MXPA98008194A - Method for decreasing oxygen content in metals for valv - Google Patents

Abstract

The present invention relates to: A method for controlling the oxygen content in metallic valve materials. This method includes deoxidation of a valve metal material, typically tantalum, niobium or alloys thereof and leaching of the material in a leaching solution at a temperature below room temperature. In one embodiment of the present invention, the leaching solution is prepared and cooled to a temperature below room temperature, prior to leaching the metallic material. It has been found that the method of the present invention decreases both the concentration of oxygen and that of fluorides in the metallic valve material, since the use of reduced leaching temperatures, supplies less oxygen for a given amount of the leaching acid, such as acid. fluorhydri

Description

METHOD FOR DECREASING OXYGEN CONTENT IN VALVE METALS BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a method for controlling the oxygen content in metallic materials for valves and, more particularly, to a method for controlling the oxygen content in tantalum, niobium powders. and alloys thereof, useful in the manufacture of capacitors and sintered anodes made of tantalum, niobium and alloys thereof. 2. Description of Related Art Valve metals can be used to make forged products such as bars, plates, sheets, wires, tubes, rods and preforms for subsequent thermomechanical processing. In addition, the capacitors can be manufactured by compressing agglomerated tantalum powders to make pellets, sintering them in an oven to form a porous body (electrode), which sometimes as a next step is subjected to deoxidation by reaction with a reactive metal, such as magnesium, and then anodizing the body in a suitable electrolyte to form a continuous dielectric oxide film on the P1642 / 98MX sintered body. As is known to those skilled in the art, valve metals generally include tantalum, niobium and alloys thereof and may also include metals of groups IVB, VB and VIB and alloys thereof. Valve metals are described, for example, by Diggle, in "Oxides and Oxide Films," Vol.l, p.94-95, 1972, Marcel Dekker, Inc., New York. Tantalum and niobium are generally extracted from their minerals in the form of powders. For example, tantalum powders, which are suitable for use in high performance capacitors, can be produced by chemical reduction, such as sodium reduction of potassium fluorotan- talate. In this process, the potassium fluoromethalate is recovered from the processed ore in the form of a dry crystalline powder. The potassium fluorotantalate is melted and reduced to metallic tantalum powder by reduction with sodium. Then, the obtained tantalum powder is washed with water and leached in acid. Dry tantalum powder is then recovered, thermally agglomerated at temperatures up to approximately 1500 ° C and crushed to a granular consistency. Typically, the granular powder is then deoxidized in the presence of an absorber material that P1642 / -98MX at elevated temperatures of approximately 1000 ° C has greater affinity for oxygen than valve metal and is then leached in acid to remove residual metal contaminants and metal oxides. The powder is then dried, compressed to pellet, sintered to a porous body and subjected to a suitable electrolyte depreciation to form a continuous dielectric oxide film on the sintered body. Such deoxidation process is described by Kumar, in U.S. Patent No. 5,242,481. In an alternative method, the powder is produced by hydrugating a molten tantalum ingot, grinding the hydridered chips and dehydrating. In all cases it is possible and sometimes desirable to deoxidize the sintered anode pellet in a process similar to that described above for the powder. Metallic powders for valves, particularly tantalum, niobium powders and their alloys, suitable for the manufacture of capacitors must provide a suitable surface area when compressed and sintered. The ufV / g of the capacitor is proportional to the surface area of the sintered porous body. The greater the specific surface area after the sintering operation, the higher the ufV / g. The purity of the powder is also an important consideration in its use for manufacturing capacitors. Metal contamination P1642 / 98MX and non-metallic can degrade the dielectric oxide film in the capacitors. While high sintering temperatures can be used to remove some volatile contaminants, these temperatures also contract the porous body and its net specific surface area and, therefore, the capacitance of the resulting capacitor. Therefore, it is important to minimize the loss of the specific surface area under the sintering conditions. In the production of tantalum capacitors, for example, typically the tantalum powder is heated under vacuum to cause agglomeration thereof while avoiding the oxidation of the tantalum. However, following this treatment, the tantalum powder often picks up a considerable amount of additional oxygen because the initial surface layer of oxide enters into solution in the metal during heating and a new surface layer is formed in the subsequent exposure to air, whereby it is added to the total oxygen content of the powder. During the subsequent processing of these powders into anodes for capacitors, the dissolved oxygen can recrystallize as surface oxide and contribute to voltage discharges or short-circuit capacitor current leakage through the amorphous oxide dielectric layer.

P1642 / 98MX Just as capacitor technology is in continuous demand for metallic valve powders with larger surface areas, the requirements for oxygen management exceed the effectiveness of available oxygen control methods. Accordingly, the electrical properties of the capacitors could be improved if the oxygen content could be controlled, eg, if it is decreased or remains almost constant during the processing of the powders. A method for deoxidizing metal powders for valves, such as tantalum powder, consists of mixing alkaline earth metals, aluminum, yttrium, carbon and tantalum carbide with tantalum powder. However, alkaline earth metals, aluminum and yttrium form refractory oxides which have to be removed, for example by acid leaching, before the material can be used for the production of capacitors. Typically, post-deoxidation in acid leaching is performed using a strong mineral acid solution including, for example, hydrofluoric acid at elevated temperatures of up to 100 ° C to dissolve the refractory oxides contaminants. The carbon content must be controlled because it can also be harmful to capacitors even at levels as low as 50 ppm. Other methods have been proposed to prevent oxidation and Plß42 / 98MX provide low oxygen content, including that which uses a thiocyanate treatment or the use of reduced atmosphere from the beginning to the end of tantalum powder processing Another process to control the oxygen content in metallic materials For valves, such as tantalum, niobium and its alloys, it includes the use of absorbent materials. For example, Hard, in U.S. Patent No. 4,722,756, describes the heating of materials in an atmosphere containing hydrogen gas in the presence of a metal such as zirconium or titanium, which is more active with oxygen than tantalum or the niobium Another process for controlling the oxygen content in metallic valve materials is described by Fife, in U.S. Patent No. 4,964,906. This process involves heating a tantalum material in a hydrogen-containing atmosphere in the presence of a tantalum metal absorber having a lower oxygen concentration than that of the tantalum material. While this process provides some control of the oxygen content in metallic valve materials, there is a desire to improve the electrical properties of valve metal capacitors, particularly those made of tantalum, niobium and alloys of the same.

P1642 / 98MX, by means of the control, eg, decrease or almost constant maintenance, of the oxygen content of the metallic powders for valves. Accordingly, there is a demand for process improvements to reduce the oxygen content of those materials particularly after they have been subjected to a deoxidation process. In addition to problems with powders and capacitor applications, the high oxygen content in forged valve metal products can decrease the ductility of the products. It is therefore an object of the present invention to provide a method for controlling the oxygen content in metallic materials for valves. It is another object of the present invention to provide a method for controlling the oxygen content in metal powders for valves, such as tantalum, niobium and alloys thereof, useful for the manufacture of capacitors, particularly after the powders have been subjected to processes of deoxidation.

SUMMARY OF THE INVENTION Accordingly, the present invention is directed to a method for controlling the oxygen content in metal powders for valves, such as tantalum, niobium and P1642 / 98MX its alloys. The method includes the leaching of a metal material for deoxidized valve in an acid leaching solution at a temperature below room temperature. In a modality, the method to control the oxygen content in metallic materials for valve includes deoxidizing the metallic material for valve, preparing and cooling an acid leaching solution at a temperature lower than room temperature and leaching the deoxidized metal material in the leaching solution cold acid. It has been found that the method of the present invention decreases both the concentration of oxygen and that of fluorides in metallic valve materials, since the use of reduced acid leaching temperatures provides less oxygen for a given amount of leaching acid, like hydrofluoric acid. Another aspect of the present invention is directed to a method for producing a valve metal material, such as tantalum, niobium or alloys thereof, having a controlled oxygen content. The method includes making a metallic powder for valve and agglomerating it. The metallic powder for agglomerated valve is then deoxidized in the presence of an absorbing material having a greater affinity for oxygen than the valve metal. The material for deoxidized valve is P1642 / 98 X leaches later in an acid leaching solution at a temperature below room temperature to remove any contaminating absorbing material. In a further aspect of the invention, the metal powder for leached valve is washed and dried. The powder is then compressed to form a pellet which is sintered to form a porous body. The body is then anodized in an electrolyte to form a dielectric oxide film on the surface of the pellet. In another aspect of the present invention, a sintered body reacts with an absorbing (reactive) material, such as magnesium, which has a greater affinity for oxygen than the valve metal. The sintered body is then leached in an acid leaching solution at a temperature below room temperature and anodized in an electrolyte to form an oxide film.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a method for controlling, eg, decreasing or maintaining almost constant, the oxygen content in metallic materials for valves, such as tantalum, niobium and its alloys, which are useful in the production of capacitors, sintered anode bodies and forged products made of tantalum, niobium and alloys thereof. The method includes P1642 / 98MX leach a metal material for deoxidized valve in an acid leaching solution at a temperature below room temperature. As it is observed, the metallic powders for valve capacitor grade can be produced by several methods, among which is the chemical reduction of its ores or the fusion of a metal ingot for valve by electronic beam or vacuum arc. In the chemical reduction of a metallic powder for valve, such as tantalum powder, the potassium fluorotantalate is recovered, melted and reduced to tantalum metal powder by reduction with sodium. Then, the dry tantalum powder is recovered, thermally agglomerated under vacuum to avoid oxidation of the tantalum and crushed. Because the concentration of oxygen in the metal valve material is critical in the production of capacitors, the granular powder is deoxidized at temperatures of up to approximately 1000 ° C in the presence of an absorbing material, such as magnesium, which have a higher affinity for the oxygen that the valve material. The powder is then leached into acid to remove contaminants, including magnesium and magnesium oxide, before the material is used to produce cpacitores. Typically, acid leaching is carried out using a strong mineral acid solution between which are included, P1S42 / 98MX for example, hydrofluoric acid, nitric acid, sulfuric acid, hydrochloric acid and the like, at elevated temperatures up to 100 ° C to dissolve any metal and metal oxide contaminants. Preferably, nitric acid and / or hydrofluoric acid are used in the leach solution due to their ability to dissolve most metal and metal oxide contaminants, as well as metal fines for valves. The powder is then washed and dried, compressed to form a pellet, sintered to form a porous body and anodized in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body. In some cases, prior to anodization, the sintered body is deoxidized with magnesium in a process similar to the treatment of dust. The process of deoxidation typically follows a process of leaching with mineral acid to dissolve any contaminant. In addition, it is further recognized that the acid solution including hydrofluoric acid can also lower the concentration of oxygen by dissolving the very small (fine) particles of the valve metals. However, the use of hydrofluoric acid can provide an undesirable increase in the concentration of fluorides in the resulting particles and, consequently, undesirable corrosion in the equipment of the P1642 / 98MX process. Typically, the mineral acid solution contains less than 10% by weight of hydrofluoric acid. Preferably, less than 5% by weight of hydrofluoric acid is used in the acid leach solution to dissolve residual metal and metal oxide contaminants while minimizing the concentration of fluorides; with superlative preference, less than 1% by weight of hydrofluoric acid is used. However, it is observed that it is also desirable, a leaching solution that does not contain hydrofluoric acid, to eliminate the contamination by fluorides, is effective at decreasing the concentration of oxygen in the metal particles for valve, supplying the solution by dissolution of pollutants and fines. Traditionally, elevated temperatures (above ambient temperature of about 100 ° C) are used during post-deoxidation to acid leaching to increase the activity of the acid solution to dissolve any residual metal contaminants and metal oxides, such as magnesium and oxide. of magnesium, on the metallic material for valve. The high temperature in the post-deoxidation to the acid leaching also records the metal particles for valve and the surface area is increased, resulting in an undesirable increase of the oxygen concentration in the subsequent exposure to the atmosphere. As a result, you may need a P1642 / 98MX additional processing to control the oxygen concentration in metallic materials for valve to ensure its convenience for capacitors and related applications. However, the process of the present invention performs post-deoxidation to acid leaching at temperatures below room temperature to minimize the leaching effect on the surface area of the particles, e.g., are removed Residual metal and metal oxide contaminants while controlling the damaging etching and increasing the oxygen concentration of metallic valve materials. As is known to those skilled in the art, "room temperature" generally means an indoor temperature between about 20 ° C and 25 ° C (between about 68 ° F and 77 ° F). Because the chemical reactions during acid leaching are exothermic, the initial leaching temperature is often the lowest temperature in the process, this can be measured before or after the addition of the valve metal or during acid leaching. More typically, the leach temperature is the temperature of the acid leach solution prior to the addition of the valve metal material. In the case of the present examples (described below) the temperature of P1642 / 98MX acid leaching is defined as the temperature of the acid leaching solution prior to the addition of the metal material for deoxidized valve. It should be understood that the decrease in temperature at the start of the acid leaching process results in a temperature throughout the process that is generally lower than that which would be measured if the solution were at room temperature or higher before the addition of the metallic material for valve. For large-scale leaching, where large amounts of heat energy will be released, active cooling must be used to extract heat. In small-scale acid leaching, reagents (leach solution and / or metal valve material) can be cooled before mixing to effectively remove heat. The acid leaching solution is prepared and cooled using techniques known to those skilled in the art. For example, the acid solution and / or the metal valve material can be precooled, as can the acid leach vessel and / or ice added to the leach solution after it is added to the leach vessel . It has been found that acid leaching solutions at temperatures substantially below room temperature are more effective in removing metal contaminants Residual P1642 / 98MX and metal oxides while also controlling the resulting oxygen concentration in metallic materials for valve. The preferred temperature in the acid leach solution is below about 20 ° C; preferably superlative, below approximately 0 ° C, to effectively remove the heat of reaction between the acid leach solution and the residual metal contaminants and metal oxides and decrease the effect of the leaching solution on the surface of the metal material for valve . Although the process of the present invention is effective in controlling undesirable concentrations of oxygen, it is observed that a minimum concentration of oxygen will remain in the valve metal particles during normal processing due to its high affinity for oxygen. This level will typically be sufficient to passivate the surface of the particle. In the production of metal powders for capacitor grade valve, lower oxygen levels are preferred in the valve metal particles. For example, tantalum powders for use in capacitors preferably have less than 3000 ppm and more preferably less than 2400 ppm oxygen. It has been found that similar oxygen levels are acceptable in sintered tantalum electrode bodies. The present invention will be further illustrated with the P1S42 / 98M-X following examples which are intended to be illustrative in nature and not to be construed as limiting the scope of the invention.

EXAMPLE I Variations were evaluated in the concentration of hydrofluoric acid (HF), in the concentration of nitric acid HN03 and in the post-deoxidation temperature to the acid leaching of a tantalum powder. The concentration of HF (ml / lb of leached tantalum powder), the temperature (° C) and the concentration of HNO3 (% by weight) were varied to determine the optimal leaching conditions. These factors were varied using tantalum grade C255 powder, available from Cabot Performance Materials Division of Cabot Corporation, Boyertown, PA. Grade C255 tantalum powder is a medium to high voltage flake powder for use at 15,000-18,000 hp / g. The tantalum powder was first prepared by cooling a 600 milliliter plastic leach vessel by placing a stainless steel tray containing an ice bath and coarse salt therein. Approximately 250 milliliters of deionized water was added to the leach vessel. Then 125 milliliters of HN03 grade were added slowly with stirring.

P1642 / 98MX reagent of approximate concentration between 68% to 70% to the leach vessel. To mix the liquids, a 2-inch diameter propeller-type stirrer with plastic cover was used at approximately 425 rpm. The temperature of HN03 / water was reduced and maintained at a temperature of approximately 20 ° C. After the desired temperature was reached, about 1 pound grade C255 flake tantalum powder was added to the leach vessel with agitation. Prior to its addition to the leach vessel, the tantalum powder was subjected to a deoxidation process with magnesium. After the addition of tantalum, approximately 5 milliliters of HF reagent grade, with a concentration between 48% and 51% approximately was slowly added with agitation to the leach vessel. After the addition of HF, the contents of the leach vessel were mixed approximately 30 minutes. After the tantalum powder was leached for about 30 minutes, the stirrer was turned off and the temperature that was measured was about 5 ° C. The tantalum powder was allowed to settle and the acid was decanted. The tantalum powder was transferred to a 4,000 milliliter plastic container and washed using deionized water at room temperature. The tantalum powder was allowed to settle and the wash water was decanted. The step of P1642 / 98MX washing was repeated until the conductivity of the decanted washing water was less than 10 μM Mohs / cm. The conductivity of the water was measured using a Cole-Parmer Model 1500-00 conductometer. After the desired conductivity in the water was reached, the tantalum solution was filtered using a Buchner funnel, filter paper and vacuum pump. The wet tantalum powder was recovered and transferred to a stainless steel tray. The powder was dried in a vacuum oven at approximately 180 ° F (approximately 82 ° C) for approximately 6 hours. The dry tantalum powder was sifted through the 50 mesh and analyzed. The aforementioned process was repeated using portions of the same batch of the deoxidized tantalum powder mixture, varying the concentration of HF, the leaching temperature (defined as the temperature of the HN03 / water solution before the addition of tantalum) and the HN03 concentration, to determine the optimal leaching conditions to control the oxygen content in the tantalum powder. The ranges of each variable (including HF, HN03 and leaching temperature) and the experimental results (fluorine and oxygen concentration and BET surface area determined using the ASTM D4567 method of continuous N2 flow) are listed in the Table 1.

P1642 / 98MX TABLE 1 As reported in Table 1, a reduced acid leaching temperature results in a controlled oxygen content in the final tantalum powder. Samples 1 to 4 were evaluated at an acid leaching temperature of 20 ° C, the HF content varying between 1 and 5 milliliters of HF per pound of tantalum (Samples 1 and 2 and samples 3 and 4, respectively) and adjusting the HN03 concentration between 23.0 and 70.0 percent by weight, between samples. As expected, from Samples 1 to 4, a lower oxygen content was measured in Samples 3 and 4 due to the additional content HF P1642 / 98MX, which dissolved the smaller (fine) tantalum particles of the tantalum material. It is noted that in each of Samples 1 to 4, using a reduced acid leaching temperature, the oxygen contents were controlled to acceptable levels (less than 2100 ppm of oxygen approximately). Materials produced with minor additions of HF are preferable. The adjustment of the HN03 concentration (between Samples 1 and 2 and 3 and 4) seems to have only a minimal effect on the oxygen content of the final tantalum powder. Samples from 5 to 8 were evaluated at a leaching temperature of 80 ° C, the HF content varying between 1 and 5 milliliters of HF per pound of tantalum (Samples 5 and 6, and Samples 7 and 8 respectively) and adjusting the concentration of HN03 between 23.0 and 70.0 percent by weight, between samples. Each of these samples exceeded an oxygen range of approximately 2400 ppm. However, a lower oxygen content was measured with Samples of tantalum material 5 and 6 that used a lower HF content, because at high temperature, the increase in surface area due to HF etching predominates over the elimination of the very small particles. The total results indicate that the fluoride level of the final powder is determined by the amount of HF P1642 / 98MX used in acid leaching. Moreover, as expected, the surface areas of the particles are proportional to the oxygen content of the final powder. Therefore, it is seen that the use of low leaching temperatures is important to decrease both oxygen and fluoride, because the low temperature provides less oxygen for a given amount of HF and the lowest possible amount of HF is needed. HF to control the fluoride content in the final powder.

EXAMPLE II Variations in the concentration of hydrofluoric acid (HF) (ml / lb of leached tantalum powder) and in the post-deoxidation temperature to the acid leaching of the tantalum powder were evaluated to determine the optimal leaching conditions to control the oxygen content of the powder. These factors were varied using grade C515 tantalum powder, available from Cabot Performance Materials Division of Cabot Corporation, Boyertown, PA. C515 grade tantalum powder is a nodular powder of low to medium voltage for use from 35,000 to 45,000 CV / g. The tantalum powder was prepared by premixing 1 liter of reactive grade HN03 in a container with a P16it2 / 98MX concentration between approximately 68% and 70% and around 2 liters of deionized water. The temperature of the HN03 / water solution was lowered by placing the container in a foam bed containing an ice bath and coarse salt. A steel barrel with baffles, covered with plastic, with an approximate volume of 100 liters, was used as a leach vessel, which was previously cooled by adding in this approximately between 8 and 10 pounds of ice and enough deionized water to cover it. The vessel was rotated for approximately 10 minutes, the ice / water mixture was emptied and the vessel was rinsed with deionized water. The temperature of the HN03 / water solution was then measured with a thermocouple and was about 0 ° F (about -16 ° C). The HN03 / water solution was poured into the previously cooled leach vessel and about 5 pounds of C515 nodular grade tantalum powder were added with agitation. Prior to its addition to the leach vessel, the tantalum powder was subjected to a deoxidation process with magnesium and sieved with approximately 50 mesh to remove any chunk. After the addition of tantalum, HF reagent grade, with a concentration between approximately 49% and 51%, was added to the leach vessel slowly with stirring. After the addition of HF the contents of the leach vessel were mixed P1642 / 98MX for approximately 30 minutes. After the tantalum powder was leached for approximately 30 minutes, the agitator was turned off. The tantalum powder was allowed to settle approximately 10 minutes after deionized water was added and the acid / water solution was decanted. The tantalum powder was washed using deionized water at room temperature and a rotation period of 2 minutes. The tantalum powder was allowed to settle and the wash water was decanted. The washing step was repeated until the conductivity of the decanted washing water was less than 10 μM Mohs / cm. The conductivity of the water was measured using a Cole-Parmer Model 1500-00 conductometer. After the desired water conductivity was reached, it was decanted and the tantalum powder was filtered. The wet tantalum powder was recovered and transferred to a stainless steel tray. The powder was then dried in a vacuum oven at approximately 180 ° F (approximately 82 ° C) for approximately 6 hours. The aforementioned process was repeated, varying the concentration of HF, the leaching temperature (defined as the temperature of the HN03 / water solution before the addition of tantalum) and the concentration of HN03, to determine the optimal leaching conditions to control the oxygen content in the powder P1642 / 98MX tantalum. The ranges of each variable (including HF and leaching temperature) and the experimental results (fluorine and oxygen concentration and BET surface area determined using the ASTM D4567 method of continuous N2 flow) are listed in Table 2.

TABLE 2 As reported in Table 2, a reduced acid leaching temperature results in a controlled oxygen content in the final tantalum powder. Samples 1 and 2 were evaluated using 1 and 5 milliliters of HF per pound of tantalum, respectively, at temperatures and acid leaching of -12 ° C and -16 ° C. As expected with Sample 2 of the tantalum material, a lower oxygen content was measured due to the additional content of HF, which dissolved small tantalum particles P1642 / 98MX additional. However, as a result of the additional HF, the fluoride content in Sample 2 of tantalum material is higher. Because the oxygen content in Samples 1 and 2 is controlled, the material produced with reduced HF content (Sample 1) is preferred due to the resulting lower fluoride contents. Even though the lowest oxygen content was measured in Sample 4 of tantalum material, this is a result of the high level of HF in the leaching solution and, consequently, of the reduced surface area. An undesirably high level of fluoride was also measured in Sample 4. As noted above, it is known that elevated temperatures increase the activity of the acid solution for dissolving contaminants in metallic valve materials. However, the combination of the increased HF content at elevated temperature in Sample 4 resulted in a reduced surface area. A smaller amount of HF in the acid leaching solution at a high temperature in Sample 3 resulted in an increase in surface area, because the surface of the particles was etched and did not dissolve. This increase in surface area resulted in an oxygen content greater than 2700 ppm. The above mentioned results P1642 / 98MX also confirm that the level of fluoride is determined by the amount of HF used in the leaching solution. The same amount of HF (1 ml / lb Ta) was used in Samples 1 and 3 and in Samples 2 and 4, varying the leaching temperature. As reported, while the reduced temperatures decreased the oxygen content to acceptable levels, the fluoride content was only marginally reduced. However, it was observed that by varying the HF content as in Samples 1 and 2 and Samples 3 and 4 (1 and 5 ml / lb Ta, respectively) and using reduced temperatures for Samples 1 and 2 and elevated temperatures for In Samples 3 and 4, higher levels of fluoride were obtained in Samples 2 and 4, which used higher levels of HF in acid leaching solution. Therefore, it is observed that the use of reduced acid leaching temperatures is important to decrease both oxygen and fluoride, because the low temperatures supply less oxygen for a given amount of HF and the lowest possible amount of HF is needed. HF to control the fluoride content in the final powder.

EXAMPLE III Variations in the temperature of P1642 / 98MX post-deoxidation to the acid leaching of niobium powder to determine the optimal leaching conditions to control the oxygen content of the powder. Acid leaching temperature was varied using grade WCb-C deoxidized niobium powder, available from Cabot Performance Materials Division of Cabot Corporation, Boyertown, PA. Niobium grade grade WCb-C powder is a low surface area powder derived from ingot. The niobium grade WCb-C powder was first deoxidized by mixing 1 kilogram of sample with 0.4% magnesium in a tantalum tray. The tray was covered, placed in a retort and heated in an oven at a temperature of 750 ° C under an argon atmosphere, for approximately 1 hour. After this period, a vacuum was applied to the retort and the argon was eliminated and a final pressure of less than 400 microns was reached, which was maintained for approximately 1 hour. The retort was cooled to a temperature below about 200 ° C and removed from the oven. After the system was cooled to a temperature below 40 ° C, passively adding air before opening the retort and removing the niobium powder. The resulting deoxidized niobium powder had an oxygen content of 1767 ppm. The deoxidized niobium powder was then treated at three different acid leaching temperatures for P1642 / 98MX determine the effectiveness of the acid leaching temperature in the control of the oxygen content in the powder. The acid leach solution was prepared by premixing about 55 milliliters of reactive grade HN03 at a concentration of approximately 68% and approximately 110 milliliters of deionized water (resulting in 165 ml of HN03 solution) in a 250 milliliter plastic container. Approximately 100 grams of WbC-C grade deoxidized niobium powder was added with agitation to the leach vessel. After addition of the niobium powder, approximately 0.9 milliliters of reactive HF, with an approximate concentration of 49%, were then added to the leach vessel slowly and with stirring. After the addition, the contents of the leach vessel were mixed for about 30 minutes. After the niobium powder was leached for 30 minutes, the agitator was turned off. The niobium powder was allowed to settle for 10 minutes after adding approximately deionized water and the acid / water mixture was decanted. The niobium powder was then washed using deionized water at room temperature, allowed to settle and the wash water was decanted. The washing step was repeated until the conductivity of the decanted washing water was less than 10μMohs / cm.

P1642 / 98MX After the desired water conductivity was reached, the water was decanted and the niobium powder was filtered. The wet niobium powder was recovered and dried in a vacuum oven at approximately 85 ° C. The aforementioned process was repeated by varying the leaching temperature (defined as the temperature of the HN03 / water solution before the addition of niobium powder) to determine the optimum leaching temperature to control the oxygen content in the niobium powder. The niobium powder was added to the 23% HN03 solution at temperatures of 30 ° C, 3 ° C and 55 ° C. The acid leaching solution at 3 ° C was prepared by cooling the 23% HN03 solution in an ice / salt bath; the 55 ° C using hot deionized water (approximately 60 ° C) to form the acid / water leaching solution and using a hot water bath (between 45 ° C and 50 ° C approximately) to keep the temperature high. The experimental results (oxygen concentration) are listed below in Table 3.

P1S42 / 98MX TABLE 3 As reported in Table 3, a cold acid leaching solution results in a reduced oxygen content in the final niobium powder. The powder that was leached in the acid leach solution at 3 ° C had an average oxygen content of 473 ppm, which was approximately 100 ppm lower than the powder that was leached in the acid leaching solution at 30 ° C. The powder that was leached in the hottest acid solution (approximately 55 ° C) had an average oxygen content of 900 ppm, which is 330 ppm higher than the powder that was leached almost at room temperature and almost twice the content of oxygen from the powder that was leached in the coldest leaching solution. Therefore, the use P1642 / 98MX of reduced acid leaching temperatures is important to control (decrease) the oxygen content in metallic valve materials such as niobium powder.

EXAMPLE IV Variations in the acid leaching temperature of a non-deoxidized tantalum powder were evaluated to determine the optimum leaching conditions for the control of the oxygen content of the powder. The acid leach temperature was varied using a non-deoxidized tantalum powder grade C275, available from Cabot Performance Materials Division of Cabot Corporation, Boyertown, PA. The non-deoxidized tantalum powder had an oxygen content of 8913 ppm. The acid leach solution was prepared by pre-mixing approximately 33 milliliters of reactive grade HN03 at a concentration of approximately 68% and approximately 66 milliliters of deionized water (resulting in 99 ml of a 23% HN03 solution) in a 250 milliliter plastic container. A cold leaching solution (approximately -3 ° C) was prepared by cooling the 23% HN03 solution in an ice / salt bath. Approximately 120 grams of grade C275 non-deoxidized tantalum powder were added with agitation to the P1642 / 98MX leaching vessel. After the addition of the tantalum powder, approximately 0.3 milliliters of HF reagent grade with a concentration of approximately 49% was then added to the leach vessel slowly and with stirring. After addition of HF the contents of the leach vessel were mixed approximately for 30 minutes. A second leaching solution (approximately 37 ° C) prepared using warm deionized water was also evaluated in the treatment of approximately 120 grams of non-deoxidized tantalum powder, as described above. After the tantalum powder was leached for approximately 30 minutes, the agitator was turned off. After adding deionized water the tantalum powder was allowed to settle for about 10 minutes and the acid / water solution was decanted. The tantalum powder was then washed using deionized water at room temperature. It was allowed to settle and the washing water was decanted. The washing step was repeated until the conductivity of the decanted washing water was less than 10 μM / cm. After the desired water conductivity was reached, the water was decanted and the tantalum powder was filtered. The wet tantalum powder was recovered and dried in a vacuum oven at about 85 ° C. The process P1642 / 98MX mentioned above was repeated for each leaching solution, to determine the optimum leaching temperature to control the oxygen content in a non-deoxidized tantalum powder. The experimental results (oxygen concentration) are listed below in Table 4.

TABLE 4 As reported in Table 4, neither acid cold or hot leaching significantly decreases the oxygen content of the non-deoxidized tantalum powder. The powder that was leached in the reduced temperature acid leaching solution had an average oxygen content of 8748 ppm and the powder that was leached in the milder solution had an average oxygen content of 8797 ppm. As noted above, the content P1S42 / 98MX oxygen from the initial non-deoxidized tantalum powder was 8913 ppm. Therefore, the use of reduced acid leaching temperatures appears to be ineffective in controlling (decreasing) the oxygen content in metallic valve materials such as tantalum powder.

EXAMPLE V Variations in the acid leaching temperature of a sintered tantalum anode were evaluated to determine the optimal leaching conditions to control the oxygen content of the anode. Acid leaching temperature was varied using sintered anodes made of finished HPllO tantalum, available from Cabot Performance Materials Division of Cabot Corporation, Boyertown, PA. The anodes weighed 476 grams each, with a pressure density of 5.0 g / cc and sintered at 1570 ° C for 30 minutes. The anodes were cut into small pieces before leaching. The acid leaching solution was prepared by premixing approximately 10 milliliters of reactive grade HN03 at a concentration of approximately 68% and approximately 20 milliliters of deionized water (resulting in 30 ml of HN0323% solution) in a 100-milliliter plastic container. A cold leaching solution (approximately -3 ° C) was prepared by cooling P1S42 / 98MX the 23% HN03 solution in an ice / salt bath. Approximately 3.5 grams of tantalum anode pieces were added with agitation to the leach vessel. After the addition of the tantalum anode, then approximately 0.05 milliliters of HF reagent grade with a concentration of approximately 49% was added to the leach vessel slowly and with stirring. After the addition of HF, the contents of the leach vessel were mixed for about 30 minutes. A second leaching solution (approximately at 42 ° C) prepared using deionized water was also evaluated in the treatment of approximately 3.5 grams of the tantalum anode parts, as described above. After the tantalum anode pieces were leached for approximately 30 minutes, the agitator was turned off. The pieces of tantalum anode were allowed to settle for approximately 10 minutes after deionized water was added, the water / acid solution was decanted. The tantalum anode pieces were then washed using deionized water at room temperature. The pieces of tantalum anode were allowed to settle and the washing water was decanted. The washing step was repeated until the conductivity of the decanted washing water was less than 10 μMhos / cm.

P1642 / 98MX After the desired water conductivity was reached, the water was decanted and the tantalum anode pieces were recovered and dried in a vacuum oven at about 85 ° C. The aforementioned process was repeated for each leaching solution to determine the optimum leaching temperatures to control the oxygen content in sintered tantalum anodes. The experimental results (oxygen concentration) are listed below in Table 5.

P1642 / 98MX TABLE 5 As reported in Table 5, neither acid cold nor hot leaching significantly decreases the oxygen content of the sintered tantalum anode parts. The pieces of sintered tantalum anode that were leached in the reduced temperature leaching solution had an oxygen content P1642 / 98MX average of 2476 ppm and the dust that was leached in the hotter leach solution had an average oxygen content of 2441 ppm. The average oxygen content of the sintered tantalum anode pieces was 2517 ppm. The use of reduced temperatures in acid leaching solutions, therefore, appears to be ineffective in controlling (decreasing) the oxygen content of metallic valve materials, such as tantalum anodes, relative to warm leaching solutions.

EXAMPLE VI Variations in the acid leaching temperature of a niobium powder derived from an ingot were evaluated, to determine the optimal leaching conditions to control the oxygen content of the powder. Acid leaching temperature was varied using non-deoxidized niobium powder derived from a WCb-C ingot, available from Cabot Performance Materials Division of Cabot Corporation, Boyertown, PA. The powder was produced by hydruration and crushing a niobium ingot. The powder was then degassed in a vacuum oven. The acid leaching solution was prepared by premixing first 55 milliliters of reactive grade HN03 P1S42 / 98MX with an approximate concentration of 68% and approximately 110 milliliters of deionized water (resulting in 165 ml of 23% HN03 solution) in a 250-milliliter plastic container. A cold leach solution (approximately 0 ° C) was prepared by cooling the 23% HN03 solution in an ice / salt bath. Approximately 200 grams of niobium powder were added with agitation to the leach vessel. After addition of the niobium powder, approximately 0.5 milliliters of HF reagent grade with an approximate concentration of 49% was added slowly and with agitation to the leach vessel. After the addition of HF, the contents of the leach vessel were mixed for approximately 30 minutes. A second leaching solution (approximately 38 ° C) prepared using warm deionized water was also evaluated in the treatment of approximately 200 grams of niobium powder, as described above. After the niobium powder was leached for approximately 30 minutes, the agitator was turned off. The niobium powder was allowed to settle for approximately 10 minutes after adding more deionized water and the acid / water solution was decanted. The niobium powder was then washed using deionized water at room temperature. The niobium powder was allowed to settle and the wash water was decanted. The washing step was repeated until the P1642 / 98 X Conductivity of the decanted washing water was less than 10 μM / cm. After the desired water conductivity was reached, the water was decanted and the niobium powder was filtered. The wet niobium powder was recovered and dried in a vacuum oven at about 85 ° C. The aforementioned process was repeated for each leaching solution to determine the optimum leaching temperatures to control the oxygen content in a niobium powder derived from an ingot. The experimental results (oxygen concentration) are listed below in Table 6.

TABLE 6 P1642 / 98MX As reported in Table 6, cold acid leaching did not significantly decrease the oxygen content of ingot-derived niobium powder relative to hot acid leaching. The powder that was leached in the acid leach solution at reduced temperature had an average oxygen content of 1899 ppm and the powder that was leached in the hotter leaching solution had an average oxygen content of 1756 ppm. The average oxygen content of the ingot-derived niobium powder was 2410 ppm. Therefore, the use of reduced temperatures in the acid leach solution seems to be inefficient to control (decrease) the oxygen content of non-deoxidized metallic materials derived from valve ingots, such as niobium powder, in relation to the solutions of acid leaching more hot. Although particular embodiments of the invention have been described in detail for purposes of illustration, various changes and modifications may be made without departing from the scope and spirit of the invention. For example, the process of the present invention can also be used to control the oxygen content of forged valve metal products. Accordingly, the invention is limited only by the appended claims.

P1642 / 98MX

Claims (27)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS is claimed as property: 1. A method for controlling the oxygen content in metallic materials for valves comprising: leaching a metal material for deoxidized valve into an acid leaching solution at a temperature below room temperature.
2. 2. The method of claim 1, wherein said acid leaching solution is cooled to below room temperature prior to leaching said deoxidized valve metal material. The method of claim 1, wherein said valve metal is selected from the group comprising tantalum, niobium and alloys thereof. 4. The method of claim 3, wherein said valve metal is tantalum. The method of claim 3, wherein said valve metal is niobium. The method of claim 1, wherein said valve metal material is selected from the group comprising nodular powders, flake powders, ingot-derived powders and sintered bodies. P1642 / 98MX 7. The method of claim 1, wherein said temperature of the acid leach solution is less than about 20 ° C. 8. The method of claim 7, wherein said temperature of the leaching solution is lower than 0 ° C approximately. The method of claim 1, wherein said acid leaching solution contains a mineral acid. The method of claim 9, wherein said acid leaching solution contains less than 10% by weight of hydrofluoric acid. 11. A method for producing a metallic valve material having a controlled oxygen content, comprising the steps of: making a metallic powder for valve; agglomerating said metallic powder for valve; deoxidizing said metal powder for agglomerated valve in the presence of an absorbing material having greater affinity for oxygen than said metallic powder for valve, and leaching said metallic powder for valve in an acid leaching solution at a temperature lower than room temperature to remove contaminants of absorbing materials. P1642 / 98MX 12. The method of claim 11, wherein said acid leaching solution is cooled to below room temperature before leaching said deoxidized valve metal powder. The method of claim 11, wherein said valve metal is selected from the group comprising tantalum, niobium and alloys thereof. The method of claim 13, wherein said valve metal is tantalum. 15. The method of claim 13, wherein said valve metal is niobium. 16. The method of claim 11, wherein said metal powder for valve is thermally agglomerated under vacuum. The method of claim 11, wherein said metal powder for valve is deoxidized at temperatures up to about 1000 ° C in the presence of a magnesium-containing absorbing material. 18. The method of claim 11, wherein said acid leach solution contains a mineral acid. The method of claim 18, wherein said acid leach solution contains less than 10% by weight of hydrofluoric acid. 20. The method of claim 11, P1642 / 98MX additionally comprises the steps of: washing and drying said metal powder for acid leached valve; compressing said powder to form a pellet; sintering said pellet to form a porous body; and anodizing said porous body in an electrolyte to form an oxide film on said porous body. The method of claim 20, further comprising the steps of: deoxidizing said porous body in the presence of an absorbing material having greater affinity for oxygen than said metal for valve; and leaching said sintered porous body in an acid leaching solution at a temperature below room temperature to remove contaminants from absorbing material before anodizing said porous body. 22. The method of claim 11, wherein said temperature of the acid leach solution is less than about 20 ° C. 23. The method of claim 22, wherein said temperature of the acid leach solution is less than about 0 ° C. 24. A method to produce a metal anode P1S42 / 98MX for a valve having a controlled oxygen content, comprising the steps of: compressing a metallic powder for a valve to form a pellet; sintering said pellet to form a porous body; deoxidizing said sintered porous body in an acid leaching solution at a temperature below room temperature to remove contaminants from absorbing material before anodizing said porous body; and anodizing said porous body in an electrolyte to form a continuous dielectric oxide film on the surface of said porous body. 25. The method of claim 24, wherein said temperature of the acid leach solution is less than about 20 ° C. 26. The method of claim 24, wherein said temperature of the acid leach solution is less than 0 ° C. 27. A method for controlling the oxygen content in metallic materials for valves, comprising: leaching a metal material for deoxidized valve into an acid leaching solution wherein the P1S42 / 98MX temperature of the acid leach solution, before the addition of the metallic material for valve or at the beginning of the leaching process, is lower than the ambient temperature. P1642 / 98MX