US3030285A - Semi-continuous electrolytic process - Google Patents

Description

3,030,285 SEMI-@QN'HNUUUS ELEQTRULYTHC PRUCESS Ralph M. Sarla, Lewiston, N.Y., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed May 31, 1955, Ser. No. 512,2a7 9 Claims. (Cl. 294-64) This invention relates to an electrolytic process for the production of extremely pure reactive refractory metals of groups IV, V and VI of the periodic table.

On'e commonly employed method of producing such metals is by the electrolysis of one of their compounds dissolved in an electrolyte consisting of fused alkali and alkaline earth metal halides. Electrolytes generally em 'ployed in this connection are calcium chloride, mixtures of calcium and magnesium chlorides, mixtures of calcium chloride, sodium chloride and potassium chloride, as well as sodium and potassium chlorides, either alone or in combination.

The inherent ductility of many of these metals is adversely affected by traces of elemental impurities. In particular, minute amounts of hydrogen are capable of penetrating and transforming the metallic grain structure. Metals having this element often pass all the initial hardness and tensile strength tests, but may fail abrupt-1y after a short service period under strain.

Reactive refractory metals absorb hydrogen from several sources. One of these is moisture accumulated in the brick of metal furnace during temporary shut-down. Another is the electrolyte used in their production. Still another is live steam employed to clean dies with which these metals are forged.

In the specific case of the present commercial procedure for extracting tantalum from .its ores, usually tantalite, difficult and expensive fractional crystallization steps are necessary to remove both metallic and non-metallic impurities, which steps are followed by electrolysis of potas sium-tantalum-heptafluoride on a batch basis. During electrolysis a finely powdered, slightly impure tantalum metal is deposited at the cathode, and a portion of the fluorine present in the original salt is evolved at the anode as a highly corrosive gas. At the end of the operation the contents of the cell are allowed to solidify, removed from the cellthe metal being separated by wet chemical process. The solidified mass remaining in the cell at the end of the operation contains approximately percent of tantalum metal, the balance consisting of potassium fluoride and potassium-tantalum-heptafluoride. The last mentioned salts must, of course, be reprocessed in order to recover the tantalum that was not converted to metal during electrolysis.

A newer procedure for the electrolytic production of reactive refractory metals is disclosed and claimed in a copending application, Serial No. 467,628, filed November 8, 1954, by D. H. Barbour and R. M. Sarla (now abandoned). This procedure employs an electrolyte comprising highly purified sodium chloride, and the appropriate complex fluoride of potassium and the desired metal. The advantage of utilizing sodium chloride in such an electrolyte lies in the fact that chlorine instead of fluorine is evolved at the cathode. In this manner, corrosion defects are materially reduced. In the practice of the above-outlined method, it has been necessary to operate on a batch basis, since accumulation of the potassium and sodium fluorides in the cell and exhaustion of the sodium chloride contained alter the character of the electrolysis.

It is accordingly the primary object of the instant invention to provide a procedure which will permit practically continuous production of high melting point reactive metals by fusion electrolysis.

Another object of the invention is to provide an electrolytic process for the production of reactive metals in visibly crystalline form.

A further object of the invention is to provide a procedure which eliminates the necessity of reprocessing and repurifying electrolytic constituents.

A still further object of the invention is to provide a fusion electrolysis procedure, which procedure operates more conveniently, more efiectively, and at less cost than heretofore employed methods.

It is a still further object of the invention to provide a procedure of fusion electrolysis which substantially eliminates polarization.

A more specific object of the invention is to provide an electrolytic process for the production of tantalum metal, which process provides a product having a purity of 99.95 percent or better without requiring further purification steps.

Viewed in its broadest aspects, the instant invention contemplates a semi-continuous electrolytic process for the production of tantalum, titanium, columbium, chromium and similar metals, which process employs a fused electrolyte containing principally sodium chloride and fluoride, to which may 'be added the halide of the desired reactive metal. Such additions of reactive metal halide are made periodically during the electrolysis period, thereby prolonging the operation and providing better control thereof. The proportions of the chloride to fluoride are so adjusted in relation to the quantity of refractory metal chloride as to result in the apparent formation in situ of the complex alkali metal, refractory metal, double fluoride. The electrolysis need be interrupted only when the cathode has been coated with such a thickness of deposited metal that its physical size necessitates its removal.

In this manner a chemically continuous process is obtained, since all the elements contained in the source material are removed from the cell during operation. By employing a cell permitting a very rapid change-over of cathode, the instant process may be made mechanically continuous as Well.

The minimum relationship between alkali metal fluoride and the reactive metal chloride is illustrated by the following equations:

The above equations illustrate only four of the possible metals in groups IV, V and VI which may be electrolyzed according to the method of the present invention. Although the equations show the alkali metal as sodium, other alkali metals may be substituted therefor.

A minimum initial composition of the electrolyte is shown on the right hand side of each of the above equations. Additional sodium chloride other than that shown in the equations may be included in the electrolyte. In general, it is preferable to have some excess over the indicated quantity in order to insure that no fluorine will be evolved at any time during the electrolysis, paiticularly at the start. Such addition may also be desirable to adjust the melting point of the electrolyte.

As the electrolytic action proceeds, additional quantities of the refractory metal chlorides are added to the fused baths in quantities equal to or less than the amount corresponding to the stoichiometric equivalent of metal removed by electrolysis. As indicated above, this action may be repeated as often as desired until the metal deposited on the cathode has built up to the extent that it occupies such an undesirably large portion of the cell volume as to necessitate its removal. When such circum- For the sake of conciseness, the method of the invention will be described in detail with particular reference to tantalum, it being understood that it is not so limited.

As an example of the practice of the invention, an electrolytic bath comprising equal quantities of sodium chloride and sodium fluoride was purified to remove all traces of moisture and air by a special high temperature vacuum treatment method such as that disclosed and claimed in my copending application Serial No. 782,617, filed Dec. 24, 1958. The mixture was then fused in an electrolytic cell. 3.65 pounds of tantalum pentachloride were added to this electrolyte, and the mixture electrolyzed at the voltage of 4 volts and a current of 90 amperes for a period of 4.65 hours. At the end of this period 450 grams of tantalum metal were obtained. of tantalum pentachloride were next added to the cell, and electrolysis continued at a cell voltage of four volts, and an increased current of 95 amperes. This operation produced 828 grams of tantalum metal after 8.1 hours. In addition to the tantalum pentachloride added during this second period, two pounds of purified sodium chloride were also introduced into the cell to change the ratio of sodium chloride to sodium fluoride from 50 to 50, to 56 to 44. At the conclusion of the second electrolytic period, 6 /2 pounds of tantalum pentachloride were added to the cell. The cell voltage was raised to 5 volts, the current to 188 amperes. Proceeding under these conditions for 6.1 additional hours, 1223 grams of tantalum metal were produced. A fourth addition of tantalum pentachloride (7.7 pounds) was made to the cell, and electrolysis continued at 6 volts and 232 amperes, to give 1434 grams of tantalum metal. For the fifth electrolytic period, 6.61 pounds of tantalum pentachloride were added, and the process continued at an increased voltage of 7 volts, and an increased amperage of 365 amperes. Under these conditions 1401 grams of tantalum metal were recovered. In summary, a total of 27.55 pounds of tantalum pentachloride was added for the five electrolytic periods, and a total of 11.75 pounds of tantalum was recovered. The metal produced had a hardness varying between 84 and 147 on the Brinell scale. The current efiiciency was 74 percent.

As another example of the practice of this invention, crystals of columbium (niobium) were produced by the electrolysis of columbium pentachloride in a fused electrolyte consisting of 72.5 percent of sodium chloride and 27.5 percent of sodium fluoride vacuum melted according to procedures disclosed and claimed in my copending application Serial No. 427,886, filed May 5, 1954.

In the manner described in the preceding example, a total of 5.65 pounds of CbCl containing 882 grams of columbium metal, was added to approximately 17 pounds of the base electrolyte at a cell temperature of 800 C. The mass was electrolyzed at 5.5 volts and 330 amperes. A total of 460 grams of metal containing coarse, needlelike crystals averaging 2 to 4 millimeters in length and one millimeter in diameter were produced.

The procedure described above was repeated with TiCl to produce titanium metal. 2.5 pounds of TiCL; was added to about 17 pounds of a mixture consisting of potassium chloride and sodium fluoride in an 80-20 ratio. An electrolyzing current of about 180 amperes at a voltage of '5 volts maintained the cell in a fused state at a temperature of about 750 C. The reaction proceeded for 8 hours to give 63 grams of titanium metal.

To the salt residue from the above run was added 0.44 pound of K TiF This mixture was then electrolyzed at an amperage of 180 to 285 amperes for 13 /2 hours to 355 pounds give 153 grams of titanium metal. The cathodically deposited titanium had a Brinell hardness of 239.

In the same manner 3.2 pounds of TiCL; were added to 17 pounds of a 50-50 mixture of sodium chloride and sodium fluoride. This mixture was maintained in a fused state by passing an electrolyzing current of 225 to 400 amperes under a voltage range of 5 to 6% volts. The reaction which required 12 /2 hours, gave grams of titanium. This metal had a hardness of 226 on the Brinell scale.

Advantages of this process, in addition to those mentioned heretofore, including its semi-continuous character, arethat the metal is easily removed from the cathode, and that it deposits in the form of dendritic trees, the individual crystals of which average from 1 to 2 millimeters in diameter, with a length of 4 to 5 millimeters. Previously, reactive metals have been produced as powders in which formthey are contaminated easily, since the ratio of surface area to mass is considerably higher than in the visibly crystalline form.

The deposited metal requires only simple leaching with hot water to remove mechanically entrained electrolyte, following which step the metal may be dried and are melted in an inert atmosphere to form an ingot. Since an alkali metal chloride is included as an essential constituent of the electrolyte, the fluorine produced is not evolved as a gas, but immediately replaces chlorine to form a fluoride.

What is claimed is:

1. In the electrolytic production of reactive refractory metals of groups IV, V and VI of the periodic table, wherein a chloride of the selected reactive refractory metal is electrolyzed in a fused bath, the improvement which comprises employing a fused bath containing principally at least one simple alkali metal chloride and at least one simple alkali metal fluoride.

2. In the electrolytic production of reactive refractory metals of groups IV, V and VI of the periodic table,

wherein the most highly oxidized chloride of the selected reactive refractory metal is electrolyzed in a fused bath, the improvement which comprises employing a fused bath containing principally at least one simple alkali metal chloride and at least one simple alkali metal fluoride.

3. A process for the production of a reactive refractory metal selected from the group consisting of tantalum, columbium, titanium and zirconium comprising electrolyzing, in a fused bath containing principally at least one simple alkali metal chloride and at least one simple alkali metal fluoride, a refractory metal chloride respectively selected from the group consisting of tantalum pentachloride, columbium pentachloride, titanium tetrachloride and zirconium tetrachloride.

4. A process in accordance with claim 3, wherein said selected reactive refractory metal is tantalum and said selected refractory metal chloride is tantalum pentachloride.

5. A process in accordance with claim 3, wherein said selected reactive refractory metal is columbium and said selected refractory metal chloride is columbium pentachloride.

6. A process in accordance with claim 3, wherein said selected reactive refractory metal is titanium and said selected refractory metal chloride is titanium tetrachloride.

7. A process in accordance with claim 3, wherein said selected reactive refractory metal is zirconium and said selected refractory metal chloride is zirconium tetrachloride.

8. A semi-continuous process for the production of a reactive refractory metal selected from groups IV, V and VI of the periodic table comprising, providing in an electrolytic cell having an anode and a cathode a fused bath containing at least one simple alkali metal chloride and at least one simple alkali metal fluoride, passing an electric current between said anode and cathode, making repeated additions tosaid fused bath of said selected reactive refractory metal chloride in an amount up to the stoichiometrical equivalent of metal removed by electrolysis, until the volume of the pure, crystalline metal deposited on the cathode necessitates its removal, and then inserting a new cathode and continuing said electrol- 37815.

9. A semi-continuous process for the production of a reactive refractory metal selected from groups IV, V and VI of the periodic table comprising, providing in an electrolytic cell having an anode and a cathode a fused bath containing at least one simple alkali metal chloride and at least one simple alkali metal fluoride, passing an electric current between said anode and cathode, making repeated additions to said fused bath of the most highly oxidized metal chloride of said selected reactive refractory metal in an amount up to the stoichiometrical equivalent of metal removed by electrolysis, until the volume of the pure, crystalline metal deposited on the cathode necessitates its removal, and then inserting a new cathode and continuing said electrolysis.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. IN THE ELECTROLYTIC PRODUCTION OF REACTIVE REFRACTORY METALS OF GROUPS IV, V, AND VI OF THE PERIODIC TABLE, WHEREIN A CHLORIDE OF THE SELECTED REACTIVE REFRACTORY METAL IS ELECTROLYZED IN A FUSED BATH, THE IMPROVEMENT WHICH COMPRISES EMPLOYING A FUSED BATH CONTAINING PRINCIPALLY AT LEAST ONE SIMPLE ALKALI METAL CHLORIDE AND AT LEAST ONE SIMPLE ALKALI METAL FLUORIDE.