US2707168A - Preparation of titanium monoxide by electrolysis - Google Patents

Description

April 26, 1955 E. WAINER ETAL PRPARATION OF TITANIUM MONOXIDE BY ELECTROLYSIS led Dec. 26

United States Patent O PREPARATION OF TITANIUM MONOXIDE BY ELECTROLYSIS Eugene Wainer, Cleveland Heights, and Merle E. Sibert,

Garfield Heights, Ohio, assignors, by mesne assignments, to Horizons Titanium Corporation, Princeton, N. J., a corporation of New Jersey Application December 26, 1950, Serial No. 202,805

7 Claims. (Cl. 204-61) This invention relates to the preparation of substantially chemically pure titanium monoxide by electrolysis of titanium dioxide in fused salt bath at elevated temperatures. The pureness of the product concerns not only substantial freedom from ordinary chemical impurities, including higher oxides of titanium, but also more importantly involves physical and structural purity from a crystallographic standpoint. The importance of such purity in the titanium monoxide preparations enters not only in chemical usages for the titanium monoxide, but also particularly where it is to be employed as a source for manufacture of titanium metal. It' suitable malleable, non-brittle and workable titanium metal is to be produced, it is essential that titanium monoxide employed as raw material must be free from impurities. In accordance with the present invention, titanium monoxide may be had which meets the various requirements of purity for different usages. 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 its general aspects, the present invention involves the electrolytic production of titanium monoxide from titanium dioxide starting material. The titanium dioxide is charged into the anode compartment portion of an eleclytic cell separated into anode and cathode compartments by a porous barrier extending from the bottom of the cell upwardly at least a substantial distance toward the surface of a fused salt cell bath. The cell bath comprises at least one halide of an alkaline earth metal or an alkali metal, and advantageously a mixture of such halides. The fused bath is maintained at a temperature value below that at which any halide component of the bath decomposes, and under these conditions electrolysis of the titanium dioxide-containing bath results in electrolytic conversion of the titanium dioxide to titanium monoxide. The resulting titanium monoxide accumulates in and may be recovered from the cathode compartment of the cell. In commercial preference, the halides used desirably are the chlorides of alkaline earth metals singly or in mixture, or with addition of chlorides of sodium or potassium. A CaClz bath for instance operates at 780-925 C.; and CaClz 70 parts with NaCl 30 parts operates at around 750 C., and CaClz 60 parts with 20 parts each of NaCl and KCl operates at 70D-750 C., while CaClz 60 parts with MgCl2 40 parts operates at 80G-850 C. The range of operation in general is 70D-925 C.

Compartmented electrolytie cells in which the process of our invention may be advantageously carried out are shown in the drawings in which:

Fig. 1 is a sectional view of an inert atmosphere electrolytic cell in which the cell is divided into anode and cathode compartments by a graphite partition extending partway from the bottom of the cell to the level of eleclyte therein; and

Fig. 2 is a sectional view of an inert atmosphere cell divided into anode and cathode compartments by a porous barrier extending from the bottom of the cell to above the level of electrolyte therein.

While many procedures have been described for the preparation of titanium monoxide heretofore, these in rice general have failed to produce satisfactorily pure titanium monoxide, and the products have contained more or less of the undesirable higher oxides. For instance, it has been proposed to heat mixtures of powdered metal titanium and dioxide of titanium, or powdered titanium hydride and titanium dioxide, to elevated temperature in an inert atmosphere such as argon. Such procedures however are acedemic rather than commercial, for the reason that the raw material is disproportionately expensive. Also, it has been proposed to reduce dioxide of titanium with metals of the alkali and alkaline earth series, hydrogen, etc., under varying conditions. ln each case however, there has been the drawback that satisfactorily pure titanium monoxide is not obtained, and there is the invariable contamination by higher oxides. Such procedures also are disproportionately expensive, and there is finally the necessity of removing by-products, and highly specialized secondary equipment is required.

In electrolytic decomposition of titanium dioxide in a fused halide bath, an impure product tends to be formed unless certain precautions are taken. Higher oxides of titanium, and even traces of metal may occur. We have found, however, that if the procedure be carried out such as to prevent access of materials from the anode zone to the cathode zone, chemically pure titanium monoxide is obtained etiiciently and at high yields at the cathode zone. Under such conditions, the titanium monoxide produced is free from higher oxides of titanium, and the product is well suited for use in electrolytic preparation of malleable titanium metal.

The titanium dioxide raw material, such as pigment grade titanium dioxide, may be mixed with a temporary binder, such as 8% water with 0.5% starch, and then be pressed into pellets under pressures of the order 2 to 10 tons per square inch, and the pellets then be tired in an oxidizing atmosphere to temperatures of the order of 2300" F. lf these sintered pellets be crushed and sized to a grog particle size, e. g. 1/16 to 1A inch there is a further gain in efficiency. And by charging the material in this form, for instance, in a graphite crucible anode zone or in the vicinity of a graphite anode which is inserted in a cell, the physical condition of the material favors its retention generally in the anode zone instead of its being largely disseminated by turbulence into the cathode zone. The desired purity of titanium monoxide obtained in this manner is satisfactory, but the efficiency can be raised still higher. lf a more effective separation of anode and cathode zone be enforced, the etliciency is increased, and thus if a barrier be provided between the anode and cathode zones, even though only a fraction of the total height of the electrolyte level, this affords substantially continuous operation of the cell yielding a pure product with satisfactory etiiciency.

An advantageous electrolytic cell arrangement by which our invention can be carried out is Vshowin in Fig. 1. A graphite crucible 10 serving as the container for the fused salt electrolyte is disposed within an inert atmosphere chamber 11 equipped with an inert gas, for example, argon, inlet pipe 12 and an outlet pipe 13. An electric resistance element 14 provides the heat necessary to fuse and maintain at operating temperature the fused salt electrolyte 15 contained in the crucible 10. A graphite anode 16 extending through a packing gland 17 and a graphite cathode 18 extending through a packing gland 19 are immersed in the fused electrolyte 15. The crucible is divided into anode and cathode compartments by a solid barrier of graphite 20 between the anode and cathode compartment. In such case, the anode compartment portion is desirably roughly twice the volume of that of the cathode portion, and the top of the graphite barrier 20 is somewhat below the level of the liquid bath. This form of device is particularly effective if the feed material is supplied as relatively large particles of sintered titanium dioxide 21. Another type of cell design which has operated well is shown in Fig. 2. In this construction we employ a graphite type crucible 10 as a container for the fused electrolyte 15 and as the cell anode, while a cylindrical diaphragm 22 of sintered refractory ceramic such as Zircon or mullite having a porosity of about 20% is employed. Under these conditions, the diaphragm 22 extends above the level of the bath and is sutiiciently porous or permeable to permit ions to pass through, while at the same time holding the solid particles of titanium dioxide 21 back. The use of such a diaphragm necessitates a somewhat higher voltage because of the voltage drop across the diaphragm. Solid graphite diaphragms pierced with multitudinous fine holes also can serve. A third type of diaphragm cell (not shown) which has been used effectively is in the form of a porous refractory sleeve which completely surrounds a graphite anode. In this case, although a graphite crucible be used for the container, the graphite anode does not come directly in contact with the Crucible but is separated at the bottom by a fraction of an inch, while the porous sleeve rests on the bottom of the crucible and leaves an annular space to the graphite anode. The titanium dioxide feed material may be in such case either in the form of finely divided powder or small sintered pellets placed in the annular space between this porous sleeve and graphite anode. The importance of preventing bodily transfer of titanium dioxide into the cathode zone has been shown by us by experiment. Titanium dioxide in powdered form is practically insoluble in a molten calcium chloride bath, or in the various halide baths suitable for the present purpose. As a result of the turbulent formation of gas at the anode, there is a tendency for finely powdered titanium dioxide to become suspended throughout the entire volume of a bath. Titanium monoxide is formed at the cathode, and in view of its high reactivity, tends to entrap and hold titanium dioxide encountered. As a result, higher oxides of titanium occur in a product formed under such conditions, and irrespective of the length of time in which the electrolysis is carried out, no improvement in purity can result, since no means exists for transferring this cathode material back to the anode zone for continued purification. The use of sintered material, and particularly plus a minor amount of compartmentalization, and still more effectively, further complete compartmentalization, diaphragming, etc., prevents this kind of action, and the titanium dioxide and undesirable impurities may be retained wholly in the anode zone, with resultant production of uncontaminated titanium monoxide in the cathode zone. The reason for the effectiveness of sintered titanium dioxide is the relatively high density of such material as compared to that of the molten baths, and consequently the limitation on bodily transport into the cathode zone.

The equipment used involves provision of inert atmospheres such as pure argon or helium in cell designs such that a positive pressure of these inert gases may be available inside the electrolytic cell at all times. Crucibles used for the cells may be of graphite, although certain vitrified ceramics can also serve. The anodes are invariably of graphite or carbon, and the cathodes may be graphite or a metal such as nickel or molybdenum. The cells should be suitably formed for influx of the argon or other inert gas, addition of reagents, insertion of electrodes, etc. Argon or the like should be purified from oxygen, nitrogen, carbon and water vapor impurities, by passage through suitable purification trains, and the gas may be recirculated through such trains after use for reasons of economy.

The fused bath material comprises alkaline earth halides or mixtures of such alkaline earth halides with or without the addition of sodium and potassium chlorides or combinations thereof. Thus, the anhydrous chlorides of calcium, magnesium, strontium, and the barium may be used separately, or combinations of these may be employed, or one or more of the alkaline earth halides in combination with sodium or potassium chloride or a mixture of sodium potassium chloride. The preferred bath material from the standpoint of economy, ease of operation and the like, is generally based on calcium chloride, mixtures of calcium chloride and magnesium chloride, or calcium chloride plus one or more of sodium and potassium chloride.

The temperature of operation is generally in the range of 900-925" C. When the bath is heavily complexed so that it contains a combination of calcium and magnesium chloride and of sodium and potassium chloride, the temperature of operation may be reduced as low as 750-800 C. The mechanism of action in the operation is obscure, and we are not committed to any theory. The important fact is that it is necessary to control the reaction temperature of the electrolysis carefully, since if the temperature is allowed to rise much above 925 C., the bath itself is attacked, being electrolyzed and depleted, and calcium may be given off at the cathode. When properly carried out, the bath level remains substantially constant.

In the final analysis, the sole final reaction which takes place is a simple reduction of titanium dioxide to the desired pure titanium monoxide. The electrolysis is generally carried out with an E. M. F. of from 5 to 7 volts. If a diaphragm or compartment is used, the voltage required is of the order of 10 to l2 volts, and the amount of the increase is determined by the porosity and thickness of the diaphragm and the resulting potential drop across this diaphragm. All current densities from 1 up to at least 500 amperes per square decimeter at the cathode appear to be effective and in the small sized laboratory cells, current densities of the order of 300 to 400 amperes per square decimeter are used. It appears that the voltage and current requirements are not critical within the limits given.

It has been indicated in the foregoing that some loss in efliciency develops if the bath temperature is allowed to rise too high and the upper limit at which this loss in efficiency starts to become evident is around 1000-1050" C. Thus, for insurance purposes, the bath temperature is maintained at about 925 C. or below. In preparing the bath materials, it is an absolute necessity that these reagents be completely anhydrous and standard methods are used for preparation of the anhydrous agents.

Titanium monoxide, the desired end product, is formed as a powder in the cathode compartment and the process may be made continuous by circulating the cathode liquid into a third compartment from which the electrolyte is continuously ladled out for subsequent separation steps,

After the electrolysis is completed, the cathode material is washed free of halide by thorough water washing under a carbon dioxide atmosphere. The powder is then filtered and dried in a vacuum or at a temperature not exceeding 80 C. Under these conditions, it has been found that substantially no oxidation takes place resulting in the harmful higher oxides of titanium. The product titanium monoxide is a golden brown hard powder which when examined by X-ray procedures is shown to be chemically pure titanium monoxide.

The nature of the reactions which take place in this electrolysis are not clear, and we are not committed to any theory. The facts are that carbon monoxide is given off at the anode, no calcium metal is found in the cathode material, and if the cell is operated within the temperature limits given in the foregoing, the efficiency is extremely high, both electrically and from a chemical yield standpoint.

If too high a level of temperature is used, calcium droplets appear in the vicinity of the cathode and separate from the cathode, the course of the reaction being changed such that the bath itself is being electrolyzed to the formation of calcium, and correspondingly the efficiency both electrically and chemically falls, and the bath level also drops.

Illustrative of the procedure 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 decimeter 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 l to 1/2 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 dioxide 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 decimetei at the cathode equivalent to 200 amperes total input. Typical yields of material are of the order of while the current efficiency 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 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 a temperature in an ordinary drier not exceeding 80 C. Typical figures are the following: 500 grams of calcium chloride, 50 grams of titanium dioxide yielding 40 grains of titanium monoxide 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 in 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 titanium dioxide 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 eiiiciency is around 90% in that 40 grams of TiO are obtained in 13.5 minutes. Again, this is expected since a more effective use of the anode compartment is obtained.

Example 3.-The same general construction as described in Example l is used except that the comparutmentaliZing 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 is a porous sleeve of a Zircon porcelain consisting of substantially pure high tired zirconium silicate tired to a porosity of the order of Titanium dioxide feed material is placed in the annular space between the graphite anode and the Zircon sleeve. Material yields are quantitative while current yields are of the order of 85 to 90%.

Example 4 In this case, the crucible is a dense iired pure Zircon which is non-porous. The diaphragm of porous Zircon is placed in the middle portion of the cell and this diaphragm extends above the top of the liquid. The anode isa massive graphite rod and the cathode iS molybdenum. The anode almost fills the anode compartment. Titanium dioxide is placed in this area. Chemical yield is quantitative and current yields are 90 to 95%, 40 grams of TiO being obtained in about l2 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 oxide sinter having a particle size in the range of 1/16 to Mt of an inch. This is heaped in a pile around the anode in 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 oxide feed material is in the form of sintered tiitanium oxide grog which is placed inside this porous Zircon crucible. This method provides positive separation of titanium dioxide from the titanium monoxide 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 in a purified Zircon crucible submerged below the surface of the electrolyte. The crucible rests on the floor of the container crucible. Sintered TiOz grog is placed in the electrolyte in the anode area outside the cathode compartment. The TiO formed is retained in the removable cathode container.

There are many obvious modifications of the diaphragm and compartment type cells such as have been described in the foregoing specification as suitable for use in the process, and no attempt has been made to exhaust all the possibilities of construction. The important point to consider, however, is that it is vitally necessary to separate the anode and cathode products of reaction completely so as to prevent any possibility of the higher oxides of titanium being present in the end product. The means which have been described in this specification are sufliciently effective for the purposes for which titanium monoxide are eventually to be put.

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 substantially pure titanium monoxide which comprises providing an electrolytic cell separated into anode and cathode compartments by a porous barrier, forming a fused salt bath therein consisting essentially of at least one halide salt of the group consisting of alkaline earth metal halides and mixtures of alkaline earth metal halides and alkaline metal halides, charging solid titanium dioxide into the anode compartment only of the cell, maintaining the fused bath at a temperature within the range between about 700 C. and 925 C. and 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 monoxide from the cell.

2. The method of producing substantially pure titanium monoxide which comprises providing an electrolytic cell separated into anode and cathode compartments by a porous barrier, forming a fused salt bath therein consisting essentially of at least one halide salt of the group consisting of calcium chloride, mixtures of calcium chloride and at least one other alkaline earth metal halide and mixtures of calcium chloride and at least one alkali metal halide, charging solid titanium dioxide into the anode compartment only of the cell, maintaining the fused bath at a temperature within the range between about 700 C. and 925 C. and 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 resltllltant11 cathodically deposited titanium monoxide from t e ce 3. The method of producing substantially pure titanium monoxide which comprises providing an electrolytic cell separated into anode and cathode compartments by a porous barrier, forming a fused salt bath therein consisting essentially of at least one halide salt of the group consisting of alkaline earth metal halides and mixtures of alkaline earth metal halides and alkali metal halides, charging solid titanium dioxide into the anode compartment only of the cell, maintaining the fused bath at a temperature within the range between about 700 C. and 925 C. and 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 l to 500 amperes per square decimeter and recovering the resultant cathodically deposited titanium monoxide from the cell.

4. The method of producing substantially pure titanium monoxide which comprises providing an electrolytic cell separated into anode and cathode compartments by a porous barrier, forming a fused salt bath therein consisting essentially of at least one halide salt of the group consisting of alkaline earth metal halides and mixtures of alkaline earth metal halides, charging solid titanium dioxide into the anode compartment only of the cell, maintaining the fused bath at a temperature within the range between about 700 C. and 925 C. and under a noble gas atmosphere, passing an electrolyzing current through the fused bath between a carbonaceous 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 monoxide from the cell.

5. The method of producing substantially pure titanium monoxide which comprises providing an electrolytic cell separated into anode and cathode compartments by a porous barrier, forming a fused salt bath therein consisting essentially of at least one halide salt of the group consisting of alkaline earth metal halides and mixtures of alkaline earth metal halides and alkali metal halides, charging sintered titanium dioxide into the anode compartment only of the cell, maintaining the fused bath at a temperature within the range between about 700 C. and 925 C. and 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 monoxide from the cell.

6. The method of producing substantially pure titanium monoxide which comprised forming a fused salt bath consisting essentially of at least one halide salt of the group consisting of alkaline earth metal halides and mixtures of alkaline earth metal halides and alkali metal halides in an electrolytic cell containing an anode and a cathode maintaining the fused bath at a temperature within the range between about 700 C. and 925 C. and under a noble gas atmosphere, charging solid sintered titanium dioxide in the vicinity of the cell anode and remote from the cell cathode, 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 decimenter and recovering the resultant cathodically deposited titanium monoxide from the cell, substantially free from any higher oxides of titanium by withdrawing material from the vicinity of the cathode uncontaminated by any of the sintered titanium dioxide charged in the vi cinity of the anode and extracting the titanium monoxide from the withdrawn material.

7. The method of producing substantially pure titanium monoxide which comprises providing an electrolytic cell separated into anode and cathode compartments by a porous barrier, forming a fused salt bath therein consisting essentially of at least one halide salt of the group consisting of calcium chloride, mixtures of calcium chloride and at least one other alkaline earth metal halide and mixtures of calcium chloride and at least one alkali metal halide, charging solid titanium dioxide into the anode compartment only of the cell, maintainl References Cited in the tile of this patent UNITED STATES PATENTS Von Kugelgen Oct. 4, 1904 OTHER REFERENCES Journal of Applied Chemistry (U. S. S. R.), vol. 13 (1940), pages 5l thru 55, article by Sklarenko et al., original in Russian.

"Chemical Abstracts, vol. 34 (1940), page 7756, abstract of article by Sklarenko et al.

Titanium by Barksdale, published in 1949 by the Ronald Press Company, of New York, pages 41 and 43.

Claims (1)

1. 6. THE METHOD OF PRODUCING SUBSTANTIALLY PURE TITANIUM MONOXIDE WHICH COMPRISED FORMING A FUSED SALT BATH CONSISTING ESSENTIALLY OF AT LEAST ONE HALIDE SALT OF THE GROUP CONSISTING OF ALKALINE EARTH METAL HALIDES AND MIXTURES OF ALKALINE EARTH METAL HALIDES AND ALKALI METAL HALIDES IN AN ELECTROLYTIC CELL CONTAINING AN ANODE AND A CATHODE MAINTAINING THE FUSED BATH AT A TEMPERATURE WITHIN THE RANGE BETWEEN ABOUT 700* C. AND 925* C. AND UNDER A NOBLE GAS ATMOSPHERE, CARGING SOLID SINTERED TITANIUM DIOXIDE IN THE VINCINITY OF THE CELL ANODE AND REMOTE FROM THE CELL CATHODE, PASSING AN ELECTROLYZING CURRENT THROUGH THE FUED 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 DECIMENTER AND RECOVERING THE RESULTANT CATHODICALLY DEPOSITED TITANIUM MONOXIDES FROM THE CELL, SUBSTANTIALLY FREE FROM ANY HIGHER OXIDES OF TITANIUM BY WITHDRAWING MATERIAL FROM THE VINCINITY OF THE CATHODE UNCONTAMINATED BY ANY OF THE SINTERED TITANIUM DIOXIDE CHARGED IN THE VICINITY OF THE ANODE AND EXTRACTING THE TITANIUM MONOXIDE FROM THE WITHDRAWN MATERIAL.

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