Specialized metallurgical processes – compositions for use therei – Processes – Producing or purifying free metal powder or producing or...
Reexamination Certificate
2002-09-30
2004-09-07
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or purifying free metal powder or producing or...
C075S369000
Reexamination Certificate
active
06786951
ABSTRACT:
TECHNICAL FIELD OF INVENTION
The present invention relates to a process for the production of high surface area tantalum and/or niobium powders.
BACKGROUND OF THE INVENTION
An important application of tantalum and niobium powders is to be used in manufacturing the electrolytic capacitors. The manufacture of tantalum or niobium solid electrolytic capacitor is typically comprised of: compressing the tantalum or niobium powder to form a pellet with an embedded tantalum or niobium lead wire, sintering the pellet to a porous body, subsequently forming a continuous dielectric oxide thin film on the surface of the porous pellet by anodizing, coating the oxide film with cathode material, and the final enveloping operation. The capacitance of the capacitor is depended on the surface area of tantalum and/or niobium powders. The higher surface area of the powder used, the higher capacitance of the capacitors may be obtained. Leakage current of the capacitor is also an important parameter in evaluating the quantity of the capacitor. As the impurities degrade the dielectric properties of the oxide film, low leakage current capacitors can be obtained by using the tantalum and/or niobium powders with high purity.
There are two kinds of methods for the production of tantalum and niobium powders in the prior art. One method is electronic beam method, in which the electronic beam melted tantalum or niobium ingot was hydrogenated and then pulverized, and the obtained powder has high purity, but with low surface area, therefore result in low capacitance of the capacitors made with these powders. The other method is chemical reduction method, in which the compound containing tantalum or niobium is reduced by reducing agent, and the obtained powder is leached with acids and water.
The typical process for producing tantalum powder is reducing potassium fluorotantalate (K
2
TaF
7
) with sodium as published in U.S. Pat. No. 3,012,877. As summarized in WO 91/18121, in order to obtain high surface area tantalum powder via chemical reduction method, a certain amount of diluent, such as alkali metal halides selected from NaCl, KCl, KF and NaF was added in the raw materials to be reduced. However, if higher surface area of the powder is required, more diluent should be used in said method for producing tantalum powder. Unfortunately, when more diluent is used, tantalum powder will be contaminated with more impurities and the yield will also be decreased. Moreover, as the surface area of the powder reaches to a certain extent, it can hardly be increased even if more diluent is used in the reduction reaction. As a result, tantalum powder obtained by reducing K
2
TaF
7
with sodium in industry usually has a surface area of between 0.2~2.0 m
2
/g, it is almost impossible to produce higher surface area tantalum powder by the chemical reduction method.
U.S. Pat. No. 6,136,062 disclosed a method of producing niobium and/or tantalum powders by reducing corresponding niobium and/or tantalum oxides with magnesium metal, wherein the first reduction stage is carried out as far as an average composition corresponding to (Nb, Ta) O
x
(x=0.5~1.5), and before the second reduction stage, the reduction product from die first stage is freed from excess reducing metals and alkaline earth metal oxides formed in the reduction by washing with mineral acids. Although this process can produce high surface area niobium and/or tantalum powders, the disadvantages are that the amount of reducing agent used is too much, that the amount of acid needed is too much. Moreover, this process includes two stages of reduction, and the degree of reduction of the first stage must be critically controlled. Therefore, the process is complicated and low efficiency.
SUMMARY OF THE INVENTION
In order to solve the problem described above, the present inventors developed an economic process of producing high surface area tantalum and/or niobium powders, in which reduction of tantalum and/or niobium oxides are carried out with an alkali metal and at least one halide selected from the group consisting of halides of Mg, Ca, Sr, Ba and Ce.
The object of the invention is to provide an economic process for the production of high surface area tantalum and/or niobium powders via the reduction of corresponding tantalum and/or niobium oxides, wherein the reduction is carried out by reacting the tantalum and/or niobium oxides with at least one metal halide and an alkali metal at elevated temperature so as to form the tantalum and/or niobium powders, said metal halide is selected from the group consisting of halides of Mg, Ca, Sr, Ba and Ce.
According to a preferred embodiment of the present invention, at least one alkali metal halide is further used in the reduction as a diluent, said alkali metal halide may be selected from sodium chloride, potassium chloride, lithium chloride, potassium fluoride, sodium fluoride.
According to an embodiment of the present invention, said process comprises charging said at least one metal halide selected from the group consisting of halides of Mg, Ca, Sr, Ba and Ce, the alkali metal, the tantalum and/or niobium oxides, and the optional at least one alkali metal halide in a reactor, heating the reactor to elevated temperature so that the tantalum and/or niobium oxides are reduced to tantalum and/or niobium powders.
According to another embodiment of the present invention, said process comprises charging said at least one metal halide selected from the group consisting of halides of Mg, Ca, Sr, Ba and Ce, and the optional alkali metal halide in a reactor, heating the reactor to elevated temperature to form a molten bath, and then metering required amount of tantalum and/or niobium oxides and alkali metal to the molten bath while controlling the temperature of the reactor so that the tantalum and/or niobium oxides are reduced to tantalum and/or niobium powders.
According to the process of the present invention, the reduction is usually carried out at a temperature in the range of 400-1200° C., preferably in the range of 600-1000° C. for about 20-300 minutes so that the reduction can be carried out completely.
According to the present invention, the said alkali metals are used as reducing agent, said alkali metals are preferably selected from sodium, potassium and lithium, sodium and/or potassium are particularly preferred. The amount of alkali metals used is 1.0 to 1.3 times of the stoichiometric amount for reducing the tantalum and/or niobium oxides. According to the present invention, the halide selected from halides of Mg, Ca, Sr, Ba and Ce is used as both a diluent and as an indirect reducing agent, wherein halides of Mg and Ca are preferred. The mole amount of said metal halides used is 0.5 to 8.0 times of the mole amount of the alkali metal used.
The tantalum and/or niobium oxides used for the present invention may be any tantalum and/or niobium oxides or their mixture which is capable of being reduced to tantalum and/or niobium metal, for example, Ta
2
O
X
(x≦5), Nb
2
O
X
(x≦5). They are generally available as Ta
2
O
5
and Nb
2
O
5
.
In order to obtain high surface area tantalum and/or niobium powders as well as the sintered anodes formed from them, according to the present invention, a dopant containing N, P, S, B or Si can be further added to the above raw materials used in the reduction reaction, and/or added during the reduction reaction, and/or added after the reduction reaction.
According to the present invention, the reduction is usually carried out in a closed reactor made from refractory alloy. In order to make the reactant dispersing homogeneously in the molten salts and to prevent from local overheating, the reactor is preferably equipped with a stirrer. In addition, the reactor is preferably equipped with a heating device and a cooling device so as to control the temperature of the reactor. The said reactor as well as the stirrer, the heating device and cooling device can be any equipment that is well known to the skill in the art.
According to the process of the present invention, the reduction
He Jilin
Li Xingbang
Pan Luntao
Wen Xiaoli
Yuan Ningfeng
Fish & Neave
Gunnison Jane T.
Ningxia Orient Tantalum Industry Co., Ltd.
Wong Connie
Wyszomierski George
LandOfFree
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