JP2688452B2 - Method for producing tantalum powder with high surface area and low metal impurities - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high surface area, high purity tantalum powder.

Background Art Among various applications, tantalum powders are commonly used in the manufacture of tantalum capacitors. Solid tantalum capacitors are typically pressed tantalum powder into pellets and sintered in a furnace to form a porous body,
Then, this porous body is anodized in a suitable electrolytic solution to form a continuous dielectric oxide film on the inner and outer surfaces of the sintered porous body. The porosity at the anode thus formed is then filled with the catholyte solution. Then, the whole anode body of the clogged pores is sealed to form a capacitor.

Generally, the performance of a tantalum capacitor is judged by the capacitance, voltage capacity and leakage current of the capacitor. These properties are determined by the manufacturing process of tantalum powder and capacitors. The surface area of tantalum powder is important for producing high quality capacitors. High surface area tantalum powders are used to make high surface area anodes. The anode capacity at a given voltage is
It is directly related to the anode surface area. Therefore, high surface area anodes are desirable.

The purity of tantalum powder is also important for producing high quality capacitors. The metallic and non-metallic impurities of tantalum powder cause deterioration of the quality of the dielectric oxide film formed in the manufacturing process of the capacitor, and cause many leakage of the capacitor current. The use of less-impurity tantalum powder suppresses the degradation of the dielectric oxide film and results in less current leakage. Therefore, high purity tantalum powder is desirable.

Sintering at high temperatures in the manufacture of capacitors tends to reduce the leakage of current due to impurities. However,
This method will reduce the net surface area of the anode and hence the capacitance of the capacitor.

Tantalum powders are generally produced by one or two methods, mechanical or chemical. The mechanical method is to melt the tantalum powder with an electron beam into an ingot, hydrogenate the ingot, crush the hydride, dehydrogenate, wash with acid and heat treat.

Generally, the high-purity powder obtained by this method is used for capacitors that require high voltage or high reliability. However, mechanical methods are associated with high production costs. In addition, tantalum powders obtained by mechanical methods generally have a small surface area.

Another commonly used method of making tantalum powder is the chemical method. This chemical method involves chemically reducing a tantalum compound with an active metal, commonly referred to as a reducing reagent, and then subjecting the tantalum powder to an acid and heat treatment. Without intending to be limiting, a representative tantalum compound is potassium fluorotantalum (K ₂ TaF ₇ ),
Includes sodium fluorotantalum (Na ₂ TaF ₇ ), tantalum chloride (TaCl ₃ ), and mixtures thereof. In general,
The reducing reagent is one that can treat a tantalum compound to reduce it to tantalum metal including sodium, potassium or a mixture thereof. The powder is sometimes further ground to increase surface area or porosity. Tantalum powders obtained by chemical methods generally have a higher surface area than powders obtained by mechanical methods. However, tantalum powders obtained by chemical methods are generally more impure than powders obtained by mechanical methods.

More specifically, various methods have been adopted for producing tantalum powder by a chemical method. For example, U.S. Patent No.
It is No. 4,067,736. U.S. Pat.
Typical methods are described in the background section of No. 9.

Fluorotantalum potassium (K ₂ TaF ₇ ), tantalum salt,
Is reduced to electrolytic tantalum in a molten bath with a dilute salt such as sodium chloride. Production is regulated by electrolysis parameters of current and voltage. The yield is relatively low because the resulting concentration gradient blocks yield. The resulting tantalum powder is coarse and tends to be resinous, and will produce an anode for an electrolytic capacitor having a very low charge capacity.
Due to the chemical corrosion activity of the reaction vessel components, a significant amount of impurities are transferred to the product.

Tantalum powder can also be mixed with K ₂ TaF ₇ with a reducing reagent,
It is produced by causing an exothermic reaction in an inert atmosphere. See U.S. Pat. No. 4,231,790. The charge is heated indirectly in a closed container until the exothermic reaction occurs spontaneously. Failure to regulate this reaction produces powders with a wide range of particle sizes. This powder has a large surface area per unit weight compared to the electrolyte powder, but is classified as unsuitable for the production of anodes, especially for electrolytic capacitors.

Usually, tantalum powder is industrially produced by adding sodium to K ₂ TaF ₇ previously dissolved in a molten salt or a molten diluent. In this reaction, K ₂ TaF ₇ and diluted salt are heated in a reaction vessel to a temperature above the melting point of the salt mixture. Liquid sodium is then added. The bath is kept substantially isothermal while stirring with an internal stirrer. In order to satisfy the production of the anode of the electrolytic capacitor, it is necessary to obtain a desired particle size distribution. The charge capacity at anodes made from such powders is typically in the intermediate range, 7,000 cv
Greater than / g / g range, and generally 15000cv / g
There is no higher price. Applications of such stirred liquid phase reactions include introducing diluted salts into the stirred reaction bath. Addition of known dilute salts such as NaCl to K ₂ TaF ₇ lowers bath temperature. However, such applications lead to very fine powder agglomerates, absorption of impurities and production of overly fines.

Another method is to combine the solid diluted salt and K ₂ TaF ₇ with liquid sodium and heat the mixture until the spontaneous exothermic reaction begins. The exothermic reaction that occurs is not easily tunable and therefore has powder characteristics with a wide range of particle size distributions and various electrical properties. This type of powder requires the removal of fine and coarse particles before manufacturing the anode of an electrolytic capacitor.

As already mentioned above, the volume of tantalum pellets is a direct function of the surface area of the sintered powder. Of course,
By increasing the grams of powder per pellet,
More surface area is possible. However, from an economic perspective, developmental focus should be on means to increase the surface area of the powder used per gram. Reducing the particle size of the tantalum powder results in a product with an increased surface area per unit weight, so reducing the size avoids other adverse properties.
Methods must be developed to make tantalum particles smaller.

Various tantalum powder processing methods have been employed to maximize the production of selected small desired particle size and thus increased surface area powders. For example, U.S. Patent No. 4,149,876 a method for adjusting the particle size of tantalum powder product is disclosed by reduction method, liquid sodium herein is added to the molten bath of K ₂ TaF ₇ and diluted salts. Sodium metal is added at a high rate up to the reduction temperature. The injection rate of sodium (feed rate to the reactor) has been reported to have the opposite effect on the final product particle size. A problem with adjusting the temperature by increasing the injection of sodium is the release of heat by the means for cooling the reaction mixture in the reaction vessel. The use of cooling can in the end significantly reduce the processing time and the particle size of the resulting powder. Is reported.

Another factor contributing to the formation of high surface area tantalum powder is the high use of dilute salts such as sodium chloride in the reduction reaction. Such diluents also act as internal heat absorbers in this system.

An important requirement for producing finer particle size, higher surface area tantalum powders is the temperature at which the sodium is poured into the molten bath. Lower temperatures promote fine particle formation.

 Another important requirement to control grain shape is the reduction temperature.

As already mentioned, at temperatures from about 760 ° to about 850 ° C,
It tends to form smaller particles, while at temperatures from about 850 ° to about 1000 ° C it tends to form somewhat larger particles.

According to U.S. Pat. No. 4,149,876, the above requirements (large amount of diluent, low melting bath temperature, rapid supply of sodium and the use of cooling means to maintain a constant temperature during the growth process) are combined to achieve a uniform There is an advantage that a high-area tantalum powder having a fine particle size can be produced.

In all the reactions outlined above, when the tantalum compound is reduced with a reducing metal to produce tantalum powder, the reactants are mixed and heated in a closed vessel until the exothermic reaction spontaneously begins, or A molten bath of tantalum compound is maintained and reducing metal is fed to the bath so as to reduce the tantalum compound to tantalum powder.

In Japanese Patent Publication No. 38-8, tantalum metal suitable for metallurgical purposes is K ₂ TaF ₇ heated to a temperature of about 500 ° C. or less.
It can be produced by a method of gradually adding crystals to a bath of sodium maintained at a temperature close to the boiling point.

Japanese Examined Patent Publication No. 43-25910 examines the contents of Japanese Examined Patent Publication No. 38-8, and a document therein discloses a method for producing a tantalum product having a purity useful for metallurgy, while it is 5 μm. It is stated below and that products with particle sizes above 100 microns would be unsuitable for condenser applications. This latter document describes K ₂ Ta containing diluents.
Add F ₇ gradually to a stirred liquid sodium bath teaches an improvement of the former method. Tantalum powders with a specific surface area of less than about 750 cm ² / g between 5 and 100 microns have been obtained. However, although this article defines this product as a capacitor grade tantalum powder, by current standards this powder is of unacceptably low capacity for capacitors.

U.S. Pat. No. 4,684,399 discloses a method of producing tantalum powder by adding it to a reactor in such a manner that the tantalum compound is continuously or increased in the course of reacting with a reducing metal. The rate or increasing amount of this continuous addition can be varied depending on the desired tantalum powder product. Continuous addition or smaller increments are preferred to increase capacity.

The term continuous addition hereinafter means the period of uninterrupted addition of the tantalum compound or reducing agent.

It is also known to increase the yield of fine particles with various dopants using the method described above. U.S. Pat. Nos. 3,825,802 and 4,009,007 disclose the use of phosphorus as a method for improving the capacitance of capacitors and the fluidity of tantalum powders. U.S. Pat. No. 4,582,530 discloses the addition of sulfur as a doping agent. The use of boron and other dopants is also well known to those skilled in the art. As already explained, an important consideration in the production of tantalum powder used in capacitors is purity. Impurities commonly found in tantalum powders are classified as light (or low molecular weight) or heavy (or high molecular weight) impurities. Low molecular weight impurities are generally carbon, calcium and aluminum derived from water, fluorine, chlorine, sodium and potassium derived from reactive materials, and tantalum powders used to wash powders when contacting air or water. Includes nitrogen and hydrogen produced. In general, most of the low molecular weight impurities evaporate during the sintering process and therefore do not have a particularly adverse effect on the capacitor performance.

High molecular weight impurities include Fe, Cr, Mo, Co and other metals. High molecular weight impurities generally remain in the powder even after high temperature sintering. Therefore, if the degree of heavy impurities in the tantalum powder is not reduced in the tantalum powder formation process, they remain and adversely affect the capacitor performance.

Generally, the source of heavy impurities is the equipment used in the reduction process to produce tantalum powder. The device is a reduction cell, a lid, and an agitator in contact with the reaction material. The equipment is usually made of nickel, iron or alloys and is easily attacked by reaction components under reaction conditions.

One idea is that in the process of powder formation, a thin film of metal oxide forms on the metal surface of the reactor, and this film dissolves to form metal ions, which are mixed with the tantalum powder matrix. As a result, heavy impurities are generated.
The thin film of metal oxide is formed when the air in the reactor attacks the metal surface of the reactor. At processing temperatures of about 80 ° C. or higher, metal oxides form more rapidly. on the other hand,
Water absorbed by dilute salts or tantalum compounds should
It liberates at temperatures above 80 ° C. and attacks the metal surface of the reactor forming a thin film of metal oxide. When the diluted salt or potassium salt is brought into a molten state by heat of reaction or heating from the outside, the metal oxide thin film is dissolved in the molten material to form metal ions.

It is advantageous to produce high surface area, high purity tantalum powder by inhibiting the formation of metal oxide films on the metal surface of the reactor and by removing sources of heavy impurities during the tantalum powder manufacturing process.

SUMMARY OF THE INVENTION A new method has been discovered to prevent the formation of metal oxide films on the metal surface of a reaction vessel, thereby limiting the source of heavy impurities and producing high purity tantalum powder. According to the invention, a small amount of active ingredient having a higher thermodynamic potential and chemical activity than the metal surface of the reaction vessel used to produce the tantalum powder is added to the reactor prior to heating the reactor to the reaction temperature. Add.

Although the present invention is not limited thereto, one way to consider it as a low-scale impurity is that the air or moisture in the container reacts with the active ingredient by heating, and then reacts with the metal surface of the reaction container to form a metal oxide. To block the free air or moisture that forms the film.

The active ingredient is a compound having a higher thermodynamic potential and chemical activity than the metal of the reactor and stirrer at the reaction temperature. The terms thermodynamic potential and chemical activity relate to the equilibrium constant and kinetics of active components in an oxidation reaction.

Active ingredients include, but are not limited to, alkali metals and alkaline earth metals. Other elements such as carbon or silicon are also used provided that they satisfy thermodynamic potential and chemical activity levels as active ingredients. The preferred active ingredients are sodium and potassium. After reacting with oxygen or moisture, these metals form sodium and potassium ions as part of the molten salt and therefore do not contaminate the tantalum product. Furthermore, sodium and potassium are more active than other elements because they have low melting points and high vapor pressures. Other active ingredients oxidize and subsequently evaporate from the melt diluent or remain in the melt diluent. As to a few of these, cesium or rubidium are included in these.

The amount of active ingredient added to the reactor is that amount of active ingredient sufficient to react with some, preferably all, of the moisture and oxygen in the reactor. Generally, this amount is greater than 1 gram due to the amount of moisture and air in the reaction vessel. After introducing the active ingredient, the reactor is heated and the tantalum compound is reacted with a reducing agent to reduce it to tantalum metal by methods well known to those skilled in the art. Preferably, the tantalum compound reacts with the reducing metal to reduce it to tantalum metal. As mentioned above, the tantalum compound and / or the reduced metal are introduced into the reactor in such a way that they are continuous or increasing in the course of the reduction reaction. The rate of continuous addition, or each increasing amount, will vary based on the properties of the tantalum powder product. Continuous addition or smaller incremental additions tend to result in favorable high surface area.

Tantalum compounds are compounds that reduce to tantalum metal by reaction with a reducing metal and may be used in any convenient and desirable physical state. Such compounds are typically potassium fluorotantalum (K ₂ Ta
F ₇ ), sodium fluorotantalum (Na ₂ TaF ₇ ), tantalum chloride (TaCl ₃ ), and mixtures thereof, and also potassium fluoroniobium (K ₂ NbF ₇ ) and potassium fluorocolumbium (K ₂ CbF ₇ ). Is also chemically similar to tantalum and can be included in this definition. The preferred tantalum compound is potassium fluorotantalum.
The preferred form of addition of K ₂ TaF ₇ is solid, but other forms are satisfactory.

The reduction reaction produces tantalum powder and a metal salt. After the reduction reaction is complete, the tantalum powder and metal salt reactant is exposed to water to dissolve the salt, and then acid washed before recovering the tantalum powder. The tantalum powder is then dried, screened, doped and heat treated by methods well known to those skilled in the art.

The tantalum powder obtained by the method of the present application has a Fisher Subsieve Particle Size.
Is less than 5 microns and the BET surface area is greater than about 0.2 m ² / g. The purities of iron, nickel, chromium and molybdenum are all less than 15 ppm, in contrast to the usual levels of iron and nickel of greater than 15 ppm using similar equipment.

One advantage of the method of the present invention is that it produces a low impurity level tantalum powder.

The advantage of the preferred method of the present invention is that when tantalum and / or reducing compounds are added to the reactor in a manner that is continuous or increasing during the course of the reduction reaction, high levels of low impurities, which is particularly advantageous for the use of condensers. A surface area tantalum powder is obtained.

Other advantages of the invention will be apparent from the following more detailed description and claims.

DETAILED DESCRIPTION OF THE INVENTION According to the present invention, the thermodynamic potential and chemical activity are higher than the metal surface of the reaction vessel used before the reactor is heated to the reaction temperature to produce tantalum powder. A small amount of active ingredient is added to the reactor, and the tantalum compound with the reducing agent is then reduced in the reactor to produce tantalum powder. The active ingredient draws in air and moisture inside the reactor and removes air and moisture that reacts with the metal surface of the reactor to form an oxide film on the metal surface of the reactor. The amount of active ingredient added to the reactor is sufficient to react with at least a portion and preferably all of the air and moisture present in the reactor. After the addition of active ingredients and diluents is done, the reactor is purged with inert gas or depressurized to reduce the amount of air and moisture inside the vessel. The reactor is then heated and the reduction reaction of the tantalum compound proceeds.

The reactor used in the process of the present invention is a reactor well known to those skilled in the art used to produce tantalum powder via reduction of tantalum compounds. In general,
A typical reactor includes a reaction vessel, its lid, stirrer, thermowell, gas inlet and outlet ports, and substance inlet and outlet ports. Typically, reaction vessels and stirrers are made from pure nickel, nickel-based alloys and iron-based alloys.

As one embodiment of the present invention, high-purity, high-surface-area tantalum powder is manufactured as follows. A pure nickel reactor is prepared and checked for air tightness as described in Example 1 below. Then the charging port of the reactor is opened and a small amount of active ingredient is added to the reactor. After adding the active ingredient, a dilute salt such as NaCl is added to the reactor.
It will be appreciated by those skilled in the art to add the active ingredients after or at the same time as the diluent is added. Preferably, the active ingredient is added before adding the diluent, tantalum compound and reducing agent. After the addition of these compounds, the reactor is purged with argon for about 4 hours to further reduce contact with air and moisture. The reactor is then placed in a furnace and heated to about 500 ° C for about 4 hours.

After purging for 4 hours or preheating for the same time, the temperature is raised to between 700 and 900 ° C. and maintained there for the time required for the diluent to melt. After about 5 to 60 minutes, the stirrer is driven to stir the molten mixture and the reducing agent is introduced into the reactor, preferably in a continuous manner. On the other hand, the tantalum compound may be added before adding the reducing agent.

Preferably, the tantalum salt is fed into the reactor in progressively increasing amounts. Generally, the timing of addition while increasing the amount is
The tantalum compound is added in increasing amounts when the amount of reducing agent reaches a certain level, as it is synchronized with the addition of reducing agent. On the other hand, if the tantalum compound is continuously added to the reactor, the reducing agent is also added in an increasing amount. However, both the tantalum compound and the reducing agent may be continuously added, or both may be added in an increasing amount.

After adding the specific amounts of tantalum compound and reducing agent, either addition is stopped. The reactor is heated at the temperature and for the time required to complete the reduction reaction between the tantalum compound and the reducing agent, while the stirrer stirs the molten mixture inside the reactor.

After the reduction reaction is completed, air or water is circulated to cool the reactor. After the reactor has cooled sufficiently, the reactor is opened. Subsequently, steam and water are injected into the reactor to neutralize excess sodium.

The reaction is then mechanically converted to cubes, washed with water to remove the diluent and extract the tantalum powder.

The extracted tantalum powder is then treated with acid to remove residual impurities on the powder surface.

The tantalum powder obtained by the method of the present invention is used in a method well known to those skilled in the art.

Generally used for capacitors, tantalum powder is screened, doped, heat treated, deoxidized, pelletized, sintered to about 1400 ° C to about 1700 ° C, and anodized. The method of forming the capacitor is well known in the art, and is not particularly relevant to the present invention, so the description will be made to this extent.

The test method shown below was adopted to analyze a capacitor using the tantalum powder obtained by the method of the present invention and to determine and evaluate its physical properties.

The surface area of tantalum powders was determined by the nitrogen Brunaur, Emme.
The tt, Teller (BET) method was used. Particle size is Fi
It was performed using the sher Subsieve method (ASTM30 B330-82).
The purity of the tantalum powder was determined by using an optical emission spectrometer capable of detecting iron, nickel, chromium and molybdenum up to 5 ppm.

Further features and advantages of the invention will be apparent from the examples which follow.

Comparative Example 1 A chemical reactor, a reaction lid and a stirrer were combined to form an apparatus. These constituents are nickel, nickel-based alloys or iron-based alloys. The reactor was inspected for air leaks at 5 inches Hg (absolute) and 5 minutes of vacuum. If the pressure increases by 2 inches or less in 5 minutes, the reactor is considered to be well sealed.

The charging port of the reactor was opened and 300 lbs of diluted salt was added to the reactor.

The reactor is then placed in a conventional hopper type furnace and the reactor is heated to 60 SCFH for about 10 hours at ambient temperature of about 25 ° C.
(1 cubic foot per hour, standard cubic feet per
Hour) and purged with argon. The furnace was then heated to 225 ° C and maintained at this temperature for 4 hours.
After 4 hours, the reaction temperature was raised to about 850 ° C and held for about 40 minutes to melt the diluent. A stirrer was swirled to stir the molten mixture inside the reactor. Liquid sodium preheated to about 125 ° C. was continuously charged to the reactor at a rate of 1 pound per minute.

At this point, K ₂ TaF ₇ was added to the reactor in batches of 12 in increments of 20 pounds. When the total sodium reached 71 to 72 pounds, the addition of liquid sodium was stopped and the reactor was heated at 900 ° C. for 2 hours with continued stirring.

After 2 hours, air was blown into the reactor to cool it below about 100 ° C. Steam and then water was added to the reactor to remove residual sodium and potassium. Then the reactor was opened and water was poured out. The washed solid reactants were mechanically shaped into cubes. After washing with water to remove salt, the cube was placed in a tank and water was added. After pouring out the water
HC1 and a mixture of mineral acids including HNO ₃ was added to the reactor. The entire vessel was rotated for about 30-60 minutes to ensure complete contact between the reactants and the acid mixture. After pouring, the acid treated powder was rinsed thoroughly to remove residual acid and dried. Table 1 shows the properties of the tantalum powder obtained by the method of Comparative Example 1.

Examples 1.2.3 and 4 One pound of sodium rod was added to the reactor prepared in the method of Comparative Example 1 prior to the addition of 300 pounds of diluted salt. This sodium rod is about 2-1 / 2 inches in diameter,
It is in the shape of an 8 inch long cylinder.

After loading the reactor, the remaining steps were performed as in Comparative Example 1. The properties of the tantalum powder obtained in Examples 1-4 are shown in the following table.

Comparative Example 2 The method of Comparative Example 1 was repeated, including the reactor, compound addition, purging and heating to about 850 ° C. In this example, liquid sodium was also added at a continuous rate of 1 lb / min. Potassium fluorotantalum was added to the reactor in increments of 28 pounds over ten times.

When the total sodium addition was 83 lbs, the liquid sodium addition was stopped. The rest of the process from now on
The same procedure as in Comparative Example 1 was performed.

Example 5 Prior to adding the diluent and constituents of Comparative Example 2 to the reactor. The procedure of Comparative Example 2 was followed except that one pound of sodium in the shape of a cylinder of 2-1 / 2 inch diameter and 8 inches long was added.

Example 6 A nickel reactor was prepared and tested for air leaks according to the method set forth in Comparative Example 1. A 1/4 pound sodium rod in the shape of a 2 1/2 inch diameter, 2 inch long cylinder was used. About 200 pounds of diluent was added. The remaining steps were performed according to Comparative Example 2 and Example 5. The properties of the obtained powder are as follows.

FSS 1.35 micron BET 0.59M ² / g O ₂ 2145ppm Fe <5Ni <5Cr <5Mo <5 Comparative Example 3 A reactor was prepared according to the method described in Comparative Example 1.
Prior to heating at 500 ° C. for 4 hours, 300 lbs of diluent was charged to the reactor and purged with 60 SCFH argon for about 10 hours under ambient conditions. The furnace was then heated to 820 ° C and stirred for about 30 minutes. Liquid sodium preheated to 125 ° C was added to the reactor at a rate of 1 pound per minute. K ₂ TaF ₇ was added to the reactor in 10 pound increments of 20. The sodium addition was stopped when 60 pounds of sodium had been added. The reactor was then heated to 900 ° C., then cooled and extracted according to the procedure previously described.

 The properties of the obtained tantalum powder are shown below.

FSS 1.17 micron O ₂ 2685ppm Fe 25 Ni 35 Cr 20 Mo <5 Example 7 A reaction vessel was assembled and prepared according to the method of Comparative Example 1 and 1 pound of sodium rod was added and then 30
0 pounds of diluent was added. The vessel was then purged with 60 SCFH of argon at about 25 ° C for about 10 hours. The reactor was then heated inside the furnace at about 225 ° C for about 4 hours. The temperature of the furnace was raised to about 820 ° C. in 30 minutes and stirring was performed. Immediately after starting the addition of liquid sodium, about 28 pounds of K ₂ TaF ₇ was added. Sodium was preheated to 125 ° C and added at a rate of about 0.75 pounds per minute. K ₂ Ta for the remaining 7 episodes
The F ₇ was added to the reactor. Add 83 pounds of sodium,
The sodium addition was stopped. The remaining steps of treating the reactants were performed as described in Comparative Example 3.

The properties of the tantalum powder obtained by the example here are as follows.

FSS 1.17 micron O ₂ 3190 Fe 5 Ni 10 Cr <5 Mo <51 Example 7 except that 1 lb of sodium was added.
The properties of the tantalum powder obtained by the above method are as follows.

FSS 1.93 micron O ₂ 1895 Fe 15 Ni 45 Cr <5 Mo <5

Claims (13)

(57) [Claims] 1. An active ingredient is added to a reaction vessel to remove moisture and oxygen in the reaction vessel, the active ingredient having a higher thermodynamic potential and chemical activity than the reaction vessel, and the tantalum compound in the reaction vessel. A method of producing tantalum powder in a reaction vessel comprising reducing with a reducing agent to form tantalum powder. 2. The method of claim 1 wherein said active ingredient is selected from the group consisting of alkali metals, alkaline earth metals, oxygen and silicon. 3. Tantalum with improved purity by adding sodium to the reaction vessel to react with moisture and oxygen in the reaction vessel before heating the reaction vessel to the reduction reaction temperature and reducing potassium fluorotantalum with sodium. A method of making tantalum powder in a nickel or iron based reaction vessel comprising forming a powder. 4. The method of claim 3 wherein the sodium added to react with moisture and oxygen is at least 1 gram. 5. The method of claim 4 wherein said tantalum powder contains less than 15 ppm nickel and iron impurities. 6. A method for producing a tantalum powder by reducing a tantalum compound in a molten salt diluent with a reducing agent in an iron or nickel based reaction vessel in which air and moisture in the reaction vessel are reduced by a purge gas. , Tantalum with a low nickel and iron content by adding an active ingredient (which has a higher thermodynamic potential and chemical activity than the reaction vessel) to the reaction vessel before heating the reaction vessel to reduce the tantalum compound. An improved method of making a powder. 7. A process according to claim 6, wherein the active ingredient is added to the reaction vessel before the reaction vessel is heated to a temperature above 80 ° C. 8. The method of claim 6 wherein the reaction vessel containing the active ingredient and the salt diluent is purged to remove air and moisture prior to heating and melting the salt diluent. 9. The method of claim 8 wherein the active ingredient is selected from the group consisting of alkali metals, alkaline earth metals, carbon and silicon. 10. The method of claim 6 wherein the active ingredient is selected from the group consisting of alkali metals, alkaline earth metals, carbon and silicon. 11. The method according to claim 10, wherein the active ingredient is sodium or potassium. 12. The method of claim 11, wherein the active ingredient comprising sodium or potassium is added to the reaction vessel before heating the reaction vessel to a temperature above 80 ° C. 13. The method of claim 11 wherein the reaction vessel containing the active ingredient and the salt diluent is purged of air and moisture prior to heating the salt diluent to the molten state.