US 2950185 A
Description (OCR text may contain errors)
Aug. 23, 1960 E. G. HELLIER ETAL ,1
PRODUCTION OF TANTALUM POWDER Filed June 13, 1958 Reflux Condenser INVENTO S Ed ard 67 Ht (ll CT 2,950,185 PRODUCTION OF TANTALUM POWDER Edward G. Hellier, Hingham, and George L. Martin,
Wayland, Mass., assignors to National Research Corporation, Cambridge, Mass., acorporation of Massachusetts 1 Filed June 13, 1958, Ser. No. 741,865
Claims. (Cl. 75-.5)
This invention relates to the production of tantalum metal in pure powder form.
The principal object of the present invention is to provide an improved process for producing tantalum metal which can be converted into usefully ductile material without the necessity of high vacuum sintering operations to remove oxygen.
Another object of the invention is to provide an improved method for producing tantalum powder at high production rates :withexcellent control of the reaction to produce uniform, small particles.
Still another object of the invention is to provide a process which can be readily controlled to give high production rates in a relatively short time cycle.
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 products possessing the features, properties and the relation of components which are exemplified in the following detailed 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 schematic, diagrammatic illustration of one embodiment of the invention; and
Fig. 2 is a sectional view of a portion of Fig. 1 taken along the line 2- 2 of Fig. l. i
In the present invention, tantalum powder is produced by reducing potassium fluotantalate with sodium. Potassium fluotantalate (K TaF has a melting point of about 765 C. In the'reduction operation a molten bath of the fluotantalate is preferably confined in a reactor maintained at a temperature above the melting point of potassium fluotantalate and liquid sodium is fed to the fused bath of potassium fluotantalate to reduce'the potassium fluotantalate to tantalum powder with the pro-' duction of the byproducts sodium fluoride and potassium fluoride. For each mole of tantalum formed, there are formed at least 5 moles of sodium fluoride and 2 moles of potassium fluoride. However the byproduct may not uniformly have the ratio of 2 moles of potassium fluoride to 5 moles of sodium fluoride since sodium will reduce potassium fluoride to potassium with the formation of sodium fluoride. Accordingly, the surface of the bath where the sodium is fed may consist largely of sodium fluoride, free sodium and NaK. The NaK thus liberated can reduce the potassium fluotantalate to tantalum metal. i
Sodium fluoride has a high melting point on the order of 990 C., while potassium fluoride has a relatively lower melting point of about 840 C. A mixture of 5 moles of sodium fluoride and 2 moles of potassium fluoride has a melting point of approximately 875 to 880 :ited States Patented Aug. 23, 1960 C., which is about the boiling point of sodium and con siderably above the boiling point of potassium. It is also'above the boiling point (about 830 C. to 850 C.) of the alloy NaK which can be formed in fairly copious quantities by reaction between sodium and KF.
In operating a reactor of the type generally described above, wherein sodium is fed to the surface of the molten bath of potassium fluotantalate, a serious problem is encountered as the reaction proceeds which drastically reduces the effective rate of production of tantalum powder. This is the formation of a frozen solid crust on the surface of the liquid potassium fluotantalate bath. This crust consists largely of sodium fluoride and exists even though there is considerable solubility of sodium fluoride in potassium fluotantalate. The principal contributing factor to the formation of the crust is the great discrepancy between the density of sodium fluoride and the density of the much heavier potassium fluotantalate. Accordingly, sodium fluoride formed at the surface of the bath by reaction with potassium fluotantalate can remain at the surface and form a concentration in excess of the equilibrium solubility in the available potassium fluotantalate. Accordingly, if the surface is below 990 C., the sodium fluoride can freeze and float as a crust on top of the molten bath of potassium fluotantalate. 'Since sodium is continuously fed to this crust surface, and since this surface is above the boiling point of sodium or the boiling point (about 830 C.-850 C.) of NaK, the sodium and NaK, whichever is present, will continuously remove heat from the frozen surface by vaporizing from this surface. Since this sodium so vaporized must either be refluxed to the reactor (from a suitable condenser) or must be replaced by fresh sodium or NaK, continual cooling of the frozen NaF surface is encountered. Once the NaF surface is frozen, it is extremely diflicult to melt the surface by heating up the bath to a temperature above the melting point of sodium fluoride due to the cooling effect of the refluxing sodium. It is possible to stop the feed of sodium and to maintain the bath at an elevated temperature until no more sodium is being refluxed so that the crust can be gradually melted and redissolved in the bath. However, this is a time-consuming operation and requires maintaining the reactor at an elevated temperature considerably in excess of 900 C. for extended periods of time. This expedient also reduces the amount of production which can be obtained from a reactor of given size. As can be seen from the above discussion, the problems encountered with the formation of crust are such that they should be avoided if high production rates and simple operation of the production equipment are to be achieved. \In the present invention, formation of the solid sodium fluoride crust is prevented by violently agitating the fused salt bath with sufficient vertical components of the agitating bath so as to positively prevent the existence of any substantial concentration stratification of the byproduct fluorides and potassium fluotantalate. Thus any sodium fluoride formed at the surface which starts to freeze is immediately pulled down into the bath where '(a) it is no longer subjected to the cooling effect of vaporizing sodium and (b) is in the presence of a large excess of potassium fluotantalate for permitting equilibration of the system.
Referring now to Figs. 1 and 2, there is illustrated a very diagrammatic form of one preferred embodiment of the invention. In these figures the reactor is indicated at 10 as comprising a cylindrical vessel such as an lnconel pot. The charge of KzTaFq, illustrated at 12, is placed in the pot and the pot is then sealed by an insulated cover 14. The pot is supported in a furnace generally indicated at 16 having a plurality of electrical heating elements 18 therein. Liquid sodium is fed from a sodium supply 20 through pipe 22 onto the surface of the charge 12 of K TaF Individual drops of sodium are illustrated at 24. A reflux condenser 30 is provided in a vent line 28 for refluxing sodium vapors tending to escape through the vent line 28. A vacuum pump 32 is also connected to the vent line to permit evacuation of the interior of the reaction vessel 10. A source of inert gas 34 is also connected to the vent line to permit the introduction of a slight superatmospheric pressure of inert gas into the interior of the reaction vessel.
For agitating the bath 12, there is provided a propeller 40 carried by a propeller shaft 42. This propeller shaft extends through a suitable vacuum-tight gland 44 to a motor schematically indicated at 46. Three baffles 48 are positioned around the interior of the reaction vessel 10. These bafiles serve to discourage circumferential flow of KgTaFq and encourage vertical flow. The combination of the baflles and propellers thus illustrated gives high upward flow of the fused bath at the center of the bath and rapid downward flow at the periphery of the bath. This violent agitation of the bath thus keeps the composition of the bath substantially completely uniform throughout its depth. In one preferred embodiment of the invention the propeller will give an upward velocity to the bath of 10 to 20 feet per minute. It will also provide substantially complete recirculation of the bath in 10 or 15 seconds.
In order to more fully understand one detailed application of the invention, reference should be had to the following specific non-limiting example.
Example I One hundred pounds of KzTaFq is placed in an Inconel reactor 12 inches in diameter and 12 inches high. The inside of the reactor is provided with 3 baffles having radial dimensions of about 1 inch as illustrated in Fig. 1 and is provided with a propeller of inches in diameter with a pitch of 35 During the reduction stage of the operation the propeller is run at 175 r.p.m. The reactor is sealed, and repeatedly evacuated and back filled with argon to provide a slight positive pressure of a few rni'llimeters above atmosphere. During this repeated evacuation and back filling with argon, the reactor is heated to 200 to 400 C. to assure removal of all water vapor and other gases or vapors. The reactor is then heated to 925 C. and 34.2 pounds of sodium (3% deficient) is fed in liquid form to the reactor, sodium being spread across the surface of the molten K TaF- and being fed to the reactor at a rate of about 2.0 pounds per hour (i.e. about 2.67 pounds per hour per square foot of molten salt surface). At the end of the sodium feed the reactor temperature is held at 925 C. and the molten mass is maintained at this temperature for 4 hours. The reactor is then cooled, opened up and leached in a number of successive leaches as set forth below:
G. Leaches 12, 13, 14, 1.5 and 16-25 gals. tap water- H. Leaches 17 and l8-25 gals. of distilled water I. Leaches 19 and 20-2 gals. of absolute methanol I. Vacuum dry product at 50 C. for 8 hours.
The product consisted of tantalum powder having the following particle size distribution after being run through a Wiley mill:
and an oxygen content of 0.017%
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, shall be interpreted as illustrative and not in a limiting sense.
What is claimed is:
1. In the process 'of producing tantalum powder wherein potassium .fluotantalate is reduced to tantalum metal by means of sodium the improvement which comprises confining a bath of molten potassium fluotantalate in a reactor at a temperature above 800 C., feeding molten sodium to the surface of the bath at a rate in excess of one pound per hour per square foot of bath surface and agitating said bath sufliciently strongly to provide for rapid upward flow of the fused bath at one portion of said bath andrapid downward flow at another portion of said badh so as to prevent any substantial Stratification between the byproduct and the potassium fluotantalate.
2. The process of claim 1 wherein the agitation is sufficient to provide an upward velocity of at least five feet per minute in said bath.
3. The process of claim 1 wherein the agitation is sufficient to provide for substantially complete recirculation of the bath in less than 1 minute.
4. In the process of producing tantalum powder wherein potassium fluotantalate is reduced to tantalum metal by means of sodium the improvement which comprises confining a bath of molten potassium fluotantalate in a reactor at a temperature above 900 C., feeding molten sodium to the surface of the bath at a rate in excess of one pound per hour per square foot of bath surface, refluxing to said surface sodium and NaK evaporating from said surface, and agitating said bath sufficiently strongly to provide for rapid upward flow of the fused bath at the center of said bath and rapid downward flow at the periphery of said bath so as to prevent any substantial stratification between the byproduct, sodium .fluoride and the potassium fluotantalate.
5. In the process of producing tantalum powder wherein potassium fluotantalate is reduced to tantalum metal by means of sodium the improvement whidh comprises confining a bath of molten potassium fluotantalate in a reactor at a temperature above 900 C., feeding molten sodium to the surface of the bath at a rate in excess of one pound per square foot per hour of bath surface, refluxing to said surface sodium and NaK evaporating from said surface, and agitating said bath with a mechanical agitator at a suificiently high rate and with substantially lineal upward and downward flow to draw into the bath any sodium fluoride crystals floating on the surface of the bath.
References Cited in the file of this patent UNITED STATES PATENTS 2,069,705 Gadeau Feb. 2, 1937 2,193,364 Adamoli Mar. 12, 1940 2,823,991 Kamlet Feb. 18, 1958 2,837,426 Kamlet June 3, 1958 FOREIGN PATENTS 791,121 Great Britain Feb. 26, 1958 541,517 Canada May 28, 1957