US20080105084A1

US20080105084A1 - Method of production of tantalum powder with low impurity level

Info

Publication number: US20080105084A1
Application number: US11/929,583
Authority: US
Inventors: John Crawley; Eugeny Polyakov
Original assignee: Niotan Inc
Current assignee: Kemet Blue Powder Corp
Priority date: 2006-10-30
Filing date: 2007-10-30
Publication date: 2008-05-08

Abstract

The production of tantalum powder having a low impurity level is provided by a method in which potassium heptafluotantalate is added to a mixture of alkali metal halides in reactor vessel in which the internal surface and auxiliary input accessories of the reactor are covered with a tantalum coating. In one embodiment, the production of tantalum powder with low impurity level includes depositing a protective tantalum coating onto an internal surface of the reactor vessel and auxiliary accessories of a reactor by electrolysis of a mixture of alkali metals halides and potassium heptafluotantalate.

Description

CROSS-REFERENCE TO RELATED APPLICATION

The application claims the benefit of U.S. Provisional Application No. 60/855,697, filed Oct. 30, 2006.

FIELD OF INVENTION

The present invention relates in general to a field of powder metallurgy and, more particularly, to the production of a tantalum powder with low impurity levels.

BACKGROUND OF INVENTION

Tantalum powder is an excellent material for manufacturing of electrolytic capacitors. Tantalum powder can be produced by several methods, including mechanical methods, electrochemical reduction of tantalum compounds in molten salts and known metallothermic reduction process. Mechanical and electrochemical methods result in tantalum powder with low surface area that makes impossible to manufacture anodes for electrolytic capacitors with high specific charge. Methods of chemical reduction allow producing fine powder, which is characterized by high specific surface. Commonly, tantalum powder is commercially produced by reduction of potassium heptafluotantalate K₂TaF₇with sodium metal in alkali metal halide melt, where addition of components into a reactor is carried out in one way or another.
Along with a high specific surface, a high purity of the powder is a critical factor for capacitor production. Metallic and non-metallic impurities lead to degradation of the dielectric oxide film on a surface of tantalum powder, increasing the leakage current or causing breakdown of the capacitor. Some impurities negatively influenced on tantalum powder properties, such as alkali metals, calcium, silicon, fluorine, chlorine, are concentrated on its surface and can be essentially removed by surface treatment. Contamination by nickel, iron, and chromium occurs in more complex ways including corrosion of the equipment in alkali halide melt, accompanied with transition of these metals into a bulk of the melt in the form of ions. Tantalum reduction ions also undergo reduction by one or other mechanism:
5Ni²⁺(Fe³⁺,Cr³⁺)+2Ta=5Ni(Fe,Cr)+2Ta⁵⁺
Ni²⁺(Fe³⁺,Cr³⁺)+2Na=Ni(Fe,Cr)+2Na⁺
One feature of these reactions is their topochemical character, which leads to the appearance of impurities uniformly distributed in a bulk of tantalum grain that cannot be removed by surface treatments, such as etching.
In addition, tantalum powder produced by sodium reduction of potassium heptafluotantalate contains impurity elements derived from the starting materials and the equipment used. Usually potassium heptafluotantalate and sodium metal are used as pure as possible. The equipment including the reactor vessel, lid and stirrer is generally made of stainless steel (U.S. Pat. No. 4,954,169), nickel (U.S. Pat. No. 5,234,491) or nickel alloys (U.S. Pat. No. 4,149,876). These materials are more or less easily attacked by reaction components under temperatures of reduction process and are the source of heavier impurities, such as Ni, Fe, Cr, Co, Mo etc. as described above.
Use of tantalum as a material of a reaction vessel and lid results in considerable expenditures for tantalum powder production. Various tantalum powder techniques have been practiced in an attempt to produce tantalum powders having low level of impurities. For example, the control and decrease of reduction process temperature, cooling of a reaction mass, stepwise additions of initial components produce some decrease of impurity levels in tantalum powder (U.S. Pat. No. 5,442,978).
Some methods of relatively pure tantalum powder production are known (See e.g., U.S. Pat. No. 5,234,491). They include the addition of a small quantity of an active ingredient into a reaction mixture before the reactor vessel is heated to preset temperature. The active ingredient having a higher thermodynamic potential and chemical activity than metal surfaces of reactor vessel made of nickel, nickel based alloys or stainless steel is introduced into a reactor in the form of sodium or potassium rod prior to alkali halides, K₂TaF₇and reducing compound. Authors propose that the active ingredient attracts air and moisture present inside the vessel preventing their interaction with a melt. However, the impurity levels coming from a material of a reactor and oxygen remain significant due to the high solubility of alkali metal oxides in the melt used.
It was shown in Russian Patent RU 2 164 194 (2001), that reduced impurity level in tantalum or niobium powders could be obtained by a method wherein a salt of valve metal, alkali halides and an active ingredient are added simultaneously into a reactor made of nickel-containing material. As the active ingredient, powder of tantalum or niobium metal is used. The reactor is then heated and tantalum or niobium compound is reduced to tantalum or niobium metal by reaction with sodium metal. The active ingredient reacts with the melt and material of the reactor resulting in an intermetallic coating on the internal metallic surface of the reactor. The quantity of active ingredient added to the reactor is 0.3-3.0 w/o of the initial content of K₂TaF₇or K₂NbF₇in a melt. An intermetallic coating consisting of Ni₃Ta and Ni₂Ta or NiNb and Ni₃Nb is obtained. While corrosion resistance of the surface layer of such composition in alkaline halide melt is higher than corrosion resistance of the substrate materials, there still remains some measure of contamination of the tantalum or niobium powder by nickel and other metals.
Moreover, a thickness of intermetallic coating formed was no more than 6 microns. This method is unable even theoretically to produce tantalum or niobium metal coating onto nickel or nickel-containing surface. In addition, the use of tantalum powder as an active ingredient raises considerably the cost of the process. Thus, what is needed is an improved method for producing a high purity tantalum powder.

SUMMARY OF THE INVENTION

Disclosed and claimed herein are methods for the production of a tantalum powder in a reactor vessel. In one embodiment, a method includes depositing a tantalum coating onto an internal surface of a reactor vessel and onto at least one auxiliary accessory thereof. The method further includes adding a quantity of potassium heptafluotantalate to the reactor vessel having the tantalum coating, and adding a quantity of a mixture of alkali metal halides into the reactor vessel. Finally, the method includes reducing the potassium heptafluotantalate using sodium in the reactor vessel.
Other aspects, features, and techniques of the invention will be apparent to one skilled in the relevant art in view of the following detailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the terms “a” or “an” shall mean one or more than one. The term “plurality” shall mean two or more than two. The term “another” is defined as a second or more. The terms “including” and/or “having” are open ended (e.g., comprising). The term “or” as used herein is to be interpreted as inclusive or meaning any one or any combination. Therefore, “A, B or C” means “any of the following: A; B; C; A and B; A and C; B and C; A, B and C”. An exception to this definition will occur only when a combination of elements, functions, steps or acts are in some way inherently mutually exclusive.
Reference throughout this disclosure to “one embodiment”, “certain embodiments”, “an embodiment” or similar term means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of such phrases or in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner on one or more embodiments without limitation.
One aspect of the present disclosure relates to the production of tantalum powder having a low impurity level. In one embodiment, the method includes adding potassium heptafluotantalate and a mixture of alkali metal halides into a reactor vessel in which the internal surface and auxiliary input accessories of the reactor are covered with a tantalum coating, and reducing potassium heptafluotantalate by sodium in said reactor vessel with a protective tantalum coating. In one embodiment, the production of tantalum powder with low impurity level includes depositing said protective tantalum coating onto an internal surface of a reactor vessel and auxiliary accessories of a reactor by electrolysis of a mixture of alkali metals halides and potassium heptafluotantalate.
The electrolytic deposition of a protective tantalum coating may comprise using, as the cathode, said auxiliary accessories of a reactor and/or a reactor vessel in which a mixture of alkali metal halides and potassium heptafluotantalate are electrolyzed with the tantalum anode. The auxiliary accessories of a reactor utilized for production of tantalum powder may include elements of a reactor (a stirrer, a shell of a thermocouple, a head of a reactor, etc.) contacting with the melt during reduction of potassium heptafluotantalate by sodium. In certain embodiment, auxiliary accessories of a reactor and a reactor vessel are made of mild steel, stainless steel, nickel or nickel containing material.
In certain embodiments, the thickness of the protective coating is at least 30 microns in order to eliminate possible penetration of contaminants through the tantalum layer.
A mixture of alkali metal halides may be a mixture of alkali metals chlorides and fluorides with melting point less than 700° C. In one embodiment, the mixture of alkali metals halides may be a waste product of a tantalum powder production by sodium-reduction method. This waste product of a tantalum powder production meets a low melting point requirement, and may reduce the overall cost of tantalum powder production.
After deposition of the aforementioned coating, the melt may be pumped out from the electrolyzer with a vacuum-bucket. The electrolyzer may then be filled with inert gas and cooled. After cooling an internal surface of a reactor and/or a detail with a tantalum coating may be washed from solidified remains of the melt by water or a dilute solution of hydrochloric acid.
The steps for production of the tantalum powder include reducing of potassium heptafluotantalate by sodium metal in the presence of mixture of alkali metal halides. In certain embodiments, the mixture of alkali metal halides may be a mixture of sodium chloride and sodium and potassium fluorides. After the reduction process is completed, the reaction block of tantalum powder and metal salts may be processed by digesting with water to dissolve the salts and then be treated with acids to remove residual impurities from the tantalum powder surface. Chemical analysis (certificates) of samples of dried tantalum powder produced according to the principles of the invention demonstrates low level of impurities and can be used for manufacturing of high performance capacitors.
During sodium reduction of potassium heptafluotantalate in a reactor made of stainless steel or a heat-resisting nickel-based alloy, the temperature may reach up to 1000-1100° C. In such conditions, rather intensive interdiffusion develops on the boundary between underlying metal and a tantalum coating. Atoms of tantalum and underlying metal move towards each other and the thickness of the coating decreases. Further, this may result in the appearance of nickel, iron and other metals in the form of Ta(Fe,Ni,Cr)₂and Ta₂(Fe,Ni,Cr) intermetallic compounds in surface layer, directly contacting with a melt. Such interdiffusion leads to unavoidable contamination of a produced tantalum powder by products of corrosion, in particular, nickel (iron, chrome and other metals). The rate of moving of iron, nickel and other impurity atoms has been observed to be about 10 micron/hour within the first thirty minutes. After that the sharp decrease of their mobility caused by diffusion difficulties in the layer of Ta₂(Fe,Ni,Cr) intermetallic compound is observed. However, a thickness of the tantalum coating of at least 30 microns appears to safely prevent the appearance of such impurities on the reactor's surface over an expected reactor lifetime of 1500 hours.
Tantalum layer is deposited by electrolysis of the molten salt, in which a mix of the salts being a by-product of the process of potassium heptafluotantalate reduction by sodium metal is used. In one embodiment, this may lower the cost associated with the electrolysis process. In one embodiment, this salt mixture may be sensitive to atmospheric moisture and, as such, may be kept in a sealed metal container under an atmosphere of dry inert gas.
Cathodic electrodeposition of tantalum coating may be carried out in the vessel referred to as an electrolyzer. In one embodiment, the electrolyzer may be filled up with the above-mentioned mix of salts at the temperature exceeding its melting point. In order to avoid pyrohydrolysis, which can negatively affecting the quality of tantalum coating, this process may be carried out under an inert gas atmosphere.
The thickness of the resulting tantalum layer may depend on the selected cathodic current density as well as the duration of the plating process. Depending on the type of a plated part, the process of electrodeposition may be carried out in one way or another for which the following set of operations may be used:
A mixture of salts comprising mainly waste products of the production of tantalum powder by sodium-reduction method, with the additive potassium heptafluotantalate in quantity 5-20% of a charge, is loaded into a clean and dried electrolyzer.
The electrolyzer is closed by a hermetic cover equipped by a tantalum anode, a thermocouple, a flange for connection of a vacuum-gas line, a flange for immersing of a vacuum-bucket into the vessel and other devices if necessary. When the stirrer or other accessories are coated, they too can be mounted on a cover. The plated detail, playing a role of the cathode and the anode are electrically insulated from each other.
The electrolyzer is placed into a furnace, after which air is evacuated from inside with the vacuum pump to reduce the amount of oxygen and moisture and, continuing pump-down, is stepwise heated up to temperature 100, 200 and 400° C. The electrolyzer may then be filled with purified inert gas and heated up to the given temperature.
Electrodes with the current leads are immersed into the melt as deep as necessary and soaked until temperature in the electrolyzer is balanced.
Tantalum anode and cathode, i.e. a plated detail or the reactor vessel when its internal surface is covered with a tantalum coating are connected to the power supply and applied given voltage.
The electrolysis process is carried out during a preset time, maintaining a relatively constant temperature in the electrolyzer and cathodic current density under the given program. From reasons of minimization of power consumption and maintenance of coating quality, the temperature, may be supported in the range of 700-770° C. For electrochemical plating, a direct current or a reverse current may be used. Assuming a condition of 0≦Q_a/Q_k<0.9 is maintained, where Q_a/Q_kis the ratio of electric charge (coulombs) in anodic and cathodic parts of the electrodeposition cycle. At the selected temperature and current, a thickness of the tantalum coating depends on electric mass (number of coulombs) used on its deposition Q_eff=Q_k−Q_a. As a basis for calculations the following ratio serves: at passage through square centimeter of a surface of covered product Q_effin quantity of 1 A·h, thickness of a tantalum coating attains 800 micrometers.
After the completion of the electrodeposition coating, the current may be cut off and current leads disconnected from the power supply. With a vacuum-bucket the melt is pumped out from the electrolyzer, which may simultaneously be filled with inert gas. In one embodiment, a furnace may be switched out and the electrolyzer body cooled down to room temperature.
A cover of the electrolyzer may then be opened. The anode and the detail covered with tantalum may then be detached from the current leads. The electrolyzer may then be closed with a cover again, pumped out and filled with inert gas.
The anode and a detail with a tantalum coating are washed off with water or a dilute-solution of hydrochloric acid, according to one embodiment. Any residual salts may similarly be removed.
For repeated conducting of the process the molten mixture of salts may be filled inside the electrolyzer with the vacuum-bucket.
The following examples are provided for illustrative purposes only and should not limit the scope of the invention in any manner. It should be noted that the following examples utilize certain parameters of the tantalum sodium-reduction process described in the above-mentioned RU 2,164,194 for purposes of comparing the results only:

EXAMPLE 1

10 kg of potassium heptafluotantalate and 7.5 kg of sodium chloride were loaded into a reactor vessel manufactured of nickel. Then a reactor was placed into a container from stainless steel with a water-cooled cover and evacuated down to pressure 10⁻²Torr, filled with argon, heated up to 800° C. and soaked at this temperature for 1 hour before melting of salts. After that, temperature in the container was reduced down to 700° C. and continuously stirred for 1.3 hours, after which 3.1 kg of the melted sodium was introduced into the reactor. During reaction of reduction of tantalum temperature of melt smoothly raised up to 820° C. After soaking at this temperature for 0.5 hours, a reactor was cooled down to room temperature. Then tantalum powder with crystallized salt mixture was removed from the reactor vessel, crushed and washed by water. Later on tantalum powder was processed consistently in 10% hydrochloric acid and 1% hydrofluoric acid, taken accordingly in quantity of 1.0 and 0.5 l/kg of a powder, carefully washed out with distilled water and dried. To raise the purity of tantalum powder produced, an internal surface of a nickel reactor was covered with a thin layer of tantalum before a process of reduction.
Tantalum coating onto an internal surface of the nickel reactor was electrodeposited from the molten mixture of alkali metal halides with addition of potassium heptafluotantalate. Composition of a mix—10 w/o K₂TaF₇, the rest—the mixture of salts formed as waste products during manufacture of tantalum by sodium-reduction method, in weight proportion: KF-43.5, NaF-30.0, NaCl-26.5. Direct current processing was realized at temperature 750° C. with cathodic current density 0.1 A/cm²for 30 minutes. As a result, a 40 microns thick tantalum coating was formed on the internal surface of the reactor. Metallographic analysis of the reference sample, also made of nickel and simultaneously coated with tantalum in the same conditions, has shown that the formed tantalum coating is uniform, porousless and adherent to a substrate.

EXAMPLE 2

10 kg of potassium heptafluotantalate and 7.5 kg of sodium chloride were loaded into a reactor vessel manufactured of SS316 stainless steel. The reactor was then placed into a stainless steel container with a water-cooled cover and evacuated down to pressure 10⁻²Torr, filled with argon, heated up to 800° C., and then soaked at this temperature for 1 hour before melting of the salts. After that, the temperature in the container was reduced down to 700° C. and, at continuous stirring of the melt for 1.3 hours, 3.1 kg of the melted sodium was introduced into a reactor. During reaction of the reduction of the tantalum powder, the temperature of the melt was smoothly raised up to 820° C. After soaking at this temperature for 0.5 hours, the reactor was cooled down to room temperature. Thereafter, the tantalum powder with crystallized salt mixture was removed from the reactor vessel, crushed and washed by water.
The tantalum powder was later processed in 10% hydrochloric acid and 1% hydrofluoric acid, taken accordingly in quantities of 1.0 and 0.5 l/kg of a powder, carefully washed out with distilled water and dried. To raise the purity of the tantalum powder produced, an internal surface of the SS316 stainless steel reactor was covered with a thin layer of tantalum before a process of reduction.
Tantalum coating onto this internal surface of the SS316 stainless steel reactor was electrodeposited from the molten mixture of alkali metal halides with addition of potassium heptafluotantalate. Composition of a mix—10 w/o K₂TaF₇, the rest—the mixture of salts formed as waste products during manufacture of tantalum by sodium-reduction method, in weight proportion: KF-43.5, NaF-30.0, NaCl-26.5. Direct current process was realized at a temperature of 750° C. with cathodic current density 0.1 A/cm²within 30 minutes. As a result, a 40 micron thick tantalum coating was formed on the internal surface of the reactor. Metallographic analysis of the reference sample also made of SS316 stainless steel and simultaneously coated with tantalum in the same conditions, has shown that the formed tantalum coating is uniform, porousless and adherent to a substrate.

EXAMPLE 3

10 kg of potassium heptafluotantalate and 7.5 kg of sodium chloride were loaded into a reactor vessel manufactured of the nickel alloy Inconel 600. The reactor was then placed into a stainless steel container with a water-cooled cover and evacuated down to pressure 10⁻²Torr. The reactor was filled with argon, heated up to 800° C. and soaked at this temperature for 1 hour before melting of the salts. After that, the temperature in the container was reduced down to 700° C. while the melt was continuously stirred for 1.3 hours, after which 3.1 kg of the melted sodium was introduced into the reactor. During reduction reaction, the temperature of the melt was smoothly raised up to 820° C. After soaking at this temperature for 0.5 hours, the reactor was cooled down to room temperature.
The tantalum powder with crystallized salt mixture was then removed from the reactor vessel, crushed and washed by water. The tantalum powder was then processed consistently in 10% hydrochloric acid and 1% hydrofluoric acid, taken accordingly in quantity of 1.0 and 0.5 l/kg of a powder, carefully washed out with distilled water and dried. To raise a purity of tantalum powder produced, an internal surface of the Inconel 600 reactor was covered with a thin layer of tantalum before a process of reduction.
Tantalum coating onto an internal surface of the Inconel 600 reactor was electrodeposited from the molten mixture of alkali metal halides with addition of potassium heptafluotantalate. Composition of a mix—5 w/o K₂TaF₇, the rest—the mixture of salts formed as waste products during manufacture of tantalum by sodium-reduction method, in weight proportion: KF-43.5, NaF-30.0, NaCl-26.5. Direct current process was realized at temperature 730° C. with cathodic current density 0.05 A/cm²within 45 minutes. As a result, a 30 micron thick tantalum coating was formed on the internal surface of the reactor. Metallographic analysis of the reference sample also made of Inconel 600 and simultaneously coated with tantalum in the same conditions, has shown that the formed tantalum coating is uniform, porousless and adherent to a substrate.

EXAMPLE 4

10 kg of potassium heptafluotantalate and 7.5 kg of sodium chloride were loaded into a reactor vessel manufactured of nickel clad Inconel 600. The reactor was then placed into a stainless steel container with a water-cooled cover and evacuated down to pressure 10⁻²Torr. The reactor was then filled with argon, heated up to 800° C. and soaked at this temperature for 1 hour before melting of salts. After that, the temperature in the container was reduced down to 700° C. and, at continuous stirring of the melt for 1.3 hours, 3.1 kg of the melted sodium was introduced into the reactor. During reaction of the reduction of tantalum, the temperature of the melt was smoothly raised up to 820° C. After soaking at this temperature for 0.5 hours, the reactor was cooled down to room temperature. Thereafter, the tantalum powder with crystallized salt mixture was removed from the reactor vessel, crushed and washed by water.
The tantalum powder was later processed in 10% hydrochloric acid and 1% hydrofluoric acid, taken accordingly in quantity of 1.0 and 0.5 l/kg of a powder, carefully washed out with distilled water and dried. To raise a purity of tantalum powder produced, an internal surface of the nickel clad Inconel 600 reactor was covered with a thin layer of tantalum before a process of reduction.
Tantalum coating onto the internal surface of the nickel clad Inconel reactor was electrodeposited from the molten mixture of alkali metal halides with addition of potassium heptafluotantalate. Composition of a mix—15 w/o K₂TaF₇, the rest—the mix of salts formed as waste products during manufacture of tantalum by sodium-reduction method, in weight proportion: KF-43.5, NaF-30.0, NaCl-26.5. Direct current process was realized at temperature 750° C. with cathodic current density 0.1 A/cm²within 30 minutes. As a result, a 40 micron thick tantalum coating was formed on the internal surface of the reactor. Metallographic analysis of the reference sample also made of nickel clad Inconel 600 and simultaneously coated with tantalum in the same conditions, has shown that the formed tantalum coating is uniform, porousless and adherent to a substrate.

EXAMPLE 5

10 kg of potassium heptafluotantalate and 7.5 kg of sodium chloride were loaded into a reactor vessel manufactured of the nickel alloy Inconel 600 protected inside with a electrodeposited tantalum layer and equipped with a mild steel St 3 stirrer. The reactor was then placed into a stainless steel container with a water-cooled cover and evacuated down to pressure 10⁻²Torr, filled with argon, heated up to 800° C. and then soaked at this temperature for 1 hour before melting of the salts. After that, the temperature in the container was reduced down to 700° C. and, at continuous stirring of the melt for 1.3 hours, 3.1 kg of the melted sodium was introduced into a reactor. During reaction of reduction of the tantalum, the temperature of the melt was smoothly raised up to 820° C. After soaking at this temperature for 0.5 hours, the reactor was cooled down to room temperature. Then tantalum powder with crystallized salt mixture was then removed from the reactor vessel, crushed and washed by water.
The tantalum powder was later processed in 10% hydrochloric acid and 1% hydrofluoric acid, taken accordingly in quantity of 1.0 and 0.5 l/kg of a powder, carefully washed out with distilled water and dried. To raise the purity of tantalum powder produced, a surface of the mild steel St 3 stirrer reactor was covered with a thin layer of tantalum before a process of reduction.
Tantalum coating onto the surface of the mild steel St 3 stirrer was electrodeposited from the molten mixture of alkali metal halides with addition of potassium heptafluotantalate. Composition of a mix—10 w/o K₂TaF₇, the rest—the mixture of salts formed as waste products during manufacture of tantalum by sodium-reduction method, in weight proportion: KF-43.5, NaF-30.0, NaCl-26.5. Reverse current process was realized at temperature 720° C. within 60 minutes with cathodic current density 0.15 A/cm²and anodic current density 0.4 A/cm². Length of the cathodic part of the cycle was 100 s and of anodic part-10s. Ratio of coulombs in anodic and cathodic parts Q_a/Q_k=0.27. As a result, a 48 micron thick tantalum coating was formed on a surface of the stirrer. Metallographic analysis of the reference sample also made of mild steel St 3 and simultaneously coated with tantalum in the same conditions, has shown that the formed tantalum coating is uniform, porousless and adherent to a substrate.

EXAMPLE 6

10 kg of potassium heptafluotantalate and 7.5 kg of sodium chloride were loaded into a reactor vessel manufactured of the nickel alloy Inconel 600 protected inside with the electrodeposited tantalum layer and equipped with a nickel thermocouple shell. The reactor was then placed into a stainless steel container with a water-cooled cover and evacuated down to pressure 10⁻²Torr, filled with argon, heated up to 800° C. and soaked at this temperature for 1 hour before melting of salts. After that, the temperature in the container was reduced down to 700° C. and, at continuous stirring of melt for 1.3 hours, 3.1 kg of the melted sodium was introduced into the reactor. During reaction of reduction of tantalum, the temperature of the melt was smoothly raised up to 820° C. After soaking at this temperature for 0.5 hours, the reactor was cooled down to room temperature. The tantalum powder with crystallized salt mixture was then removed from the reactor vessel, crushed and washed by water.
Thereafter, the tantalum powder was processed in 10% hydrochloric acid and 1% hydrofluoric acid, taken accordingly in quantity of 1.0 and 0.5 l/kg of a powder, carefully washed out with distilled water and dried. To raise the purity of tantalum powder produced, a surface of the nickel thermocouple shell was covered with a thin layer of tantalum before a process of reduction.
Tantalum coating onto the surface of the nickel thermocouple shell was electrodeposited from the molten mixture of alkali metal halides with addition of potassium heptafluotantalate. Composition of a mix—10 w/o K₂TaF₇, the rest—the mix of salts formed as waste products during manufacture of tantalum by sodium-reduction method, in weight proportion: KF-43.5, NaF-30.0, NaCl-26.5. Reverse current process was realized at temperature 770° C. within 70 minutes with cathodic current density 0.15 A/cm²and anodic current density 0.3 A/cm². Length of the cathodic part of the cycle was 100 s and of anodic part-5 s, Q_a/Q_k=0.1. As a result, a 74 micron thick tantalum coating was formed on a surface of the thermocouple shell. Metallographic analysis of the reference sample also made of nickel and simultaneously coated with tantalum in the same conditions, has shown that the formed tantalum coating is uniform, porousless and adherent to a substrate.
While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art. Trademarks and copyrights referred to herein are the property of their respective owners.

Claims

1. A method for production of a tantalum powder in a reactor vessel, comprising:

depositing a tantalum coating onto an internal surface of a reactor vessel and onto at least one auxiliary accessory thereof;

adding a quantity of potassium heptafluotantalate to the reactor vessel having the tantalum coating;

adding a quantity of a mixture of alkali metal halides into the reactor vessel; and

reducing the potassium heptafluotantalate using sodium in the reactor vessel.

2. The method of claim 1, wherein depositing comprises depositing the tantalum coating by electrolysis of a molten salts mixture.

3. The method of claim 1, wherein reducing comprises using as a cathode said at least one auxiliary accessory in which the mixture of alkali metal halides and potassium heptafluotantalate is electrolyzed with the tantalum coating.

4. The method of claim 1, wherein said at least one auxiliary accessory comprises one or more of a stirrer, a shell of a thermocouple and a head of the reactor vessel.

5. The method of claim 1, wherein said mixture of alkali metal halides comprises a waste products of a tantalum powder production by sodium-metallothermic process.

6. The method of claim 1, wherein said reactor vessel is comprised of one of a mild steel, stainless steel, nickel and nickel-containing material.

7. The method of claim 1, wherein the tantalum coating has a thickness of not less than 30 micron.

8. The method of claim 1, wherein said mixture of alkali metal halides comprises a mixture of sodium chloride, sodium and potassium fluorides.

9. A method for production of a tantalum powder in a reactor vessel, comprising:

adding a quantity of potassium heptafluotantalate to the reactor vessel, wherein the reactor vessel and at least one auxiliary accessory thereof have a tantalum coating;

reducing the potassium heptafluotantalate using sodium in the reactor vessel.

10. The method of claim 9, wherein the tantalum coating is formed by electrolysis of a molten salts mixture.

11. The method of claim 9, wherein reducing comprises using as a cathode said at least one auxiliary accessory in which the mixture of alkali metal halides and potassium heptafluotantalate is electrolyzed with the tantalum coating.

12. The method of claim 9, wherein said at least one auxiliary accessory comprises one or more of a stirrer, a shell of a thermocouple and a head of the reactor vessel.

13. The method of claim 9, wherein said mixture of alkali metal halides comprises a waste products of a tantalum powder production by sodium-metallothermic process.

14. The method of claim 9, wherein said reactor vessel is comprised of one of a mild steel, stainless steel, nickel and nickel-containing material.

15. The method of claim 9, wherein the tantalum coating has a thickness of not less than 30 micron.

16. The method of claim 9, wherein said mixture of alkali metal halides comprises a mixture of sodium chloride, sodium and potassium fluorides.