US 3635693 A
A block of tantalum or niobium is first stripped of superficial impurities by immersion in an acid bath, for instance hydrofluoric acid. Thence it is dried in a vacuum and saturated with hydrogen under high pressure and heat. This embrittles the body which is fragmented into coarse metal-hydride particles. The brittle fragments are crushed to a powder of the desired fineness and the powder is cleaned in boiling acid then degassed in a vacuum under heat to transform the interstitial hydride to the pure-metal powder.
Description (OCR text may contain errors)
Friedrich et aI.
[ Ian. i
 METHOD OFPRQDUCING TANTALUM FOREIGN PATENTS OR APPLICATIONS 0R NIOBIUM POWDER FROM 236,694 4/1959 Australia ..7s/0.5 COMPACT BODIES 746,061 3/1956 Great Britain .75/0.s I 676,359 12/1963 Canada ..75/0.5  321?" Meyer 680,534 2/1964 Canada ..75/0.5 y 697,222 11/1964 Canada ..75/0.5  Assignee: Hermann C. Starck, Berlin, Germany Primary Examiner-Byland Bizot  Filed 1969 Assistant Examiner-W. W. Stallard  93 93 AttorneyKarl F. Ross and Herbert Dulbno ABSTRACT  U.S.Cl. ..75/0.5 BB l 51 161. c1. ..B22f9/00' A block 0f tantalum or moblum first PP of superficlal 58 Field of Search ..75/0.5 impurities by immersiml acid bath, for instance hydrofluoric acid. Thence it is dried in a vacuum and satu- 56] References Cited rated with hydrogen under high pressure and heat. This embrittles the body which is fragmented into coarse metal- UNITED STATES PATENTS hydride particles. The brittle fragments are crushed to a powder of the desired fineness and the powder is cleaned in 3,295,951 1/1967 F ncham et al. ....75/O.5 boiling acid then degassed in a vacuum under heat to trans 31323914 6/l967 Fmcham et aL form the interstitial hydride to the pure-metal powder. 3,4l5 639 12/1968 Daendliker et al ....75/0.5 3.473915 10/196) Pierret ..75/0.5 5 Claims, 1 Drawing Figure W hn as 1 g 2 I 5 6 Safurafing 3 Withdrawing HF Bath Drying 3 Wifh H, Free H,
w w w L w l l l l l /O' Torr 800- 300-400 Torr Surface Cleaning [4000c Geog/000C Coarse Iron Removal Metal-Hydride 7 Removal of Hydrogen Wifh HC/ Comminufion Powder In j v IO'-'/0" Torr Mere! Powder F 3 METHOD OF PRODUCING TANTALUM OR NlIOlBHUM POWDER FROM COMPACT BODIES Our present invention relates to a method of producing tantalum or niobium powder from compact blocks or bars of these metals.
In order to produce metallic tantalum or niobium, an impure powder containing these elements is usually electronbeam smelted in a vacuum of about torr. This allows an extremely high degree of purity to be obtained since any impurities are volatilized by this process.
Tantalum capacitors on the other hand require extremely pure tantalum powder for their manufacture, in particular if they are to be used with voltages superior to 35 volts.
However, it is virtually impossible to mechanically fragment tantalum or niobium bars or blocks due to their extreme ductility in the pure state. An attempt has been made to make powder of tantalum and niobium by turning a block of one of these metals on a lathe and treating the shavings with hydrogen to embrittle them. These shavings can be crushed and degassed. Here however, a great many impurities are added during the turning operations, which impurities either contaminate the final product or must be painstakingly removed.
Unfortunately, it has proven impossible heretofore to saturate directly blocks of tantalum or niobium with hydrogen.
It is therefore the principal object of our invention to provide an improved method of reducing compact tantalum or niobium blocks or bars into a powder of a purity at least equal to that of the original blocks or bars which is simpler and easier to carry out than the above-described and other earlier techniques.
We have now discovered, surprisingly, that it is not necessary to comminute the compact bodies (i.e., continuous bars, billets, etc. cast from a melt or sintered bodies) prior to using hydrogen-saturation technique and that such saturation can be effected upon removal of surface films of impurities which appear to act as barriers to hydrogen penetration, but were not heretofore recognized as the reason why earlier attempts at saturation were unavailing.
We do this, according to one feature of our invention, by first thoroughly cleaning the bars or blocks in an acid bath preferably of concentrated hydrofluoric acid. This is effective to eliminate the minute coating of impurities on the surface of tantalum or niobium blocks, which is so thin as to be immeasurable, preventing hydrogenation or saturation of the blocks by hydrogen. Thus, agitation of the bar in a bath of, for instance, hydrofluoric acid removes this coating or film of foreign matter.
After this the remaining acid is washed off and the block is dried in high vacuum (e.g., less than 10" Torr) under heat (e.g. 800 to l,400 C.).
The dried and stripped block is then saturated with hydrogen. This is carried out, in accordance with another important feature of our invention, in a pressurizable induction furnace. The pressure of hydrogen gas is maintained around 300-400 Torr and the temperature is held, in stages, at points between 600 C. and l,000 C., 200 C. and 500 C., and 60 C. and 120 C.
After saturation with hydrogen, although here by saturation only saturation to a point where the quantity still absorbable is nominal is meant, the oven is opened to ambient pressure, temperature and to the atmosphere. The hydrogen is absorbed into the interstices of the crystal lattice to cause fragmentation and defect structures when the pressure is released. The resulting powder contains hydrogen in the form of the interstitial hydride of tantalum or niobium. The result is that the block which was greatly embrittled by absorption of hydrogen spontaneously fragments into a coarse metalhydride powder of great brittleness.
These fragments are comminuted by crushing, for example in a pebble or ball mill, and the impurities (mainly iron impurities) thereby acquired are removed by boiling the resultant powder of the desired fineness in an acid, for example hydrochloric acid.
Finally, the cleansed powder is dehydrated or degassed in a high vacuum (e.g., l0"'-l0" Torr) at elevated temperatures (e.g., 600 to 1,200 C.) to produce a fine powder of a purity at least equal to that of the original block.
Direct-coupling induction heating, without intervening insulation, using a water-cooled coil prevents extra buildup of impurities on the block. This is a particular advantage of our invention.
These and other objects, features and advantages of our invention will be more fully described with reference to the following example, with reference to the sole FIGURE of the drawing which shows the method or process according to our invention in schematic representation.
As shown in the FIGURE, a block W of tantalum is first soaked in an agitated bath 1 of 40 percent hydrofluoric acid for 24 to 48' hours. This thoroughly removes any film of foreign matter on the surface of the block W and simultaneously etches the surface.
On removal from this bath 1, it is washed at a washing station 2 with distilled water until all of the acid removed, as indicated by the attainment of a neutral condition in the was water.
At a drying station 4 the block W is placed in an induction furnace 3 which can withstand extremes of gas pressure. Here the furnace 3 is heated to 8001,400 C. and evacuated l0" Torr) to thoroughly dry the block W. This heating is preferably, to avoid contamination, carried out by induction with an uninsulated coil, the bar being directly coupled in terms known in the art.
Subsequently the block W is saturated with hydrogen in the furnace 3. The saturation is carried out in three stages, the gas pressure being maintained constant in all stages as hydrogen is absorbed by the block W. First it is heated at around 600 to l,O00 C., then only from 200 to 500 C. and then the heating is shut off. Due to the exothermicity of the reaction between the hydrogen and the tantalum, the temperature remains somewhat elevated to 60 C.) for several hours more while the block continues to absorb hydrogen. In fact it has been found that this temperature is maintained as long as hydrogen continues to be absorbed and. the conclusion of absorption (saturation) is signalled by a further temperature drop.
Block 6 represents the next step wherein the free hydrogen is withdrawn from the furnace 3 which is opened to the air (block 7). At this point the original block has spontaneously become a pile of metal-hydride fragments F.
Thence the fragments F are crushed in a pebble or ball mill 8 to a powder of the desired fineness. The powder is cleaned in an acid bath 9.
Finally the powder is degassed (transformed from the hydride to the elemental metal) in a chamber 10 under a high vacuum with a pressure of around 600-l ,200 C.). This produces a very pure tantalum powder which s hydrogen poor.
It is to be understood that the above method would be carried out in substantially the same manner for a block or bar of niobium.
Below a detailed EXAMPLE is given to more fully illustrate our method.
Five electron-beam smelted bars of tantalum (or niobium) each weighing 50 kg. are etched for 24 hours in a solution of 40 percent hydrofluoric acid.
Then they are repeatedly washed with distilled water until the water, on testing, is neutral.
After this the damp bars are set upright in the water-cooled floor of a high-vacuum induction furnace. This furnace has a water-cooled uninsulated heating coil and the bars are set in the furnace 3 like a star to insure even heating. The junction of a thermocouple is centered in the middle of the star of bars about level with the middle of the induction coil.
In this manner the room-temperature furnace is first evacuated to 10 Torr for at least one hour. Subsequently the bars are heated through direct inductive coupling to l,200 C. This temperature is maintained until the pressure remains constant at 10- Torr for 1 hour.
Afterwards the temperature is dropped to 800 C. and pure hydrogen is pumped in to a pressure of 400 Torr. Once the pressure sinks to 300 Torr due to absorption by the bars, it is again pumped up to 400 Torr and maintained there with constant renewal. Once the bars can no longer effectively absorb any hydrogen, which takes about one hour and requires about 64 liters of hydrogen, the temperature is dropped to 450 C. with the pressure kept at 400 Torr. After 6 hours and around 318 liters of hydrogen no more hydrogen can be absorbed, and the heating is turned off. On cooling to around 100 C. around 2,350 liters of hydrogen are absorbed over a period of 3 hours. The temperature remains at this level for quite a while since the reaction is exothermic, taking 8 hours to drop to 80 C., 3 more hours to reach 70 C. and 4 more to reach 60 C. Meanwhile 8,950 liters of hydrogen are consumed. In the subsequent 18 hours the temperature drops to 20 C. and 3,850 liters of additional hydrogen are consumed.
Finally the remaining hydrogen is pumped off and the furnace is opened to ambient pressure and atmosphere. The original compact tantalum bars are at this stage reduced to coarse tantalum-hydride powder of a grain size of 0.2-5 mm. forming a pile on the bottom of the furnace.
These fragments are comminuted in a ball mill of pure iron with iron balls to a powder of a grain size less than 150 p.. The impurities of iron hereby acquired are eliminated in a bath of boiling hydrochloric acid.
The resultant powder is then freed from hydrogen in a high vacuum to 10 Torr) at about 800 C. The final product is tantalum powder free from impurities.
The improvement described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art, all such modifications being considered within the spirit and scope of the invention except as limited by the appended claims.
1. A method of making powder from a solid body in the form of a bar of tantalum or niobium, the method comprising the steps of:
stripping the surface of said body to eliminate foreign matter thereon by treating said body for 24 to 48 hours with aqueous hydrofluoric acid, and thereafter washing said body until it is free from the acid;
saturating the stripped body with hydrogen to embrittle the body by heating said body at a temperature of 800 to l,400 C. in a high-vacuum chamber and at least for a period of 1 hour at a pressure of at most 10 Torr, and thereafter treating said body with hydrogen at an elevated temperature and pressure in at least three stages including a first stage in which the stripped body is heated at a temperature of substantially 600 to l,000 C., a second stage at a temperature of 200 to 500 C. and a third stage at a temperature of 60 to C., the hydrogen being introduced in at least one of these stages under a gas pressure of at least 300 torr;
fragmenting the embrittled body to form a coarse metalhydride powder therefrom and degassing the coarse powder under a high vacuum at a temperature of 600 to 1,200 C.
2. The method defined in claim 1 wherein said body is etched for 24 to 48 hours in aqueous hydrofluoric acid at a concentration of at least 40 percent.
3. The method defined in claim 2 wherein the hydrofluoric acid is removed by washing with water, further comprising the step of drying the washed body prior to saturating same with hydrogen.
4. The method defined in claim 1 wherein the body is heated inductively.
5. The method defined in claim 1 wherein said bar is stripped by treating it with agitation for a period of about 24 hours with a 40 percent solution of hydrofluoric acid, and
washing the bar thereafter with distilled water until the wash water assumes a neutral character; said is sub ected while its surface remains damp from the washing thereof to a vacuum of better than 10 Torr. in an induction furnace until the vacuum is maintained at that level for at least 1 hour; heating said bar inductively in said vacuum to a temperature of at least 1,200 C. until the pressure at this temperature remains constant for at least 1 hour at 10" Torr; thereafter reducing the temperature of said bar to a temperature in the range of 600 to l,00 C. and exposing said bar to hydrogen at a pressure of about 400 Torr; upon a decrease in the pressure in said furnace by about 25 percent by absorption of hydrogen interstitially by said bar, increasing the pressure of hydrogen in said furnace again to at least 400 Torr until the bar is saturated with hydrogen at said temperature in said range of 600 to l,000 C.; thereafter lowering the temperature of said bar to substantially 200 to 500 C. and saturating said bar at the latter temperature at 300 to 400 Torr pressure with hydrogen; thereafter lowering the temperature of the bar to 60 to 120 C. and saturating the bar at this latter temperature at 300 to 400 Torr pressure with hydrogen; evacuating hydrogen from said furnace and recovering said coarse metal-hydride powder therefrom; comminuting the coarse metal-hydride powder to a maximum particle size of about microns; and subjecting the comminuted product to high vacuum at a temperature of about 600 to 1,200 C.