US2707169A

US2707169A - Preparation of titanium metal by electrolysis

Info

Publication number: US2707169A
Application number: US202806A
Authority: US
Inventors: Morris A Steinberg; Merle E Sibert; Alfred A Topinka
Original assignee: Horizons Titanium Corp
Current assignee: Horizons Titanium Corp
Priority date: 1950-12-26
Filing date: 1950-12-26
Publication date: 1955-04-26
Anticipated expiration: 1972-04-26

Description

United States Patent OfiFice 2,707,169 Patented Apr. 26, 1955 PREPARATION OF TITANIUM METAL BY ELECTROLYSIS Morris A. Steinberg, Lakewood, Merle E. Sibert, Garfield Heights, and Alfred A. Topinka, Cleveland, Ohio, assignors, by mesne assignments, to Horizons Titanium forporation, Princeton, N. J., a corporation of New ersey No Drawing. Application December Serial No. 202,806

7 Claims. (Cl. 204-64) Titanium metal is an important commercial material in view of the unusual properties which the metal ex hibits. At present, it is being made in several ways, all of which are complicated and expensive in spite of the ready availability of the raw materials of titanium in large quantities. Briefly, the procedures now used for the manufacture of titanium are the following:

At the present the only method used commercially is the reduction of titanium tetrachloride with magnesium followed by subsequent distillation of the magnesium chloride by-product in a vacuum.

As indicated, titanium has many unusual physical and chemical properties. In order to be workable and generally useful, the metal has to be supplied in extraordinarily pure form. Small percentages of oxygen, nitrogen, or carbon embrittle the metal markedly so that it cannot be handled by metal working procedures. Under these conditions, great care is taken to eliminate these undesirable elements from being present while the titanium is being formed. The literature on methods for preparation of titanium metal is exceptionally complex. Many broad claims to preparation of pure metal have been made which subsequent work has shown to be inaccurate. One of the reasons for this inaccuracy is that the carbides, nitrides, and lower oxides of titanium exhibit metal-like characteristics though all are very brittle materials and unless the work done was followed with x-ray examination, there could be no assurance that titanium was prepared.

Relatively little work has been done in an attempt to prepare the metal by electrolysis. Earlier work alleges that titanium metal is formed by the electrolysis of titanium dioxide suspended in fused salts such as calcium chloride, combinations of calcium chloride and sodium. chloride, and the like. However, work of more recent date has shown that in all cases the product formed is highly impure titanium monoxide and no evidence of metal deposition was found. Repetition of these experiments by us has substantiated the fact that only impure titanium monoxide mixed with substantial amounts of other oxides of titanium such as the sesquioxide is formed.

In accordance with the present invention, the metal titanium is produced by electrolytic reduction of a monoxide of titanium in a fused salt electrolyte such as halides and mixtures of halides, particularly of group II. And, metal of high purity may be had, and the process involves excellent efliciency. Other objects and advantages of the invention will appear from the following description.

To the accomplishment of the foregoing and related ends, said invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be employed.

In general, our process entails the electrolysis under cover of an inert atmosphere of a fused mixture of pure titanium monoxide and one or more alkaline earth halides with or without an admixture of alkali metal chloride, viz., sodium chloride, potassium chloride, or combination of potassium and sodium chlorides, under conditions where anode and cathode areas are physically separated. The alkaline earth halides referred to are any of those of calcium, strontium, barium 'or magnesium, and while bromides, fluorides and iodides can be used, from the standpointof costs and availability and in the case of fluorides their lower solubility for ultimate washing out, the preferred halides and mixtures are the single chloride of calcium or magnesium; mixtures of calcium and magnesium chlorides; mixture of calcium and sodium chlorides; mixtures of calcium, sodium and potassium chlorides; calcium and potassium chlorides; or the corresponding mixtures of magnesium chlorides and calcium chlorides. Through the various combinations possible among such halides, a range of temperatures can be obtained for the electrolysis to eliminate undesired side reactions. The preferred baths from the standpoint of ease of operation, economy, and the like, are generally based on calcium chloride, mixtures of calcium chloride and magnesium chloride or calcium chloride plus one or more of sodium and potassium chloride. Illustrative baths are:

1. Calcium chloride.

2. 60 parts of calcium chloride and 40 parts of magnesium chloride.

3. 70 parts of calcium chloride, 30 parts of sodium chloride.

4. 60 parts of calcium chloride, 20 parts of sodium chloride, 20 parts of potassium chloride.

5. The chlorides of barium, strontium or magnesium.

Bath 1 operates at 780-925" C., Bath 2 operates at 800 to 850 C., Bath 3 operates at round 750 C., Bath 4 operates at 700 to 750 C. And the range of operation in general is 700-925 C.

In the electrolytic decomposition of titanium monoxide there is a tendency for oxygen to be included in the metal formed at the cathode, unless special precautions be taken. The presence of oxygen in such metal is verified by x-ray examinations, and the metal so formed is brittle. In accordance with our invention, this tendency is obviated. And, we take particular care to prevent transfer of undesired material to the cathode zone. By employment of diaphragm or compartment type electrolytic cells in which a separation is maintained between anode and cathode zones, transfer of undesired material to the cathode zone may be prevented. The procedure involves the addition of pure titanium monoxide to the anhydrous molten salts. This mixture is fused in a properly made graphite crucible and melted in an argon, helium or similar inert atmosphere. The furnace used is substantially air tight with respect to infiux of outside atmosphere and is provided with electrode openings and holes for control of the temperature. In practice, the graphite crucible is heated by electrical resistance means and the assembly is provided with an argon inlet at the bottom of the furnace. The argon is purified so as to be free of gases such as hydrogen, oxygen, nitrogen, Water vapor, and the like. Positive pressure of argon is maintained inside the furnace at all times to prevent the entrance of atmospheric gases. This argon is recirculated through purifying chambers for reuse in the furnace chamber proper. The graphite crucible may act as the anode and the cathode is a suspended rod of graphite or metals. Nickel and molybdenum are most satisfactory at present as cathode material and titanium metal itself has been used successfully as the cathode. instead of the graphite crucible itself being used as the anode, a graphite rod inserted in the electrolyte is suitable: and the electrolysis takes place between the graphite rod and cathodes of the type described, under conditions specified in later portions of this description.

In general, the electrolysis operation is carried out at about 900 to 925 C. In the case of the TiO-calcium chloride mixtures, the melting point is of the order of 780 C. At the temperature of electrolysis, a fluid readily electrolyzable melt is obtained. In electrolysis, a voltage of 4 to 5 volts is used and current densities which are effective for metal deposition range from 1 up to at least 500 amperes per decimeter square of cathode. The temperature is not permitted to rise above 925 C. during the electrolysis since as a result of the nature of the reactions which take place, some calcium metal may be lost by volatilization, thus resulting in a noticeable drop in efficiency. The titanium metal product which forms at the cathode develops as a sponge composed of minute and closely interlocked crystals. After completion of the electrolysis, this sponge is thoroughly washed and freed of electrolyte and then is dried in an inert atmosphere at 100 C. Subsequently, the sponge is compressed and sintered to 100% density by heating in a vacuum to yield fused cast ductile titanium. The sponge metal may also be fused in a vacuum to yield cast ductile titanium. The yields are chemically quantitative with current efficiencies of 60% or better.

The necessity for the use of completely chemically pure titanium monoxide free from the higher oxides of titanium may be indicated by study of the electrolytic reactions. It may be shown that if any of the higher oxides of titanium are present in the anode compartment, then titanium monoxide is formed in the cathode compartment and this titanium monoxide is deposited along with the metal. This is apparently the source of the oxygen content of metal deposited at the cathode when the anode feed material is not free from oxides higher than the monoxide.

Titanium monoxide has a limit of solubility in the electrolyte, and if the amount of titanium monoxide which is added to the bath exceeds this solubility limit, it appears that particles of powdered titanium monoxide suspended in the bath can be carried by cataphoresis to the cathode, thereby causing contamination of the metal, as above-noted. However, if the titanium monoxide be added to the bath at a rate such that the amount present at all times never exceeds the solubility limit of the monoxide in the bath, this is prevented. Also, as aforenoted, the use of diaphragms or compartmentalized cells constructed to prevent physical transfer of suspended titanium monoxide, prevents contamination. Again, in some cases we may employ a titanium monoxide feed in the form of large pellets or chunks of sintered titanium monoxide which rest against the bottom of the graphite anode, and this successfully prevents possibility of loose powdered titanium monoxide being dispersed into the bath and carried to the cathode zone. Sintered anodes of TiO may also be used. Again, loose fitting sintered porous sleeves may be used to surround either anode or cathode for barrier purposes. Thus, using one or more of these expedients, in all cases we take care to prevent transfer of titanium monoxide particles to the cathode zone.

The degree of solubility of titanium monoxide in the various halide baths described is not known with certainty, but it appears to be of the order of 5% at least, and in calcium chloride alone it is much higher. Under these conditions, the rate of addition of the titanium monoxide is maintained so that the total concentration in the bath is always less than this figure and the rate of addition is a function of the total amount of flux in the bath versus the disappearance of the titanium monoxide as metal at the cathode as a result of the current density normally used. In view of the extremely high etficiency of this electrolyzing process, assurance is always present that excess titanium monoxide does not exist.

While such metered addition of feed is effective, it represents a procedure somewhat difficult to control and thus the diaphragm cells are preferred. The diaphragm cells which have been used successfully serve chiefly to divide the anode and the cathode areas into compartments so that the products formed at these two electrodes cannot mix with each other or the material in each compartment cannot be mixed with the other. In the use of such diaphragm or compartment cells, the crucibles are vitrified high temperature porcelains or graphite. Suitable porcelains are mullite or Zircon fired to zero porosity. The anode is a graphite rod which extends almost to the bottom of the crucible and the anode and cathode compartments are separated by a porous diaphragm made of coarse sintered zircon whose pores are smaller than the particle size of the titanium monoxide generally present. A graphite diaphragm may also be used. The cathode is graphite or a metal such as nickel or molybdenum. Modifications of this construction involve the use of porous crucibles for the anode compartment. Successful separating barriers have also been made by using a graphite crucible as the anode and separating the anode and cathode compartments with the porous zircon barrier. When these porous barriers are used, the voltage of operation is increased to about 10 to 12 volts, the excess being required to overcome the potential difference offered by the resistance of the porous barrier when it operates as an insulator.

Probably the simplest type of compartmental cell is embodied in a graphite crucible which is relatively deep compared with its width. In this case, the barrier is solid and is of a height such that the electrolyte will extend over the top of this solid barrier placed in the center of the crucible. Since the titanium monoxide is considerably heavier than the calcium chloride electrolyte, the powdered titanium monoxide remains completely in the anode compartment. The anode compartment is maintained relatively large in comparison with the cathode compartment in this case so as to have as much graphite surface exposed as possible. This type of submerged barrier compartment type cell is relatively easier to operate than the other types of barriers previously described.

The use of sintered pellets of titanium monoxide as feed is the simplest of all the afore-mentioned expedients, but it is somewhat more expensive. Pellets of pure titanium monoxide are desirably sintered to maximum density, and the pieces are provided on the order of A" in diameter. In this case, since complete assurance exists that no titanium monoxide goes into suspension, it is maintained in a simple cell without a barrier. The titanium monoxide pellets are placed against the bottom of a crucible, and additions of such pellets are made from time to time. Under these conditions, substantially pure titanium metal is obtained at the cathode with practically no oxygen content. Again, a compartment cell using the solid graphite barrier, with the electrolyte level above the top of this compartment may be employed, and again the coarse lumps of titanium monoxide may be applied as feed material. In such case, it is not necessary to keep the rate of addition of titanium monoxide below its solubility limit in the halide baths.

The nature of the reactions which take place in the electrolysis is not clear, and we are not committed to any theory. The desirable temperature range depends somewhat upon the particular halide composition, and properly the operating temperature is between 700 and 925 C. If, however, the temperature be allowed to exceed the proper operating range the course is different. The halide bath itself is then attacked in electrolysis, and the bath level decreases with the electrolysis thereof.

Illustrative of procedure in accordance with the invention are the following examples:

Example 1.A heated electrolytic cell is provided in which the anode is a graphite crucible which contains the melt. The cathode is a sheet of molybdenum 0.5 square decimeters in area. The cell is compartmentalized by the insertion of a solid graphite barrier in the central portions of the cell such that the anode compartment is roughly twice the size of the cathode compartment. This barrier extends to within about 1 to /2" from the top of the electrolyte level. The electrolyte is fused CaClz. The cell is operated in the presence of a positive pressure of purified argon. The pure titanium monoxide feed is provided as a powder in the anode compartment. Electrolysis is carried out at 900 C. with a voltage of about 8 volts and a current of 400 amperes per square decimeter at the cathode equivalent to 200 amperes total input. Typical yields of material are of the order of while the current efiiciency is of the order of 60% so that the cell is run for twice the theoretical time. This is expected since all of the anode area is not used in the electrolytic process. On cooling, the bath is white with no trace of blue tinge. The bath is leached in water under carbon dioxide atmosphere and the product is recovered by filtering and washing. It is then subsequently dried in a vacuum or in an ordinary drier at a temperature not exceeding 80 C. Typical figures are the following: 500 grams of CaClz, 50 grams of titanium monoxide yielding 37 grams of titanium metal in 20 minutes at a current of 200 amperes.

Example 2.A modification of the cell structure used in Example 1 yields somewhat higher current efficiency. The same type of compartmentalized cell is used, but 11'] this case, the graphite crucible is electrically inert. A massive graphite anode almost filling the anode compartment is inserted in the cell so that it is close to the walls of the graphite crucible, but is not touching. The titamum monoxide powder is placed in the annular space between the graphite anode and the graphite crucible. Under these conditions, the same general chemical yields are developed as in Example 1, but the current efficiency is around 90% in that 37 grams of titanium metal are obtained in 15 minutes. Again, this is expected since a more effective use of the anode compartment for electrolytic purposes is obtained.

Example 3.The same general construction as described in Example 1 is used except that the compartmentalizing graphite barrier is. omitted. The anode in this case is a graphite rod which extends almost to the bottom of the crucible. Surrounding the graphite rod 1s a porous sleeve of a zircon porcelain consisting of substantially pure high fired zirconium silicate fired to a porosity of the order of Titanium monoxide feed material is placed in the annular space between the graphite anode and the zircon sleeve. Material y1elds are quantitative while current yields are of the order of 85 to 90%.

Example 4.-In this case, the crucible is a dense fired pure zircon which is non-porous. The d1aphragm of porous zircon is placed in the middle portion of the cell and this diaphragm extends above the top of the 11qu1d. The anode is a massive graphite rod and the cathode 1s molybdenum. The anode almost fills the anode compartment. Titanium monoxide is placed in this area. Chem1- cal yield is quantitative and current yields are 90 to 95%, 37 grams of titanium metal being obtained in about 12 minutes at 200 amperes.

Example 5 .--A cell without compartments is provided made of graphite. The same cell structure as in Example 3 is used. The feed material is crushed titanium monoxide sinter having a particle size in the range of to of an inch. This is heaped in a pile around the anode 1n the anode sections of the cell.

Example 6.--A graphite crucible is provided as the container. Again the cathode is a molybdenum sheet. A porous zircon crucible is used as the anode compartment in which a massive graphite anode is placed. Titanium monoxide feed material is in the form of sintered titanium monoxide grog which is placed inside this porous zircon crucible. This method provides positive separation of titanium monoxide from the titanium metal form simply by lifting the porous crucible out of the bath at any desired time.

Example 7.-A graphite crucible is container and anode. The cathode is compartmentalized by placement 1n a rectified zircon crucible submerged below the surface of the electrolyte. This zircon crucible rests on the floor of the anode crucible. Sintered titanium monoxide grog is placed in the electrolyte in the anode area outside the cathode compartment. The titanium metal formed is retained in the removable cathode container.

Specific and effective baths used in the foregoing are the following: calcium chloride operating at 900-925" C.; 60 parts of calcium chloride in 40 parts of magnesium chloride operating at about 850 C.; calcium chloride 70 parts, parts of sodium chloride operating at 750 C.; 60 parts of calcium chloride, 20 parts of sodium chloride, 20 parts of potassium chloride operating at 700-750 C.; the halides of barium, strontium, and magnesium may be used separately or in combinations with the other halides indicated. These latter are not preferred for reasons of economy.

Various modifications of diaphragm and compartment type cells above-described may be employed, as readily seen. The important factor however is, as pointed out, that transfer of raw material to the cathode zone be prevented, and whichever of the indicated expedients be used, this central figure of preference of transfer of titaniugi monoxide particles to the cathode zone is taken care 0 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 tllie fgllowing claims or the equivalent of such be emp oye We therefore particularly point out and distinctly claim as our inventlon:

The method of producing substantially pure metallic titanium in an electrolytic cell which comprises: preparing a fused electrolyte consisting essentially of at least one halide salt of the group consisting of alkaline earth metal halides and mixtures of alkaline earth metal halides with alkali metal halides, introducing substantially pure titanium monoxide free from higher oxides of titanium into the said fused electrolyte to form a fused salt cell bath, maintaining the fused salt cell bath in a fused state at a temperature not in excess of 925 C. under a noble gas atmosphere, passing an electrolyzing current through the fused bath between an anode and a cathode in contact with said bath at a cell voltage such as to establish a cathode current density within the range of l to 500 amperes per square decimeter and recovering the resultant cathodically deposited titanium.

2. The method of producing substantially pure metallic titanium in an electrolytic cell which comprises: preparing a fused electrolyte consisting essentially of at least one halide salt of the group consisting of alkaline earth metal halides and mixtures of alkaline earth metal halides with alkali metal halides, introducing substantially pure titanium monoxide free from higher oxides of titanium into the said fused electrolyte to form a fused salt cell bath, maintaining the fused salt cell bath in a fused state at a temperature not in excess of 925 C. under an argon atmosphere, passing an electrolyzing current through the fused bath between an anode and a cathode in contact with said bath at a cell voltage such as to establish a cathode current density within the range of 1 to 500 amperes per square decimeter and recovering the resultant cathodically deposited titanium.

3. The method of producing substantially pure metallic titanium in an electrolytic cell which comprises: preparing a fused electrolyte consisting essentially of at least one halide salt of the group consisting of alkaline earth metal halides and mixtures of alkaline earth metal halides with alkali metal halides, introducing in solid form substantially pure sintered titanium monoxide free from higher oxides of titanium into the said fused electrolyte to form a fused salt cell bath, maintaining the fused salt cell bath in a fused state at a temperature not in excess of 925 C. under a noble gas atmosphere, passing an electrolyzing current through the fused bath between an anode and a cathode in contact with said bath at a cell voltage such as to establish a cathode current density within the range of l to 500 amperes per square decimeter and recovering the resultant cathodically deposited titanium.

4. The method of producing substantially pure metallic titanium in an electrolytic cell which comprises: preparing a fused electrolyte consisting essentially of at least one halide salt of the group consisting of alkaline earth metal halides and mixtures of alkaline earth metal halides with alkali metal halides, introducing substantially pure titanium monoxide free from higher oxides of titanium into the said fused electrolyte to form a fused salt cell bath, maintaining the fused salt cell bath in a fused state at a temperature not in excess of 925 C. under a noble gas atmosphere, passing an electrolyzing current through the fused bath between an anode and a cathode in contact with said bath at a cell voltage. such as to establish a cathode current density within the range of 1 to 500 amperes per square decimeter, recovering the resultant cathodically deposited titanium, and replenishing the titanium monoxide by feeding same into the bath at a rate suflicient to replace the titanium monoxide content which has been depleted by the deposition of titanium at the cathode but at a rate insufficient to exceed the solubility of the bath for titanium monoxide.

5. The method of producing substantially pure metallic titanium in a compartmented electrolytic cell which comprises: preparing a fused electrolyte consisting essentially of at least one halide salt of the group consisting of alkaline earth metal halides and mixtures of alkaline earth metal halides with alkali metal halides, introducing substantially pure titanium monoxide free from higher oxides of titanium into the said fused electrolyte in the anode compartment of the cell to form a fused salt cell bath, maintaining the fused salt cell bath in a fused state at a temperature not in excess of 925 C. under a noble gas atmosphere, passing an electrolyzing current through the fused bath between an anode and a cathode in contact with said bath at a cell voltage such as to establish a cathode current density within the range of 1 to 500 amperes per square decimeter and recovering the resultant cathodically deposited titanium.

6. The method of producing substantially pure metallic titanium in an electrolytic cell which comprises: preparing a fused electrolyte consisting essentially of at least one member of the group consisting of calcium chloride, magnesium chloride, and mixtures of calcium chloride and magnesium chloride, introducing substantially pure titanium monoxide free from higher oxides of titanium into the said fused electrolyte to form a fused salt cell bath, maintaining the fused salt cell bath in a fused state at a temperature not in excess of 925 C. under a noble gas atmosphere, passing an electrolyzing current through the fused bath between an anode and a cathode in contact with said bath at a cell voltage such as to establish a cathode current density within the range of 1 to 500 amperes per square decimeter and recovering the resultant cathodically deposited titanium.

7. The method of producing substantially pure metallic titanium in an electrolytic cell which comprises: preparing a fused electrolyte consisting essentially of calcium chloride and at least one alkali metal halide from the group consisting of sodium chloride and potassium chloride, introducing substantially pure titanium monoxide free from higher oxides of titanium into the said fused electrolyte to form a fused salt cell bath, maintaining the fused salt cell bath in a fused state at a temperature not in excess of 925 C. under a noble gas atmosphere, passing an electrolyzing current through the fused bath between an anode and a cathode in contact with said bath at a cell voltage such as to establish a cathode current density within the range of 1 to 500 amperes per square decimeter and recovering the resultant cathodically deposited titanium.

References Cited in the file of this patent UNITED STATES PATENTS 2,148,345 Freundenberg Feb. 21, 1939 2,302,604- Dolbear Nov. 17, 1942 OTHER REFERENCES Journal of Applied Chemistry (U. S. S. R.), vol. 13 (1940), pages 51-55, article by Sklyarenko et al.; abstracted in Chemical Abstracts, vol. 34 (1940), page 7756.

Titanium, by Barksdale, published in 1949 by The Ronald Press Company (New York), pages 41 and 43.

Claims

1. THE METHOD OF PRODUCING SUBSTANTIALLY PURE METALLIC TITANIUM IN AN ELECTROLYTIC CELL WHICH COMPRISES: PREPARING A FUSED ELECTROLYTE CONSISTING ESSENTIALLY OF AT LEAST ONE HALIDE SALT OF THE GROUP CONSISTING OF ALKALINE EARTH METAL HALIDES AND MIXTURES OF ALKALINE EARTH METAL HALIDES WITH ALKALI METAL HALIDES, INTRODUCING SUBSTANTIALLY PURE TITANIUM MONOXIDE FREE FROM HIGHER OXIDES OF TITINIUM INTO THE SAID FUSED ELECTROYTE TO FORM A FUSED SALT CELL BATH, MAINTAINING THE FUSED SALT CELL BATH IN A FUSED STATE AT A TEMPERATURE NOT IN EXCESS OF 925* C. UNDER A NOBLE GAS ATMOSPHERE, PASSING AN ELECTROLYZING CURRENT THROUGH THE FUSED BATH BETWEEN AN ANODE AND A CATHODE IN CONTACT WITH SAID BATH AT A CELL VOLTAGE SUSH AS TO ESTABLISH A CATHODE CURRENT DENSITY WITHIN THE RANGE OF 1 TO 500 AMPERES PER SQUARE DECIMETER AND RECOVERING THE RESULTANT CATHODICALLY DEPOSITED TITANIUM.