US2813068A - Production of titanium by fused salt electrolysis - Google Patents

Description

United States Patent c 2,813,068 Ice Datented Nov. 12, 1957 PRODUCTION OF TITANIUM BY FUSED SALT ELECTRQLYSIS No Drawing. Application December 21, 1951, Serial No. 262,886

4 Claims. (Cl. 204-64) Titanium metal is an important commercial material in view of the unusual physical and chemical properties which the metal exhibits. In the many methods reported for the production of titanium, there has been mostly a failure to produce a metal of commercial purity such that the titanium metal could be either hot or cold-worked and fabricated. Most of the methods described in the literature for production of titanium actually produce an alloy of titanium with oxygen or nitrogen which makes the commercial application of the product impractical due to the fact that these gas metal alloys are exceptionally hard and brittle. This is particularly true of the previously reported methods proposed for the production of titanium by fused salt electrolysis. However, if the drawbacks with respect to oxygen and nitrogen, and other embrittling causes can be eliminated, electrodeposition is attractive for the recovery of titanium. Such a process may be made continuous and produce titanium of the highest purity once a proper procedure has been developed.

As disclosed by the United States Bureau of Mines on page 3 of Information Circular 7381 published November 1946, many investigators have tried to prepare metallic titanium by electrolysis but without success. Several fused salt electrolytes were investigated on a small scale to determine their effectiveness for production of the metal; Such electrolytes have been disclosed in the literature. Mixtures of sodium and potassium fluotitanate with or without added titanium dioxide and mixtures of sodium and potassium fluorides with titanium dioxide are examples of such procedures.

In U. S. Patent No. 1,835,025 to Dn'ggs et al. on the production of certain rare metals, a possible method for the production of group IV rare refractory metals is disclosed. In this method a double halide of the group IV metal was dissolved in a mixture of molten potassium and sodium chloride and then electrolyzed. In a paper entitled Contribution to the problem of production of metallic titanium by electrolysis of non-aqueous media, by S. I. Sklyarenko and Y. N. Lipkes, in the Russian journal Zhurnalprikladnoikhimii, vol. 13, No. 1, pp. 51-55 (1940), the work of Driggs and co-workers as disclosed in the above listed patent was repeated in detail by these authors. In all cases, these workers found that What was deposited on the cathode was not pure metallic titanium, but a mixture of lower oxides which apparently con- .tain, in some cases, a certain amount of metallic titanium. .To date, no process has been disclosed on the fused salt electrolysis in the literature or elsewhere which has yielded titanium metal of high enough purity to be commercially feasible. As explained before,in order to be generally workable and useful, the metal has to be in extraordinarily pure form. Extremely small percentages of oxygen and nitrogen or carbon embrittle the titanium markedly so that it cannot be handled by metal working procedures. Under these conditions, great care must be taken to eliminate these undesirable elements from being present while the titanium is being formed. In addition i to these stringent requirements, the current-efiiciencies disclosed by prior workers were exceptionally low so that irrespective of metal purity, the economics with respect to power requirements would be adverse.

The literature on methods for preparation of titanium metal is exceptionally complex. Many broad claims for preparation of pure metal have been made which subsequent work has shown to be inaccurate. A main reason for this inaccuracy is that the carbides and nitrides and lower oxides of titanium exhibit metal-like characteristics.

.All of these materials present in very small quantities in the metal make the end product quite brittle. Unless verified with present day known chemical and X-ray techniques, there could be no assurance that pure titanium was prepared.

In accordance with the present invention, titanium metal of high purity may be had such as to be applicable in a manner and in kinds of usages not heretofore possible. Other objects and advantages of the invention will appear from the following description.

In general, our process involves electrolysis of a specially prepared bath containing a titanium ion in a valence less than four, under cover of an inert atmosphere. These baths consist of fused mixtures of reduced alkali fiuotitanates in which the titanium has a valence of less than four, and one or more alkaline earth or alkali halides, under conditions which avoid all embrittling agents.

The alkaline earth halides referred to are those of calcium, magnesium, strontium, and barium. The alkali halides referred to are those of potassium and sodium. These halides may be used singly or in mixture. Chlorides are preferable, being of lower cost. For example, mixtures such as calcium and sodium chloride, calcium, sodium, and potassium chloride, sodium chloride and potassium chloride, and the like may be used. While all these mixtures are effective, preferred base baths which have been used with the greatest effectiveness, are sodium chloride or a mixture of sodium and potassium chloride. From the standpoint of simplicity, sodium chloride is preferred. We have found that a mixture of sodium chloride with the reduced potassium fluotitanate will melt at a temperature around 700 C. and an economic electrolysis can be carried out over a range of temperature from 700 to 800 C. With the eutectic mixture of potassium and sodium chloride as a base bath, the electrolysis of the reduced potassium fluotitanate may be carried out at a temperature as low as 550 C. The potassium and sodium compounds are preferred as the alkali metal fluotitanates, being more economical.

Our novel process involves the electrolytic decomposition of a potassium fluotitanate in a fused halide bath in which the titanium has a valence of less than four, and in specialized conditions. In contrast to this, examination of the simple electrolysis of purified recrystallized KzTiFa in which thetitanium has a valence of four and in which the potassium titanium fluoride is dissolved in fused sodium chloride, has shown that an impure product is always obtained and that the metal which forms at the cathode is always brittle. In addition, the yields are low and the current efficiency is exceptionally low so that irrespective of the product obtained by this procedure, the economics would be adverse. Invariably the presence of oxygen in this product is verified by X-ray determination and in oxygen analysis by chemical means. This embrittlement and oxygen content is found no matter how long the electrolysis is carried out. Any variation in bath composition, time, temperature of electrolysis, electrolytic characteristics of the procedure and the like have relatively little effect on the elimination of the brittle nature of the metal. Thus, examination of the procedures disclosed by Driggssand co-worke'rs leads us to the same con,

clusions found by Sklyarenko and Lipkes. And thus, it is evident that the electrolysis of normal pure potassium titanium fluoride in which the titanium has a valence of four in a fused salt bath results in a product which is always brittle and usually due to the presence of oxygen or oxides of titanium. The adversities of low efficiencies are also universally found.

We have found however how to obtain pure metal at high current efliciencies. The purity of the metal-is at once seen from the fact that the product may be fused to a billet which is malleable, ductile, and capable of being cold-worked into a useful shape.

We carry out electrolysis in an atmosphere which is completely free ofoxygen, nitrogen, or carbon in gaseous form. This atmosphere is chemically purified argon or helium, argon being preferred. In addition, the elec trolysis is carried on in such a, manner as to insure removal of any combined water or oxygen normally present even in carefully purified fluotitanates. And in addition further, the electrolysis is carried out in such a manner so as to insure that all of the titanium present in the bath is provided for electrolytic purposes from the bath at a valence of less than four. The fluotitanate in which the titanium is present .at less than a valance of four will be hereinafter designated as a reduced fluotitanate. We have several method for providing the reduced fluotitanate in proper chemical condition. The first and most important is to obtain the compound .by electrolysis of a normal fluotitanate, i. e., one in which the titanium is present in the valence of four, at a :low voltage usually less than three volts. Under these conditions, the titanium reduces quantitatively from the :tetra-valent stateto the tri-valent state without deposition of metal. In addition, however, an equally important result is also developed. Through the medium of electrolysis at voltages less than :three, the oxygen present as an impurity in the fluotitanate is removed through electrolytic action. If water is present, both hydrogen and oxygen are removed, hydrogen being removed at the cathode and oxygen at the anode. Under normal conditions, the time required to reduce the tetravalent titanium to tri-valent titanium by electrolysis is more than ample to remove all oxygen and hydrogen. We have found further that this is a perfectly safe procedure in that no metal is deposited. Once all the titanium has been reduced to the tri-valent state and all oxygen and hydrogen has been removed by this preliminary electrolytic treatment of the fused bath, then the voltage is increased into the range of five or higher at which the decomposition takes place with the deposition of metallic titanium in coarse crystal form at the cathode and at high current efiiciency. Another procedure invol'vles the use of a properly prepared reduced fluotitanate as a bath constituent dissolved in the various halides aforestated.

iThe preliminary electrolysis used for reduction of normal potassium titanium fluoride, i. e., a state in which the valence of titanium is four and in which the salt contains some oxygen, and water or both is, as has been stated before, carried out at a voltage less than that required to reduce the tri-valent titanium to metal and at relatively low current densities. Other methods for first making the reduced potassium titanium fluoride may be employed where desired.

In the practice of our invention, for instance, from 25 to 50 parts of the proper fluotitanate is mixed with 50 to 75 parts of sodium chloride or a mixture of sodium chloride and potassium chloride. In this range, the proportions of 35 parts of potassium titanium fluoride and 65 parts of sodium chloride are preferred. This mixture is fused in a graphite crucible maintained in an argon or helium atmosphere. The furnace used is substantially air-tight with respect to influx of outside atmosphere and is provided with electrode openings and holes forcontrol of temperature.

The course of the reaction is as described above. If

a four va'lent titanium compound such as KzTiFs is used, the bath after fusion is electrolyzed at voltages less than three, for instance 0.5-3, at current densities not exceeding 100 amperes per square decimeter. The time of electrolysis is seldom more than 3 hours and is dependent on the amount of titanium required for reduction. If the bath consists of potassium titanium fluoride and sodium chloride, the electrolysis is normally carried out at approximately 700 to 750 C. If a bath consists of a eutectic mixture of sodium and potassium chloride and potassium titanium fluoride, the electrolysis may be carried out at 550 to 600 C.

Once the preliminary electrolysis period is over, all the titanium is in a valence state less than four. Then both the voltage and the current are increased. The voltage is normally increased to 5 to 8 volts and the current density to the order of 200 to 500 amperes per square decimeter. As a result of the electrolysis in this high voltage stage, a titanium metal sponge is deposited in a tightly adhering mass at the cathode. The size of the crystals comprising this sponge can be controlled by the temperature of operation of the bath, the current density employed, and the concentration of reduced fluotitanates. High temperatures, low concentrations of reduced titanium ion and low current densities favor the development of large crystals, whereas low temperatures, high concentrations and high current densities favor the development of fine crystals. The argon or helium as the atmosphere is carefully purified of all gases such as hydrogen, oxygen, nitrogen, Water vapor, carbonaceous gases and the like. A positive pressure of this inert pure gas is maintained inside the furnace at all times to prevent the entrance of atmospheric gases.

In the electrolytic operation, the graphite crucible may act as the anode and the cathode is a suspended rod of graphite or metal such as nickel, molybdenum and titanium metal itself. A graphite rod or plate inserted in the electrolyte is suitable as the anode and the electrolysis may take place between the graphite rod and cathodes of the type described under conditions specified or the crucible itself may be the anode.

In the main electrolysis, the conditions for deposition of the metal are as above-stated, five to eight volts in current densities of the order of at least 200 amperes per square decimeter. Current densities greater than 500 amperes per square decimeter of cathode area have been used with success.

After the electrolysis is completed, the cathode is withdrawn from the bath and allowed to cool in the inert atmosphere of argon ;until it has cooled to a temperature of at least 200 C. After reaching this temperature, the cathode with its deposit may be removed from the electrolytic cell and processed for recovery of the metal. Subsequent to the washing and drying operation, the sponge is compressed and sintered to a 100% density by heating in vacuum to yield solid ductile titanium. In addition, the sponge may be fused under pure argon to a billet. A suitable washing operation for the sponge prior to fusion is leaching with water, treatment with dilute sulfuric acid and then final washing with water to remove the sulfuric acid and drying in a vacuum dryer to its final condition.

The current efficiency for deposition of metal from fully reduced titanates used as original feed material generally is in excess of Much of the difference between 80 and is due to the requirement for preliminary electrolysis to eliminate water and oxygen, when fully reduced titanates are used as original feed material. When a four valent fluotitanate is used as original raw material and the elimination of oxygen and water is combined with the reduction of tetra-valent to lower valent titanium, then the overall current efficiency is usually in the range of 25 to 60% of which the difference between 50 and 100 for example is made up .of 30 to 35% for the chemical reduction process. The balance is needed to remove oxygen and water.

Thus, the novel aspects of our invention are based on the preliminary electrolysis cycle below three volts to eliminate oxygen and water, a reduction cycle to remove all tetra-valent titanium from the bath before actual deposition of metal begins, and finally the electrolyzing metal from the reduced titanium compound, i. e., a compound in which the titanium exists in a valence less than four as original feed material. Basically, then, the invention is electrolytic reduction of the pure reduced fluotitanate for the production of pure titanium metal in which the titanium in the reduced fluotitanate has a valence less than four.

Illustrative examples are as follows:

Example 1.One pound of the compound KzTiFs was mixed with 3.5 pounds of pure sodium chloride and then dried at 110 C. for 18 hours. This mixture was added to a heated graphite crucible maintained at a temperature of roughly 800" C. A one-inch diameter graphite cathode was inserted in the melt and the temperature dropped to 755 C.

-A preliminary treatment was initiated at a D. C. voltage of three volts and a total current of 15 amperes. All during the heating and preliminary electrolytic treatment, purified argon at the rate of 1.5 liters per minute was passed through the cell in order to insure a completely oxygen-free atmosphere and neutral as far as contamination with respect to titanium is concerned. This preliminary electrolysis was continued for 2% hours. After this operation all of the double fluoride was reduced so that all of the titanium was present in the trivalent state 'at least and all oxygen and hydrogen was removed from the bath as evidenced by the cessation of bubbling at cathode and anode. As showing that the reduction was complete, a portion of the bath cooled to room temperature exhibited a deep purple color. Analysis showed all of the titanium compound was present in a valence less than four.

After completion of the preliminary electrolysis, the carbon cathode was removed and the bath was skimmed to remove any carbon or other impurities floating on the surface. The titanium cathode was then placed in the molten bath and electrolysis was continued at a temperature of 733 C. at a voltage of 6 volts on a current of 150 amperes equivalent to a current density of 420 amperes per square decimeter of cathode surface. The electrolysis was continued for two hours and 10 minutes. On completion of the electrolysis, the cathode was removed from the bath, but left in the upper region of the cell and allowed to cool in the argon atmosphere to room temperature. After removal and washing of the cathode, 60 grams of coarse crystalline titanuim flake and powder was recovered. This represented a metal yield of approximately 65% and a current efficiency of 45%. Chemical analysis showed the titanium to have a purity of 99.4%.

Example 2.A mixture of 380 grams of KzTiFs and 3 pounds of sodium chloride was dried as in Example 1 and added to a carbon crucible inside the specially constructed electrolytic cell previously heated to 870 C. After addition of the salt mixture, the temperature dropped to 770 C. Preliminary electrolysis was carried out under a purified argon atmosphere at a voltage varying between 0.8 and 1.4 volts and a current of amperes. A carbon cathode was again used for the preliminary electrolysis. The preliminary electrolysis was continued for 2 hours and 15 minutes which on analysis proved to be suflicient to reduce all the titanium to a valence less than four. The carbon cathode was removed and the bath skimmed and a molybdenum cathode inserted. Electrolysis was now begun at 150 amperes and at a voltage varying between 6.1 and 6.8. The current density during electrolysis was varied between. 420 and 560 amperes per square decimeter at the cathode. Time of electrolysis was 2 hours and 20 minutes. After electrolysis, the cathode was removed and cooled as in Example 1 and the powder washed and treated as previously for titanium metal recovery. A metal recoveryof 60 grams was obtained equivalent to a current efliciency of 28.5% and a metal recovery of 78%. Titanium metal recovered analyzed 98.6%. I

Example 3.This was run as before except that the bath contained 16% KzTiFs, the balance being sodium chloride. Preliminary electrolysis was carried out as before. Electrolysis proper was carried out from 6.8 to 8 volts and at current densities at the cathode of 400 to 5 60 amperes per square decimeter and an average temperature of 750 C. was used. In this case, again, a yield of 60 grams of titanium was obtained at a metal recovery of 87% and a current eificiency of 51%. Metal analyzed 99.7% pure and was cold workable after fusion. A repetition of the procedure as carried out in Example 3 yielded 61 grams of coarse titanium metal showing a yield of 90% of the total available metal exhibiting a purity of 99.8% and a current efiiciency of 56%.

Example 4.-The same as Example 3 except that the electrolyte consisted of 15 parts of KaTiFs, 40 parts of NaCl and 45 parts of potassium chloride. The electrolysis was carried out at 620 C. at a current density of 420 amperes per square decimeter at the cathode and a voltage varying between 6 and 8 volts. A yield of metal equivalent to 94% of the total metal in the bath exhibiting a purity of 99.8% and equivalent to a current efiiciency of 56% was obtained.

Example 5 .15 parts of a compound corresponding by analysis to the formula K2TiF5 and parts of sodium chloride were melted in the electrolytic cells under purified argon and subjected to preliminary electrolysis at 2 volts and 50 amperes per square decimeter at the cathode for a period of 30 minutes. Bubbling at the cathode and anode, indicating the elimination of all oxygen and hydrogen, ceased after about 12 minutes. Electrolysis was carried out at voltages varying between 6 and 7 volts and at current densities varying between 400 and 520 amperes per square decimeter at the cathode. After electrolysis, a total metal recovery equivalent to 96% of the metal in the bath was obtained of purity analyzing 99.9%. The current efiiciency was 86%. This increase in current efficiency was no doubt due to the lack of need for reducing the tetra-valent compound to the tri-valent compound and for elimination of any substantial quantities of oxygen and water vapor.

Example 6.15 parts of a compound corresponding to the formula KsTiFs and 85 parts of sodium chloride were treated as in Example 5. Again current efiiciencies in excess of 85%, metal yields in excess of and a purity of the order of 99.8% was obtained. Again, as in the previous example, the time required for preliminary electrolysis did not extend past a few minutes.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

We therefore particularly point out and distinctly claim as our invention:

1. The method of producing metallic titanium which comprises forming a fused bath composed essentially of an alkali metal fluotitanate in which the titanium has a valence of four and at least one halide of the group of alkali metal and alkaline earth metal halides, electrolyzing said bath at a voltage not in excess of three volts for a period of time sufiicient to effect reduction of substantially all of the titanium component of said fiuotitanate to a fluotitanate in which the titanium has a valence less than four, and electrolyzing the resulting bath at a voltage above the decomposition voltage of the titanium in said.

fluotitanate having a valence less than four with the resulting c'athodic deposition of metallic titanium.

2. The method of producing metallic titanium which comprises forming a fused bath composed essentially of an alkali metal fluotitanate' in which the titanium has a valence of four and at least one halide of the group of alkali metal and alkaline earth metal halides, electrolyzingsaid bath at a voltage not in excess of three volts for a period of time sufficient to reduce substantially all of the titanium component of the fluotitanate to a valence of three, and electrolyzing the resulting bath at a voltage above the decomposition voltage of the trivalent titanium in the reduced fiuotitanate with the resulting cathodic deposition of metallic titanium.

3. The method of producing metallic titanium which comprises forming a fused bath composed essentially of potassium fiuotitanate in which the titanium has a valence of four and at least one alkali rnetalchloride, electrolyzing said bath at a voltage not in excess of three volts for a period of time su'fficient to effect reduction of the tetravalent titanium to trivalent titanium, and electrolyzing the resulting bath at a voltage above the decomposition voltage of the trivalent titanium in said fluotitanate with the resulting cathodic deposition of metallic titanium.

4. The method of producing metallic titanium which comprises forming a fused bath composed essentially of a eutectic mixture of potassium flilotitanate'in which the titanium" has a valence of four. and potassium chloride, electrolyzing said bath at a voltage not in excess of three for a period of time suflicient to'effect reduction of substantially all of the tetravalent titanium to trivalent titanium; and eleetrol'yzing the resulting bath at a voltage above the-decomposition voltage of the trivalent titanium 10 with the resulting cathodic deposition of metallic titanium.

References Cited in the file of this patent UNITED STATES PATENTS 1,835,025 Driggs et a1. Dec. 8, 1931 FOREIGN PATENTS 615,951 Germany July 16, 1935 OTHER REFERENCES Australian Journal of Applied Science, vol. 2, N0. 3, September 1951, pp. 385-97. Article by Cordner et al. Eastman (abstract), Ser. No. 791,465, April 29, 1952.

Claims (1)

1. THE METHOD OF PRODUCING METALLIC TITANIUM WHICHH COMPRISES FORMING A FUSED BATH COMPOSED ESSENTIALLY OF AN ALKALI METAL FLUOTITANATE IN WHICH THE TITANIUM HAS AA VALENCE OF FOUR AND AT LEAST ONE HALIDE OF THE GROUP OFOF ELKALI METAL AND ALKALINE EARTH METAL HALIDES, ELECTROLYZING SAID BATH AT A VOLTAGE NOT IN EXCESS OF THREE VOLTS FOR A ALL OF THE TITANIUM COMPONENT OF SAID FLUOTITANATE TO A FLUOTITANATE IN WHICH THE TITANIUM HAS A VALENCE LESS THAN FOUR, AND ELECTROLYZING THE RESULTING BATH AT A VOLTAGE ABOVE THE DECOMPOSITION VOLTAGE OF THE TITANIUM IN SAID FLUOTITANATE HAVING A VALENCE LESS THAN FOUR WITH THE RESULT-ING CATHODIC DEPOSITION OF METALLIC TITANIUM.