US3647420A - Process for producing high-purity niobium and tantalum - Google Patents

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 4 torr. at a temperature of about 1,800* 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* C. and nevertheless sufficient to effect a final reduction. Preferably the latter temperature is about 1,700* C. The resulting high-purity metal (i.e., tantalum or niobium), may be used in electrolytic capacitors.

Description

United States Patent Restelli Mar. 7, 1972 [54] PROCESS FOR PRODUCING HIGH- PURITY NIOBIUM AND TANTALUM [72] Inventor: Attilio Restelli, Binningen, Switzerland [73] Assignee: Hermann C. Starck Berlin, Berlin, Germany [22] Filed: June 4,1969

[21] Appl.No.: 830,542

[30] Foreign Application Priority Data June 6, 1968 Switzerland ..8380/68 [52] US. Cl ..75/84 [51] Int. Cl. C22b 51/00 [58] Field of Search ..75/84, 0.5; 176/91 SP, 67; 252/301.1

[56] References Cited UNITED STATES PATENTS 3,114,629 12/1963 Downing et a1 ..75/84 3,144,328 8/1964 Doty ..75/84 3,231,408 1/1966 Huddle .....l76/89 Re26,294 11/1967 Sowman et a1 ..176/67 Daendliker et al ..75/5 BB 3,499,753 3/1970 Daendliker ..75/5 BB Primary ExaminerReuben Epstein Attorney-Karl F. Ross [57} 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. 1n the first stage, the mixture is subjected to a high vacuum at the order of 10" torr. at a temperature of about 1 ,800 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 C. and nevertheless sufficient to effect a final reduction. Preferably the latter temperature is about l,700 C. The resulting high-purity metal (i.e., tantalum or niobium), may be used in electrolytic capacitors.

8 Claims, No Drawings PROCESS FOR PRODUCING I-IIGII-PURITY NIOBIUM AND TANTALUM l. 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 oxygenrich 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 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 highpurity 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 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. 609,00l filed 13 Jan. 1967 (now US. 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 torr and at a temperature below 2,000 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 firststage 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 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, 1 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:

t: H hC (N-l-l 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 leg. 700 to l.000 t1). is permeable to hydrogen. for example. a nickel-chromi- .llll-llOn 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 and l.000 C.

lurprisingly. the intermediate stage wherein a finely divided .iarbon film is precipitated on the surface of the partially reduced metal. permits the second reaction stage to be carried out at temperatures well below l.000 C. preferably at about llS purity and grain structure. most suitable for high quality i imter anodes of niobium and tantalum in high-capacity elecirolytic condensers. Moreover. in the course of the process. the concentrations of nonrefractory impurity metals are reduced to less than five p.p.m.

l. SPECIFIC EXAMPLES The following examples are illustrative of the present process.

EXAMPLE] Thirty Kilograms (kg) of tantalum pentoxide powder intimately mixed with a fine annealed graphite a 99.6 percent purity in an amount of 4.040 grams lg.) and pressed into tablets weighing two grams ig.) each. The tablets are uniformly heated in a highvacuum furnace to a temperature of l.800 C. to react the graphite with the oxide and form carlhonmonoxide. The reaction temperature is maintained as long .is carbonmonoxide is evolved and the vacuum brought to a iubatmospheric or negative pressure value of about 1 to 5-l0 orr.

The resulting partially reduced tantalum. constituting the first-stage product. has an average oxygen content of 1.720 ppm. and a carbon content of 65 p.p.m.

Of this first-stage reduction product. i .2 kg. is charged .into a retort composed of the nickel alloy known as lnconel "r00. {A suitable lnconel alloy may consist of 77-105 percent iy weight nickel. l4il percent by weight chromium. 0.21-0.05 percent by weight copper. 0.5:1 percent by weight iron. 0.5103 percent by weight manganese. 10.75 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 C. whereupon 38.000 torr X liter (corrected to 0 C.) of methane is added in portions. This .sorresponds approximately to 26.7 g. of carbon. The amount if carbon taken up is determined by the cumulative pressure tlifference. As the methane contacts the interior of the retort and the prereduced metal therein. it pyrolyzes and deposits :1 iinely divided carbon film over these surfaces. The methane pyrolysis results in a gradual reduction of the pressure from the starting level. During pyrolysis. hydrogen lS released and iiiffuses 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.

he resulting mixture is then subjected to second-stage "eduction according to the present invention as described in connection with the first reduction stage under vacuum at a temperature of l.850 C. at which the mass held until the pressure within the high-vacuum furnace is reduced to 2Xl0 torr. The resulting tantalum has an oxygen content of 530 p.p.m. and a carbon content of p.p.m.

The tantalum is particularly suited for use in electrolytic capacitors and can be formed into plates as described in US. Pat. No. 3.430.108 or the above-identified application Ser. No. 718.929.

EXAMPLE ll As described in Example l. 30 kg. of tantalum pentoxide is ntimately mixed with 4.040 g. of graphite powder. pressed .nto tablets and sintered in vacuo. The resulting first-stage "ECIUCUOH product has an oxygen content of 2.020 p.p.m. and .1 carbon content of 40 p.p.m.

The intermediate carbon correction is carried out with 23 xg. of butane whereby l [.700 torr X liters at 800C. is reacted o pyrolytically precipitate the carbon film.

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

The second-stage reduction is carried out by sintering in iacuum (see Example I) at a temperature of l.850 C. to obam a nigh-purity tantalum product with only [30 p.p.m. of ixvgen.

EXAMPLE Ill Eighteen g. of tantalum tablets recovered from a first-stage fiaCIlOl'l of tantalum pentoxide and graphite powder as .iescribed 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 :reated in a retort of lnconel 600 at 900 C. with butane in an mount of 25.000 torr X liters. The average carbon content of the thoroughly mixed mass. after deposition of the finely dirided pyrolytic carbon. is found to be about 2.650 p.p.m. The mass is sintered in vacuum at a temperature of l.850 C. as in Example l to yield tantalum containing 250 p.p.m. oxygen and tlO-QO p.p.m. carbon. The tantalum is hydrogenated and TllllCd to a fine powder (see applications Ser. No. 609.00l and No. 718.929) by conventional techniques and is thereafter iubiected to dehydrogenation to obtain a metal powder with in average particle size of seven microns. This powder conraining 1.650 p.p.m. oxygen. 50 p.p.m. nitrogen and p.p.m. carbon is characterized by a low content of metallic impurities. The total amount of nickel. chromium. manganese. magriesium. 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 iDBClfiC gravity of 8.4 g/cm: and sintered for 30 minutes at 950 C. in high vacuum. The resulting anode had an electrical capacity of about 6.240;; FV. The breakdown voltage of the electrode in 0.1 percent H PO is determined to be in excess of 250 volts. The tantalum powders of Example I and ll yield similar results.

What is claimed is:

A process for producing high-purity niobium and tantalum from a corresponding metal oxide. comprising the steps .l. reducing the metal oxide by intimately mixing it with eleiiiental carbon and subjecting the resulting mixture to an cievated temperature in vacuo to produce a prereduced iroduct with an oxygen content of about 500 to 10.000 p.p.m.. the elemental carbon mixed with the metal oxide vieing present in an amount less than the quantity "BQUII'Cd to stoichiometrically react all of the oxygen of he oxide to form carbon monoxide and said oxygen is iresent 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" 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 and 1,000 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 to l,000 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,,H where n is an integer ranging between l and 8.

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

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

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

Claims (6)

1. 2. The process defined in claim 1 wherein said prereduced product is reacted with said hydrocarbon at a temperature between 700* and 1,000* C.
2. 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* to 1,000* C.
3. 4. The process defined in claim 3 wherein said reaction vessel is composed of nickel-chromium-iron alloy.
4. 5. The process defined in claim 3 wherein said hydrocarbon is a paraffinic alkane of the general formula CnH(2n 2) where n is an integer ranging between 1 and 8.
5. 6. The process defined in claim 5 wherein steps (a) and (c) are each carried out at a temperature below 2,000* 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 4 torr.
6. 8. The process defined in claim 7 wherein step (c) is carried out at a temperature of about 1,700* C.