Process For Producing High-purity Niobium And Tantalum

Abstract

A process for producing high-purity niobium and tantalum wherein the oxide of the metal is intimately mixed with carbon, e.g., fine graphite, in an amount such that the oxygen is present in a slight excess beyond the quantity of carbon required to react stoichiometrically with the metal oxide. In the first stage, the mixture is subjected to a high vacuum at the order of 10.sup..sup.-4 torr. at a temperature of about 1,800.degree. C. to carry out an initial reduction, the reduced material containing about 500 to 10,000 parts per million (p.p.m.) of oxygen. In an intermediate stage, the partly reduced product is combined with finely divided carbon pyrolytically precipitated from a hydrocarbon in a retort permeable to hydrogen (at elevated temperature), so that the carbon is uniformly distributed over the surface of the partly reduced product. In the second state, the mixture of the partially reduced product and the finely divided pyrolytically precipitated carbon is subjected to a temperature below to 2,000.degree. C. and nevertheless sufficient to effect a final reduction. Preferably the latter temperature is about 1,700.degree. C. The resulting high-purity metal (i.e., tantalum or niobium), may be used in electrolytic capacitors.

Description

1. FIELD OF THE INVENTION

My present invention relates to a process for the production of high-purity metallic tantalum and niobium and, more particularly, to a process for producing tantalum or niobium metal low in oxygen and carbon and particularly suitable for use in electrolytic capacitors.

2. BACKGROUND OF THE INVENTION

It has been proposed heretofore to produce metallic tantalum and/or niobium by reduction of the oxides of these metals with the corresponding carbide or with elemental carbon (e.g., in the form of graphite) in high-vacuum furnaces at elevated temperatures.

Such processes have, however, the disadvantage that, when carbides are required, these carbides must be produced as intermediate products at additional costs. Whether or not the carbide is used, it has been found to be difficult using these earlier techniques to obtain an end product both low in oxygen and low in carbon and capable of being used successfully as electrolytic-condenser plates.

One of the problems arising in the prior art systems is that the intermediate product of the high-temperature reaction is a metal/oxygen/carbon system which is created during the initial reduction stage. When the precise quantity of carbon necessary to react stoichiometrically with all of the oxygen is used, it is found to be impossible, in practice, to maintain such precise stoichiometry throughout the entire charge.

This difficulty arises from the fact that the mobility of oxygen in the oxide/oxygen/carbon system is relatively high whereas that of carbon is relatively low. Limited, although hardly avoidable, temperature differentials within the charge result in a concentration of oxygen at certain localities therewithin while carbon-rich locations are found elsewhere. In fact, these localized carbon concentrations can not adequately be brought into intimate relationship with oxygen-rich areas even during prolonged heating after mixture; also any interaction between carbon and oxygen ceases while the charge contains proportionately large quantities of both. The residual oxygen appears to be present, to a certain extent at least, in the form of suboxides which can be volatilized by an increase in temperature at the end of the reduction stage.

The volatilization step has the additional disadvantage that the vaporized metal suboxides can not be recovered and result in a loss of the starting material. Moreover, there is a tendency, at the elevated temperatures which must be used to vaporize the suboxides, for the metal suboxides to react with the material forming the reaction vessel or crucible and result in a sloughing of crucible material into the reacting mass. The latter difficulty introduces a further impurity, which may render the recovered metal unsuitable for use in electrolytic capacitors.

Still further, since the temperature required for volatilizing the superfluous oxygen in the form of suboxides of the metal generally range above 2,000.degree. C., there occurs a sintering or fusing of the reactant material to itself and to the reaction vessel; this makes more difficult the removal of the charge from the reaction vessel. In addition the fused or sintered mass must be broken up or comminuted, thereby giving rise to a comminution step and a formation of fresh surfaces subject to atmospheric oxidation. Such oxidation detrimentally influences the ability to use the metal as sintered anodes in electrolytic capacitors of the type mentioned earlier.

3. OBJECTS OF THE INVENTION

It is, therefore, the principal object of the present invention to provide an improved process for the production of high-purity tantalum and niobium which is particularly suited for use as a sintered plate in an electrolytic capacitor and which avoids the disadvantages of the prior art processes mentioned earlier.

Another object of this invention is to provide a process for producing high-purity tantalum and niobium which yields a product low in oxygen and carbon and which does not require mechanical comminution with the disadvantages thus entailed.

Yet another object of this invention is to provide a process for the production of high-purity tantalum and niobium which can be operated at temperatures below 2,000.degree. C., thereby avoiding sintering of the mass, consequent difficulty of removing the mass from the reaction vessel, and the necessity of comminuting the mass.

It is still further an object of the instant invention to provide a process for the production of tantalum and niobium, from the oxides thereof, which reduces loss of the metal in the form of its suboxides, precludes contamination of the metal with material derived from the reaction vessel, and results in a product with a lower oxygen content than has been attainable heretofore.

Yet a further object of the instant invention is to provide a process for the production of high-purity niobium and tantalum which extends principles set forth in the commonly assigned copending applications Ser. No. 609,001 filed 13 Jan. 1967 (now U.S. Pat. No. 3,499,753) and Ser. No. 718,929 (now abandoned) and filed 4 Apr. 1968 by myself and Gustav Daedliker.

4. SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, in a two-stage process for reducing niobium and tantalum oxides, wherein, in the first stage, the oxide of the metal to be recovered in a low-oxygen, low-carbon condition suitable for use in electrolytic capacitors, is intimately mixed with carbon, e.g., in the form of finely divided graphite, in an amount just less than the quantity required to stoichiometrically react all of the oxygen of the material to form carbon monoxide; the first-stage reduction process is carried out in a vacuum furnace under a negative pressure of the order of 10.sup..sup.-4 torr and at a temperature below 2,000.degree. C. to yield a product containing about 500 to 10,000 parts per million (p.p.m.) of oxygen. An oxygen excess of at most 1 percent thus remains in the first-stage reduction produce. In a second step of the instant process, intermediate to two reduction stages, I deposit upon the surfaces of the reduced product of the first stage a finely divided elemental carbon obtained from the pyrolytic decomposition of a hydrocarbon, especially a paraffinic alkane having a carbon number ranging between one and eight.

In this intermediate step, the precipitated pyrolytic carbon is intimately mixed with the reduced product of the initial stage, whereupon the mixture of finely divided carbon and partially reduced oxide, wherein the carbon content now is stoichiometrically equal to the quantity necessary to react all of the oxygen remaining, is reacted at a temperature below to 2000.degree. C. in the second reduction stage and at reduced pressure to yield a final product which may be used in electrolytic capacitors as will be apparent hereinafter.

By pyrolytic precipitation of carbon in finely divided form uniformly over the surfaces of the prereduced or partially reduced product of the first stage and following this precipitation by an intimate mixture of the partially reduced oxide and precipitated carbon, I an able to completely eliminate the tendency toward the formation of segregation zones containing carbon-rich or oxygen-rich materials which are incapable of interacting. Moreover the volatilization stage can be completely eliminated inasmuch as no substantial proportion of suboxide remains.

According to an important feature of this invention, the precipitation of the finely divided carbon film is effected by pyrolysis at or above the pyrolyzing temperature of the gaseous hydrocarbon and especially a hydrocarbon of the paraffin series on a heated metal surface according to the formula:

C.sub.n H.sub. (2n.sub.+2) nC (N+1) H wherein n is an integer ranging from one to eight.

The reaction vessel, according to the present invention, is a material which, at elevated temperatures (e.g., 700.degree. to 1,000.degree. C.), is permeable to hydrogen, for example, a nickel-chromium-iron alloy. By evacuating the furnace containing the sealed retort, I initially am able to effect a hydrogen diffusion from the interior of the vessel and thereby control the amount of precipitated carbon which appears to from in reproducible ratios upon the wall of the vessel and upon the surfaces of the charge. The evacuated hydrogen is burned off. Best results have been found with pyrolysis temperatures ranging between 700.degree. and 1,000.degree. C.

Surprisingly, the intermediate stage wherein a finely divided carbon film is precipitated on the surface of the partially reduced metal, permits the second reaction stage to be carried out at temperatures well below 2,000.degree. C. preferably at about 1,700.degree. C. when extremely low pressures (of the order of 10.sup..sup.-4 torr.) are employed.

A further advantage of the present process is that the product is a high-purity highly porous metal which may easily be converted to low-oxygen powders and incorporated in the plates of an electrolytic capacitor. Such material is, thanks to its purity and grain structure, most suitable for high quality sinter anodes of niobium and tantalum in high-capacity electrolytic condensers. Moreover, in the course of the process, the concentrations of nonrefractory impurity metals are reduced to less than five p.p.m.

5. SPECIFIC EXAMPLES

The following examples are illustrative of the present process.

EXAMPLE I

Thirty Kilograms (kg.) of tantalum pentoxide powder is intimately mixed with a fine annealed graphite .gtoreq.99.6 percent purity in an amount of 4,040 grams (g.) and pressed into tablets weighing two grams (g.) each. The tablets are uniformly heated in a high-vacuum furnace to a temperature of 1,800.degree. C. to react the graphite with the oxide and form carbonmonoxide. The reaction temperature is maintained as long as carbonmonoxide is evolved and the vacuum brought to a subatmospheric or negative pressure value of about 1 to 5.sup.. 10.sup..sup.-4 torr.

The resulting partially reduced tantalum, constituting the first-stage product, has an average oxygen content of 1,720 p.p.m. and a carbon content of 65 p.p.m.

Of this first-stage reduction product, 19.2 kg. is charged into a retort composed of the nickel alloy known as Inconel 600. (A suitable Inconel alloy may consist of 77.+-.0.5 percent by weight nickel, 14.+-.1 percent by weight chromium, 0.2.+-.0.05 percent by weight copper, 6.5.+-.1 percent by weight iron, 0.5.+-. 0.3 percent by weight manganese, 1.+-.0.75 percent by weight silicon, 0.08 to 0.2 percent by weight carbon and is permeable by hydrogen at elevated temperatures.)

The retort is placed bodily in the high-vacuum furnace and heated to a temperature of 900.degree. C. Whereupon 38,000 torr .times. liter (corrected to 0.degree. C.) of methane is added in portions. This corresponds approximately to 26.7 g. of carbon. The amount of carbon taken up is determined by the cumulative pressure difference. As the methane contacts the interior of the retort and the prereduced metal therein, it pyrolyzes and deposits a finely divided carbon film over these surfaces. The methane pyrolysis results in a gradual reduction of the pressure from the starting level. During pyrolysis, hydrogen is released and diffuses through the wall of the retort, the retort being hermetically sealed except for diffusion through its walls.

The retort is then discharged and the mixture of pyrolytically discharged carbon thoroughly mixed with the tantalum pellets. The average carbon content is determined as 690 p.p.m. and 12.0 g. of carbon are calculated as taken up by the metal.

The resulting mixture is then subjected to second-stage reduction according to the present invention as described in connection with the first reduction stage under vacuum at a temperature of 1,850.degree. C. at which the mass held until the pressure within the high-vacuum furnace is reduced to 2.times.10.sup..sup.-4 torr. The resulting tantalum has an oxygen content of 530 p.p.m. and a carbon content of 70 p.p.m.

The tantalum is particularly suited for use in electrolytic capacitors and can be formed into plates as described in U.S. Pat. No. 3,430,108 or the above-identified application Ser. No. 718,929.

EXAMPLE II

As described in Example I, 30 kg. of tantalum pentoxide is intimately mixed with 4,040 g. of graphite powder, pressed into tablets and sintered in vacuo. The resulting first-stage reduction product has an oxygen content of 2,020 p.p.m. and a carbon content of 40 p.p.m.

The intermediate carbon correction is carried out with 23 kg. of butane whereby 11,700 torr .times. liters at 800.degree. C. is reacted to pyrolytically precipitate the carbon film.

The carbon-coated tablets are thoroughly mixed and analyzed and the average carbon content is found to be about 900 p.p.m.

The second-stage reduction is carried out by sintering in vacuum (see Example I) at a temperature of 1,850.degree. C. to obtain a high-purity tantalum product with only 130 p.p.m. of oxygen.

EXAMPLE III

Eighteen g. of tantalum tablets recovered from a first-stage reaction of tantalum pentoxide and graphite powder as described in the previous Examples, and having an oxygen content of 4,420 p.p.m. and a carbon content of 20 p.p.m. is treated in a retort of Inconel 600 at 900.degree. C. with butane in an amount of 25,000 torr .times. liters. The average carbon content of the thoroughly mixed mass, after deposition of the finely divided pyrolytic carbon, is found to be about 2,650 p.p.m. The mass is sintered in vacuum at a temperature of 1,850.degree. C. as in Example I to yield tantalum containing 250 p.p.m. oxygen and 80-90 p.p.m. carbon. The tantalum is hydrogenated and milled to a fine powder (see applications Ser. No. 609,001 and No. 718,929) by conventional techniques and is thereafter subjected to dehydrogenation to obtain a metal powder with an average particle size of seven microns. This powder containing 1,650 p.p.m. oxygen, 50 p.p.m. nitrogen and 80 p.p.m. carbon is characterized by a low content of metallic impurities. The total amount of nickel, chromium, manganese, magnesium, aluminum, silicon, calcium, copper, titanium, zirconium and iron is less than five p.p.m.

This powder is formed into sintered anodes for condensers as described generally in the aforementioned application and U.S. patent. More particularly, 1.98 g. of the powder is pressed into an anode, with a diameter of 6.7 mm. and a specific gravity of 8.4 g./cm..sup.3 and sintered for 30 minutes at 1,950.degree. C. in high vacuum. The resulting anode had an electrical capacity of about 6,240.mu. FV. The breakdown voltage of the electrode in 0.1 percent H.sub.3 PO.sub.4 is determined to be in excess of 250 volts. The tantalum powders of Example I and II yield similar results.

Claims

What is claimed is:

1. A process for producing high-purity niobium and tantalum from a corresponding metal oxide, comprising the steps of:

a. reducing the metal oxide by intimately mixing it with elemental carbon and subjecting the resulting mixture to an elevated temperature in vacuo to produce a prereduced product with an oxygen content of about 500 to 10,000 p.p.m., the elemental carbon mixed with the metal oxide being present in an amount less than the quantity required to stoichiometrically react all of the oxygen of the oxide to form carbon monoxide and said oxygen is present in an excess of at most one percent beyond that stoichiometrically calculated to react with the elemental carbon;

b. heating the prereduced produce and contacting same with a gaseous hydrocarbon to pyrolytically precipitate elemental carbon on the prereduced product to form an intimate combination of the pyrolytically precipitated carbon and the prereduced product; and

c. subjecting the combination of pyrolytically precipitated carbon and the prereduced product to a second reducing stage at a temperature below about 2,000.degree. C. but sufficient to react the pyrolytically precipitated carbon with oxygen retained in the prereduced product.

2. The process defined in claim 1 wherein said prereduced product is reacted with said hydrocarbon at a temperature between 700.degree. and 1,000.degree. C.

3. The process defined in claim 2 wherein said prereduced product is reacted with said hydrocarbon in a sealed reaction vessel having a hydrogen-permeable wall at a temperature of 700.degree. to 1,000.degree. C.

4. The process defined in claim 3 wherein said reaction vessel is composed of nickel-chromium-iron alloy.

5. The process defined in claim 3 wherein said hydrocarbon is a paraffinic alkane of the general formula C.sub.n H.sub.(2n.sub.-2) where n is an integer ranging between 1 and 8.

6. The process defined in claim 5 wherein steps (a) and (c) are each carried out at a temperature below 2,000.degree. C.

7. The process defined in claim 6 wherein steps (a) and (c) are each carried out at a pressure of the order of 10.sup..sup.-4 torr.

8. The process defined in claim 7 wherein step (c) is carried out at a temperature of about 1,700.degree. C.