Method for preparing gaseous fluorides

Abstract: The method for preparing a gaseous metallic fluoride is here disclosed which comprises reacting a metal or its oxide with a fluorine gas or nitrogen trifluoride gas, the aforesaid method being characterized by comprising the steps of mixing the metal or its oxide with a molding auxiliary comprising a solid metallic fluoride which does not react with fluorine and nitrogen trifluoride; molding the resulting mixture under pressure; and contacting the molded pieces with the fluorine gas or nitrogen trifluoride gas, while the molded pieces are heated.

Description: BACKGROUND OF THE INVENTION (i) Field of the Invention The present invention relates to a method for preparing a gaseous fluoride by reacting the element or its oxide with a fluorine gas or nitrogen trifluoride gas. (ii) Description of the Prior Art A gaseous fluoride of the present invention means a fluoride which is in a gaseous state in the vicinity of a temperature at which a element or its oxide is reacted with a fluorine gas or nitrogen trifluoride gas. Typical examples of the gaseous fluorides include compounds such as tungsten hexafluoride (WF₆), molybdenum hexafluoride (MoF₆), antimony pentafluoride (SbF₅), niobium pentafluoride (NbF₅), tantalum pentafluoride (TaF₅), titanium tetrafluoride (TiF₄), silicon tetrafluoride (SiF₄), germanium tetrafluoride (GeF₄) and arsenic trifluoride (AsF₃). These compounds are utilizable in a variety of industrial fields. For example, WF₆, MoF₆ and the like are expected as raw materials of electrodes for semiconductors. In particular, tungsten silicide (WSi₂) and molybdenum silicide (MoSi₂) which can be prepared from WF₆ and MoF₆ as raw materials are noticed as wire materials for large scale integrated circuits (LSI). In addition, the above-mentioned gaseous metallic fluorides, inclusive of WF₆ and MoF₆, are used as various fluorinating agents and as optical materials. Heretofore, the gaseous fluoride has been generally prepared by a method which comprises contacting/reacting an element or its oxide with a fluorine (F₂) gas or nitrogen trifluoride (NF₃) gas at a high temperature of some hundreds degree centigrade. In this case, when the high-purity gas is desired, the element is preferably used as the raw material. However, in usual cases, the oxide is employed as the raw material because of being inexpensive and thus economical. Due to the strong toxicity and very high activity, the F₂ gas is diluted with an inert gas such as a nitrogen (N₂) gas or argon (Ar) gas so that the concentration of the F₂ gas may be in the range of about 10 to 30% by volume, when used in the above-mentioned contact reaction. As described above, the F₂ gas is very toxic, and therefore it is advantageous to use NF₃ from the viewpoint of safety, though there is a little cost increase Hereinafter, the element or its oxide will be called "element or the like" inclusively. In the above-mentioned reaction between the element or the like and F₂ or NF₃, the produced gaseous fluoride is led out of the reaction system and is then cooled at a temperature less than its boiling point in order to collect the product. Therefore, a carrier gas is necessary in the reaction. Since the carrier gas is introduced together with the F₂ gas or NF₃ gas into a reactor, an inert gas such as a nitrogen (N₂) gas, helium (He) gas or argon (Ar) gas is usually used as the carrier gas. When the element or the like is reacted with the F₂ gas or NF₃ gas (hereinafter referred to sometimes as "F₂ gas or the like"), the element or the like is usually used in the form of powder so as to promptly and effectively perform the contact of the element or the like with the F₂ gas or the like. This reaction is carried out by introducing the F₂ gas or NF₃ gas into the powder layer of the oxide on a fluidized bed or fixed bed disposed in the reactor. However, in such a method, there is the following problem. That is, in the fluidized bed system, the diluted F₂ gas or the like passes through the layer of the element powder or the like being fluidized, and therefore the fine powder of the element or the like is entrained in the produced gaseous fluoride, with the result that the purity of the product deteriorates. Furthermore, the yield of the product is limited to about 80% or so based on the F₂ gas, and also in this point, the above-mentioned method is insufficient. On the other hand, in the case of the fixed bed system, the reaction of the element powder or the like and the F₂ gas or the like is performed only on the surface of the layer of the element powder or the like, and thus the contact area of the element powder or the like with the F₂ gas or the like is restricted, so that the yield is inconveniently low. In addition, there is also the problem that the produced gaseous fluoride is contaminated with plenty of the unreacted F₂ gas or the like. Moreover, as the reaction proceeds the element powder or the like is more and more finely ground, and the finely ground metal powder is entrained in the produced gas, which leads to the deterioration in the product purity. In order to increase the yield in the fixed bed system, it is necessary to increase the contact area of the element powder or the like and the F₂ gas or the like, which requires a fairly large reactor. In this reaction, the F₂ gas or NF₃ used as the raw material, are very corrosive and for this reason, expensive nickel is usually used as the construction material of the reactor. which results in the noticeable increase of the costs. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for preparing a gaseous fluoride by which the contamination of the gaseous fluoride with the element powder is prevented and by which the desired gaseous fluoride can be manufactured in high yield and at low cost. The present invention is directed to a method for preparing a gaseous fluoride of a metal or of arsenic, phosphorous, sulfur, silicon or boron by reacting the element or its oxide with a fluorine gas or nitrogen trifluoride gas, said method being characterized by comprising the steps of mixing said element or its oxide with a molding auxiliary comprising a solid metallic fluoride which does not react with fluorine and nitrogen trifluoride; molding the resulting mixture under pressure; and contacting the molded pieces with said fluorine gas or nitrogen trifluoride gas, while said molded pieces are heated. The gaseous fluoride is a compound represented by MFm wherein M is at least one simple element selected from the group consisting of elements in the groups IIIA, IIIB, IVA, IVB, VA, VB, VIA and VIB of the periodic table, and m is an integer of 3 to 6. Typically, the gaseous fluoride is at least one compound selected from the group consisting of tungsten hexafluoride (WF₆), molybdenum hexafluoride (MoF₆), antimony trifluoride (SbF₃), antimony pentafluoride (SbF₅), niobium pentafuoride (NbF₅), arsenic trifluoride (AsF₃), phosphorus trifluoride (PF₃), boron trifluoride (BF₃), tantalum pentafuoride (TaF₅), titanium tetrafluoride (TiF₄), silicon tetrafluoride (SiF₄), germanium tetrafluoride (GeF₄), sulfur hexafluoride (SF₆) and uranium hexafluoride (UF₆). The solid metallic fluoride as the molding auxiliary which does not react with fluorine and nitrogen trifluoride is at least one compound selected from the group consisting of fluorides of metals in the groups IA, IIA and IIIB of the periodic table. Typically, the solid metallic fluoride is at least one compound selected from the group consisting of lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), rubidium fluoride (RbF), cesium fluoride (CsF), beryllium fluoride (BeF₂), magnesium fluoride (MgF₂), calcium fluoride (CaF₂), strontium fluoride (SrF₂), barium fluoride (BaF₂), aluminum fluoride (AlF₃), gallium fluoride (GaF₃), indium fluoride (InF₃), thallium fluoride (TlF₃) and aluminum sodium fluoride (Na₃AlF₆). The simple element or its oxide which is the raw material is a simple element or its oxide corresponding to the above-mentioned gaseous fluoride. The element or its oxide which is the raw material is mixed with the molding auxiliary, the amount of the element or its oxide being in the range of 30 to 70% by weight. Afterwards, the mixture is molded in a shape of cylinder, ring or chrysanthemum having a typical diameter of about 5 to 15 mm under a pressure of 500 to 10000 bar (0.5 to 10 t/cm²). The reaction of the thus molded pieces with the fluorine gas or nitrogen trifluoride gas is carried out at a temperature of 150 to 700°C under a pressure of 1 to 10 bar (1 to 10 kg/cm²)for a period of 1 to 20 hours. At this time, it is preferred that the molded pieces are disposed as a filler layer in a reactor and the fluorine gas or nitrogen trifluoride gas is caused to pass through this layer. It is also preferred that prior to the reaction, the molded pieces are subjected to a heating treatment in a reducing gas atmosphere at a temperature of 500 to 850°C for a period of 1 to 500 hours. DETAILED DESCRIPTION OF THE INVENTION A gaseous fluoride which can be prepared in the present invention is a fluoride which can be synthesized by usually reacting fluorine or nitrogen trifluoride (hereinafter referred to simply as "fluorine or the like") with an element or its oxide (hereinafter referred to simply as element or the like"), and the above-mentioned fluoride is in a gaseous state at a reaction temperature at which the simple element or the like is reacted with the F₂ gas or the like, for example, at a temperature of 300°C or more. Examples of such a fluoride compound include tungsten hexafluoride (WF₆), molybdenum hexafluoride (MoF₆), antimony pentafluoride (SbF₅), antimony trifluoride (SbF₃), niobium pentafluoride (NbF₅), tantalum pentafluoride (TaF₅), titanium tetrafluoride (TiF₄), silicon tetrafluoride (SiF₄), silicon hexafluoride (SiF₆), germanium tetrafluoride (GeF₄), arsenic trifluoride (AsF₃), uranium hexafluoride (UF₆), phosphorus trifluoride (PF₃), boron trifluoride (BF₃) and sulfur hexafluoride (SF₆). Needless to say, the element used in the present invention is a element constituting the above-mentioned gaseous fluoride. This element is selected from elements in the groups IIIA, IIIB, IVA, IVB, VA, VB, VIA and VIB of the periodic table, and preferable typical examples of the simple element include tungsten (W), molybdenum (Mo), antimony (Sb), niobium (Nb), arsenic (As), phosphorus (P), boron (B), tantalum (Ta), titanium (Ti), silicon (Si), germanium (Ge), sulfur (S) and uranium (U). The oxide used in the present invention is the oxide of each element mentioned above and can provide the gaseous fluoride through the reaction with the F₂ gas or the like. Typical examples of the oxide include tungsten oxide (WO₃), molybdenum oxide (MoO₃), uranium oxide (UO, UO₂ and U₃O₈), arsenic oxide (As₂O₅), antimony oxide (Sb₂O₅), niobium oxide (Nb₂O₅), tantalum oxide (Ta₂O₅), titanium oxide (TiO₂) and germanium oxide (GeO₂). With regard to the raw material element used in the present invention, its shape is not particularly limited, but it is preferably in a powdery state having a particle diameter of 1 to 50 µm, preferably 3 to 20 µm. This reason is as follows: In the present invention, it is necessary that the element is mixed with a molding auxiliary and the resulting mixture is then molded under pressure to form molded pieces. In this case, in order to obtain the product in high yield, it is preferable that the element and molding auxiliary in the molded pieces are mixed as uniformly as possible, and hence the element is conveniently in the state of the powder. In addition, when the element is in the state of the powder, the molding under pressure is also easy. Next, reference will be made to the molding auxiliary. In the present invention, a solid fluoride which does not react with the fluorine or the like is used as the molding auxiliary. This solid fluoride is preferably in the state of a solid even at a temperature at which the element or the like reacts with the F₂ gas or the like. Typical examples of such a solid fluoride include metallic fluorides of the group IA such as lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), rubidium fluoride (RbF) and cesium fluoride (CsF); metallic fluorides of the group IIA such as beryllium fluoride (BeF₂), magnesium fluoride (MgF₂), calcium fluoride (CaF₂), strontium fluoride (SrF₂) and barium fluoride (BaF₂); metallic fluorides of the group IIIB such as aluminum fluoride (AlF₃), gallium fluoride (GaF₃), indium fluoride (InF₃) and thallium fluoride (TlF₃); and a double salt such as aluminum sodium fluoride (Na₃AlF₆). They may be used singly or in combination. Since the solid metallic fluoride is mixed with the metal or the like and is then molded under pressure, it preferably takes a powdery state having a particle diameter of about 1 to 20 µm, as in the case of the element or the like. In the present invention, the element is mixed with the solid metallic fluoride which is the molding auxiliary, and the resulting mixture is then molded under pressure. In this case, the amount of the element or the like in the mixture is in the range of 30 to 70% by weight based on the total weight of both the element and the molding auxiliary. When the content of the element or the like is too high, the reactivity is improved, but the strength of the molded pieces deteriorates, so that the molded pieces tend to break into powder unpreferably. Inversely, when the element content is too low, the reactivity drops, though the strength of the molded pieces is sufficient. In the present invention, the mixture of the element or the like and the solid metallic fluoride is preferably molded under pressure by the use of a tableting machine, and in this case, tableting pressure is in the range of about 500 to 10000 bar (0.5 to 10 t/cm²), preferably about 1 to 3000 bar (1 to 3 t/cm²). The molded pieces may take any shape, so long as the the shape can be obtained by the use of the usual tableting machine, and preferable examples are shapes of cylinder, ring, cubic, sphere, gear wheel shape and ribbed shape. Moreover, the size of the molded pieces is not particularly limited, either, and it depends upon the size of the reactor and the handling ease of the molded pieces themselves. In effect, the molded piecess can take any size, so long as they can be obtained by the use of the tableting machine. Preferebly, the molded pieces have an effective diameter of 1/5 to 1/20 of the diameter of the reactor. For example, in the case that the shape of the molded pieces is cylindrical, it is preferred that the diameter of the molded pieces is 1/5 to 1/20 of the diameter of the reactor and the height thereof is 1/2 of this diameter, i.e., about 1/10 to 1/40 of the diameter of the reactor. Therefore, when the diameter of the reactor is 10 cm, the diameter and height of the molded pieces are preferaably in the range of 5 mm⊘ to 2 cm⊘ and in the range of 2.5 mmh to 1 cmh, respectively. Also with regard to the molded pieces having other shapes, these standards can be applied. In the preferable molded pieces, the content of water is low, and therefore it is preferred that the element and solid fluorine compound are dried to remove water therefrom prior to the molding. Next, reference will be made to a method for preparing the gaseous metallic fluoride by the use of the thus prepared molded pieces and the F₂ gas or the like. In the present invention, the material for the reactor, in which the metal or the like will be reacted with the F₂ gas or the like, is usually nickel in consideration of anticorrosion to the fluorine or the like. The shape of the reactor is not particularly limited, but a cylindrical shape is preferable from the viewpoint of the ease of manufacture. The convenient usage of the reactor is as follows: Under the reactor uprightly stood, a perforated plate is disposed, and the reactor is then filled with the above-mentioned molded pieces so that these pieces may be put on the perforated plate. Afterward, the F₂ gas or the like is introduced into the reactor through the perforated plate from its underside. In this case, heating of the reactor is easily carried out by disposing a heater or the like on the outer periphery of the cylindrical reactor. In the present invention, the reactor is filled with the molded pieces comprising the element or the like and the molding auxiliary, as described above. While the molded pieces are heated at a predetermined temperature, the F₂ gas or the like is introduced into the reactor from its underside, so that the element in the molded pieces is reacted with the F₂ gas or the like, thereby preparing the desired gaseous fluoride. Needless to say, a reaction temperature depends upon the kind of gaseous fluoride and the kinds of raw materials. Usually, the reaction temperature is in the range of 150 to 700°C, preferably 200 to 600°C. Typically, the reaction temperatures for the kinds of gaseous fluorides are as shown in Table 1. Gaseous Fluorides Reaction Temperature (°C) Tungsten hexafluoride (WF₆) 250-500 Molybdenum hexafluoride (MoF₆) 250-500 Antimony pentafluoride (SbF₅) 300-500 Niobium pentafluoride (NbF₅) 300-550 Tantalum pentafluoride (TaF₅) 300-550 Titanium tetrafluoride (TiF₄) 300-550 Germanium tetrafluoride (GeF₄) 250-500 Arsenic trifluoride (AsF₃) 250-500 Uranium hexafluoride (UF₆) 250-500 Arsenic tetrafluoride (AsF₅) 250-500 The F₂ gas introduced into the reactor, when having a high concentration, is toxic, and therefore it is preferred that the F₂ gas is diluted with an inert gas such as an N₂ gas or Ar gas so as to be used at a concentration of about 5 to 40% by volume. In the case of the NF₃ gas, it may be directly used without dilution, but for the ease of handling, the NF₃ gas, when used, may be conveniently introduced thereinto together with a carrier gas of an inert gas such as N₂ or Ar. In this case, the NF₃ gas and carrier gas may be introduced into the reactor separately or in the form of a mixture thereof. The heating of the reactor is easily carried out by disposing a heater or the like on the outer periphery of the cylindrical reactor, as described above. At the time of the reaction, the pressure is not particularly limited, and needless to say, reduced pressure is also acceptable, but usually the pressure is in the range of atmospheric pressure to about 10 bar (10 kg/cm²). A reaction time depends upon the above-mentioned temperature, but it is usually in the range of 1 to 20 hours, preferably about 2 to 10 hours. The gaseous fluoride obtained through the reaction contains some of the inert gas used to dilute the F₂ gas or the like and the unreacted F₂ gas or the like, and therefore the fluoride is cooled below its liquefaction temperature to remove the inert gas and F₂ gas or the like therefrom. In a preferable embodiment of the present invention, the molded pieces are previously heated in a reducing gas atmosphere and are then brought into contact with the fluorine gas or the like under heating. This procedure permits preventing a white solid, which would comprise high-boiling compounds, from adhering to or, in an awful case, clogging pipes, valves and the like attached to the reactor. Therefore, the above-mentioned embodiment enables the stable operation of the reactor for a long period of time. Now, reference will be made to the heating treatment of the molded pieces in the reducing gas atmosphere. While the reactor filled with the molded pieces is heated, the reducing gas is introduced into the reactor. As the reducing gas, a usual gas such as an H₂ gas, NH₃ gas or CO gas can be used. The reducing gas can be used at a concentration of 100%, but it is preferably diluted with an inert gas such as an N₂ gas or He gas in consideration of safety. A heating temperature depends slightly upon the kind of element but is usually in the range of 500 to 850°C. When the heating temperature is less than 500°C, the formation of the white solid cannot be inhibited perfectly during the reaction with the F₂ gas or NF₃ gas which will be described hereinafter. Inversely, when it is in excess of 850°C, energy is only wasted and the reactor filled with the molded pieces must be made from a more expensive material, though the purpose of this invention can be achieved. The heating time is in the range of 1 to 100 hours in the case that the metal is used, and it is in the range of about 2 to 500 hours in the case that the metallic oxide is used. The completion of the heating operation can be considered to be at the point of time when the water content in the reducing gas at the outlet of the reactor drops to several ppm or less. In the present invention, it is preferred that the water content in the molded pieces is low, but additional heating to remove water therefrom is not particularly necessary, since the water in the molded pieces is removed therefrom by the above-mentioned heating. The molded pieces which have been heated in the reducing gas atmosphere in this manner are then brought into contact with the F₂ gas or NF₃ gas, thereby preparing the gaseous fluoride. In this case, the molded pieces which have undergone the heating treatment are preferably handled not to be exposed to an oxidizing atmosphere such as air. Consequently, it is preferred that the heating treatment of the molded pieces is carried out while they are placed in the reactor, and after the heating treatment, the molded pieces are brought into contact with the F₂ gas or NF₃ gas while still placed in the reactor. EXAMPLES Now, the present invention will be described in detail in reference to examples, but it should not be limited to these examples. Example 1 Metallic tungsten powder (iron content = 50 ppm) of 99.9% purity which had been previously dried at 120°C for 2 hours in an N₂ gas atmosphere was mixed sufficiently with NaF powder (first class grade chemical) which had been previously dried in like manner, in a weight ratio of 1:1, and the resulting mixture was then tableted under a tableting pressure of 2000 bar (2 t/cm²) by the use of a small-sized tableting machine to obtain cylinder-shaped pieces each having a diameter of 5 mm and a height of 5 mm. Compressive strength of the molded pieces was extremely high 270 bar (270 kg/cm²) in terms of an average value. Next, a vertical reactor made of nickel and having an inner diameter of 19 mm and a height of 600 mm was filled in the central portion thereof with the molded pieces. Afterward, an N₂ gas having atmospheric pressure was introduced into the reactor from its underside at a flow rate of 300 Nml/minute for about 2 hours, while the filler layer of the molded pieces was heated at about 100°C. After the stop of the introduction of the N₂ gas, an F₂ gas diluted with an N₂ gas and having a concentration of about 30% by volume and atmospheric pressure was introduced into the reactor from its underside at a flow rate of 300 Nml/minute for 3 hours, while the filler layer of the molded pieces was heated at a temperature of 380 to 400°C, whereby reaction was performed. A WF₆-containing gas generated in the reactor was led into a refrigerant trap cooled to a temperature of -80°C, so that the gas was liquefied and collected therein. After the reaction, the trap was evacuated by a vacuum pump to remove the N₂ gas used to dilute the F₂ gas and the unreacted F₂ gas. The yield amount of WF₆ was 69 g, and its yield ratio is high, 96% based on fluorine. In addition, the molded pieces in the reactor maintained the predetermined shape without breaking. Since it was difficult to quantitatively analyze the content of the metal tungsten present in the collected WF₆, the content of iron (Fe) which was present in a relatively great amount as an impurity in tungsten was instead measured by high-frequency induction combination plasma (ICP) analysis. As a result, it was apparent that the content of iron was 0.05 ppm or less. This result indicates that metal tungsten which was the raw material was prevented substantially from flying about. Examples 2 to 8 The same procedure as in Example 1 was repeated with the exception that tungsten as the element was replaced with each metal shown in Table 2 and sodium fluoride as the molding auxiliary was also replaced with each solid metal fluoride shown in Table 2 in each amount shown in Table 2, in order to obtain molded pieces in each amount shown in Table 2 (prior to the molding, each element and molding auxiliary were dried as in Example 1). The same reactor as used in Example 1 was filled with the molded pieces in each amount shown in Table 2, and an F₂ gas was introduced thereinto under reaction conditions shown in Table 2. Afterward, following the same procedure as in Example 1, a variety of gaseous fluorides were prepared. In this case, conditions of drying the molded pieces prior to the introduction of the F₂ gas into the reactor were the very same as in Example 1. Yield amounts, yield ratios and Fe contents of the thus obtained gaseous fluoride products are set forth in Table 2. The yields in the respective examples were as high as in Example 1, and the elements did not fly about as in Example 1. Additionally, in all the examples, after the completion of the reaction, it was confirmed that the molded pieces did not break. Comparative Example 1 In the bottom portion of a lateral reactor made from nickel and having a diameter of 38 mm and a length of 600 mm was placed 100 g of the same previously dried metallic tungsten powder as used in Example 1 as uniformly as possible, and the reactor was then heated up to about 100°C. Afterward, an N₂ gas was introduced into the reactor from the left end portion thereof at a flow rate of 300 Nml/minute for about 2 hours, followed by drying metallic tungsten. After the stop of the introduction of the N₂ gas, an F₂ gas diluted with an N₂ gas was introduced into the reactor from the left end portion thereof under the same conditions as in Example 1, while the metallic tungsten layer was heated at a temperature of 380 to 400°C as in Example 1, and a WF₆-containing gas generated in the reactor was cooled, thereby collecting WF₆. The yield amount of WF₆ was 33 g, and its yield ratio was 46% based on fluorine, which values were so low that they did not reach even the half levels of the values in Example 1. In addition, the content of Fe in WF₆ was 0.9 ppm, which was indicative that the product was not a little contaminated with the metallic tungsten powder. Example 9 Metallic tungsten powder (iron content = 50 ppm) of 99.9% purity which had been previously dried at 120°C for 2 hours in an N₂ gas atmosphere was mixed sufficiently with NaF powder (first class grade chemical) which had been previously dried in like manner, in a weight ratio of 1:1, and the resulting mixture was then tableted under a tableting pressure of 2000 bar (2 t/cm²) by the use of a small-sized tableting machine to obtain cylinder-shaped pieces each having a diameter of 5 mm and a height of 5 mm. Compressive strength of the molded pieces was extremely high, 280 bar (280 kg/cm²) in terms of an average value. Next, a vertical reactor made of nickel and having an inner diameter of 19 mm and a height of 600 mm was filled in the central portion thereof with the molded pieces. Afterward, an N₂ gas having atmospheric pressure was introduced into the reactor from its underside at a flow rate of 300 Nml/minute for about 2 hours, while the filler layer of the molded pieces was heated at about 100°C. The flow rate of the N₂ gas was then lowered to 100 Nml/minute, and while the filler layer of the molded pieces was heated at a temperature of 380 to 400°C, an NF₃ gas having atmospheric pressure was introduced into the reactor at a flow rate of 80 Nml/minute, whereby reaction was performed for 2 hours. A WF₆-containing gas generated in the reactor was led into a refrigerant trap cooled to a temperature of -80°C, and the gas was liquefied and collected therein. After the completion of the reaction, the trap was evacuated by a vacuum pump to remove the N₂ gas used as a carrier gas, a secondarily formed N₂ gas and the unreacted NF₃ gas. The yield amount of WF₆ was 60 g, and its yield ratio was as high as 94% based on WF₆. In addition, the molded pieces in the reactor maintained the predetermined shape without breaking. Since it was difficult to quantitatively analyze the content of the metal tungsten present in the collected WF₆, the content of iron (Fe) which was present in a relatively great amount as an impurity in tungsten was instead measured by high-frequency induction combination plasma (ICP) analysis. As a result, it was apparent that the content of iron was 0.05 ppm or less. This result indicates that metal tungsten which was the raw material was prevented from flying about. Examples 10 to 16 The same procedure as in Example 9 was repeated with the exception that tungsten as the metal was replaced with each element shown in Table 3 and sodium fluoride as the molding auxiliary was replaced with each solid metal fluoride shown in Table 3 in each amount shown in Table 3, in order to obtain molded pieces in each amount shown in Table 3 (prior to the molding, each element and molding auxiliary were dried as in Example 1). The same reactor as used in Example 9 was filled with the molded pieces in each amount shown in Table 3, and the molded pieces were dried under the same conditions as in Example 9. Afterward, an NF₃ gas and carrier gas were introduced thereinto under reaction conditions shown in Table 3, and following the same procedure as in Example 9, a variety of gaseous fluorides were prepared. In this case, conditions of drying the molded pieces prior to the introduction of the NF₃ gas into the reactor were the very same as in Example 9. Yield amounts, yield ratios and Fe contents of the thus obtained gaseous fluoride products are set forth in Table 3. The yields of the respective examples were as high as in Example 9, and the metals did not fly about as in Example 9. Additionally, in all the examples, after the completion of the reaction, it was confirmed that the molded pieces did not break. Comparative Example 2 In the bottom portion of a lateral reactor made of nickel and having a diameter of 25 mm and a length of 600 mm was placed 100 g of the same previously dried metallic tungsten powder as used in Example 9 as uniformly as possible, and the reactor was then heated up to about 100°C. Afterward, an N₂ gas having atmospheric pressure was introduced into the reactor from the left end portion thereof at a flow rate of 300 Nml/minute for about 2 hours, followed by drying metallic tungsten. Afterward, the flow rate of the N₂ gas was lowered to 100 Nml/minute and an NF₃ gas having atmospheric pressure was introduced into the reactor from the left end portion thereof at a flow rate of 80 Nml/minute under the same conditions as in Example 9, while the metallic tungsten layer was heated at a temperature of 380 to 400°C as in Example 9, whereby reaction was performed for 2 hours. Afterward, a WF₆-containing gas generated in the reactor was cooled to collect WF₆ in the same manner as in Example 9. The yield amount of WF₆ was 28 g, and its yield ratio was 44% based on fluorine, which values were so low that they did not reach even the half levels of the values in Example 9. In addition, the content of Fe in WF₆ was 1.2 ppm, which was indicative that the product was not a little contaminated with the metallic tungsten powder. Example 16 Tungsten oxide (WO₃) powder (iron content = 55 ppm) of 99.9% purity which had been previously dried at 120°C for 2 hours in an N₂ gas atmosphere was mixed sufficiently with NaF powder (first class grade chemical) which had been previously dried in like manner, in a weight ratio of 1:1, and the resulting mixture was then tableted under a tableting pressure of 2000 bar (2 t/cm²) by the use of a small-sized tableting machine to obtain cylinder-shaped pieces each having a diameter of 5 mm and a height of 5 mm. Compressive strength of the molded pieces was extremely high, 260 bar (260 kg/cm² ) in terms of an average value. Next, a vertical reactor made of nickel and having an inner diameter of 19 mm and a height of 600 mm was filled in the central portion thereof with the molded pieces. Afterward, an N₂ gas having atmospheric pressure was introduced into the reactor from its underside at a flow rate of 300 Nml/minute for about 2 hours, while the filler layer of the molded pieces was heated at about 120°C. After the stop of the introduction of the N₂ gas, while the filler layer of the molded pieces was heated at a temperature of 380 to 400°C, an F₂ gas diluted with the N₂ gas and having a concentration of about 20% by volume and atmospheric pressure was introduced into the reactor from its underside at a flow rate of 300 Nml/minute for 3 hours, whereby reaction was performed. A WF₆-containing gas generated in the reactor was led into a refrigerant trap cooled to a temperature of -80°C, and the gas was liquefied and collected therein. After the completion of the reaction, the trap was evacuated by a vacuum pump to remove the N₂ gas used to dilute the F₂ gas, the unreacted F₂ gas and a secondarily formed O₂ gas which slightly remained in the liquefied WF₆. The yield amount of WF₆ was 45 g, and its yield ratio was as high as 94% based on fluorine. In addition, the molded pieces in the reactor maintained the predetermined shape without breaking. Since it was difficult to quantitatively analyze the content of the metal tungsten present in the collected WF₆, the content of iron (Fe) which was present in a relatively great amount as an impurity in tungsten was instead measured by high-frequency induction combination plasma (ICP) analysis. As a result, it was apparent that the content of iron was 0.05 ppm or less. This result indicates that tungsten oxide which was the raw material was prevented substantially from flying about. Examples 17 to 23 The same procedure as in Example 16 was repeated with the exception that tungsten oxide as the oxide was replaced by each of the oxides shown in Table 4 and sodium fluoride as the molding auxiliary was replaced with each solid metal fluoride shown in Table 4 in each amount shown in Table 4, in order to obtain molded pieces in each amount shown in Table 4 (prior to the molding, each oxide and molding auxiliary were dried as in Example 16). The same reactor as used in Example 16 was filled with the molded pieces in each amount shown in Table 4, and an F₂ gas was introduced thereinto under reaction conditions shown in Table 4, and following the same procedure as in Example 16, a variety of gaseous fluorides were prepared. In this case, conditions of drying the molded pieces prior to the introduction of the F₂ gas into the reactor were the very same as in Example 16. Yield amounts, yield ratios and Fe contents of the thus obtained gaseous fluoride products are set forth in Table 4. The yields of the respective examples were as high as in Example 16, and the oxide did not fly about as in Example 16. Additionally, in all the examples, after the completion of the reaction, it was confirmed that the molded pieces did not break. Comparative Example 3 In the bottom portion of a lateral reactor made from nickel and having a diameter of 25 mm and a length of 600 mm was placed 100 g of the same previously dried tungsten oxide powder as used in Example 16 as uniformly as possible, and the reactor was then heated up to about 100°C. Afterward, an N₂ gas was introduced into the reactor from the left end portion thereof at a flow rate of 300 Nml/minute for about 2 hours, followed by drying tungsten oxide. After the stop of the introduction of the N₂ gas, an F₂ gas diluted with an N₂ gas was introduced into the reactor from the left end portion thereof under the same conditions as in Example 16, while the tungsten oxide layer was heated at a temperature of 380 to 400°C as in Example 16, and a WF₆-containing gas generated in the reactor was then cooled to collect WF₆. The yield amount of WF₆ was 22 g, and its yield ratio was 46% based on fluorine, which values were so low that they did not reach even the half levels of the values in Example 16. In addition, the content of Fe in WF₆ was 0.8 ppm, which was indicative that the product was not a little contaminated with the tungsten oxide powder. Example 23 Tungsten oxide powder (iron content = 50 ppm) of 99.9% purity which had been previously dried at 120°C for 2 hours in an N₂ gas atmosphere was mixed sufficiently with NaF powder (first class grade chemical) which had been previously dried in like manner, in a weight ratio of 1:1, and the resulting mixture was then tableted under a tableting pressure of 2000 bar (2 t/cm²)by the use of a small-sized tableting machine to obtain cylinder-shaped pieces each having a diameter of 5 mm and a height of 5 mm. Compressive strength of the molded pieces was extremely high, 265 bar (265 kg/cm²)in terms of an average value. Next, a vertical reactor made of nickel and having an inner diameter of 25 mm and a height of 600 mm was filled in the central portion thereof with the molded pieces. Afterward, an N₂ gas having atmospheric pressure was introduced into the reactor from its underside at a flow rate of 300 Nml/minute for about 2 hours, while the filler layer of the molded pieces was heated at about 100°C. The flow rate of the N₂ gas was then lowered to 100 Nml/minute, and while the filler layer of the molded pieces was heated at a temperature of 380 to 400°C, an NF₃ gas having atmospheric pressure was introduced into the reactor at a flow rate of 70 Nml/minute, whereby reaction was performed for 2 hours. A WF₆-containing gas generated in the reactor was led into a refrigerant trap cooled to a temperature of -80°C, and the gas was liquefied and collected therein. After the completion of the reaction, the trap was evacuated by a vacuum pump to remove the N₂ gas used as a carrier gas, a secondarily formed N₂ gas and O₂ gas and the unreacted NF₃ gas. The yield amount of WF₆ was 52 g, and its yield ratio was as high as 93% based on fluorine. In addition, the molded pieces in the reactor maintained the predetermined shape without breaking. Since it was difficult to quantitatively analyze the content of tungsten oxide present in the collected WF₆, the content of iron (Fe) which was present in a relatively great amount as an impurity in tungsten oxide was instead measured by high-frequency induction combination plasma (ICP) analysis. As a result, it was apparent that the content of iron was 0.05 ppm or less. This result indicates that metal tungsten oxide which was the raw material was prevented from flying about. Examples 24 to 30 The same procedure as in Example 23 was repeated with the exception that tungsten oxide as the oxide was replaced with each oxide shown in Table 5 and sodium fluoride as the molding auxiliary was replaced with each solid metal fluoride shown in Table 5 in each amount shown in Table 5, in order to obtain molded pieces in each amount shown in Table 5 (prior to the molding, each metallic oxide and molding auxiliary were dried as in Example 23). The same reactor as used in Example 23 was filled with the molded pieces in each amount shown in Table 5, and the molded pieces were dried under the same conditions as in Example 23. Afterward, an NF₃ gas and carrier gas were introduced thereinto under reaction conditions shown in Table 5, and following the same procedure as in Example 23, a variety of gaseous fluorides were prepared. In this case, conditions of drying the molded pieces prior to the introduction of the NF₃ gas into the reactor were the very same as in Example 23. Yield amounts, yield ratios and Fe contents of the thus obtained gaseous metallic fluoride products are set forth in Table 5. The yields of the respective examples were as high as in Example 23, and the oxides did not fly about as in Example 23. Additionally, in all the examples, after the completion of the reaction, it was confirmed that the molded pieces did not break. Comparative Example 4 In the bottom portion of a lateral reactor made of nickel and having a diameter of 25 mm and a length of 600 mm was placed 100 g of the same previously dried tungsten oxide powder as used in Example 23 as uniformly as possible, and the reactor was then heated up to about 100°C. Afterward, an N₂ gas having atmospheric pressure was introduced into the reactor from the left end portion thereof at a flow rate of 300 Nml/minute for about 2 hours, followed by drying tungsten oxide. The flow rate of the N₂ gas was then lowered to 100 Nml/minute and an NF₃ gas having atmospheric pressure was introduced into the reactor from the left end portion thereof at a flow rate of 70 Nml/minute under the same conditions as in Example 23, while the tungsten oxide layer was heated at a temperature of 380 to 400°C, whereby reaction was performed for 4 hours. Afterward, a WF₆-containing gas generated in the reactor was cooled to collect WF₆ in the same manner as in Example 23. The yield amount of WF₆ was 48 g, and its yield ratio was 43% based on fluorine, which values were so low that they did not reach even the half levels of the values in Example 23. In addition, the content of Fe in WF₆ was 1.0 ppm, which was indicative that the product was not a little contaminated with the tungsten oxide powder. Example 30 First, 1.5 kg of metallic tungsten powder of 99.9% purity was mixed sufficiently with 1.5 kg of NaF powder (first class grade chemical), and the resulting mixture was then tableted under a tableting pressure of 2000 bar (2 t/cm²) by the use of a small-sized tableting machine to obtain 3 kg of cylinder-shaped pieces each having a diameter of 15 mm and a height of 5 mm. Compressive strength of the molded pieces was extremely high, 270 bar (270 kg/cm²) in terms of an average value. Next, a vertical reactor made of nickel and having an inner diameter of 75 mm and a height of 1,000 mm was filled in the central portion thereof with the molded pieces. Afterward, an N₂ gas (diluent gas) and H₂ gas (reducing gas) having atmospheric pressure were both introduced into the reactor from its underside at a flow rate of 300 Nml/minute for about 10 hours, while the filler layer of the molded pieces was heated at about 700°C, whereby the molded pieces were thermally treated. Next, the H₂ gas in the reactor was perfectly replaced with the N₂ gas. Afterward, the flow rate of the N₂ gas was lowered to 100 Nml/minute, and while the filler layer of the molded pieces was heated at a temperature of 380 to 400°C, an NF₃ gas having atmospheric pressure was introduced into the reactor at a flow rate of 100 Nml/minute, whereby reaction was performed for 50 hours. A WF₆-containing gas generated in the reactor was led into a collector vessel cooled to a temperature of -80°C, so that the gas was liquefied and collected therein. After the completion of the reaction, the collector vessel was evacuated by a vacuum pump to remove the N₂ gas used as a carrier gas, a secondarily formed N₂ gas and the unreacted NF₃ gas. The yield amount of WF₆ was 1,960 g, and its yield ratio was as high as 98% based on the NF₃ gas. Furthermore, WF₆ was removed from the collector vessel by distillation at a little higher temperature than the boiling point of WF₆, and the interior of the collector vessel was then observed. However, it was confirmed that neither the white solid nor metallic tungsten were present therein. Examples 31 to 37 The same procedure as in Example 30 was repeated with the exception that metallic tungsten was replaced with each simple element shown in Table 6, and NaF as the molding auxiliary was replaced with each solid metallic fluoride shown in Table 6 in each amount shown in Table 6 by the use of each tableting pressure shown in Table 6, in order to obtain molded pieces. The same reactor as used in Example 30 was filled with the molded pieces, and a reducing gas and dilute gas shown in Table 6 were introduced into the reactor under reaction conditions shown in Table 6 as in Example 30 to thermally treat the molded pieces. Afterward, an F₂ gas or NF₃ gas and a carrier gas were introduced into the reactor under reaction conditions shown in Table 6, and following the same procedure as in Example 30, a variety of gaseous fluorides were prepared. In this case, yield amounts and yield ratios of the thus obtained gaseous fluoride products are set forth in Table 6, and the yields in the respective examples were as high as in Example 30. Additionally, in the same manner as in Example 30, it was inspected whether or not the simple metal and a white solid were present in the collected gaseous fluorides. As a result, their presence was not confirmed. Comparative Example 5 The reactor used in Example 30 was filled with 3 kg of the molded pieces obtained in the same manner as in Example 30, and while the filler layer of the molded pieces was heated at a temperature of 700°C, an N₂ gas as an inert gas having atmospheric pressure was then introduced into the reactor at a flow rate of 300 Nml/minute until the water content in the gas at an outlet of the reactor had reached 5 ppm, whereby the water in the molded pieces was removed therefrom. Next, an NF₃ gas and N₂ gas were introduced into the reactor under the same conditions as in Example 30, in order to prepare WF₆. However, 30 hours after the introduction of these gases had been commenced, the pressure in the reactor rose, and the amount of the gas coming from the outlet of the reactor decreased extremely, so that it became impossible to successively feed the NF₃ gas and N₂ gas. At this point of time, therefore, the reaction was brought to an end. After the stop of the reaction, this cause was inspected. As a result, it was found that a pipe disposed between the outlet of the reactor and a collector vessel was clogged with a white solid. In addition, the thus collected WF₆ was distilled off as in Example 30, and the interior of the collector vessel was observed, and the presence of the white solid was confirmed.

Claims: A method for preparing a gaseous fluoride of a metal or of arsenic, phosphorous, sulfur, silicon or boron by reacting the element or its oxide with a fluorine gas or nitrogen trifluoride gas, said method being characterized by comprising the steps of mixing said element or its oxide with a molding auxiliary comprising a solid metallic fluoride which does not react with fluorine and nitrogen trifluoride; molding the resulting mixture under pressure; and contacting the molded pieces with said fluorine gas or nitrogen trifluoride gas, while said molded pieces are heated. A method for preparing a gaseous fluoride according to Claim 1 wherein said gaseous fluoride is represented by MFm in which M is at least one element selected from the group consisting of elements in the groups IIIA, IIIB, IVA, IVB, VA, VB,VIA and VIB of the periodic table, and m is an integer of 3 to 6. A method for preparing a gaseous fluoride according to Claim 2 wherein said gaseous fluoride is at least one compound selected from the group consisting of tungsten hexafluoride (WF₆), molybdenum hexafluoride (MoF₆), antimony trifluoride (SbF₃), antimony pentafluoride (SbF₅), niobium pentafluoride (NbF₅), arsenic trifluoride (AsF₃), phosphorus trifluoride (PF₃), boron trifluoride (BF₃), tantalum pentafluoride (TaF₅), titanium tetrafluoride (TiF₄), silicon tetrafluoride (SiF₄), germanium tetrafluoride (GeF₄), sulfur hexafluoride (SF₆) and uranium hexafluoride (UF₆). A method for preparing a gaseous fluoride according to Claim 1 wherein said solid metallic fluoride which does not react with fluorine and nitrogen trifluoride is at least one compound selected from the group consisting of fluorides of metals in the groups IA , IIA and IIIB of the periodic table. A method for preparing a gaseous fluoride according to Claim 4 wherein said solid metallic fluoride which does not react with fluorine and nitrogen trifluoride is at least one compound selected from the group consisting of lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), rubidium fluoride (RbF), cesium fluoride (CsF), beryllium fluoride (BeF₂), magnesium fluoride (MgF₂), calcium fluoride (CaF₂), strontium fluoride (SrF₂), barium fluoride (BaF₂), aluminum fluoride (AlF₃), gallium fluoride (GaF₃), indium fluoride (InF₃), thallium fluoride (TlF₃) and aluminum sodium fluoride (Na₃AlF₆). A method for preparing a gaseous fluoride according to Claim 1 wherein said element is at least one selected from metals in the groups IIIA, IIIB, IVA, IVB, VA, VB, VIA and VIB of the periodic table. A method for preparing a gaseous fluoride according to Claim 6 wherein said element is at least one selected from the group consisting of tungsten (W), molybdenum (Mo), antimony (Sb), niobium (Nb), arsenic (As), phosphorus (P), boron (B), tantalum (Ta), titanium (Ti), silicon (Si), germanium (Ge), sulfur (S) and uranium (U). A method for preparing a gaseous fluoride according to Claim 1 wherein said oxide is at least one selected from the group consisting of tungsten oxide (WO₃), molybdenum oxide (MoO₃), uranium oxide (UO, UO₂ and U₃O₈), arsenic oxide (As₂O₅), antimony oxide (Sb₂O₅), niobium oxide (Nb₂O₅), tantalum oxide (Ta₂O₅), titanium oxide (TiO₂) and germanium oxide (GeO₂). A method for preparing a gaseous fluoride according to Claim 1 wherein the amount of said element or its oxide in the mixture is in the range of 30 to 70% by weight based on said mixture of said element or its oxide and said molding auxiliary. A method for preparing a gaseous fluoride according to Claim 1 wherein said mixture of said element or its oxide and said molding auxiliary is molded under a pressure of 0.5 to 10 t/cm². A method for preparing a gaseous fluoride according to Claim 1 wherein said molded pieces have a shape of a cylinder, ring, cubic sphere, gear wheel shape or ribbed shape. A method for preparing a gaseous fluoride according to Claim 1 wherein said reaction of said molded pieces with said fluorine gas or nitrogen trifluoride is performed at a temperature of 150 to 700°C. A method for preparing a gaseous fluoride according to Claim 1 wherein said reaction of said molded pieces with said fluorine gas or nitrogen trifluoride is performed under a pressure of 1 to 10 kg/cm². A method for preparing a gaseous fluoride according to Claim 1 wherein said reaction of said molded pieces with said fluorine gas or nitrogen trifluoride is performed for a period of 1 to 20 hours. A method for preparing a gaseous fluoride according to Claim 1 wherein when the molded pieces are brought into contact with said fluorine gas or nitrogen trifluoride gas, said molded pieces are present in the form of a filler layer. A method for preparing a gaseous fluoride according to Claim 1 wherein after thermally treated in a reducing gas atmosphere, said molded pieces are brought into contact with said fluorine gas or nitrogen trifluoride gas under heating. A method for preparing a gaseous fluoride according to Claim 1 wherein said heating treatment in said reducing gas atmosphere is performed at a temperature of 500 to 850°C. A method for preparing a gaseous fluoride according to Claim 16 wherein said heating treatment in said reducing gas atmosphere is performed for a period of 1 to 100 hours in the case that the raw material is said element, or for a period of 2 to 500 hours in the case that the raw material is said oxide.

Inventors: HARADA ISAO MR, IWANAGA NARUYUKI MR, KIKKAWA AKIO MR, NISHITSUJI TOSHIHIKO MR, YODA YUKIHIRO MR, HARADA, ISAO, MR., IWANAGA, NARUYUKI, MR., KIKKAWA, AKIO, MR., NISHITSUJI, TOSHIHIKO, MR., YODA, YUKIHIRO, MR.
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1989
Main classification: C01B9
IPC codes: C01B35, C01B25, C01B17, C01B9, C01B33, C01G41