U.S. patent number 3,647,420 [Application Number 04/830,542] was granted by the patent office on 1972-03-07 for process for producing high-purity niobium and tantalum.
This patent grant is currently assigned to Hermann C. Starck Berlin. Invention is credited to Attilio Restelli.
United States Patent |
3,647,420 |
Restelli |
March 7, 1972 |
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.
Inventors: |
Restelli; Attilio (Binningen,
CH) |
Assignee: |
Hermann C. Starck Berlin
(Berlin, DT)
|
Family
ID: |
4339418 |
Appl.
No.: |
04/830,542 |
Filed: |
June 4, 1969 |
Foreign Application Priority Data
Current U.S.
Class: |
75/622 |
Current CPC
Class: |
C22B
34/24 (20130101); B22F 9/20 (20130101) |
Current International
Class: |
C22B
34/00 (20060101); B22F 9/16 (20060101); B22F
9/20 (20060101); C22B 34/24 (20060101); C22b
051/00 () |
Field of
Search: |
;75/84,.5 ;176/91SP,67
;252/301.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Epstein; Reuben
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.
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.
* * * * *