3NaF), there are periodically made corrective additions containing AIF BaCl and NaCl. The fluorinating treatment by means of NH F.HF transforms a portion of the barium chloride into barium fluoride, the presence of which changes the density, the fusion point and the selectivity of the bath which, as a result of such changes, may be rendered unsuitable for the aluminum refining operation. Accordingly, and following applicants further improvement, the procedure adopted is as follows:

parts of aluminum fluoride are carefully mixed with 3 parts of ammonium bifluoride, NH F.HF. This is heated between 400 and 500 C. with constant stirring and while proceeding very carefully and slowly at the beginning in order to avoid vaporization (entrainment). The end of the reaction is indicated by cessation of bub bling. The heating is preferably carried out in a graphite crucible, proceeding with successive additions of the mixture to be heated in order to avoid violent evaporation (entrainment). An aluminum stirrer can be used.

When the bubbling has ceased, the hot aluminum fluoride is poured into an aluminum basin and permitted to cool. 'As soon as it is cold, it is immediately mixed with the two other correcting salts BaCl and NaCl which previously have been carefully dried at 120 C. There is obtained in this manner an addition bath free of oxides.

and capable of giving excellent results in the refining of aluminum.

The present invention finds a particularly important application in the electrolytic refining of titanium and zirconium. The metallurgy of titanium as well as of zirconium produces a metal which is frequently contaminated with impurities, as for example, oxygen and nitrogen. As a result, an appreciable percentage of the resultant metal, sometimes between 40% and 50%, does not possess the necessary ductility and constitutes scrap. The production cost of ductile titanium and zirconium is appreciably increased as a result of this circumstance and it becomes mandatory to treat this scrap. It is afurther object of the present invention to use the improved process disclosed herein in the treatment of such titanium and zirconium scrap.

Electrolytic refining processes of metals have been known for a long time: The crude metal is used as an anode which is dissolved little by little by the anions of the electrolyte and is deposited in a pure state onthe cathode. This process is especially used in refining aluminum and beryllium, using as electrolytes molten alkaline vor alkalineeearth halides containing ac'ertain proportion of the halide of the metals which it is desired to refine, so as to maintain in the electrolyte a certain concentration of cation of the metal to be deposited, for example, between 35 and 115 gr. of this metal per kgm. of bath.

As a special feature of this invention, applicant has applied the present invention to improve this general refining method, which is also applicable to the refining of titanium and zirconium.

In accordance with the present invention, there is manufactured ductile titanium and Zirconium having a Brinell hardness less than 120, and frequently less than 100, by the electrolytic refining of scrap metal in a bath preferably composed of molten alkali chlorides; the necessary quantity of titanium or zirconium ions is obtained preliminarily by acting with a tetrachloride, TiCL, or ZrCl on titanium or zirconium scrap, which constitutes the anode element of the refining cell. An important feature of the improvedprocess according to the invention, consists in eliminating all trace of oxygen in the electrolyte prior to starting the refining operation; indeed, it has been observed that when such elimination is suitably carried out, there is obtained at the beginning of the refining operation metallic titanium the Brinell hardness of which is less than 100. A similar improvement has been found in the case of the zirconium also. i

In a preferred embodiment of the invention, there is used as deoxidizing agent ammonium bifluoride, NH F.HF. To this end, the procedure is as follows:

NaCl or NaCl+KCl are mixed in the dry condition, with for example, 10% of NH F.HF. The mixture is heated to between 300 and 400 C. to start the deoxi dizing effect. A small quantity of titanium or zirconium chips of the order of a few percent of the bath is added, in order to reduce the sulphates into sulfides; thereupon, the chlorides are molten at a maximum temperature of 900 C. Under these conditions, the sulfides are destroyed by the residual bifluoride which completes in a thorough manner the deoxidization and de-sulfurization. This operation can be carried out in air but, preferably, is carried out in an argon atmosphere as is also the subsequent electrolysis.

During the deoxidizing operation, a portion of the alkali chlorides is transformed into fluorides which do not in any fashion interfere with the refining operation.

The titanium or zirconium tetrachloride is then injected into the interior of abasket containing the scrap of the metal to be refined, and which will subsequently form the anode assembly during the electrolysis. In the presence of a large excess of metal, the TiCl or ZrCl; is transformed almost completely. into bichlorides.

The injection of the tetrachloride can be stopped when the bath contains about 40 grms. of titanium or zirconium per kgm. of electrolyte, but the injection can be contined until there is obtained, for example, 60 to grms. of the metal to be refined per kgm.

The electrolytic refining is thereafter carried out in known manner without it being necessary to reinject the tetrachloride.

At the cathode, there are obtained needles of titanium or zirconium which may attain 4 centimeters in length and a cross-section of several-square millimeters. The portion of this deposit which does not pass through a 100 mesh screen attains or exceeds 95 The metal yield frequently attains The following example will give an idea of the purity of the produced metal:

Parts per million Oxygen Very light traces Titanium thus prepared, when tested with a 10 mm. ball and a pressure of 3 tons has a Brinell hardness varying between 130 and 73, with an average of 100. Obviously, this constitutes an excellent ductile titanium.

The present invention also comprehends an electrolysis cell which enables the refining of titanium or zirconium to be carried out in a continuous manner using the resistance of the bath to maintain the electrolyte in molten condition without the use of an external heat supply.

Figures 1 and 2 show an embodiment of an electrolysis cell in accordance with the invention, wherein Figure 1 shows a vertical sectional view along the line 1-1 of Figure 2, of the electrolysis cell, and

Figure 2 is a plan View of the cell partly in section, the section plane being along line 22 of Figure 1.

In these figures; 1 designates the external metallic casing of the cell, 2 the heat insulation jacket, 3 the interior casing which may be of sheet iron. The space between the casings 1 and 3, filled with heat insulation bricks, constitutes a tight assembly in which a vacuum can be made through connection 4. 5 is a circular iron basket made, for example, of a sufliciently close metallic screen; this basket, filled with titanium or zirconium scrap, is connected at 6 to the positive pole.

The cathodes 7 are formed of metallic tubes which are closed at their lower ends; they dip into the molten electrolyte the level of which is represented by the line 8-8. These cathodes are extended upwardly by tubes 9, 9 of a smaller diameter, which makes it possible to remove them when the titanium or zirconium deposit has become too thick; these tubes are connected at their upper ends to the negative pole by the conductor 16.

The muds accumulate at the bottom of the cell at 11, and can be periodically removed through a lock chamber 12 closed with a tight cover 13.

A cylindrical space or chamber 14 extends above the electrolysis cell; it is lined at its circumference by the basket 5 containing the titanium or zirconium scrap and is traversed by the rods 9 carrying the cathodes. The cylindrical chamber is fixed at its lower end to the electrolysis cell by means of a tight joint 15.

To the upper portion of the cylindrical chamber 14, there is fixed by a tight joint 16 an assembly of cylindrical lock chambers 17 (one per cathode) which can be closed near their lower ends by valves 18; the height of the lock chambers above the valves is suflicient so that the cathodes 9 can be withdrawn therein. At the upper part of each lock chamber, there is disposed a vacuum and argon connection 19.

Openings 20 enable the introduction, when necessary, of an iron bar to facilitate the descent of metal scrap to be refined into the basket 5, or of a tube for introducing TiCl or ZrCl to form the titanium or zirconium cations before the start of the electrolysis.

The quantity of titanium or zirconium scrap contained in the basket permits continuous operation of the electrolysis for a period of to days, but there are provided three scrap feeding lock chambers 21 disposed above the iron ring of the cover of the cylindrical chamber 14, at the side of the openings 20. By'the use of these lock chambers, the scrap metal to be formed falls directly into the basket 5.

As the electrolysis progresses, it is necessary to remove and change the cathodes one after the other; too thick a deposit of refined metal prevents normal operation of the electrolysis due to the risk of short circuit between anode and cathode. The cathode, which it is desired to remove, is first lifted into the cylindrical chamber in order that it may drain; it is then introduced into its lock chamber 17, and the valve 18 thereof is closed; it cools therein and the lock chamber can then be opened at its upper end for the removal of the cathode coated with titanium or zirconium, and for the replacement of the cathode.

Following closure of the lock chamber, ,a' vacuum is made therein; it is then filled with argon; the valve 18 is then opened, and the new cathode is lowered into the molten electrolyte.

The same lock chambers which serve for the transfer of the cathodes can advantageously be used for the introduction of the bath previously treated asset out above.

The electrolysis cell in accordance with this invention makes it possible to obtain a continuous refining operation. The compact form of the cell makes it possible to maintain the electrolyte in a liquid state by the heat evolved by the passage of the electric current therethrough. The cell can operate with current strengths of thousands of amperes.

While in the preferred method of refining zirconium and titanium according to this invention use is made of molten alkali chlorides, it is understood that other halides of alkali metals, as well as halides of alkaline earth metals can be employed, such as bromides, iodides and fluorides of alkaline and alkaline earth metals and magnesium.

The term preparation as used in the appended claims covers both the production of metals, as by reduction of their compounds, as well as the refining of impure metals.

I claim:

1. In the preparation of metals selected from the group consisting of aluminum, beryllium, titanium, and zirconium, by the igneous electrolysis of a bath comprising a mixture of halides and a halide of the metal to be prepared, said metal halide being contaminated with an oxide of said metal, the improvement of inhibiting the deposition of said oxide in the cell which consists in: treating the bath with an agent capable of exerting a fluorinating action on said oxide and of transforming it into a fluoride whereby the bath is maintained in a substantially fluid and transparent condition and substantially free of the metallic oxide.

2. Process according to claim 1, wherein the fluorinating agent is a compound selected from the class consisting of gaseous hydrofluoric acid, ammonium bifluon'de, alkali metal bifluorides and alkaline earth metal bifluorides.

3. Process according to claim 1, wherein the fluorinating agent is ammonium bifluoride which is reacted with the bath at a temperature within the range of 200600 C.

4. Process according to claim 1, wherein the halide constituents of the bath contain residual moisture.

5. Process according to claim 1 wherein the mixture of halides consists of alkali and alkaline earth metal chlorides and fluorides.

6. In the preparation of metals selected from the group consisting of aluminum, beryllium, titanium and zirconium, by the igneous electrolysis of a bath comprising a mixture of metallic chlorides and fluorides including that of the metal to be prepared, said fluorides being contaminated with a metallic oxide, the improvement of inhibiting the deposition of said oxide in the cell which consists in: treating the fluoride constituents of the mixture at an elevated temperature with a fluorinating agent selected from the class consisting of gaseous hydrofluoric acid, ammonium bifluoride, alkali metal bifluorides and alkaline earth metal bifluorides, whereby the oxide is transformed into a fluoride, cooling the so-treated metal fluoride, drying the chloride constituents of the bath, mixing the dried chlorides with the cooled metal fluoride and using the resultant mixture as an additive for the electrolysis bath.

7. In an electrolytic process for treating scrap of a metal selected from the group consisting of titanium and zirconium to produce a refined ductile product, the steps comprising: heating at least one halide of metal selected from the class consisting of: alkali and alkaline earth metals, and wherein said halide is contaminated with oxygen-containing compounds of the metal to be prepared;

contacting the heated halide with a bifluoride and further heating the halide to a, temperature sufiicient to melt the same, whereby the oxygen-containing compounds react with the bifluoride and are deoxidized; contacting scrap of the selected metal with the molten deoxidized halide, and injecting a tetrachloride of the selected metal into the scrap, thereby forming bichloride of the selected metal. a

8. Process according to claim 7, characterized in that a mixture of sodium and potassium chlorides is treated with ammonium bifiuoride and heated to a temperature 0f.'300-400 CL, comminuted metal to be refined is added to the heated chloride mixture, and the heating is continued to a temperature not in excess of 900 C. to melt the chlorides.

References Cited in the file of this patent UNITED STATES PATENTS 578,633 Gooch Mar. 9, 1897 736,020 Roepper Aug. 11, 1903 786,244 Blackmore Mar. 28, 1905 842,256 Seward et a1. Jan. 29, 1907 2,451,494 Johnson 'Oct. 19, 1948 2,731,402 Topinka et a1. Jan. 17, 1956 10 2,755,240 Normore et al. July 17, 1956 FOREIGN PATENTS 713,446 Great Britain Aug. 11, 1954 OTHER REFERENCES The Production of Zirconium by Fused Salt Elev trolysis, AEC Report NYC 3117, J an. 1, 1952.

Claims (1)

1. IN THE PREPARATION OF METALS SELECTED FROM THE GROUP CONSISTING OF ALUMINIUM, BERYLLIUM TITANIUM, AND ZIRCONIUM, BY THE IGNEOUS ELECTROLYSIS OF A BATH COMPRISING A MIXTURE OF HALIDE BEING CONTAMINATED WITH AN PREPARED SAID METAL HALIDE BEING CONTAMINATED WITH AN OXIDE OF SAID METAL, THE IMPROVMENT OF INHIBITING THE DEPOSITION OF SAID OXIDE IN THE CELL WHICH CONSISTS IN: TREATING THE BATH WITH AN AGENT CAPABLE OF EXERTING A FLUORINATING ACTION ON SAID OXIDE AND OF TRANSFORMING IT INTO A FLUORIDE WHEREBY THE BATH IS MAINTAINED IN A SUBSTANTIALLY FLUID AND TRANSPARENT CONDITION AND SUBSTANTUALLY FREE OF THE METALLIC OXIDE.

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