US 2951021 A
Description (OCR text may contain errors)
Aug. 30, 1960 Argon C22 W. 0. Di PIETRO ELECTROLYTIC PRODUCTION OF TITANIUM Filed March 28, 1952 To Scrubber and Chloridizer Coolo Argon C12 TiCl4 Anode Lead Ti C2 Vupor} Condenser "Ticl4 Liqt Argon -7 4.4-- Ti Fines Argon agg qo FIG. 2
Ccrl'hode Lead Ti Porficles (Large) FIG. l
Fili'er Filfer T' Powder L Tl Powder \1 I INVENTOR.
QC y 9 WILLIAM O. DiFlETRQ ATTOR NEY Unite States Patent ELECTROLYTIC PRODUCTION OF TITANIUM Wiiiiarn 0. Di Pietro, Water-town, Mass, assignor to National Research Corporation, Cambridge, Mass, a corporation of Massachusetts Filed Mar. 28, 1952, Ser. No. 279,159
6 Claims. (Cl. 204-64) This invention relates to the production of metals and more particularly to the production of titanium in an electrolytic cell.
A principal object of the present invention is to provide a process for producing titanium in an electrolytic cell which permits the collection of relatively large-particles of titanium with separate removal of these large particles of titanium from the cell.
Another object of the invention is to provide a process for producing titanium in an electrolyticcell which gives a high conversion of titanium tetrachloride to titanium metal.
Still another object of the invention is to provide an electrolytic process for producing titanium which minimizes a number of the corrosion problems ordinarily encountered in electrolytic cells of this type.
Gther objects of the invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises the process involving the several steps and the relation and the order of one or more of such steps with respect to eachof the others which are exemplified in the-following detailed dis closure, and the scope of the application of which will be indicated in the claims.
For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:
Fig. l is a diagrammatic, schematic, sectional view of one embodiment of the invention; and
Fig. 2 is a diagrammatic, fragmentary, enlarged view of aportion of the apparatus of Fig. 1.
In the production of titanium by electrolytic techniques it is quite desirable that the titanium manufacture be achieved-in eachelectrolytic cell at the highest possible rate with the highest possible power efficiency. In the present invention these objectives are achieved by providing, in the cell, an electrolyte whichcomprises a molten mixture of halides, and particularly the chlorides from the group consisting of the alkali metal chlorides and the alkali earth metal chlorides. To this electrolyte there is fed a chloride of titanium, preferablytitanium tetrachloride, and the'cell is operated at a sufliciently high voltage so that the cathode potential is above the discharge potential of one of the alkali metal or alkali earth metal ions. This method of operation has the advantage that it seems to prevent the production of titanium crystals which adhere to the cathode. In this manner either the alkali metal or the alkali earth metal is liberated at the cathode. For'convenience, the alkali metal or alkali earth metal released at the cathode is sometimes referred to hereinafter as the electrolyte metal. This electrolyte metal serves as a reducing agentwhich can reduce the titanium tetrachloride in the cell either to titanium metal or to a lower chloride. This aspect of the invention is particularly important in those cases where the electrolyte has a low solubility for the titanium tetrachloride, or Where the rate of dissolution of the titanium tetrachloride in the electrolyte is a limiting factor. In any particular case it is difiicult to state whether or not the titanium tetrachloride is completely reduced by the alkali metal or alkali earth metal to titanium metal, or whether the reduction takes place in stages, the tetrachloride being first reduced to a lower chloride which has a higher solubil-ity in the electrolyte, and then being reduced, in solution, to titanium. In eithercase much higher production rates are achieved, since the electrolyte metal will be completely used up, either by forming the more soluble lower chloride, or by reducing a titanium chloride to titanium.
In addition to the above features of the invention, there is another aspect of considerable practical importance. That aspect involves the control of particle size of the product titanium. Very small titanium particles, on the order of a micron or so, are enormously reactive with water, oxygen, nitrogen, and other contaminants. They are extremely difficult to handle and are highly undesirable as reaction products. In the present invention the particle size is controlled by recirculating the fine titanium powders, which are initially formed in thecell, into the space or zone between the anode and the cathode where the particle size is built up by a .mechanism which is not particularly well understood. This mechanism may involve the wetting of the titanium particles by alkali metal or alkali earth metals emanating from the cathode. It may be the formation of a surface lower chloride on the titaniumparticles which enhances the agglomeration thereof. Equally, it maybe a.high temperature sintering of the particles due to .thermal reduction of the -titanium chloride by alkali metal or alkali earth metal in the vicinity of a group of titanium particles. It is not unlikelythat particle growth may involve the simultaneous operation of a number of the above mechanisms.
In one preferred embodiment of the invention the circulation of titanium particles is achieved by circulating the electrolyte into the electrolysis zone or space between the anode and the cathode, this circulation being at a sufiiciently fast rate to carry withit all of the fine titanium particles, but not being sufficiently violent so as to pick up those larger titanium particles which can be readily handled in subsequent purification steps. This circulaation'rnay be achieved by utilizing the gas lift of the chlorine generated at the anode to force the electrolyte upwardly between the anode and the cathode. Additional circulation may be achieved by mechanical circulation means. I
Since the power loss in an electrolytic cell is a direct function of the IR drop in theelectrolytic cell, it is desirable to maintain the anode to cathode spacing as small as possible. The circulation of theelect-rolyte Within the cell, as provided in the present. invention, contributes considerably to the achievement of desired close anode to cathode spacing. This is due to the fact that the circulation of electrolyte continuously removes titanium from the space between the cathode and anode, .and also continuously sweeps titanium particles from the cathode so as to prevent short-circuiting of the cell. This circulation sweeps both :the little titanium particles'and the big titanium particles outof the electrolysis .zone between the anode and cathode and :returns the smaller titanium particles ,to this .zone to provide -for their fur ther growth. 1
Another feature of the present invention is directed to minimizing corrosion of the titanium cathode by the chlorine which is released in the cell at the anode. is achievethin a preferred form lot the invention, by maintaining the cathode immersed below the 'leyel of the electrolyte. This has the additional advantage that "the electrolyte can be recirculated up over the top of. the cathode so that the large titanium particles can -be collected in a settling zone on the inactive side of the cathode, and the fine titanium particles can be recirculated into the electrolysis zone between the cathode and anode.
Referring now to Figs. 1. and 2, there is shown one preferred embodiment of the invention where like numbers refer to like elements in both figures. In these figures, represents an electrolytic cell having therein an anode 12 and a cathode 14. The anode preferably surrounds the cathode, these two electrodes being formed as concentric cylinders in the preferred embodiment shown. The cathode and anode are preferably spaced less than one inch apart and have no current-restricting diaphragms or screens therebetween. This arrangement provides for high power efficiency in operation of the cell. The cell 10 contains a predetermined amount of molten salt 16 comprising the electrolyte, the level of this electrolyte being preferably above the top of the cathode 14. The anode preferably extends to the top 20 of the cell and is carried by this top 2! the top serving as the positive current lead to the anode. The cathode 14 is preferably supported by a plurality of conductors 22 and 22a. As shown, conductor 22a is preferably hollow and serves to provide a feed for a gas such as argon to a distributing ring 23 mounted adjacent the bottom of the cathode.
The titanium tetrachloride is introduced into the cell by means of a pipe 24, this titanium tetrachloride preferably being introduced as liquid drops 25 from a titanium tetrachloride supply 26. The flow of titanium tetrachloride into the cell is preferably controlled by a valve 28. The chlorine (indicated at 29) which is generated at the anode, escapes from the top of the cell to a pipe 30 along with the argon and excess titanium tetrachloride. A condenser 32 is provided for condensing titanium tetrachloride vapors which escape with the argon and chlorine, and for returning this titanium tetrachloride to the supply 26. The cell is preferably operated slightly above atmospheric pressure to provide for thorough purging of chlorine from the cell by the excess. titanium tetrachloride vapors and also to prevent inleakage of air into the cell. The chlorine and argon pass through the condenser 32 into the pipe '24 where the chlorine and argon are preferably separated so that the argon can be recycled and the chlorine may be utilized, such as by the chlorination of titanium-bearing ores or slag, to manufacture titanium tetrachloride.
The fine titanium particles formed in the cell are recycled into the electrolysis zone 18 between the anode and cathode. The larger titanium particles settle to the bottom 36 of the electrolytic cell where their removal is controlled by means of a valve 38, the larger titanium particles being periodically withdrawn by the operation of the valve and directed to one of the two filters 40. In these filters, the titanium particles are separated from the bulk of the salt comprising the electrolyte and are withdrawn for subsequent purification and consolidation into the form of ingots and the like. The salt which is separated from the titanium particles in filters 40 is recirculated to the cell 10 by means of pumps 42 and pipe 44. If desired, some of this salt may be introduced beneath the surface of the electrolyte to aid in the circulation of the electrolyte in the cell. With this modification the salt from the filters can be introduced, for example, through the pipe 22a and the distributing ring 23, the holes 54 in the distributing ring 23 being directed upwardly to direct the entering salt upwardly into space 18. Additionally, a minor percentage of the salt (or argon) can be introduced into space 36 to give a slight upward travel of the electrolyte in space 36 to preven inadvertent settling therein of titanium fines.
In a preferred embodiment of the invention this purification of the titanium particles is achieved by washing with a low-melting-point salt such as aluminum trichloride, as set forth more, fully in the copending applica- 4 tion of Findlay et al., Serial No. 252,564, filed October 22, 1951, now Patent 2,760,858.
Referring now more particularly to Fig. 2, there is shown an enlarged detailed'view of the conductor 22a which supplies current to the cathode 14 and argon to distributing ring 23. The hollow conductor 22a enters the electrolytic cell 10 through a hollow extension 50. This extension 50 preferably includes an insulating end plate 52 and is provided with a cooling coil 53 which is arranged to freeze the electrolyte 16a within extension This frozen electrolyte 16a serves as a high resistance path between the conductor 22a and the wall 10 of the electrolytic cell. The distributing ring 23 is provided with a'plurality of holes 54 through which the argon passes to form fine bubbles in the space between the anode and the cathode. These argon bubbles serve as a gas lift to supplement the gas lift created by the chlorine bubbles released at the anode. By the generation of the two gas lifts operating within the relatively confined electrolysis zone 18, a strong upward circulation of the electrolyte is achieved within this zone. This lift causes circulation of the electrolyte out of the top of the zone 18, to the settling zone inside of the cylindrical cathode 14, and back into the zone 18 at the bottom thereof. As the electrolyte re-enters the bottom of the zone 18, it carries in suspension the fine titanium particles which do not have sufficient mass to cause their settling to the space 36 at the bottom of the cell 10.
In a preferred embodiment of the invention the electrolytic cell 10 is formed of a suitable metal, such as molybdenum, nickel, stainless steel, or the like. To the extent that the alkali metal or alkali earth metal generated at the cathode can be kept out of the electrolyte in the neighborhood of the wall 10 of the electrolytic cell, this wall may be formed of a refractory or carbon.
The circulation of the electrolyte upwardly between the cathode and anode in the electrolysis zone 18 serves to prevent migration of alkali metal into the space outside of the anode 12. Thiscirculation of the electrolyte thus serves an additional advantage in providing a simpler cell construction. The anode 12 .is preferably formed of dense carbon or graphite so as to confine the chlorine and titanium tetrachloride vapors to the central portion of the cell. Thus, these vapors cannot reach the outer wall 10 of the electrolysis cell. The cathode 14 is preferably formed of a metal, such as molybdenum or the like, which is relatively inert to the electrolyte and also the alkali metal or alkali earth metal generated at the surface of the cathode. Since the cathode is immersed below the level of the bath, it is maintained relatively free from corrosion by chlorine vapors above the surface of the bath. The top 20 of the cell, at least that portion thereof which is contacted by the titanium tetrachloride and chlorine vapors. is preferably faced with carbon or the like to prevent attack thereof.
In one preferred .embodiment of the invention the cell is operated under the conditions set forth in Example I.
When the cell is operated in accordance with the conditions set forth in Example I, the titanium tetrachloride is preferably introduced in the form of liquid droplets by suitably controlling the valve28. As is apparent from Example I, approximately a twelvefold excess of titanium tetrachloride (over that necessary for supplying the titanium' produced in the cell) is introduced to the cell. This. serves to maintain a high partial pressure of titanium'tetrachloride vapor in the cell so that the chlorine generated 'at the anodeis rapidly driven out of the cell by the excess titanium tetrachloride vapors. I As described more fully in the copending application of'Di Pietro, Serial No. 279,160, filed on even date herewith (now abandoned), when utilizing the specific electrolyte of Example I, the top portion of the cell is maintained at approximately 525 C. to 550 C. so as to prevent the'formation of a crust which might plug up the chlorine escape line 30. This crust is believed to be due to the formation of .a compound between titanium tetra- :chloride and potassium chloride. This compound is considered'to be unstable at temperatures over about 500 and elimination of this crust has been successfully accomplished by maintaining the space immediately above the bath at a temperature somewhat above 500 C. A separate heater (not shown) may be employed for maintaining the desired high temperature at the top of the cell. This additional heater for the top of the cell may be particularly desirable in those cases where the ce'llisioperated with very low 1 R losses since the titaniumtetrachloride droplets introduced into the top of the cell will have a considerable cooling eifect upon the top of the electrolyte bath due to the heat of vaporization :of titanium tetrachloride. The total heat balance in the *system,'however, may be such that this additional heating is not essential, since heat is also generated within 'the cell 'by the exothermic reaction of the alkali metal or alkali earth metal with the titanium tetrachloride.
The argon flow into the space 18 is preferably maintained su'fiiciently high so as to obtain adequate circulation of the electrolyte upwardly within the electrolysis zone 18. As explained previously, this circulation may -':be due only to the gas lift of the chlorine bubbles rising up the surface of the anode 12. In the preferred form shown this circulation is augmented by the gas lift of the argon. The speed of circulation of the electrolyte is naturally a function of the height of the space 18 as well as the amount of chloride and/ or other gas rising in this space 18. In general, it is desired that sufiicient circulation be obtained so that all particles of titanium having a size, less than about 300 microns be maintained in suspension in the electrolyte and be carried back into the zone 18. All titanium particles having a diameter greater than about 300 microns may be allowed to settle into the-bottom 36 of the cell where they may be periodically removed by operation of the valve 38. An electrolyte velocity on the order of about 10 cm. per second will be sufiicient to carry the fine titanium particles back into the electrolysis zone, assuming more or less porous agglomerates of titanium having about 50% voids. Due to the fact that the electrolyte moves upwardly into the zone 18 between the anode and cathode, the conditions for separation of larger titanium particles from the electrolyte stream are optimum. Since the smaller titanium particles are readily carried in suspension in the electrolyte stream, they are readily circulated into the zone 18 while the larger titanium particles, due to their higher mass, readily settle out of the electrolyte stream and collect in the space 36 at the bottom of the cell.
The electrolyte and titanium particles in the space 36 are fed alternatively to the two filters 40 where gross quantities of the salt are removed from the titanium particles. The remainder of the electrolyte salt may then be removed from the titanium by vacuum-leaching or by washing with a low-melting-point salt, such as aluminum trichloride, as described more fully in the above identified Findley et a1. application, Serial No. 252,564. The thus purified titanium powder is then preferably vacuumleached to remove the aluminum trichloride, and may be consolidated in accordance with usual techniques, such as by melting in an arc furnace to form an ingot.
In connection with the operation of the cell, it is extremely important that all oxides and nitrides be kept out of the electrolyte and that no air be permitted to contact the electrolyte during the operation of the cell. If there are any impurities in the salts comprising the electrolyte, these salts should be purified by known .tec. niques prior to production of titanium in the cell. Equally, the argon which is introduced to the cell .must be maintained completely freeof nitrogen, oxygen, and Water vapor.
In still other embodiments of the invention the conditions of operation of the cell are preferably the same as described in connection with the discussion of Example I. However, in this case the electrolytes and temperatures may be as set forth in the following examples.
Example II Electrolyte:
35 mole percent NaCl 45 mole percent MgCl Temperature:
above M.P. of 430 C.
Example III Electrolyte:
67 mole percent KCl 33 mole percent MgCl Temperature:
above M.P. of 430 (3., preferably above 500 C.
Example IV Electrolyte:
35 mole percent BaCl 65 mole percent LiCl Temperature:
above M.P. of 510 C.'
Example V Electrolyte:
50 mole percent SrCl 50 mole percent NaCl Temperature:
above M.P. of 565 C.
While preferred embodiments have been described above, numerous modifications thereof may be made without departing from the spirit of the invention. For example, numerous other electrolytic bath compositions may be employed. In general, it is desirable that the bath have a low melting point. It is highly desirable that the cathode to anode spacing in the cell be maintained as close as possible, and that the cell be operated at a sufiiciently high voltage so that the cathode potential is above the discharge potential of at least one of the alkali metal or alkali earth metal ions of the chlorides comprising the bath.
While specific methods of creating circulation of the electrolyte have been discussed above, numerous other means may be employed. A mechanical stirrer may be inserted in the cell, or the electrolyte may be physically pumped from the cell and reintroduced into the cell between the anode and cathode so as to furnish the circulating energy. Equally, other methods of feeding titanium tetrachloride may be employed, such as those shown in the copending application of Benner and Chadsey, Serial No. 233,203, filed June 23, 1951, wherein the titanium tetrachloride is introduced below the surface of the bath. A somewhat similar arrangement is shown in the copending application of Hood et al., Serial No. 279,064, filed on even date herewith (now abandoned). Equally, the cathode may be made porous and titanium tetrachloride can be fed into the electrolyte from the interior of the porous cathode. In these latter embodiments of titanium tetrachloride feed, the titanium tetrachloride itself may furnish a gas lift for obtaining additional circulation of the electrolyte.
While it is preferred that titanium tetrachloride be utilized as the titanium compound which is introduced into the cell, other chlorides of titanium, such as titanium trichloride or titanium dichloride, may be employed. However, since these lower chlorides are more expensivestarting materials, the tetrachloride is preferred. Equally, the electrolyte may include halides other than the chlorides so as to lower the melting point of the electrolyte. From the standpoint of cheapness and corrosion resistance the chlorides are preferred.
This application is in part a continuation of the copending application of Findlay et al., Serial No. 252,564, filed October 22, 1951, now Patent 2,760,858.
Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting sense.
What is claimed is:
1. The process of producing titanium within a closed electrolytic cell, said cell containing a cathode and an anode defining an electrolysis zone through which electrical currents will substantially continually pass between said cathode and anode, said cell containing an electrolyte consisting essentially of at least one halide selected from the group consisting of the alkali metals and the alkali earth metals, the steps which comprise feeding titanium tetrachloride to said cell, electrolyzing said electrolyte at a sufficiently high voltage to give a cathode potential which is above the discharge potential of at least one of the alkali metal or alkali earth metal ions of the halides comprising the electrolyte to electrolyze said electrolyte halide to its metal, at least partially reducing said titanium tetrachloride by means of said electrolyte metal, circulating said electrolyte through the electrolysis zone between said cathode and anode to remove from said zone titanium particles created in said zone, collecting larger particles of titanium formed in said cell, said collection being accomplished within said electrolytic cell in a collection zone removed from the electrolysis zone, and recirculating into the electrolysis zone between the anode and cathode those titanium particles formed in the electrolysis zone which are too small to be collected in the collection zone and which have a diameter less than about 300 microns.
2. The process of claim 1 wherein said electrolyte consisting of a mixture of alkali metal chlorides and circulation of said titanium fines is at least partially achieved by gas lift of the chlorine generated at the anode.
3. The process of claim 1 wherein said circulation is achieved by introducing a gas into said cell adjacent the bottom of the zone between the anode and cathode,
7 r 4. The process of claim 1 wherein said gas is an inert gas.
-5.- The process of claim 1 wherein said circulation is sufficiently rapid to sweep from said cathode substantially all titanium particles adhering thereto.
'6. The process for producing titanium which comprises the steps of providing in a closed, electrolytic cell an electrolyte consisting essentially of a molten mixture of chlorides from the group consisting of the alkali metals and the alkali earth metals, introducing titanium tetrachloride into said cell, electrolyzing said electrolyte at a voltage sufliciently high so that the cathode potential is above the discharge potential of at least one of the alkali metal or alkali earth metal ions of the chlorides comprising the electrolyte to liberate one of said alkali or alkali earth 'metals, maintaining said electrolyte at a temperature above that temperature at which the electrolyte has an appreciable solubility for titanium tetrachloride, and circulating the electrolyte within the cell to remove titanium particles from the space between the anode and cathode and to carry fine titanium particles back into said space, the circulation of electrolyte being sufficient to sweep out of said space substantially all titanium particles created in said space, passing the circulating electrolyte through a setting zone where particles of titanium larger than about 300 microns are decanted from the circulating electrolyte, the velocity of the electrolyte in the settling zone being maintained sufficiently high that titanium particles smaller than about- 300 microns do not settle from the circulating electrolyte.
References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Australian Journal of Applied Science, vol 2, No. 3, September 1951, pages 358 thru 367, paper by Cordner et al. I