PROCESS FOR PRODUCING NIOBIUM METAL OF AN ULTRAHIGH PURITY

Abstract: A process for producing niobium metal of an ultrahigh purity, which comprises iodizing niobium metal or niobium chloride containing at least tantalum as an impurity, thermal­ ly reducing the iodized product, and then thermally decom­ posing the reduced product.

Description: The present invention relates to a process for producing niobium metal of an ultrahigh purity. More particularly, it relates to a process for producing niobium metal of an ultrahigh purity useful for the production of electronic materials, particularly super conductive thin films. Heretofore, a purity of 99.9% has been the upper limit for the purity of so-called high purity niobium metal. No process has been known which is capable of efficiently producing niobium metal having an ultrahigh purity of at least 99.99%. For a process for producing niobium metal by the thermal decomposition of a metal iodide, there has been known a closed system method wherein the iodization of niobium metal and the thermal decomposition of the iodized product are conducted in the same closed container to precipitate the metal on a heated wire, or a flow method in which niobium iodide is introduced into a decomposition chamber, whereupon the metal is precipitated on a heated wire. This flow method has an advantage that the iodide can be purified prior to the thermal decomposition. However, both of the above methods have problems such that the decomposition rate of the iodide is very slow (0.01 - 0.02 g/cm².h), and the decomposition temperature is required to be as high as at least 1000 o C, whereby it is hardly possible to avoid the reaction of the precipitated metal with the material constituting the container. Further, it has been reported that in the case of titanium metal, the decomposition rate can be improved by high-frequency heating of the metal in the form of a rod under reduced pressure so that a gaseous iodide is thermally decomposed (Research Report No. 3, 1982, Kinzoku Zairyo Gijutsu Kenkyusho, p 292 - 302). However, it is difficult to obtain niobium metal of an ultrahigh purity by this method. Further, the decomposition rate is not yet satisfactory, and there still remains a problem that the productivity is poor. It is an object of the present invention to produce niobium metal of an ultrahigh purity which could not be obtained by the conventional methods. Namely, it is an object of the present invention to provide niobium metal having a purity of at least 99.99% with high production efficiency. The present invention provides a process for producing niobium metal of an ultrahigh purity, which comprises iodizing niobium metal or niobium chloride containing at least tantalum as an impurity, thermally reducing the iodized product, and then thermally decomposing the reduced product. Now, the present invention will be described in detail with reference to the preferred embodiments. In the accompanying drawings, Figure 1 illustrates an apparatus for continuous iodization useful for the iodization reaction of the present invention. Figure 2 illustrates an apparatus for the thermal reduction. Figure 3 illustrates an apparatus for the thermal decomposition. The process steps of the present invention may be represented by the following reaction formulas. (1) Iodization step Nb(Ta) + 5/2I₂→Nb(Ta)I₅ or Nb(Ta)Cl₅ + 5HI→Nb(Ta)I₅ + 5HCl (2) Thermal reduction step Nb(Ta)I₅→NbI₃↓(TaI₅↑) (3) Second iodization step NbI₃ + I₂→NbI₅ (4) Thermal decomposition step NbI₅→Nb + 5/2I₂ or NbI₃→Nb + 3/2I₂ Now, the present invention will be described step by step in further detail. (1) Iodization step Niobium metal used as the starting material in the present invention, hereinafter referred to as "crude niobium metal") contains at least tantalum, and it further contains trace amounts of other components such as iron, aluminum, silica, tungsten, zirconium, nickel, chromium, cobalt, thorium and sodium. In addition to the crude niobium metal, niobium chloride may be employed for the iodization. The iodization reaction may be conducted either in a batch system or in a continuous system. However, the continuous system is preferred from the viewpoint of the productivity and economy. The iodization proceeds at a high rate at a temperature of 300 o C or higher. Therefore, the reaction temperature is not critical so long as it is at least 300 o C. However, it is usual to employ a reaction temperature of from 400 to 600 o C. After the completion of the reaction, the iodide is purified by distillation and recovered as a high purity iodide, which is then supplied to the subsequent step of the thermal reduction. In the distillation step, niobium iodide is separated from iodides of the trace amount impurities by the difference in the precipitation temperatures, whereby the trace amount impurities will be reduced to a level of about 1/10. (2) Thermal reduction step The thermal reduction treatment of the iodide is conducted in an inert gas atmosphere or in a hydrogen gas atmosphere or under reduced pressure at a temperature of from 200 to 600 o C, preferably from 250 to 450 o C. Namely, the iodide is introduced into the container and heated under reduced pressure or by using, as a carrier gas, an inert gas such as argon, helium or nitrogen, or a hydrogen gas. With respect to the separation of niobium from the impurities like tantalum, in the case of an inert gas atmosphere, the higher niobium iodide (NbI₄₋₅) starts to undergo a conversion to a lower homologue by the liberation of iodine at a temperature of about 200 o C, and starts to form the lower niobium iodide (NbI₃) at a temperature of from about 300 to about 350 o C, while the higher tantalum iodide (TaI₄₋₅) does not undergo a conversion to a lower homologue, whereby due to the substantial difference in the vapour pressures between the lower niobium iodide and the higher tantalum iodide, the impurities like tantalum will be removed from niobium. At a temperature of higher than 600 o C, the lower niobium iodide starts to vapourize, and it is not preferable to employ such a high temperature for the reduction according to the present invention. In the case where the thermal reduction is conducted in a hydrogen gas atmosphere, the lowering phenomenon of the niobium iodide starts to proceed at a temperature of 100 o C, and the lower niobium iodide starts to form at a temperature of from about 250 to about 300 o C. Namely, the stebilization temperature of the lower niobium iodide is lower by about 50 o C than in the case where the inert gas is used. Whereas, the thermal behavior of the higher tantalum iodide does not substantially change. Therefore, the difference in the vapour pressures between the lower niobium iodide and the higher tantalum iodide increases, whereby the yield of the niobium iodide will be improved. There is no particular restriction as to the temperature raising rate. However, it is usual to employ a rate of about 500 o C/min taking into the yield and the purification efficiency into consideration. In this step, the impurities like tantalum contained in the niobium iodide will be reduced to a level of from 1/10 to 1/100, whereby the lower niobium iodide having a high purity will be recovered. (3) Second iodization step This step is not an essential step in the present invention. However, this step is one of the useful steps to obtain niobium metal having a higher purity. This step is conducted substantially in the same manner as the iodization step for niobium metal as described above. (4) Thermal decomposition step This step is one of the important steps to obtain niobium metal of an ultrahigh purity in the present invention. Namely, this step is a step wherein the lower niobium iodide (NbI₃) or the higher niobium iodide (NbI₄₋₅) is thermally decomposed to obtain niobium metal having an ultrahigh purity. The thermal decomposition temperature is usually at least 700 o C, preferably at least 800 o C. There is no particular restriction as to the pressure, but it is usual to employ a pressure of not higher than 13.3 mbar (10 Torr) taking the decomposition efficiency and the purification efficiency into consideration. There is no particular restriction as to the heat source, which may be high-frequency induction heating or infrared heating. However, it is one of the preferred methods in the present invention that by using a high-frequency induction heating apparatus, a low temperature plasma is generated under vacuum to decompose the iodide and thereby to precipitate niobium metal of an ultrahigh purity. Here, the frequency for the high-frequency induction heating is preferably from a few MHz to a few tens MHz. Heretofore, a temperature of at least 1000 o C has been required for the thermal decomposition. Whereas, according to the thermal decomposition by means of this high-frequency induction heating apparatus, the decomposition can adequately be conducted at a temperature of about 800 o C by activating the metal iodide by the generation of the low temperature plasma, and the decomposition rate can be improved remarkably i.e. from 10 to 100 times. Further, the purity of niobium metal obtained by this step can be as high as at least 99.99%, and the niobium metal will be useful for electronic materials for which an ultrahigh purity is required, particularly as a starting material for super conductive thin films or alloys. Now, the present invention will be described with reference to the drawings. Figure 1 illustrates an apparatus for continuous iodization employed for the iodization reaction of the present invention. Figure 2 illustrates an apparatus for the thermal reduction. Likewise, Figure 3 illustrates an apparatus for the thermal decomposition. Referring to Figure 1, reference numeral 1 indicates a pot for supplemental iodine designed to supplement iodine consumed as the iodides. Reference numeral 2 indicates an iodine reservoir, and numeral 3 indicates a closed iodine feeder (e.g. an electromagnetic feeder), designed to supply iodine in the form of powder quantitatively to an iodine vapourizer 4. The iodine gasified here, is then sent to a reactor 6, and reacted with crude niobium metal supplied from a crude niobium metal pot 7 quantitatively and falling onto a perforated plate 5, whereby niobium iodide is formed. The formed niobium iodide is precipitated in a niobium iodide purification tower 9, and the purified niobium iodide is collected into a niobium iodide collecting pot 8. Unreacted iodine and iodides of impurities are led to an iodine distillation tower. The iodides of impurities are collected into a pot 10, and the purified iodine gas is led to an iodine quenching trap 12 cooled by a cooling medium. Here, the iodine gas is rapidly cooled by an inert gas cooled by a condenser 13, and formed into a powder, which is again fed back to the iodine reservoir 2. Thus, niobium iodide having a high purity is continuously produced, and at the same time, iodine is recycled in a completely closed system. Referring to the operational method more specifically, the degassing and dehydration are conducted by vacuuming the entire system at a level of not higher than 1.33 x 10⁻²mbar (10⁻² Torr), by heating the system to a temperature of at least about 300 o C, and by maintaining the condition for a long period of time. Then, iodine is supplied in a proper amount to the iodine vapourizer heated to a temperature higher than the boiling point of iodine, and the entire system is made under an iodine atmosphere. Further, when the respective portions reach the predetermined temperatures, crude niobium metal is supplied for iodization. Referring to Figure 2, reference numeral 21 indicates a carrier gas inlet, numeral 22 indicates a reaction tube for the thermal reduction, and numeral 23 indicates niobium iodide. A proper amount of the carrier gas is introduced from the carrier gas inlet 21 into the reaction tube for the thermal reduction in which niobium iodide 23 is placed, and the thermal reduction is conducted. The vapourized impurities such as the higher tantalum iodide are collected by an impurity collecting trap 24. Thus, the purified lower niobium iodide remains in the reaction tube 22, and is recovered, whereas the iodides of impurities 25 accumulate in the impurity collecting trap 24. Reference numeral 26 in Figure 2 indicates an exhaust gas line. In Figure 3, reference numeral 31 indicates a purified niobium iodide gas inlet, numeral 32 indicates a low temperature plasma, numeral 33 indicates a high frequency induction heating coil, numeral 34 is a seed metal, numeral 35 indicates a gas outlet. From the inlet 31, the purified niobium iodide is introduced in the form of a gas, and decomposed ink the vicinity of the seed metal 34 (most preferably niobium metal i.e. the same as the precipitating metal) heated to a high temperature by the high frequency induction heating coil 33, whereupon niobium metal deposits on the seed metal. At the same time, argon gas is supplied form the gas inlet 31 to generate a stabilized low temperature plasma 32 below the seed metal 34, and the purified niobium iodide gas is activated in the plasma. Surprisingly, by such a method, the thermal decomposition of the purified niobium iodide can be conducted at a temperature lower by about 200 o C than the conventional decomposition temperature, and yet the decomposition rate is improved by from 10 to 100 times. For the generation of the low temperature plasma and for the decomposition, a reduced pressure of not higher than 1.33 to 2.66 mbar (1 to 2 Torr) is sufficient when the purified niobium gas iodide and argon gas flow in the system. Unreacted iodine and liberated iodine are removed from the gas outlet 35 and then recovered for reuse. Now, the present invention will be described in detail with reference to Examples. However, it should be understood that the present invention is by no means restricted by these specific Examples. 1. Examples for iodization step EXAMPLE 1-1 By using the apparatus as shown in Figure 1, crude niobium metal was continuously iodized under the following conditions. Conditions (1) (2) Iodine supply rate 13 g/min 13 g/min Niobium supply rate 1 g/min 1 g/min Iodine vapourizer temperature 200 o C 220 o C Iodization temperature 500 o C 550 o C Tower top temperature of the iodide purification tower 250 o C 180 o C Tower top temperature of the iodine purification tower 185 o C 190 o C Tower bottom temperature of the iodine purification tower 200 o C 200 o C Niobium iodide forming rate 6.4 g/min 7.5 g/min The purification effects by the production of niobium iodide under the above conditions are shown in Table 1. (1) (2) Ta Fe Al Ta Fe Al Crude niobium metal (ppm) 2000 20 30 2000 20 30 Impurities (as calculated as niobium) in the iodide (ppm) 180 2 5 200 3 6 Metal impurities other than Ta, Fe and Al were less than 1 ppm. The ratio of bound iodine in the formed niobium iodide is shown in Table 2. Table 2 Nb (wt.%) Bound iodine (wt.%) Free iodine (wt.%) I/Nb (molar ratio) (1) 12.95 87.03 0.02 4.92 (2) 12.90 87.05 0.05 4.94 EXAMPLE 1-2 Examples for the iodization step where niobium chloride was used as the starting material, will be given. EXAMPLE 1-2-1 10 g of niobium pentachloride having a particle diameter of from 10 to 100 µm obtained by the chlorination and purification of commercially available ferroniobium, was supplied (0.15 g/min) to the reaction tube in a counter current relation with HI, and HI containing 2% of I₂ was introduced at a rate of 0.7 g/min. The reaction zone was preliminarily heated to 150 o C. The iodide collected at the lower portion of the reaction tube was niobium pentaiodide (NbI₅) comprising 12.3% of Nb, 0.4% of free iodine and 87.3% of bound iodine. The yield was 97%. EXAMPLE 1-2-2 Niobium pentachloride as used in Example 1-2-1 was heated to 200 o C, and supplied (0.15 g/min) to a horizontal type reactor by using argon gas as the carrier gas. HI gas and I₂ gas (partial pressure: 133 mbar (100 mmHg)) were supplied at a rate of 0.7 g/min. The reaction temperature was kept at 300 o C. Niobium pentaiodide thereby obtained was 25 g. Free iodine was 0.2%. The yield was 95%. 2. Examples for thermal reduction step EXAMPLE 2-1 An apparatus as shown in Figure 2 was used. 50 g of niobium iodide (NbI₅) containing 0.12% by weight of tantalum iodide (TaI₅) (obtained by iodizing niobium containing 2000 ppm of tantalum) was employed as the starting material iodide. The thermal reduction was conducted for 2 hours to remove tantalum by using 100 ml/min of argon gas as the carrier gas. The temperature raising rate was 500 o C/min. The Ta content (based on Nb) in the remained niobium iodide and the yield of Nb are as shown in Table 3. Thermal reduction temperature (°C) Ta content (based on Nb) (ppm) Yield of Nb (%) 250 500 87 300 50 92 350 30 83 400 10 87 450 9 85 EXAMPLE 2-2 The thermal reduction was conducted under the same conditions as in Example 2-1 except that 100 ml/min of hydrogen gas was used as the carrier gas. The results are shown in Table 4. Table 4 Thermal reduction temperature (°C) Ta content (based on Nb) (ppm) Yield of Nb (%) 200 800 99 250 150 98 300 10 98 350 5 97 400 4 96 As shown above, the yield was remarkably improved by using hydrogen gas. EXAMPLE 2-3 Table 5 shows the results on the Ta content (based on Nb) in the remained niobium iodide and the yield of Nb in the cases where the temperature raising rate was differentiated at levels of 150 o C/min, 300 o C/min and 500 o C/min by using the same starting material iodide as used in Examples 2-1 and 2-2 and 100 ml/min of hydrogen as the carrier gas at a thermal reduction temperature of 300 o C or 400 o C for a thermal reduction time of 2 hours. Thermal reduction temperature (°C) Temperature raising rate (°C/min) Ta content (based on Nb) (ppm) Yield of Nb (%) 300 150 35 87 300 12 94 500 10 98 400 150 32 85 300 6 91 500 4 96 EXAMPLE 2-4 The thermal reduction was conducted by using the same starting material iodide and the sane apparatus as used in Examples 2-1 and by vacuuming the apparatus to maintain the interior under reduced pressure. The results are shown in Table 6. Thermal reduction temperature (°C) Ta content (based on Nb) (ppm) Yield of Nb (%) 200 230 98 300 120 95 400 92 89 500 132 72 3. Examples for second iodization step EXAMPLE 3-1 By using the same apparatus as used in the first iodization step, the lower niobium iodide instead of the crude niobium metal, was continuously iodized. The conditions for the second iodization are shown below, and the quality of the niobium iodide thereby obtained is shown in Table 7. Conditions Iodine supply rate 13 g/min Lower iodide supply rate 13 g/min Second iodization temperature 500 o C Tower top temperature of iodide purification tower 250 o C Ta Fe Al Impurities (as calculated as niobium) in the lower niobium iodide (ppm) 30 4 7 Impurities (as calculated as niobium) in the purified iodide (ppm) 25 2 2 4. Examples for thermal decomposition EXAMPLE 4-1 By using an apparatus as shown in Figure 3, the niobium iodide purified in the above-mentioned step was thermally decomposed. The conditions for the thermal decomposition are as shown below. The frequency of the high frequency induction heating apparatus was 4MHz to generate a low temperature plasma. A niobium metal rod having a diameter of 10 mm and a length of 25 mm was used as a seed metal rod. Conditions (1) (2) Thermal decomposition temperature 800 o C 1000 o C Niobium iodide supply rate 60 g/h 60 g/h Vacuum degree (2x10⁻¹Torr) 2.66 x 10⁻¹mbar (2x10⁻¹Torr) 2.66 x 10⁻¹mbar Argon gas flow rate 10-20ml/min 10-20ml/min The results of the thermal decomposition are shown in Table 8. Nb precipitation rate (1) (2) 1.0 g/cm³.h 4.0 g/cm³.h Analytical values (ppm) Ta 7 10 Fe <1 <1 Al <1 <1 O 10 10 H <1 <1 C 25 25 The total amount of other components was not higher than 1 ppm. As described in the foregoing, the precipitation rate is remarkably improved over the conventional methods, and Nb having an ultrahigh purity of at least 99.99% was obtained. EXAMPLE 4-2 Table 9 shows the decomposition efficiency and the purification effects in the cases where the vacuum degree was differentiated at levels of atmospheric pressure 39.9 mbar, (30 Torr), 13.3 mbar (10 Torr), 5.32 mbar (4 Torr) and 0.26 mbar (0.2 Torr) without generating a plasma by using the same apparatus and a high frequency heating apparatus of 400 kHz. Decomposition efficiency (%) Ta concentration (ppm) Atmospheric pressure 18 24 (30 Torr) 39.9 mbar 20 20 (10 Torr) 13.3 mbar 38 15 (4 Torr) 5.32 mbar 40 12 (0.2 Torr 0.26 mbar 53 10 EXAMPLE 5 Working Examples will be given which show the entire process of the present invention comprising a series of the above described steps. The conditions of the respective steps are shown in Table 10. The purification states and the analytical values of the final niobium of an ultrahigh purity thereby obtained are shown in Table 11. As shown above, it is possible to obtain Nb having an ultrahigh purity of at least 99.99% by purifying crude niobium metal having a poor purity (from 99 to 99.9%) by the process of the present invention.

Claims: A process for producing niobium metal of an ultrahigh purity, which comprises iodizing niobium metal or niobium chloride containing at least tantalum as an impurity, thermally reducing the iodized product, and then thermally decomposing the reduced product. The process according to Claim 1, wherein the temperature for the thermal reduction is from 200 to 600 o C. The process according to Claim 1, wherein the thermal reduction is conducted in an inert gas atmosphere or under reduced pressure. The process according to Claim 1, wherein the temperature for the thermal decomposition is at least 700 o C. The process according to Claim 1, wherein the thermal decomposition is conducted by a low temperature plasma. The process according to Claim 1, wherein the thermal decomposition is conducted under atmospheric pressure or under reduced pressure. The process according to Claim 1, wherein the niobium metal of an ultrahigh purity has a purity of at least 99.99%. The process according to Claim 1, which includes an additional step of iodizing the thermally reduced product between the steps of the thermal reduction and the thermal decomposition.

Inventors: KUDO, MASAYUKI, NISHIZAWA, KEIICHIRO, SUDO, HAJIME
Applicants: TOSOH CORPORATION
Application year: 1986
Main classification: C22B34
IPC codes: C22B34, C23C16, C22B4