US3002905A - Process for electrowinning titanium from lower valent titanium alkali chlorides - Google Patents

Description

Oct. 3, 1961 BR ET AL 3,002,905

PROCESS FOR ELECTROWINNING TITANIUM FROM LOWER VALENT TITANIUM ALKALI CHLORIDES Filed May 27, 1955 E2 or C (Anode) L gage Cai'ho/yfe INVEN TORS, Abner B r arm er 3,002,905 PROCESS FOR ELECTRUWINNlNG TTTANIUM FROM LOWER VALENT TITANKUM AEKALE CHLORIDES Abner Brenner, Chevy Chase, Md, and Joseph M. Sherfey, Arlington, Va, assignors to the United States of America as represented by the @ecretary of the Filed May 27', 1955, Ser. No. 511,319 1 Claim. (@l. %64) (Granted under Title 35, US. Code (1.952), see. 2st) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to use of any royalty thereon.

This invention relates to a process for electron inning titanium from a trivalent compound dissolved in a fused bath of alkali halides.

, Because of the widespread interest in titanium at the present time the production of the metal by electrolysis has received much attention. Several electrolytic processes have been developed and the process which will prove to be the most satisfactory for commercial production of titanium will depend on the convenience and economy in producing the initial source of titanium, the readiness with which the metal can be isolated from the fusedsalt bath and the purity of the metal obtained. Prior art electrochemical procedures for depositing titanium from fused salt baths are found in Japanese Patent No. 3,859 (1952); British Patent Nos. 678,807 and 682,919 (1952); Australian Patent No. 256,951 (1951); and Argentine Patent No. 81,510 (1951). These patents deal with the passage of TiCl, vapor over a cathode in mersed in a fused salt bath. The resulting solution may contain titanium in either the divalent or trivalent state. In the British Patent No. 698,151 hydrogen is passed with the TiCl, vapor into the bath to effect the reduction. A few publications have also appeared dealing With the deposition of titanium from fused halide melts into which the titanium has been introduced in the form of titanium trichloride, TiCl for which see: G. D. P. Cordner and A. W. Worner, Australian J. Applied Science, vol. 2, page 358 (1951); Shinzo Okada, Makoto Kawane and Mitsunao Takahashi, Bull. Eng. Research Inst, Kyoto Univ., vol. 6, page 57 (1954); J. Chem. Soc. Japan, Ind. Chem. Soc, vol. 56, page 410 (1953); A. Brenner and S. Senderoif, J. Electrochem. Soc., vol. 99, page 2230 (1952). However, this method of introducing titanium does not prove commercially feasible.

'Three recent US. patents, Nos. 2,707,168, 2,707,169 and 2,707,170 deal with the electrodeposition of titanium using titanium monoxide as the source of titanium.

Titanium metal in commercial quantities is now being produced solely by the Kroll process, an inherently expensive batch operation involving the chemical reduction of titanium tetrachloride with metallic magnesium. Because of the high cost of this'method there is much current interest in the development ofan economically feasible process for electrowinning titanium metal.

When conducted on a laboratory scale this is easily accomplished. A wide variety of titanium compounds yield a deposit of metallic titanium when electrolyzed in any one of the many possible molten salt baths. These processes are beset with many difficulties, both technical and economic, which prevent their utilization on a commercial scale.

One such process that has been extensively investigated 'utilizes a bath consisting of potassium fluotitanate (K TiF dissolved in a molten salt bath, such as a eutectic mixture of sodium chloride (NaCl) and potassium chloride (KCl). From a commercial standpoint this system has many objectionable features. In the first are a Patented @ct. 3, 1961 place the solute (KgTiFs) is prepared from an aqueous solution, resulting in a product which is diflicult to render anhydrous. Moreover this bath operates well only at a relatively high temperature (800900 C.) and high current density (200500 amp./dm. The fluoride content of this bath constantly increases, necessitating its periodic disposal.

One of the most serious objections to this bath is the high cost of the solute K TiF This is caused by the difiiculties encountered in preparing it in a pure state and the relatively high cost of the hydrofluoric acid used in its preparation.

The obvious solution to these last difliculties lies in the use of titanium tetrachloride or compounds derived from it in place of K TiF Titanium tetrachloride is comparatively cheap and is being produced in a very pure state on a commercial scale. It is an unfortunate fact, however, that this compound is a volatile liquid which is not retained in an adequate concentration even in a lowmelting solvent such as potassiumdithium chloride eutectic (melting point 350 C.). The difficulty resulting from the volatility of titanium tetrachloride can be avoided by use of either of the lower valent titanium chlorides, titanium trichloride (TiCl or titanium dichloride (TiCl neither of which is volatile. Both of these compounds have been used in fused melts to electrodeposit titanium on a laboratory scale and offer many advantages. Titanium trichloride, for example, is readily soluble in the low temperature potassium chloride-lithium chloride eutectic bath and yields titanium metal with a high current efficiency.

An object of the present invention therefore is to disclose a process for electrowinning titanium from a trivalent compound dissolved in a fused vath of alkali or alkali earth halides which involves two essential conditions: (1) the solute in the fused bath is a trivalent compound of titanium introduced into the bath in solid form and having the approximate composition corresponding to the formula NaTiCl (2) the use of a glass diaphragm to enclose the cathode chamber to prevent mixing of the catholyte and the anolyte. I

The use of either of the two simple lower valent chlorides of titanium, in view of their low volatility and good solubility, has obvious advantages, but to develop a commercially feasible process based on them, two problems must be overcome which had not been solved prior to our work. In the first place there is no known method for producing them in anhydrous condition on a commercial sca e. In fact, they are laboratory curiosities. See Apparatus for the Preparation of Anhydrous Ti tanium Ill Chloride and Titanium III Bromide, Journal of Research of the National Bureau of Standards, vol. 46, No. 4, April 1951, J. M. Sherfey.

The second problem which would be common to any electrolytic process based on a reduced titanium compound solute, is the necessity of preventing anodic oxidation of the solute. Without such prevention the current efiiciency would be greatly lowered and, in the case of a chloride bath; volatile TiC1 would be formed and lost from the bath.

The use of diaphragms to effect separation of the catholyte and anolyte is a common commercial practice in electrodeposition from aqueous solutions. However, the successful use of such a diaphragm in a production scale molten salt bath has never been reported in the literature. Few materials can withstand the conditions which exist in such baths. Aluminum oxide has some utility, but porous diaphragms composed of this material permit serious mixing of catholytc and anolyte if the pores are large and have too high an electrical resistance if the pores are small. Furthermore, although porous diaphragms decrease mixing resulting from convection they do not affect mixing caused by electrical migration of ions.

It can thus be seen that an electrowinning process which would utilize an inexpensive and ea ily prepared lower valent titanium chloride as a solute. in a cell having a satisfactory diaphragm would have many obvious advantages over the Kroll process. In the prior art patents discussed above no suitable diaphragm was shown to have been developed. FIG. 1 of Patent. No. 2,707,168 shows a graphite barrier 20 and in FIG. 2 a ceramic diapbragm, 22, of zircon or mullite having a porosity of 20 percent is shown, neither of which provides complete separation of anode and cathode compartments.

A diaphragm which prevents mixing by electrical mi gration is proposed herein. The solutes are described in our copending application Serial No. 310,147,;filed on September 17, 1952 (now Patent 2,765,270). Briefly it consists of any one of a. series of stable reduced. alkali titanium halide complexes produced by the.- reaction of an alkali metal with a tetrahalide of titanium. The composition of one such compound approximates that represented by the formula NaTiCh.

To prevent oxidation of the compound the cell must be equipped with an eflicient diaphragm. It has been demonstrated by the applicants (see our. copending application Serial No. 448,396, filed on August 30, 1954) and others (see Ingeberg, U.S. Patent No. 1,299,947), that glass is an ideal material for construction of such a diaphragm as it efiects absolute separation of the catholyte and anolyte except for the passage of a positive ion, ordinarily sodium, from the anolyte through the glass membrane and into the eatholyte. Another obvious advantage stems from the fact that a glass diaphragm simplifies. the protection of the cell from contamination by atmospheric gases. Such protection is always necessary when electrodepositing titanium from a high temperature bath. However, when a glass diaphragm is used it is only the cathode compartment that needs protection. The entire anode compartment can be exposed to the air. The proposed cell alfords considerable flexibility in the. choice of an anolyte melt in that it must meet only two requirements. First, the maximum permissible melting point is that temperature above which the diaphragm is. too soft to have adequate mechanical strength. The glass diaphragm may take the form of a semi-molten, horizontal layer in the cell, in which case the melting point maybe higher. Second, the cation in the anolyte must be the same as the current carrying cation in the glass. Unless thi is the case, electrolysis will change the composition of the glass. Thus, a sodium glass diaphragm would require a sodium salt anolyte. A process employing these principles has been operated successfully on a laboratory scale. The catholyte consisted of KCl-LiCl eutectic containing about percent by Weight of NaTiCl contained in a Pyrex glass vessel that also served as a diaphragm. The top of the vessel was fitted with an air-tight lid through which passed a tungsten rod which served as cathode. Thev top of the vessel was also provided with an airlock which permitted removal and replacement of the cathode while an inert atmosphere was maintained in the catholyte chamber.

The lower portion of the glass vessel containing the catholyte was immersed in an open vessel containing the. anolyte which was a low melting mixture of molybdic oxide (M06 and sodium molyhdate (Na M O Carbon or iron was used as an anode.

This cell was operated at a temperature somewhat below the softening point of the Pyrex diaphragm (550- 600 (2.). The current density was about 2.0-20.0 anr1p./dm. although a considerable variation of this value is permissible. The cathode current efiiciency based on trivalent titanium was in excess of 90 percent. Essentially all of the titanium introduced as a solute was recovered in the form of metallic crystals or powder.

The following two variations of the process were demonstrated on a test-tube scale: (1) a divalent sodium titanium chloride can be used as a solute. Its preparation and the preparation of NaTiCl are similar except that the reactants, sodium and titanium tetrachloride, are in the molar ratio of two to one respectively. The product of this reaction forms a solution containing divalent titanium ions when dissolved in the molten salt melt. (2) Metallic molybdenum was deposited from a'cell which was identical to the cell described above for the electro deposition of titanium excepting the substitution of a trivalent molybdenum complex (K Mocl for the trivalent titanium complex (NaTiCh).

A typical example of the operating of our process follows: 10 grams of sodium titanium halide, NaTiCl' and 40 grams of potassium chloride-lithium chloride eutectic were placed in a Pyrex test tube provided with a rubber stopper having three holes. A nickel or tungsten rod was passed through the center hole to serve as the cathode. Through the other two holes tubes were passed for flushing the cell with argon. This test tube assembly comprised the cathode compartment. The bottom part of the tube containing the mixed salts was placed in a molten bath consisting of sodium molybdate and molybdenum trioxide at a temperature of 575 C. A current of about 1. ampere was passed between the rod serving as cathode and an iron rod placed in the outer vessel of fused molybdate to serve as anode. After about 10 percent more than the theoretical amount of current had passed, the contents of the test tube were extracted with water and the titanium collected on a filter as a powder. The yield of titanium was over 90 percent.

An apparatus in which the above processes may be carried out is illustrated in the accompanying drawing.

it should be. clearly understood, however, that the above illustration is solely by way of example, and is not to be construed as a limitation upon the spirit or scope of the appended claim.

This case is a continuation-in-part of our application Serial No. 448,396, filed on August 6, 1954 (now abandoned).

We claim as our invention:

The process of electrowinning titanium from lower valent titanium alkali chlorides contained in the catholyte of an electrolytic cell in which the anolyte consists of a mixture of M00 and Na MoO which process comprisesmaintaining a separation of said anolyte from said catholyte by means of a solid glass diaphragm impervious to the passage of the catholyte or anolyte, passing an electric current through said cell and said diaphragm and recovermg titanium as a powder.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Cordner et al.: Australian Journal of Applied Science," vol. 2, September 1951, pp. 358-367.

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