PROCESS FOR PURIFYING NITROGEN-FLUORIDE

Abstract: A process for continuously producing NF₃ and a process for removing O₂, N₂, etc. from NF₃ containing them are disclosed. This process comprises introducing an additional gas such as He, Ne, etc. in the liquefaction and con­ centration of NF₃ for preventing contamination with a H₂ gas. In addition, a process of introducing one or more of He, Ne and Ar as a third component in low temperature distillation of the resulting liquefied NF₃ is also disclosed. This invention enables continuous operation safely for a long time and a highly pure NF₃ can be obtained since O₂, N₂, etc. in NF₃ can be effectively removed. ABSTRACT The present invention has as its technical theme to provide a liquefactive and condensable method that enables the reaction step of forming NF₃ to be operated safely and continuously and purifying method that eliminates O₂, N₂, etc. in NF₃. The present invention is directed to a method wherein liquefaction and condensation of NF₃ is carried out with an additive gas such as He, and Ne introduced in order to prevent H₂ from mixing. The present invention is also directed to a purifying method wherein deeply cooled distillation of liquefied NF₃ is carried out with one or more of He, He, and Ar introduced as a third component gas. According to the present invention, a safe long-term countinuous operation has been made possible. Since O₂, N₂, etc. in NF₃ can be eliminated efficiently, highly pure NF₃ can be obtained.

Description: The present invention relates to a method for preparing purified liquid nitrogen-fluoride, in particular to a method of purifying NF₃ formed in the production of NF₃. NF₃ is a colorless gas having such physical properties that the boiling point is -129 °C, and the melting point is -208 °C. Since the activity of NF₃ is suitably weak as a fluorine source in comparison with fluorine (F₂), and the toxicity thereof is low, conventionally NF₃ was used as a fluorine source in adjusting fluoroolefins, and was used as an oxidizing agent of high energy fuels. Also, in recent years, NF₃ as a cleaning agent for CVD apparatuses in the field of electronic materials as well as as a dry etching gas for very large-scale integrated circuits has become to be used. There are many processes for the production of NF₃. As major processes thereof can be mentioned, for example, a molten salt electrolytic process using an ammonium acid fluoride (U.S. Patent No. 3,235,474), a process wherein an ammonium acid fluoride in a molten state is reacted with gaseous fluorine (Japanese Patent Publication No. 8926/1980), and a process wherein an ammonium complex of a metal fluoride in the solid state is reacted with F₂ in the elemental state (Japanese Patent Publication No. 21724/1987). In any of these processes, in the reaction step of producing NF₃, it is required that an inert gas is introduced as a carrier gas into the reaction system in order to prevent explosion to improve the safety or in order to control the reaction suitably, and as this inert gas, inexpensive nitrogen (N₂) gas is employed in most cases. Even in the case wherein the reaction is carried out without making a particular introduction of a carrier gas from the outside, in any of the above processes, N₂ gas is formed as a by-product, and remains mixed in the formed NF₃ gas. Therefore, when NF₃ produced in any of the above processes is to be used for various applications as mentioned above, it is required to eliminate impurities such as N₂ gas, nitrous oxide (N₂)), and carbon dioxide (CO₂). However, since generally N₂ gas (N₂ gas as a carrier gas, and N₂ gas as a by-product) is contained in a considerable amount in the NF₃ produced in any of the above processes, in order to eliminate the N₂ gas, the so-called NF₃ condensing step wherein the N₂ gas is eliminated after the reaction step is joined to follow the reaction step. (Note that, in some cases, a purifying step of eliminating impurities other than N₂ gas such as N₂O and CO₂ as mentioned above, and hydrogen fluoride (HF) that has remained unreacted or has been produced as a by-product is also placed between the reaction step and the condensation step.) In this case, although there are various ways of condensing NF₃, a process of liquefying NF₃ is described in US Patent 4091081 using a refrigerant is commonly used as the most effective process since impurities will not be introduced and the facilities are simple in comparison with other processes wherein compression by using a compressor is effected. As the refrigerant for liquefying, use is made of a liquefied gas having a boiling point lower than that of NF₃ such as liquid nitrogen, liquid air, and liquid argon, and when, of these, liquid nitrogen is used, it is the most preferable because, for example, the condensation of NF₃ becomes easy, N₂ is an inert substance, therefore the use thereof is safe, and N₂ is inexpensive. However, in the case wherein NF₃ is produced by using apparatus including the production steps as mentioned above, when the NF₃ formed in the condensation step is cooled and liquefied by using liquid nitrogen as a refrigerant, because N₂ gas that is a carrier gas and N₂ gas that has been formed as a by-product are also partly liquefied together with NF₃, the pressure in the condensation step is decreased, and the pressure in the whole reaction system is also decreased. Since the formation reaction of NF₃ is carried out continuously, such a state makes quite difficult the control of the reaction in the reaction step, and safety problems arise, making impossible the reaction to be continued for a long period of time. This is described using the molten salt electrolytic process as an example as follows. For example, in the case wherein, in order to produce NF₃, a molten salt electrolysis is carried out in an NH₄F/HF system using, as raw material, acid ammonium fluoride or ammonium fluoride and hydrogen fluoride or a KF/NH₄F/HF system that is formed by adding as a raw material, acid potassium fluoride or potassium fluoride to the NH₄F/HF system, NF₃ gas is released from the anode of the electrolytic cell while H₂ gas is released from the cathode. If the NF₃ gas and the H₂ gas are mixed, they will explode, and therefore, in order to avoid the mixing, as shown in Fig. 3, the electrolytic cell is provided with a partition 13 for separating the anode 14 and the cathode 15 in such a manner that the partition 13 is fixed to a lid plate 16 with a suitable distance between it and the bottom plate of the electrolytic cell, so that the electrolytic cell is formed with an anode chamber 18 and a cathode chamber 19 (the partition is positioned in such a way that the electrolyte can move freely between the anode chamber and the cathode chamber at the bottom part of the electrolytic cell.) to prevent the NF₃ gas and the H₂ gas from mixing. In order to prevent the partition at the time of the electrolysis from having a bipolarization phenomenon, the partition is made of a fluorocarbon resin or has the surfaces coated with a fluorocarbon resin. As the fluorocarbon resin used for the partition in the present invention, any of generally known ones can be suitably used such as polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene/hexafluoropropylene copolymer, tetrafluoroethylene/ethylene copolymer, tetrafluoroethylene/perfluoroalkylvinyl ether copolymer, and chlorotrifluoroethylene/ethylene copolymer. The NF₃ gas and the H₂ gas released respectively from the anode chamber and the cathode chamber of the electrolytic cell are led to the outside of the system, and then the NF₃ gas is led to the next step, that is, the condensation step while the H₂ gas is utilized as a by-product or is burnt and then released into the atmosphere. Therefore it is impossible to keep the pressure in the anode chamber and the pressure in the cathode chamber at the same level all the time, resulting in a pressure difference. As a result, the level of the electrolyte in the anode chamber becomes different from the level of the electrolyte in the cathode chamber. As the pressure difference becomes great, the difference in the level of the electrolyte surface becomes greater between the anode chamber and the cathode chamber, and when this difference exceeds a certain limit, either the NF₃ gas or the H₂ gas passes below the partition, and then mixes with the other gas thereby leading in the end to formation of a gas within the explosion limit. In order to obviate this, an inert gas is introduced as a carrier gas into the anode chamber and the cathode chamber of the electrolytic cell respectively in suitable amounts to control the difference. Thus, the term "carrier gas" means a gas that is fed into the reaction tank for the purpose of improving the safety and controlling the reaction. In this case, as the carrier gas, a gas that is inert to NF₃ gas and H₂ gas, and can be easily separated from the NF₃ gas in the subsequent purifying step for example N₂, or Ar is used, and since, of these gases, N₂ gas is inexpensive, it is preferable. Further, in the molten salt electrolytic process, since N₂ is formed as a by-product in a considerable amount in the reaction step, and the thus formed N₂ interfuses into the produced NF₃, it is also preferable to use N₂ gas as the carrier gas. The NF₃ gas continuously produced in the electrolytic cell is then cooled with liquid nitrogen as stated above to be liquefied and condensed. (It should be noted that, in the case of the molten salt electrolytic process, since for example hydrogen fluoride, etc. are included as unreacted material, it is preferable that a purifying step of eliminating the hydrogen fluoride, etc. is placed between the reaction step and the liquefying/condensing step.) However, since, in this liquefying/condensing step, the N₂ gas introduced as a carrier gas into the electrolytic cell in the reaction step is also partly liquefied, the pressure in the liquefying/condensing apparatus decreases, and due to this the pressure in the anode chamber of the electrolytic cell also decreases, leading to a pressure difference between the anode chamber and the cathode chamber. As a result, for the same reason as stated before, the hydrogen gas will mix into the NF₃ gas formed in the anode chamber thereby forming an explosive gas. Since it is impossible to prevent completely the drop of the pressure in the anode chamber that will lead to the formation of the explosive gas only by the control of the supply of N₂ gas into the anode chamber, there is a serious problem that the operation cannot be continued for a long period of time to produce NF₃. Therefore, it is desired to develop a method that can solve the above-mentioned problems, is safe, and can produce NF₃ continuously for a long period of time, and more specifically to develop a method of recovering NF₃ by liquefaction and condensation from a mixed gas of NF₃ obtained by a molten salt electrolytic process and a carrier gas. NF₃ is produced by the various processes as mentioned above, and generally the thus produced NF₃ contains relatively large amounts of impurities such as nitrous oxide (N₂O), carbon dioxide (CO₂), oxygen (O₂), and nitrogen (N₂). Therefore, for example, when the NF₃ is to suitably be used as a cleaning agent for CVD apparatuses in the field of electronic materials or as a dry etching agent or the like in the production of LSI's, it is required that the above-mentioned impurities are eliminated as far as possible to make the NF₃ highly pure. As a method of eliminating the above-mentioned impurities, usually a method is used wherein the NF₃ is brought in contact with an adsorbent such as zeolites thereby eliminating the impurities, because the method is the most effective and simple. However, although the treatment with a zeolite can eliminate efficiently impurities having relatively high boiling points such as N₂O, and CO₂ present in the NF₃, the treatment can hardly eliminate components having relatively low boiling points such as O₂, and N₂, and a method of eliminating them has not yet been known. Although distillation technique is effective and is commonly used generally as means of separating components having different boiling points, when the inventors attempted distillation technique to eliminate low-boiling components in the NF₃, the low-boiling components could not efficiently be separated in spite of the fact that there is a sufficient difference in the boiling point between NF₃ and the low-boiling components, and it was found that for example N₂ remained in an amount of at least as much as thousands ppm in the NF₃. Development of a method of eliminating efficiently low-boiling components such as O₂, and N₂ to yield highly pure nitrogen fluoride is desired. A first object of the present invention is to provide a method for preparing purified liquid NF₃ safely and continuously for a long period of time in the production of NF₃, and to provide a process of recovering highly pure NF₃ gas from a mixed gas of NF₃ gas and a carrier gas in a liquefying/condensing step. A second object of the present invention is to provide a method of eliminating efficiently low-boiling impurities such as O₂, and N₂ from the produced NF₃, and capable of producing NF₃ having an ultra-high purity of 99.99 wt. %, or so-called four-nines % pure NF₃. In solving the above objects the present invention is directed to a method for preparing purified liquid NF₃ which method comprises leading a mixed gas of nitrogen - fluoride and a carrier gas comprising at least one of nitrogen, argon, helium and neon to a cooling step, cooling said mixed gas in the co-presence of an additive gas comprising helium and / or neon whose boiling point is lower than that of nitrogen gas and that is not dissolved in liquid nitrogen - fluoride to obtain liquefied nitrogen - fluoride mainly comprising nitrogen - fluoride, and recovering the obtained liquefied nitrogen - fluoride. As the gas whose boiling point is lower than that of nitrogen gas and that is not dissolved in liquid nitrogen trifluoride, preferably use is made of helium or neon. The gas whose boiling point is lower than that of nitrogen gas and that is incompatible with nitrogen trifluoride is added in the liquefying/condensing step, and also includes the carrier gas containing helium or neon that has been present in the reaction tank, or helium and/or neon itself that has been added itself as a carrier gas, and thus the term "in the copresence" means this. The present invention is also directed to a method of purifying nitrogen - fluoride, wherein nitrogen - fluoride gas mixed with impurity gases comprising oxygen or nitrogen is cooled to be liquefied and is subjected to, or liquefied nitrogenfluoride is subjected to low temperature distillation in the copresence of a third component gas comprising helium, argon or neon whose boiling point is lower than that of nitrogen - fluoride, and that is not dissolved in liquid nitrogen - fluoride. In the present invention, when a mixed gas of NF₃ and a carrier gas is led into a step (liquefying/condensing step) of cooling NF₃ joined after a reaction step of forming NF₃, particularly joined after a reaction step of forming NF₃ by an molten salt electrolytic process, and the NF₃ is liquefied and condensed, the liquefaction/condensation is carried out in the copresence of a gas whose boiling point is lower than that of nitrogen gas, and that is not dissolved in liquid. The term "in the copresence" is intended to mean that the gas whose boiling point is lower than that of nitrogen gas, and that is incompatible with NF₃ may include, in addition to one added in the liquefaction/condensation step, one that has been added to the carrier gas, or one that has served as the carrier gas itself. In the present invention, as the gas whose boiling point is lower than that of nitrogen gas, and that is not dissolved in liquid NF₃ is preferably used for example helium (He), or neon (Ne). The gas whose boiling point is lower than that of nitrogen gas, and that is not dissolved liquid NF₃, i.e., a gas other than nitrogen, is abbreviated hereinafter to an additive gas. The additive gas may comprise a single gas or a mixed gas of one or more gases. In the present invention, as the apparatus for liquefying and condensing NF₃, generally use is made of an apparatus comprising a closed container for collecting liquefied NF₃ equipped with a gas inlet and a gas discharge outlet (hereinafter abbreviated to a collecting container), and a refrigerant tank for cooling the collecting container. In order to avoid allowing air to enter the collecting container due to the back-flow of the additive gas, the N₂ gas as a by-product, and the carrier gas (usually N₂ gas) used in the reaction step that are to be discharged, it is preferable that a sealing tank filled with a liquid inert to these gases such as water is provided at the tip of the gas discharge outlet, and after these gases to be discharged are bubbled through the liquid in the sealing tank, they are discharged into the atmosphere. In the present invention, the NF₃ gas formed in the reaction step containing the N₂ gas, the carrier gas which may contain He and/or Ne), etc. is cooled in the copresence of an additive gas to liquefy and condense the NF₃, and this operation is carried out under a pressure that is the same as or slightly higher than the normal pressures, and is lower than the pressure in the reaction step. It is preferable that the pressure is in the range of 0 to 0.5 kg/cm² G. The control of this pressure is carried out by the amount of the additive gas that is copresent (by the amount of the additive gas introduced into the reaction tank and/or the collecting container). However, since this additive gas is accompanied by part of the NF₃ when the additive gas is discharged outside of the system from the discharge outlet of the collecting container together with the carrier gas from the reaction step, in order to prevent the loss of NF₃ due to this accompaniment as far as possible, it is unfavorable to introduce, into the collecting container, an excess of the additive gas including, if added, the amount of the additive gas added to the carrier gas, and therefore generally the control of the pressure is carried out with the amount of the additive gas being 0.01 to 10 times by volume as much as the amount of the NF₃ gas generated in the reaction step. If the volume ratio is less than 0.01, the effect of the addition of the third component gas is not obtained, while if the volume ratio is over 10, the loss of NF₃ becomes great. The amount of NF₃ accompanying the discharged gases is also affected by the cooling/condensing temperature of the NF₃, and it is preferable that this temperature is as low as possible within the below mentioned range. That is, the temperature range is up to the liquefying temperature of NF₃ and down to the liquefying temperature of N₂ gas. In other words, the temperature range is preferably -129 °C to -196°C. There is no particular limitation on the method of introducing the additive gas used in the present invention into the collecting container, thus various methods can be used, and there is a method wherein He and/or Ne is used as the carrier gas itself, or a method wherein He and/or Ne is previously mixed with the carrier gas that is for example N₂. If the additive gas is added in the liquefying/condensing step, it is carried out simply for example as shown in Fig. 1 by a method wherein an introducing pipe 3 of the additive gas is connected to an introducing pipe 2 of the gas (a mixed gas consisting of NF₃, N₂, the carrier gas, etc.) generated in the reaction step that extends to the collecting container 1, or as shown in Fig. 2 by a method wherein an introducing pipe 2 of the reaction gas, and a introducing pipe 3 of the additive gas are provided separately to the collecting container 1 to introduce them into the collecting container 1. It is preferable that the introducing pipes to the collecting container 1 are not placed into the liquefied NF₃ in the collecting container 1. Since the liquefied NF₃ that has thus been recovered by the liquefaction/condensation still contains relatively large amounts of impurities that are components having relatively high boiling points such as N₂O, and CO₂, and components having relatively low boiling points such as O₂, and N₂, it is required that the NF₃ is freed from these impurities to be purified. Of these impurities, N₂O, and CO₂ that are impurities having relatively high boiling points can be eliminated by a prior known method, wherein it is brought in contact with an adsorbent such as active alumina or a zeolite, more efficiently than by means of deeply cooled distillation. For that reason, it is preferable that, before or after deeply cooled distillation, the NF₃ is brought in contact with an adsorbent such as active alumina or a zeolite so that NO₂ and CO₂ may be eliminated. A method of eliminating N₂O, CO₂, HF, etc. that are relatively-high boiling point impurity components by using active alumina as an adsorbent is described in more detail as an example. If a zeolite is used, the method is carried out in the same way. It is preferable to use active alumina having a particle size of 8 to 12 mesh. The temperature is 0 to -125 °C, the pressure is 0 to 5 kg/cm² G, the contact time is 10 sec to 1 min, and generally the gas is passed through a packed column loaded with active alumina, so that impurities are adsorbed and eliminated. There is no particular limitation on active alumina, and although any active alumina that is usually commercially available can be used, it is preferable to use one in the form of particles having a higher surface area that has been heated to 250 to 900 °C, preferably 250 to 600 °C to be dehydrated. If this temperature is lower than 250 °C, water remains in the active alumina, and when NF₃ gas is passed through the active alumina layer, the ability to eliminate N₂O, and CO₂ lowers. In contrast, if the temperature is higher than required, it is not preferable because such problems arise that not only the energy is lost, but also the container for heat-treating the active alumina such as the column is attacked. Although the dehydration treatment of active alumina by heating may be carried out in air, it is preferable that the dehydration treatment is carried out under a flow of an inert gas such as nitrogen gas free from water since the heating is effected for the purpose of evaporating the water contained in the active alumina, and also it is preferable the dehydration treatment is carried out under reduced pressure while the gas is sucked. Although it is enough if the heating is carried out for 30 min at the above-mentioned temperature and in the above-mentioned atmosphere, generally the heating is carried out for 1 to 2 hours to make the dehydration sure. The active alumina that has been dehydrated by heating is cooled below normal temperatures by standing or with force, and, in this case, it must be avoided that water comes into the alumina. Therefore, to do that, it is preferable that the dehydration treatment of active alumina by heating is carried out with the active alumina loaded for example in a column, then after the dehydration the column is cooled, and NF₃ gas is passed through the active alumina layer. It is preferable that the temperature at which the NF₃ gas is passed is as low as possible, and generally the temperature that is used is 0 °C or below. However, since the boiling point of NF₃ is -129 °C, the operation becomes difficult in practice if the temperature is at or below -129 °C. Therefore, NF₃ gas is passed at 0 to -125 °C in the present invention. Although the pressure of the NF₃ gas under which the NF₃ is passed is not particularly limited, the pressure is preferably, for example, on the order of 0 to 5 kg/cm² G because that pressure is easy to handle. The concentration ranges of impurities in the NF₃ before and after the treatment with active alumina are as follows: N₂O, 0.1 to 1 %, CO₂, 0.1 to 1 %, O₂, 0.1 to 1 %, and N₂, 0.1 to 1 %, in the crude NF₃ before the treatment with active alumina; and N₂O, 0.1 to 10 ppm, CO₂, 0.1 to 10 ppm, O₂, 0.1 to 1 %, and N₂, 0.1 to 1 %, in the NF₃ after treatment with active alumina. That is, low-boiling impurities such as O₂, and N₂ cannot be eliminated by adsorption on active alumina. Although, of the above two purification methods, either one can be carried out first, generally after N₂O, and CO₂ that are relatively-high boiling components are brought in contact with active alumina to be eliminated, low-boiling components are eliminated by the present deeply cooled distillation method. In the present invention, as stated above, preferably N₂ and CO₂ that are relatively-high boiling components are eliminated previously, and then the NF₃ gas containing as main impurity gas O₂ and/or N₂ is cooled to be liquefied. Although as the refrigerant used for the liquefaction can be used a material whose boiling point is lower than that of nitrogen fluoride such as liquid nitrogen, liquid air, liquid argon, and LNG, of these gases, liquid nitrogen is the most preferable because it is inexpensive and inert. For example, it is favorable to combine the liquefaction with a process of vaporizing LNG because the energy can be saved. Also a method wherein the heat of the refrigerant of the refrigerator is exchanged directly for the heat of the crude NF₃ can be effectively employed. If purification is carried out after the production of NF₃, this liquefying step is not required because the liquefied NF₃ obtained in the liquefying/condensing step in the production step can be obtained as raw material directly. In the present invention, the thus liquefied NF₃ is subjected to deeply cooled distillation so that the NF₃ may be freed from low-boiling components such as O₂, and N₂ contained therein and purified, and when the deeply cooled distillation is carried out, a third component whose boiling point is lower than that of NF₃ and that is not dissolved in liquid NF₃ is made copresent. As such a third component, helium, neon, and argon can be exemplified. The third component may comprise a single gas or a mixture of two or more gases. In the present invention, as a specific method of making the third component copresent in the crude NF₃, various methods can be used such as a method wherein the third component is fed together with the crude NF₃ into the distillation column as shown in Fig. 4, a method wherein the crude NF₃ and the third component are fed into the distillation column through separate feeding ports as shown in Fig. 5, a method wherein after the inside atmosphere of the distillation column is replaced with the third component, the crude NF₃ is fed into the distillation column as shown in Fig. 6, or a method wherein the third component in the gaseous form is bubbled through the crude NF₃ liquefied and staying in the bottom of the distillation column as shown in Fig. 7. Of these methods, the method wherein the third component in the gaseous form is bubbled through the crude NF₃ liquefied and staying in the bottom of the distillation column is particularly preferable because the efficiency of eliminating low-boiling components such as O₂ and N₂ is high. Although the methods shown in Figs. 4 to 7 are of continuous distillation type, if the method is carried out batch-wise, a method as shown in Fig. 8 wherein the third component in the gaseous form is bubbled through the liquefied crude NF₃, or a bubble stirring method as shown in Fig. 9 wherein a stirring machine is provided to stir the crude NF₃ is preferable because the efficiency of eliminating low-boiling components is high. When the method wherein the third component in the gaseous form is bubbled through the NF₃ in the liquid form is used, the more finely dispersed the bubbles, the higher the efficiency of eliminating low-boiling components, and therefore it is preferable to use a dispersing nozzle for bubbling the third component. When the deeply cooled distillation is carried out under normal pressures in any one of the above methods, it is required to prevent air from coming into the distillation apparatus system. Fig. 10 is a diagram showing a detailed mode of carrying out the methods given in the above drawings, and if the deeply cooled distillation is effected under normal pressures, the low-boiling components and the third component are discharged into the atmosphere after they are bubbled through a water sealing tank provided on their way, so that air can be prevented from coming into the distillation apparatus system. In the present invention, the deeply cooled distillation is carried out at a temperature at which part of the NF₃ is in the liquid state, that is, at a temperature ranging between up to the liquefying temperature of NF₃ and down to the solidifying temperature of NF₃ and at which the copresent third component will not liquefy. Consequently, the crude NF₃ must be in a state partly liquefied in the distillation column, and this can be done by cooling the distillation column with a refrigerant such as liquid nitrogen, liquid air, and liquid argon and insulating the distillation column. At the time when the crude NF₃ is fed to the distillation column, although it is not necessarily required that the crude NF₃ is in the liquefied state, and the crude NF₃ may be in the gaseous state, it is preferable that the crude NF₃ is fed to the distillation column with it being in the liquefied state because the loss of NF₃ can be prevented as far as possible in that case. In the present invention, although the operation is simple when the distillation is carried out with the pressure in the distillation column under normal pressures, it is preferable to carry out the distillation under reduced pressure in order to improve the efficiency of eliminating low-boiling components, and the higher the vacuum, the higher the efficiency of elimination. As means of reducing the pressure in the distillation column, use can be made, for example, of a method wherein suction is carried out from the top of the distillation column by using a commonly known decompression device such as a vacuum pump or an aspirator, or a method wherein the mixed gas of the third component and the low-boiling components discharged from the top of the distillation column is liquefied by using liquid helium. Although there is no particular limitation on the amount of the third component that is made copresent in the crude NF₃ when the deeply cooled distillation is carried out, the larger the amount of the third component is, the better it is, in view of the effect of elimination of low-boiling components such as O₂, and N₂. However, the third component is accompanied by part of the NF₃ when it is discharged outside of the system together with the low-boiling components from the top of the distillation column. Therefore, as the copresent amount of the third component is increased, the amount of the loss of the NF₃ increases, so that it is preferable in practice that the volume ratio of the third component in the gaseous state to the crude NF₃ in the liquid state is in the range of from 0.01 to 10. If the volume ratio is less than 0.01, the effect of the addition of the third component gas cannot be observed similarly to the case of the liquefying/condensing step, while if the volume ratio is over 10, it is not preferable because the loss of NF₃ becomes great. There is no particular limitation on the way of feeding the third component to the distillation column, and the third component can be fed to the distillation column continuously, batch-wise or intermittently. Gases other than NF₃, such as N₂F₂ or N₂F₄ can also be applied equivalently to the present purifying method. Figs. 1 and 2 are explanatory diagrams, showing Examples of the present method of introducing an additive gas into a collecting container for liquefying and condensing NF₃ is liquefied and condensed, and Fig. 3 is a flow sheet, showing a reaction apparatus, and a liquefying/condensing apparatus of the apparatus for producing NF₃ used in the Examples and a Comparative Example: wherein the reference numerals account for the following, respectively: 1: a collecting container, 2: an introducing pipe of the gas generated in the reaction step, 3: an additive gas introducing pipe, 4: an exhaust pipe, 5: liquefied NF₃, 6, 7, 8, and 9: valves, 10: a refrigerant tank, 11: a refrigerant, 12: an electrolytic cell, 13: a partition, 14: an electrode (anode), 15: an electrode (cathode), 16: a lid plate, 17: an electrolytic bath, 18: an anode chamber, 19: a cathode chamber, 20, and 21: carrier gas introducing pipes, 22, 23: valves, 24: a piping, 25: a hydrogen gas discharging pipe, 26, and 27: valves, 28: a hydrogen gas concentration meter, 29, and 30: pressure gauges, 31: an HF eliminating apparatus, 32: a sealing tank, 33: a sealing liquid, and 34: an exhaust gas atmosphere discharging pipe. Figs. 4 to 9 are explanatory diagrams, showing distillation apparatuses illustrating the mode of each Example of the present invention, and Fig. 10 is a flow sheet, showing the distillation apparatus used in Examples and Comparative Examples; wherein the reference numerals account for the following, respectively: 100: a distillation column or collecting container, 200: a crude nitrogen fluoride feed line, 300: a third component feed line, 400: a low-boiling component and third component exhaust line, 500: a purified nitrogen fluoride drawing line, 600: a refrigerant tank, 700: a water sealing tank, 800: a vacuum pump, 900: a normal pressure exhaust line, 100: a vacuum exhaust line, 110: an insertion pipe, and 120, 130, 140, 150, 160, and 170: valves. The best modes of carrying out the present invention are described with reference to Examples. In the following Examples, and Comparative Examples, the percentages and parts per million quoted represent percent and part per million by volume. The method of analyzing the gases used in the Examples and Comparative Examples was gas chromatography. Example 1 NF₃ was produced by a molten salt electrolytic process using an apparatus shown in Fig. 3. That is, after acid ammonium fluoride and hydrogen fluoride as raw materials were charged in prescribed amounts into an electrolytic cell 12, the mixture was heated to 130 to 135 °C to form an electrolytic bath 17 of an NH₄F/HF system. Thereafter, while N₂ gas as a carrier gas was being introduced at a flow rate of 90 to 110 mℓ/min through a carrier gas introducing pipe 20 into an anode chamber 18, and was also being introduced at a flow rate of 80 to 90 mℓ/min through a carrier gas introducing pipe 21 into a cathode chamber 19, and the electrolytic bath 17 was kept at the above mentioned temperature, an electric current of 50 amperes was passed from an anode 14 to a cathode 15 to carry out the electrolysis. About 80 mℓ of NF₃ gas per min and about 350 mℓ of H₂ gas per min were released from the cathode and the anode respectively. The H₂ gas containing the carrier gas (N₂) generated at the cathode was discharged into the atmosphere through a hydrogen gas discharging pipe 25. On the other hand, the NF₃ gas containing the carrier gas (N₂), etc. generated at the anode was led into an HF eliminating apparatus 31 via a piping 24, where the contained HF was eliminated, and then the NF₃ gas was led into the collecting container 1 via an introducing pipe 2. The collecting container 1 was previously cooled with it immersed in a refrigerant (liquid nitrogen) 11 that filled a refrigerant tank 10 as shown in Fig. 3. As stated above, the NF₃ gas containing N₂ gas, etc. was introduced into the cooled collecting container 1, and He gas as an additive gas was also introduced at a flow rate of 40 to 50 mℓ/min through an introducing pipe 3 into the cooled collecting container 1 thereby liquefying and condensing the NF₃. The liquefied NF₃ was stored in the collecting container 1, and the He gas, the N₂ gas, etc. were discharged through an exhaust pipe 4, and a sealing tank 32 from an exhaust gas atmosphere discharging pipe 34 into the atmosphere. During that the concentration of the H₂ in the anode chamber 18 of the electrolytic cell 12 was measured continuously by a hydrogen gas concentration meter, and it was found that, at the point when 100 hours had passed since the start of the electrolysis, the H₂ concentration was less than 1 %, and the state was enough to allow the electrolysis to be carried out further. Example 2 Example 1 was repeated, except that as the carrier gas to be introduced to the anode a mixed gas of N₂ at a flow rate of 90 to 110 mℓ/min, and He (an additive gas) at a flow rate of 40 to 50 mℓ/min was used, as the carrier gas to be introduced to the cathode N₂ was introduced at a flow rate of 120 to 140 mℓ/min, and other additive gas was not introduced into the liquefying/condensing step, thereby carrying out the electrolysis and the liquefaction/condensation of the NF₃. The H₂ concentration in the anode chamber 18 of the electrolytic cell 12 at the point when 100 hours had passed since the start of the electrolysis was less than 1 % similarly to Example 1, and the state was enough to allow the electrolysis to be carried out further. Example 3 Example 1 was repeated, except that Ne gas was used as the additive gas to be introduced into the collecting container 1, thereby carrying out the electrolysis and the liquefaction/condensation of the NF₃. The H₂ concentration in the anode chamber 18 of the electrolytic cell 12 at the point when 100 hours had passed since the start of the electrolysis was less than 1 % similarly to Example 1, and the state was enough to allow the electrolysis to be carried out further. Comparative Example 1 Example 1 was repeated, except that the introduction of the additive gas to the collecting container 1 was stopped, thereby carrying out the electrolysis, and the liquefaction/condensation of the NF₃. After 2 hours of the start of the electrolysis, the H₂ gas concentration in the anode chamber 18 of the electrolytic cell 12 increased to about 5 % and reached near the explosion limit, and the state became very dangerous, so that it could not help stopping the electrolysis. Example 4 Crude nitrogen trifluoride (NF₃) that had been subjected to adsorption operation by using commercially available active alumina (in the form of particles having a pore diameter of 5 Å, and a particle diameter of 24 to 48 mesh) to eliminate N₂O, CO₂, etc. thereby having the qualities shown in Table 1 was purified by deeply cooled distillation using the apparatus shown in Fig. 10. That is, after the insulating container 600 was filled with liquid nitrogen to cool the collecting container (having an internal volume of 1 ℓ) 100, the gaseous NF₃ having the qualities shown in Table 1, and helium gas as a third component were fed at a flow rate of 50 mℓ/min and a flow rate of 3 mℓ/min respectively into the collecting container through the lines 200 and 300. The NF₃ liquefied in the collecting container, and the helium gas that was made copresent and fractionally distilled gaseous low-boiling components (oxygen, nitrogen, etc.) were discharged through the water sealing tank 700. (Thus, the inside of the system was kept approximately at the atmospheric pressure.) When the feed of the NF₃ gas reached 100 g, the valves 120 and 130 were closed to stop the feed of the NF₃ gas and helium gas, then after the valve 160 on the way to the water sealing tank 700 was closed, the valve 170 was opened to exhaust the gaseous low-boiling components and helium gas in the system by the vacuum pump 800. After the completion of the exhaustion, the temperature of the collecting container 100 was brought to the normal temperatures, and the liquefied NF₃ in the collecting container 100 was made gaseous and was analyzed. The results are as shown in Table 2 that oxygen and nitrogen were eliminated considerably. Example 5 Example 3 was repeated using the apparatus (Fig. 10) used in Example 4, except that, instead of exhausting the helium gas and the gaseous low-boiling components generated by the deeply cooled distillation by passing them through the water sealing tank 700, they were exhausted through the line 100 by using the vacuum pump 800 to reduce the pressure in the collecting container 100, thereby carrying out deeply cooled distillation of crude NF₃ having the qualities shown in Table 1. The pressure in the system at the time of the deeply cooled distillation was 10 mm Hg abs. The NF₃ in the collecting container 100 was made gaseous and analyzed. The results are as shown in Table 2 that the contents of oxygen and nitrogen were reduced far more than in Example 4. Example 6 In the apparatus shown in Fig. 10, the apparatus was remodeled in such a manner that the insertion pipe 110 was extended to the bottom of the collecting container 100 so that helium gas might be bubbled into liquefied NF₃. After the insulating container 600 was filled with liquid nitrogen to cool the collecting container 100, 100 g of gaseous crude NF₃ having the qualities shown in Table 1 were fed into the collecting container 100 and were liquefied in the collecting container 1. Helium gas was fed at a flow rate of 100 mℓ/min into the liquefied crude NF₃ for 30 min to bubble. The helium gas and evaporated low-boiling components were exhausted through the water sealing tank 700 in the same way as in Example 1 (the pressure in the system was at the atmospheric pressure). The volume ratio of He/NF₃ was about 0.1. Then, in the same way as in Example 4, the inside of the system was exhausted by the vacuum pump 800 to remove the copresent gas. The obtained liquefied NF₃ was made gaseous and was analyzed in the same way as in Example 4. The results are as shown in Table 2 that oxygen and nitrogen were eliminated considerably, thereby giving highly pure NF₃. Example 7 Example 6 was repeated, except that the vacuum pump 800 was used when bubbling was effected using helium gas, and the pressure in the system at the time of the bubbling was 100 mm Hg abs. The analysis of the obtained NF₃ gas is shown in Table 2. Thus, very highly pure NF₃ gas containing trace amounts of oxygen and nitrogen was obtained. Comparative Example 2 Example 4 was repeated, except that the feed of helium gas that was the third component was stopped. The contents of oxygen and nitrogen in the obtained NF₃ gas were as shown in Table 3, indicating that they were scarcely eliminated. Comparative Example 3 Example 5 was repeated, except that the feed of helium gas was stopped. The contents of oxygen and nitrogen in the obtained NF₃ gas were as shown in Table 3, indicating that the elimination rates were very poor though they were not so low as in Comparative Example 2. According to the present method of liquefying and condensing NF₃, as a carrier gas itself a gas whose boiling point is lower than that of nitrogen such as He and/or Ne and that is not dissolved in liquid NF₃ is used in a reaction step in the production of NF₃, or He and/or Ne is contained in a carrier gas such as nitrogen gas other than He and Ne in the reaction step, or a mixed gas of NF₃ and a carrier gas is led into a step (liquefying/condensing step) of cooling NF₃ that follows the reaction step, and a gas such as He and/or Ne is added thereto, so that the NF₃ gas is liquefied and condensed in the copresence of such a gas, thereby providing a quite simple method. In the conventional processes, H₂ gas mixed into the formed NF₃ gas, which attended a risk of explosion, and to prevent such a risk a long-term continuous operation was impossible, whereas by carrying out the present method, as shown in Examples, such a risk of explosion in the production of NF₃ can be obviated completely, a safe long-term continuous operation becomes possible, and therefore the possibility of the utilization of the present invention in the industry is high. Further, according to the present method of purifying NF₃, a method is provided wherein crude NF₃ is subjected to deeply cooled distillation in the copresence of a third component that is incompatible with nitrogen fluoride such as helium, neon, and argon, and therefore it has become possible to eliminate low-boiling components such as O₂, and N₂ in crude NF₃ which could not be eliminated by the conventional purifying methods. Further, as shown in Examples, if said deeply cooled distillation is carried out under reduced pressure, or in said deeply cooled distillation, if a third component is bubbled through the liquefied crude NF₃, low-boiling components can be separated and eliminated with quite good efficiency. Impurities contained in crude NF₃ other than low-boiling components such as N₂O, and CO₂ can be eliminated by combining the present method with an other known purifying method wherein the crude NF₃ is brought in contact with an adsorbent such as active alumina and zeolites, so that quite highly pure nitrogen fluoride having for example a purity of 99.99 % or over suitable for the application in the field of electronic materials can be provided. Although 99.9 % NF₃ can be used as a cleaning agent for CVD apparatuses in the field of electronic materials, it is preferable to use NF₃ having a purity of 99.99 % or over because other gases used for CVD have a purity of 99.99 % or over and if 99.9 % NF₃ is used for CVD, it is considered that the furnace will be contaminated. As dry etching gas for very large-scale integrated circuits, dry etching gases other than NF₃ having a purity of 99.99 % or over cannot be used. Such highly pure NF₃ has not been provided up to now, and meets well the recent desire in the field of electronic materials, and the possibility of the utilization in the industry is high.

Claims: A method for preparing purified liquid nitrogen - fluoride, which method comprises leading a mixed gas of nitrogen - fluoride and a carrier gas comprising at least one of nitrogen, argon, helium and neon to a cooling step, cooling said mixed gas in the copresence of an additive gas comprising helium and / or neon whose boiling point is lower than that of nitrogen gas and that is not dissolved in liquid nitrogen - fluoride to obtain liquefied nitrogen - fluoride mainly comprising nitrogen - fluoride, and recovering the obtained liquefied nitrogen - fluoride. A method as claimed in claim 1, wherein helium and / or neon gas itself is used as carrier gas to be introduced into the nitrogen - fluoride gas, or a carrier gas containing helium and / or neon is used as a carrier gas to be introduced into the nitrogen - fluoride gas, and the mixed gas of said carrier and the nitrogen - fluoride gas is led into the cooling step so that the helium gas and / or neon gas are copresent with the nitrogen - fluoride gas. A method as claimed in claim 1, wherein as a carrier gas to be introduced into the nitrogen - fluoride gas is used nitrogen and / or argon gas, the mixed gas is led into the cooling step, and helium gas and / or neon gas is added to the cooling step so that the helium gas and / or neon gas are copresent with the nitrogen - fluoride gas. A method as claimed in claim 1, wherein the nitrogen - fluoride gas is obtained by a molten salt electrolytic process. A method as claimed in claim 4, wherein the nitrogen - fluoride gas is obtained by a molten salt electrolytic process that uses an electrolytic cell equipped with a partition of a fluorocarbon resin or a partition having surface coated with a fluorocarhon resin. A method as claimed in claim 4, wherein a mixed gas of a carrier gas and nitrogen - fluoride gas obtained by a molten salt electrolytic process is continuously led into the cooling step. A method as claimed in claim 1, wherein the additive gas is copresent with nitrogen - fluoride in a volume ratio of 0.01 to 10 to the nitrogen - fluoride. A method of purifying nitrogen - fluoride, wherein nitrogen - fluoride gas mixed with impurity gases comprising oxygen or nitrogen is cooled to be liquefied and is subjected to, or liquefied nitrogen - fluoride is subjected to low temperature distillation in the copresence of a third component gas comprising helium, argon or neon whose boiling point is lower than that of nitrogen - fluoride, and that is not dissolved in liquid nitrogen - fluoride. A method as claimed in claim 8, wherein the third component gas is bubbled through the liquefied nitrogen - fluoride. A method as claimed in claim 8, wherein the low temperature distillation is carried out under reduced pressure. A method as claimed in claim 8, wherein the low temperature distillation is a single distillation. A method as claimed in claim 8, wherein said nitrogen fluoride is obtained by a molten salt electrolytic process. A method as claimed in claim 12, wherein said nitrogen is obtained by a molten salt electrolytic process that uses an electrolytic cell equipped with a partition of a fluorocarbon resin or a partition having surfaces coated with a fluorocarbon resin. A method as claimed in claim 8, wherein the third component gas is copresent with nitrogen fluoride in a volume ratio of 0.01 to 10 to the nitrogen fluoride. A method as claimed in one or more of claims 1 to 14, wherein nitrogen trifluoride is used as nitrogen - fluoride.

Inventors: HARADA, ISAO 7-2, SAKO-MACHI 5-CHOME HIKOSHIMA, HYAKUTAKE, HIROYUKI 1541-2-3-5, YABE-CHO, IWANAGA, NARUYUKI 7-1, SAKO-MACHI 5-CHOME, NISHITSUJI, TOSHIHIKO 9-6, SAKO-MACHI 5-CHOME
Applicants: MITSUI TOATSU CHEMICALS, INC.
Application year: 1988
Main classification: F25J1
IPC codes: F25J3, C01B21, F25J1, C25B1