US2714575A

US2714575A - Production of metallic titanium

Info

Publication number: US2714575A
Application number: US297157A
Authority: US
Inventors: Wainer Eugene; Merle E Sibert
Original assignee: Horizons Titanium Corp
Current assignee: Horizons Titanium Corp
Priority date: 1952-07-03
Filing date: 1952-07-03
Publication date: 1955-08-02
Anticipated expiration: 1972-08-02

Description

United ftates atent PRODUCTION on METALLTC TITANIUM Eugene Wainer, Cleveland Heights, and Merle E. Sibert,

Garfield Heights, @hio, assignors, by mesne assignments, to Horizons Titanium Corporation, Princeton, N 3., a corporation of New Jersey N Drawing. Application duty 3, 1952, Serial No. 297,157

9 (Ilaims. (Cl. 204--7ll) This invention relates to the production of metallic titanium and, more particularly, to improvements in the electrolytic production of metallic titanium.

The esteemed qualities of titanium metal are predicated upon its availability in malleable or workable state and the latter can be attained only when the metal is produced substantially free of contained oxygen. The principal sources of oxygen in electrolytically deposited titanium are oxygen or water vapor in the atmosphere above the electrolytic bath from which the metal is deposited and moisture contained in the salts which are used as the electrolytic bath. it is a common and effective practice to exclude air from the atmosphere above the bath by sweeping this atmosphere with an inert gas such as argon, helium, or the like, and substantially all traces of moisture can be removed from the bath by electrolysis under conditions which will effect dissociation of any contained water without deposition of any significant amount of titanium metal. These expedients are effective in preventing contamination of the cathodically deposited titanium by oxygen, but subsequent handling of the deposited metal for its recovery in the form of an ingot or other useful shape tends to introduce a substantial amount of extraneous oxygen into the metal. This contamination is caused by the necessity of freeing the deposited metal from entrained salts when it is removed from the cell, such separation being generally effected by washing the deposited metal with Water and with dilute acid. Of necessity, washing and leaching to remove entrained salts is sufficiently drastic to cause some surface oxidation of the particles of titanium metal. When these particles of surface-oxidized metal are subsequently melted in an inert atmosphere, it is generally found that the re sulting titanium ingot contains such an amount of oxygen that it is brittle and unworkable. Accordingly, in order to lower the ratio of surface area to volume of such particles and thus lower the oxygen contamination of the resulting ingot, it is advantageous to produce the metal particles in the largest size possible.

In the production of metallic titanium by electrolysis of an alkali metal fluotitanate in a diluent bath composed of one or more alkali and alkaline earth metal halides, it has been found possible heretofore to deposit the titanium metal in the form of particles 60 to 80 microns in average size provided that an inert cell atmosphere is maintained and that the bath is subjected to preelectrolysis at a decomposition pdtential below that of the titanium salt but sui'licient to {insure decomposition of all free or combined water in the bath components. However, at least 30% of the particles so produced have a size less than about 45 microns and only a small percentage of the particles are as large as 100 to 159 microns. The purity of the metal ingot produced from such particles is only about 99% Ti, the balance being predominantly oxygen.

We have now discovered that the particle size of the cathodically deposited titanium produced by the aforementioned electrolysis of an alkali metal fluotitanate in a fused diluent salt bath can be remarkably increased by incorporating in the bath during electrolysis a small but significant amount of certain extraneous salts. The salts which we have found to be useful are the halides of metals which are normally and stably tetravalent, the metal component further being characterized by the ability to enter into solid solution in titanium, and these salts will therefore be referred to hereinafter as extraneous tetravalent metal halides. The halides of zirconium, hafnium and thorium are representative of such extraneous tetravalent metal halides and function effectively in increasing the particle size of the eiectrodeposited metal pursuant to our invention.

The form of the halides of the aforementioned extraneous tetravalent metals may be either that of the simple tetrahalide or that of a double halide such as the double halide of the tetravalent metal and an alkali metal, or it may comprise the product of fusing together the tetrahalide of the tetravalent metal and an alkali metal halide. The simple tetrahalides such as the tetrachlorides, tetratluorides, and the like, are available and are useful in the practice of our invention although these salts generally contain a significant amount of entrained and combined water which must, of course, be removed. The fused mixtures of simple salts, such for example as a fused mixture of zirconium tetrachloride and sodium chloride in substantially equimolar proportions, are less contaminated with entrained moisture than the simple tetrahalides of the tetravalent metal. The double fluorides of these tetravalent metals and alkali metals, on the other hand, can be produced in a form largely free of entrained and combined moisture and are for this reason presently preferred for the practice of our invention. The aforementioned double fluorides of the tetravalent metal and of either sodium or potassium may be readily prepared by the addition of the alkali metal fluoride to a solution of the tetravalent metal tetrafiuoride whereupon the double halide is separated by crystallization from a saturated solution of the salt. Thus, the double fluoride of potassium and zirconium (also known as potassium fluozirconate, l lzZrl s) may be produced by adding a stoichiometric quantity of potassium fluoride to a zirconium tetrafiuoride solution and effecting subsequent crystallization of the double fluoride from the resulting solution.

The amount of extraneous tetravalent metal halides which are useful in practicing our invention range upwardly from 0.2% of the tetravalent metal halide by weight of the alkali metal fiuotitanate content of the bath to which it is added. That is, the amount of extraneous tetravalent halide should be sufficient to yield a titanium metal cathode deposit containing at least 0.5% by weight of the tetravalent metal component of the extraneous halide. increasing amounts of the extraneous tetravalent halides up to about 5% by weight on the aforementioned basis appear to produce a progressive increase in the particle size of the elcctrodeposited metallic titanium which appears to reach a maximum at about a 5% addition of the tetravalent metal halide. Amounts of the tetravalent metal halide greater than 5% may, of course, be used although such larger amounts appear to produce no further beneficial effect on the particle size of the electrodeposited metallic titanium but merely increase the amount of the added tetravalent metal component in the titanium deposit at the cathode.

The increase in particle size of the electrodeposited metallic titanium obtained by the practice of our invention is outstanding. For example, under electrolytic conditions which, in the absence of an extraneous tetravalent metal halide pursuant to our invention, will yield a metallic titanium cathode deposit of which about 30% is of particle size smaller than 325 mesh and the balance com- 3 prises plus 325 mesh crystals having a maximum size of about ll50 microns, the mere addition of an extraneous tetravalent metal halide in accordance with our invention results in an increase in the particle size of the deposited titanium so that all of the particles are larger than 325 mesh and a major proportion of the crystals are at least one-quarter inch long. The resulting increase in particle size not only facilitates washing of entrained salts from the cathode deposit but minifies the surface oxidation of the washed cathode deposit to the extent that the oxygen content of the titanium metal obtained by melting the crystals into massive form is generally well below 0.1% by weight. The massive titanium so produced is exceptionally malleable because of its low oxygen content and is readily amenable to cold working, and its purity is such that its total content of elements other than titanium and the metal component of the extraneous tetravalent metal halide does not exceed 0.01% by weigh of the titanium.

The. metal component of the extraneous tetravalent metal halide used in the practice of our invention is deposited at the cathode along with the titanium. However, each of these metal components, such as zirconium, hafnium and thorium, forms a solid solution with titanium when the cathode deposit is melted down into n massive form, and its inclusion in the titanium does not markedly impair the ductility, malleability, strengthweight ratio, or any other desirable physical or chemical characteristic of the titanium metal. In special circumstances where purity of the titanium metal product is at a premium, the amount of tetravalent metal halide added to the electrolytic cell bath pursuant to our invention should obviously be within the lower portion of the aforementioned useful range, but in general we have found that the presence of the tetravalent metal in the titanium cathode deposit is so unobjectionable that the primary consideration is the attainment of a titanium metal particle size such as to produce the minimum amount of oxygen in the massive titanium metal produced from that deposit compatible with a useful degree of cold Workability.

The salts useful in forming the bath from which the metallic titanium is deposited are those conventionally used for this purpose and comprise the alkali metal and alkaline earth metal halides. Thus, sodium chloride may be used as the sole component of the diluent bath, or mixtures of the aforementioned halides, such as a eutectic mixture of sodium chloride and potassium chloride, may be used where it is desired to obtain a lower melting point bath. However, sodium chloride alone forms a diluent bath which permits electrolysis within a temperature range of about 700 to 1000 C. and is so relatively inexpensive and of such high purity that we presently prefer to use this salt as the diluent bath component.

The alkali metal fluotitanates from which titanium metal may be obtained by electrolysis in the aforementioned fused salt bath are sodium and potassium fluotitanates, NazTiFs and K2TiF6, respectively. These salts are generally obtained by simple crystallization from an aqueous solution thereof, and in this form they are of adequate purity to yield titanium metal of exceptional purity. However, the purification or" these salts to reduce their total content of aluminum, chromium, iron and vanadium to at least below 0.02% by weight of the salt, as described and claimed in the copendin application of Topinka, McKenna and Carlton, Serial No. 297,158, filed July 3, 1952, leads to a further increase in the particle size of the titanium metal particles which are produced pursuant to the practice of our present invention. Moreover, when the extraneous tetravalent metal halide is added in the form of the alkali metal double halide, we have found it advantageous to similarly purify this double fluoride so as to obtain the maximum advantages of such purification. When purification of the alkali obtained by the practice of our invention.

metal fluotitanate is adopted in order to obtain the maximum increase in titanium metal particle size, the necessary purification may be obtained simply by dissolving the commercial grade of alkali metal fluotitanate in such an amount of hot water as to form a saturated solution at a temperature of about 90 C. and by subsequently cooling this saturated solution to ambient temperature with the resulting recrystallization of the fluotitanate with the desired degree of purity. The same technique is applicable to the purification of the alkali metal double fluorides of zirconium, hafnium and thorium. The increase in particle size of the electrodeposited titanium obtainable by the use of such purified alkali metal fluo titanates is a particularly useful complement to the in- I crease in particle size of the titanium which can be obtained by the practice of our present invention, the improvement attributable to the use of the purified fluotitanate being particularly useful where the requisite purity of the titanium metal dictates the use of an amount of an extraneous tetravalent metal halide near the lower limit of the useful range of such halide in the practice of our invention.

The amount of the alkali metal fluotitanate which is incorporated in the diluent salt bath in practicing our invention is not critical within the range of 0.5 to 35 mol per cent of the diluent bath component, but within this range we have found it advantageous to use between 10 to 25 mol per cent of the fluotitanate regardless of whether it is used in the commercial grade or purified form.

Although it has heretofore been considered necessary to remove moisture from the alkali metal fluotitanatediluent salt bath not merely to the extent possible by vacuum drying but to the much greater extent made possible by pro-electrolysis of the bath, we have found that such pro-electrolysis is not essential to the quality of particle size of the electrodeposited titanium metal Thus, we have found it to be wholly satisfactory to subject the bath components merely to vacuum drying at temperatures up to about 200 C., a vacuum of the order of one millimeter of mercury or less being sufficient for this purpose of producing a cell bath which is substantially anhydrous. When the bath components are thus vacuum dried, they may be used directly in the operation of an electrolytic cell in the practice of our invention without degrading the quality or particle size of the electrodeposited titanium.

The practice of our invention does not necessitate any modification in the cell geometry or operating conditions which have been previously ascertained to be useful in the electrolysis of alkali metal fiuotitanates in a diluent salt bath. Thus, the cell voltage should be such as to provide the necessary decomposition voltage for the alkali metal fluotitanate while nevertheless being below the decomposition voltage of the diluent bath components. Current densities up to about 400 amperes per square decimeter are also not critical inasmuch as with any specific current density up to that limit an improvement in the particle size of the deposited titanium is noted when the extraneous tetravalent metal halide is used in accordance with our invention as compared with the particle size previously obtainable under identical conditions without the addition of such halide.

' Within the aforementioned range, current densities of the order of 200 to 400 amperes per square decimeter are particularly useful and lead to commercially acceptable current efiiciencies of at least We have ascertained, however, that the maximum improvement in particle size of the deposited titanium achieved at any cell voltage and current density used in practicing our invention is obtained when the voltage gradient between the cell anode and cathode is at least one volt per centimeter. For example, with a cell voltage of about 6 to 8 volts, We have found it advisable to maintain all portions of the anode and cathode immersed in the fused bath within a maximum mutual spacing of 2 inches. It must, be understood, however, that the increase in particle size obtainable by adopting the aforementioned optimum values of current density and voltage gradient are cumulative with respect to the increase in particle size obtained simply by the use of an extraneous tetravalent metal halide according to our invention. Thus, under all cell conditions which result in the electrodeposition of metallic titanium from the alkali metal fiuotitanate and diluent salt bath, the use of an extraneous tetravalent metal halide pursuant to our invention will, under the same cell conditions, effect a substantial increase in the particle size of the electrodeposited titanium. Adoption of the other optimum cell conditions recited hereinbefore merely accentuates this increase in particle size and therefore they are not essential conditions but merely optimum conditions of cumulative value.

The products of the electrolysis consist of titanium metal deposited at the cathode and chlorine gas evolved at the anode. The titanium metal further contains an amount of the tetravalent metal component of the extraneous tetravalent metal halide approximately proportional to the relative amounts of said extraneous halide and the alkali metal fluotitanate in the cell bath. No fluorine is evolved from the cell and consequently the concentration of fluorides in the bath is progressively built up by successive additions of the alkali metal fluotitanate. We have found it generally advisable to continue electrolysis of the alkali metal fiuotitanate in the aforementioned diluent bath only until the molar ratio of potassium fluoride to sodium fluoride in the bath reaches a value of about 2:1. At this point, we have found it desirable to replace the diluent bath, the spent bath being appropriately treated for recovery of its potassium and fluorine components.

The following specific examples are illustrative but not limitative of the practice of our invention:

Example I Potassium fluotitanate (KzTiFs) and potassium fluozirconate (KzZrFs) were purified as described hereinbefore by simple recrystallization and the purified salts were dried in air at a temperature of 135 C. in order to remove entrained water. A mixture of salts was then prepared composed of 34-0 parts of the purified and air dried potassium fluotitanate, 3 parts of the purified and air dried potassium fluozirconate and 1800 parts of chemically pure sodium chloride, all parts being by weight. The mixture was vacuum dried at 180 C. for 12 hours under a vacuum of one millimeter of mercury or better.

An electrolytic cell was heated to a temperature of 850 C. in a dry argon atmosphere and thereupon the aforesaid vacuum dried mixture of salts was transferred to the heated cell. Heating of the cell and its charge was continued until the salt mixture was reduced to the molten state. An iron cathode was then inserted into the bath to a depth of about 2 inches and spaced from the anode a maximum of 1.5 inches. The cell temperature was promptly lowered to 750 C. and electrolysis of the fused bath was then carried out at this tempera ture in an argon atmosphere while maintaining the cell voltage at 4.8 to 6.1 volts. The current density during the electrolysis period of 92 minutes varied between 300 and 400 amperes per square decimeter. At the conclusion of the electrolysis, the bath was cooled so that the cathode could be removed at a temperature sufficiently low to insure against oxidation of the sponge-like cathode deposit of the titanium metal. The titanium was separated from the cathode, was washed with sulfuric acid, and was then washed with water and dried in a vacuum. The yield of washed and dried titanium metal product was determined to be 68 parts by weight and represented (i a yield efficiency of 97% at a current efliciency of 65%. All particles of the washed product were retained on a 100 mesh screen and most of these particles were in the form of thick stubby crystals ranging in width from 0.2 to 0.4- inch. The product obtained after fusion of the washed particles in an inert atmosphere was exceptionally malleable and ductile and contained 1.4% zirconium and 0.05% oxygen.

Example II The procedure described in Example i was repeated with a single variation, to wit, the use of 15 parts of purifled potassium fluozirconate in lieu of the 3 parts by weight used in Example I. The titanium product obtained from the cathode comprised 73 parts by weight and represented a 99% yield efliciency at a current efliciency of 85%. All of the product after washing and drying had a particle size such that it was retained by a 20 mesh screen, and the majority of the particles had widths ranging from 0.2 to 0.5 inch. After compacting and melting the crystalline product in an inert atmosphere, an extremely ductile massive product was obtained containing 6.7% zirconium and 0.04% oxygen.

Example III The procedure described in Example I was duplicated with the exception that 6 parts of potassium hafnium fluoride (KzHfPs) were substituted for the 3 parts of potassium fluozirconate. After washing and drying, the cathode product was found to comprise 69 parts by weight, thus equivalent to a yield efliciency of about 98% at a current eiiiciency of 70%. All of the particles in the product were retained on a 60 mesh screen and the average particle size was approximately 0.3 inch.

Example I V The procedure described in Example I was again repeated with the exception that 6 parts of recrystallized potassium thorium fluoride (KT11F5) were substituted for the potassium fluozirconate. The yield of cathode metal was 70 parts by weight and represented a yield efficiency of 99% at a current efficiency of All of the washed and dried product was retained on a 60 mesh screen and the majority of the particles of this product ranged between 0.2 and 0.3 inch in width.

It will be seen, accordingly, that the incorporation of an extraneous tetravalent metal halide along with the alkali metal fluotitanate in a diluent fused salt bath in accordance with our present invention results in a remarkable increase in the particle size of the electrodeposited titanium metal. The practice of our invention thus results in the electrolytic production of titanium metal capable of being converted to ingot form with only such a minute amount of oxygen that the ingot metal is malleable and readily amenable to cold Working. A further advantage resides in the fact that the practice of our invention eliminates the necessity of subjecting the fused salt bath to pre-electrolysis and therefore contributes to the art a substantial saving in the power requirements heretofore considered necessary for the electrolytic production of metallic titanium. It has been observed heretofore that at bath temperatures suiiiciently high to maintain fused salt conditions, and thus establish conditions making possible pro-electrolysis of the bath, moisture in the bath tended to cause hydrolysis of the titanium salt. Such hydrolysis results in the combination of the oxygen component of the water with the titanium salt to form a titanium-oxygen compound which is not decomposed prior to the titanium electrolysis, and consequently hydrolysis of the titanium salt has in the past resulted in significant oxygen-contamination of the electrodeposited titanium. This tendency toward hydrolysis is eliminated by the practice of our invention inasmuch as it makes possible adequate dehydration of the bath components by simple vacuum drying at temperatures not substantially in excess of 200 C. Our invention thus makes vacuum 1 drying of the cell charge wholly adequate and generally suflicient in and of itself as a means of producing a substantially anhydrous cell charge. These advantages, coupled with a significant improvement in yield efficiencies, characterize the practice of our invention and are a measure of its contribution to the art.

We claim:

1. In the method of producing metallic titanium by electrolysis of an alkali metal fiuotitanate in a substantially anhydrous fused salt bath consisting essentially of an alkali metal fluotitanate and at least one halide salt from the group consisting of alkali metal halides and alkaline earth halides, the improvement which comprises incorporating in the bath a halide of an extraneous tetravalent metal of the group consisting of zirconium, hafnium, and thorium in an amount comprising at least about .2% by weight of the alkali metal fluotitanate in the bath, thereby increasing the particle size of the electrodeposited titanium.

2. 1n the method of producing metallic titanium by electrolysis of an alkali metal fluotitanate in a substantially anhydrous fused salt bath consisting essentially of an alkali metal fluotitanate and at least one halide salt from the group consisting of alkali metal halides, the improvement which comprises incorporating in the bath a halide of an extraneous tetravalent metal of the group consisting of zirconium, hafnium, and thorium in an amount comprising at least about .2% by Weight of the alkali metal fluotitanate in the bath, thereby increasing the particle size of the electrodeposited titanium.

3. In the method of producing metallic titanium by electrolysis of an alkali metal fiuotitanate in a substantially anhydrous fused salt bath consisting essentially of an alkali metal fluotitanate and at least one halide salt from the group consisting of alkali metal halides and alkaline earth halides, the improvement which comprises incorporating in the bath a halide of an extraneous tetravalent metal of the group consisting of zirconium, hafnium and thorium in an amount from about 2% to about 5% by weight of the alkali metal fluotitanate in the bath, thereby increasing the particle size of the electrodeposited titanium.

4. In the method of producing metallic titanium by electrolysis of an alkali metal fiuotitanate in a substantially anhydrous fused salt bath consisting essentially of an alkali metal fiuotitanate and at least one halide salt from the group consisting of alkali metal halides, the improvement which comprises incorporating in the bath a halide of an extraneous tetravalent metal of the group consisting of zirconium, hafnium, and thorium in an amount from about .2% to about 5% by weight of the alkali metal fiuotitanate in the bath, thereby increasing the particle size of the electrodeposited titanium.

5. in the method of producing metallic titanium by electrolysis of an alkali metal fluotitanate in a substantially anhydrous fused salt bath consisting essentially of an alkali metal fiuotitanate and at least one halide salt from the group consisting of alkali metal halides and alkaline earth halides, the improvement which comprises incorporating in the bath a double fluoride of an alkali metal and a tetravalent metal of the group consisting of zirconium, hafnium, and thorium in an amount irom 2% to about 5% by weight of the alkali metal fluotitanate in the bath, thereby increasing the particle size of the electrodeposited titanium.

in the method of producing metallic titanium by electrolysis of an alkali metal fluotitanate in a substantially anhydrous fused salt bath consisting essentially of an alkali metal fluotitanate and at least one halide salt from the group consisting of alkali metal halides, the improvement which comprises incorporating in the bath a double fluoride of an alkali metal and a tetravalent metal of the group consisting of zirconium, hafnium, and thorium in an amount from .2% to about 5% by Weight of the alkali metal fluotitanate in the bath, thereby increasing the particle size of the electrodeposited titanium. 7. in the method of producin' metallic titanium by electrolysis or" an alkali metal fluotitanate in a substantially anhydrous fused salt bath consisting essentially of iluotitanate and at least one halide salt no group consisting of alkali metal halides and earth halides, the improvement which comprises n'corpor: 1

r g in the bath a double fluoride of an alkali metal and zirconium in an amount from 2% to about 5% by W of the alkali metal fluotitanate in the bath, thereby '1 ng the particle size of the electrodeposited titanium.

8. the method of producing metallic titanium by electrolysis of an alkali metal fluotitanate in a substantia 1y anhydrous fused salt bath consisting essentially of an alkali metal r'luotitanate and at least one halide salt from the group consisting of alkali metal halides and alkaline earth halides, the improvement which comprises incorporating in the bath a double fluoride of an alkali metal and hafnium in an amount from 2% to about 5% by weight of the alkali metal fluotitanate in the bath, thereby increasing the particle size of the electrodeposited titanium.

9. In the method of producing metallic titanium by electrolysis of an alkali metal fiuotitanate in a substantially anhydrous fused salt bath consisting essentially of an alkali metal fluotitanate and at least one halide salt from the group consisting of alkali metal halides and alkaline earth halides, the improvement which comprises incorporating in the bath a double fluoride of an alkali .etal and thorium in an amount from .2% to about 5% by weight of the alkali metal fluotitanate in the bath, thereby increasing the particle size of the electrodeposited titanium.

References Cited in the file of this patent UNlTED STATES PATENTS 1,815,054 Driggs July 21, 1931 1,821,176 Driggs et al. Sept. 1, 1931 1,835,025 Driggs et al Dec. 8, 1931 1,874,090 riggs Aug. 30, 1932 OTHER REFERENCES Journal of Applied Chemistry (U. S. S. R.), vol. 13 (1940), pages 51 thru 55. Copy in Library of Congress.

Claims

1. IN THE METHOD OF PRODUCING METALLIC TITANIUM BY ELECTROLYSIS OF AN ALKALI METAL FLUOTITANATE IN A SUBSTANTIALLY ANHYDROUS FUSED SALT BATH CONSISTING ESSENTIALLY OF AN ALKALI METAL FLUOTITANATE AND AT LEAST ONE HALIDE SALT FROM THE GROUP CONSISTING OF ALKALI METAL HALIDES AND ALKALINE EARTH HALIDES, THE IMPROVEMENT WHICH COMPRISES INCORPORATING IN THE BATH A HALIDE OF AN EXTRANEOUS TETRAVALENT METAL OF THE GROUP CONSISTING OF ZIRCONIUM, HAFNIUM, AND THORIUM IN AN AMOUNT COMPRISING AT LEAST ABOUT .2% BY WEIGHT OF THE ALKALI METAL FLUOTITANATE IN THE BATH, THEREBY INCREASING THE PARTICLE SIZE OF THE ELECTRODEPOSITED TITANIUM.