US 2847297 A
Description (OCR text may contain errors)
Aug. 12, 1958 w. 0'. D! PIETRO METHOD OF PRODUCING TITANIUM CRYSTALSQ Filed Aug. 23. 1952 H20 and 20 TiC2 (2))- I82 TiC1 (1) TiClg (5) Frozen Self A Reducing Agcnf (c.g.Na) in j 26 SuH FIG.
2 Sheets-Sheet 1 Coolanf o ui' SqH' (0.9. Na Cl) Reducing Agcni' (.g. Na)- Ti Powder INVENTOR. W/'///'am Q Dip/z fro ATTORNEY tates Patent Ofiice 2,847,297 Patented Aug. 12, 1958 METHOD OF PRODUCING TITANIUM CRYSTALS William 0. Di Pietro, Watertown, lvlass assignor to National Research Corporation, Cambridge, Mass, a corporation of Massachusetts Application August 23, 1952, Serial No. 306,026
2 Claims. c1. 75-845) This invention relates to.the production of metals and more particularly to the production of the group IVa metals, such as titanium, zirconium and the like. This application is in part a continuation of my copending application Serial No. 279,160, filed March 28, 1952, now abandoned.
A principal object of the present invention is to provide an improved process for the production of group IVa metals, such as titanium, zirconium and the like, in the form of relatively coarse particles.
Another object of the invention is to provide an improved process and apparatus for producing titanium and the like at high production rates with relatively low capital equipment and labor costs.
Another object of the present invention is to minimize corrosion and heat transfer problems arising from the thermal reduction of titanium tetrachloride and like materials.
Other 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 each of the others, and the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following'detailecl disclosure, 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 drawings wherein:
Fig. 1 is a diagrammatic, schematic, partially sectional view of one embodiment of the invention; and
Fig. 2 is a diagrammatic flow sheet illustrating the related equipment and processing steps utilized with the Fig. 1 embodiment.
The present invention is primarily directed to the production of group IVa metals, and will be specifically discussed in connection with the production of titanium by the reduction of a titanium tetrahalide by the use of a metal reducing agent such as the alkali metals lithium, sodium and potassium or the alkaline earth metals magnesium or calcium. In a preferred embodiment of the invention the titanium tetrahalide starting material is titanium tetrachloride, and metallic reducing agent is sodium.
In the prior art numerous attempts have been made to devise commercially practicable methods for producing titanium by the thermal reduction of titanium tetrachloride with metallic reducing agents such as sodium, magnesium and the like. Examples of such processes are described in the patents to Kroll No. 2,205,854, Maddex No. 2,556,763, Freudenberg No. 2,148,345, Blue No. 2,567,838 and Winter No. 2,586,134. These prior art processes give low production rates, require large capital investment, and the product titanium is either 'free reaction chamber.
in the form of fine powder (usually mixed with lower chlorides of titanium) or is in the form of a bulky sponge containing much entrapped alkali metal or alkaline earth metal chlorides. In the present invention many of the difiiculties of the prior art are overcome by so operating the process that relatively large particles of titanium are produced, and providing for accurate control of the reaction temperature.
In a preferred form of the invention there is provided a fused salt bath which is maintained in an air- Within the reaction chamber there is provided an atmosphere of titanium tetrachloride above the bath. A liquid metal reducing agent is introduced into this titanium tetrachloride atmosphere, the reducing agent forming titanium trichloride gas. The titanium trichloride gas is condensed above the bath and is returned to the bath where it is dissolved in the fused salt comprising the bath and where it is further reduced to titanium metal. In a preferred embodiment of the invention, the liquid metal reducing agent is introduced into the reaction chamber by being .fed into the fused salt bath below the surface thereof. Since the reducing'agent is lighter than the 'bath, it travels upwardly in the fused salt bath, some of this liquid metal reducing agent reaching the surface of the bath so'as to react with the titanium tetrachloride atmosphere to form the titanium trichloride which is dissolved in the bath. Most ofthe reducing agent, however," is used up during its upward travel in the bath by reducing the dissolved lower chloride of titanium to titanium metal. During the course of this reaction, the fused salt bath is vigorously agitated so as to cause circulation of the titanium particles in the bath, this circulation greatly increasing the particle size of the resultant titanium. Periodically, the titanium particles are removed from the reaction chamber and are separated from thefused salt comprising the bath.
In a preferred embodiment of the invention, as mentioned previously, the metal reducing agent'is a liquid and comprises sodium, while the fused salt bath comprises sodium chloride. This arrangement has the advantage that the sodium reduction of titanium tetrachloride forms sodium chloride as a by-product and therefore the composition of the fused salt bath remains substantially constant. 7
Since the reduction of titanium tetrachloride by sodium is a highly exothermic reaction, it is essential that tremendous quantities of heat be dissipated in the reactor. In the present invention this heat is removed in two ways. The first method of removing heat is to cool the reactor walls (at least those'portions thereof in contact with the fused salt) by a liquidmetal heatexchange medium which may, for convenience, be a sodium-potassium alloy. In one embodiment of the invention the cooling of the reactor walls is sufficiently rapid so as to maintain a layer of frozen sodium chloride adjacent the metallic walls, thereby protecting these Walls against corrosion. In one preferred embodiment of the invention this frozen layer of salt is relatively thick and consequently limits the heat transfer through the salt layer to the reaction chamber. Since this removal of heat through the walls of the reaction chamber is not particularly effective, the preferred embodiment of the.
invention also includes condensation of the titanium tettrachloride atmosphere adjacent the top of the reaction A chamber, the heat of condensation being removed by a cooling medium such as water. The condensed tit-anium tetrachloride is allowed to drip from the top of the reaction chamber on to the hot surface of the molten sodium chloride. As it strikes this hot surface, it is immediately vaporized, thereby removing large quan- 3 tities of heat from the surface of the molten salt bath. This evaporation of titanium tetrachloride from the surface of the salt bath has been utilized experimentally to remove approximately 9700 B. t. u. per hour per square foot of molten salt surface. This rate of heat removal is equal to a rate of titanium production of approximately 1.5 lbs. per hour per square foot of salt surface, ignoring all other heat removal accomplished by removal of hot salt and hot titanium from the reactor, and removal of heat by cooling the reactor walls. This titanium production rate is by no means an upper limit and can be greatly increased, the only limitation being that it is undesirable to feed sufficient titanium tetrachloride to the surface of the bath to cause the freezing of this surface. This freezing will be inhibited, even at high rates of titanium tetrachloride feed thereto, by high salt circulation rates.
Referring now to Fig. 1, there is shown one preferred form of apparatus embodying the present invention. This apparatus includes a reactor which defines therewithin a reaction chamber 12. This reaction chamber holds a charge of molten salt 14- in which titanium particles 15 are held in suspension. The reactor 10 is preferably designed so as to have an outer shell 16 therearound, this outer Shell 16 providing, with reactor 10, a space 17 for a heat-transfer medium which can be a liquid metal, such as sodium or a sodium-potassium alloy. The top 18 of the reactor 10 is also provided with cooling means which preferably comprise water pipes 20 for permitting a high rate of circulation of relatively cold water within the top 18. Pipes 22 similarly provide for the circulation of a cooling agent in the space 17 between reactor 10 and the outer shell 16.
As mentioned previously, a reducing agent (e. g. sodium) is preferably introduced into the reaction chamber below the level of the salt bath 14 which is confined Within this reaction chamber. As illustrated this sodium is introduced by means of a pipe 26. The titanium tetrachloride which is to be reduced to titanium metal may be introduced, as a gas, by means of a pipe 28, to the space above the level of the salt bath 14. This titanium tetrachloride may be introduced in sufficient quantities to maintain about one atmosphere of pressure of titanium tetrachloride above the salt bath. A stirrer 30 is also preferably provided within the reactor 10 so as to violently agitate the salt bath to break up the bubbles of sodium into very small particles. This stirrer 30 also maintains the bath in agitation so as to provide for circulation within the bath of small particles 15 of titanium suspended in the bath, these particles being sufiiciently agitated so that all of the smaller particles are kept in suspension while the larger particles are allowed to settle to the bottom of the reactor.
In general it is desired that the fused salt, during circulation, have an upward component of velocity sufficient so that all particles of titanium having a size less than about 300 microns will be maintained in suspension in the fused salt. An upward fused salt velocity on the order of 10 cm. per second is sufficient to maintain the titanium particles in suspension, assuming more or less porous agglomerate of titanium particles having about 50% voids.
In one preferred embodiment of the invention, the bottom of the reactor is provided with a relatively narrow tube 32 which constitutes an outlet for salt and titanium particles generated in the reactor. This outlet may be normally closed by providing a plug of frozen salt therein, this salt being kept frozen by means of a cooling coil 33. Positioned below the outlet tube 32 is a filter 34 in which gross quantities of salt are separated from the titanium particles.
Referring now to Fig. 2 there is shown more clearly the operation of the device of Fig. l and the relationship of the auxiliary equipment, preferably employed with this type of reactor. In Fig. 2, where like numbers refer to like elements of Fig. 1, there is shown a number of additional items of equipment such as sodium supply tank 36 from which the liquid sodium is fed by means of a sodium feeding mechanism 38 into the feed pipe 26. A similar supply 40 is provided for the titanium tetrachloride, the liquid titanium tetrachloride being fed to a vaporizer 42 so as to provide a source of vapors for the pipe 28 leading to the reactor. A coolant temperature controller 44 is also provided for controlling the temperature of the heat exchange medium which is used adjacent the walls of the reactor. This coolant temperature controller is used to remove heat from the reactor walls at predetermined rates so as to maintain the desired thick layer of frozen salt adjacent the inside of the reactant walls. Equally, the coolant temperature controller may, for initially starting the reactor, be used to melt an initial charge of salt placed in the reactor.
The titanium powder, contaminated with salt, obtained from the filter 34 is purified in a purifying step illustrated schematically at 46. This step may comprise a Water leaching or a vacuum leaching operation, depending upon the ultimate particle size of the titanium particles. The purified titanium powder is melted to form a titanium ingot or the like in a suitable melting chamber 48. The salt which is separated from the titanium salt suspension in the filter 34 is preferably fed to an electrolysis cell 50 where it is electrolyzed to sodium and chlorine, the sodium being fed back to the sodium supply 36 and the chlorine being fed to a usual titanium tetrachloride manufacturing plant 42 where it is reacted with carbon and titanium dioxide.
In the operation of the device of Figs. 1 and 2, a charge of pure salt is placed in the reactor 10 and all air and other contaminants are removed from the interior of the reactor, preferably by evacuating the reactor. An inert gas may be then introduced into the reactor to displace any residual quantities of air and the salt may be brought up to a temperature slightly above its melting point by suitable heat transfer with hot liquid sodium which is circulated in the space 17 between the outer shell 16 and the outer surface of the reactor 10. When the reactor has been brought up to temperature, an atmosphere of titanium tetrachloride gas may be created by feeding titanium tetrachloride vapors into the reactor. At this time the cooling water flow to the head of the reactor 18 is preferably turned on so as to maintain the head 18 at a temperature slightly below the boiling point of titanium tetrachloride, thus condensing some of the titanium tetrachloride. A layer of frozen salt is now preferably formed on the inner surface of the reactor wall by suitably lowering the temperature of the wall coolant below the melting point of the salt. Some of the hot salt will creep up the wall above the level thereof so that this frozen salt layer will extend above the salt bath. At this point sodium may be fed into the pipe 26, the agitator breaking the sodium into very small particles which float to the surface of the bath 14. As these droplets of sodium reach the surface of the bath 14, they immediately react with the titanium tetrachloride atmosphere to form titanium trichloride vapors, probably some titanium dichloride and some titanium metal. Titanium trichloride vapors are condensed on the top 18 along with the liquid titanium tetrachloride and the condensed titanium trichloride is washed back to the surface of the molten bath 14- by means of the titanium tetrachloride liquid. The titanium trichloride is dissolved in the salt bath, while the titanium tetrachloride liquid flashvaporizes from the surface of the bath removing large quantities of heat from this bath. The dissolved titanium trichloride is reduced in the bath by means of the finely dispersed sodium droplets and forms titanium metal.
During the reduction operation, the stirrer 30 is rotated at a high speed so as to break up, into very small globules, the larger particles of molten sodium which are fed into the lower portion of the molten salt bath 14.
Ihe action of the stirrer 30 also serves to agitate the whole bath 14 so as to maintain in suspension the smaller particles of titanium which are produced as a result of the reduction reaction. The circulation of these small particles of titanium within the bath and also into contact with the titanium tetrachloride atmosphere above the surface of the bath provides for particle growth of the individual titanium particles. When the particles become sufficiently large (depending upon the speed of circulation of the bath) they will settle out of the bath and collect adjacent the bottom of the reactor 10. Prior to removal of the titanium particles by draining the salt from the reactor, it is preferred to discontinue the in-' troduction of titanium tetrachloride and to introduce an excess of the reducing agent so as to reduce to metal substantially all of the lower chlorides of titanium dissolved in the salt bath. In order to remove the titanium particles and the excess salt generated during the reduction operation, the frozen salt plug in the outlet tube 33 is melted, thus permitting the salt in the bottom portion of the reactor to flow down into the filter 34 carrying the suspended titanium particles therewith. The salt in the outlet tube 32 is preferably refrozen before all of the salt has been drained out of the reactor and so that operation can be continued immediately.
In an alternative embodiment of the invention, the salt may be removed from the reactor without being first purged of lower chlorides of titanium. However, in this case, it is preferred that the efiiuent salt be treated with a reducing agent such as sodium or magnesium and the like prior to water leaching so as to prevent hydrolysis of any lower titanium chlorides dissolved in the salt.
The filtered titanium particles, from which the major proportion of salt has been removed, are transferred to a usual purification step shown at 46 in Fig. 2 where the residual salt may be removed by Water leaching or vacuum leaching, as the case may be. The reduction of residual lower chlorides of titanium may also take place in this purification step 46. The thus purified titanium is then preferably transferred to a melting chamber 48, where the titanium is melted to form an ingot or other shape. If desired, alloying ingredients may be added during this melting.
While the above description of the invention has been limited to one preferred embodiment thereof, it is apparent that numerous modifications may be made in the previously described invention without departing from the scope thereof. For example, it is quite possible and in some cases highly desirable to utilize a mixture of reducing agents such as sodium-potassium alloy. This has the distinct advantage that the alloy is liquid at room temperature, thus simplifying the handling, metering and storage of the reducing agent. This embodiment also has the advantage that the resultant mixture of chlorides, which are the by-product of the reduction step, has a much lower melting point than the individual chlorides. When a sodium-potassium alloy is used as the reducing agent, the sodium chloride-potassium chloride mixture from the reactor can be treated with sodium metal in accordance with the teachings of British Patent 590,274 to form a sodium-potassium alloy. In this way substantially all of the potassium content of the effluent salt may be recovered while the residual sodium chloride can be discarded or fed to an electrolytic cell for recovery of the sodium and chlorine therefrom. The same reasoning applies with respect to other mixtures of reducing agents such as mixtures of sodium and magnesium or.
sodium and calcium.
Equally, other halides of titanium, zirconium, and the like may be utilized. However, it is preferred that the lower halide of the metal being reduced be soluble in the salt bath held in the reactor 10. When the other halides sublime prior to melting, as is the case with zirconium tetrachloride, it is apparent that the pressure in the reactor must be maintained above atmospheric pressure in order to condense the halide and to feed it back as a liquid to the surface of the salt bath. When other halides of titanium and zirconium are employed, the temperatures of the top of the reaction chamber and the pressure in the reaction chamber are preferably as set forth in the following table.
Tabl'el Tem of Pressure, Compound 18 'QFg. 1) Atm.
Tron 136 1 T114- 377 1 230 1 437 25 450 15 499 6.3
When the zirconium tetrahalides are employed it is preferred that the pressure in the coolant space 17 be equal to the pressure within the reaction chamber so as to prevent an undue stress on the relatively hot inner wall of the reactor 10.
In like manner numerous alternative embodiments. of the apparatus can be employed. For example, the condensation of the titanium tn'chloride may take place on a condensing surface above the surface of the salt bath and a mechanical scraper may be provided for feeding the condensed trichloride back into the bath. From the standpoint of corrosion, it is preferred that this condensing surface be maintained at the. lowest possible temperature, 'but it may, in certain instances, be impractical to maintain this condensing surface at a temperature below the boiling point of titanium tetrachloride. In this case the titanium tetrachloride may be condensed outside of the reactor and the liquid titanium tetrachloride may then be fed back to the reactor, where it is re-evaporated from the surface of the molten salt bath. In this embodiment the amount of liquid titanium tetrachloride fed back to the surface of the salt bath may be controlled as a function of the temperature of the bath. Equally, some titanium tetrachloride may be fed to the salt bath below the surface thereof, this embodiment having the advantage that the titanium tetrachloride passing upwardly through the bath is in a position to contact small titanium particles suspended in the bath and thus encourage particle growth by the formation of titanium lower chloride surface strata on the small titanium particles. This feed of titanium tetrachloride below the bath may also assist in the stirring of the bath.
It is equally apparent from the above description that numerous other arrangements of the mechanical elements constituting the reactor are feasible. For example, the v'alving arrangement shown may comprise a normal plug valve or the like in lieu of the frozen salt plug illustrated. Equally, titanium tetrachloride may be introduced into the reactor as a liquid rather than as a gas. Additionally the apparatus may be made continuous or semi-continuous by providing for more or less continuous removal of excess salt and titanium particles from the reactor. In this case a weir may be provided for removal of the salt and a mechanical conveyer can be employed for removal of the titanium particles.
Since certain changes may be made in the above process and apparatus 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 drawings, shall be interpreted as illustrative and not in a limiting sense.
What is claimed is:
1. In .a method of producing titanium, the step of growing titanium crystals in a molten salt bath containing a dissolved lower titanium chloride by substantially continuously introducing into the molten salt bath a liquid metal reducing agent comprising a metal from the group consisting of the alkali metals and the alkaline earth metals magnesium and calcium to form some free titanium metal in the salt bath, maintaining the titanium metal so formed in the presence of dissolved lower chloride in the molten salt bath to permit the growth of titanium crystals of substantial size while reduction of said dissolved titanium lower chloride to titanium metal continues within the molten salt bath adjacent the growing titanium crystals, and thereafter separating the titanium crystals from the salt bath.
2. The process of claim 1 wherein the solution of dissolved lower titanium chloride is produced by reducing titanium tetrachloride to the lower chloride in the presence of molten salt in a separate zone from the zone where the titanium crystal growth is achieved.
References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Journal of Metals, April 1950, pp. 634-640. Metal Powder Report, vol. 7, No. 4, December 1952,
pp. 50-51. 15 Ltd.
Published in London by Powder Metallurgy